Double SideBand Suppressed Carrier (DSB-SC)

Enjoy

1. Communication Theory Double-sideband Suppressed-Carrier (DSB-SC)

2. 2 DSB-SC: Principle (1) The carrier is suppressed before transmission (not transmitted) Power efficiency increases: Transmit power requirement decreases Bandwidth requirement: Twice of the bandwidth of the message signal Envelope detection can’t be used DSB-SC signal: DSB-SC signal generation:

3. 3 DSB-SC: Principle (2) DSB-SC Demodulation: After LPF: Synchronous/ Coherent / Homodyne Demodulator (Detector)

4. 4 DSB-SC: Modulators (1) 1. Multiplier Modulator: 2. Non-linear Modulator: Characteristics of non-linear element: The carrier signal does not appear at the input of the final BPF The bridge acts as a balanced bridge for the carrier only As the modulator is balanced with respect to one input, it is called single balanced modulator

5. 5 DSB-SC: Modulators (2) 3. Switching Modulators: The multiplication operation is replaced by a simpler switching operation Multiplication can be achieved multiplying m(t) by any periodic signal φ (t) of the fundamental frequency ω c Thus,

6. 6 DSB-SC: Modulators (3) 3. Switching Modulators ( contd …) w ( t ) as φ ( t ): f c ≥ 2 B

7. 7 DSB-SC: Modulators (4) 3. Switching Modulators ( contd …) A. Diode-bridge Modulator: Series-bridge diode modulator Shunt-bridge diode modulator When terminal ‘c’ is + ve with respect to terminal ‘d’, all four diodes conduct As D 1 and D 2 (also D 3 and D 4 ) are matched, terminal ‘a’ and terminal ‘b’ are short circuited When terminal ‘d’ is + ve with respect to ‘c’, all four diodes are open Thus, switching of m ( t ) at a frequency of f c is achieved It is a single balanced modulator Diode-bridge works as a switch

8. 8 DSB-SC: Modulators (5) 3. Switching Modulators ( contd …) B. Ring Modulator: Positive Half-cycle: D1 and D3 conduct Negative Half-cycle: D2 and D4 conduct (A double-balanced modulator)