This document discusses a ring modulator method for generating a double sideband suppressed carrier (DSB-SC) signal using amplitude modulation. It has advantages like a stable output and not requiring external power. The operation involves using diodes in a ring configuration to selectively pass or block the carrier signal depending on the polarity of the modulating signal. Coherent detection can then be used to recover the message signal from the DSB-SC by synchronizing the local carrier signal.

1. EC8491-Communication Theory
Dr.Arun Chokkalingam
Professor
Generation of DSB-SC using
Diode Ring Modulator or chopper
Modulator
Amplitude Modulation

2. Ring Modulator Method
▪ Advantages of Ring Modulator
▪ Stable output
▪ It does not require external power to activate the diodes
▪ Longer life
▪ Low maintenance

4. Operation
▪ Mode-1 (Carrier Suppression)
▪ Modulating or message signal=0
▪ Positive half cycle of the carrier:
▪ Diode D1 and D2 –forward biased
▪ Diode D3 and D4 – Reverse biased
▪ The current through transformerT2 they are equal and opposite and cancel each other .
▪ So the carrier is suppressed in positive half cycle
▪ Negative half cycle of the carrier
▪ Diode D1 and D2 –Reverse biased
▪ Diode D3 and D4 – forward biased
▪ The current through transformerT2 they are equal and opposite and cancel each other .
▪ So the carrier is suppressed in positive half cycle

5. Cont..
▪ Mode-2 ( both carrier and Message signal present)
▪ Positive half-cycle of Modulating Signal with carrier
▪ In the positive half-cycle of the carrier, D1 and D2 are ON and secondary ofT1 is applied as it is across
the primary ofT2. Hence, during the positive half cycle of carrier, the output ofT2 is positive
▪ In the negative half-cycle of the carrier, the diodes D3 and D4 are turned ON and the secondary
of T1 is applied in a reversed manner across the primary ofT2 .Hence the primary voltage ofT2 is
negative and output voltage also becomes negative .
▪ Negative half-cycle of Modulating Signal with carrier
▪ When modulating signal reverses the polarities, the operation of the circuit is same as that in the
positive half-cycle discussed earlier .
▪ Now, the only difference is that the diode pair D3 D4 will produce a positive output voltage
whereas D1 D2 will produce a negative output voltage as shown in the waveforms of fig.5 .

7. Coherent or Synchronous Detection (For detection of message
signal from DSBSC/SSBSC)
If both message and local carrier synchronized 100% - output of demodulator maximum
If both message and local carrier synchronized minimum - output of demodulator is minimum