Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 1  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. Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 2 Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND Figure 8-1. Generation of vestigial sideband (VSB).

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 3  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. Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 4  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. Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 5 Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND Figure 8-2. Television video spectrum.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 6 Chapter VIII. Sideband Systems VESTIGIAL SIDEBAND Figure 8-3. VSB compensation filter response.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 7  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. Double-Sideband/Suppressed-Carrier

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 8  EXAMPLE 8-1 : Determine the power savings when the, carrier is suppressed in a regular AM signal modulated to an index of 100%.  Solution: P t = (1+ m 2 /2)P c, P sb = P c.m 2 /2. The power savings is (P t - P sb )/P t = 1/(1+m 2 /2) = 1/1.5 = 66.7% for DSB-SC transmission. Double-Sideband/Suppressed-Carrier

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 9  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. Double-Sideband/Suppressed-Carrier

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 10  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 tone- modulated DSB-SC signal, the output of a peak detector will be the "cusp" signal of Figure Fig Result of noncoherent demodulation of DSB-SC. Double-Sideband/Suppressed-Carrier

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 11  The circuit used for producing a double- sideband/suppressed-carrier type AM signal is shown in Figure 8-4.  This circuit is a double-balanced mixer in which the diode pair's D 1 -D 2 and D 3 -D 4 are alternately switched on and off by the high-frequency carrier signal v c (t).  The carrier signal could be a sinusoid or squarewave at frequency f c ; either way, its amplitude is much larger than that of the information (modulation) signal m(t). BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 12 BALANCED MODULATOR Figure 8-4. Balanced ring modulator.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 13  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 D 1 and D 2 become low-impedance devices for one-half of the RF cycle, while D 3 and D 4 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 D 3 and D 4 "on" while reverse-biasing D 1 and D 2.  The output signal v o (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 T 2. BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 14 BALANCED MODULATOR Figure 8-5. Balanced modulator Phase reversals.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 15  The balance is confirmed by following current from a positive polarity v c (t) into the center tap of T 1, then splitting and flowing through both. D 1 and D 2, converging at the center tap of T 2 and returning to the v c (t) source.  The opposite flowing currents in the primary of T 2 induce voltages of equal magnitude and opposite polarity in the T 2 secondary, which therefore cancel each other. BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 16  The squarewave switching function can be written with an amplitude of A=  /2 as v c (t) = sin2  f c t +(1/3)sin2  f c )t +… +(1/n)sin2  nf c )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 time- varying signals v c (t) and the information signal m(t).  Hence, the output is m(t) x v c (t) = v o (t) = m(t).sin2  f c t + (1/3)m(t).sin2  f c )t + higher odd harmonics (8-2) BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 17  To illustrate that Equ. (8-2) indeed represents a DSB- SC signal, let the modulation signal be a 2-Vpk audio tone of frequency f m = 5kHz so that m(t) = Asin2  f m t = 2sin2  (5kHz)t V.  Also, let the carrier frequency be f c = 45 kHz. Substituting into Equ. (8-2) yields a modulated output signal of v o (t) = A. sin2  f m t. sin2  f c t + (A/3). sin2  f m. sin2  f c )t +… = 2. sin2  (5kHz)t. sin2  (45kHz)t + (2/3). sin2  (5kHz)t. sin2  135kHz)t +… BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 18  By the use of the trigonometric identity sinA. sinB = (1/2)[cos(A-B) - cos(A+B)], v o (t) is seen to be v o (t) = (A/2)cos2  (f c -f m )t – (A/2)cos2  (f c +f m )t + (A/6)cos2  (3f c -f m )t – (A/6)cos2  (3f c +f m )t +… (8-3) = cos2  (40kHz)t – cos2  (50kHz)t + (1/3)cos2  (130kHz)t – (1/3)cos2  (140kHz)t +… (8-4) BALANCED MODULATOR

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 19  Figure 8-6 shows a sketch of Equ. (8-4) in both time and frequency domains.  If v o (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. BALANCED MODULATOR Figure 8-6. Wideband DSB-SC signal.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 20 BALANCED MODULATOR Figure 8-7. DSB-SC after filtering higher harmonics.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 21  Figure 8-11 shows the transmitted DSB-SC phasors (a), and the correct relationship between the reinserted carrier and sidebands (b). A phase error  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. Phase Distortion in the Demodulation of Suppressed-Carrier Systems

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 22 Phase Distortion in the Demodulation of Suppressed-Carrier Systems Figure Phasor representation of DSB-SC. (a) DSB-SC. (b) DSB-SC with carrier “reinserted”-AM. (c) Carrier reinserted with wrong phase. Figure Result of phase distortion due to reinserted- carrier 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 23  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 (envelope- delay distortion), will make the problem even worse. Phase Distortion in the Demodulation of Suppressed-Carrier Systems

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 24  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 band- width 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. Single-Sideband/Suppressed-Carrier

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 25  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. The Sideband-Filter Method

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 26 The Sideband-Filter Method Figure 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 27  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 single- sideband signals. The Sideband-Filter Method

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 28  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 The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 29 The Phase Method of SSB-SC Generation Fig SSB transmitter block diagram (phase method).

