Question

Which of the following schemes suffer(s) from the threshold effect?

• A. AM detection using envelope detection
• B. AM detection using synchronous detection
• C. FM detection using discriminator
• D. SSB detection using synchronous detection

Hint:

The threshold effect is specific to FM detection using discriminators. Below threshold SNR, output becomes unacceptably noisy.

Answer 1

The Correct Option is C

The threshold effect is a phenomenon in analog modulation systems, especially in FM systems, where noise performance degrades rapidly below a certain signal-to-noise ratio (SNR). Here's the breakdown:
- In FM with discriminators (used for demodulating FM signals), when the carrier-to-noise ratio (CNR) falls below a threshold (typically 10 dB), the output SNR falls off drastically.
- This is called the threshold effect, and it severely limits the performance of the receiver in low SNR environments.
- AM envelope detection does not exhibit this sharp drop in performance. It degrades more linearly with noise.
- Synchronous detection is more robust and can demodulate even in low SNR conditions.
- Therefore, only FM detection using discriminators suffers from threshold effect.