Also known as: SAW filter, surface acoustic wave filter
A SAW (surface acoustic wave) filter is a compact, sharp band-pass filter built on a piezoelectric substrate.1 In an SDR front end it acts as a preselector — passing only the wanted band and strongly rejecting out-of-band signals that would otherwise desensitise or overload the receiver. A single SAW device can give the kind of steep skirts that would take many stages of an LC filter, in a package a few millimetres across.
How it works
A SAW filter turns the electrical signal into a mechanical wave and back again. Two interdigital transducers (IDTs) — interleaved comb-shaped metal electrodes — are deposited on a piezoelectric crystal such as lithium niobate or quartz.2 Applying RF to the input IDT flexes the substrate and launches a surface acoustic wave that ripples across the chip at roughly 3,000–4,000 m/s — about a hundred-thousandth the speed of the electromagnetic wave. The output IDT converts that mechanical wave back to electricity.
Because the wave is so slow, one acoustic wavelength is tiny, and the geometry of the electrode fingers sets the response with great precision. The finger spacing fixes the centre frequency; the number of fingers and their weighting sets the bandwidth and skirt steepness. The result is a fixed-frequency band-pass with excellent selectivity and no tuning. The trade-off is insertion loss — a passive SAW typically costs a few dB, which is why a receive SAW is usually followed immediately by a low-noise amplifier to restore the noise budget.
Variants
- Band-pass SAW — the common receive preselector (ADS-B 1090 MHz, GPS L1 1575 MHz, ISM bands).
- SAW resonator / oscillator — a one-port device used to stabilise oscillators, close cousin of the crystal filter.
- SAW delay line — exploits the slow acoustic velocity to build compact delays for radar and correlation receivers.
- For applications needing far higher rejection or power handling than a SAW can offer, a cavity filter is the bulkier alternative.
In practice
The canonical hobby use is the 1090 MHz ADS-B chain: a SAW plus LNA “green” filter board sits between the antenna and an RTL-SDR so that nearby cellular (700–900 MHz, 1800 MHz) and broadcast transmitters cannot swamp the tuner. Similar filtered front ends exist for the 137 MHz weather-satellite band and for GPS. Because a SAW is fixed-frequency, it only helps the one service it was cut for — it is preselection, not a tunable filter.
Relevance to SDR
SAW-based filter-and-amp modules are ubiquitous in SDR reception of ADS-B, ACARS, weather satellites, and GNSS, precisely because broadband dongles have weak inherent selectivity and overload easily. GopherTrunk decodes the resulting samples but does not manage the filter; on a trunking site crowded with strong pager, cellular, and broadcast signals, a band-appropriate SAW or cavity filter ahead of the SDR is often what turns an unusable, intermod-riddled spectrum into a clean control channel GopherTrunk can lock.
Sources
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Surface acoustic wave — Wikipedia, on the surface-acoustic-wave devices used to build compact sharp band-pass filters. ↩
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Interdigital transducer — Wikipedia, on the comb electrodes that launch and receive the acoustic wave and set a SAW filter’s response. ↩