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© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Electronic DevicesNinth Edition
Floyd
Chapter 19
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Modulation
Modulation is the process of converting a signal containing information to a form suitable for transmission from a transmitter to a receiver. Circuits designed for this conversion are called modulators.
SummarySummary
Audio signals are far too low in frequency for direct radio transmission. The signal is impressed onto a carrier, which is a high-frequency wave called RF (for radio-frequency) that can be radiated from an antenna.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Amplitude Modulation
Amplitude modulation (AM) is the process of varying the amplitude of the carrier with the information signal such that the carrier amplitude varies in the same way as the information.
SummarySummary
Amplitudemodulatedenvelope CarrierFor an AM radio station, the
carrier is the assigned frequency for that station. The commercial AM broadcast band extends from 535 kHz to 1605 kHz and each station has a 10 kHz bandwidth (with some exceptions).
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
AM Radio Receiver
SummarySummary
The receiver recovers the audio signal from the modulated carrier. Since the early 1920’s all receivers convert signals from various stations to a single intermediate frequency (IF).The IF amplifier is optimized for this frequency and contributes most of the gain for the receiver. This greatly simplified tuning different stations.
540 kHz–1640 kHzelectromagnetic waves
Soundwaves
Localoscillator,
LO
455 kHzmodulated IF
Audio modulatedcarrier fc
455 kHz modulatedIF amplified Amplified
audio
Audio
Tuned to fc
fc + 455 kHz
Audio andpower
amplifiersDetectorMixerRF
amplifierIF
amplifier
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
AM Radio Receiver
SummarySummary
The idea for the IF amplifier is credited to Edward Armstrong, in 1918. This concept is what defines a superheterodyne receiver. He was credited with numerous inventions for radio and held 42 patents.
540 kHz–1640 kHzelectromagnetic waves
Soundwaves
Localoscillator,
LO
455 kHzmodulated IF
Audio modulatedcarrier fc
455 kHz modulatedIF amplified Amplified
audio
Audio
Tuned to fc
fc + 455 kHz
Audio andpower
amplifiersDetectorMixerRF
amplifierIF
amplifier
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Frequency Modulation
In frequency modulation (FM), the modulating signal, which carries information, varies the instantaneous frequency of the carrier signal. The amplitude of the frequency modulated signal remains fixed.
The assigned frequencies for FM broadcasting is between 88 and 108 MHz. NTSC television also uses FM for the audio portion of the TV signal, which is transmitted just above the video signal within the channel.
SummarySummary
Modulating signal
Frequency-modulated carrier
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
FM Radio Receiver
The superheterodyne FM receiver has many of the same blocks as the AM receiver. The main difference is the manner of recovering the signal. The FM receiver uses a discriminator (or phase-locked loop) rather than a detector to recover the modulating signal. Note that the IF frequency is 10.7 MHz, but some receivers use double-conversion to 455 kHz.
SummarySummary
De-emphasisnetwork
DiscriminatorLimiter
88 MHz–108 MHzelectromagneticwaves
Audio
MixerRFamplifier
IFamplifier
10.7 MHz FMFM fc
fc + 10.7 MHz
Localoscillator
Audio andpower
amplifiersSound
10.7 MHzAmplified FM
10.7 MHzLimited FM
Compensatedaudio
Amplifiedaudio
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Linear Multiplier
A linear multiplier generates the scaled product of two input voltages. A two-quadrant multiplier has only positive outputs whereas a four-quadrant multiplier can accept either polarity.
SummarySummary
V OU
T, ou
tput
vol
tage
(vol
ts)
+10
+8.0
+6.0
+4.0
+2.0
0
–2.0
–4.0
–6.0
–8.0
–10–10 –8.0 –6.0 –4.0 –2.0 0 +2.0 +4.0 +6.0 +8.0 +10
VX, input voltage (volts)
+
–
K = 110
V Y= +10 V
VY= +6.0 V
VY = +2.0 V
VY = 0 VVY = –2.0 V
VY = – 6.0 VV
Y = –10 V
KXYXYThe transfer curve for a four-
quadrant multiplier is shown. On the transfer curve, one input is along the x-axis, the other is represented by the blue lines. The output is the product multiplied by a scale factor, K and is read on the y-axis.
