Microwave electronically tunable filters

Design procedures are presented for varactor tuned
microwave bandpass and bandstop filters constructed in
Microwave Integrated Circuit form. The tunable bandpass
filters were based on a combline filter incorporating novel
input and output coupling networks which compensate for the
frequency dependence of the coupling between the resonators.
Using this filter, tuning over an octave band with an acceptable response and approximately constant passband bandwidth is possible. The tunable bandstop filters consisted of a uniform impedance main line with capacitively decoupled resonators located at intervals along it. A novel design technique is presented for evaluating the correct phase shifts between the resonators of this filter in order to obtain the optimum symmetrical frequency response. Detailed computer analysis of the filters, including both varactor and integrated circuit loss, and the effects of tuning is presented. The measured performances of several practical devices, constructed in Suspended Substrate Stripline, and operating in the frequency range 2-10 GHz are presented, these agree closely with computed performances.