A quite different technology is based on SAW resonators. A resonator can be made using a transducer in between two SAW reflectors. The reflectors are arrays of metal strips with spacing /2 often called gratings. The resonator has two gratings forming a resonant cavity, with an IDT in the cavity to couple it to the electrical terminals, as in Fig.1. The response of this device is basically a one-pole resonance.

A transverse-coupled resonator (TCR) consists of two identical resonators fabricated close together, as in Fig. 2 and relies on acoustic coupling between the two resonators. The waves in one resonator extend slightly outside its physical structure, and this enables some energy to leak from one resonator to the other.

This couples the two resonators, and the device gives a 2-pole response. The use of resonances enables very narrow bandwidths to be obtained. In fact, this device is limited to bandwidths below about 0.2% because the coupling between the two resonators is weak. Insertion losses are typically 1–2 dB. Because the input and output transducers are in different tracks, not facing each other, the stop band rejection can be good. It is common to cascade two devices to improve this (giving a ‘4-pole’ filter), and a rejection of around 50 dB is obtainable. The response near the pass band is approximately that of a 4-pole filter, so the shape factor is not so small. The substrate is almost always quartz.