Question

A DSB-SC signal can be demodulated using

• A. Low pass filter
• B. Synchronous detector
• C. Phase discriminator
• D. Envelop detector

Hint:

Standard AM (with carrier): Envelope detector (simple).
DSB-SC (suppressed carrier): Synchronous detector (requires carrier recovery).
SSB-SC (suppressed carrier): Synchronous detector.
FM/PM: Frequency/Phase discriminator, PLL detector.

Answer 1

The Correct Option is B

DSB-SC stands for Double Sideband Suppressed Carrier. In this modulation scheme, the carrier component is suppressed from the modulated signal, which only contains the two sidebands.
(a) Low pass filter: A low pass filter is a component used in demodulation (e.g., to extract the message signal after multiplication with a carrier), but it is not a complete demodulation method by itself for DSB-SC.
(b) Synchronous detector (Coherent detector): This is the standard method for demodulating DSB-SC signals. It involves multiplying the received DSB-SC signal with a locally generated carrier signal that is perfectly synchronized in phase and frequency with the original carrier used at the transmitter. The product is then passed through a low-pass filter to recover the message signal.
(c) Phase discriminator: This is used for demodulating frequency modulated (FM) or phase modulated (PM) signals.
(d) Envelope detector: This is used for demodulating standard AM signals (AM with full carrier), where the carrier component is present and the envelope of the modulated signal follows the message signal (provided \(m \le 1\)). It cannot be used for DSB-SC because the envelope of a DSB-SC signal does not directly represent the message signal (due to phase reversals when the message signal crosses zero). Therefore, a DSB-SC signal is demodulated using a synchronous detector. \[ \boxed{\text{Synchronous detector}} \]