SINGLE SIDEBAND (SSB)

Cont’d…ssb Conventional AM – wasteful of transmission power and bandwidth. Suppressing carrier signal – reduces the transmitted power Suppressing one of the sidebands -reduces the bandwidth. Leads to single sideband (SSB) modulation system.

Cont’d..ssb Generation of SSB Filter method Phase-shift method

Cont’d…ssb Filter method A good suppression of sideband if the filter is effective. It cannot be used for a very high RF bands of signal. Balanced Modulator Sideband FilterDSBSCSSBCarrier signal Balanced Modulator Sideband Filter DSBSC SSB Carrier signal Information signal

Cont’d…ssb Phase shift method The size of the circuit is smaller Can be used at any RF band without any response Easy to convert from one sideband to another. Generate LSB – shifted negatively Generate MSB – shifted positively

Cont’d…ssb Balanced Modulator 1 Modulator 2 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…ssb Power Transmitted, Pt Pt = Pt DSB-SC / 2 = PC Pm/2 Where, Pc = carrier power Pm= information power

Cont’d… Advantages The spectrum space occupied by the SSB signal is only half of DSB-SC and DSB-FC. This greatly conserves spectrum space and allows more signals to be transmitted in the same frequency range. Less interference between signals.

Coherent demodulation Cont’d…ssb Demodulation Coherent scheme The received signal is heterodyned /mixed with a local carrier signal which is synchronous (coherent) with the carrier used at the transmitting end. LPF X SSB cos wct Coherent demodulation

VESTIGIAL SIDEBAND (VSB)

Cont’d… vsb Also called asymmetric sideband system. Compromise between DSB & SSB. Easy to generate. Bandwidth is only ~ 25% greater than SSB signals. Derived by filtering DSB, one pass band is passed almost completely while just a trace or vestige of the other sideband is included.

Cont’d…vsb AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C Mainly used for television video transmission.

Cont’d…vsb VSB Frequency Spectrum fc LSB MSB Carrier VSB SVSB(t) = Ac cos wct + (mAc / 2) cos (wc + wm)t

Cont’d…vsb Average transmitted power Pt = Pc + (1/2) Pm Pc In general Pt = Pc + k Pm Pc Where 0.5<k<1

Cont’d…vsb Characteristics Odd symmetry about carrier frequency A relative response of ½ at fc

Cont’d…vsb Demodulation Using 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…vsb 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.

COMPARISON

Summary DSB-SC Less transmitted power – all the transmitted power is useful. Wide bandwidth. Requires a coherent carrier at the receiver – increased complexity. Suited for point to point communication involving one Tx and one Rx which would justify the use of increased Rx complexity.

Cont’d… Conventional Sidebands are transmitted in full with the carrier. Simple to demodulate ( envelope detector). Wide bandwidth. Used in commercial AM radio broadcasting - one TX, many receivers

Cont’d… SSB Good bandwidth utilisation (information signal BW = modulated signal BW) Good Power efficiency. Demodulation is harder compares to conventional – exact filter design & coherent demodulation. Widely use for long distance transmission. Preferred in systems requiring a minimal BW – multi channel carrier telephony.

Cont’d… VSB Compromise solution between DSB & SSB. Has lower power, less bandwidth than DSB. Higher power, slightly greater bandwidth than SSB. Standard for transmission of TV (low frequency component).