DCT - ozQRP.comozqrp.com/docs/DCT_manual_V1.pdf · 2019. 8. 14. · DCT Construction Manual –...
Transcript of DCT - ozQRP.comozqrp.com/docs/DCT_manual_V1.pdf · 2019. 8. 14. · DCT Construction Manual –...
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DCT Construction Manual – Issue 1 Page 1
DCT DSB/CW TRANSCEIVER
CONSTRUCTION MANUAL
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DCT Construction Manual – Issue 1 Page 2
CONTENTS
1 Introduction .............................................................................................................................. 6
2 DSB vs SSB .................................................................................................................................. 7
3 DC receiver ................................................................................................................................ 8
4 DSB Transmitter ...................................................................................................................... 9
5 CW Transmitter ..................................................................................................................... 10
6 DCT Block Diagram ............................................................................................................... 11
7 Circuit Description ................................................................................................................ 12
7.1 Microcontroller ........................................................................................................................... 12 7.2 Connector Board ......................................................................................................................... 12 7.3 VFO ................................................................................................................................................... 13 7.4 Receive Product Detector ........................................................................................................ 13 7.5 Receive Audio Amplifier .......................................................................................................... 13 7.6 Receive Audio Filters ................................................................................................................ 13 7.7 Headphone Amplifier ................................................................................................................ 13 7.8 Microphone Amplifier ............................................................................................................... 14 7.9 Balanced Modulator .................................................................................................................. 14 7.10 Transmit Pre-driver................................................................................................................... 15 7.11 Transmit Driver ........................................................................................................................... 15 7.12 Transmit Power Amplifier ...................................................................................................... 15 7.13 Power Supply and RX/TX switching ................................................................................... 16
8 Parts List ................................................................................................................................... 26
8.1 Main Board SMD Parts .............................................................................................................. 26 8.2 Main Board Through Hole Parts ........................................................................................... 29 8.3 Connector Board Parts ............................................................................................................. 30 8.4 Other parts .................................................................................................................................... 30 8.5 Band Specific Parts ..................................................................................................................... 30
9 Construction ............................................................................................................................ 31
9.1 General ............................................................................................................................................ 31 9.1.1 Recommended tool list ................................................................................................................... 31
9.1.2 Winding toroids................................................................................................................................. 31
9.2 Balanced Modulator .................................................................................................................. 32 9.2.1 Trimpot ................................................................................................................................................. 32
9.2.2 Transformer ........................................................................................................................................ 32
9.3 Transmit Low Pass Filter ......................................................................................................... 33 9.3.1 Capacitors ............................................................................................................................................ 33
9.3.2 Coils ........................................................................................................................................................ 33
9.4 Receive Bandpass Filter ........................................................................................................... 34 9.4.1 Capacitors ............................................................................................................................................ 34
9.4.2 Coil .......................................................................................................................................................... 34
9.5 Pre-Driver ...................................................................................................................................... 35 9.5.1 BD139 transistor .............................................................................................................................. 35
9.5.2 Transformer ........................................................................................................................................ 35
9.5.3 Filter Capacitors ................................................................................................................................ 36
9.5.4 Filter Coil .............................................................................................................................................. 36
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9.6 Driver .............................................................................................................................................. 37 9.6.1 BD139 transistor .............................................................................................................................. 37
9.6.2 Transformer ........................................................................................................................................ 37
9.7 LCD ................................................................................................................................................... 38 9.7.1 Trimpot ................................................................................................................................................. 38
9.7.2 Male header ......................................................................................................................................... 38
9.7.3 standoffs ............................................................................................................................................... 38
9.7.4 Female header .................................................................................................................................... 38
9.8 Connectors and Switches ......................................................................................................... 39 9.8.1 Relay....................................................................................................................................................... 39
9.8.2 BNC ......................................................................................................................................................... 39
9.8.3 DC ............................................................................................................................................................ 39
9.8.4 Slide Switch ......................................................................................................................................... 39
9.9 RF Power Amplifier ................................................................................................................... 40 9.9.1 Trimpot ................................................................................................................................................. 40
9.9.2 Output Transformer ........................................................................................................................ 40
9.9.3 IRF510 ................................................................................................................................................... 41
9.10 Connector Board ......................................................................................................................... 43 9.10.1 Board Assembly ................................................................................................................................ 43
9.10.2 Standoffs ............................................................................................................................................... 44
9.10.3 Connector Board Installation ...................................................................................................... 44
9.11 Controls .......................................................................................................................................... 45 9.11.1 Rotary Encoder .................................................................................................................................. 45
9.11.2 Potentiometers .................................................................................................................................. 45
10 Connector board jumper settings .................................................................................... 46
11 Making connections.............................................................................................................. 47
11.1 Power Supply ............................................................................................................................... 47 11.1.1 DC power supply ............................................................................................................................... 47
11.1.2 Internal Battery ................................................................................................................................. 47
11.2 Antenna .......................................................................................................................................... 47 11.3 Morse Key ...................................................................................................................................... 47 11.4 Headphones .................................................................................................................................. 47 11.5 Speaker/Microphone ................................................................................................................ 48 11.6 Microphone ................................................................................................................................... 48
12 First Time Power Up ............................................................................................................ 49
13 Configuration .......................................................................................................................... 50
13.1 To access Configuration: .......................................................................................................... 50 13.2 Navigating Configuration ........................................................................................................ 50 13.3 Configuration Items ................................................................................................................... 50
13.3.1 Firmware .............................................................................................................................................. 50
13.3.2 Callsign .................................................................................................................................................. 50
13.3.3 Frequency ............................................................................................................................................ 50
13.3.4 Freq Steps ............................................................................................................................................ 51
13.3.5 30S Freq Save ..................................................................................................................................... 51
13.3.6 Freq Calibrate ..................................................................................................................................... 51
13.3.7 Volts Calibrate .................................................................................................................................... 51
13.3.8 Pwr Mtr Cal ......................................................................................................................................... 51
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14 Testing and alignment ......................................................................................................... 52
14.1 General ............................................................................................................................................ 52 14.2 Receive ............................................................................................................................................ 52 14.3 Transmit ......................................................................................................................................... 53
14.3.1 PA bias setup ...................................................................................................................................... 53
14.3.2 Carrier Balance .................................................................................................................................. 53
14.3.3 Voice Test ............................................................................................................................................. 54
14.3.4 CW Test ................................................................................................................................................. 54
15 Final assembly ........................................................................................................................ 56
15.1 Case holes ...................................................................................................................................... 56 15.2 Installing the lid .......................................................................................................................... 56 15.3 Mounting in the case ................................................................................................................. 56
16 Operation ................................................................................................................................. 57
16.1 LCD ................................................................................................................................................... 57 16.2 Menu System ................................................................................................................................ 58 16.3 DSB Receive .................................................................................................................................. 59
16.3.1 Tuning steps ....................................................................................................................................... 59
16.3.2 AF Gain .................................................................................................................................................. 59
16.3.3 Attenuator ........................................................................................................................................... 59
16.3.4 Filter ....................................................................................................................................................... 59
16.3.5 RIT ........................................................................................................................................................... 59
16.4 DSB Transmit ............................................................................................................................... 59 16.5 CW Receive .................................................................................................................................... 60
16.5.1 Filter ....................................................................................................................................................... 60
16.6 CW transmit .................................................................................................................................. 60 16.6.1 Semi break in ...................................................................................................................................... 60
16.6.2 Answering A Call ............................................................................................................................... 60
16.6.3 CW drive ............................................................................................................................................... 60
16.6.4 Key delay .............................................................................................................................................. 61
16.6.5 Sidetone ................................................................................................................................................ 61
16.6.6 Automatic CQ Caller ......................................................................................................................... 61
16.7 Getting on the air ........................................................................................................................ 61
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List of Figures
Figure 1 Direct conversion receiver ......................................................................................................................... 8
Figure 2 DSB generation ................................................................................................................................................ 9
Figure 3 CW Transmitter ............................................................................................................................................ 10
Figure 4 DCT Block diagram ..................................................................................................................................... 11
Figure 5 Microcontroller ............................................................................................................................................ 17
Figure 6 Connector Board .......................................................................................................................................... 18
Figure 7 Main Board Connector Board interface.............................................................................................. 19
Figure 8 VFO and LCD .................................................................................................................................................. 20
Figure 9 Power Supply ................................................................................................................................................ 21
Figure 10 Product detector, Attenuator, Audio Filters and Switching .................................................... 22
Figure 11 Headphone amplifier ............................................................................................................................... 23
Figure 12 Mic Amp, Balanced Modulator, DC switch, Pre Driver and Filter ......................................... 24
Figure 13 Driver, Power Amplifier, LPF and Relay.......................................................................................... 25
Figure 14 Transmit Low pass Filter components ............................................................................................ 33
Figure 15 BD139 Lead identification .................................................................................................................... 35
Figure 16 Typical DC circuit voltages. ................................................................................................................... 55
Figure 17 Case holes ..................................................................................................................................................... 56
Change History Date Issue Comments
10/8/2019 1 First release
Acknowledgment.
