--- title: "Requirements of a good radio receiver; superheterodyne" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:good-receiver-superheterodyne" source: https://www.bctengineer.com/q/semester_6/communication_systems/good-receiver-superheterodyne site: BCT Engineering --- # Requirements of a good radio receiver; superheterodyne Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2070 Bhadra (CS I), 3+5 marks - 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/good-receiver-superheterodyne - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question What are the requirements for a good radio receiver? Explain the operation of a superheterodyne receiver. ## Short answer Good receiver requirements include sensitivity, selectivity, fidelity, image rejection, and AGC. The superheterodyne receiver converts all stations to a fixed intermediate frequency using a mixer and local oscillator, then amplifies and filters at the IF for uniform performance across the band. ## Answer #### Requirements of a good radio receiver 1. **Sensitivity:** ability to pick up weak signals and give a usable output (measured in µV for a given output and SNR). 2. **Selectivity:** ability to select the wanted station and reject adjacent channels. 3. **Fidelity:** ability to reproduce all message frequencies equally, without distortion. 4. **Image frequency rejection:** high rejection of the image $f_{si} = f_s + 2f_{IF}$. 5. **Good SNR / low noise figure** in the front end. 6. **Stability** and simple, single-knob tuning over the whole band. 7. Automatic gain control so output stays steady as signal strength changes. #### Superheterodyne receiver A superheterodyne receiver converts every incoming station to one fixed **intermediate frequency (IF)** (455 kHz for AM broadcast), and does most of the amplification and filtering at that fixed frequency. ```text Antenna | [RF amp]-->[Mixer]-->[IF amp]-->[Det]-->[AF amp]-->Spk fs ^ 455 kHz | | | ^ | | [Local osc] +--AGC--+ | fLO=fs+fIF +--ganged tuning--+ ``` **Blocks** 1. **RF stage:** a tuned amplifier at the signal frequency $f_s$. It improves sensitivity and noise figure and rejects the image frequency before mixing. 2. **Local oscillator and mixer:** the oscillator is tuned together with the RF stage (ganged) so that $f_{LO} = f_s + f_{IF}$. The mixer is a non-linear stage giving sum and difference frequencies; the difference $f_{LO} - f_s = f_{IF}$ is selected. The message is kept, only the carrier is moved. 3. **IF amplifier:** several fixed-tuned stages at 455 kHz with bandwidth about 10 kHz. Since the frequency is fixed, they give high gain and sharp, constant selectivity for all stations. Most of the receiver gain comes from here. 4. **Detector:** an envelope detector recovers the audio. It also gives a DC voltage proportional to carrier strength. 5. **AGC:** this DC level controls the gain of RF and IF stages, keeping the output nearly constant for strong and weak stations. 6. **AF amplifier and speaker:** raise the audio to the power needed to drive the loudspeaker. **Example:** for a station at 1000 kHz, $f_{LO} = 1455$ kHz and $f_{IF} = 455$ kHz. The image is $1000 + 2(455) = 1910$ kHz, which the RF stage must reject. **Advantages:** uniform selectivity and sensitivity across the band, high gain without instability, simple tuning. **Problems:** image frequency and local oscillator tracking. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Selectivity of a receiver means A. reproducing all audio frequencies B. picking up weak signals C. controlling gain D. rejecting adjacent channels while selecting the wanted station Answer: D. Sensitivity is about weak signals; fidelity about audio reproduction. (Key point: Selectivity) 2. In the superheterodyne receiver, fLO is A. fIF / fs B. 2 fs C. fs + fIF D. fs - fIF only at 455 kHz Answer: C. The difference fLO - fs gives the fixed IF. (Key point: LO frequency) 3. Why does most gain and selectivity come from the IF amplifier? A. It is tuned to each station B. It works at a fixed frequency, giving high gain and constant selectivity C. It has no bandwidth D. It operates at audio Answer: B. Fixed-tuned stages are stable and sharp. (Key point: IF stage) 4. For a 1000 kHz station and IF 455 kHz, the image frequency is A. 1910 kHz B. 1455 kHz C. 910 kHz D. 545 kHz Answer: A. fs + 2 fIF = 1000 + 910. (Key point: Image) ## 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× - [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: 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 - [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — Wireless Communication, chapter 5 - [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2× - [Requirements of a good pipe material](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arequirements-good-pipe-material) — Water Supply Engineering, chapter 8, asked 2×