---
title: "AM vs FM superheterodyne receiver; AGC"
subject: "Communication Systems"
chapter: 3
key: "semester_6/communication_systems:am-fm-superhet-agc"
source: https://www.bctengineer.com/q/semester_6/communication_systems/am-fm-superhet-agc
site: BCT Engineering
---

# AM vs FM superheterodyne receiver; AGC

Communication Systems · Chapter 3: Angle Modulation

- Asked in: 2072 Magh (CS I), 3+2 marks
- Video: [EC 208 ACE -Automatic Gain Control (AGC)- Dr. Deepa Elizabeth George](https://www.youtube.com/watch?v=Ti-mBq9P_Kg) — Toc H Kochi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/am-fm-superhet-agc
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3

## Question

List the major differences between AM and FM superheterodyne receiver. Why are superheterodyne receivers provided with automatic gain control (AGC) mechanism?

## Short answer

AM receivers operate MW band, IF 455 kHz, envelope detector, no limiter; FM receivers operate VHF 88-108 MHz, IF 10.7 MHz, use limiter and FM detector (discriminator/ratio detector), with de-emphasis. AGC adjusts RF/IF gain using a DC voltage from the detector to prevent overload from strong signals, compensate fading, and maintain constant output volume.

## Answer

#### Differences between AM and FM superheterodyne receivers

| Point | AM receiver | FM receiver |
|---|---|---|
| RF band | MW/SW, 0.54–1.65 MHz (MW) | VHF, 88–108 MHz |
| Intermediate frequency | 455 kHz | 10.7 MHz |
| IF bandwidth | about 10 kHz | about 200 kHz |
| Limiter | Not used | Used before detector to remove amplitude noise |
| Detector | Envelope (diode) detector | Discriminator, ratio detector or PLL |
| De-emphasis | Not used | De-emphasis network (75 μs) after detector |
| Gain control | AGC | AFC common; limiter does most amplitude control |
| Noise immunity | Poor | Good (capture effect, limiter) |

#### Why AGC is used

**Automatic gain control (AGC)** automatically adjusts the gain of the RF and IF amplifiers according to the strength of the received signal, using a DC voltage taken from the detector output.

It is needed because:

1. Signals from near and far stations differ widely in strength (microvolts to hundreds of millivolts). Without AGC, the volume changes when tuning between stations.
2. **Fading** makes the strength of one station vary with time; AGC keeps the output level nearly constant.
3. Strong signals would **overload** the RF/IF stages and the detector and cause distortion; AGC reduces gain for strong signals.
4. Weak signals get full gain, which improves sensitivity.

## Check yourself

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

1. What are the standard IFs?
   A. AM 455 kHz, FM 10.7 MHz
   B. AM 10.7 MHz, FM 455 kHz
   C. Both 455 kHz
   D. AM 38 kHz, FM 19 kHz
   Answer: A. FM needs wider IF and higher IF. (Key point: IF values)

2. What does the FM limiter do?
   A. Detects the envelope
   B. Removes amplitude variations/noise before the detector
   C. Applies pre-emphasis
   D. Selects the pilot
   Answer: B. FM carries info in frequency. (Key point: Limiter)

3. Why is AGC used?
   A. To increase selectivity
   B. To tune the local oscillator
   C. To keep output level nearly constant despite signal strength differences and fading
   D. To remove image frequency
   Answer: C. It adjusts RF/IF gain from detector DC and avoids overload. (Key point: AGC purpose)

## 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×
- [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×
- [Short note: Superheterodyne receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-superheterodyne) — Communication Systems, chapter 2, asked 3×
- [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 efficiency and complexity; coherent ASK error](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-fm-bw-efficiency-ask-error) — Communication Systems, chapter 9, asked 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

## 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
- [AM versus FM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-vs-fm-compare) — Data Communication, chapter 5, asked 2×
- [Modulation; AM and FM modulated waves](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amodulation-am-fm-wave) — Basic Electronics Engineering, chapter 5
- [Horizontal control specification for major traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-horizontal-control-specification) — Surveying II, chapter 1