A method for assembly and for setting a narrowband - crystal filter in brueckenschaltung

摘要

A process for the manufacture and adjustment of a narrow-band bridge connected crystal filter is described. The crystal filter is connected in series with a capacitor in one branch of the bridge and the series connection is shunted by a parallel arrangement of a fixed and a variable capacitor. The input of the filter includes a tunable transformer with a centre-tapped secondary one end of which is connected to the crystal-containing bridge branch. The band width of the transformer is much greater than the design band width of the filter. In assembly, first the series resonant frequency of the crystal is measured and then the value of the capacitor to be connected in series therewith is calculated and selected on the basis of the measurement before connecting it in the circuit. The parallel connected variable capacitor is then used to adjust the poles of the filter and the transformer is tuned to adjust the attenuation. IMAGE