CHAPTER 2 – CHAPTER 2 – Part 3Part 3

AM Modulation – Types of AM AM Modulation – Types of AM MethodsMethods

(SSB and VSB)(SSB and VSB)

Why SSB?Why SSB?• Shortage of AM and DSB signals is

basic information is transmitted TWICE (2X).Once in each sidebandBoth sideband have identical informationNo solid reason to transmit both!!

• One sideband may be suppressed, the remaining sideband is SSB signal.

• Bandwidth SSB signal, BWSSB = fm(max)

Benefits of SSB SignalsBenefits of SSB Signals• All power transmitted (one-half of power in

DSB method) represents intelligence.• Occupy half spectrum space than AM and

DSB signals and less interference.• Transmit stronger information and more

reliable over longer distances.• Susceptible to less noise due to restricted

signal BW and receiver BW (major adv for long-distance comm.)

• Experience less fading due to ionosphere.

How Fading Affect Signal?How Fading Affect Signal?• Fading means signal alternately

increase and decrease in strength as pick-up by the receiver.

• The ionosphere bends all signals compositions back to earth at slightly different angles resulting canceling out or adding up of the signals.

• Fading does not occur in SSB since only one sideband transmitted.

Characteristic of SSBCharacteristic of SSB• When no information is present, no RF

signal is transmitted (unusual condition) – e.g voice pause

• A carrier modulated by a single tone audio signal generates a constant frequency pure sin wave at the rate of either the upper or lower frequency.

• Modulating voice (varying amplitude and freq) will create a complex RF SSB which varies in freq and amplitude over narrow spectrum bounded by voice BW.

SSB Power ContentSSB Power Content• In practice an SSB transmitter generates

both sidebands and a switch is used to select either USB or LSB transmission.

• Since SSB transmitter sends no carrier the carrier power, PC = 0.

• The transmitter output is expressed in peak envelope power (PEP) – maximum power produced on voice amplitude peaks.

PEP ComputationPEP Computation• The PEP output power is computed by,

• The PEP input power is computed by,

R

V

R

VoutPEP prms

2)(

22

max)( IVinPEP s Vs = dc supply voltage of final

amplifier stage

Imax = maximum final amplifier peak

current

V = Voltage across tx antenna

R = antenna chac. impedance

Eq (2.17)

Eq (2.18)

Contd…Contd…• PEP occurs only occasionally.• Voice amplitude peaks are produced only

when very loud sound generated or words emphasized.

• During normal speech levels, the average power is typically only one-fourth or one-third of the PEP value with typical human speech.

• Typical SSB transmitters designed to handle the average power level on continuous basis not the PEP.

Example 1Example 1• Calculate the input and output PEP

assuming that a voice signal produces a 120-Vpp signal across a 50- transmitting antenna and the dc supply of the final amplifier stage is 300 V with a peak current of 0.6 A. Hence evaluate the power handling capability of a typical SSB transmitter for this system.

Solution 1Solution 1

• PEP (in) = VsImax = (300V)(0.6A)

• = 180 W

• PEP (out) = Vp2/2R = (60)2/[2x50]

• = 36 W

• Pavg = 1/4[PEP(out)] to 1/3[PEP(in)]

• = 1/4[36W] to 1/3[36 W]• = 9 W to 12 W

Example 2Example 2• A voice signal with a frequency range

of 300 to 3000 Hz modulates a 14.3-MHz RF carrier. Determine the frequency limit for the output signal produced by SSB AM modulator and its BW.

• Draw the time-domain wave and spectrum using a constant 2 kHz sine wave tone.

Solution 2Solution 2• The SSB signal may use either USB or LSB.• If chosen USB: Frequency band extends

from [14.3M + 300] to [14.3M + 3000] i.e. 14,300,300 Hz to 14,303,000 Hz.

• If chosen LSB: Frequency band extends from [14.3M – 3000] to [14.3M – 300] i.e. 14,297,000 Hz to 14,299,700 Hz.

• BW = fUSB(highest) –fUSB(lowest) or

fLSB(highest) – fLSB(lowest) = 2.7 kHz

Solution 2Solution 2

Solution 2Solution 2

Variations of SSB OperationsVariations of SSB Operations• Single sideband suppressed-carrier

(SSBSC or simply SSB) – used for HF mobile communications.

