FM demodulator

• US 4,675,882 A
• Filed: 09/10/1985
• Issued: 06/23/1987
• Est. Priority Date: 09/10/1985
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Lillie, Ross J., Jasper, Steven C.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Chin, Stephen
• Application Number:
  US06/751,913
• Time in Patent Office: 651 Days
• Field of Search: 375/44, 375/45, 375/78, 375/80, 375/82, 375/94, 375/97, 375/39, 329/50, 329/103, 329/110, 329/122, 329/124, 455/214, 455/337
• US Class Current: 375/324
• CPC Class Codes: H04L 27/1566 using synchronous sampling

Abstract

A digital FM demodulator is described. According to the principles of the present invention, a digital FM signal is demodulated by processing sampled input vectors of a quadrature FM digital signal centered approximately zero frequency. The sampled quadrature input vectors are quantized to lie within a predetermined magnitude range. The quantized vectors are coupled to a phase accumulator which generates a current coarse phase value related to input vectors of the quadrature FM signal. The output of the phase accumulator is also coupled to a vector rotation means which rotates the input vector to a predetermined quadrant to determine a fine phase value based on said rotated input signal vector. The fine and coarse values are then summed and output as a composite phase value, and digitally filtered by 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.

Claims

1. A method of digitally demodulating a received angle-modulated signal, said method comprising the steps of:

(a) inputting digitized quadrature samples of a signal centered approximately at zero frequency, said samples indicating a composite signal vector;

(b) scaling said samples to a desired magnitude within a predetermined range;

(c) calculating the nearest octant within which the scaled composite signal vector lies, said nearest octant comprising a coarse phase range value;

(d) rotationally scaling the scaled composite signal vector to lie within a range between -π

/4 to +π

/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) filtering said composite of phase angle samples to produce a sequence of demodulated message samples; and

(i) outputting said demodulated message samples to an output register.

2. 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 -π

/4 to +π

/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.

3. 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 -π

/4 to +π

/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.

4. 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 -π

/4 to +π

/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.