Bandpass filter with N+1 transmission zeroes using frequency-variant couplings

A bandpass filter topology which can generate N+1 transmission zeroes is presented. These zeroes are created using frequency-variant couplings. A design procedure, derived from the transmission line equivalent circuit, is described and used to evaluate a first approximation of the filter dimensions. The final dimensions are obtained with an optimization routine in order to take into account the frequency variation of the couplings. The experimental validation presented shows that the proposed topology can provide an improved out-of-band rejection compared to conventional filters. The order 3 bandpass filter centered at 10 GHz, with 4 transmission zeroes at 8, 9, 11, and 12 GHz, presents an insertion loss of 1.9 dB for a 500 MHz bandwidth.