Equalization filter compensating for distortion in a surface acoustic

Abstract

The preferred embodiment improves the modulation accuracy of a communication system having a surface acoustic wave (SAW) device filtering a modulated signal. An equalizer filter is used to predistort the signal to be filtered so as to equalize the SAW filter response. In the preferred approach, the equalizer also provides baseband filtering. In this particular embodiment, the original baseband filter impulse response is convolved with the inverse of the actual SAW filter impulse response. The resulting values provide the coefficients for a digital filter (i.e., a modified baseband filter) forming part of a DQPSK modem. When the signal filtered by the modified baseband filter is then up-converted and filtered by the SAW filter, the predistortion caused by the modified baseband filter equalizes the SAW filter distortion, and only the modulation errors of the original baseband filter occur. In other embodiments, the equalizer filter characteristics are derived solely from the SAW filter impulse response or frequency response to equalize any distortion resulting from the SAW filter. In the preferred embodiment, the coefficients for the digital filter are stored in a ROM lookup table.