Phase locked loop method of synchro-to-digital conversion

Claim 1

In a system having a primary winding coupled with a source of energization signal and a plurality of secondary windings each having an alternating signal induced therein with an amplitude dependent upon the angular position of said primary winding relative to said secondary windings, a circuit for determining the angular position, comprising: input means coupled to the plurality of secondary windings for providing at least two alternating input signals carrying amplitude information related to the amplitudes of the alternating signals from the plurality of secondary windings; controllable oscillator means for generating cyclical signals having a frequency and phase varying in proportion to signals at an error input; a phase locked loop for controlling the oscillator means to cause the cyclical signals to have a frequency variable in direct proportion with the frequency of the energization signal and a phase directly proportional with said angular position, including analog gate means coupled to the input means for sampling the alternating input signals to develop sampled signals, means coupling the oscillator means to the analog gate means for causing the analog gate means to sample portions of the alternating input signals in response to the phase of the cyclical signals, and error means connecting the analog means to the error input for varying the phase of the cyclical signals under control of the sampled signals; and phase comparator means coupled to the oscillator means and to the source for comparing the relative phase between the cyclical signals and the energization signal to determine said angular position.