DSB-SC & SSB

DSB-SC MODULATION

A DSB-SC signal is an AM signal of the form:

S(t)=Ac m(t) coswct

Normal AM signal :

S(t)=Ac[1+um(t)] coswct =Ac coswct +Ac u m(t) coswct

FREQUENCY SPECTRUM OF DSB-SC SIGNAL

Let M(f) be the Fourier transform of m(t), then the fourier transform of the DSB-SC is:

S(f)= Ac/2 M(f-fc)+Ac/2 M(f+fc)

Notes: No impulses are present at ±fc; no carrier

transmitted. The transmission bandwidth is 2W (same as

normal AM). Power efficiency= (Power in the sideband/

total transmitted power) * 100% = 100%

SSB MODULATION Here only one of the two sidebands of a DSB-SC

signal is retained while the other sideband is suppressed.

This means that the bandwidth requirement is half that for DSB-SC.

This saving in B.W comes at the expense of increasing modulation and demodulation complexity.

In SSB modulation we eliminate the carrier and one sideband, thus less transmitted power is needed, and approximately 83% power efficiency is achieved.

GENERATION OF SSB SIGNAL A SSB signal can be generated either by:

I. Filtering Method.II. Phase discrimination method.

o Filtering Method: The SSB signal is obtained by selecting either the

upper- side band(USB) or the lower-Sideband (LSB) of the DSB-SCby means of a suitable band pass filter.

A band pass filter with appropriate BW and center frequency is used to pass the desired side band only.

Phase Discrimination method: This method is based on the time

representation of the SSB signal:S(t)=Acm(t)cos wct ±Ac m*(t) sin wct

(-) : upper side band is retained.m*(t): the hilbert transform of m(t).

Notes:

• TP5 represents (DSB-SC)Q which is the quadrature component:

(DSB-SC)Q = cos2π(fc-fm)t - cos2π(fc+fm)t

• TP6 represents the (DSB SC)I which is the in phase componen:

(DSB-SC)I = cos2π(fc-fm)t+ cos2π(fc+fm)t

• To generate the SSB signal, an adder is used to obtain the desired side band:

- Lower side-band:

LSSB= (DSB-SC)I+ (DSB-SC)Q= 2cos2π(fc-fm)t

- Upper side-bad:USSB= (DSB-SC)I- (DSB-SC)Q=

2cos2π(fc+fm)t

NORMAL AM, DSB-SC, AND SSB SIGNALS:

DSB-SC AND SSB DEMODULATION

In this experiment , a coherent detector is used to recover the message signal.

S(t) is multiplied by a locally generated signal at the receive which has the same frequency and phase as c(t) at the transmitter, then it passes through a low-pass filter.

EXAMPLE ON DSB-SC DEMODULATION:

Consider s(t) represents a DSB-SC signal, after the multiplication process the signal will be:

V(t)= s(t) Ac* cos wct= Ac Ac*/2 m(t)+ Ac Ac*/2 m(t) cos 2wct

The high frequency term can be eliminated using a low pass filter.