SAW transformer-filter

Low-loss Surface Acoustic Wave (SAW) resonator-type filters, such as double mode SAW filters (or Coupled Resonator Filters, DMS/CRF), can be considered as transformers. Such a RF SAW “transformer-filter” could operate directly on one of the ISM bands and be useful, for example, in “wake-up” radios, since the RF voltage can be raised by passive means prior to rectification. Here, we propose to use a CRF-type of low-loss filter in combination with a few special techniques to increase the output impedance/voltage. The main idea is to separate the output transducers in sections and connect the sections in series. Several variants of this SAW transformer were designed, fabricated, and measured. We have used a so-called “degenerated” CRF with three long IDTs, the output transducers being subdivided into series-connected sections. An estimated transformer coefficient of around 10 was achieved.