Surface wave filter filtering delay transfer function

Surface acoustic wave filter comprising:

• longitudinal resonant cavities, forming an integer number N, equal to at least 2, of parallel filtering channels each including;
• an input transducer and an output for each of said filtering channels having a direct path between each said input and output transducers;
• at least two reflecting gratings in each of said filtering channels;
• the transmission amplitudes being, at least for each said direct path, substantially the same for these N channels;
• wherein, the difference in propagation length in the direct path is defined by the center-point distance from said input transducer to a corresponding said output transducer and is, for two consecutive channels, substantially equal, to within an integer number of wavelengths (.lambda.) at the central filter frequency (fo), to one Nth of this wavelength (.lambda.), so that for this central frequency (fo), the vector resultant of the signals of these N channels which correspond to the direct paths is substantially zero and that in consequence there is substantially cancellation of the delay line transfer function outside the stop band of the filter;
• and
• wherein, each center-point distance from each said input and said output transducers to a closest said reflecting grating are equal.