1 Contents Theory Implementation –Transmitter –Detector Synchronous Square Power analysis Summary

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

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

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

5 DSB-SC - Implementation AM mod. AM mod. Carrier E c cos  c t 0.5 m(t) -0.5 m(t) DSB-SC E c m(t) cos  c t + E c ( m(t) cos  c t E c ( m(t) cos  c t Balanced modulator Ring modulator Square-law modulator AM mod. AM mod.

6 DSB-SC - Detection Synchronous detection Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t) = cos  c t Local oscillator c(t) = cos  c t Condition: Local oscillator has the same frequency and phase as that of the carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

7 DSB-SC - Synchronous Detection Case 1 - Phase error Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t) = cos(  c t+  ) Local oscillator c(t) = cos(  c t+  ) Condition: Local oscillator has the same frequency but different phase compared to carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

8 Phase Synchronisation - Costa Loop Recovered signal When there is no phase error. The quadrature component is zero When  0, y ip (t) decreases, while y qp (t) increases X X VCO DSB-SC E c cos (  c t+  ) LPF Phase discriminator Phase discriminator In-phase 0.5E c m(t) cos  V phase (t) y ip (t) X. 0.5E c m(t) sin  90 o phase shift LPF E c sin (  c t+  ) Quadrature-phase y qp (t) The out put of the phase discriminator is proportional to 

9 DSB-SC - Synchronous Detection Case 1 - Frequency error Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t)=E c cos(  c t+  ) Local oscillator c(t)=E c cos(  c t+  ) Condition: Local oscillator has the same phase but different frequency compared to carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

10 DSB-SC - Square Detection Regenerated carrier z(t) Band pass filter DSB-SC S i (t) C Squaring circuit g =x 2 Squaring circuit g =x 2 g(t)g(t)  by 2 Multiplier Low pass filter Message signal y(t)y(t) 2c2c g(t) = S i 2 (t) = B 2 cos 2  m t cos 2  c t = B 2 (½ + ½ cos 2  m t )(½ + ½ cos 2  c t ) = B 2 /4 [1 + ½ cos 2(  c +  m )t + ½ cos 2(  c -  m )t + cos 2  m t + cos 2  c t ] y(t) = B 2 /4 cos 2w c tz(t) = B 2 /4 cos w c t

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

12 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