Bandpass–Bandstop Filter Cascade Performance Over Wide Frequency Tuning Ranges

A tunable, substrate integrated, high Q bandpass-bandstop filter cascade is demonstrated that is capable of providing up to 100 dB of isolation between two dynamically selectable frequencies of interest. Evanescent-mode cavity filters are used in the cascade to allow high quality factors and wide tuning ranges. It is shown that it is theoretically possible to tune the cascade circuit's passband and transmission nulls independently over more than an octave in frequency while displaying good performance. General coupling matrix theory, ABCD matrix theory, and measurements are shown that describe the behavior of the filter cascade over the entire frequency tuning range. The high, dynamic isolation provided by cascade circuits has potential to be useful in concurrent transmit-receive systems, shared aperture systems, and spectral environments with strong co-site interference.