Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
 The 4.2-MHz video signal of commercial broadcast
television is transmitted as vestigial sideband (VSB)
signal. As illustrated in Figure 8-1, the baseband
video signal modulates the carrier in a regular
double-sideband/full-carrier (DSB-FC) modulator.
 Before power amplification, this AM signal enters the
vestigial sideband filter that eliminates most of the
lower sideband.
 The reason for using VSB is to minimize the
transmission spectrum (bandwidth) while
maintaining an easily demodulated AM signal; the
demodulated low-frequency response of the recovered
signal will also be better than for single sideband.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
1
Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
Figure 8-1. Generation of vestigial sideband (VSB).
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
 As seen in Figure 8-2, both sidebands of video signals
below 0.75 MHz are transmitted, but only one sideband
of the video signals above 0.75MHz is transmitted.
 The low frequency video power will be twice that of the
high-frequency signals.
 If no compensation is provided, the low frequencies will
be overemphasized in the picture, and the fine details
will be of relatively low contrast (washed out).
 This form of frequency distortion is compensated for in
TV receiver IF amplifiers.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
 Vestigial sideband compensation is accomplished before
the demodulation process by providing IF filtering as
illustrated in Figure 8-3.
 The frequency response of the IF amplifier is designed
to roll off linearly between 0.75 MHz of the carrier so
that the high video frequencies are emphasized in the IF.
 The demodulated video will come out with the same
relative amplitudes as it had at the studio.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
Figure 8-2. Television video spectrum.
5
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Chapter VIII. Sideband Systems
VESTIGIAL SIDEBAND
Figure 8-3. VSB compensation filter response.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Double-Sideband/Suppressed-Carrier
 The amplitude-modulation technique called
double-sideband/suppressed-carrier (DSB-SC)
has an important advantage over regular AM
(DSB-FC): The carrier is suppressed during the
modulation process.
 As a result, most of the power in a regular AM
transmission, which provides no information,
is eliminated.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Double-Sideband/Suppressed-Carrier
 EXAMPLE 8-1 :
Determine the power savings when the, carrier is
suppressed in a regular AM signal modulated to
an index of 100%.
 Solution:
Pt = (1+ m2/2)Pc, Psb = Pc.m2/2.
The power savings is
(Pt - Psb)/Pt = 1/(1+m2/2)
= 1/1.5
= 66.7%
for DSB-SC transmission.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Double-Sideband/Suppressed-Carrier
 The power savings of Example 8-1 has its price, however.
As will be obvious by simple inspection of the waveform
of a DSB-SC signal, an AM rectifier cannot be used to
demodulate DSB-SC.
 Demodulation can be achieved only if a locally generated
carrier signal is introduced.
 It must not only have exactly the correct frequency (be
frequency-coherent) but also have a phase very close to
what the carrier would have if it had been transmitted;
 that is, DSB-SC demodulation must also be
approximately phase-coherent.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Double-Sideband/Suppressed-Carrier
 The peak detector discussed for demodulation of
regular AM (DSB-FC) will not yield the correct result
for DSB-SC.
 For instance, when the input is the sinusoidal tonemodulated DSB-SC signal, the output of a peak
detector will be the "cusp" signal of Figure 8-10.
Fig. 8-10. Result of noncoherent demodulation of DSB-SC.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
10
BALANCED MODULATOR
 The circuit used for producing a doublesideband/suppressed-carrier type AM signal is shown
in Figure 8-4.
 This circuit is a double-balanced mixer in which the
diode pair's D1-D2 and D3-D4 are alternately switched
on and off by the high-frequency carrier signal vc(t).
 The carrier signal could be a sinusoid or squarewave at
frequency fc; either way, its amplitude is much larger
than that of the information (modulation) signal m(t).
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
Figure 8-4. Balanced ring modulator.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
 Figure 8-5 shows how the carrier causes alternate reversals, of
the polarity of the modulation input signal.
 In part A the carrier is positive and diodes D1 and D2 become
low-impedance devices for one-half of the RF cycle, while D3
and D4 are essentially open-circuited by reverse bias.
 In part B the modulation signal is coupled to the output
with reverse polarity because the carrier signal has
switched D3 and D4 "on" while reverse-biasing D1 and D2.
 The output signal vo(t) is merely m(t) alternately multiplied by
+1 and -1 due to the carrier's switching of the diodes.
 It should be recognized that due to the balanced output circuit,
the carrier signal ideally is not coupled to the secondary of T2.
13
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
Figure 8-5. Balanced modulator Phase reversals.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
14
BALANCED MODULATOR
 The balance is confirmed by following current from
a positive polarity vc(t) into the center tap of T1, then
splitting and flowing through both. D1 and D2,
converging at the center tap of T2 and returning to
the vc(t) source.
