Digital frequency measurement system and method with automatic frequency control

A digital frequency measurement system comprising:

• first and second digital differentiators responsive to first and second digital quadrature signals representative of first and second quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined frequency for generating first and second differentiated signals;
• first and second processing circuits for combining the first differentiated signal with the second digital quadrature signal and the second differentiated signal with the first digital quadrature signal to provide first and second combined signals;
• a combining circuit responsive to said first and second combined signals for generating a density signal having a pulse density proportional to the frequency of said input signals;
• and
• a digital signal processor responsive to said density signal for providing the average value of said pulse density and determining the midpoint of the maximum and minimum signal level representative of said input signals to generate a digital signal that represents said center frequency of said input signals.

A digital frequency measurement system comprising:

• an analog-to-digital converter responsive to first and second analog quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined IF frequency for generating first and second digital quadrature signals that represent said input signals;
• first and second digital differentiators responsive to said first and second digital quadrature signals for generating first and second differentiated signals;
• first and second processing circuits for combining the first differentiated signal with the second digital quadrature signal and the second differentiated signal with the first digital quadrature signal to provide first and second combined signals;
• a combining circuit responsive to said first and second combined signals for generating a density signal having a pulse density proportional to the frequency of said input signals;
• and
• a digital signal processor responsive to said density signal for providing the average value of said pulse density and determining the midpoint of the maximum and minimum signal level representative of said input signals to generate a digital signal that represents said center frequency of said input signals.

A single-ended digital frequency measurement system comprising:

• a digital differentiator responsive to a digital representation of a modulated input signal that represents binary data having a center frequency equal to a predetermined frequency for generating a differentiated signal;
• a processing circuit for combining said digital representation of said input signal by said differentiated signal to provide a combined signal whose pulse density is proportional to the frequency of said input signal;
• and
• a digital signal processor responsive to said combined signal for providing the average value of said pulse density and for determining a midpoint of the maximum and minimum signal level representative of said input signal to generate a digital signal that represents said center frequency of said input signal.

A single-ended digital frequency measurement system comprising:

• a digital differentiator responsive to a digital representation of an analog FSK modulated input signal that represents binary data having a center frequency equal to a predetermined IF frequency for generating a differentiated signal;
• a processing circuit for combining said digital representation of said FSK modulated input signal by said differentiated signal to provide a combined signal whose pulse density is proportional to the frequency of said input signal;
• and
• a digital signal processor responsive to said combined signal for providing the average value of said pulse density and for determining a midpoint of the maximum and minimum signal level representative of said input signal to generate a digital signal that represents said center frequency of said input signal.

A single-ended digital frequency measurement system comprising:

• a limiter responsive to a modulated input signal that represents binary data having a center frequency equal to a predetermined frequency for generating a digital signal that represents said input signal;
• a digital differentiator responsive to said digital signal for generating a differentiated signal;
• a processing circuit for combining said digital representation of said input signal by said differentiated signal to provide a combined signal whose pulse density is proportional to the frequency of said input signal;
• and
• a digital signal processor responsive to said combined signal for providing the average value of said pulse density and for determining a midpoint of the maximum and minimum signal level representative of said input signal to generate a digital signal that represents said center frequency of said input signal.

A single-ended digital frequency measurement system comprising:

• a digital differentiator responsive to a digital representation of an analog FSK modulated input signal that represents binary data having a center frequency equal to a predetermined IF frequency for generating a differentiated signal;
• a multiplier circuit responsive to said digital representation of said input signal and said differentiated signal for multiplying said digital representation of said input signal by said differentiated signal to provide a multiplied signal whose pulse density is proportional to the frequency of said input signal;
• a digital filter responsive to said multiplied signal for providing an output signal representing the average value of said pulse density;
• and
• an envelope detector circuit responsive to said output signal for determining a midpoint of the maximum and minimum signal level representative of said input signal and providing a digital signal that represents said center frequency of said input signal.