Thanks to Peter Parker VK3YE for his invaluable assistance with prototype testing and
suggestions for improvement during the development of this kit.
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DCT Construction Manual – Issue 1 Page 6
1 INTRODUCTION The DCT (Double sideband and CW Transceiver) is an inexpensive and easy to build QRP kit for the 40M or 80M band. At the core of the transceiver is a microcontroller that controls a digital VFO and LCD, and the transceiver switching and control functions. Building the DCT is quick and easy. All components mount on the PCB and there is no discrete wiring required. Most parts are SMD and these come pre-installed on the PCB. Only a few through-hole parts need installing. The PCB is a high quality double sided type with ground plane, solder mask and silk screen. The receiver doesn’t require any alignment and the only setup required for the transmitter is setting the power amplifier bias and adjusting the balance modulator to null out the DSB carrier. A kit of parts for the DCT with everything you need, including an enclosure and silkscreened lid
is available from www.ozQRP.com.
DCT Specifications and features:
1. Size 196mm x 112mm x 60mm excluding knobs and connectors.
2. Sensitive direct conversion receiver.
3. Double Sideband voice transmitter.
4. CW transmitter with semi break-in.
5. Power output variable from 0 to 5W.
6. Stable digital VFO.
7. Customizable frequency tuning steps.
8. Tuning, AF gain, Mic gain and CW drive controls.
9. Easy access menu system.
10. Callsign entry.
11. Automatic CQ caller.
12. Use with low cost handheld speaker/mic or separate microphone and headphones.
13. Internal battery or external supply. Range 11 to 17V
14. Available in 40M or 80M versions.
15. Selectable low impedance dynamic or Electret microphone.
16. Compatible with Baofeng low cost handheld speaker/mic.
17. Backlit LCD displays frequency, transmit power, receive signal strength, battery voltage
and transceiver status.
18. Selectable audio filters: narrow 800Hz BPF or 2.3KHz LPF.
19. 3.5mm stereo headphone connector.
20. 3.5mm straight Morse key socket.
21. Power slide switch.
22. 2.1mm DC power socket – centre pin positive.
23. BNC antenna socket.
24. Carrier suppression up to 50dB.
25. All spurious transmit outputs better than -46dBc.
26. Receive current approximately 80mA.
27. Transmit current approximately 900mA at maximum power output.
28. Reverse polarity protection included.
http://www.ozqrp.com/
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2 DSB VS. SSB It may seem unusual that the DCT is a double sideband (DSB) transceiver when most voice
stations on the air use single sideband (SSB). The reason for this is simple – for the amateur kit
builder a DSB transceiver is less expensive, less complicated and easier to build and align than a
SSB transceiver. This is mainly because a DSB transceiver doesn’t have the complication of a
crystal filter, IF amplifier and multiple mixers used in a SSB rig.
While a DSB rig has many advantages for the home builder, there are some things to consider.
Firstly, a DSB transmitter occupies twice the bandwidth of a SSB transmitter. On a quiet band
this does not cause any problems but on a crowded band it may not be as easy to find a free spot
to operate without interfering with nearby stations. Secondly, the direct conversion receiver has
equal response to both sidebands. This means you hear signals on both upper and lower
sidebands simultaneously. This results in a slightly higher noise level and the possibility of
hearing two separate stations at the same time.
There are, however, a couple of nice advantages when a direct conversion receiver is used with
a DSB transmitter. Firstly, you can operate with SSB stations using Upper Sideband (USB) or
Lower Sideband (LSB) without having to change controls or move frequency. Secondly, being
able to hear both sidebands means that you can check for other stations on both sides of your
frequency before transmitting and avoid interfering with them.
Finally, DSB transmissions are entirely compatible with SSB transceivers, and in fact most operators won’t be aware unless you tell them.
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3 DC RECEIVER The receiver in the DCT is a direct conversion (DC) type. A DC receiver can receive both SSB and
CW signals. The main difference compared to a superhet receiver is that the VFO operates on
the same frequency as the received signal and there are no intermediate frequency stages.
A simplified example of a direct conversion receiver for 40M is shown in Figure 1.
Signals from the antenna are presented to the product detector, and mixed with the VFO signal.
The output of the product detector contains sum and difference frequency signals. The sum
frequency of 7.101MHz + 7.100MHz (14.201MHz) is easily filtered out by a low pass audio filter.
However the difference frequency of 7.101MHz – 7.100MHz (1KHz) can pass through the filter
and be heard in the headphones. This is the upper sideband response as the antenna signal
frequency of 7.101MHz is above the 7.100MHz VFO frequency.
Note that there is also another antenna signal that can be heard. This is the lower sideband
signal at 7.099MHz. This would also produce a 1KHz tone in the headphones.
This ability to simultaneously detect both upper and lower sidebands is an important
characteristic of a direct conversion receiver.
Figure 1 Direct conversion receiver
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4 DSB TRANSMITTER
Figure 2 shows the simplified diagram of a Double Sideband generator as used in the DCT. The
balanced modulator is a form of mixer and the term balanced refers to the fact that the original
input signals are suppressed at the output due to the balanced circuit configuration.
The first input to the balanced modulator is from the VFO. The second input is audio from the
microphone amplifier. The dominant outputs are the sum and difference frequencies, that is, the
sum and difference of the carrier and audio frequencies. In this 40M example, 7.101MHz
(7.100MHz + 1KHz) upper sideband, and 7.099MHz (7.100MHz – 1KHz) lower sideband.
Again, the important thing to note is that only the sidebands are present at the output as the
carrier and audio signals have been suppressed by the action of the balanced mixer.
The waveform in Figure 2 at top right shows the DSB output signal in the time domain, or how it
would be seen on an oscilloscope. Note the overlapping envelope shape that follows the audio
waveform. The diagram at bottom right shows the DSB output signal in the frequency domain
and how it would be seen on a spectrum analyser. The horizontal axis is frequency and the
vertical axis is amplitude. The dotted vertical line in the middle indicates the suppressed carrier
frequency.
By contrast, if this was a SSB transmitter, there would be a crystal filter placed after the mixer
and one of the sidebands would be filtered out. However, it would then be necessary to add
another mixer to move the SSB signal onto the wanted transmit frequency.
Figure 2 shows a single 1KHz tone for the audio signal. This is done to make it easier to
understand the process involved. In practice there will be a range of voice band frequencies in
the microphone signal, but the same principle applies.
Figure 2 DSB generation
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5 CW TRANSMITTER The DCT as well as being a DSB voice transmitter can transmit CW. In this mode the balanced
modulator is again used but the microphone amplifier is turned off. The VFO applies a carrier
frequency signal but because the modulator is balanced there will be no output without any
other input. A simplified diagram of the DCT CW transmitter is shown in Figure 3. In practice
the key is not directly connected to the DC switch, but monitored by the microcontroller which
in turn controls the DC switch.
When the key is operated the DC switch injects a DC current into the modulator. This causes it
to become unbalanced and generate a CW carrier wave. When the key is opened again the DC
current goes to zero and the carrier stops.
In Figure 3 the top waveform is the output carrier signal that gets amplified and passed to the
antenna. The bottom waveform shows the DC switch output. When the DC switch is high the
carrier is turned on, and when it is low it is turned off.
Figure 3 CW Transmitter
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6 DCT BLOCK DIAGRAM
Figure 4 DCT Block diagram
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7 CIRCUIT DESCRIPTION
7.1 MICROCONTROLLER
The microcontroller in the DCT (U2) is an Atmel ATmega328 running an internal 8MHz clock. It
controls the LCD, VFO and transceiver switching and control functions. There are limited output
pins on U2 so a 74HC595 port expander chip (U1) is used. U1 takes clk, data and latch lines from
U2 and outputs up to 8 individual control lines. Not all are used in this circuit.
The 16 character by 2 line LCD is controlled via a standard 4 wire data bus and control lines.
