Question

The threshold effect in demodulators is
1. The rapid fall of output SNR when the input SNR falls below a particular value
2. Exhibited by all the demodulators when the input SNR is low
3. Exhibited by all AM suppressed carrier coherent demodulators
4. Exhibited by correlation receivers

Answer 1

Correct Answer - Option 1 : The rapid fall of output SNR when the input SNR falls below a particular value

• When the value of the input signal to noise ratio falls below a particular value (called Threshold value), a rapid fall of the output signal to noise ratio is observed by the receiver. This is known as the Threshold effect in demodulation.
• The threshold effect occurs due to the presence of large noise in the modulated signal.

 

Important Point:

• Due to the threshold effect, the detection of the message signal becomes difficult.
• The threshold effect can be improved by applying feedback in demodulator circuits such as phase-locked loops and frequency locked loops. So that the output SNR value does not fall rapidly.

 

The relation between input SNR and output SNR is

(SNR)_Output = 3β².(SNR)_Input

Where, β = modulation index in FM.