The Superheterodyne Receiver

Selectivity is measured relative to the centre frequency. A 10 kHz channel at 1 MHz needs a Q of 100; the same channel at 100 MHz needs a Q of 10 000 — and a tuneable filter that stays that sharp across a band is not a practical component.

1. 1Multiply the incoming signal by a local oscillator, producing sum and difference frequencies.
2. 2Tune the oscillator so the wanted station's difference always lands at a fixed intermediate frequency.
3. 3Filter and amplify at that fixed IF, with a filter that never has to move.
4. 4Demodulate from the IF.

At a fixed frequency, a ceramic or crystal filter gives steep skirts and a flat passband cheaply. Most of the receiver's gain and all of its selectivity live in a stage designed once. AM broadcast standardised on 455 kHz, FM on 10.7 MHz.

Two input frequencies — one above the oscillator, one below — differ from it by the same amount, so both land on the IF. The unwanted one is the image, at 2 × IF from the wanted station, and nothing after the mixer can separate them.

• An RF preselector before the mixer rejects it. It can be broad, because the image is 2×IF away rather than one channel away.
• A higher IF pushes the image further out, making the preselector easier.
• But a higher IF makes the IF filter's job harder — the Q problem returns.
• Double conversion resolves it: a high first IF to kill the image, a low second IF for selectivity.

RF preselector

Broad tuned circuit before the mixer. Rejects the image; contributes little selectivity.

Local oscillator

The only tuned element that must track accurately, staying exactly one IF from the wanted station.

Mixer

A deliberately non-linear stage producing sum and difference products.

IF amplifier and filter

Fixed frequency, so it carries most of the gain and all of the selectivity.

AGC

A feedback loop measuring IF level and reducing gain ahead of it, so a 60 dB range of station strengths gives similar output.

Detector

Envelope for AM, discriminator or PLL for FM.

Armstrong's 1918 architecture is still in phones, Wi-Fi, radar and test equipment because the insight — translate the signal so the hard filter never moves — is independent of the technology implementing it.

What changed is that the IF is now often sampled directly and processed in software. Integrated receivers increasingly use zero-IF or low-IF architectures to avoid the image entirely, trading it for DC offset and I/Q imbalance problems instead.