A low loss SAW matched filter using unidirectional transducers under the compensation method of temperature deviations

An extremely low loss Surface Acoustic Wave (SAW) matched filter for use in a 2.4 GHz wireless LAN Spread Spectrum (SS) demodulator is described. The output correlation peak of the matched filter is 3dB larger than the input level at a 50 Ω input and output impedance. This low loss SAW matched filter is fabricated on 128° Y-X LiNbO₃ substrate and consists of a floating electrode type unidirectional transducer (FEUDT) and a coded group type unidirectional transducer (CGUDT). The phase error caused by the temperature coefficient of delay is compensated for by using the temperature dependent change of local frequency in the spread spectrum receiver. Re-excited SAWs on the coded electrodes are canceled out by the good self correlation characteristics of the spreading code. Under the critical conditions of a 2.4 GHz wireless LAN (Data rate is 1.4 Mbps, C/N>2 dB, that is E_b/N₀>13.6 dB, and operation temperature is from -20°C to +60°C.), good performance which is less than 10^-5 bit error rate is achieved