Topic 3 Oscilloscope and Signal Generator
Transcript of Topic 3 Oscilloscope and Signal Generator
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-1
TOPIC 3
OSCILLOSCOPE AND SIGNAL GENERATOR
3.1 Introduction to Oscilloscope
3.1.1 Define of Oscilloscopes
Oscilloscope is a device that allows the amplitude of electrical signals, whether they be
voltage, current, power, etc., to be displayed primarily as a function of time ( the basic
instrument for the study all types of waveforms).
3.1.2 Explain the functions of Oscilloscope.
The main functions of oscilloscope are:
i. Measure the voltage (AC or DC)
ii. Measure the time and frequency
iii. Measure the phase differential between two waveforms
3.1.3 Classify types of Oscilloscope (analog and digital).
Oscilloscopes can be classified into two categories:
i. Analog →Works with continuously variable voltages.
→Works by directly applying a voltage being measured to an electron
beam moving across the oscilloscope screen. The voltage deflects the
beam up and down proportionally, tracing the waveform on the screen.
This gives an immediate picture of the waveform.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-2
ii. Digital →Works with discrete binary numbers that represent voltage samples.
→Samples the waveform and uses an analog-to digital converter (or
ADC) to convert the voltage being measured into digital information.
It then uses this digital information to reconstruct the waveform on
the screen.
3.1.4 Draw block diagram of an analog oscilloscope.
Figure 3.1: An analog oscilloscope block diagram
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-3
3.1.5 Draw block diagram of a digital oscilloscope.
Figure 3.2: A digital oscilloscope block diagram
3.1.6 Explain the basic functional block diagram of an analog and digital
oscilloscope
Basic functional block diagram of an analog oscilloscope
i. When you connect an oscilloscope probe to a circuit, the voltage signal travels
through the probe to the vertical system of the oscilloscope. Figure 2 is a
simple block diagram that shows how an analog oscilloscope displays a measured
signal.
ii. Depending on how you set the vertical scale (volts/div control), an attenuator
reduces the signal voltage or an amplifier increases the signal voltage.
iii. Next, the signal travels directly to the vertical deflection plates of the cathode
ray tube (CRT). Voltage applied to these deflection plates causes a glowing dot to
move. (An electron beam hitting phosphor inside the CRT creates the glowing
dot.) A positive voltage causes the dot to move up while a negative voltage causes
the dot to move down.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-4
iv. The signal also travels to the trigger system to start or trigger a "horizontal
sweep." Horizontal sweep is a term referring to the action of the horizontal
system causing the glowing dot to move across the screen. Triggering the
horizontal system causes the horizontal time base to move the glowing dot
across the screen from left to right within a specific time interval. Many
sweeps in rapid sequence cause the movement of the glowing dot to blend into a
solid line. At higher speeds, the dot may sweep across the screen up to 500,000
times each second.
v. Together, the horizontal sweeping action and the vertical deflection action
trace a graph of the signal on the screen. The trigger is necessary to
stabilize a repeating signal. It ensures that the sweep begins at the same
point of a repeating signal.
Basic functional block diagram of a digital oscilloscope
i. Some of the systems that make up digital oscilloscopes are the same as those in
analog oscilloscopes; however, digital oscilloscopes contain additional data processing
systems. (See Figure 3.) With the added systems, the digital oscilloscope
collects data for the entire waveform and then displays it.
ii. When you attach a digital oscilloscope probe to a circuit, the vertical system
adjusts the amplitude of the signal, just as in the analog oscilloscope.
iii. Next, the analog-to-digital converter (ADC) in the acquisition system samples
the signal at discrete points in time and converts the signals voltage at these
points to digital values called sample points. The horizontal systems sample clock
determines how often the ADC takes a sample. The rate at which the clock
"ticks" is called the sample rate and is measured in samples per second.
iv. The sample points from the ADC are stored in memory as waveform points.
More than one sample point may make up one waveform point.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-5
v. Together, the waveform points make up one waveform record. The number of
waveform points used to make a waveform record is called the record length.
The trigger system determines the start and stop points of the record. The
display receives these record points after being stored in memory.
