Abstract: A multi-pole filter for use in radio apparatus includes a plurality of isolated ceramic resonators each having an associated varicap diode network to enable electronic tuning respective of ceramic resonators. The inputs of adjacent ceramic resonators are coupled via a first coupling path and also via a second coupling path, the first coupling path being composed of a fixed capacitance network and the second coupling path being composed of a capacitance network including the varicap diode networks associated with the adjacent resonators. The selective values of the various capacitances forming the first and second coupling paths are such that the overall coupling between the resonators remains substantially constant during the electronic tuning of the resonators by the varicap diode network.

Abstract: A band pass filter for radio frequencies uses helical resonators arranged to have a transmission notch on the high frequency side of the pass band. A plurality of resonators housed in a shield are formed by quarter-wavelength transmission lines wound in a helix, with adjacent resonators being wound in opposite directions and grounded at opposite ends to provide a transmission notch at the high frequency side of the passband. One or more grounded shielding pins are arranged between adjacent resonators to adjust the frequency of the notch to be at or near an image frequency. The grounded end of each of the resonators may be coupled to a varicap diode network for electronically tuning the filter.