Wide bandwidth low cost SAW notch filters

SAW notch filters have been fabricated in the past with limited success. A prevailing problem has been stopband null depth and bandwidth versus a reasonably broad passband response. Typically, notch filters developed in the past have achieved either one or the other. Notch filters using the Single Phase Unidirectional Transducer (SPUDT) SAW notch element have achieved good null depth and null bandwidth, but have had little success in achieving a broad passband response. Other filters, used in the CATV industry, have achieved very good broad passband responses, but have produced notch bandwidths that are too narrow to reliably operate over a large temperature range. SAW notch filters of the past have been physically large, at times over 17 mm in length and therefore required expensive packaging. Also, these filters have required large and expensive tuning components to achieve the desired performance. The physical size requirements resulted in circuits that were limited in applications In an effort to solve these problems, a new approach has been considered where a SAW element was optimized for high input impedance at the notch center frequency. A Coupling of Modes model was used to predict the performance of the notch element and the notch network. The result was a SAW notch element under 7 mm in length that could easily fit in a TO-39-5 package. Tuning for this notch network required only two small surface mount inductors. Typical passband performance was less than -7.0 dB from 350 MHz to 1.5 GHz.