3.1.7 Compare advantages and disadvantages digital Oscilloscope with analog
Oscilloscope in term of accuracy.
a. Digital Oscilloscope
Advantages Disadvantages
High-accuracy measurements Can be more costly
Display storage Can be less intuitive to operate
because
they typically have more
features)
Bright, well-focused display at
virtually sweep speed
Pre-trigger viewing capability
Peak/glitch detection
Automatic measurements
Computer, printer/plotter
connectivity
Waveform processing capability
including waveform math functions
Display modes like averaging and
infinite persistence
Self calibration
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-6
b. Analog Oscilloscope
Advantages Disadvantages
Familiar controls Low accuracy
Instantaneous display updating for
real- adjustments Display flicker and/or dim display
Direct, dedicated controls for
often used
No pre-trigger viewing capability
Adjustments like vertical
sensitivity, time base speed, trace
position and trigger level low cost.
Limited bandwidth
Higher cost of ownership
Limited measurement capability
3.2 Understand the basic control functions of Oscilloscope
3.2.1 Explain function of Display Controls.
1) POWER SWITCH ON/OFF
Main power switches of the instrument. When this switch is turned on, the
LED above the switch is also turned ON
2) POWER LAMP
This LED lamp lights when power is turned ON
3) INTENSITY KNOB
It can control the brightness of the spot or trace.
4) FOCUS KNOB
After obtaining appropriate brightness with intensity, adjust focus for clearest
line
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-7
5) TRACE ROTATION KNOB
This knob is used to correct the horizontal trace when it becomes slanted
with respect to the horizontal scale, due to the effect of magnetic fields
6) SCALEILLUM KNOB
This is used to adjust scale brightness. If this knob is turned clockwise,
brightness is increased. This feature is useful for operation in dark places,
or when taking pictures
7) CAL 0.5V TERMINAL
Outputs a 0.5V p-p 1 KHz rectangular wave for calibrating probes
8) GND TERMINAL
This is a grounding terminal
3.2.2 Explain function of Vertical Controls.
1) CH1 INPUT CONNECTOR
This is a BNC connector used for vertical input CH1. The signal applied to this
connector when in the X-Y mode becomes the X-axis signal
2) CH2 INPUT CONNECTOR
This is a BNC connector used or vertical input CH2. The signal applied to this
connector when in the X-Y me ie becomes the Y-axis signal
3) AC-GND-DC SWITCH
Select following input coupling options for CHI and CH2
AC: blocks dc signal component allowing only AC signal to pass into attenuator
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-8
GND: input signal is switch off and attenuator is grounded
DC: dc coupling, all signal are directly connected to attenuator
4) VOLTS/DIV SELECTOR SWITCH
This is a step attenuator switch adjusting the vertical deflection sensitivity.
Set to the position which displays the input signal at the most convenient height
on the CRT
5) VARIABLE KNOB
The fine adjustment is used for varying the vertical-axis deflection sensitivity
continuously. If this knob is completely counterclockwise the vertical sensitivity
is reduced to less than 1/2.5 of VOLTS/DIV switch setting. This knob is used
for comparing two waveform and rise time measurement. However this knob is
normally in the locking position.
PULL X5 MAG
When the pull x5 Mag is pulled out, the vertical axis gain is magnified 5 times,
the maximum sensitivity becomes 1mV/div
6) 20MHz BANDWIDTH
The frequency bandwidth of vertical axis is limited to 20MHz. This knob can be
used when you cannot synchronize the signal by high frequency noise or expanded
trace
7) ALT/CHOP
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-9
When the vertical mode is in dual, this button can display ALT and CHOP mode. ALT
mode is a sequential display mode with one cycle of signal between CHI and CH2.
CHOP mode is a sequential display mode with a frequency step of approximately
220MHz between CHI and CH2
8) POSITION
Used to move the CHI or CH2 trace up or down on the CRT screen
9) INVERT SWITCH
When the invert push button is pressed, the polarity of the input signal applied
to CH2 is inverted. This function is convenient when 2 waveforms of difference
are compared, or for displaying the CHI and CH2 difference waveform using ADD
10) MODE SELECTOR SWITCH
Selects vertical axis operating mode
CHI: only the signal applied to CH1 is displayed on the screen
CH2: only the signal applied to CH2 is displayed on the screen
DUAL: when both CH1 and CH2 buttons are pushed in the signals applied to CH1
and CH2 input are displayed on the screen
ADD: display the algebraic sum of the CHI and CH2 input voltage
3.2.3 Explain function of Horizontal Controls.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-10
1) A TIME/DIV
Selects sweep speed from 0.05µs/div to 0.2µs/div in 21 calibrated steps
2) HORIZONTAL POSITION
The trace can be moved in a horizontal direction. Tuning this knob clockwise moves
the trace towards the right, turning the knob is counterclockwise moves the trace
towards the left
PULL X10 MAG
When pulled out, the trace will be magnified by a factor of 10 times. The sweep
time becomes 1/10 of the indicated on-the time/div switch, (eg. 100µs/div
becomes 10µs/div for X10 MAG). To magnify a portion of a waveform: Move the
waveform of interest to the center gratitude on the horizontal scale
3) A, B, ALT (H DISPLAY)
This switch selects the sweep method of A, B. When both A and B buttons
are pushed in, it was display B sweep which was duplicated by A sweep and 2
trace of B sweep simultaneously.
