Tunable bandpass filter

摘要

A bandpass filter has a combline structure having a plurality of cascaded nodes. A plurality of nodes in the filter are connected both to resonant elements (a.k.a. resonators) and non-resonant elements (including elements having inductances and/or capacitances that do not resonate in a predetermined frequency band of interest). The resonant frequencies of the resonant elements may be adjusted, in order to adjust the location of the center frequency and/or the width of the passband of the filter. The characteristics of the resonant and non-resonant elements are selected such that the poles of the filter, when plotted on the complex plane, move substantially along the imaginary axis when the resonant frequencies are adjusted, without substantial movement along the real axis. The resulting bandpass filter has substantially constant losses and substantially constant absolute selectivity over a relatively wide range of bandwidths.

主权项

1. A filter for receiving an input signal and providing a filtered output signal, the filter comprising:
an input coupling adapted to receive the input signal; an output coupling adapted to provide the filtered output signal; at least three transmission-line couplings forming a main transmission line; at least four nodes connected in series between the input coupling and the output coupling, each node being connected to an adjacent node through one of the transmission-line couplings; at least four resonant elements, each coupled to a different node of the at least four nodes and each having an adjustable resonant frequency; and at least four non-resonant elements, each connected to a different node of the at least four nodes, wherein:
(i) at least one of the non-resonant elements is one of (a) a capacitor coupled to ground and (b) a structure that is equivalent to a capacitor coupled to ground;(ii) at least one of the non-resonant elements is one of (a) an inductor coupled to ground and (b) a structure that is equivalent to an inductor coupled to ground;(iii) at least two of the resonant elements provide at least two transmission zeros at at least two different frequencies in a lower stopband of the filter;(iv) at least two of the resonant elements provide at least two transmission zeros at at least two different frequencies in an upper stopband of the filter; and(v) each of the non-resonant elements provides the coupling between one of the nodes along the main transmission line and a corresponding one of the resonant elements, such that the filter has a bandpass filtering characteristic between the lower and upper stopbands.