• Vestigial sideband (VSB) – used worldwide for television (picture) transmissions.

• Single sideband full-carrier (SSBFC) – used in a low-distortion, compatible AM broadcasting system whose transmitted signal can be received by standard AM and SSB receivers.

Contd…Contd…• Single sideband reduced-carrier (SSBRC) – a

pilot carrier sys widely used in transmarine point-to-point radiotelephony.

• Single sideband controlled-carrier (SSBCC) – another pilot carrier sys used in which power is independent of modulation and output Pavg is constant.

• Independent sideband (ISB) also called twin-sideband suppressed-carrier (TSBSC) – transmit two independent sidebands, each containing different intelligence, frequently used for HF point-to-point radiotelephony and primarily used in military communications.

• We will discuss on the first THREE only.

Comparing Spectrum Comparing Spectrum of AM Methodsof AM Methods

Figure 23 (a) & (b)

Contd…Contd…

Figure 23 (c) and (d)

Two Basic Methods forTwo Basic Methods for SSB Generation SSB Generation

• Filter method.• Phase cancellation method.• Both methods use some form of balanced

modulator to suppress the carrier but differ in the suppression of the unwanted sideband.

• Commonly used SSB generators consist of balanced modulator followed by an extremely selective crystal, ceramic or mechanical filter or phasing circuit.

Balanced ModulatorsBalanced Modulators• The balanced modulator (BM) is a symmetrical

circuit that delivers an output signal containing the sum and difference frequencies of the two inputs.

• In theory the injected carrier is balanced out so that it disappear in the output.

• Practical design characteristics determine the extent to which the carrier can be balanced out. – Existing BM suppress the carrier from 30 dB to 40 dB

below the PEP.– The SSB filter further suppresses the carrier by an

additional 20 dB.– Total carrier suppression of from 50 dB to 60 dB can be

reasonable be expected from SSB transmitter systems.

Refer Figure 24

Simplified Block Diagram of Simplified Block Diagram of a Filter-type SSB generatora Filter-type SSB generator

Figure 24

Filter MethodFilter Method• The widely used technique is filter method

which uses a BP filter to pass one sideband while reject the other.

• The filters used in SSB transmitters must have very fast rise and fall slopes and flat BW characteristics.

• Crystal filters are commonly used in SSB systems because they have very high Q (selectivity) values.

• This method is not suitable for high frequency signals (VHF and above).

Simplified Block Diagram of a Simplified Block Diagram of a Filter-type SSB TransmitterFilter-type SSB Transmitter

Figure 25

SSB Transmitter ActionSSB Transmitter Action

Figure 26

Sideband FiltersSideband Filters• Voice transmission for adequate speech

intelligibility – audio frequency from 100 Hz to 3000 Hz

• After carrier suppression, one sideband rejected without affecting the other.

• Closeness between sidebands require high Q-filters for sharply defined skirts and narrow pass band and flat band-pass characteristics to pass the desired sideband and reject the other.

Phase-cancellation MethodPhase-cancellation Method• The phase-cancellation method uses two matched

balanced modulators, both of which suppress the carrier and have a double-sideband output.

• The two sideband signals are combined by a summing circuit producing only the desired sideband.

• Advantage of this method is SSB signal at the operating frequency can be generated without the frequency conversion.

• Disadvantage is the phase shifts and amplitudes of the signals to the summing circuit must be maintained.

Contd…Contd…• This method can be used at higher

frequency band (VHF and above).• Reconfigurable for any sideband

output i.e. upper or lower-generated SSB signal.

• In case of upper sideband modulation, the modulated output is phase-shifted by /2.

Block Diagram of Phase-Block Diagram of Phase-shift SSB Transmittershift SSB Transmitter

Figure 27

Upper Sideband Upper Sideband SSB ModulatorSSB Modulator

Figure 28

Lower SidebandLower Sideband SSB Modulator SSB Modulator

Figure 29

Mathematical Analysis Mathematical Analysis of SSB Signalsof SSB Signals

• Recall from previous part that the output of balanced modulator (DSB signal) is

• The expression is DSB signal contains two sideband frequencies.

ttVV

ttVV

ttmVtV

mcmcmc

cmmc

ccDSBam

)(cos)(cos2

coscos

cos)()()(

Eq (2.19)

Contd…Contd…• If one sideband is removed either by

filtering or canceling method, taking USB to remain hence

• Since all the transmitted power goes into USB wave, yield

tVVtV mc

mcUSB )(cos

2)(

R

VV

R

VV

P mc

mc

T 822

2

2

Eq (2.20)

Eq (2.21)

Contd…Contd…• Demodulation of a single sideband signal

is achieved by multiplying it with a locally generated synchronous carrier at the receiver.

