RF Filter (SAW / LC / Crystal)
Filter
Frequency-selective gatekeepers that pass the band you want and reject everything else — the reason your WiFi ignores GSM towers. SAW filters put a precise bandpass in a 2 mm package; LC filters handle power; crystal filters give razor-sharp selectivity.
What it looks like
RF Filter (SAW / LC / Crystal)
A typical filter — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Centre frequency / cutoff
MHzThe band it passes (bandpass) or the corner where rolloff starts (low/high-pass).
Bandwidth
MHz / kHzPassband width: a 2.4 GHz WiFi SAW passes ~100 MHz; an SSB crystal filter passes just 2.4 kHz.
Insertion loss
dBLoss inside the passband — 1–3 dB typical for SAW; every dB before the LNA raises the receiver noise figure directly.
Stopband rejection
dBHow hard out-of-band signals are suppressed — 30–50 dB typical.
Impedance
Ω50 Ω in RF chains; some SAW filters need specific LC matching networks per datasheet.
Markings
SAW filters: tiny SMD cans with a part code (e.g. B3550, TA0395A) — datasheet lookup required. LC filter modules mark cutoff frequency; IF cans are stamped 10.7M or 455K.
Standard values
SAW: 433.92 MHz, 868/915 MHz, 1575.42 MHz (GPS), 2441 MHz (WiFi/BT). IF: 455 kHz ceramic, 10.7 MHz crystal. Harmonic LPFs sold per amateur band (7 MHz, 14 MHz...).
How to choose
1) Receiver front-end: SAW at the exact band, minimum insertion loss. 2) Transmitter output: LC low-pass sized for the power, cutting harmonics to legal limits. 3) Narrowband IF selectivity: crystal/ceramic filters. 4) Check terminating impedance — filters only meet their spec when driven and loaded as designed.
Pinout & package
SAW: 3–6 pad SMD (in, out, grounds). IF cans: 3–5 pins through-hole. LC modules: SMA in/out boxes. Feed-through EMI filters: threaded bulkhead barrels.
Example circuits
- GPS receiver: antenna → SAW (1575 MHz) → LNA → SAW → mixer
- 7 MHz QRP transmitter output low-pass filter cleaning harmonics
- Superheterodyne FM radio: 10.7 MHz crystal filter setting selectivity
- 455 kHz ceramic filter in an AM receiver IF strip
Common failures
Cracked SAW from board flex or ESD (band goes deaf), detuned LC from a shifted core slug, moisture shifting cavity filters, and simply the wrong-band filter installed. Filters otherwise rarely die — suspect them last.
How to test
A NanoVNA in S21 mode sweeps the filter and draws its actual passband — compare against the datasheet curve for centre frequency, insertion loss and skirts. Crude go/no-go: signal generator at centre frequency in, power/RSSI meter out.
Substitutes
Same centre frequency, bandwidth and impedance interchange across brands. An LC filter can be hand-wound to replace a module (many calculators exist); nothing hand-made replaces a SAW's size and precision at GHz.