17 - SAW Filters in Digital Communications

Abstract

SAW filters find application in two important areas of digital radio and digital communications. One area concerns their use as spectral-shaping filters for efficient data transmission with low bit-error-rate (BER) for the filter fractional bandwidths of 70% or more. Such filters are called Nyquist filters. The other one relates to the use of SAW transversal filters in clock-recovery circuits for regenerative repeaters in optical fiber data communications systems. This chapter highlights the principles of Nyquist filter design for digital radio communications and emphasizes on the design and use of SAW filters in such systems. Many factors can degrade the BER performance and increase the inter-symbol interference within a digital radio system. These include (a) amplitude and group delay ripple in the Nyquist filters, (b) misaligned modulator and demodulator circuitry, phase noise in the timing recovery circuit, power amplifier nonlinearities, and noise in the transmission channel. SAW filters for digital radio must meet stringent specifications if the BER within the system is to be kept to a minimum, which means minimizing all possible second-order degradation due to diffraction, bulk waves, circuit loading, and IDT end-effects. The IDT end-effect by itself can yield unacceptable passband amplitude and group delay ripple, thereby degrading the performance.