4) X-Y
Displays the CH1 and CH2 input signal as an X, Y graph. The vertical deflection
signal is applied to the CH1 input and the horizontal deflection signal is
applied to the CH2 input. The CH2 Vertical position control is used for the
positioning the X, Y display on the vertical axis the horizontal position
control positions the X, Y display on horizontal axis of CRT.
5) TRACE SEPERATION
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-11
This trace separation controls the vertical position interval of A sweep
and B sweep at the sweep mode is A ALT B.
6) DLY POS
Adjusts to starting with B sweep during the A sweep periods.
7) VARIABLE
When this knob is turned all the way clockwise (cal) the sweep is indicated
by the A time/diy switch, if the knob is turned all the way
counterclockwise the sweep is less than ½.5 of the A time/div setting.
During normal operation, this knob is turned to the CAL position.
3.2.4 Explain function of Trigger Controls.
1) B TRIS'D
This knob selects between continuous delay and triggered delay. For
continuous delay (normal state), the B sweep starts immediately after the
sweep delay time determined by A time/div switch (19) and delay pos (25).
For triggered delay (the knob is pushed), the sweep starts with B trigger
signal after the continuous delay time.
2) TRIGGER SOURCE SELECTOR SWITCH
Selects sweep trigger signal source.
INT: The input signal applied to CH1 or CH2 becomes the trigger signal
CH2: The input signal applied to CH2 becomes the trigger signal
LINE: The power line frequency becomes the trigger signal source
EXT: The external signal applied to EXT input becomes the trigger signal.
This is used when the trigger signal is external the vertical input
signal
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-12
3) TRIG LEVEL KNOB
This control sets the amplitude point on the trigger waveform that will start
the sweep.
Pull Slope knob
Selects the polarity of the slope the trigger source waveform will start the sweep.
(+) Slope is selected when the pull slope knob is in normal position
(-)Slope is selected when the pull slope knob is pulled out.
4) TRIG-MODE SWITCH
Auto: Sweep continuously runs in the auto sweep mode. A trace will be displayed
even when there is no input signal or when the input waveform is not triggered. A
stationary waveform will be displayed when the input waveform is properly
triggered.
Norm: A trace will be displayed only when the input waveform is present and is
properly triggered. There will be no trace displayed on the CRT if there is no
input signal or if the input signal is not synchronized. Normal sweep is used when
the input signal's frequency is less than 2LHz.
TV-H: Effective when trig mode is set to TV, and is used when the horizontal
of the TV signal is to be synchronized.
TV-V: Effective only when trig mode is set to TV, and is used when the vertical
of the TV signal is to be synchronized.
*Both TV-V and TV-H are synchronized only when the trigger signal is (-).
6) HOLD OFF
By the operation of Hold Off, complicated repetitive signals can be captured.
3.3 Understand the Oscilloscope Probes
3.3.1 Define of Oscilloscope Probes.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-13
A probe is more than a cable with a clip-on tip. It is a high-quality connector,
carefully designed not to pick up stray radio and power line noise. Probes are designed
not to influence the behavior of the circuit you are testing. However, no measurement
device can act as a perfectly invisible observer.
3.3.2 Classify types of Oscilloscope Probes.
Figure 3.3: Classify types of Oscilloscope Probes
3.4 Understand the application of Oscilloscope
3.4.1 Prepare procedure for calibrate Oscilloscope.
1. Turn on oscilloscope. Allow it to warm up for approximately 10 minutes. Letting the
oscilloscope warm up prevents damage to its cathode ray tube.
