FM demodulator

A digital demodulator apparatus having improved linearity, said apparatus comprising:

• (a) means for inputting a sampled input vector comprising a quadrature signal centered approximately zero frequency;
• (b) scaler means for quantizing said inputted sampled quadrature signal within a predetermined range;
• (c) phase accumulator means for generating a current coarse phase value related to input vectors of the quadrature signal;
• (d) vector rotation means for rotating said input vector to a quadrant within a range between -.pi./4 to +.pi./4 radians;
• (e) means for determining a fine phase value based on said rotated input signal vector;
• (f) summing means for summing said fine and coarse phase values and outputting a composite phase value;
• and
• (g) filtering means for filtering said composite of phase angle samples to produce a sequence of demodulated message samples.

A method of digitally demodulating a received frequency modulated signal, said method comprising the steps of:

• (a) inputting digitized quadrature samples of a signal centered approximately at zero frequency, said samples indicating an input signal vector;
• (b) scaling said samples to a desired magnitude within a predetermined range;
• (c) calculating the nearest octant within which the scaled signal vector lies, said nearest octant comprising a coarse phase value range;
• (d) rotationally scaling the scaled signal vector to lie within a range between -.pi./4 to +.pi./4 radians;
• (e) calculating a second value equal to the tangent of the phase of the rotationally scaled signal vector;
• (f) calculating a third value equal to the phase angle of said signal vector by deriving the arctangent of said second value, said third value comprising a fine phase angle value;
• (g) summing the fine phase angle and coarse phase angle values to produce a composite phase angle sample equivalent to the phase angle of the input signal vector;
• (h) subtracting the value of the previous composite phase angle sample from the value of the current composite phase angle sample to produce a demodulated message sample;
• and
• (i) outputting said demodulated message sample to an output register.

A digital frequency modulation demodulator apparatus having improved linearity, said apparatus comprising:

• (a) means for inputting a sampled input vector comprising a quadrature of frequency modulation digital signal centered approximately zero frequency;
• (b) scaler means for quantizing said inputted sampled quadrature signal within a predetermined range;
• (c) phase accumulator means for generating a current coarse phase value related to input vectors of the quadrature FM signal;
• (d) vector rotation means for rotating said input vector to a quadrant within a range between -.pi./4 to +.pi./4 radians;
• (e) means for determining a fine phase value based on said rotated input signal vector;
• (f) summing means for summing said fine and coarse phase values and outputting a composite phase value;
• and
• (g) filtering means for subtracting the value of the previous composite phase angle sample from the value of the current composite phase angle sample to produce a demodulated message sample.