There are two pulse width modulated (PWM) outputs from U2. The first on pin 13 generates
sine wave audio tone frequencies. The high frequency 32KHz PWM carrier is filtered out by R3
and C2. The tones are fed to the audio amplifier for system beeps and for CW sidetone. The
second output on pin 14 is low pass filtered to provide a variable DC voltage and used to vary
the gain of the headphone amplifier.
The analog to digital converter in U2 receives two inputs. One on pin 22 from the receiver audio
to display signal strength on the LCD, and the second on pin 19 from the RF power amplifier to
indicate transmit power on the LCD.
PTT and key presses are monitored on pins 24 and 23 respectively. After processing they are
repeated by U2.
A rotary encoder is used as the tuning control. It produces quadrature signals when rotated and
one line (REA) is fed into the microcontroller interrupt input (pin 32) and becomes the
reference input. Once an interrupt is detected the state of the other line (REB) is read and the
direction determined. The rotary encoder also incorporates a press button switch input on U2
pin 1 and is used for a number of functions as described later. U2 incorporates pull-up resistors
for the encoder inputs so they are normally held high and capacitors C10 and C11 help limit
encoder switch bounce.
The Menu pushbutton is monitored on pin 30 and is normally pulled high by an internal pull-up
resistor.
7.2 CONNECTOR BOARD
The connector board is a small sub-board that is soldered into the main board via an 8 pin
header. It holds the microphone, headphone, CW key connectors and the Menu pushbutton. The
connector board signals are shown below:
Pin Signal Description
1 GND common ground
2 SPKR output from audio amp
3 PTT_IN input from PTT
4 KEY_IN input from CW key
5 GND common ground
6 MIC_SIG input from microphone
7 PB1 input from Menu push button
8 5VA 5V for Electret microphone
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7.3 VFO
A Si5351A clock generator chip (U5) is used as the VFO. A 25MHz crystal supplies the master
clock on pins 2 and 3. The Si5351A is controlled over the I2C bus by microcontroller U2. The
3.3V power supply rail is applied to pins 1 and 7. Capacitors C8 andC12 and a ferrite bead help
guard against instability and decouple the IC from the power supply. The RX VFO appears on pin
10 and the TX VFO output is on pin 9.
MOSFETs Q1 and Q2 form a bi-directional level shifter between the 5 volt microcontroller logic
level and the 3.3V logic level of the VFO.
7.4 RECEIVE PRODUCT DETECTOR
Signals from the antenna are passed through a bandpass filter comprised of L1, C18 and C19.
The ratio of the two capacitors provides an impedance conversion between the 50 ohm antenna
and the approximate 1K ohm single ended input impedance of the product detector U6. Diodes
D1 and D2 protect U6 from overvoltage spikes.
U6 is a SA612 and takes antenna signals on pin 1 and produces balanced audio on pins 4 and 5.
The 3.3V pk-pk VFO signal is reduced through a resistive divider network (R16 and R17) to
around 500mV pk-pk. Power for U6 comes from a 6.8V Zener diode and filtered by C23 and C24.
7.5 RECEIVE AUDIO AMPLIFIER
The output of U6 is input to differential amplifier U7a with a gain of around 10 times or 20dB.
To keep high frequencies out of the audio, low pass filtering is applied through capacitors C29
and C31.
Non-inverting amplifier U7b is configured as a switchable gain amplifier. With transistors Q3
and Q4 off the gain of U7b is unity or 0dB. When transistor Q3 is turned on capacitor C32 is
connected to ground via R26 and the gain increases to around 10 or 20dB. When Q3 is off and
transistor Q4 is turned on the gain is around 3.2 or 10dB.
The switching is controlled by the microcontroller to act as an adjustable receive attenuator.
7.6 RECEIVE AUDIO FILTERS
The output of the audio amplifier is applied to a 2.3KHz low pass filter formed with U8a.
Connected directly to the output of U8A is U8b configured as an 800Hz bandpass filter.
A half rail power supply or pseudo ground for the op-amps is formed by R19 and R20.
The filter outputs are switched through to the AF gain control via electronic switches. The low
pass filter switch uses Q5 and Q7, while the band pass filter switch uses Q6 and Q8. The
switches are under control of the microcontroller and selects which filter is in circuit. To enable
a filter, the associated MOSFET gate is taken high which pulls the drain low and the base of the
PNP transistor low. Audio can now pass via the low impedance of the transistor emitter to
collector path via C38 to the AF gain control VR2. Sampled audio is taken via C37 and DC biased
to half 5V for the signal meter A/D input of the microcontroller.
7.7 HEADPHONE AMPLIFIER
The headphone amplifier U9 is a LM4875. It is a bridge-tied load (BTL) output device normally
powering a loudspeaker in balanced mode, but in this circuit it is used in a single ended
headphone mode. This reduces the overall power output available but is adequate to drive
headphones and also the small speaker in handheld speaker/mics.
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Pin 2 is the audio input and receives the receiver audio as well as tones generated by the
microcontroller.
Power is supplied by a 5 volt regulator REG3 and filtered by C47 and C48. The audio output is
via capacitor C49 to the connector board. A 100 ohm resistor (R2) is placed across the
headphone line to guard against instability.
A feature of U9 is that it has a DC voltage controlled gain pin (pin 4). A voltage of about 3.5 volts
gives maximum gain and minimum gain is about 1.2 volts. The chip can be completely shut off
(muted) if the voltage is between 0.5 and 1 volt.
The gain is controlled by a microcontroller PWM output filtered by R44 and C46 to provide a
smooth DC control signal.
7.8 MICROPHONE AMPLIFIER
Microphone signals are coupled via capacitor C50 into the common emitter amplifier transistor
Q10. Capacitor C51 prevents RF getting into the base circuit. The gain is set to 20 mainly by
resistor R61. The output is applied to a discrete 2.3KHz low pass filter formed around transistor
Q12. The object here is to limit the transmitted bandwidth as we are occupying twice the
bandwidth of a SSB signal. The Mic gain control is the emitter resistor of Q12. The output of the
filter is amplified about 3 times and buffered by Q13. The low value collector resistor ensures a
low impedance drive to the modulator.
Power to the microphone amplifier is supplied via electronic switch Q5 and Q11. This allows the
microcontroller to turn off the amplifier in DSB receive and CW modes.
7.9 BALANCED MODULATOR
The balanced modulator is formed with T1, D3, D4, VR5 and L2. The TX VFO signal is applied to
the primary winding of T1. The secondary winding of T1 is made up of 2 windings where the
start of one winding is connected to the end of the second winding to form a centre tap. The
centre tap is effectively at ground potential to RF due to capacitor C60. The wiper of the
trimmer resistor is at ground potential for audio frequencies and DC due to choke L2.
The signal from TX VFO induces alternating current in the secondary of T1. During one half of
the waveform the Anode of D3 will be positive and the Cathode of D4 will be negative with
respect to ground. This causes both diodes to conduct equally resulting in the voltage at the
wiper to be zero. When the TX VFO polarity reverses, the Anode of D3 will be negative and the
Cathode of D4 will be positive and both diodes will be biased off. Again the voltage at the wiper
will be zero. This is called the balanced state because no VFO signal is being output.
In DSB mode an audio signal is injected into the centre tap and the mixer balance is upset
because the audio also induces current in the diodes. Because the audio signal changes state
much less frequently than the carrier signal the instantaneous diode currents are not equal. As a
result a signal is now developed at the wiper of VR5 and is a double sideband suppressed
carrier waveform.
Due to variations in component parameters the balance is not exact and if not compensated for
the some VFO signal (carrier) would be always present. Trimpot VR5 is used to balance the
diode currents in each side and bring the modulator into balance. In practice up to 50dB of
carrier suppression can be achieved.
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In CW mode, when the key is pressed, the microcontroller takes the KEY_OUT signal high and a
voltage appears across the CW drive potentiometer VR4. The wiper supplies current to the base
of Q14 and in turn a current through R69. This current passes into the centre tap of T1 and to
ground via choke L2.
A DC current in the modulator upsets the balance because one diode is turned on more than the
other and so RF is present at the trimpot wiper. Capacitor C59 is used to create a small ramp up
and down of the base voltage and hence modulator current to prevent sharp carrier on off
transitions that cause key clicks.
7.10 TRANSMIT PRE-DRIVER
The small signal from the modulator requires three stages of amplification to lift it to the 5 watt
level at the antenna. The first stage is the pre-driver formed around BD139 transistor Q15. It is
biased to 25mA collector current and has a large amount of negative feedback to stabilize
performance. Its loaded gain is low but its main purpose is to act as a buffer stage between the
modulator and the following filter.
