---
title: "Short note: FM radio receiver"
subject: "Communication Systems"
chapter: 3
key: "semester_6/communication_systems:short-note-fm-radio-receiver"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fm-radio-receiver
site: BCT Engineering
---

# Short note: FM radio receiver

Communication Systems · Chapter 3: Angle Modulation

- Asked in: 2072 Asoj (CS I), 4 marks
- Video: [FM radio receiver in block diagrams](https://www.youtube.com/watch?v=Sgi_XLvYN28) — Shubham Rai
- 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-fm-radio-receiver
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3

## Question

Write a short note on FM radio receiver.

## Short answer

An FM receiver is a superheterodyne with RF 88-108 MHz, IF 10.7 MHz, bandwidth ~200 kHz. It uses a limiter to strip amplitude noise, a discriminator/ratio detector for FM demodulation, and a de-emphasis network (75 μs) to restore flat audio after pre-emphasis. AFC stabilizes the local oscillator.

## Answer

An FM broadcast receiver is a **superheterodyne receiver** working in the 88–108 MHz band with an **IF of 10.7 MHz**. It differs from an AM receiver mainly in the limiter, the FM detector and the de-emphasis network.

```text
Ant
 |  +----+  +-----+  +-------+  +---------+  +-------+
 +->| RF |->|Mixer|->|  IF   |->| Limiter |->|  FM   |
    |amp |  |     |  |10.7MHz|  |         |  | det.  |
    +----+  +-----+  +-------+  +---------+  +-------+
              ^                                  |
         +---------+    +-------+  +-------+     v
         | Local   |<---|  AFC  |  | Audio |<-[De-emph]
         |  osc.   |    +-------+  |  amp  |-> Speaker
         +---------+               +-------+
```

**Blocks:**

- **RF amplifier:** selects and amplifies the weak VHF signal; improves image rejection and noise figure.
- **Mixer and local oscillator:** convert every station to the fixed IF of 10.7 MHz ($f_{LO} = f_s + 10.7$ MHz).
- **IF amplifier:** provides most of the gain and selectivity, with a bandwidth of about 200 kHz.
- **Limiter:** clips the IF signal to a constant amplitude, removing amplitude noise and interference.
- **FM detector:** discriminator, ratio detector or PLL converts frequency variations into audio voltage.
- **De-emphasis (75 μs RC network):** attenuates high audio frequencies that were boosted by pre-emphasis at the transmitter, which reduces high-frequency noise.
- **AFC:** a DC voltage from the detector corrects drift of the local oscillator.
- **Audio amplifier and speaker:** reproduce the sound. In stereo receivers a stereo decoder is placed after the detector.

## Check yourself

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

1. What intermediate frequency does a standard FM broadcast superheterodyne receiver use?
   A. 455 kHz
   B. 10.7 MHz
   C. 88 MHz
   D. 200 kHz
   Answer: B. The FM broadcast receiver converts every station in the 88-108 MHz band to a fixed IF of 10.7 MHz (455 kHz is the AM IF). (Key point: IF of FM receiver)

2. What is the purpose of the limiter in an FM receiver?
   A. To increase the receiver gain
   B. To select the wanted station
   C. To clip the IF signal to constant amplitude and remove amplitude noise
   D. To correct local oscillator drift
   Answer: C. The limiter holds the IF amplitude constant, so amplitude noise and interference are removed and only frequency variations reach the detector. (Key point: Role of the limiter)

3. Why does an FM receiver contain a 75 μs de-emphasis network?
   A. To restore high audio frequencies that were boosted by pre-emphasis and reduce high-frequency noise
   B. To boost the high audio frequencies further
   C. To remove the 10.7 MHz IF
   D. To generate the local oscillator voltage
   Answer: A. De-emphasis attenuates the high audio frequencies boosted at the transmitter, undoing pre-emphasis and cutting high-frequency noise. (Key point: De-emphasis)

4. For a station at fs, what is the local oscillator frequency, and what does AFC do?
   A. fLO = fs - 10.7 MHz; AFC amplifies the audio
   B. fLO = fs + 10.7 MHz; AFC uses a DC voltage from the detector to correct oscillator drift
   C. fLO = fs; AFC removes the image frequency
   D. fLO = 10.7 MHz; AFC selects the station
   Answer: B. The LO is placed 10.7 MHz above the signal, and AFC feeds a detector DC voltage back to correct LO drift. (Key point: Local oscillator and AFC)

## Related in Communication Systems

- [FM superheterodyne radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afm-superheterodyne-receiver) — Communication Systems, chapter 3, asked 2×
- [Short note: AM radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-am-radio-receiver) — Communication Systems, chapter 2
- [Requirements of a good radio receiver; superheterodyne](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Agood-receiver-superheterodyne) — Communication Systems, chapter 2
- [AM vs FM superheterodyne receiver; AGC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-fm-superhet-agc) — Communication Systems, chapter 3
- [Stereo FM transmitter and receiver; output of mono receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-mono-receiver) — Communication Systems, chapter 3
- [Superheterodyne AM receiver; why standard AM in broadcasting](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Asuperhet-am-receiver-standard-am) — Communication Systems, chapter 2, asked 2×

## Related in other subjects

- [AM superheterodyne radio receiver diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aam-superheterodyne-receiver) — Basic Electronics Engineering, chapter 5