Effect of stray coupling on the balance of a differential LSAW front-end resonator-filter for wireless/mobile circuits

Differential-mode LSAW resonator-filters are used in front-end mobile wireless circuitry for low-level signal coupling to active differential mixers. These are typically the first/third mode types, cascaded to give four-pole operation over fractional bandwidths up to about 5 percent. At signal levels below about -100 dBm, insertion loss differential imbalance of about 1 dB can degrade the active mixer response. One possible imbalance considered here may be that due to stray coupling or layout connections. This paper employs LSAW theory as well as lumped-equivalent coupling-of-modes-in-time (COMT) modelling to examine the effects of stray capacitance on the differential output of an 883-MHz four-pole differential LSAW resonator-filter on 43° LiTaO₃. Good agreement is obtained between theory and experiment.