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 mvgr03@kluniversity.in

Double Side Band Suppressive Carrier DSB-SC

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

Block Schematic Diagram of DSB-SC

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:

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

Multi Tone DSB-SC Modulation

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

Baseband DSB-SC Spectrum z Transmission Bandwidth of DSB-SC is 2 W Hz

DSB-SC Generation (DSB-SC Modulators)

1. Multiplier Modulator

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 .

2. Balanced Modulator + -

3. Switching Modulators BPF is designed with Centered

3. Switching Modulators

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

4. Ring Modulator

DSB-SC Demodulation

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

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’

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

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.

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

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.

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.

Hilbert Transform

Hilbert Transform

Hilbert Transform (Phase Shifter)

Frequency Response

Inverse Hilbert Transform:

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