A 3 pole low pass filter is formed with C64, C66 and L1 and is included to reduce the harmonic
content from the modulation stage. The addition of C65 across L1 forms a parallel circuit to
create a deep notch at the second harmonic.
7.11 TRANSMIT DRIVER
Transmit signal from the pre-driver is applied to the driver stage built around transistor Q16. A
BD139 works well here when biased with about 50mA of collector current. The design is well
proven using both shunt and series feedback to provide low input and output impedance and
good stable gain on the low HF bands.
7.12 TRANSMIT POWER AMPLIFIER
The power amplifier (Q17) is an IRF510 MOSFET and can provide in excess of 5 Watts PEP from
a 13.8 V drain supply. The output from the driver is applied across resistor R82 and becomes
the AC drive component for Q17 gate. Zener diode ZD2 and trimpot VR6 provides a stable and
variable DC gate voltage to place Q17 just into conduction for linear service. There is a short
ramp up of the gate voltage when switching to TX state as capacitor C71 charges and is included
to provide a smooth gate voltage transition.
The drain load for Q17 is a broadband bi-filar wound transformer (T4) and this configuration
was found to provide maximum output into a 50 ohm load. The waveform from Q17 can be high
in harmonics and so a 7 pole low pass filter is included to reduce the level of harmonic and other
spurious energy to an acceptable level.
The transmit signal is sampled by capacitor C88 and ground referenced by R88. The signal is
rectified by D6 and filtered by C89. This voltage is reduced in resistive divider R86 and R87 and
input to the microcontroller A/D to display power output on the LCD bar graph.
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7.13 POWER SUPPLY AND RX/TX SWITCHING
When the PTT is operated the TX/RX relay is energized and the transmit signal is passed to the
antenna. When the PTT is not operated the relay switches the antenna through to the receive
circuits. The relay also switches power to the TX and RX sections as required.
A ‘P’ channel power FET (Q19) protects the components in case the power supply is connected
in reverse. It works like this. For a P channel MOSFET to be turned on the gate must be at least
4V negative with respect to the source. The internal structure of a MOSFET has a diode
connected between the drain and source terminals and is reverse biased in its usual
application. However in this circuit if power is applied with the correct polarity, the internal
diode will be biased on and the source voltage will be around a volt below the drain voltage.
The gate is effectively at ground potential because of the 10K resistor and so the gate voltage is
more than 4V negative with respect to the source. This turns on the MOSFET and the drain to
source becomes a very low resistance, bypassing the diode, and supplying power to the board.
If the power supply is connected with reverse polarity the internal MOSFET diode will not
conduct. As a result the gate to source voltage never exceeds -4V and the MOSFET remains off
protecting the board. Zener diode ZD3 protects the gate from voltage spikes.
Un-switched 12V is distributed around the board to linear regulators to power the
microcontroller, VFO and headphone amplifier.
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Figure 5 Microcontroller
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Figure 6 Connector Board
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Figure 7 Main Board Connector Board interface
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Figure 8 VFO and LCD
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Figure 9 Power Supply
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Figure 10 Product detector, Attenuator, Audio Filters and Switching
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Figure 11 Headphone amplifier
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Figure 12 Mic Amp, Balanced Modulator, DC switch, Pre Driver and Filter
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Figure 13 Driver, Power Amplifier, LPF and Relay
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DCT Construction Manual – Issue 1 Page 26
8 PARTS LIST
8.1 MAIN BOARD SMD PARTS
Capacitors
Des Value Des Value
C1 100nF 50V 10% Ceramic MLCC C41 10nF 50V 10% Ceramic MLCC
C2 10nF 50V 10% Ceramic MLCC C42 10nF 50V 10% Ceramic MLCC
C3 1nF 50V 10% Ceramic MLCC C43 100nF 50V 10% Ceramic MLCC
C4 1nF 50V 10% Ceramic MLCC C44 10uF 25V electro
C5 1nF 50V 10% Ceramic MLCC C45 1uF 50V 10% Ceramic MLCC
C6 100nF 50V 10% Ceramic MLCC C46 1uF 50V 10% Ceramic MLCC
C7 100nF 50V 10% Ceramic MLCC C47 100uF 25V electro
C8 100nF 50V 10% Ceramic MLCC C48 100nF 50V 10% Ceramic MLCC
C9 100nF 50V 10% Ceramic MLCC C49 100uF 25V electro
C10 10nF 50V 10% Ceramic MLCC C50 100nF 50V 10% Ceramic MLCC
C11 10nF 50V 10% Ceramic MLCC C51 10nF 50V 10% Ceramic MLCC
C12 1uF 50V 10% Ceramic MLCC C52 100uF 25V electro
C13 1uF 50V 10% Ceramic MLCC C53 10uF 25V electro
C14 100nF 50V 10% Ceramic MLCC C54 10nF 50V 10% Ceramic MLCC
C15 100nF 50V 10% Ceramic MLCC C55 4.7nF 50V 10% Ceramic MLCC
C16 1uF 50V 10% Ceramic MLCC C56 100nF 50V 10% Ceramic MLCC
C17 100nF 50V 10% Ceramic MLCC C57 1uF 50V electro
C20 1nF 50V 10% Ceramic MLCC C58 100nF 50V 10% Ceramic MLCC
C21 100nF 50V 10% Ceramic MLCC C59 100nF 50V 10% Ceramic MLCC
C22 1nF 50V 10% Ceramic MLCC C60 10nF 50V 10% Ceramic MLCC
C23 10uF 25V electro C61 100nF 50V 10% Ceramic MLCC
C24 100nF 50V 10% Ceramic MLCC C62 100nF 50V 10% Ceramic MLCC
C25 47nF 50V 10% Ceramic MLCC C63 100nF 50V 10% Ceramic MLCC
C26 100uF 25V electro C67 100nF 50V 10% Ceramic MLCC
C27 47nF 50V 10% Ceramic MLCC C68 100nF 50V 10% Ceramic MLCC
C28 47nF 50V 10% Ceramic MLCC C69 100nF 50V 10% Ceramic MLCC
C27 470pF 50V 10% Ceramic MLCC C70 100nF 50V 10% Ceramic MLCC
C30 100uF 25V electro C71 1uF 50V electro
C31 470pF 50V 10% Ceramic MLCC C72 100nF 50V 10% Ceramic MLCC
C32 1uF 50V electro C73 100nF 50V 10% Ceramic MLCC
C33 2.2nF 50V 10% Ceramic MLCC C74 100nF 50V 10% Ceramic MLCC
C34 10nF 50V 10% Ceramic MLCC C75 100nF 50V 10% Ceramic MLCC
C35 100uF 25V electro C76 100uF 25V electro
C36 4.7nF 50V 10% Ceramic MLCC C77 100uF 25V electro
C37 100nF 50V 10% Ceramic MLCC C78 100nF 50V 10% Ceramic MLCC
C38 1uF 50V electro C79 100nF 50V 10% Ceramic MLCC
C39 10nF 50V 10% Ceramic MLCC C88 22pF C0G 50V 10% Ceramic MLCC
C40 10nF 50V 10% Ceramic MLCC C89 100nF 50V 10% Ceramic MLCC
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DCT Construction Manual – Issue 1 Page 27
Resistors
Des Value Des Value
R1 10K 1/8W 1% 0805 R45 1K 1/8W 1% 0805
R2 100Ω 1/8W 1% 0805 R46 100K 1/8W 1% 0805
