SAW Filter
A SAW filter is a two-port piezoelectric device with one set of interdigital transducer (IDT) fingers that converts RF voltage to surface acoustic waves and a second IDT that converts the wave back into voltage; the geometry imposes a sharp passband (typical 1–100 MHz BW) at frequencies up to several GHz with insertion loss as low as 1 dB. Examples: Murata SAFFB942MFA0F, Qorvo SAW85901, Skyworks 856920.
On a polished quartz, lithium niobate, or lithium tantalate substrate, photolithography defines aluminium IDT fingers spaced at half a wavelength of the desired centre frequency. An AC voltage on the input IDT excites a Rayleigh surface wave that travels at ~3 km/s; only frequencies at which the IDT fingers add coherently launch efficiently. A second IDT some millimetres away catches the wave and converts it back to RF voltage. Apodisation (varying finger overlap) tailors the passband shape to specifications.
In plain terms
A guitar string with finely spaced frets: pluck the string with electricity and only certain frequencies (matching the fret spacing) survive a trip down and back, while everything else is absorbed.
Why designers use it
- Reject the 'image frequency' in superheterodyne receivers with sharp 50 dB stopband adjacent to the passband.
- Implement narrow-bandwidth band-select filters in cellular handsets that LC filters cannot achieve at these frequencies.
- Provide a cheap, repeatable IF filter (10–500 MHz) without the assembly variation of LC ladders.
- Substitute for ceramic filters in TV tuners, sensor remote keys, and ISM-band receivers.
Best for
- Cellular RX
- TV IF
- ISM-band radios
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When the selected part cannot meet the required center frequency, bandwidth, insertion loss or rejection.
- When its power or temperature behavior is unsuitable; compare specific SAW, temperature-compensated SAW and BAW products rather than using a universal frequency cutoff.
Common mistakes
- Skipping the impedance-matching shunt inductor — the SAW IDT presents a shunt capacitance that detunes the passband and adds 3 dB of return-loss-driven loss.
- Mounting too close to a switching regulator — magnetic-field coupling into the substrate causes the passband to ripple at the regulator's switching frequency.
Where you will find it
- A 4G LTE smartphone's receive chain uses a band-specific SAW filter (Qorvo 855952 for Band 28 700 MHz) immediately after the LNA: the SAW provides 30 dB of rejection of out-of-band cellular and broadcast TV interferers, allowing the radio to extract weak desired signals next to a high-power FM broadcast tower.
- A car keyless-entry remote uses a 433 MHz SAW filter (Murata SAFEA433MZAA0F) to define the receiver's narrow band: the 200 kHz BW filter rejects nearby ISM-band interferers, allowing the receiver in the car to wake on the OOK packet at a reliable −110 dBm sensitivity.
- A digital TV ATSC tuner uses a wide SAW (44 MHz BW at 44 MHz IF, e.g., Murata SAFKB44.0MZ70R) to select the 6 MHz channel after downconversion: the SAW gives near-rectangular passband with 50 dB stopband, far better than any analog LC filter and at a fraction of the BOM cost.
A short history
A SAW filter is a passive RF device that uses the piezoelectric effect to convert an electrical signal into a surface acoustic wave on a crystal substrate and back again, with interdigital transducers whose electrode spacing sets the passband. This structure gives sharp frequency selectivity and is often used to filter signals in mobile handset RF front ends, TV tuner stages, and wireless receivers.