A digital frequency measurement system with automatic frequency control for an FSK demodulator comprising:

• a digital frequency measurement system responsive to first and second digital quadrature signals representative of first and second analog quadrature FSK modulated input signals that represent binary data having a center frequency equal to a predetermined IF frequency varied by positive and negative frequency deviations for providing a digital signal that represents said center frequency of said input signals;
• and
• a feedforward automatic frequency control system comprising:
• a subtraction circuit responsive to said digital signal and a predetermined digital representation of a preferred IF frequency for subtracting said digital signal from said predetermined digital representation of said preferred IF frequency to generate a multiplied error signal,
• a scaling adjustment circuit responsive to said multiplied error signal for introducing a scaling coefficient to remove the multiplied factor from said multiplied error signal and generate an error signal that represents the frequency error between said center frequency at said predetermined IF frequency and said preferred IF frequency, and
• a delay control circuit responsive to said error signal, a digital representation of the frequency deviation of said input signals, and a digital representation of the ratio of said predetermined IF frequency to said frequency deviation for generating first and second control signals to adjust the delays of cosine and sine correlators of an FSK demodulator to compensate for said error signal.

A digital frequency measurement system with automatic frequency control for a demodulator comprising:

• a digital frequency measurement system responsive to first and second digital quadrature signals representative of first and second quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined frequency varied by positive and negative frequency deviations for providing a digital signal that represents said center frequency of said input signals;
• and
• a feedforward automatic frequency control system comprising:
• a comparing circuit for comparing said digital signal to a predetermined digital representation of a preferred frequency to generate a frequency error signal,
• an adjustment circuit for introducing a coefficient to remove a multiplied factor from said frequency error signal and generating an error signal that represents the frequency error between said center frequency and said preferred frequency, and
• a delay control circuit for generating first and second control signals to adjust the delays of a demodulator to compensate for said error signal.

A digital frequency measurement system with automatic frequency control for an FSK demodulator comprising:

• a digital frequency measurement device comprising:
• first and second digital differentiators responsive to first and second digital quadrature signals representative of first and second quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined frequency varied by positive and negative frequency deviations for generating first and second differentiated signals,
• first and second processing circuits for combining the first differentiated signal with the second digital quadrature signal and the second differentiated signal with the first digital quadrature signal to provide first and second combined signals,
• a first combining circuit responsive to said first and second combined signals for generating a density signal having a pulse density proportional to the frequency of said input signals, and
• a digital signal processor responsive to said density signal for providing the average value of said pulse density and determining the midpoint of the maximum and minimum signal level representative of said input signals to generate a digital signal that represents said center frequency of said input signals;
• and
• a feedforward automatic frequency control system comprising:
• a comparing circuit for comparing said digital signal and a predetermined digital representation of a preferred frequency to generate a frequency error signal,
• an adjustment circuit for introducing a coefficient to remove a multiplied factor from said frequency error signal to generate an error signal that represents the frequency error between said center frequency and said preferred IF frequency, and
• a delay control circuit for generating first and second control signals to adjust the delays of cosine and sine correlators of an FSK demodulator to compensate for said error signal.

A digital frequency measurement system with an automatic frequency control comprising:

• a digital frequency measurement system responsive to first and second digital quadrature signals representative of first and second quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined frequency varied by positive and negative frequency deviations for providing a digital signal that represents said center frequency of said modulated input signals;
• and
• a feedback automatic frequency control loop comprising:
• a first combining circuit for subtracting said digital signal from a predetermined digital representation of said preferred frequency to generate a multiplied error signal,
• an adjustment circuit for introducing a coefficient to remove the multiplied factor from said multiplied error signal and generate an error signal that represents the frequency error between said center frequency and said preferred IF frequency,
• an automatic frequency control system responsive to said error signal for generating a filtered frequency error signal and providing loop stability,
• a summer circuit responsive to said filter error signal and a fine frequency adjustment value for combining said filtered frequency error signal and said fine frequency adjustment value to generate a composite input signal,
• a phase lock loop for removing said frequency error and generating an adjusted local oscillator signals having said frequency error removed, and
• a quadrature generator circuit responsive to said adjusted local oscillator signal for generating sine and cosine quadrature signals representative of said adjusted local oscillator signal.