2. A green light is show on the CRT screen when the oscilloscope is ready. The green
light should run across the screen horizontally and should be in the center of the
screen. Adjust the position knob on the oscilloscope by and turn it clockwise or
counter clockwise until this line is at the center of the screen. If the line
shows up as a green dot moving across the screen, adjust the time/div knob until
the dot appear as a line and then center it.
3. Usually an oscilloscope has two channels CH1 and CH2. Connect your oscilloscope
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-14
probe to CH1.
4. Find the voltage selector switch and set it to AC volts.
5. Find CAL connector. It looks like a small enclosed hook, similar to the eye of a
needle.
6. Attach the oscilloscope probe to CAL connector. Ground the ground wire. A peak
to peak square wave one volt above the center division and one volt below the center
division. This means the oscilloscope is correctly calibrated at 2 Vpp.
3.4.2 Construct connection between Signal Generator and Oscilloscope for signal
measurement.
Figure 3.4: Construct connection between Signal
Generator and Oscilloscope
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-15
3.4.3 Use the Oscilloscope to measure voltage, frequency, time and phase angle in
sinusoidal and non-sinusoidal waveform from signal generator.
Figure 3.5: Sinusoidal and non-sinusoidal waveform
a. Voltage,V (V) :
= No. vertical division X volt/div X probe x1 or x10
b. Period, T (s) :
= No. Horizontal division X time/div
c. Frequency measurement, f(Hz):
= 1 / period
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-16
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-17
3.4.4 Construct connection between simple circuits to Oscilloscope for
in circuit waveform measurement.
Figure 3.6: Simple Circuit
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-18
3.4.5 Evaluate the specification of analog Oscilloscope
To use an analogue oscilloscope, three basic setting accommodate an incoming
signal:
a. The attenuation or amplification of the signal
Use the volt/div control to adjust the amplitude of the signal before it is
applied to the vertical deflection plates.
b. The time base
The time/div control to set amount of time per division represented
horizontally across the screen.
c. The trigger of the oscilloscope
Use the trigger level to stabilize a repeating signal, as well as triggering on
a single event.
Also adjust the focus and intensity controls to create a sharp and visible
display.
3.5 Introduction to signal generator
Define of signal generator.
A signal generator is a test device which generates an alternating voltage
signal suitable for test purposes. It is, in effect, a small radio transmitter generating
a signal of any desired frequency. The signal may be either modulated or
unmodulated and is used for the following checks or tests:
a. Alignment of tuned circuits, sensitivity measurements, and
approximate frequency measurements.
b. For frequency measurements, its use is limited because it is not a
frequency meter and cannot be used as a frequency standard.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-19
The signal generator is used primarily in the alignment of tuned circuits. A
signal generator is classified according to its frequency and is one of two types:
audio frequency or radio frequency.
a. Audio frequency generators produce signals with a frequency range
from 20 Hz to 20 kHz.
b. Radio-frequency generators produce signals covering a range of
frequencies from 10 kHz to 10 GHz. Many radio-frequency generators
have audio outputs separately available through front panel jacks.
These outputs are normally 100 Hz and 400 Hz.
When using the generator, the output test signal is coupled into the circuit
being tested, and its progress through the equipment is traced by the use of high-
impedance indicating devices such as vacuum-tube voltmeters or scopes. In many
signal generators, calibrated networks of resistors, called attenuators, are provided.
These are used to regulate the voltage of the output signal and also provide correct
impedance values for matching the input impedance of the circuit under testing.
Accurately calibrated attenuators are used, because the signal strength must be
regulated to avoid overloading the circuit receiving the signal.
3.5.1 Classify types of signal generator.
There are many types of signal generators. They may be classified roughly by
frequency into audio signal generators, video signal generators, radio frequency
generators, frequency-modu lated RF generators, and special types which
combine all of these frequency ranges.
http://www.tpub.com/content/armycomsystems/ss06026/ss060260064.htm
3.5.1.1 Audio signal generators.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-20
a. Audio signal generators produce stable audio-frequency signals used
for testing audio equipment. Video signal generators produce signals
which include the audio range and extend considerably further into the
RF range. These generators are used in testing video amplifiers and
other wideband circuits.
b. In both audio and video generators, the major components include a
power supply, an oscillator, one or more amplifiers, and an output
control. Voltage regulation circuits are necessary to ensure stability
of the oscillator in the generators which derive power from 115-volt AC
sources. In portable generators, battery power supplies are usually
used, and these require no voltage regulation.
c. In the audio and video generators of the beat-frequency type, the
output frequency is produced by mixing the signals of two radio
frequency oscillators, one of which is fixed in frequency and the other
variable. The difference in frequency of the two is equal to the desired
audio or video frequency.
