Patent ID: 7006562

Claim:
A phase demodulator for measuring a phase difference between a phase-modulated test signal I s (ωt)=2 k 1 cos(ωt+ψ s ) and a phase-modulated reference signal I r (ωt)=2 k 2 cos(ωt+ψ r ), the test and reference signals having fixed carrier frequencies (ω), a phase difference (Δψ) being equal to (ψ s −ψ r ), said phase demodulator comprising: an amplitude control device for adjusting amplitudes of the test and reference signals to satisfy the condition k 1 =k 2 =k; a differential amplifier, coupled to said amplitude control device, for receiving the amplitude-adjusted test and reference signals from said amplitude control device, for obtaining an intensity difference between the amplitude-adjusted test and reference signals, and for amplifying the intensity difference to generate an amplitude-modulated output I out (ωt) equal to |4 γk sin(½Δψ)|sin(ωt), where γ is the gain of said differential amplifier; and a signal processing device comprising: an amplitude demodulator coupled to said differential amplifier, said amplitude demodulator demodulating the amplitude-modulated output from said differential amplifier to obtain an output that is related to the phase difference (Δ 104 ); and a phase comparator, coupled to said amplitude control device, for determining a sign of the phase difference (Δψ) from the amplitude-adjusted test and reference signals and for determining an increasing or decreasing direction of change in the phase difference (Δψ).