The threshold effect in a demodulator is

Concept

A demodulator is judged by how its output signal-to-noise ratio (SNR) responds to the SNR present at its input. A detector that acts linearly on the received waveform keeps the two proportional at every input level. A detector that acts non-linearly — an envelope detector, or an FM frequency discriminator — only behaves that way while the carrier stays large compared with the noise: once the noise envelope becomes comparable to the carrier, the noise captures the detector and suppresses the message, so the output SNR collapses far faster than the input SNR is falling. The input SNR at which that collapse sets in is called the threshold, and the abrupt degradation below it is the threshold effect.

Application

1. Above threshold, the output SNR of a non-linear detector varies in step with the input SNR — a decibel of extra input SNR buys about a decibel at the output (for wideband FM it buys more, through the FM improvement factor).

2. At the threshold the proportional relation breaks down. The commonly quoted break points are an input SNR of roughly 10 dB for an envelope detector and about 10–12 dB for an FM discriminator.

3. Below threshold the curve turns sharply downward: a further drop of a few decibels at the input costs many decibels at the output, because noise-driven clicks and envelope reversals now dominate the detected waveform.

4. So the effect is defined by the shape of that curve — the rapid fall in output SNR once the input SNR drops below a particular value. That is the description the stem is asking for.

Cross-check

The effect is a behaviour of the SNR transfer curve, not a label for a family of receivers, so any statement that identifies it with a class of hardware misses what is being defined:

• Coherent (synchronous or product) detection, including A.M. suppressed-carrier coherent detection, multiplies the incoming signal by a locally regenerated carrier and low-pass filters the result. That is a linear operation, so the output SNR stays proportional to the input SNR right down to very small input SNRs — no threshold appears.

• A correlation receiver is the matched-filter-equivalent structure used for digital detection; its output SNR at the sampling instant is fixed by the symbol-energy-to-noise-density ratio, again with no abrupt knee.

• And because coherent detection shows no such knee, the behaviour cannot be attributed to every demodulator at low input SNR.

Hence the threshold effect is the rapid fall in output signal-to-noise ratio that occurs once the input signal-to-noise ratio falls below a particular value.