R3 4.7K 1/8W 1% 0805 R47 1K 1/8W 1% 0805
R4 1K 1/8W 1% 0805 R48 100K 1/8W 1% 0805
R5 1K 1/8W 1% 0805 R49 4.7K 1/8W 1% 0805
R6 1K 1/8W 1% 0805 R50 4.7K 1/8W 1% 0805
R7 30K 1/8W 1% 0805 R51 100K 1/8W 1% 0805
R8 10K 1/8W 1% 0805 R52 100K 1/8W 1% 0805
R9 4.7K 1/8W 1% 0805 R53 1K 1/8W 1% 0805
R10 4.7K 1/8W 1% 0805 R54 100K 1/8W 1% 0805
R11 22K 1/8W 1% 0805 R55 470K 1/8W 1% 0805
R12 4.7K 1/8W 1% 0805 R56 4.7K 1/8W 1% 0805
R13 2.2K 1/8W 1% 0805 R57 470Ω 1/8W 1% 0805
R14 2.2K 1/8W 1% 0805 R58 4.7K 1/8W 1% 0805
R15 4.7K 1/8W 1% 0805 R59 100K 1/8W 1% 0805
R16 470Ω 1/8W 1% 0805 R60 4.7K 1/8W 1% 0805
R17 100Ω 1/8W 1% 0805 R61 100Ω 1/8W 1% 0805
R18 470Ω 1/8W 1% 0805 R62 470Ω 1/8W 1% 0805
R19 4.7K 1/8W 1% 0805 R63 10K 1/8W 1% 0805
R20 4.7K 1/8W 1% 0805 R64 56K 1/8W 1% 0805
R21 10K 1/8W 1% 0805 R65 560Ω 1/8W 1% 0805
R22 10K 1/8W 1% 0805 R66 100Ω 1/8W 1% 0805
R23 100K 1/8W 1% 0805 R67 1K 1/8W 1% 0805
R24 470Ω 1/8W 1% 0805 R68 10K 1/8W 1% 0805
R25 100K 1/8W 1% 0805 R69 2.2K 1/8W 1% 0805
R26 1K 1/8W 1% 0805 R70 100Ω 1/8W 1% 0805
R27 100K 1/8W 1% 0805 R71 470Ω 1/8W 1% 0805
R28 3.9K 1/8W 1% 0805 R72 2.2K 1/8W 1% 0805
R29 10K 1/8W 1% 0805 R73 100Ω 1/8W 1% 0805
R30 4.7K 1/8W 1% 0805 R74 100Ω 1/8W 1% 0805
R31 1K 1/8W 1% 0805 R75 47Ω 1/8W 1% 0805
R32 10K 1/8W 1% 0805 R76 470Ω 1/8W 1% 0805
R33 10K 1/8W 1% 0805 R77 2.2K 1/8W 1% 0805
R34 10K 1/8W 1% 0805 R78 10Ω 1/8W 1% 0805
R35 100K 1/8W 1% 0805 R79 27Ω 1/8W 1% 0805
R36 10K 1/8W 1% 0805 R80 10Ω 1/8W 1% 0805
R37 4.7K 1/8W 1% 0805 R81 560Ω 1/8W 1% 0805
R38 470Ω 1/8W 1% 0805 R82 47Ω 1/8W 1% 0805
R39 62K 1/8W 1% 0805 R83 1K 1/8W 1% 0805
R40 3.3K 1/8W 1% 0805 R84 100K 1/8W 1% 0805
R41 4.7K 1/8W 1% 0805 R85 10K 1/8W 1% 0805
R42 10K 1/8W 1% 0805 R86 27K 1/8W 1% 0805
R43 130K 1/8W 1% 0805 R87 10K 1/8W 1% 0805
R44 4.7K 1/8W 1% 0805 R88 2.2K 1/8W 1% 0805
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DCT Construction Manual – Issue 1 Page 28
Semiconductors
Des Value Des Value
D1 1N4148W signal diode SOD-123 Q10 MMBT3904 NPN transistor SOT23-3
D2 1N4148W signal diode SOD-123 Q11 MMBT3906 NPN transistor SOT23-3
D3 1N4148W signal diode SOD-123 Q12 MMBT3904 NPN transistor SOT23-3
D4 1N4148W signal diode SOD-123 Q13 MMBT3904 NPN transistor SOT23-3
D5 1N4148W signal diode SOD-123 Q14 MMBT3904 NPN transistor SOT23-3
D6 1N4148W signal diode SOD-123 Q18 BSS138 N-ch MOSFET SOT23-3
ZD1 6.8V 500mW Zener SOD-123 Q19 NDT452AP P-ch MOSFET SOT223-4
ZD2 6.8V 500mW Zener SOD-123 REG1 78L05 regulator SOT89-3
ZD3 6.8V 500mW Zener SOD-123 REG2 MCP7100-3.3 regulator SOT23-3
Q1 BSS138 N-ch MOSFET SOT23-3 REG3 78L05 regulator SOT89-3
Q2 BSS138 N-ch MOSFET SOT23-3 U1 74HC595 Ser to Par SOIC-16
Q3 MMBT3904 NPN transistor SOT23-3 U2 ATmega328PA Micro 32TQFP
Q4 MMBT3904 NPN transistor SOT23-3 U5 S15351 Oscillator 10MSOP
Q5 BSS138 N-ch MOSFET SOT23-3 U6 SA612 Mixer SOIC-8
Q6 BSS138 N-ch MOSFET SOT23-3 U7 LM833 Dual op-amp SOIC-8
Q7 MMBT3906 PNP transistor SOT23-3 U8 LM833 Dual op-amp SOIC-8
Q8 MMBT3906 PNP transistor SOT23-3 U9 LM4875 headphone amp SOIC-8
Q9 BSS138 N-ch MOSFET SOT23-3 X1 25MHz crystal 5x3.2
Coils
Des Value Des Value
FB1 Ferrite bead 0805 L2 100uH 1812
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DCT Construction Manual – Issue 1 Page 29
8.2 MAIN BOARD THROUGH HOLE PARTS
Resistors
Des Value Des Value
VR1 20K vertical trimpot VR4 10K LIN 9mm vertical potentiometer
VR2 10K LOG 9mm vertical potentiometer VR5 500Ω multi-turn vertical trimpot
VR3 10K LOG 9mm vertical potentiometer VR6 20K vertical trimpot
Semiconductors
Des Value Des Value
Q15 BD139 transistor TO-126 Q17 IRF510 N-ch Power MOSFET TO-220
Q16 BD139 transistor TO-126
Connectors
Des Value Des Value
J1 BNC connector RA LCD1 16 pin male header
SK3 2.1mm DC connector RA
Switches
Des Value Des Value
SW1 SPDT slide switch RA K1 DPDT relay
RE1 Rotary encoder with switch vertical
Coils
Des Value Des Value
T1 FT37-43 5:3+3 T3 FT37-43 8T:3T
T2 FT37-43 8T:3T T4 FT50-43 5T+5T
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DCT Construction Manual – Issue 1 Page 30
8.3 CONNECTOR BOARD PARTS
Resistors
Des Value Des Value
J1 2.5mm stereo socket R1 2.2K 1/4W 5% resistor
J2 3.5mm stereo socket R2 22 Ω 1/4W 5% resistor
J3 3.5mm stereo socket SK1 8 pin male header RA
J4 3.5mm stereo socket SK2 2 x 6 pin male header
PB1 6mm tactile switch RA
8.4 OTHER PARTS
QTY Value
1 16 x 2 blue backlight LCD display
4 Knobs with screw
1 Silkscreen printed lid
1 plastic case
4 metal 20mm spacers
4 nylon 12mm spacers
12 3mm x 6mm screw - zinc
4 3mm x 6mm screw - black
4 No. 4 self tapping screw - black
4 3mm star washers
1 3mm x 10mm screw - zinc
1 3mm nut
1 TO-220 insulating washer
1 3mm insulating bush
4 shunts
4 self-adhesive feet
8.5 BAND SPECIFIC PARTS
See section 9 for component details.
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DCT Construction Manual – Issue 1 Page 31
9 CONSTRUCTION
9.1 GENERAL
The DCT is built on a high quality fiberglass PCB. The PCB is doubled sided with the majority of
the tracks on the bottom side with the top side forming a ground plane.
To assist construction the component overlay is screen printed on the top side and a solder
mask is included to help guard against solder bridges.
The ground plane is substantial and can sink quite a bit of heat from low wattage soldering irons
so ensure you use a good quality iron that can sustain the power required. You may find that
sometimes solder doesn’t appear to flow through to the top side. This is not necessarily a
problem because the plated through holes make a connection to the top side automatically.
Another point to consider is that plated through holes consume more solder than non-plated
holes and makes it more difficult to remove components. So check the value and orientation of
components before soldering!
To make assembly much simpler the PCB comes with all SMD parts already soldered. The
construction only requires the installation of the through hole parts and mechanical assembly.
9.1.1 RECOMMENDED TOOL LIST Below is a minimum list of tools required to build the DCT:
soldering iron
solder
side cutters
drill
reamer
small file
small knife or scalpel
metric rule
Phillips head screwdriver
Small flat blade screwdriver
9.1.2 WINDING TOROIDS Winding toroids is a task that many first time builders find difficult. However with patience and
attention to detail it’s not that hard. Here are a few tips:
Cut the wire to length before starting.
Whenever a wire passes through the centre of the toroid, it is counted as a turn, so count
inside not outside.
For windings with many turns it is easier to pass the wire though the hole to half way.
Wind on half the number of turns with one end and then repeat for the other end.