Transfer curve
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
The Linear Multiplier
SummarySummary
V OU
T, ou
tput
vol
tage
(vol
ts)
+10
+8.0
+6.0
+4.0
+2.0
0
–2.0
–4.0
–6.0
–8.0
–10–10 –8.0 –6.0 –4.0 –2.0 0 +2.0 +4.0 +6.0 +8.0 +10
VX, input voltage (volts)
+
–
K = 110
V Y= +10 V
VY= +6.0 V
VY = +2.0 V
VY = 0 VVY = –2.0 V
VY = – 6.0 VV
Y = –10 V
KXYXY
Transfer curve
From the transfer curve, what is VOUT when Vx = +6 V and Vy = 8 V?
See red lines for the inputs and the green line for the output, which is 4.8 V. This is the product of the inputs and the scale factor of 1/10.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Applications for linear multipliers include modulators and phase detectors in communication systems as well as processing circuits such as these.
V KV2V
KVX
XVV
YY
R2
R1
VX
KVYVOUT
0 V
VY
VOUT= –VXVY
–
+
I2
I1
R2
R1
VX
KVOUT
0 V VOUT= VX
–
+
I2
I1
2
Multiplier Squaring circuit
Square root circuitDivide circuit
1. 2.
3. 4.
The Linear Multiplier
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Amplitude modulation is a multiplication process, so the linear multiplier is ideally suited as a modulator.
Amplitude Modulation
Amplitudemodulator
Carrier signal
Modulating signal
fc
fm
fout
If the gain of the carrier frequency is made to vary sinusoidally at a certain frequency, the output will vary with the shape of the sinusoidal signal.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Recall from basic studies that a sinusoidal voltage can be expressed as v = Vpsin 2ft. If you multiply two sinusoidal voltages of different frequencies together, the result is the product of two sine terms:
Amplitude Modulation
1 2 1( ) 2( ) 1 2(sin 2π )(sin 2π )p pv v V V f t f t
A trigonometric identity for the product of two sinusoidal functions is(sin A)(sin B) = ½ [cos (A – B) – cos (A + B)]
By applying this identity to the equation for v1v2, the following result is obtained.
1( ) 2( ) 1( ) 2( )1 2 1 2 1 2cos 2π( ) cos 2π( )
2 2p p p pV V V V
v v f f t f f t
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
The equation on the previous slide (equation 19-2) illustrates an important idea. The product of two frequencies creates two new frequencies, which are the difference frequency and the sum frequency of f1 and f2.
Amplitude Modulation
f
f
1
f1 f1
2
f2 f2f
When the two frequencies have the same amplitude, the difference and sum frequencies are ½ this amount.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Amplitude Modulation
1( ) 2( ) 1( ) 2( )1 2 1 2 1 2cos 2π( ) cos 2π( )
2 2p p p pV V V V
v v f f t f f t
From the equation for the product of two sine waves, what is the relative amplitude of the output signals if f1 = ½ f2?
f
f
1
f1 f1
2
f2 f2f
1( ) 2( )
2p pV V
Let V2(p) = 1; then V1(p) = ½. The amplitude of each output
is given by:
Substituting, 1
1( ) 2( ) 2 12 2 4
1p pV V
The outputs are ¼ the size of V2(p).
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Amplitude Modulation
For communication systems, there are several different types of modulators available. Balanced modulators multiply the carrier signal, fc, and the modulating signal, fm, together, resulting in only sidebands in the output spectrum. If there is no modulating signal, there is no output.This is called double sideband suppressed carrier modulation. As in other cases, a sine wave modulating signal is assumed.
Suppressed carrier modulation is efficient, but it is not used in commercial radio broadcasting. One problem is that the receiver must insert the missing carrier at the correct frequency to avoid distortion.
fm fmfc ff ccf
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Amplitude Modulation
In standard amplitude modulation, the carrier is added to the signal before transmission. A dc level representing the carrier peak is added to the signal.
