---
title: "Short note: Threshold effect in FM"
subject: "Communication Systems"
chapter: 9
key: "semester_6/communication_systems:short-note-threshold-effect-fm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-threshold-effect-fm
site: BCT Engineering
---

# Short note: Threshold effect in FM

Communication Systems · Chapter 9: Noise in Communication Systems

- Asked in: 2071 Magh (old course), 4 marks; 2068 Jestha (old course), 5 marks; 2080 Bhadra (CS II), 4 marks; 2075 Chaitra (CS II), 5 marks (asked 4 times)
- Video: [Threshold effect in FM and AM](https://www.youtube.com/watch?v=7NO-4Qi0cdM) — GATE ACHIEVERS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-threshold-effect-fm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-9

## Question

Write a short note on threshold effect in demodulation of FM.

## Short answer

The FM threshold effect is a sharp drop in output SNR when the input carrier-to-noise ratio falls below about 10 dB. Below threshold, noise occasionally overpowers the carrier, causing click noise; above threshold, output SNR improves greatly, proportional to the modulation index squared.

## Answer

The **threshold effect** in FM is the sharp drop in output signal-to-noise ratio of an FM receiver when the input carrier-to-noise ratio (CNR) falls below a certain value, called the **threshold**, typically about **10 dB** (about 13 dB in some texts).

#### Above threshold

When the carrier is much stronger than the noise, noise only causes small phase perturbations. The output SNR grows linearly with input CNR and FM gives a large improvement over AM:

$$
(SNR)_o=\frac{3}{2}\beta^2\,(SNR)_c\quad(\text{tone modulation})
$$

#### Below threshold

```text
 (SNR)o dB
   |              /  FM above threshold
   |             /
   |            /
   |     ------/  <- knee (threshold, ~10 dB CNR)
   |    /
   |   /   sharp fall below threshold
   +---------------------------> (SNR)c dB
```

When noise amplitude occasionally exceeds the carrier amplitude, the resultant phasor can **encircle the origin**, causing a sudden $2\pi$ phase jump. The discriminator output (derivative of phase) then shows sharp impulses, heard as **clicks**. As CNR falls, clicks become frequent, the noise rises rapidly, and the signal is "captured" by noise. The output SNR then falls much faster than the input CNR and the FM advantage is lost.

#### Consequences and remedies

- Large $\beta$ improves SNR above threshold but needs more bandwidth, so more noise enters and the threshold rises; this sets a practical limit on $\beta$.
- **Threshold extension** (lowering the threshold by a few dB) uses an FM demodulator with feedback (FMFB) or a **phase-locked loop** demodulator, which track the signal with a narrower effective bandwidth.
- Pre-emphasis/de-emphasis improves SNR above threshold but does not remove the threshold.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the threshold effect in FM?
   A. A rise in output SNR when bandwidth is reduced
   B. The maximum frequency deviation
   C. Sharp drop in output SNR when input CNR falls below about 10 dB
   D. Loss of carrier due to over-modulation
   Answer: C. Below the CNR threshold, output SNR falls much faster than input CNR. (Key point: Threshold effect definition)

2. What causes the clicks below threshold?
   A. Carrier frequency drift
   B. The noise phasor encircles the origin, producing 2-pi phase jumps and impulses in the discriminator
   C. Pre-emphasis boosting highs
   D. Limiter overload by the message
   Answer: B. A 2-pi phase jump differentiates into a spike heard as a click. (Key point: Cause of clicks)

3. Above threshold, for tone modulation, what is (SNR)o?
   A. (3/2) beta^2 (SNR)c
   B. beta (SNR)c
   C. (SNR)c / beta^2
   D. 2 (SNR)c
   Answer: A. Output SNR is (3/2) beta^2 times the carrier SNR, giving FM its improvement. (Key point: Above-threshold SNR)

4. Which techniques lower the threshold?
   A. Increasing beta
   B. Pre-emphasis alone
   C. Using a wider IF filter
   D. FM feedback (FMFB) and PLL demodulators
   Answer: D. FMFB and PLL demodulators track the signal with narrower effective bandwidth; pre-emphasis does not remove the threshold. (Key point: Threshold extension)

## Related in Communication Systems

- [Binary error probability; threshold effect in FM and mitigation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aerror-prob-threshold-effect) — Communication Systems, chapter 9, asked 2×
- [Threshold effect in FM; error probability of coherent PSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Athreshold-effect-psk-error) — Communication Systems, chapter 9
- [Threshold effect in non-coherent FM detection and its correction](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Athreshold-noncoherent-fm-correction) — Communication Systems, chapter 9
- [Figure of merit of non-coherent FM; threshold effect](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afom-noncoherent-fm-threshold) — Communication Systems, chapter 9
- [FM demodulation using PLL](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afm-demod-pll) — Communication Systems, chapter 3, asked 2×
- [PLL lock and capture range; FM demodulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apll-lock-capture-fm-demod) — Communication Systems, chapter 3