How a superheterodyne receiver works

Almost every receiver you will ever use is a superheterodyne. Understanding the signal path makes a whole cluster of exam questions straightforward.

The problem it solves

A receiver has to select one signal out of an enormous range of frequencies and amplify it enormously. Building a filter that is sharp and tunable across a whole band is difficult and expensive.

The superhet's trick: instead of moving the filter, move the signal.

The signal path

Antenna → RF amplifier → Mixer → IF amplifier and filter → Detector → AF amplifier → Speaker

with a Local Oscillator feeding the mixer.

The mixer and the local oscillator

The mixer combines the incoming signal with a local oscillator running at a frequency you control. Mixing two frequencies produces sum and difference products. The receiver selects the difference — a fixed intermediate frequency, or IF.

Tune the local oscillator, and whichever incoming signal you want lands on that same fixed IF.

Why that is clever

The IF never changes. So the IF filter never has to change either. You can build one very sharp, very high-quality fixed filter and use it for every signal, at every frequency, across every band.

That single idea is why the design has dominated for a century.

The two performance figures

Sensitivity — the ability to receive weak signals. Set mainly by the noise figure of the front end.

Selectivity — the ability to reject signals on adjacent frequencies. Set mainly by the IF filter bandwidth.

These are different things and exams test whether you know which is which:

Sensitivity is about weak. Selectivity is about nearby.

Matching the filter to the mode matters: about 2.4 kHz for SSB, 500 Hz or less for CW, 15 kHz for FM. Using an SSB filter for CW means you hear four other CW signals alongside the one you want.

The controls

AGC — Automatic Gain Control. Automatically reduces gain on strong signals and increases it on weak ones, so signals of wildly different strengths all arrive at a comfortable listening level. Fast AGC suits CW, slow suits SSB.

Squelch. Mutes the audio when no signal is present. Essential on FM, where an idle channel produces loud rushing noise. It sets a threshold; signals below it never reach the speaker.

RIT — Receiver Incremental Tuning. Shifts the receive frequency slightly without moving the transmit frequency, for working a station that is slightly off yours.

The dummy load

Not part of the receiver, but tested alongside it. A dummy load is a non-inductive resistor — normally 50 Ω — that absorbs your transmitter output and converts it to heat instead of radiating it.

It lets you test, tune and measure without putting a signal on the air. Using one is basic courtesy and, where your testing would cause interference, a regulatory obligation.