High data rate spread spectrum demodulators using low-loss SAW matched filters

Compared with the SAW convolver and the digital correlator, the SAW matched filter (tapped delay line) has many advantages in commercial wireless LAN applications, because of its high efficiency and simple configuration. A new type of SAW device for the spread spectrum (SS) transceiver and a low-loss SAW matched filter for the wireless LAN system is described. Two matched filters which correspond to bi-phase data modulation and bandpass filters for the intermediate frequency (IF) section are combined into one device. This SAW matched filter fabricated on ST cut quartz has been practically used in commercial wireless LAN systems and has good temperature stability. To reduce the conversion loss further we have developed an extremely low loss SAW matched filter. The output correlation peak of the matched filter is 3 dB 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 unidirectional transducers. The phase error caused by the temperature coefficient of delay is compensated by using a temperature dependent change of local frequency in the spread spectrum receiver. 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 is achieved with less than a 10^-5 bit error rate