amplitude modulation

Overview of DSB-SC, DSB-LC and SSB

ZEESHAN TALLAT (156)TAIMUR SAEED (133)

ASAD MAHMOOD (026)

CLASS : BEE-2D

AMPLITUDE MODULATIONIn amplitude modulation (AM), the amplitude of the carrier is proportional to the message signal.

DSB-SCm(t)cos2πfct [M(f + fc) + M(f - fc)]Upper sideband (portion outside ±fc)Lower sideband (portion inside ±fc)If message signal’s BW is B hz, then

modulated signal has a BW of 2B hz. Modulated signal does not contain a

discrete component of the carrier frequency.

DSB-SC (cont….)To avoid overlapping of

modulated spectra, fc ≥ B

fc / B >> 1 to avoid distortion caused by transmit antenna in broadcast.

Synchronous detection (coherent detection) method is used for demodulation.

DSB-SC (cont…)E(f) = M(f) + [M(f + 2fc) + M(f -

2fc)]Low pass filters will recover ½

M(f), which is the baseband (message) signal.

To remove factor of ½, use 2cos2πfct as carrier.

Efficiency = η = 100 %

DSB-LC ØLC(t) = [A + m(t)] cos2πfctØLC(t) [M(f + fc) + M(f - fc)] +

[δ(f + fc) + δ(f - fc)] Rectifier detection or envelope

detection may be used for demodulation.

For envelope detection to properly detect baseband signal

I. fc >> BW of m(t)II. A + m(t) ≥ 0

DSB-LC (cont….)Major drawback is the wastage of

carrier power.Useful message information

resides in the sideband power. Total power is the sum of carrier

(wasted) power and sideband (useful) power.

Efficiency = η =

DSB-LCFor tone modulation

Efficiency = η = 100%η max = 33% at µ = 1.

SSB (single sideband)In DSB, both sidebands (upper and lower)

contain complete information about the baseband signal and bandwidth = 2B hz.

To improve spectral efficiency, SSB is used.

Transmits either LSB or USB with bandwidth = B hz.

ØSSB(t) = m(t)cos2πfct ± mh(t)sin2πfctmh(t) is Hilbert transform of m(t).Hilbert transform is an ideal phase shifter.

SSB (cont…)H(f) = - j sgn(f)

= - j ; f > 0 and j ; f < 0Coherent demodulation is used.Phase shift method or selective

filtering method are commonly used to generate SSB signals.

Drawback is the difficulty to design ideal bandpass filter.

Spectral efficiency of SSB is twice that of DSB.

SSB + CSSB signal with a carrier (SSB +

C).ØSSB+C(t) = Acos2πfct +

m(t)cos2πfct ± mh(t)sin2πfct

For A >> m(t), SSB+C can be demodulated by envelope detector.

Efficiency is very low.

OverviewDSB-SC DSB-LC SSB

100 % efficiency less efficient since carrier power is wasted

100 % efficiency

Low spectral efficiency

Low spectral density (same as DSB-SC)

Spectral density twice than DSB

Difficult demodulation because of generation of same carrier frequency at transmitter and receiver

Easier demodulation i.e. envelope detection but A + m(t) ≥ 0

Coherent demodulation (same as DSB-SC) but in SSB + C envelope detector may be used when A >> m(t)