GB925738A - Improvements in and relating to quartz crystal filters - Google Patents

Abstract

925,738. Crystal filter networks. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Aug. 21, 1959 [Aug. 22, 1958], No. 27080/58. Class 40 (8). [Also in Group XL (c)] A quartz crystal filter bridge circuit, Fig. 2a, has two parallel branches, one comprising a quartz crystal X<SP>TAL</SP> and an external resistance Rs in series, and the other comprising in series a second external resistance Rs<SP>1</SP> substantially of the same value as Rs and an external capacitance Cp<SP>1</SP>, which can be variable, of a value substantially equal to the parallel capacitance Cp, Fig. 2b, in the equivalent crystal circuit, both branches being connected to an output load resistance Ro. A variable capacitor can be connected in series with the branch including the quartz crystal to vary the centre frequency. Means can be provided for feeding to the two branches split signals of equal amplitude and in mutual anti-phase such as a wide-band transformer with a centre-tapped secondary, a paraphasing valve or transistor circuits. Two filter circuits of equal bandwidth and giving the same amplitude output at resonance can be used to form part of a narrow range frequency discriminator. The two filters are off-tuned by an amount equal to their bandwidth. Signal sources of low output impedances are used to drive both filters with negligible cross-coupling effects, application of equal amplitude signals to both filter inputs thus being ensured. The filter outputs are reversely rectified, using for instance reversed diodes. Buffer stages, such as cathode followers, may be arranged between the filters and the rectifiers. The outputs of the rectifiers are algebraically added together by either a resistive network or a feed-back amplifier adder. If the input is sinusoidally frequency-modulated, at a value of modulation index less than 0.2, at a constant deviation, and at a variable rate, an amplitude attenuation of the output A.C. component and an output phase shift is obtained. Such a discriminator has a bandwidth of approximately 0.7 of the bandwidth of the individual filters at the modulating rate, and the slope of the attenuation curve is eventually 12 dbs. per octave.