Phase determining apparatus for measuring the phase of a tone signal of known tone frequency relative to a reference signal of the same frequency, comprising in combination:
- input means for supplying the tone signal;
- clipping means for hard limiting the tone signal to produce a clipped signal having either of two instantaneous amplitudes;
- a reference signal source for generating first and second square wave signals comprising quadrature components of a reference signal having a repetition rate equal to said tone frequency and either of two instantaneous amplitudes substantially equal to those of the clipped signal;
- first and second digital multipliers for multiplying the clipped signal and each of the first and second square wave signals to produce first and second product signals respectively, each having a first value when the instantaneous amplitudes of its multiplicand signals are substantially equal and a second value when the instantaneous amplitudes of its multiplicand signals are different;
- sampling means for sampling the first and second product signals at a predetermined sampling rate to provide first and second sample trains respectively;
- a first shift register having an input terminal connected to receive the first sample train, an output terminal for subsequently supplying a first sample train, and a predetermined number of storage locations through which samples are shifted seriatim at said sampling rate between the input and output terminals;
- a first reversible counter having a count control terminal connected to receive the first sample train from said first shift register, and an enabling terminal for receiving a signal to permit counting in response to samples received at the count control terminal, said first reversible counter being operable to count in one sense in response to samples having said first value and operable to count in the opposite sense in response to samples having said second value;
- first enabling means operable to supply a signal to the enabling terminal of said first reversible counter only when the values of the samples at the input and output terminals of said first shift register are different, whereby said first reversible counter produces a first count indicative of the magnitude and sense of the difference in numbers of samples having first and second values in said first shift register;
- a second shift register having an input terminal connected to receive the second sample train, an output terminal for subsequently supplying the second sample train, and a predetermined number of storage locations through which samples are shifted seriatim at said sampling rate between the input and output terminals;
- a second reversible counter having a count control terminal connected to receive the second sample train from said second shift register, and an enabling terminal for receiving a signal to permit counting in response to samples received at the count control terminal, said second reversible counter means being operable to count in one sense in response to samples having said first value and operable to count in the opposite sense in response to samples having said second value;
- second enabling means operable to supply a signal to the enabling terminal of said second reversible counter only when the values of the samples at the input and output terminals of said second shift register are different, whereby said second reversible counter produces a second count indicative of the magnitude and sense of the difference in numbers of samples having first and second values in said second shift register;
- processing means for receiving counts indicative of the products of a clipped signal and each of quadrature components of a reference signal, and computing therefrom the phase angle between the clipped signal and the reference signal;
- and
- transfer means for transferring the first and second counts from said reversible counters to said computing means, whereby said computing means is operable to compute the phase angle of the tone signal relative to the reference signal.