• Detectors using this principle are product detectors which use balanced modulator circuit.

• The carrier must be as closely synchronized in frequency (≤100Hz) and phase with the original carrier in the transmitter to avoid distortion in the modulated output.

Contd…Contd…• To demonstrate the demodulation process,

multiply the received signal with Vccosct yields,

• Low-pass filtering easily remove the 2nd term which is the upper side freq at the second harmonic of the local carrier freq, leaving only the 1st term which is required information.

ttVV

tVtVV

tV

mcmmc

ccmcmc

received

)2(coscos2

cos)(cos2

)(

2

Eq (2.22)

Demodulation usingDemodulation using carrier reinsertion carrier reinsertion

Figure 30

SSB Receiver (demodulator)SSB Receiver (demodulator)• SSB receivers are superheterodyne but

differ from standard AM receivers in that additional circuitry must be used to restore the suppressed carrier.

• Since SSB transmission is used extensively in HF communications systems, SSB receivers must be capable of receiving signal in HF band (3 MHz to 30 MHz).

Block Diagram of Block Diagram of SSB ReceiverSSB Receiver

Figure 31

SSB ApplicationsSSB Applications• SSB Signals are

popularly used in telephone systems as well as in two-way radio.

• Two-way SSB communications is used in the military, CB radio and radio amateurs.

Disadvantage of SSBDisadvantage of SSB• SSB transmitters are more

complicated hence more expensive than standard AM transmitters – due to added cost and system complexity involved in providing better frequency stability in the transmitter and receiver.

VESTIGAL SIDEBAND

(VSG)

Vestigal Sidebands (VSB)Vestigal Sidebands (VSB)Overcomes the complexity problem

of coherent demodulation.Obtained by passing one pass band

mostly and a vestige, just a trace, of the other sideband of an AM/DSB-SC modulation.

Easy to generate.Bandwidth is ~ 25% greater than

SSB signals.

Contd…Contd…Also called asymmetric sideband

system.Compromise between DSB & SSB.Low frequency including DC

components can also be transmitted and received.

Average transmitted power equals SSB + small power of carrier.

Contd…Contd…AM wave is applied to a vestigial sideband

filter, producing a modulation scheme – VSB + C

Mainly used for television video transmission.

VSB allows recovery of the base band signal with a receiving system (envelope detector) intended for AM signal.

Avoid the necessity of coherent demodulation.

Introduce some distortion.

VSB SpectrumVSB Spectrum

Eq (2.23)

Figure 32

Demodulation of VSB SignalDemodulation of VSB SignalUsing envelope detector.Mathematically written asSVSB(t) = Ac cos ct + (mAc / 2) cos(c+m)t

= Ac (1 + (m/2)cos m t) cos c t –

(mAc /2) sin c t sin m t

Eq (2.24)

COMPARISONCOMPARISON

Table 1

Summary of Various Summary of Various AM MethodsAM Methods

DSB-SCo Less transmitted power – all the

transmitted power is useful.o Wide bandwidth.o Requires a coherent carrier at the

receiver – increased complexity.o Suited for point to point communication

involving one Tx and one Rx which would justify the use of increased Rx complexity.

Contd…Contd…Conventionalo Sidebands are transmitted in full with

the carrier.o Simple to demodulate ( envelope

detector).o Wide bandwidth.o Used in commercial AM radio

broadcasting - one TX, many receivers

Contd…Contd…SSBo Good bandwidth utilisation (information

signal BW = modulated signal BW)o Good Power efficiency.o Demodulation is harder compares to

conventional – exact filter design & coherent demodulation.

o Widely use for long distance transmission.o Preferred in systems requiring a minimal

BW – multi channel carrier telephony.

Contd…Contd…VSBo Compromise solution between DSB &

SSB.o Has lower power, less bandwidth

than DSB.o Higher power, slightly greater

bandwidth than SSB.o Standard for transmission of TV (low

frequency component).