Use a knife, scalpel or sandpaper to remove the enamel from the ends of the wires and
then tin with solder before inserting in the PCB.
To twist a bifilar winding, a battery drill with a cup hook in the chuck makes it easy.
Hold one end firm and loop the other end over the cup hook. Run the drill until the
desired number of twists per centimetre is achieved.
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DCT Construction Manual – Issue 1 Page 32
9.2 BALANCED MODULATOR
9.2.1 TRIMPOT Insert the 500 ohm multi-turn trimpot into the PCB and solder in place. The adjustment screw faces out from the edge of the PCB.
9.2.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.
Designator Winding Length Turns
T1 Primary 100mm 5
Secondary 100mm x 2 3+3
The secondary is a bifilar winding. Take the two lengths and place alongside each other. Then, holding one pair of ends firm, rotate the other pair of ends until there are about 3 twists per centimetre. The combined secondary wires are then wound on the toroid as one. Scrape enamel off the end of each wire and spread them apart. Determine the start and end of each of the secondary windings with a multimeter set to continuity. Take the start of one winding and the end of the other and twist them together to form a centre tap. Tin the wire ends with solder before inserting into the PCB.
Primary
Start 1 Start 2
End 1
End 2
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DCT Construction Manual – Issue 1 Page 33
9.3 TRANSMIT LOW PASS FILTER
9.3.1 CAPACITORS The PCB has provision for two capacitors at each location, however only one is required and
installed as shown below.
Designator 80M 40M
Marking Value Marking Value
C80 821 820pF 471 470pF
C81 - - - -
C82 152 1500pF 821 820pF
C83 - - - -
C84 152 1500pF 821 820pF
C85 - - - -
C86 821 820pF 471 470pF
C87 - - - -
9.3.2 COILS All coils are wound on red T37-2 powdered iron toroids using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to cover about 80% of the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.
Designator 80M 40M
Wire length Turns Wire length Turns
L4 360mm 25 280mm 18
L5 360mm 26 280mm 19
L6 360mm 25 280mm 18
Figure 14 Transmit Low pass Filter components
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DCT Construction Manual – Issue 1 Page 34
9.4 RECEIVE BANDPASS FILTER
9.4.1 CAPACITORS Select and install the capacitors as shown below. The kit may be supplied with disc ceramic,
MLCC ceramic or a mix of types.
Designator 80M 40M
Marking Value Marking Value
C18 561 560pF 471 470pF
C19 222 2200pF 152 1500pF
9.4.2 COIL The coil is wound on a red T37-2 powdered iron toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to evenly over the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.
Designator
80M (3.5 – 3.7) 80M (3.7- 3.9) 40M
Wire length
Turns Wire
length Turns
Wire length
Turns
L1 440mm 33 420mm 31 250mm 18
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DCT Construction Manual – Issue 1 Page 35
9.5 PRE-DRIVER
9.5.1 BD139 TRANSISTOR Figure 15 shows how to identify the BD139 pins. With the transistor placed on a flat surface,
and the writing facing up, the Base lead is on the right hand side.
Figure 15 BD139 Lead identification
Insert the transistor into the PCB with the Base lead towards capacitor C61. Push down until it
rests against the PCB and solder in place.
9.5.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.
Designator Winding Length Turns
T2 Primary 140mm 8
Secondary 80mm 3
Scrape enamel off the end of each wire and tin the wire ends with solder before inserting into the PCB.
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DCT Construction Manual – Issue 1 Page 36
9.5.3 FILTER CAPACITORS Select and install the capacitors as shown below. The kit may be supplied with disc ceramic,
MLCC ceramic or a mix of types.
Designator 80M 40M
Marking Value Marking Value
C64 681 680pF 391 390pF
C65 221 220pF 151 150pF
C66 681 680pF 391 390pF
9.5.4 FILTER COIL The coil is wound on a red T37-2 powdered iron toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to cover about 80% of the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.
Designator 80M 40M
Wire length Turns Wire length Turns
L3 290mm 21 200mm 14
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DCT Construction Manual – Issue 1 Page 37
9.6 DRIVER
9.6.1 BD139 TRANSISTOR Insert the transistor into the PCB with the Base lead towards capacitor C67. Push down until it
rests against the PCB and solder in place. See Figure 15 for lead identification.
9.6.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.
Designator Winding Length Turns
T3 Primary 140mm 8
Secondary 80mm 3
Scrape enamel off the end of each wire and tin the wire ends with solder before inserting into the PCB.
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DCT Construction Manual – Issue 1 Page 38
9.7 LCD
9.7.1 TRIMPOT Insert a 20K trimpot into position VR1 and solder.
9.7.2 MALE HEADER Insert a 16 pin male header into the PCB with the long pins facing upwards. Solder a single pin
first and check the strip is sitting flat and at right angles to the PCB before soldering the
remaining pins. If the kit is supplied with a strip longer than 16 pins, simply snap off the
unwanted pins.
9.7.3 STANDOFFS Mount four 12mm nylon spacers into the LCD mounting holes using 3mm x 6mm long screws.
9.7.4 FEMALE HEADER Insert the 16 pin female header into the LCD from the rear. Solder a single pin first and check it
is sitting flat and at right angles to the PCB before soldering the remaining pins.
Don’t install the LCD at this stage. This will be done once the main PCB is finished.
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DCT Construction Manual – Issue 1 Page 39
9.8 CONNECTORS AND SWITCHES
9.8.1 RELAY One end of the relay has a line printed on top of the case for identification. This side faces
towards the centre of the PCB. Once inserted hold against the PCB and solder the pins.
9.8.2 BNC Insert the BNC connector into the PCB and solder. Make sure it sits flat against the board. The
large mounting posts will need a reasonable wattage iron for the solder to flow properly.
9.8.3 DC Insert the 2.1mm DC connector into the PCB and solder. Make sure it sits flat against the board.
9.8.4 SLIDE SWITCH Insert the slide switch into the PCB and solder. Make sure it sits flat against the board.
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DCT Construction Manual – Issue 1 Page 40
9.9 RF POWER AMPLIFIER
9.9.1 TRIMPOT Insert a 20K trimpot into position VR6 and solder.
9.9.2 OUTPUT TRANSFORMER The output transformer is wound on a FT50-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.
Designator Length Turns
T4 150mm+150mm 5
The winding is a single bifilar winding. Take the two lengths and place alongside each other. Then, holding one pair of ends firm, rotate the other pair of ends until there are about 3 twists per centimetre. The combined secondary wires are then wound on the toroid as one. Scrape enamel off the end of each wire and spread them apart. Determine the start and end of each of the windings with a multimeter set to continuity. It does not matter which one is labeled winding one or two. Take the start of one winding and the end of the other and twist them together to form a centre tap. Tin the wire ends with solder before inserting into the PCB.
The centre tap goes into the pad closest to C75.
Start 1
Start 2 End 1
End 2
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DCT Construction Manual – Issue 1 Page 41
9.9.3 IRF510
The IRF510 MOSFET is a static sensitive device. While it is unlikely it will be damaged, it is best to use anti-static precautions such as not directly touching the pins and using a grounded wrist strap.
Bend the leads up at right angles 6mm from the edge of the plastic housing.
Place a bush on the 3mm x 10mm screw and insert into the IRF510 tab hole. Place the insulating
washer over the screw onto the heatsink tab.
Insert the assembly from the bottom side of the PCB, so that the 3 bent leads come up though
the holes in the PCB and the screw protrudes through the PCB mounting hole.
Check the insulating washer is aligned correctly, then place a nut on the screw and tighten.
Solder the leads.
Using a multimeter set to continuity, check that there are no short circuits between the IRF510 tab and the PCB ground.
6mm
3mm nut
Insulating
washer
3mm x 10mm screw
Bush
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DCT Construction Manual – Issue 1 Page 42
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DCT Construction Manual – Issue 1 Page 43
9.10 CONNECTOR BOARD
Note that the connector board has parts inserted from both top and bottom side. Follow the
steps below closely and check the parts are positioned correctly before soldering. Removing
parts later will be difficult without proper de-soldering equipment.
9.10.1 BOARD ASSEMBLY
1 Insert the 2 resistors into
the PCB from the top side. R1 is 2.2K and R2 is 22 ohms. Splay the leads slightly underneath, solder and trim the excess.
2 Mount the 2.5mm jack
socket (J1) into the PCB. Ensure it sits flat before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.
3 Mount the three 3.5mm
jack sockets (J2, J3 and J4) into the PCB. Ensure they sit flat and not rotated before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.
