Digital frequency measurement system and method with automatic frequency control

• US 20050137815A1
• Filed: 09/09/2004
• Published: 06/23/2005
• Est. Priority Date: 09/09/2003
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Hunt, William, Quinlan, Philip E., Mulvaney, Kenneth J., Crowley, Patrick G.
• Granted Patent: US 7,352,831 B2
• Time in Patent Office: Days
• US Class Current: 702/75
• CPC Class Codes: H03D 3/007 by converting the oscillations into two quadrature related signals H03D3/245 takes precedence
• CPC Class Codes: H03D 3/02 by detecting phase difference between two signals obtained from input signal H03D3/28 - H03D3/32 take precedence; muting in frequency-modulation receivers H03G3/28; limiting arrangements H03G11/00
• CPC Class Codes: H04L 27/144 with demodulation using spectral properties of the received signal, e.g. by using frequency selective- or frequency sensitive elements
• CPC Class Codes: H04L 27/1525 using quadrature demodulation
• CPC Class Codes: H04L 27/156 with demodulation using temporal properties of the received signal, e.g. detecting pulse width

Abstract

A digital frequency measurement system including 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 IF frequency for generating first and second differentiated signals, first and second processing circuits responsive to the first and second digital quadrature signals representative of the modulated input signals and the first and second differentiated signals for multiplying the first differentiated signal by the second quadrature digital representation of the input signals and multiplying the second differentiated signal by the first quadrature digital representation of the input signals to provide first and second multiplied signals, a combining circuit responsive to the first and second multiplied signals for generating a density signal having a pulse density proportional to the frequency of the input signals, a digital filter responsive to the density signal for providing an output signal representing the average value of the pulse density, and an envelope detector circuit responsive to the output signal for determining the midpoint of the maximum and minimum signal level representative of the input signals and providing a digital signal that represents the center frequency of the input signals.

Claims

1. 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 IF frequency for generating first and second differentiated signals;

first and second multiplier circuits responsive to said first and second digital quadrature signals and said first and second differentiated signals for multiplying the first differentiated signal by the second digital quadrature signal and multiplying the second differentiated signal by the first digital quadrature signal to provide first and second multiplied signals;

a subtraction circuit responsive to said first and second multiplied signals for generating a density signal having a pulse density proportional to the frequency of said input signals;

a digital filter responsive to said density 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 the midpoint of the maximum and minimum signal level representative of said input signals and providing a digital signal that represents said center frequency of said input signals.

2. The digital frequency measurement system of claim 1 further including an analog-to-digital converter responsive to said first and second analog quadrature modulated input signals at said predetermined IF frequency and varied by positive and negative frequency deviations for generating said first and second digital quadrature signals.

3. The digital frequency measurement system of claim 2 in which said analog-to-digital converter includes a limiter.

4. The digital frequency measurement system of claim 2 in which said analog-to-digital converter includes a one bit analog-to-digital converter.

5. The digital frequency measurement system of claim 2 in which said analog-to-digital converter includes a saturated analog-to-digital converter.

6. The digital frequency measurement system of claim 1 further including sine and cosine correlators.

7. The digital frequency measurement system of claim 2 in which said frequency deviations are in the range of about 100 Hz to 5 MHz.

8. The digital frequency measurement system of claim 1 in which said predetermined IF frequency is any frequency greater than or equal to zero.

9. The digital frequency measurement system of claim 1 in which said modulated input signals include any modulated signal whose envelope level is greater than zero.

10. The digital frequency measurement system of claim 1 in which said modulated input signals are chosen from the group consisting of FSK modulated input signals, GFSK modulated input signals, MSK modulated input signals, PSK modulated input signals, OPSK modulated input signals and OQPSK modulated input signals.

11. 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.

12. The digital frequency measurement system of claim 11 in which said digital signal processor includes a digital filter responsive to said density signal for providing an output signal representing said average value of said pulse density.

13. The digital frequency measurement system of claim 12 in which said digital signal processor includes an envelope detector responsive to said output signal for determining said midpoint and providing said digital signal.

14. The digital frequency measurement system of claim 12 in which said digital signal processor includes an averaging filter responsive to said output signal for determining said midpoint and providing said digital signal.

15. The digital frequency measurement system of claim 12 in which said digital signal processor includes a low bandwidth filter responsive to said output signal for determining said midpoint and providing said digital signal.

16. The digital frequency measurement system of claim 11 in which said modulated input signals are chosen from the group consisting of FSK modulated input signals, GFSK modulated input signals, MSK modulated input signals, PSK modulated input signals, OPSK modulated input signals and OQPSK modulated input signals.

17. The digital frequency measurement system of claim 11 in which said predetermined frequency includes an IF frequency greater than or equal to zero Hz.

18. 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.

19. The digital frequency measurement system of claim 18 in which said analog-to-digital converter includes a limiter.

20. The digital frequency measurement system of claim 18 in which said analog-to-digital converter includes a saturated analog-to-digital converter.

21. 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.

