--- title: "FM superheterodyne radio receiver" subject: "Communication Systems" chapter: 3 key: "semester_6/communication_systems:fm-superheterodyne-receiver" source: https://www.bctengineer.com/q/semester_6/communication_systems/fm-superheterodyne-receiver site: BCT Engineering --- # FM superheterodyne radio receiver Communication Systems · Chapter 3: Angle Modulation - Asked in: 2080 Chaitra (CS I), 8 marks; 2071 Magh (CS I), 8 marks (asked 2 times) - Video: [5.9 FM AM Super Heterodyne Receiver](https://www.youtube.com/watch?v=FtQQBpYBC0I) — Ali Muqaibel - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/fm-superheterodyne-receiver - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-3 ## Question Explain the operation of FM superheterodyne radio receiver. ## Short answer The FM superheterodyne receiver mixes the received 88–108 MHz signal with a local oscillator to produce a fixed 10.7 MHz IF, amplifies and limits it to remove amplitude variations, then demodulates with a frequency discriminator. De-emphasis reduces high-frequency noise, and AFC locks the local oscillator to keep the IF centered. ## Answer An **FM superheterodyne receiver** converts the received FM station (88–108 MHz) to a fixed intermediate frequency of **10.7 MHz**, amplifies and limits it, and then recovers the audio with a frequency discriminator. #### Block diagram ```text Ant | [RF amp]->[Mixer]->[IF amp]->[Limiter]->[Discri-] 88-108M ^ 10.7 MHz [ minator] | | BW 200k | | [Local osc]<------AFC---------------+ | fLO=fs+10.7M | +--ganged--+ [De-emph]<------+ | [AF amp]-->Spk ``` #### Working of each block 1. **RF amplifier:** tuned to the wanted station $f_s$. It gives gain at a low noise figure (important at VHF), improves sensitivity and rejects the image frequency $f_s + 2(10.7)$ MHz. 2. **Local oscillator and mixer:** the oscillator is ganged with the RF tuning so that $f_{LO} = f_s + 10.7$ MHz. The mixer produces sum and difference frequencies; the difference, 10.7 MHz, is selected. The frequency deviation (up to ±75 kHz) is unchanged by mixing. 3. **IF amplifier:** several fixed-tuned stages at 10.7 MHz with a bandwidth of about 200 kHz (Carson bandwidth for $\Delta f = 75$ kHz, $W = 15$ kHz is 180 kHz). Provides most of the gain and the selectivity. 4. **Amplitude limiter:** clips the IF signal to a constant amplitude, removing amplitude variations caused by noise, interference and fading. The information is in the frequency, so nothing useful is lost. This is the main reason FM is less noisy than AM. 5. **Discriminator (FM detector):** converts frequency variations into voltage variations, e.g. a Foster–Seeley discriminator, ratio detector, slope detector or PLL. Output is proportional to $k_fm(t)$. 6. **De-emphasis network:** an RC low-pass filter with 75 µs (or 50 µs) time constant. It undoes the transmitter's pre-emphasis and reduces high-frequency noise. 7. **AF amplifier and speaker:** amplifies the audio to drive the loudspeaker. In stereo sets a stereo decoder sits between discriminator and AF stage. 8. **AFC (automatic frequency control):** the DC part of the discriminator output shows any mistuning; it is fed back to a varactor in the local oscillator to keep the IF centred at 10.7 MHz. #### Differences from AM superheterodyne | Point | AM receiver | FM receiver | |---|---|---| | Signal range | 0.535–1.605 MHz | 88–108 MHz | | IF | 455 kHz | 10.7 MHz | | IF bandwidth | 10 kHz | 200 kHz | | Limiter | Not used | Used | | Detector | Envelope detector | Discriminator / PLL | | Control loop | AGC | AFC (and AGC) | | De-emphasis | No | Yes | **Example:** for a station at 100 MHz, $f_{LO} = 110.7$ MHz and the image is at 121.4 MHz, outside the FM band. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main job of the amplitude limiter? A. Remove amplitude variations from noise, interference and fading, since information is in frequency B. Convert frequency changes to voltage C. Select the wanted station D. Restore low audio frequencies Answer: A. Clipping to constant amplitude removes AM noise without losing information; this is why FM is less noisy than AM. (Key point: Role of the limiter) 2. What does the discriminator do? A. Boosts the IF gain B. Converts frequency variations into voltage variations proportional to m(t) C. Generates the local oscillator D. Rejects the image frequency Answer: B. The FM detector's output is proportional to kf m(t). (Key point: Discriminator) 3. How does AFC keep the IF centred at 10.7 MHz? A. It fixes the RF amplifier gain B. It narrows the IF bandwidth C. It adds de-emphasis D. The DC part of the discriminator output drives a varactor in the local oscillator Answer: D. Mistuning shows as a DC offset at the discriminator, which is fed back to retune the oscillator. (Key point: AFC) ## Related in Communication Systems - [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: FM radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fm-radio-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× - [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: AM radio receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-am-radio-receiver) — 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 - [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — Wireless Communication, chapter 5