A digital frequency measurement system with automatic frequency control for a demodulator comprising:

• a single-ended digital frequency measurement system comprising:
• a digital differentiator responsive to a digital representation of a modulated input signal that represents binary data having a center frequency equal to a predetermined frequency for generating a differentiated signal,
• a processing circuit for combining said digital representation of said input signal by said differentiated signal to provide a combined signal whose pulse density is proportional to the frequency of said input signal, and
• a digital signal processor responsive to said combined signal for providing the average value of said pulse density and for determining a midpoint of the maximum and minimum signal level representative of said input signal to generate a digital signal that represents said center frequency of said input signal;
• and
• a feedforward automatic frequency control system comprising:
• a comparing circuit for comparing said digital signal to a predetermined digital representation of a preferred frequency to generate a frequency error signal,
• an adjustment circuit for introducing a coefficient to remove a multiplied factor from said frequency error signal and generate an error signal that represents the frequency error between said center frequency and said preferred frequency, and
• a delay control circuit for generating first and second control signals to adjust the delays of a demodulator to compensate for said error signal.

A digital frequency measurement system with an automatic frequency control comprising:

• a single-ended digital frequency measurement system comprising:
• a digital differentiator responsive to a digital representation of a modulated input signal that represents binary data having a center frequency equal to a predetermined frequency for generating a differentiated signal,
• a processing circuit for combining said digital representation of said input signal by said differentiated signal to provide a combined signal whose pulse density is proportional to the frequency of said input signal, and
• a digital signal processor responsive to said combined signal for providing the average value of said pulse density and for determining a midpoint of the maximum and minimum signal level representative of said input signal to generate a digital signal that represents said center frequency of said input signal;
• and
• a feedback automatic frequency control loop comprising:
• a first combining circuit for subtracting said digital signal from a predetermined digital representation of said preferred frequency to generate a multiplied error signal,
• an adjustment circuit for introducing a coefficient to remove the multiplied factor from said multiplied error signal and generate an error signal that represents the frequency error between said center frequency and said preferred IF frequency,
• an automatic frequency control system responsive to said error signal for generating a filtered frequency error signal and providing loop stability,
• a summer circuit responsive to said filter error signal and a fine frequency adjustment value for combining said filtered frequency error signal and said fine frequency adjustment value to generate a composite input signal,
• a phase lock loop for removing said frequency error and generating adjusted local oscillator signals having said frequency error removed, and
• a quadrature generator circuit responsive to said adjusted local oscillator signal for generating a local oscillator signal representative of said adjusted local oscillator signal.

A method for measuring the center frequency of modulated input signals comprising the steps of:

• generating first and second differentiated signals from first and second digital quadrature signals representative of first and second quadrature modulated input signals that represent binary data having a center frequency equal to a predetermined frequency;
• combining the first differentiated signal with the second digital quadrature signal and combining the second differentiated signal by the first digital quadrature signal to generate first and second combined signals;
• generating a density signal having a pulse density proportional to the frequency of said input signals from said first and second combined signals;
• providing an output signal representing the average value of said pulse density;
• and
• determining the midpoint of the maximum and minimum signal level representative of said input signals to provide a digital signal that represents said center frequency of said input signals.

A method for measuring the center frequency of a modulated input signal comprising the steps of:

• generating a differentiated signal from a digital representation of a modulated input signal that represents binary data having a center frequency equal to a predetermined frequency;
• combining said differentiated signal with said digital representation of said input signal to generate a combined signal having a pulse density proportional to the frequency of said input signal from said combined signal;
• providing an output signal representing the average value of said pulse density;
• and
• determining the midpoint of the maximum and minimum signal level representative of said input signal to provide a digital signal that represents said center frequency of said input signal.