The document discusses Double Sideband Suppressed Carrier (DSB-SC) modulation. DSB-SC modulation involves suppressing the carrier wave before transmission, which increases power efficiency by decreasing transmission power requirements and doubles the bandwidth requirement compared to the message signal bandwidth. DSB-SC signals cannot be detected using envelope detection. There are several ways to generate DSB-SC signals including using a multiplier modulator, nonlinear modulator, switching modulator like a diode bridge modulator or ring modulator. The switching modulators replace multiplication with a simpler switching operation to modulate the message signal onto the carrier wave.

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

2. 2
DSB-SC: Principle (1)
    ttmt cSCDSB  cos
 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:
          ttmtmttmttte cccSCDSB  2cos
2
1
2
1
coscos 2
 
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 ωc
Thus,

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

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 D1 and D2 (also D3 and D4) 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 fc 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)