---
title: "Superheterodyne AM receiver; why standard AM in broadcasting"
subject: "Communication Systems"
chapter: 2
key: "semester_6/communication_systems:superhet-am-receiver-standard-am"
source: https://www.bctengineer.com/q/semester_6/communication_systems/superhet-am-receiver-standard-am
site: BCT Engineering
---

# Superheterodyne AM receiver; why standard AM in broadcasting

Communication Systems · Chapter 2: Amplitude Modulation

- Asked in: 2065 Kartik (CS I), 6+2 marks; 2064 Shrawan (CS I), 6+2 marks (asked 2 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/superhet-am-receiver-standard-am
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2

## Question

Explain the working principle of the superheterodyne AM receiver with the help of block diagram. Why standard AM is used in AM radio broadcasting?

## Short answer

Superheterodyne receiver converts incoming RF to a fixed intermediate frequency (455 kHz) for high gain and selectivity. It uses RF amplifier, mixer, local oscillator, IF amplifier, envelope detector, AGC, and audio amplifier. Standard AM is used because it allows simple envelope detection and the carrier aids AGC.

## Answer

A **superheterodyne receiver** converts the incoming RF signal of any station to a fixed **intermediate frequency (IF)**, 455 kHz for AM broadcast, where most of the gain and selectivity are obtained.

#### Block diagram

```text
 Antenna
   |
 [RF amplifier]--->( Mixer )--->[ IF amplifier ]
   ^  fs              ^             455 kHz
   |                  | fLO             |
   +--- ganged ---[Local osc]      [ Detector ]
        tuning                          |
                    AGC <---------------+
                                        |
                  Speaker <--[ AF amplifier ]
```

#### Working principle
1. **Antenna and RF amplifier:** the tuned RF stage selects the desired station frequency $f_s$ (535–1605 kHz), amplifies it, improves SNR and rejects the **image frequency**.
2. **Local oscillator and mixer (frequency changer):** the LO is ganged with the RF tuning so that it always runs IF above the signal:

$$
f_{LO}=f_s+f_{IF}
$$

   The mixer produces $f_{LO}\pm f_s$; the difference $f_{LO}-f_s=455$ kHz is selected. The modulation (sidebands) is kept unchanged, only shifted to IF.
3. **IF amplifier:** several fixed-tuned stages at 455 kHz with bandwidth about 10 kHz. Since frequency is fixed, they give high, uniform gain and sharp selectivity (adjacent-channel rejection) for all stations.
4. **Detector:** a diode envelope detector recovers the audio from the IF signal.
5. **AGC (automatic gain control):** a DC voltage proportional to the carrier level from the detector controls the gain of RF and IF stages, keeping output nearly constant for strong and weak stations and preventing overload.
6. **AF amplifier and loudspeaker:** amplify the audio to the required power and convert it to sound.

**Example:** to receive 1000 kHz, $f_{LO}=1455$ kHz. The image is $f_s+2f_{IF}=1910$ kHz, which must be rejected by the RF stage.

**Advantages:** constant selectivity and sensitivity over the band, high gain without instability, good adjacent-channel rejection. **Disadvantages:** image-frequency problem, mixer noise, tracking alignment needed.

#### Why standard AM is used in AM broadcasting
1. **Simple, cheap receivers:** standard AM (DSB-FC) can be demodulated by a simple **envelope detector**; no carrier recovery or synchronization is needed.
2. **One transmitter, millions of receivers:** it is economical to put extra power (the carrier) in one costly transmitter so that every listener can use a cheap receiver.
3. The transmitted carrier also helps **AGC** and tuning.
4. Historical compatibility with the huge base of existing receivers.

The cost is poor power efficiency (at most 33.3%) and twice the bandwidth of SSB, which are acceptable for broadcasting.

## Check yourself

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

1. In an AM superhet, the LO frequency is:
   A. fs - fIF
   B. fs + fIF
   C. fs
   D. 2 fs
   Answer: B. The LO runs IF above the signal so fLO - fs = 455 kHz for every station. (Key point: Local oscillator)

2. For a 1000 kHz station and 455 kHz IF, the image frequency is:
   A. 545 kHz
   B. 1455 kHz
   C. 1910 kHz
   D. 910 kHz
   Answer: C. fsi = fs + 2 fIF = 1000 + 910 = 1910 kHz, rejected by the RF stage. (Key point: Image frequency)

3. Why are IF stages good for selectivity?
   A. They are fixed-tuned, so give high uniform gain and sharp selectivity
   B. They retune for each station
   C. They have no gain
   D. They reject noise by AGC
   Answer: A. Fixed frequency lets designers use sharp, high-gain stages for all stations. (Key point: IF amplifier)

4. Why is standard AM used for broadcasting?
   A. It is most power-efficient
   B. Receivers are simple and cheap using an envelope detector
   C. It uses the least bandwidth
   D. It needs carrier recovery
   Answer: B. Spending power on a carrier at one transmitter allows millions of cheap envelope-detector receivers. (Key point: Why standard AM)

## 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×
- [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
- [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
- [Short note: Superheterodyne receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-superheterodyne) — Communication Systems, chapter 2, asked 3×
- [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
- [CRO block diagram and working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acro-block-diagram) — Basic Electronics Engineering, chapter 7
- [Communication system and AM block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-am) — Basic Electronics Engineering, chapter 5
- [Working principle of dc generator with diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-diagram) — Electrical Machines, chapter 3