Many techniques have been developed to obtain low-loss SAW filters without the ripple problem of transversal filters. One method is to employ modified IDTs designed to generate SAWs preferentially in one direction. Commonly, some IF filters use Single Phase Unidirectional Transducers (SPUDTs) for this purpose. The unidirectional behaviour is obtained by using electrodes with different widths. The design involves relatively narrow line widths, restricting the frequencies obtainable, so these devices are usually applied at frequencies below 1 GHz. For this type of device the reflections, and hence the ripples, are minimized when the specified loading is used. So, unlike the transversal filter above, the ripples might be large if no matching components are used. These devices usually have quartz substrates, giving bandwidths up to 2% and losses of typically 5-10 dB.

Compared with a transversal filter, a SPUDT filter has much less loss for a given amount of ripple. In addition, the SPUDTs can often be designed to have a length shorter than that of a corresponding transversal filter. This is advantageous for a compact system such as a mobile phone, and it also reduces the cost of the substrate material and the package. A typical SPUDT Filter characteristic is shown in Fig 1.