GB1005496A - Frequency discriminator - Google Patents

Abstract

1,005,496. Frequency discriminators. STANDARD TELEPHONES & CABLES Ltd. June 5, 1964, No. 23420/64. Heading H3A. A frequency discriminator comprises a bridge formed from impedances 8, 9, a series resonant piezo-electric crystal 1, and a balance impedance 2, and the output across diagonal 10, 11 is compared in phase with that across the balance arm impedance 2 in a phase detector to indicate the sense and magnitude of any frequency deviation from the series resonance frequency. The output from arm 2 may be passed through an amplitude limiter and amplifier and that from the other arm through a 90 degrees phase shift network so that the phase detector output is zero at resonance. Further, another bridge formed from the input transformer secondary 16, a second output winding 15 of the transformer, a capacitor 18 of value equal to the shunt capacitance of the crystal 1 and the crystal 1 itself, may be used as shown to balance out the crystal shunt capacity and avoid any bridge diagonal output at a parallel resonance frequency of the crystal.