22. The digital frequency measurement system of claim 21 in which said digital signal processor includes a digital filter responsive to said combined signal for providing an output signal representing said average value of said pulse density.

23. The digital frequency measurement system of claim 21 in which said digital signal processor includes an envelope detector responsive to said output signal for determining said midpoint and providing said digital signal.

24. The single-ended digital frequency measurement system of claim 21 further including an analog-to-digital converter responsive to said modulated input signal for generating said digital representation of said analog modulated input signal.

25. The single-ended digital frequency measurement system of claim 21 in which said analog-to-digital converter includes a limiter.

26. The single-ended digital frequency measurement system of claim 21 in which said analog-to-digital converter includes a saturated analog-to-digital converter.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. The system of claim 31 in which said predetermined frequency includes an IF frequency having a frequency greater than or equal to zero Hz.

33. The system of claim 31 in which said delay circuit is 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 and generates said first and second control signals.

34. The system of claim 31 in which said first and second control signals adjust said delays of said cosine and sine correlators by τ

, where

$\mathrm{\tau <br><br>}=\frac{1}{4<br><br>\left(\mathrm{Fdev}+\frac{\mathrm{Ferr}}{K}\right)},$

where Fdev is said frequency deviation of said input signals, Ferr is said frequency error, and K is equal to said predetermined IF frequency divided by said frequency deviation.

35. The system of claim 31 further including an analog-to-digital converter responsive to said first and second quadrature modulated input signals for generating said first and second digital quadrature signals.

36. The system of claim 35 in which said analog-to-digital converter includes a limiter.

37. The system of claim 35 in which said analog-to-digital converter includes a saturated analog-to-digital converter.

38. The system of claim 31 in which said frequency measurement system includes a digital differentiator having first and second digital differentiators, first and second processing circuits, a combining circuit, a digital filter, and an envelope detector for generating said digital signal that represents said center frequency.

39. The system of claim 31 further including a quadrature receiver responsive to a high frequency modulated input signal and cosine and sine local oscillator signals for generating said first and second analog quadrature FSK modulated input signals.

40. The system of claim 39 further including an RF frequency synthesizer responsive to a crystal oscillator reference input signal for generating said cosine and sine local oscillator signals.

41. The system of claim 40 in which said RF frequency synthesizer includes a phase lock loop.

42. The system of claim 40 in which said RF frequency synthesizer includes a fractional-N frequency synthesizer.

43. The system of claim 40 in which said RF frequency synthesizer includes an integer-N frequency synthesizer.

44. The system of claim 40 further including a crystal oscillator for generating said crystal oscillator reference input signal.

45. 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.

46. The digital frequency measurement system of claim 45 further including an analog-to-digital converter responsive to said first and second analog quadrature modulated input signals at said predetermined frequency and varied by positive and negative frequency deviations for generating said first and second digital quadrature signals.

47. The digital frequency measurement system of claim 45 in which said analog-to-digital converter includes a limiter.

48. The digital frequency measurement system of claim 45 in which said analog-to-digital converter includes a saturated analog-to-digital converter.

49. The system of claim 45 in which said delay circuit is 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 and generates said first and second control signals.

50. 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.

51. The system of claim 50 in which said phase lock loop is responsive to said composite input signal, a predetermined number (N), and a crystal oscillator reference input signal and generates said adjusted local oscillator signal.

52. The system of claim 50 in which said phase lock loop includes a fractional-N frequency synthesizer.

53. The system of claim 50 in which said phase lock loop includes an integer-N frequency synthesizer.

54. The system of claim 50 further including a quadrature receiver responsive to said sine and cosine quadrature signals and a high frequency FSK modulated input signal for generating said first and second analog quadrature FSK modulated input signals having said frequency error removed.

55. The system of claim 50 further including a demodulator responsive to first and second analog quadrature FSK modulated input signals.

56. The system of claim 50 in which said digital frequency measurement system includes first and second differentiators responsive to said first and second digital quadrature signals representative of said first and second quadrature modulated input signals for generating said first and second differentiated signals, first and second processing circuits for combining said first differentiated signal with said second digital quadrature signal and combining said second differentiated signal with said first digital quadrature signal to provide first and second combined signals, a second 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 signal, and a digital signal processor for providing the average value of said pulse density and for determining the midpoint of the maximum and minimum signal level representative of said FSK modulated input signals to generate a digital signal that represents said center frequency of said input signals.

57. 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.

58. 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.

59. The system of claim 58 in which said phase lock loop is responsive to responsive to said composite input signal, a predetermined number (N), and a crystal oscillator reference input signal and generates said adjusted local oscillator signal.

60. The system of claim 58 in which said phase lock loop includes a fractional-N frequency synthesizer.

61. The system of claim 58 in which said phase lock loop includes an integer-N frequency synthesizer.

62. The system of claim 58 further including a quadrature receiver responsive to said local oscillator signal and a high frequency FSK modulated input signal for generating said modulated input signal having said frequency error removed.

63. The system of claim 58 further including a demodulator responsive to said modulated input signal.

64. 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.

65. 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.