This is called double sideband full carrier modulation. The spectrum in the yellow box represents 100% modulation by a sine wave.
Vc(p)
Vm(p)sin 2fmt.
Vc(p)sin 2fct.
V c(p) + Vm(p)sin 2fmt.
The output will have a carrier term, as given in Equation 19-3.
fm fmfc ff ccf
output
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Amplitude Modulation
The line spectrum shown on the previous slide represents a sine wave because it is a pure frequency. With music and voice, the spectrum is no longer lines but varies according to the information being transmitted. The green boxes represent the limits to the spectrum for a commercial AM station.In general, commercial stations are restricted to ±5 kHz on either side of fc to avoid interference with other stations. f f +5 kHzf 5 kHz c cc
f
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
AM Mixer and Demodulator
The receiver mixes the incoming RF signal local oscillator in a mixer (linear multiplier) to produce the IF. After mixing, the original modulation signal can be recovered using a linear multiplier followed by a low-pass filter.
IF
0
Low-passfilter
Audio modulated IFAudio
Filter responsef
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
IF Amplifier
The IF amplifier is a tuned-resonant amplifier. The resonant frequency is 455 kHz for AM radios, which is less than the RF to insure there is no interference from the input signal.
450455460f (kHz)
IF
An IF amplifier passes only the signals of interest because it has a relatively narrow bandwidth (10 kHz) that is the same bandwidth as broadcast stations; further it is less prone to oscillate than RF amplifiers.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Audio Amplifier
After detection, the signal is amplified by an audio amplifier. One popular audio amplifier in IC form is the LM386. A typical low-power amplifier is shown.
10 C20.1 mF
mFmF
mF
R2–
+
1.0 k
Input fromdetector
R110 kW W
W
C1
1
Volumecontrol
C30.0022
C4+ 9 V
LM386
C510
C60.047
R3
47
C7
220
(1)(6)
(2)
(3) (7)
(4)
(8)(5)
mF
mF mF
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Frequency Modulation
In an FM signal, the carrier frequency is increased or decreased by the modulating signal. FM signals are less susceptible to noise because most noise changes the amplitude of the signal.
FM modulators often use a varactor diode to change the frequency according to the modulating signal. A varactor diode VCO is shown in which Vm modifies the resonant frequency.
VFM
Vm
+
–
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Frequency Modulation
In applications where the frequency deviation is smaller (such as mobile FM radios), a crystal-controlled modulator can be used. The varactor can still modify the output frequency of the modulator.The varactor is reverse-biased by R1 and R2. Positive alterations of Vm increase the reverse bias.
+VCC
RER2
C
C
4
3
C C1 2
VFM
Vm
XTAL
R1
Varactordiode
RFC
RFC
RFC
Does a positive alteration increase or decrease the frequency?An increase in reverse-bias will decrease C and increase frequency.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
The Phase-Locked Loop
The phase-locked loop is used extensively in communication systems as an FM modulator, demodulator, frequency generation and multiplication. It is feedback circuit consisting of a phase-detector, a low-pass filter, and a VCO.
Vifi
Output is proportional tophase difference of Vi and Vo.Phase
detectorθ
Low-passfilter
Voltage-controlledoscillator
VCO
Vo fo
The phase detector is a linear multiplier and therefore the output contains the sum and difference frequencies between Vi and Vo. The low-pass filter sends the difference frequency back to the VCO as a control voltage.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
The Phase-Locked Loop
PLLs are characterized by a lock range and a capture range.
Lock range, f lock
Capture range, fcap
f0
A popular PLL is the LM565. The basic VCO frequency is set by external components and is given by
The lock range is the range of frequencies over which the PLL can track the incoming frequency. The capture range is the range of frequencies over which the PLL can acquire lock.
(3)
(5)(4)
(7)(2)
(1)(9)(8)
(10)
–V
(6)
+V
LM565
Diff inputs Output
Phase comparatorinput
VCO output
Ref output
Low-pass filter
3.6 kW
R1C1
C2
VCO
Phasedetector Amplifier
01 1
1.24
fR C
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
The Phase-Locked Loop
As an FM demodulator, the VCO input on the PLL is compared to the IF, which is a frequency modulated signal. As the IF deviates from the center frequency, the voltage on the VCO control line varies in proportion to the IF changes, forming the demodulated output.
