Suppression of reflection coefficients of surface acoustic wave filters using quadrature hybrids

This paper proposes a simple technique to suppress the reflection coefficients S₁₁ and S₂₂ of surface acoustic wave (SAW) filters. Two identical SAW filters are sandwiched in between two quadrature hybrids, where their two ports are used as input and output, and others are terminated by matched loads. First, it is shown by simulation that both |S₁₁| and |S₂₂| are suppressed to be less than -20 dB without deteriorating the transmission characteristics. Next, two hybrids using microstrip lines were fabricated, and two RF SAW filters for GSM850 were sandwiched in between them. The result showed that the maximum |S₁₁| within the passband was improved from -12.5 dB to -21.7 dB, i.e., 9.2 dB suppression of |S₁₁| was achieved by sacrificing only the insertion loss of less than 0.6 dB. We also attempted to replace the microstrip lines with lumped elements. In this case, the maximum |S₁₁| within the passband was improved more than 7.5 dB with the increased insertion loss of less than 1.5 dB. Finally, simple discussion is given on the inclusion of the transformer function in the quadrature hybrid