1. DSB-SC AM Tx signal  AM Tx signal spectrum
Also called DSB-LC  Double Side Band - Large Carrier
AM Tx signal spectrum
Discrete delta functions represent sinusoidal carrier
Delta functions provide DC term when s(t) is shifted to baseband
DSB-SC is also an amplitude modulated signal but the carrier term present in AM is suppressed
Double Side Band – Suppressed Carrier
ECE 4710: Lecture #20

2. DSB-SC DSB-SC Tx signal 
m(t) must have zero DC component for SC
DSB-SC signal spectrum is identical to AM except the delta functions (LC) are removed
Modulation Efficiency = 100% since no power used on carrier
Best case AM efficiency was 50%
Sideband power is four times that of AM signal for same peak level for Tx output signal  10 log (4) = 6 dB difference
ECE 4710: Lecture #20

3. DSB-SC Power ECE 4710: Lecture #20
Let peak Tx output signal power be the same for AM and DSB-SC
Same peak Tx output so
For demonstration let MAM = 1 then MDSB-SC = 2
Sideband power   M 2 and
ECE 4710: Lecture #20

4. Encoding, Pulse Shaping,
DSB-SC Tx
Antenna
Mixer
Gain = Ac
Analog
Input
Baseband
LPF
Baseband
Amplifier
Power
Amplifier
cos(2pfct)
DSP
Digital
Input ˜ fc
Encoding, Pulse Shaping,
Error Coding, DAC, etc.
Carrier
Oscillator
ECE 4710: Lecture #20

5. Mixer = Product Detector
DSB-SC Rx
Antenna
Mixer = Product Detector ~
Low Noise
RF Amp
LPF
Baseband
Amplifier
DSP ˜ fc
Local
Oscillator
Digital or
Analog
Output
NOTE: SuperHeterodyne Rx often used but Zero-IF is shown for simplicity
ECE 4710: Lecture #20

6. DSB-SC Spectrums f ECE 4710: Lecture #20 fc LPF 2fc BandpassSignal
Desired Baseband Signal
BandpassSignal
Frequency Doubled Signal
LPF
NOTE: No LC present!!
ECE 4710: Lecture #20

7. DSB-SC Product Detection
Product Detector or Mixer: converts bandpass signal s(t) back into a baseband signal via another frequency translation
Product Detector  multiplication of r (t) with cos(2p fc t)
Product = multiplication & Detector = result is  original m(t)
Envelope Detector  normally not possible
Only possible if m(t) is always > 0  e.g. unipolar line code
If m(t) is polar or bipolar then it will have + and - values
180° phase change occurs in carrier when m(t) transitions from + to -
r (t) = A m(t) cos(2p fc t) so for m(t) > 0  cos(2p fc t)
and for m(t) < 0  -cos(2p fc t) = cos(2p fc t + 180°)
ECE 4710: Lecture #20

8. DSB-SC Product Detection
+cos cos +cos
1. Envelope of s(t) = | s(t) |  m(t)  cannot use envelope detector!!
2. For m(t) > 0  +cos(2p fc t) & for m(t) < 0  -cos(2p fc t)
° phase change between + m(t) & - m(t)
4. Sign of m(t) is stored in phase of carrier
5. Must have phase information from carrier to recover m(t)  coherent detection  mixer is a coherent detector
ECE 4710: Lecture #20

9. DSB-SC Product Detection
Desired Baseband Signal
Frequency Doubled Signal
After LPF then frequency doubled signal is eliminated and
ECE 4710: Lecture #20

10. DSB-SC Product Detection
What happens if there is a phase or frequency error in cosine supplied by local oscillator in Rx?
After LPF term the frequency doubled term is gone and
whereas for no errors we had !!
ECE 4710: Lecture #20

11. DSB-SC Product Detection
Phase Error 
Non-linear distortion of information signal m(t)
For Dq always < 20° then error is small since cos(20°) = 0.94
For Dq = ±90° then Rx signal is completely eliminated
For random Dq the performance is not acceptable  channel propagation distance is unknown so received signal phase is random!
ECE 4710: Lecture #20

12. DSB-SC Product Detection
Frequency Error 
m(t) is modulated by time-varying cos(2p Df t)!!
Low frequency cosine (assuming Df <<< fc) distorts m(t) and periodically eliminates signal
Completely unacceptable
** DSB-SC requires perfect knowledge of frequency and phase of Tx carrier must be present in the Rx **
ECE 4710: Lecture #20

13. DSB-SC Synchronization
Frequency and phase synchronization required between Tx and Rx in DSB-SC 
Coherent Detection
Product Detector = Coherent Detector if oscillator is completely synchronized
How do we synchronize Tx and Rx?
Radar System  pass copy of Tx carrier to Rx
Not possible for vast majority of communication systems
Pilot Carrier  transmit copy of carrier outside spectrum for carrier recovery in Rx
Carrier Recovery  PLL or Squaring Loop for carrier recovery in Rx
ECE 4710: Lecture #20

14. Pilot Carrier f ECE 4710: Lecture #20 -0.5fc 0.5fc fc LPF 2fc
Frequency Doubled Signal
BPF to get 0.5 fc and then  2 multiply to recover carrier for local oscillator
LPF
2fc
2fc
Desired Baseband Signal
ECE 4710: Lecture #20

15. Squaring Loop Recovery
Square Law Device  Full wave diode rectifier
ECE 4710: Lecture #20

16. Costas PLL Recovery
ECE 4710: Lecture #20

17. DSB-SC Detection
Complicated circuitry added to Rx for coherent detection of DSB-SC signals
Good performance for low S/N at Rx input
No distortion in recovered baseband signal spectrum
Allows for data signals with non-zero power near DC in PSD
Significant cost added to Rx design and manufacturing for coherent detection
Squaring loop and Costas PLL have a 180° phase ambiguity
At initial start up of loop either type can lock on wrong polarity for carrier phase  ambiguity between +m(t) and –m(t)
ECE 4710: Lecture #20

18. DSB-SC Detection 180° phase ambiguity Ambiguity solutions:
Not a problem for audio signal
no auditory difference for +m(t) vs. –m(t)  tone not affected by sign!!
Problem for polar data signal
Ambiguity solutions:
Send test signal with a priori known phase to lock phase of recovery
Use differential encoding so that “1” and “0” stored in phase change rather than absolute phase value
ECE 4710: Lecture #20

19. 180° Carrier Phase Transitions:
DSB-SC Data Signal
If m(t) is digital data signal like polar NRZ we have
Binary Phase Shift Keying = BPSK
Special case of DSB-SC for a Polar NRZ
m(t)
Must have coherent detection for BPSK to
measure absolute phase
Can use non-coherent detection with DBPSK +1 -1 +1 -1
180° Carrier Phase Transitions:
cos(2pfct)
cos(2pfct)
ECE 4710: Lecture #20

20. AM vs. DSB-SC ModulationType Advantages Disadvantages AM (DSB-LC)
1. Inefficient use of power
2. Poor performance for
low S/N
3. High power & expensive
Tx for good Rx
4. Can’t use for most data
signals AM
(DSB-LC)
1. Envelope Detection
2. Simple & Cheap Rx’s
1. Good performance
at low S/N
2. Can use for all data
signals
3. Power efficient
1. Synchronization of f
and q for coherent
detection
2. Complicated and
expensive Rx’s
DSB-SC
ECE 4710: Lecture #20