SINGLE SIDEBAND SINGLE SIDEBAND (SSB)(SSB)

Cont’d…ssbCont’d…ssb

Conventional AM – wasteful of Conventional AM – wasteful of transmission power and bandwidth.transmission power and bandwidth.

Suppressing carrier signal – reduces Suppressing carrier signal – reduces the transmitted powerthe transmitted power

Suppressing one of the sidebands -Suppressing one of the sidebands -reduces the bandwidth.reduces the bandwidth.

Leads to single sideband (SSB) Leads to single sideband (SSB) modulation system.modulation system.

Cont’d..ssbCont’d..ssb

Generation of SSBGeneration of SSBFilter methodFilter methodPhase-shift method Phase-shift method

Cont’d…ssbCont’d…ssb

Filter methodFilter methodA good suppression of sideband if the A good suppression of sideband if the

filter is effective.filter is effective.It cannot be used for a very high RF It cannot be used for a very high RF

bands of signal.bands of signal.

Balanced Modulator Sideband Filter

DSBSC SSB

Carrier signal

Information signal

Cont’d…ssbCont’d…ssb

Phase shift methodPhase shift methodThe size of the circuit is smallerThe size of the circuit is smallerCan be used at any RF band without any Can be used at any RF band without any

responseresponseEasy to convert from one sideband to Easy to convert from one sideband to

another.another.Generate LSB – shifted negativelyGenerate LSB – shifted negativelyGenerate MSB – shifted positivelyGenerate MSB – shifted positively

Cont’d…ssbCont’d…ssb

BalancedModulator 1

Balanced Modulator 2

Phase shifter

Phase shifter

+

Information signal

Carrier signal

Output Signal, aot

Am cos wmt

Am cos (wmt + 90)

Ac cos (wct + 90)

A2(t)

A1(t)

Cont’d…ssbCont’d…ssb

Power Transmitted, PtPower Transmitted, PtPtPt = = P Ptt DSB-SC DSB-SC / 2/ 2

== P PCC P Pmm/2/2

Where, Pc = carrier powerWhere, Pc = carrier power

Pm= information powerPm= information power

Cont’d…Cont’d…

Advantages Advantages The spectrum space occupied by the The spectrum space occupied by the

SSB signal is only half of DSB-SC and SSB signal is only half of DSB-SC and DSB-FC.DSB-FC.

This greatly conserves spectrum space This greatly conserves spectrum space and allows more signals to be and allows more signals to be transmitted in the same frequency transmitted in the same frequency range.range.

Less interference between signals.Less interference between signals.

Cont’d…ssbCont’d…ssb DemodulationDemodulation

Coherent schemeCoherent schemeThe received signal is heterodyned The received signal is heterodyned

/mixed with a local carrier signal which /mixed with a local carrier signal which is synchronous (coherent) with the is synchronous (coherent) with the carrier used at the transmitting end.carrier used at the transmitting end.

LPF X

SSB

cos wct

Coherent demodulation

VESTIGIAL SIDEBAND VESTIGIAL SIDEBAND (VSB)(VSB)

Cont’d… vsbCont’d… vsb

Also called asymmetric sideband system.Also called asymmetric sideband system. Compromise between DSB & SSB.Compromise between DSB & SSB. Easy to generate.Easy to generate. Bandwidth is only ~ 25% greater than SSB Bandwidth is only ~ 25% greater than SSB

signals.signals. Derived by filtering DSB, one pass band is Derived by filtering DSB, one pass band is

passed almost completely while just a passed almost completely while just a trace or vestige of the other sideband is trace or vestige of the other sideband is included.included.

Cont’d…vsbCont’d…vsb

AM wave is applied to a vestigial AM wave is applied to a vestigial sideband filter, producing a sideband filter, producing a modulation scheme – VSB + Cmodulation scheme – VSB + C

Mainly used for television video Mainly used for television video transmission.transmission.

Cont’d…vsbCont’d…vsb

VSB Frequency SpectrumVSB Frequency Spectrum

fcfc

LSB MSB

CarrierVSB

SVSB(t) = Ac cos wct + (mAc / 2) cos (wc + wm)t

Cont’d…vsbCont’d…vsb

Average transmitted powerAverage transmitted power

PPtt = P = Pcc + (1/2) P + (1/2) Pmm P Pcc

In generalIn generalPPtt = P = Pcc + + kk P Pmm P Pcc

Where Where 0.5<k<10.5<k<1

Cont’d…vsbCont’d…vsb

CharacteristicsCharacteristicsOdd symmetry about carrier frequencyOdd symmetry about carrier frequencyA relative response of ½ at fA relative response of ½ at fcc

Cont’d…vsbCont’d…vsb

DemodulationDemodulationUsing envelope detectorUsing envelope detector

SVSB(t) = Ac cos wct + (mAc / 2) cos (wc + wm)t

= Ac (1 + (m/2)cos wmt) cos wct –

(mAc /2) sin wc t sin wmt

Cont’d…vsbCont’d…vsb

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

Avoid the necessity of coherent Avoid the necessity of coherent demodulation.demodulation.

Introduce some distortion.Introduce some distortion.

COMPARISONCOMPARISON

SummarySummary

DSB-SCDSB-SCo Less transmitted power – all the Less transmitted power – all the

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

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

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

Cont’d…Cont’d…

ConventionalConventionalo Sidebands are transmitted in full with Sidebands are transmitted in full with

the carrier.the carrier.o Simple to demodulate ( envelope Simple to demodulate ( envelope

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

broadcasting - one TX, many broadcasting - one TX, many receiversreceivers

Cont’d…Cont’d…

SSBSSBo Good bandwidth utilisation (information Good bandwidth utilisation (information

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

conventional – exact filter design & conventional – exact filter design & coherent demodulation.coherent demodulation.

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

BW – multi channel carrier telephony.BW – multi channel carrier telephony.

Cont’d…Cont’d…

VSBVSBo Compromise solution between DSB & Compromise solution between DSB &

SSB.SSB.o Has lower power, less bandwidth Has lower power, less bandwidth

than DSB.than DSB.o Higher power, slightly greater Higher power, slightly greater

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

frequency component).frequency component).