How do SAW and BAW filters work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

SAW (surface acoustic wave) and BAW (bulk acoustic wave) filters convert electrical signals into acoustic waves in piezoelectric materials. Because acoustic velocity is about 10⁵ times lower than the speed of light, resonators are micrometer-scale, giving high Q in millimeter packages. SAW dominates below roughly 2–3 GHz; BAW, including FBAR and solidly mounted resonators, covers higher bands used by LTE, 5G NR and Wi-Fi.

How does a SAW filter work?

Interdigital transducers (IDTs) on a piezoelectric substrate such as lithium tantalate or lithium niobate launch and receive surface waves; the electrode pitch sets the frequency (pitch ≈ half the acoustic wavelength). Resonator-based (ladder) designs give low loss. Temperature-compensated SAW (TC-SAW) adds an oxide layer to reduce drift.

How does a BAW filter work?

A thin piezoelectric film (commonly aluminum nitride, sometimes scandium-doped) between electrodes resonates in thickness mode; resonance is set by film thickness. FBARs suspend the film over an air cavity; SMRs use a Bragg reflector. Ladder and lattice topologies of resonators form band-pass filters.

What are the trade-offs?

Steep skirts and small size; essential for closely spaced cellular duplex bands.
Limited power handling compared with cavities.
Fractional bandwidth limited by piezoelectric coupling coefficient.
Temperature coefficients matter for narrow guard bands.

Frequently asked questions

Why do phones need so many filters?

Each supported band and carrier-aggregation combination needs its own filtering or multiplexers.

Can SAW work at 5 GHz?

Advanced SAW variants reach higher frequencies, but BAW is more common above about 3 GHz.

Are acoustic filters used in base stations?

Mostly cavity and ceramic filters are used there because of power handling.

Categories: Filters and passives

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