Digital quadrature demodulator

Claim 1

A demodulation system for demodulating an angle modulated input signal comprising: quadrature analog-to-digital sampling means for providing a first quadrature sample signal E 1 (t) of said input signal and a second quadrature sample signal E 2 (t) of said input signal, said sample signals being defined by the equations: E 1 (t) = A 1 cosωt + φ E 2 (t) = A 1 sinωt + φ where A 1 is the vector representing the instantaneous amplitude of the input signal, ω is the instantaneous frequency of the input signal, t is the time when the sample signal is taken, and φ is the initial phase of the input signal; digital processing means for operating on a pair of said quadrature sample signals to provide an output signal related to the instantaneous phase angle of said input signal; means for mixing the input signal with a locally generated signal twice, once in each of two channels; means for shifting the phase of a locally generated signal by π/2 radians in one channel; means for filtering the sum and residual original frequencies of said mixed signals to obtain difference frequencies in quadrature phase relationship to one another; and means for simultaneously sampling said difference frequencies to form an output signal related to the instantaneous phase angle of said input signal.