1. Analog Communications (DSB-SC)
Dr. M. Venu Gopala Rao
A.M.I.E.T.E, M.Tech, Ph.D(Engg)
Cert. in R.S.T ( City & Guild’s London Institute, London)
F.I.E.T.E, L.M.I.S.T.E, I.S.O.I., S.S.I., M.I.A.E.
Professor, Dept. of ECE, K L University

2. Double Side Band Suppressive Carrier DSB-SC

3. DSB-SC Introduction Single Tone, Multi tone and Baseband signals
Time domain description
Frequency domain description
Spectrum
2. Modulators
Multiplier Modulator
Balanced Modulator
Switching Modulator
Ring Modulator
3. Demodulators
Coherent Detection.
(Phase Error and Frequency Error)
4. Main |Points of DSB-SC

4. Block Schematic Diagram of DSB-SC

5. Single Tone DSB-SC Modulation
Time Domain Description: Let a message signal Then DSB-SC signal as a function of time is represented by Frequency Domain Description:

6. Single Tone DSB-SC Modulation and its Spectrum
s(t) undergoes a phase reversal whenever m(t) crosses zero

7. Multi Tone DSB-SC Modulation

8. Baseband Signal DSB-SC Modulation
Let the message signal is assumed to be band limited to ‘W’ Hz.
Time Domain Description
Then the standard form of a DSB-SC modulated signal as a function of time is represented by
Frequency Domain Description

9. Baseband DSB-SC Spectrumz
Transmission Bandwidth of DSB-SC is 2 W Hz

10. DSB-SC Generation (DSB-SC Modulators)

11. 1. Multiplier Modulator

12. 1. Multiplier Modulator
Here the modulating is directly achieved by multiplying with , using an analog multiplier whose output is proportional to the product of two input signals.
Typically, such a multiplier may be obtained from a variable gain amplifier in which the gain parameter (such as of a transistor) is controlled by one of the signals, say
When the carrier signal is applied at the input of this amplifier, the output is proportional to .

13. 2. Balanced Modulator + -

14. 3. Switching Modulators
BPF is designed with Centered

15. 3. Switching Modulators

16. 4. Ring Modulator (Double Balanced Modulator
During +ve cycle D1 and D3 conducts and ‘a’ is connected to ‘c’ and ‘b’ is connected to ‘d’, m(t) is multiplied with carrier
During -ve cycle D2 and D4 conducts and ‘a’ is connected to ‘d’ and ‘b’ is connected to ‘c’ , -m(t) is multiplied with carrier

18. 4. Ring Modulator

19. DSB-SC Demodulation

21. Coherent/Synchronous Detection
The Local oscillator frequency and phase is assumed to be synchronized with transmitted carrier

22. Any discrepancy in the frequency and phase of local carrier give rise to a distortion in the detector output.
Consider the following two situations.
The local oscillator has an ideal frequency, but arbitrary phase difference measured with respect to the carrier is referred to as ‘Phase Error’.
The local oscillator has identical phase but a difference frequency with respect to carrier is referred to as ‘Frequency error’

23. Phase Error
Let the carrier is where being the phase difference between the local oscillator signal and the carrier at the transmitter.

24. Phase Error The demodulated output is maximum when
Thus the demodulator output is proportional to
and undistorted when the phase error is constant
The demodulated output is maximum when
Minimum (zero) when This results in distortion of the demodulated output .
Necessary arrangement should be made at the receiver end to maintain the local oscillator in perfect synchronism, in both frequency and phase with the transmitted carrier wave.

25. Frequency Error
Suppose that the local oscillator signal where is frequency error. The resulting signal will be un-acceptable if is comparable to the baseband signal frequency

26. Conclusions
Necessary arrangement should be made at the receiver end to maintain the local oscillator in perfect synchronism, in both frequency and phase with the transmitted carrier wave.

27. Main Points of DSB-SC
The Carrier power is eliminated, so that efficiency increased in compare with standard AM.
But the Transmission Bandwidth is same as standard AM, that is two times of message signal bandwidth.
The phase of DSB-SC modulated signal is phase reversed when the modulating signal m(t) crosses zero .
Coherent detection is needed for DSB-SC: Obtaining the carrier phase is biggest challenges in all demodulators.

28. Hilbert Transform

29. Hilbert Transform

30. Hilbert Transform (Phase Shifter)

31. Frequency Response

32. Inverse Hilbert Transform:

34. End