Design of ladder-type SAW/BAW filters with constant group delay

This paper describes design of radio frequency (RF) surface and bulk acoustic wave (SAW/BAW) filters with constant group delay. First, the bandpass LC filter is designed based on the traditional Bessel filter design. Then LC resonators in the filter are replaced by SAW/BAW resonators with finite capacitance ratio γ. Finite γ generates passband ripples in group delay. However, it is shown they can be suppressed by optimal design of SAW/BAW resonators. Norton's first transform is also applied to the designed filter so as to reduce variation of resonator shunt capacitances. Finally, it is shown that extreme flat group delay with the deviation of 3 ns in passband is realizable over the frequency range of ±15 MHz at the center frequency of 980 MHz.