1. Double Side Band Suppressed Carrier
Professor Z Ghassemlooy
Electronics and IT Division
School of Engineering
Sheffield
Hallam
University
U.K.

2. Contents Theory Implementation Power analysis Summary Transmitter
Detector
Synchronous
Square
Power analysis
Summary

3. Double Side Band Suppressed Carrier
c - m c c + m
Carrier
USB
LSB
Single frequency
From AM spectrum:
Carries signal c carries no information m.
Carries signal consumes a lot of power more than 50%
Question: Why transmit carrier at all?
Ans:
Question: Can one suppress the carrier?
Ans.: Yes, just transmit two side bands (i.e DSB-SC)
But what is the penalty?
System complexity at the receiver

4. DSB-SC - Theory General expression:
Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore:
Information signal m(t) = Em cos mt
Thus
upper side band
lower side band

5. DSB-SC - Waveforms Mixer (Multiplier) Notice: No carrier frequency
B = 2m
Notice: No carrier frequency

6. DSB-SC - Implementation
Balanced modulator +
Ec ( m(t) cos ct AM
mod.
0.5 m(t)
Carrier
Ec cos ct -
DSB-SC
Ec m(t) cos ct +
Ec ( m(t) cos ct AM
mod.
-0.5 m(t)
Ring modulator
Square-law modulator

7. DSB-SC - Detection Synchronous detection Multiplier Low pass filter
Message signal
Local oscillator
c(t) = cos ct
Condition:
Local oscillator has the same
frequency and phase as that of the
carrier signal at the transmitter.
information
high frequency m
2c-m
2c+m
Low pass filter

8. DSB-SC - Synchronous Detection
Case 1 - Phase error
Multiplier
Low pass
filter
Message signal
DSB-SC
Local oscillator
c(t) = cos(ct+)
Condition:
Local oscillator has the same
frequency but different phase
compared to carrier signal at the
transmitter.
information
high frequency m
2c+m
2c-m
Low pass filter

9. Phase Synchronisation - Costa LoopX
VCO
DSB-SC
Ec cos (ct+)
LPF
Phase
discriminator
In-phase
0.5Ec m(t) cos 
Vphase(t)
yip(t)
Recovered
signal X.
0.5Ec m(t) sin 
90o
phase shift
LPF
Ec sin (ct+)
Quadrature-phase
yqp(t)
When there is no phase error. The quadrature component is zero
When  0, yip(t) decreases, while yqp(t) increases
The out put of the phase discriminator is proportional to 

10. DSB-SC - Synchronous Detection
Case 1 - Frequency error
Multiplier
Low pass
filter
Message signal
DSB-SC
Local oscillator
c(t)=Eccos(ct+)
Condition:
Local oscillator has the same
phase but different frequency
compared to carrier signal at the
transmitter.
information
high frequency m
2c+m
2c-m
Low pass filter

11. DSB-SC - Square Detection
Squaring circuit
g =x2
g(t)
y(t)
DSB-SC
Si(t)
Band pass filter
 by 2
2c
Regenerated carrier
z(t)
Multiplier
Message
signal
Low pass filter
g(t) = Si2(t) = B2 cos2 mt cos2 ct
= B2 (½ + ½ cos 2 mt )(½ + ½ cos 2 ct )
= B2/4 [1 + ½ cos 2(c + m)t + ½ cos 2(c - m)t + cos 2mt + cos 2 ct ]
y(t) = B2/4 cos 2wct
z(t) = B2/4 cos wct

12. DSB-SC - Power The total power (or average power):
The maximum and peak envelop power

13. DSB-SC - Summary Advantages: Lower power consumption Disadvantage:
- Complex detection
Applications:
- Analogue TV systems: to transmit colour information
- For transmitting stereo information in FM sound broadcast
at VHF