(7)(2)
(1)(9)(8)
FM input Demodulated output(10)
–V
R1
(3)
(5)
(4)
(6)
+V
C1
C2
LM565
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
SummarySummary
Fiber Optics
Fiber optic cables transmit light rather than electrical pulses. Light can be transmitted in the cable with extremely low loss because of a phenomena known as total internal reflection. In recent years, fiber optic cables have replaced many miles of copper wire for phone lines, cable TV, industrial plants, and other large communication systems.
Signalsource User
Electricalto light
conversion
Transmitter
Fiber optic cable
Receiver
Opticalconnector
Light toelectrical
conversion
Opticalconnector
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Selected Key TermsSelected Key Terms
Amplitude modulation
Frequency modulation
Four-quadrant multiplier
A communication method in which a lower frequency signal modulates (varies) the amplitude of a higher frequency signal (carrier).
A communication method in which a lower frequency intelligence-carrying signal modulates (varies) the frequency of a higher frequency signal.
A linear device that produces an output voltage proportional to the product of two input voltages.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
Selected Key TermsSelected Key Terms
Phase-locked loop (PLL)
Lock range
Capture range
Fiber optics
A device for locking onto and tracking the frequency of an incoming signal.
The range of frequencies over which a PLL can maintain lock.
The range of frequencies over which a PLL can acquire lock.
The use of light for the transmission of information through tiny fiber cables.
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
1. The normal bandwidth of commercial AM radio stations is
a. 5 kHz
b. 10 kHz
c. 20 kHz
d. 455 kHz
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
2. The IF amplifier in an AM superheterodyne receiver has a bandwidth of
a. 5 kHz
b. 10 kHz
c. 100 kHz
d. 455 kHz
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
3. In a FM signal, amplitude variations in the transmitted signal
a. depend on the amplitude of fm
b. depend on the frequency of fm
c. both of the above
d. none of the above
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
4. The plot shows the inputs and output for a four-quadrant multiplier. This is called the
a. transfer curve
b. response curve
c. product curve
d. load response V OU
T, ou
tput
vol
tage
(vol
ts)
+10
+8.0
+6.0
+4.0
+2.0
0
–2.0
–4.0
–6.0
–8.0
–10–10 –8.0 –6.0 –4.0 –2.0 0 +2.0 +4.0 +6.0 +8.0 +10
VX, input voltage (volts)
+
–
K = 110
V Y= +10 V
VY= +6.0 V
VY = +2.0 V
VY = 0 VVY = –2.0 V
VY = – 6.0 VV
Y = –10 V
KXYXY
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
5. There are two inputs to the circuit, VX and VY. With proper scaling of the resistors, the output can represent
a. – (VX + VY)
b. – (VX – VY)
c. – (VX VY )
d. – (VX / VY )
R2
R1
VX
VY
VOUT
–
+
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
6. Amplitude modulation is a
a. summing process
b. multiplication process
c. squaring process
d. division process
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
7. The spectrum shown shows a carrier that is modulated by
a. a sine wave
b. a square wave
c. a triangle wave
d. voice and musicfm fmfc ff cc
f
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
8. The circuit shown is
a. an FM modulator
b. an AM modulator
c. a VCO
d. a mixer
+VCC
RER2
C
C
4
3
C C1 2
VFM
Vm
XTAL
R1
Varactordiode
RFC
RFC
RFC
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
9. The block diagram represents a phase-locked loop. The yellow box represents a
a. low-pass filter
b. VCO
c. phase detector
d. input amplifier
OutputInput
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
10. A phase-locked loop can be used as a
a. FM modulator
b. FM demodulator
c. frequency generator
d. all of the above
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th editionThomas L. Floyd
QuizQuiz
Answers:
1. b
2. b
3. d
4. a
5. d
6. b
7. a
8. a
9. c
10. d