GB809550A - Phase insensitive synchronously tuned filter - Google Patents

Abstract

809,550. Changing wave shape. COLLINS RADIO CO. Feb. 24, 1956, No. 5872/56. Class 38 (2). [Also in Groups XL (b) and XL (c)] Relates to a synchronous filter of the type in which an input A.C. signal 10, Fig. 2A, is applied through a high resistance 13a, Fig. 4, alternately to two capacitors 18a, 19a by means of a switch 17a operating cyclically at a frequency f such that a square wave 28, Fig. 2C of fundamental frequency f is produced at the common contact of the switch 17a by an input sinusoidal signal of frequency f ; when the frequency of the input signal differs from f, the charges on the two capacitors tend to be equal and the arrangement functions in effect as a high-Q narrow-band filter, e.g. when the frequency f is 24 c/s the effective band-pass is 0.03 and the Q is 750. With such a filter the amplitude of the output waveform 28, Fig. 2C, is proportional to cos # where # is the phase difference between the switching cycle, Fig. 2B, and the input waveform 10 and according to the present invention an output signal of amplitude independent of # is obtained by using a second synchronous filter 13b, 17b, 18b, 19b, whose switch 17b is operated in phase quadrature with the switch 17a, the input signal being applied in parallel to both filters and the outputs therefrom being combined in a summing network comprising two cathode followers 42, 43. As shown in Fig. 4, each switch is of the single-pole double-throw type and is actuated by an operating coil, the two coils 26a, 26b being energized in phase quadrature from a two-phase generator 64 operating at the frequency f. In a modification, Fig. 5, each switch comprises a fixed contact 114 which engages two arcuate contacts 121, 122, insulatingly mounted on a rotating earthed shaft 163, the two storage capacitors 118, 119 being connected between the shaft and the contacts 121, 122 and the two sets of contacts 121a, 122a and 121b, 122b being relatively displaced by 90 degrees. When the invention is applied to a conical scan radio direction-finder, the shaft 63 of the generator 64, Fig. 4, or the shaft 163, Fig. 5, of the rotary switches is rotated synchronously with the spin component of the scan. If electronic switches are employed, the arrangement may be used for filtering radio frequency signals.