4 Mount the right angle
tactile pushbutton switch (PB1) into the PCB. Ensure it sits flat and at right angle to the PCB before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.
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DCT Construction Manual – Issue 1 Page 44
5 Insert the two 6 pin header
strips into the bottom side of the PCB. The short pins go into the PCB pads. Ensure they sit flat and at right angle to the PCB before soldering in place.
6 Mount the right angle 8 pin
header strip in the top side of the PCB. The short pins go into the PCB pads. Ensure it sits flat and at right angle to the PCB before soldering in place.
7 Fit the cap to the
pushbutton and the shunts to the dual 6 pin header. See section 10 on selecting the shunt positions.
9.10.2 STANDOFFS Install the four 20mm metal standoffs in the corners of the main board using 3mm x 6mm long
screws. Insert a 3mm star washer between the standoff and the PCB.
9.10.3 CONNECTOR BOARD INSTALLATION Insert the Connector board into the 8 pin header pads labeled SK1.
Place the lid on top of the four standoffs and maneuver the connector board so that the
connectors and pushbutton pass through the holes in the lid. Temporarily screw the lid down in
two places to prevent the connector board moving. Turn the assembly over and solder the
header pins. Remove the lid.
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DCT Construction Manual – Issue 1 Page 45
9.11 CONTROLS
9.11.1 ROTARY ENCODER The rotary encoder is the tune control and is installed on the right hand side of the LCD.
Insert it into the PCB and ensure it sits flat and vertical and solder one of the small pins. To
check it’s positioned correctly, place the lid onto the 4 corner standoffs. Check the encoder shaft
is on the centre of the hole in the lid. If not, gently push the encoder in the required direction.
Then carefully take off the lid without moving the encoder and solder the remaining pins.
9.11.2 POTENTIOMETERS There are 3 pots to install. VR2 and VR3 are logarithmic taper while VR4 is a linear taper type.
Use the table below to identify them. The code can be seen on the underside of the pot body.
Designator Function Value Code
VR2 AF Gain 10K LOG A103
VR3 Mic Gain 10K LOG A103
VR4 CW Drive 10K LIN B103
Install the potentiometers in the same fashion as the rotary encoder, using the lid placed on top
as an alignment guide.
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DCT Construction Manual – Issue 1 Page 46
10 CONNECTOR BOARD JUMPER SETTINGS The DCT connector board has a jumper block that configures the DCT for three types of
microphones.
1. Handheld speaker/microphone (Electret).
2. Dynamic microphone.
3. Individual Electret microphone
Electret microphones differ from dynamic types in that they contain an amplifier which requires
a bias power supply. This is the main reason for the jumpers - to enable the bias power line for
Electret types. A secondary reason is to reconfigure the microphone connector between the
non-standard speaker/mic configuration and a more common configuration when an individual
microphone is used.
Microphone Type Jumper Settings Notes
Handheld speaker/microphone
Tip – not used Ring – mic + bias
Sleeve – PTT (GND is via speaker connector)
Dynamic
Tip – mic Ring – PTT
Sleeve - GND
Individual Electret
Tip – mic + bias Ring – PTT
Sleeve - GND
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DCT Construction Manual – Issue 1 Page 47
11 MAKING CONNECTIONS
11.1 POWER SUPPLY
11.1.1 DC POWER SUPPLY The DCT can be powered from an external power supply between 11 and 17V.
The 2.1mm DC connector centre pin is positive and the outer barrel is negative or ground.
11.1.2 INTERNAL BATTERY For portable operation the DCT can be powered from an internal battery pack. Connection is
made by soldering wires onto the two pads on the PCB marked 12V.
The power switch on and off positions are reversed when changing power sources.
11.2 ANTENNA
The DCT requires a matched 50 ohm impedance antenna to be connected to the BNC connector
for proper operation.
11.3 MORSE KEY
A straight Morse key is connected to the Key connector. The Key connector is a stereo 3.5mm
jack, however the key cable can have a stereo or mono plug. If using a stereo plug the ring
circuit is not used.
Sleeve - Ground Ring – not used Tip - key
11.4 HEADPHONES
The DCT headphone socket is a stereo 3.5mm jack intended to connect a set of 32 ohm stereo
headphones. The Connector board has a 22 ohm resistor in series to limit the power level.
Sleeve - Ground Ring – Right Tip - Left
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DCT Construction Manual – Issue 1 Page 48
11.5 SPEAKER/MICROPHONE
The DCT is compatible with a commonly available BAOFENG handheld speaker/mic. This uses
two connectors, a 2.5mm stereo jack for the speaker and ground connections plus a 3.5mm
stereo jack for the microphone signal and PTT line.
3.5mm Sleeve - PTT Ring – mic signal + bias voltage Tip – not used
2.5mm Sleeve - Ground Ring – not used Tip - speaker
11.6 MICROPHONE
If a separate microphone is used it can be either an Electret type or a dynamic type.
The DCT connector is a 3.5mm stereo jack with the ring not used.
Sleeve - Ground Ring – not used Tip – mic signal
For proper operation of the microphone the jumper settings on the Connector board must be set to the correct positions. See section 10.
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DCT Construction Manual – Issue 1 Page 49
12 FIRST TIME POWER UP With the DCT main board finished it’s time to power up, but first make a thorough visual
inspection of the PCB. Pay particular attention to part placement, solder bridges across pads and
dry solder joints around coil wires.
The kit comes with a pre-programmed microcontroller so once power is applied it should come
to life.
Plug in the LCD and secure with four 3mm x 6mm long screws.
Connect a 50 ohm dummy load to the BNC antenna connector. This is to terminate the
transmitter if something goes wrong.
If a bench top power supply is used, set it to 13.8V and current limit to around 200mA.
If the power supply does not have a current meter use a multimeter to measure supply
current.
Plug a speaker/microphone or headphones into the Connector board.
Rotate the PA bias trimpot fully counter clockwise.
Rotate all pots fully anticlockwise.
Plug the power into the DC connector and move the slide switch to the ON position.
The current should be around 80mA. If far from this turn off immediately and look for
errors.
The LCD backlight should come on and 2 beeps should be heard. Adjust the LCD contrast
control VR1 until the characters are visible and clear. At power on an opening screen is
displayed, followed a few seconds later by the operating screen.
Referring to Figure 16 check the voltages around the circuit for receive mode.
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DCT Construction Manual – Issue 1 Page 50
13 CONFIGURATION Before testing and operation can commence the DCT must be configured. Configuration holds
settings that only need to be set once or changed infrequently.
The minimum items to set at this stage are the Frequency and Freq Steps. The other items can
be set later as needed.
13.1 TO ACCESS CONFIGURATION:
Turn off power to the DCT.
Hold down the Menu button and turn on power.
Wait until the LCD displays Configuration and 4 beeps are heard.
Release the Menu button.
13.2 NAVIGATING CONFIGURATION
Pressing the Menu button steps though the configuration items one by one. As each new
item is displayed a beep is heard.
Settings are displayed for each item if they have already been programmed.
Options within a configuration item are selected and set with the Tune knob, either
rotating or pressing as described below.
By simply stepping through the items with the Menu button, previously set values will
not be altered.
At the completion of all items the settings are saved in non-volatile memory and
operation returns to normal.
13.3 CONFIGURATION ITEMS
13.3.1 FIRMWARE This screen displays the current installed firmware version and is not editable.
13.3.2 CALLSIGN This item allows the entering of a callsign of up to 10 characters. This is used for two purposes.
Firstly to personalize the DCT by displaying the callsign in the opening screen, and secondly,
sent as part of the automatic CQ caller.
Press the Tune knob to step through the character positions.
Rotate the Tune knob to select and set the required character.
To clear and start again, press the Tune knob down for more than 1 second.
The available characters are digits 0-9, letters A-Z, and a slash (‘/’).
13.3.3 FREQUENCY This sets the initial frequency and band of operation.
Press the Tune knob to step through the digit positions.
Rotate the Tune knob to select the required digit.
The range is 100KHz to 100MHz.
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DCT Construction Manual – Issue 1 Page 51
13.3.4 FREQ STEPS As the tuning knob is rotated the frequency changes value by a fixed step.
This item allows the selection of the number of available steps and the value of each step.
Rotate the Tune knob to select the desired step as shown in the LCD top row.
Press the Tune knob to add the step to the list of selected steps. These are shown in the
bottom row.
To clear the selections and start again press and hold the Tune knob down for more than
1 second.
A minimum of 1 step and a maximum of 4 steps can be programmed.
