| 摘要 |
An RF filter comprises a set of resonant shunt branches, each branch including a resonant element BR and being in parallel with a non-resonant element BN such as a capacitor. The elements BR and BN are coupled by further non-resonant elements J. The resonant shunt branches provide transmission zeros (figure 16a) corresponding to the resonant frequencies of the resonant elements, the intervening stop-band lobes 36 being described as sub-bands. The non-resonant elements may be adjusted by a controller 124 to provide one or more reflection zeros in a selected sub-band, thereby providing a narrow pass-band (38, figure 16a) within the stop-band. The controller responds to a temperature sensor 126 by moving the position of the reflection zero or zeros within the sub-band so that the pass-band frequency is maintained despite thermal drift. The resonant elements 18, which may also be adjusted in dependence on temperature, may comprise then-film lumped structures such as planar spirals, zig-zag serpentine elements, single coils and double coils. These elements may comprise high temperature superconductors. The adjustable non-resonant elements may comprise varactors and switched capacitors. |