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 30  The inputs to the top balanced modulator are sin(  m t) and sin(  c t).  The multiplied output is sin(  m t). sin(  c t), which by trigonometric identity is sin(  m t). sin(  c t) = (1/2)[cos(  c -  m )t - cos(  c +  m )t] (8-5)  The inputs to the bottom balanced modulator are cos(  m t) and cos(  c t).  The output of this modulator is cos(  m t). cos(  c t) = (1/2)[cos(  c -  m )t + cos(  c +  m )t] (8-6) The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 31  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 = V o (t) = (1/2)[cos(  c -  m )t - cos(  c +  m )t] + (1/2)[cos(  c -  m )t + cos(  c +  m )t] = cos(  c -  m )t. (8-7) The various time waveforms and corresponding frequency spectra for single-tone modulation are shown in Figure The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 32 The Phase Method of SSB-SC Generation Figure Time and frequency spectra for phase method of producing SSB-SC.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 33  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  2 -  1, where  2 = 1/R 2 C 2 is the high-frequency cutoff and  1 is set by the other RC time constant. The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 34 The Phase Method of SSB-SC Generation Figure Wideband 90° phase-shift circuit.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 35  EXAMPLE 8-2: Determine the minimum frequency stability required if a 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/ MHz = 1.85 ppm (or 1.85 x ) x 100% = %.  On a short-term basis this is achievable with a crystal oscillator. However, one should be concerned about oscillator and received-signal noise. The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 36  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. The Phase Method of SSB-SC Generation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 37 Frequency Division Multiplexing (FDM) Figure 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 38 Figure Typical FDM receiver. Frequency Division Multiplexing (FDM)

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 39 Frequency Division Multiplexing (FDM)

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 40 Figure Analog telephone FDM hieracrchy. Frequency Division Multiplexing (FDM)

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 41 Figure Analog telephone FDM frequency spectrum. Frequency Division Multiplexing (FDM)

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 42  Quadrature multiplexing is illustrated in Figure The two modulation signals m 1 (t) and m 2 (t) modulate the quadrature carriers sin  c t and cos  c t 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. QUADRATURE MULTIPLEXING

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 43 QUADRATURE MULTIPLEXING Figure Quadrature multiplexing of two channels. The output is the sum of two orthogonal DSB-SC signals on the same carrier.

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 44  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 -0.44V i, where V i is the maximum in-phase carrier voltage of positive polarity, and the quadrature carrier has an amplitude of -0.9V q, then the TV demodulator should interpret the resulting color as green. QUADRATURE MULTIPLEXING

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 45 QUADRATURE MULTIPLEXING Figure Quadrature multiplexing receiver. The two information signals are m 1 (t) and m 2 (t).

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 46  The receiver LO or beat frequency oscillator (BFO) is synchronized to the incoming signal; that is, the oscillator frequency is exactly  c.  The LO signal 2cos(  c t) is split and phase-shifted to quadrature LO signals V LOi = 2cos(  c t) and V LOq = 2sin(  c t), 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. QUADRATURE MULTIPLEXING

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 47  The quadrature mixer output is V LOq x V QM = (2sin  c t)[m 1 (t)sin  c t + m 2 (t)cos  c t] (8 -8a) = 2m 1 (t)sin  c t. sin  c t + 2m 2 (t)sin  c t. cos  c t (8-8b) = m 1 (t)cos(  c -  c )t - m 1 (t)cos(  c +  c )t + m 2 (t)sin(  c -  c )t + m 2 (t)sin(  c +  c )t (8-8c) = m 1 (t) - m 1 (t)cos2  c t + m 2 (t)sin2  c t (8-9)  Note that the mixer output is the m 1 (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  c.  Since the low-pass filters (LPF) of Fig have a cutoff frequency of just below  c, the output signal from the "quadrature" branch is V Q (t) = m 1 (t) (8-10) A similar analysis shows that V I (t) = m 2 (t). QUADRATURE MULTIPLEXING

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 48  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. Costas Loop for Suppressed Carrier Demodulation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 49 Figure Costas loop includes BFO-synchronizing for demodulation of all types of suppressed-carrier signals. Costas Loop for Suppressed Carrier Demodulation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 50  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  LO =  c and only a small phase error difference (  e ) between the two signals will exist.  Because of the small phase error, Equ. 8-8c becomes V LO x V QM = m 1 (t).cos[(  c -  c )t +  e ] - m 1 (t).cos[(  c +  c )t +  e ] + m 2 (t).sin[(  c -  c )t +  e ] + m 2 (t).sin[(  c +  c )t +  e ] = m 1 (t).cos  e - m 1 (t).cos(2  c t +  e ) + m 2 (t).sin  e + m 2 (t).sin(2  c t +  e ) (8-11) and Equ. (8-10) becomes V Q (t) = m 1 (t).cos  e (8-12a) and V I (t) = m 2 (t).sin  e (8-12b) Costas Loop for Suppressed Carrier Demodulation

Prof. J.F. Huang, Fiber-Optic Communication Lab. National Cheng Kung University, Taiwan 51  Now the output of the badeband product detector becomes V BB-PD (t) = V Q (t) x V I (t) = m 1 (t)m 2 (t).cos  e sin  e = m 1 (t)m 2 (t).[sin(  e –  e ) + sin(  e +  e )]/2 = (1/2).m 1 (t)m 2 (t).sin(2  e ) (8-13)  The low-pass filter preceding the VCO will have a cutoff frequency sufficiently low to integrate the varying information signals m 1 (t) and m 2 (t) so that the average (dc) voltage applied to keep the VCO tracking any received- carrier frequency drifts will be (8-14a) which is approximately proportional to 2  e, and Equ. 8-14a becomes V o = K(2  e ) (8-14b) Costas Loop for Suppressed Carrier Demodulation