The basic filter of Fig.1 can be modified in many ways. The electrode overlaps can be varied so that the SAW beams generated by individual gaps have different widths. This means that the response becomes a linear sum of terms, with amplitudes proportional to the overlaps.

The filter is a ‘transversal filter,’ whose impulse response is basically given by the sequence of overlaps. This has a remarkable property - it means that any response at all can be produced, subject to limitations due to finite length and second-order effects. Thus, filters with very complex responses, including dispersion, can be produced.

SAW transversal filters can satisfy extremely exacting performance requirements. For example, the following performance can be achieved:

• in-band ripple can be as low as 0.2 dB p-p,
• stop-band rejection can be 60 dB,
• and shape factors (ratio of bandwidths at 3 dB and 40 dB points) can be as low as 1.1.

These parameters are often the primary considerations when a new SAW filter design is undertaken, and the results are achieved using sophisticated computer optimization methods for design, including compensation for various second-order effects (e.g. diffraction). The main limitation comes about because of unwanted reflections of the SAWs. COM DEV continuously works on its proprietary SAW design software in an ongoing effort to improve rf performance. To obtain low loss, the IDTs can be electrically matched to the source and load (using one or two matching components), but in this condition the IDTs reflect incident SAWs quite strongly. This is a consequence of the fact that the transducers are bidirectional, generating waves equally in two directions. The result is an unwanted signal due to multiple reflections of the waves, giving ripples in the amplitude and phase of the response. This effect is called Triple Transit Response and the ripples are often unacceptable; to minimize them it is necessary to adjust the matching or loading. For this reason, the insertion loss of high-performance transversal filters is usually quite high, for example in the order of 20 dB or more. The length of a transversal filter is related to its skirt width, so filters with narrow skirts are generally very long.