The steps may have any combination of 1Hz, 10Hz, 50Hz, 100Hz, 500Hz, 1KHz, 5KHz or 10KHz.
13.3.5 30S FREQ SAVE This item enables automatic saving of the current frequency in EEPROM so that it is recalled the
next time the DCT is powered on. This is performed only once per frequency change and only
after a 30 second idle period has elapsed. If the encoder is operated so that there is less than 30
seconds between changes no saves will be performed. Pressing the encoder button toggles the
selection.
Options are Yes and No.
13.3.6 FREQ CALIBRATE This item allows trimming of the VFO output frequency in firmware so that the operating
frequency matches the displayed frequency.
Rotating the Tune knob changes the value.
Pressing the Tune knob switches between plus and minus.
Range is -990Hz to +990Hz in 10Hz steps.
13.3.7 VOLTS CALIBRATE The microcontroller measures the battery voltage using an analog to digital converter through a
resistive divider. Due to the tolerance in components the displayed value may not be the same
as the battery voltage. This option allows the displayed value to be trimmed so it is spot on.
Rotating the Tune knob changes the value.
Pressing the Tune knob switches between plus and minus.
The range is -0.9V to +0.9V in 0.1V steps.
13.3.8 PWR MTR CAL This option adjusts the LCD transmit bar graph to display the correct power. After calibration
each bar is approximately equal to 1W of output power.
Connect a 50 ohm load and power meter to the antenna connector.
Plug in a Morse key and turn the CW drive control to minimum.
Press the Tune knob briefly to place the DCT in CW test mode.
Operate the Morse key and adjust the CW drive level to show 5W on the external power
meter.
Rotate the Tune knob until the bar graph shows exactly 5 full bars. Do not hold the key
down for long periods otherwise the PA may get too hot.
The number in the top row of the LCD is a calibration number used by the firmware and
is displayed for guidance only.
The range is -50 to +50.
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DCT Construction Manual – Issue 1 Page 52
14 TESTING AND ALIGNMENT
14.1 GENERAL
The minimum tools and accessories needed to get the DCT tested and aligned are listed below:
Digital multimeter.
50 ohm dummy load capable of dissipating at least 5 Watts.
Power supply capable of 13.8V DC regulated at 1 Amp or a suitable battery.
Small flat blade adjustment screwdriver.
These additional instruments are recommended to fully test and align the DCT.
QRP wattmeter or oscilloscope.
An audio signal generator.
An RF signal generator.
Figure 16 lists typical voltages at various points around the board and can be used to verify operation.
14.2 RECEIVE
Plug in a speaker/microphone or headphones.
Set the Tune control to around the middle of the range.
Turn the AF gain control to halfway. You should hear some low level hiss indicating that
the audio stages are working.
Apply an RF signal of about 100uV to the antenna connector and rotate the Tune control
until a clear tone is heard in the headphones. If no signal is heard check the receive
circuit by tracing the signal from the product detector back through the audio chain.
Check the AF gain control operates.
Check that the LCD bar graph moves up and down as the signal is tuned.
Using the Menu button select the various attenuator settings and verify operation.
Check the BPF and LPF can be selected via the Menu button.
If you don’t have an RF signal generator simply plug in an antenna and listen. You will get a good idea if the receiver is working correctly by comparing to another receiver.
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DCT Construction Manual – Issue 1 Page 53
14.3 TRANSMIT
During the transmitter testing phase operate the PTT for only short periods until fully set up. If the PTT is operated for long periods with high PA current the output MOSFET will get very hot.
Remove the RF signal source and reconnect the dummy load. It will be an advantage to
have either a QRP power meter or oscilloscope to measure the power output.
Turn the Mic gain pot fully counter-clockwise.
Ensure the PA Bias trimpot VR6 is fully counter clockwise.
Operate the PTT. Check the power supply current. This is the idle transmit current and
should be about 160mA. Anything far from this indicates a problem and should be
investigated.
Write the idle transmit current value down so that it can be used to measure and set the
PA bias current.
Check the TX mode DC voltages as shown in Figure 16. If there are any off scale readings
turn off immediately and look for construction errors.
14.3.1 PA BIAS SETUP Operate the PTT and using a screwdriver slowly rotate the PA Bias trimpot VR6
clockwise. At about half rotation the power supply current should start to rise gradually
and smoothly. Any excessive spikes in current indicate instability and need to be
investigated.
Keep increasing until the power supply current is around 150mA more than the idle
transmit current written down in the step above. The increase in current is mainly due
to the output MOSFET starting to conduct and move into linear operation, but some may
be because we have not balanced the carrier yet causing some power output.
14.3.2 CARRIER BALANCE Operate the PTT and adjust carrier balance trimpot VR5 for minimum power output.
Initially if the power is high enough you can use the LCD bar graph as a monitor, but as
the mixer reaches balance the output power will decrease significantly and the bar
graph will not indicate any power. At this point it’s necessary to use some other way to
monitor the power output to do the final balancing. The following are some suggested
methods:
o Using a second receiver placed nearby listen to the transmitted signal. Ensure
you are not listening directly to the VFO signal as this is on the same frequency.
o Connect an oscilloscope across the dummy load.
o Connect a sensitive RF probe across the dummy load.
o Connect a spectrum analyser across the dummy load. An attenuator may be
necessary.
In all cases it’s simply a matter of monitoring the transmitted signal for minimum output
while carefully adjusting VR5.
With the carrier balanced rotate the bias trimpot VR6 fully counter-clockwise again to
turn off the MOSFET.
Operate the PTT. Slowly rotate the bias trimpot clockwise again until the power supply
current is 150mA higher than the idle transmit current recorded earlier. Typically this
will be 320mA. The output stage bias current is now set.
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DCT Construction Manual – Issue 1 Page 54
14.3.3 VOICE TEST Configure the connector board for a dynamic microphone.
Make a cable with 3 wires soldered to the tip, ring and sleeve of a 3.5mm stereo plug.
Apply an audio signal generator to the tip and sleeve wires set to 1KHz at around 50mV.
Operate the PTT by shorting the ring wire to ground.
Slowly rotate the Mic gain trimpot clockwise while monitoring the power output. The
power output should increase smoothly without any sudden dips or surges and you
should be able to easily achieve 5 Watts output. At this stage the LCD bar graph may not
indicate 5 bars. This can be calibrated later.
Remove the signal generator and check that the RF output goes to zero.
Remove the temporary cable and re-configure the connector board if using an Electret
microphone.
Plug in a microphone and operate the PTT. Speaking into the microphone should result
in RF output, and the LCD bar graph should move up and down.
14.3.4 CW TEST Set the mode to CW with the Menu button.
Plug a key into the Key connector.
Press the Key down. The DCT should go into transmit. Adjust the CW drive control to
verify the power can be set between 0 and at least 5W.
Verify the LCD power meter operates.
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DCT Construction Manual – Issue 1 Page 55
Location V DC Receive V DC Transmit
Q19 Source 13.8 13.8
REG1 out 5 5
REG2 out 3.3 3.3
U6 pin 8 6.8 Note 1 0
U7 pin 8 11.5 0
U7 pin 3 5.7 0
U7 pin 1 5.7 0
U7 pin 7 5.7 0
U8 pin 1 5.7 0
U8 pin 7 5.7 0
REG3 out 5 5
U9 pin 5 2.5 2.5
Q12 collector 0 9
Q10 collector 0 4
Q13 collector 0 4.25
Q15 emitter 0 1.45
Q16 emitter 0 1.55
ZD2 cathode 0 6.8 Note 1
Q17 gate 0 4.3 Note 2
Notes: 1. Zener voltage can vary by +- 5%. 2. PA bias adjusted for 150mA idle current.
DC readings taken with a digital multimeter. Power supply voltage set at 13.8V DC.
Figure 16 Typical DC circuit voltages.
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DCT Construction Manual – Issue 1 Page 56
15 FINAL ASSEMBLY
15.1 CASE HOLES
The case requires holes to be cut in the side so that the connectors and switch can protrude.
Using Figure 17 as a guide, mark out the positions of the hole and cutouts with a pencil or
scriber.
Drill a central pilot hole for the round hole and open up with a reamer or round file.
Drill a series of holes around the inside perimeter of the rectangular cutouts, and finish off with
a small flat or square file.
Figure 17 Case holes
15.2 INSTALLING THE LID
The lid is mounted on top of the main board and held in place by the four 20mm metal spacers.
Make sure the connector board jacks and pushbutton fit neatly inside the lid holes.
Screw the l