(1) Audio signal generators often include RC oscillators in which the
audio frequency is directly produced. In these a resistance-
capacitance circuit is the frequency-determining part of the
oscillator. The frequency varies when either the resistance or
the capacitance is changed in value.
(2) In commercial generators, however, the capacitance alone is often
chosen as the variable element. The change in frequency which
can be produced by this method is limited, and it is usually
necessary to cover the entire range of the generator in steps.
This is accomplished by providing several RC circuits, each
corresponding to a portion of the entire range of frequency
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-21
values. The circuits in the oscillator are switched one at a time
to give the desired portion of the audio range.
d. The amplifier section of the block diagram (fig 3-2) usually consists of
a voltage amplifier and one or two power amplifiers. These are
coupled by means of RC networks, and the output of the final power
amplifier is often coupled to the attenuator, or output control, by means
of an output transformer.
Figure 3-2: Block diagram of audio or video signal generator
e. The output control section provides a means of matching the output
signal to the input of the equipment under test and regulating the
amplitude of the signal.
3.5.2 Classify types of signal generator.
i. Video signal generator : a device which outputs predetermined video
and/or television waveform and other signals used to stimulate faults in,
or aid in parametric measurements of television and video systems.
ii. Pitch generator: a types of signal generator optimized for use in audio and
acoustics applications. Sophisticated pitch generators will also include
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-22
sweep generators a function which varies the output frequency over a
range. Pitch generators are typically used in conjunction with sound level
meters, when measuring the acoustics of a room or a sound reproduction
system, and/or with oscilloscopes or specialized audio analyzers.
iii. Arbitrary waveform generators (AWG): Sophisticated signal generators
which allow the user to generate arbitrary waveforms, within published
limits of frequency range, accuracy and output level. Unlike function
generators, which are limited to a simple set of waveforms; an AWG allows
the user to specify a source waveform in a variety of different ways.
3.5.3 Explain Standard Signal Generators.
Produces known and controllable voltages
Used as power source for measurement of gain, signal to noise ratio,
bandwidth, standing wave ratio, and other properties.
Extensively used in testing of radio receiver and transmitter
The output signal can be Amplitude Modulated (AM) or Frequency
Modulated (FM)
3.5.4 Explain function generators
Produce different waveforms of adjustable frequency
Common output waveform are sine, square, triangular and sawtooth
The frequency may be adjusted, from a fraction of a hertz to several
hundred kHz
3.5.5 The front panel of a signal generator
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-23
Figure 3.7: The front
panel of a
signal
generator
3.5.6 Describe the function of the following items located on the panel of a signal
generator :
a. Frequency Selection Group
- Range switch: Provide seven fixed decades of frequency
- Multiplier: Variable potentiometer allowing frequency setting between fixed
range.
b. Sweep Group
- The sweep group can frequency sweep any of its function outputs. It could be
swept up or down in frequency using linear or log sweeps. Unlike function
generators, there are no annoying discontinuities or band-switching artifacts
when sweeping through certain frequencies. Two sweep marker frequencies can
be specified. When the sweep crosses either of the marker frequencies, a TTL
transition is generated at the rear-panel output to allow synchronization of
external devices.
TOPIC 3: OSCILLOSCOPE AND SIGNAL GENERATOR
EE101(MEASUREMENT) Page 3-24
c. Amplitude Modulation Group
- To provide 20dB of attenuation of the output waveform selected by function
switch.
d. DC offset Group
- To allow the DC level of the output waveform to be set as desire.
e. Function or Waveform group
- To provide selection of desired output waveform. (Square, triangle and sine
waveforms are provided)
f. Output Group
- Used to adjust the amplitude of the generator’s out signal. The group consists
of the amplitude control knob, the three attenuation buttons and the fused
50Ω BNC connector.
3.5.7 Construct simple circuit and see signal generator as signal source to the
circuit
Figure 3.8: Simple circuit construction of signal generator