amplitude modulation
TRANSCRIPT
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)