 The opposite flowing currents in the primary of T2
induce voltages of equal magnitude and opposite
polarity in the T2 secondary, which therefore cancel
each other.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
 The squarewave switching function can be written
with an amplitude of A=p/2 as
vc(t) = sin2pfct +(1/3)sin2p(3fc)t +…
+(1/n)sin2p(nfc)t
(8-1)
where n and all previous harmonics are odd only.
 The circuit physically performs a function that is
mathematically equivalent to multiplication of timevarying signals vc(t) and the information signal m(t).
 Hence, the output is
m(t) x vc(t) = vo(t)
= m(t).sin2pfct + (1/3)m(t).sin2p(3fc)t
+ higher odd harmonics
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
(8-2)
16
BALANCED MODULATOR
 To illustrate that Equ. (8-2) indeed represents a DSBSC signal, let the modulation signal be a 2-Vpk audio
tone of frequency fm = 5kHz so that
m(t) = Asin2pfmt = 2sin2p(5kHz)t V.
 Also, let the carrier frequency be fc = 45 kHz.
Substituting into Equ. (8-2) yields a modulated output
signal of
vo(t) = A.sin2pfmt.sin2pfct
+ (A/3).sin2pfm.sin2p(3fc)t +…
= 2.sin2p(5kHz)t.sin2p(45kHz)t
+ (2/3).sin2p(5kHz)t.sin2p(135kHz)t +…
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
17
BALANCED MODULATOR
 By the use of the trigonometric identity
sinA.sinB = (1/2)[cos(A-B) - cos(A+B)],
vo(t) is seen to be
vo(t) = (A/2)cos2p(fc-fm)t – (A/2)cos2p(fc+fm)t
+ (A/6)cos2p(3fc-fm)t
– (A/6)cos2p(3fc+fm)t +…
(8-3)
= cos2p(40kHz)t – cos2p(50kHz)t
+ (1/3)cos2p(130kHz)t
– (1/3)cos2p(140kHz)t +…
(8-4)
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
 Figure 8-6 shows a sketch of Equ. (8-4) in both time and
frequency domains.
 If vo(t) is filtered so that only the first set of sidebands are
transmitted, then the harmonics are missing and the result
is shown in Figure 8-7.
Figure 8-6. Wideband DSB-SC signal.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
BALANCED MODULATOR
Figure 8-7. DSB-SC after filtering higher harmonics.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Phase Distortion in the Demodulation
of Suppressed-Carrier Systems
 Figure 8-11 shows the transmitted DSB-SC phasors (a),
and the correct relationship between the reinserted
carrier and sidebands (b). A phase error Df will result
in the AM phasor signal of (c).
 The resultant signal in c is a combination of AM and
phase modulation, and the demodulated information
which might be that of Figure 8-12a would come out
like 8 -12b with severe phase distortion.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Phase Distortion in the Demodulation
of Suppressed-Carrier Systems
Figure 8-11. Phasor representation of DSB-SC.
(a) DSB-SC. (b) DSB-SC with carrier “reinserted”-AM.
(c) Carrier reinserted with wrong phase.
Figure 8-12. Result of phase distortion due to reinsertedcarrier phase error. (a) Transmitted. (b) shows the result of
a phase distortion due to phase error of reinserted carrier.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
22
Phase Distortion in the Demodulation
of Suppressed-Carrier Systems
 The demodulated signal has the correct fundamental
frequency, but the phase distortion has greatly altered
the information.
 The phase distortion problem is worse in DSB-SC than
in SSB-SC because of the complication introduced by
having the two sidebands.
 Also, transmission-channel phase shifts, which are not
linear between the upper and lower sidebands (envelopedelay distortion), will make the problem even worse.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Single-Sideband/Suppressed-Carrier
 Single-sideband/suppressed-carrier (SSB-SC) is an
amplitude modulations technique used for its
outstanding power and bandwidth efficiency.
 By eliminating the carrier and one sideband, a power
savings of over 83% is realized. Additionally, the bandwidth required for SSB-SC is theoretically one-half that
required when both sidebands are transmitted.
 As is the case for DSB-SC, the advantages are somewhat
offset by the need for carrier recovery and reinsertion at
the receiver.
 The phase and frequency accuracy requirements are not
as critical for single-sideband as they are for DSB-SC.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
24
The Sideband-Filter Method
 Figure 8-13 shows a block diagram for an SSB-SC
transmitter. The heart of this system is the balanced
modulator and sideband filter. The information to be
communicated is amplified and fed to the balanced
modulator.
 Also fed to the modulator is an intermediate-frequency
(IF) carrier that is frequency- and phase- locked to a
stable reference generator in the frequency synthesizer.
25
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Sideband-Filter Method
Figure 8-13. Single-sideband transmitter block diagram
(sideband-filter method). Either upper or lower sideband
Filtering may be chosen.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
26
The Sideband-Filter Method
 The DSB-SC output of the balanced modulator is fed
to a sideband filter where the unwanted sideband is
eliminated.
