---
title: "Short note: Superheterodyne receiver"
subject: "Communication Systems"
chapter: 2
key: "semester_6/communication_systems:short-note-superheterodyne"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-superheterodyne
site: BCT Engineering
---

# Short note: Superheterodyne receiver

Communication Systems · Chapter 2: Amplitude Modulation

- Asked in: 2076 Baisakh (CS I), 4 marks; 2074 Bhadra (CS I), 4 marks; 2070 Magh (CS I), 5 marks (asked 3 times)
- Video: [Super Heterodyne Receiver (Basics, Working, Block Diagram, Image Frequency, Pros & Cons) Explained](https://www.youtube.com/watch?v=CaQtJRheqB4) — Engineering Funda
- 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-superheterodyne
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2

## Question

Write a short note on superheterodyne receiver.

## Short answer

A superheterodyne receiver converts all incoming station frequencies to a fixed intermediate frequency (455 kHz for AM, 10.7 MHz for FM) using a mixer and local oscillator. Fixed-tuned IF amplifiers provide most of the gain and selectivity. Its main disadvantage is image frequency interference at f_s + 2f_IF.

## Answer

A **superheterodyne receiver** converts every received station frequency to a fixed **intermediate frequency (IF)** (455 kHz for AM broadcast, 10.7 MHz for FM) before the main amplification and detection. It is used in almost all radio and TV receivers.

```text
Ant->[RF amp]->(Mixer)->[IF amp]->[Detector]->[AF amp]->Spkr
       ^          ^        455k        |
       |        [LO]                  AGC
       +--gang tuning--+   <-----------+
```

**Working:**
1. **RF stage:** tunes to the wanted station $f_s$, amplifies it and rejects the image frequency.
2. **Mixer and local oscillator:** the LO is ganged with RF tuning so that $f_{LO}=f_s+f_{IF}$. The mixer gives $f_{LO}-f_s=f_{IF}$ for every station.
3. **IF amplifier:** fixed-tuned, high-gain stages with steep skirts provide most of the **gain and selectivity**.
4. **Detector:** an envelope detector recovers audio; it also gives a DC level for **AGC**, which controls RF/IF gain to keep output steady.
5. **AF amplifier and loudspeaker** reproduce the sound.

**Advantages:** uniform gain and selectivity over the whole band, good adjacent-channel rejection, high sensitivity, stable fixed-frequency amplifiers.
**Disadvantages:** **image frequency** $f_{si}=f_s+2f_{IF}$ interference (e.g. station at 1000 kHz has image at 1910 kHz), extra mixer noise and more complex circuitry.

## Check yourself

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

1. What is the main idea of the superheterodyne receiver?
   A. Amplify each station at its own carrier frequency
   B. Convert every station to a fixed intermediate frequency before most amplification
   C. Remove the need for a detector
   D. Use a different local oscillator frequency equal to the station
   Answer: B. The mixer shifts every station to a fixed IF, where fixed-tuned stages give uniform gain and selectivity. (Key point: Purpose of IF)

2. If fs = 1000 kHz and fIF = 455 kHz, what is the local oscillator frequency and the image frequency?
   A. 545 kHz and 1455 kHz
   B. 1455 kHz and 1910 kHz
   C. 1455 kHz and 1455 kHz
   D. 1910 kHz and 1455 kHz
   Answer: B. fLO = fs + fIF = 1455 kHz; image fsi = fs + 2 fIF = 1910 kHz. (Key point: LO frequency)

3. What is the role of AGC in the receiver?
   A. Generate the local oscillator
   B. Use the detector DC level to control RF/IF gain so output stays steady
   C. Filter the audio signal
   D. Tune the receiver automatically
   Answer: B. The detector gives a DC level proportional to carrier strength, which reduces gain for strong stations. (Key point: AGC)

## Related in Communication Systems

- [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×
- [FM superheterodyne radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afm-superheterodyne-receiver) — Communication Systems, chapter 3, asked 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

## 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