Suppression of reflection coefficients of surface acoustic wave filters by 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 -20dB 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 -12.5dB to -21.7dB, i.e., 9.2dB suppression of |S₁₁| was achieved by only sacrificing the insertion loss of 0.6dB. It was also attempted to replace the microstrip lines with lumped elements. In the case, the maximum |S₁₁| within the passband was improved more than 7.5dB with the maximum increased insertion loss of 1.5dB.