Patent GB-1045751-A (GB)
Title: Improvements in or relating to signal filter systems
IPC: H03J7/02
Filed: 1963-04-09
Abstract: 1,045,751. Automatic phase control systems; modulators; frequency discriminators; multiplex signalling; semi-conductor oscillator circuits. GENERAL PRECISION Inc. April 9, 1963 [April 9, 1962], No. 14232/63. Headings H3A, H3R, H3T and H4L. A signal filter system for filtering a plurality of phase-displaced variable frequency input signals having a varying phase sequence indicative of direction, comprises modulator means responsive to each of the input signals and to a plurality of controlled-frequency oscillator output signals for modulating the input signals to provide a sideband signal; a frequency selective amplifier tuned to a predetermined frequency and amplifying the sideband signal to produce a further signal; frequency discriminator means tuned to the predetermined frequency and connected to the further signal to provide an oscillator frequency control signal which varies according as the further signal tends to vary above or below the predetermined frequency; and demodulator means receiving the further signal and oscillator signals and providing a plurality of output phase-displaced signals having the same variable frequency and phase relationship as the input signal. The input signals may be derived from a shaft position digitizer or an interferometric measuring device, and a pair of such signals in phase quadrature are applied to input terminals 10 and 20, Fig. 1, and pass through units 11 and 21 to a two-phase balanced modulator 30. The signals are also inverted in units 12 and 22 and the inverted signals are fed to the modulator. A four-phase output from an oscillator 31 is also fed to modulator 30, and it is shown that the output of modulator 30 is a single sideband, components at the other sideband and at carrier frequency cancelling out. The frequency of oscillator 31 is controlled by a discriminator 34 so that the sideband is of a fixed frequency, e.g. 180 kc/s., and this fixed frequency is amplified by tuned amplifier 32 which eliminates all noise outside a narrow band about the intelligence signal. The output from amplifier 32 passes to an amplitude limiter stage 33, which improves signal/noise ratio by removing amplitude noise, and to discriminator 34 and through a buffer amplifier 38 to a two-phase demodulator 41. Four quadrature-related signals from oscillator 31 are also fed to demodulator 41, the output of which is a pair of signals which have the same frequency and phase relationship as the original input signals and which are suitable for operating a reversible counter. The level of the output signals from amplifier 32 is sensed and used to control variable attenuator units 11 and 21 in the input circuit to prevent any limiting effects occurring in modulator 30 over a wide range of input signal amplitudes. Modulators.-Signals X and Y in phase quadrature at terminals 10 and 20, Fig. 2a, pass directly to transistors 102 and 104, and through inverter units U-101 and U-102 to transistors 101 and 103. The outputs of the four transistors 101-104 pass respectively to transistors 105-108 which are gated by the four phases #1-#4 of pulses supplied from oscillator 31 (not shown in Fig. 2a). The outputs are added at terminal 104 to provide a single sideband at either the sum or the difference between the frequency of the input signals and the pulse frequency; this sideband is amplified by tuned amplifier 32. Frequency discriminators.-An input signal at terminal 120 (Fig. 3) passes through transistors T201 and T202 to the primary winding of transformer T2. Transformers T2 and T3 function as a single discriminator transformer and may comprise two interconnected high Q toroidal transformers. The secondary winding of T3 is tuned to centre frequency by capacitance C207 and the phase relationships are such that at resonance the voltages e 2  and e 3  across the two halves of the secondary winding of T3 are in quadrature with the voltage e 4  across secondary winding 204 of T2. Consequently at resonance voltages e 5  and e 6 , which are the vector sums of e 4  with e 2  and e 3  respectively, are equal. For frequencies away from resonance these voltages differ, and the consequent difference at terminal 220 is integrated by transistors T205 and T206. Semi-conductor oscillator circuits.-The frequency of the oscillator circuit shown in Fig. 5 is determined by the voltage on terminal 222, which controls the current flowing through transistor T501 acting as a constant current source. This current is the charging current for capacitor C501, which allows T502 to conduct when sufficiently charged. The pulse at the collector of T502 is applied through winding 507 of transformer T 5  to the base of transistor T504 causing it to conduct. This produces a voltage drop in winding 509 of T5 which is coupled back to the transistor base regeneratively via winding 507, thus generating a pulse in blocking oscillator fashion. This pulse is also coupled to transistor T503; allowing it to discharge capacitor C501, after which another charging cycle begins.