Also known as: IF, intermediate frequency
The intermediate frequency (IF) is the fixed, lower frequency that a superheterodyne receiver shifts the wanted signal down to before filtering and digitising.1 Mixing the variable input frequency to a constant IF lets the receiver use one well-designed filter regardless of the tuned channel, which is the central insight that made selective, tunable radios practical.
How it works
A mixer multiplies the incoming RF by a local oscillator (LO), producing sum and difference frequencies. The receiver keeps the difference, |f_RF − f_LO|, and tunes the LO so this difference always lands on the same IF. To change channels you retune the LO, not the IF filtering. That fixed IF is where the receiver puts its sharpest, most selective filter and most of its gain — a filter that would be extremely hard to build as a tunable stage but is straightforward at one fixed frequency.
The catch is the image frequency: because the mixer responds to |f_RF − f_LO|, a second input on the far side of the LO, offset by twice the IF, also falls on the IF and interferes. A higher IF pushes this image further from the wanted signal so a front-end filter can reject it more easily — the classic tension in IF selection is image rejection (favouring a high IF) versus cheaper, sharper filtering (favouring a low IF).
Variants
- High-IF (single or multiple conversion). Traditional communications receivers use one or more high IFs (e.g. 10.7 MHz, 45 MHz, 21.4 MHz) for strong image rejection and selectivity.
- Low-IF. The signal is placed at a small non-zero IF, close enough to baseband to digitise directly yet offset enough to avoid the DC artefacts of zero-IF.
- Zero-IF (direct conversion). The IF is 0 Hz — the signal is mixed straight to baseband. Simple and cheap, but it introduces a DC-offset spike and IQ imbalance to manage; see the direct-conversion receiver.
In practice
Many SDR front ends use a low-IF or zero-IF architecture because it feeds the ADC conveniently and pushes the remaining selectivity into software filters. Others, such as some SDRPlay and Airspy designs, use a genuine IF stage before digitising. Once samples reach the software domain, GopherTrunk performs a digital down-conversion — its own numerically controlled oscillator and filters act as an ideal, drift-free IF stage, translating each channel to baseband without the image and filter-tolerance compromises of analog IF hardware.
Sources
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Intermediate frequency — Wikipedia, on superheterodyne downconversion to a fixed IF. ↩