A voltage controlled filter and a radio communications device comprising the same

摘要

In order to obtain a substantially constant passband for a filter of, for example, a radio communications device, a first notch is provided in an upper side of the filter characteristic to define a slope for the upper side of the filter characteristic, thereby substantially fixing the bandwidth of the passband. The notch is generated by high impedance coupling between a capacitor 24, varactors 26, 28 and a resonant circuit 30, 32. A notch on the lower side of the filter characteristic is generated by mutual coupling between inductors 38, 40. A voltage 36 controlled e.g. by a microprocessor, shifts the centre frequency and the pass-band. An alternative arrangement includes two pairs of varactors and separate parallel tuned circuits coupling the filter to input, output terminals. Further notches are introduced by the parallel tuned circuits. The inductors e.g. 32, 38, 40 may be formed by microstrip lines.