A 400-CPS Tuning Fork Filter

The tuning fork filter as a new circuit component offers certain advantages in terms of frequency stability and large values of Q. An eleven-element comb filter is described in which each filter element has a bandwidth of 0.5 cps or a Q of 800, a 26-db insertion loss, and a 40-db rejection ratio. The frequency is constant to within 0.01 per cent, linearity to within one per cent, and Q to within ten per cent for a 40-db range of input. The temperature compensating effect of nickel used in the bimetallic fork is referred to an anomaly in its temperature-elasticity characteristic. The dedecoupling of the input-output electromagnetic transducers is described, and a novel method of Q adjustment in the form of shading coils is developed. The shape of the resonance curve shows that the tuning fork filter can be considered as equivalent to a single-tuned RLC circuit. The fork response at submultiples of its resonant frequency is due to slight harmonic generation in the drive circuit; the nonharmonic overtones are accurately predicted by cantilever theory. A 30-PPM position sensitivity is shown to be due to the change in the effective fork stiffness caused by gravity.