A new low impedance balanced drive structure for SAW transversely coupled resonator filters

Waveguide coupling of SAW resonators is widely used to make low loss compact filters. The classically used structure for this kind of device consists of two tracks separated by a common ground bussbar. To obtain a balanced drive using this structure, it is necessary to split the transducer in two parts connected electrically in series. This can result in load and source impedances which are too high for system requirements. We present a new multitrack balanced drive waveguide coupled resonator filter structure exhibiting both low impedances and small crystal sizes. A general analysis procedure has been developed to analyse this new structure. This procedure is suitable for an arbitrary number of layers and for any number of electrical ports. Results of a GSM/DCS 1800 IF filter at 188 MHz using the new structure are presented. Typical performances are a 3 dB bandwidth of 250 kHz, a 20 dB bandwidth of 500 kHz and a 6 dB insertion loss