Band pass filter with rectangular frequency characteristic - uses pair of matched capacitance diodes to achieve very sharp cut on and cut off

摘要

A band pass filter with an almost rectangular frequency characteristic, uses a pair of matched capacitance diodes to achieve this. The circuit is simpler and cheaper than those tranditionally used to achieve such characteristics, such as quartz crystal or magnetostrictive based devices and consists of a symmetrical arrangement of two circuits, each comprising an inductance (1, 2) a capacitance (45) a capacitance diode (6, 7) and a resistance (8, 9) together with a coupling capacitance (3). - The capacitance diodes have matched, but different voltage/capacitance characteristics, one being convex and one concave. Each circuit has a band pass characteristic of roughly right angled triangular shape, with the sharp cut off being at the lower frequency in one case and the higher in the other. The composite circuit has sharp cut off both at the lower and higher frequencies.