Tunable transmission zeroes bandpass filter with external quality factor control

A methodology to control the frequency transmission zeroes (FTZs) of a dual-mode resonator based elliptic-response filter is presented. Varactor loaded feeding structures are introduced for exciting a stub-perturbed ring-resonator to control the external quality factors associated with even and odd modes of the resonator. The technique allows for control of FTZs on either sides of the passband without any significant change in the bandwidth or operating frequency of the filter. Because of the high-Q nature of ring resonator, assymetric frequency responses are achieved with an in-band insertion loss <2.9 dB and out-of-band suppression >55 db at 190 MHz from the center-frequency of 2.2 GHz. Additionally, the filter is provided with tunable source-load coupling to isolate the movement of two FTZs.