Also known as: superheterodyne receiver, superhet
A superheterodyne receiver uses a mixer driven by a local oscillator to shift a chosen band down to a fixed intermediate frequency (IF) — or to baseband — where it is easier to filter and detect.1 Invented by Edwin Armstrong, it has been the dominant receiver architecture for a century and underlies most SDR front-ends.
How it works
A mixer multiplies the incoming RF by the local-oscillator tone, producing sum and difference frequencies. The difference — the IF — is a fixed frequency chosen by the designer, and tuning is just changing the LO frequency so the wanted band lands on that fixed IF. The genius of the scheme is that the hard part of the receiver — a sharp, high-gain, well-shaped filter and the detector — can be built once, at one frequency, instead of being made tunable across the whole spectrum. Fixed-frequency crystal, ceramic, or SAW filters at the IF give selectivity that a tunable RF filter never could.
The unavoidable cost is the image: two input frequencies, one above and one below the LO by the IF amount, both mix down to the same IF. A signal at the image frequency slips through as though it were the wanted station. Suppressing it needs an RF pre-selector filter or an image-reject mixer topology, so image rejection is a headline spec of any superhet.
Variants
- Single-conversion — one mixer/LO/IF stage; simple, adequate for many bands.
- Double- or triple-conversion — successive IFs (e.g. a high first IF to push the image far away, then a low second IF for tight filtering) used in high-performance communications receivers.
- Quadrature down-conversion — the SDR form: mix to two IF or baseband channels 90° apart to produce IQ, which resolves the image mathematically rather than by RF filtering. A zero-IF direct-conversion receiver is the degenerate case where the IF is 0 Hz.
In practice
Most consumer SDRs are quadrature receivers under the hood: an R820T-based RTL-SDR mixes to a low IF, while Airspy and many others mix to zero-IF baseband. Either way the classic superhet blocks — LNA, mixer, LO, IF filter — are present, just followed by an ADC instead of an analog detector. Everything after the ADC, including a further software mix in a digital down-converter, is a digital continuation of the same idea.
Relevance to SDR
Understanding the mixer/LO explains why “tuning” an SDR is simply setting a number, and why image rejection and pre-selection still matter even when the detector is software. In GopherTrunk the RF tuning is done in the radio’s superhet front-end; the software then applies a second, digital down-conversion to pick out individual channels from the digitised band.
Sources
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Superheterodyne receiver — Wikipedia, on mixing a band down to a fixed intermediate frequency. ↩