TABLE OF CONTENTS When the rejection of a signal is required, it is natural to think in terms of a notch. Just as a true bandpass filter offers improved selectivity over a single resonator, the bandstop filter offers improved rejection over a simple notch or even a cascade of notches. However, the general realization difficulties of distributed structures are worsened by particular difficulties associated with the bandstop structure.
The distributed bandstop structures can theoretically be transformed directly from a conventional L-C bandstop filter. However, in practice, this direct transformation gives unrealistic line impedances, often on the order of hundreds of ohms. Kuroda's identities can be used to help this process, inserting admittance inverters and forcing the line impedances to become somewhat more reasonable. An edge coupled bandstop filter is also available, but is only realizable for very narrow bandwidths.
An exact set of design equations is available for 1st to 5th order stub bandstop filters [1]. These equations give impedances for both cascade and stub lines. All lines are 90 degrees long. While there is no mathematical limit to the bandwidth of this filter, the stub impedances quickly become impractical as the bandwidth decreases below about 50 percent for printed structures or about 30 percent for mechanical structures. If a narrower bandwidth is required, the open-circuited stub resonators can be replaced with capacitively coupled short-circuit resonators.
The synthesis equations are based on Kuroda's identity. This identity allows shifting stubs and conversion of a series stub to a shunt...