 The single remaining sideband is at an intermediate
frequency and must be up-converted in a mixer to the
desired transmission frequency.
 After filtering the mixer signal products, the SSB-SC
signal is amplified in linear power amplifiers (LPAs)
and coupled to the antenna or perhaps to coaxial
transmission lines for multiplexing with other singlesideband signals.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
27
The Phase Method of SSB-SC Generation
 By properly combining two DSB-SC signals in which
either the upper or the lower sidebands are exactly out
of phase, a single-sideband signal can be produced.
 The equal-frequency sidebands which are out-of-phase
will cancel, and the in-phase sidebands reinforce each
other to become the transmitted sideband. The block
diagram is shown in Figure 8-14.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
Fig. 8-14. SSB transmitter block diagram (phase method).
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
 The inputs to the top balanced modulator are
sin(wmt) and sin(wct).
 The multiplied output is sin(wmt).sin(wct),
which by trigonometric identity is
sin(wmt).sin(wct)
= (1/2)[cos(wc-wm)t - cos(wc+wm)t]
(8-5)
 The inputs to the bottom balanced modulator are
cos(wmt) and cos(wct).
 The output of this modulator is
cos(wmt).cos(wct)
= (1/2)[cos(wc-wm)t + cos(wc+wm)t]
(8-6)
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
 Equation (8-5) describes DSB-SC with the upper
sideband having opposite polarity to the upper
sideband of Equ. (8-6).

The output of the summing network is the addition of
Eqs. (8-5) and (8-6); that is,
SSB-SC output = Vo(t)
= (1/2)[cos(wc-wm)t - cos(wc+wm)t]
+ (1/2)[cos(wc-wm)t + cos(wc+wm)t]
= cos(wc - wm)t.
(8-7)

The various time waveforms and corresponding
frequency spectra for single-tone modulation are shown
in Figure 8-15.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
31
The Phase Method of SSB-SC Generation
Figure 8-15. Time and frequency spectra for phase method
of producing SSB-SC.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
 With integrated circuitry, keeping the phases of modulator
inputs and outputs constant when temperatures and power
supply voltages are changing is not so simple.
 Furthermore, while the carrier phase-shift network at a single
frequency is simple enough, the wideband audio network is
required to shift the phase by exactly 90° over the full audio
frequency range.
 The circuit used for this has traditionally been the all-pass
network, which is implemented with RC branches (Fig. 8-16).
 The desired bandwidth of this network is set between w2 - w1,
where w2 = 1/R2C2 is the high-frequency cutoff and w1 is set by
the other RC time constant.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
33
The Phase Method of SSB-SC Generation
Figure 8-16. Wideband 90° phase-shift circuit.
34
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
 EXAMPLE 8-2:
Determine the minimum frequency stability required if a
27.065-MHz oscillator is used to demodulate an SSB-SC
voice transmission on Citizen's Band (CB) channel 9.
Give the answer in percent and parts per million (ppm).
 Solution:
The demodulated voice signal will be barely intelligible
if the oscillator drifts by 50 Hz. This is 50Hz/27.065 MHz
= 1.85 ppm (or 1.85 x 10-6) x 100% = 0.000185%.
 On a short-term basis this is achievable with a crystal
oscillator. However, one should be concerned about
oscillator and received-signal noise.
35
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
The Phase Method of SSB-SC Generation
 Receivers for SSB-SC are usually a double-conversion
type; that is, there are two mixers and two IF systems.
 Also, to achieve the frequency stability required when
multichannel operation is employed, the LOs and
BFO are synchronized to a highly stable reference
oscillator.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Frequency Division Multiplexing (FDM)
Figure 8.2-1. Typical FDM transmitter. (a) Input spectra
and block diagram; (b) baseband FDM spectrum.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
37
Frequency Division Multiplexing (FDM)
Figure 8.2-2. Typical FDM receiver.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Frequency Division Multiplexing (FDM)
39
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Frequency Division Multiplexing (FDM)
Figure 8-18. Analog telephone FDM hieracrchy.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
40
Frequency Division Multiplexing (FDM)
Figure 8-19. Analog telephone FDM frequency spectrum.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
41
QUADRATURE MULTIPLEXING
 Quadrature multiplexing is illustrated in Figure 8-20.
The two modulation signals m1(t) and m2(t) modulate
the quadrature carriers sinwct and coswct in balanced
modulators.
 The modulated signals are filtered (not shown) to
eliminate nonlinear mixer products then linearly added
to form the quadrature-multiplexed (QM) signal.
42
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
QUADRATURE MULTIPLEXING
Figure 8-20. Quadrature multiplexing of two channels. The output
is the sum of two orthogonal DSB-SC signals on the same carrier.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
QUADRATURE MULTIPLEXING
 The QM signal is merely the sum of two orthogonal (90°)
DSB -SC signals with the same suppressed-carrier
frequency.
