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

# Short note: AM radio receiver

Communication Systems · Chapter 2: Amplitude Modulation

- Asked in: 2072 Magh (CS I), 4 marks
- Video: [Superhetrodyne Receiver - Modern Electronic Communications - Application of Electronics](https://www.youtube.com/watch?v=rJ1MWlh9_ao) — Ekeeda
- 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-am-radio-receiver
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2

## Question

Write a short note on AM radio receiver.

## Short answer

The superheterodyne receiver converts the RF station (535–1605 kHz) to a fixed 455 kHz intermediate frequency. It consists of an RF amplifier, mixer/local oscillator, multi‑stage IF amplifier for selectivity, a diode envelope detector, AGC to keep output constant, and an audio amplifier.

## Answer

The standard **AM radio receiver** is the **superheterodyne receiver**. It converts every received station frequency to a fixed **intermediate frequency (IF)** of 455 kHz, where most of the gain and selectivity are provided.

```text
 Antenna
   |
 [RF amp]->[Mixer]->[IF amp]->[Envelope]->[Audio]->Spkr
   ^          ^      455 kHz   [detector]  [ amp ]
   |          |                    |
   +-gang-[Local osc]          AGC -> RF/IF amps
         fLO = fs + 455 kHz
```

**Stages**

1. **RF amplifier:** tuned to the station $f_s$ (535–1605 kHz); improves sensitivity and rejects the image frequency.
2. **Mixer + local oscillator:** produces $f_{LO} - f_s = 455$ kHz. LO and RF tuning are ganged (tracking).
3. **IF amplifier:** fixed-tuned, high-gain stages with sharp filters; give most of the selectivity (adjacent-channel rejection).
4. **Detector:** diode envelope detector recovers the audio.
5. **AGC:** DC part of detector output controls the gain of RF/IF stages, keeping output level constant for strong and weak stations.
6. **Audio amplifier** drives the loudspeaker.

**Image frequency:** $f_{si} = f_s + 2f_{IF}$, rejected mainly by the RF stage. Image rejection ratio $\alpha = \sqrt{1 + Q^2\rho^2}$, $\rho = \frac{f_{si}}{f_s} - \frac{f_s}{f_{si}}$.

**Advantages:** uniform selectivity and gain across the band, high sensitivity, stable operation.

## Check yourself

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

1. The standard AM radio receiver is a:
   A. TRF receiver with no mixer
   B. Superheterodyne receiver converting all stations to a fixed 455 kHz IF
   C. Crystal receiver
   D. Direct conversion receiver at 0 Hz
   Answer: B. The superhet mixes each station with a local oscillator to the fixed IF of 455 kHz. (Key point: Receiver type and IF)

2. For a station at fs, the local oscillator frequency is:
   A. fs - 455 kHz
   B. fs
   C. fs + 455 kHz
   D. 2 fs
   Answer: C. fLO - fs = 455 kHz, so fLO = fs + 455 kHz. (Key point: Local oscillator frequency)

3. The IF amplifier mainly provides:
   A. Image rejection only
   B. Local oscillation
   C. Detection of audio
   D. Most of the gain and adjacent-channel selectivity
   Answer: D. Being fixed-tuned with sharp filters, the IF stage gives most of the gain and selectivity. (Key point: IF amplifier role)

4. The image frequency is:
   A. fs + 2 fIF, mainly rejected by the RF stage
   B. fs - fIF
   C. fs + fIF
   D. 2 fs, rejected by the IF stage
   Answer: A. fsi = fs + 2 fIF also mixes to the IF, so it must be rejected before the mixer by the RF stage. (Key point: Image frequency)

## 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
- [Short note: FM radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fm-radio-receiver) — Communication Systems, chapter 3
- [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
- [Phase error in synchronous DSB-SC detection; synchronized receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aphase-error-dsbsc-costas) — Communication Systems, chapter 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