SAW filters including one-focus slanted finger interdigital transducers

Abstract

The present paper describes a new filter type, slanted finger interdigital transducer (SFIT) that allows fast analysis as a precondition for fast optimization. Therefore, filter structure and analysis are especially suitable for optimizing unidirectional SFIT (USFIT) filters. For analysis, a SFIT filter is usually divided into many narrow channels. Every channel is considered to be a subfilter and is analyzed separately. Therefore, the total filter analysis is very time consuming. For reducing computing time, we suggest a one-focus structure. In contrast to conventional SFIT filters, not only the finger and gap widths but also the spaces between transducers of different channels differ by one and the same scaling factor. As a consequence, all prolonged finger edges of both transducers intersect in one point called focus. As a result, the parameters of all subfilters can be calculated from the parameters of only one subfilter by simple frequency scaling. Consequently, the total time for analysis is essentially reduced. However, one-focus SFIT filters with continuous finger edges show a deep minimum within the passband. This problem can be overcome by using stepped one-focus structures.

A novel weighted method for layered SAW filters using slanted finger interdigital transducers

“…In the 1990s, Yatsuda et al [6][7][8][9][10] demonstrated many useful techniques to realize SFIT-based SAW filters with wide-band and large side lobe suppression, low loss and low ripple, using the withdrawal-weighting method, the aperture weighting method and the floating electrodes slanted finger unidirectional transducer (FE-SFUDT) method. In addition, Balashov and Baek [11] employed the coupling-of-modes (COM) model to design matching circuits for SFIT-based SAW filters, and Martin and Steiner [12,13] proposed SAW filters with one-focus SFIT to save computation time on analysing conventional SFIT. It is worth noting that the wide-band feature of the SFIT SAW device has also led to several important applications, such as the nondestructive evaluation of thin films [14], actuating micro droplet [15] and measurement of band gaps in phononic crystals [16].…”