 To illustrate the principle for a color television application,
the color information is transmitted as a vector
determined by the amplitude and polarity of the
quadrature carriers operating at a frequency of
approximately 3.58 MHz relative to the, video carrier.
 If, for example, the in-phase carrier has an amplitude of
-0.44Vi, where Vi is the maximum in-phase carrier voltage
of positive polarity, and the quadrature carrier has an
amplitude of -0.9Vq, then the TV demodulator should
interpret the resulting color as green.
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
QUADRATURE MULTIPLEXING
Figure 8-21. Quadrature multiplexing receiver. The two
information signals are m1(t) and m2(t).
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
45
QUADRATURE MULTIPLEXING
 The receiver LO or beat frequency oscillator (BFO) is
synchronized to the incoming signal; that is, the
oscillator frequency is exactly wc.
 The LO signal 2cos(wct) is split and phase-shifted to
quadrature LO signals VLOi = 2cos(wct) and VLOq =
2sin(wct), which are the LO inputs to the in-phase and
quadrature mixers, respectively.
 The mixer outputs are simply the products of their two
input signals -- the received QM signal and the
individual LOs.
46
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
QUADRATURE MULTIPLEXING
 The quadrature mixer output is
VLOq x VQM = (2sinwct)[m1(t)sinwct + m2(t)coswct]
= 2m1(t)sinwct.sinwct + 2m2(t)sinwct.coswct
= m1(t)cos(wc - wc)t - m1(t)cos(wc + wc)t
+ m2(t)sin(wc - wc)t + m2(t)sin(wc + wc)t
= m1(t) - m1(t)cos2wct + m2(t)sin2wct
(8 -8a)
(8-8b)
(8-8c)
(8-9)
 Note that the mixer output is the m1(t) information signal and
two 2nd-harmonic mixer products (both DSB-SC), which are
easily filtered out with a low-pass filter set just below wc.
 Since the low-pass filters (LPF) of Fig. 8-21 have a cutoff
frequency of just below wc, the output signal from the
"quadrature" branch is
VQ(t) = m1(t)
(8-10)
 A similar analysis shows that VI(t) = m2(t).
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
47
Costas Loop for Suppressed
Carrier Demodulation
 When a pilot signal is transmitted (as is the case for
telephone FDM, color TV and FM stereo) a simple
phase-1ocked loop will lock onto the pilot for
"synchronization.“
 Unfortunately, a suppressed-carrier signal with no pilot
has no fixed spectral component on which to lock-up a
phase-locked loop.
 The sidebands contain information leading to the
whereabouts of the missing carrier.
 Figure 8-22 shows the additional circuit necessary to
extract the required information.
 It is a dc-coupled product detector (mixer) known as
a phase detector.
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
48
Costas Loop for Suppressed
Carrier Demodulation
Figure 8-22. Costas loop includes BFO-synchronizing for
demodulation of all types of suppressed-carrier signals.
49
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Costas Loop for Suppressed
Carrier Demodulation
 The analysis of the loop for VCO control is as follows: If the
VCO (the receiver local oscillator) is locked to the incoming
carrier, then wLO = wc and only a small phase error difference
(qe) between the two signals will exist.
 Because of the small phase error, Equ. 8-8c becomes
VLO x VQM = m1(t).cos[(wc-wc)t + qe] - m1(t).cos[(wc+wc)t + qe]
+ m2(t).sin[(wc-wc)t + qe] + m2(t).sin[(wc+wc)t + qe]
= m1(t).cosqe - m1(t).cos(2wct + qe)
+ m2(t).sinqe + m2(t).sin(2wct + qe)
(8-11)

and Equ. (8-10) becomes
and
VQ(t) = m1(t).cosqe
(8-12a)
VI(t) = m2(t).sinqe
(8-12b)
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Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
Costas Loop for Suppressed
Carrier Demodulation
 Now the output of the badeband product detector becomes
VBB-PD(t) = VQ(t) x VI(t)
= m1(t)m2(t).cosqesinqe
= m1(t)m2(t).[sin(qe – qe) + sin(qe + qe)]/2
= (1/2).m1(t)m2(t).sin(2qe)
(8-13)
 The low-pass filter preceding the VCO will have a cutoff
frequency sufficiently low to integrate the varying
information signals m1(t) and m2(t) so that the average (dc)
voltage applied to keep the VCO tracking any receivedcarrier frequency drifts will be
(8-14a)

which is approximately proportional to 2qe, and Equ. 8-14a
becomes
Vo = K(2qe)
(8-14b)
Prof. J.F. Huang, Fiber-Optic Communication Lab.
National Cheng Kung University, Taiwan
51