Surface Acoustic Wave
Understanding SAW Filters
SAW filters exploit the fact that acoustic waves travel about 100,000x slower than electromagnetic waves. This means a SAW filter at 1 GHz has features measured in micrometers, enabling extremely compact filters that can be mass-produced using semiconductor photolithography.
How SAW Filters Work
- Input transducer (IDT): Interdigitated electrode pattern converts electrical signal to acoustic surface wave.
- Propagation: Acoustic wave travels along the piezoelectric substrate surface.
- Output transducer: Converts acoustic wave back to electrical signal.
- Frequency selection: The electrode spacing determines the center frequency. Filter shape is defined by electrode apodization.
SAW vs BAW vs LC
| Parameter | SAW | BAW/FBAR | LC |
|---|---|---|---|
| Frequency | 10 MHz - 3 GHz | 500 MHz - 6 GHz | DC - 100 GHz |
| Q | 500-2000 | 1000-3000 | 50-500 |
| Size | Small | Very small | Large |
| Cost | $ |
Frequently Asked Questions
What is a SAW filter?
A SAW filter converts electrical signals to acoustic waves on a piezoelectric crystal surface, processes them through the electrode pattern geometry, and converts back to electrical signals. They provide excellent selectivity in a compact, low-cost package.
Where are SAW filters used?
SAW filters are ubiquitous in consumer wireless devices: cellular phone IF and RF filters, GPS receiver front ends, Wi-Fi channel selection, TV tuners, and remote control receivers. Billions are manufactured annually.
What limits SAW filter frequency?
SAW filter electrode spacing must be half the acoustic wavelength. Above 3 GHz, the features become too small for reliable manufacturing. BAW (Bulk Acoustic Wave) and FBAR filters extend acoustic filter technology to 6+ GHz.