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Filters in radios
Roofing, crystal, mechanical and DSP filters.
Roofing, crystal, mechanical and DSP filters.
A filter passes some frequencies and blocks others. A radio uses a whole series of them, from broad ones at the antenna to very narrow ones near the speaker, because each filter protects the stages after it and the last ones decide what you hear. The narrower and steeper the final filter, the better the receiver separates a signal from its neighbours (see Sensitivity, selectivity and dynamic range).
A filter has a passband (what it lets through), skirts (how it falls off) and a stop-band (what it rejects). Two filters of the same 2.4 kHz width can behave very differently: one with steep skirts removes a strong signal 2 kHz away, one with gentle skirts lets it through. The ratio of widths at two attenuation levels (often −60 dB to −6 dB) is the shape factor: closer to 1 means steeper skirts.
| Type | Where | Character |
|---|---|---|
| LC (coils and capacitors) | RF band-pass, output low-pass | cheap, broad, gentle skirts; works at any frequency |
| Ceramic | low-frequency IFs such as 455 kHz | inexpensive, moderate steepness |
| Crystal (quartz) | IFs and roofing filters, typically megahertz | very steep and stable; each width needs its own set of crystals |
| Mechanical | 455 kHz IFs in older high-end radios | steep and rugged, narrow |
| DSP | digital stage after the ADC | any width and shape, adjustable; cannot protect the analog stages before it |
That last point is why even an SDR still needs good analog filters ahead of its converter: DSP cannot un-overload an amplifier.