Methods and apparatus for digital frequency generation in atomic frequency standards

• US 5,714,910 A
• Filed: 12/19/1994
• Issued: 02/03/1998
• Est. Priority Date: 12/19/1994
• Status: Expired due to Fees

• Current Assignee: Efratom Time and Frequency Products, Inc.
• Original Assignee: Efratom Time and Frequency Products, Inc.
• Inventors: Skoczen, Gerold L., Pasin, Jaral Del, Cashin, William F. Jr.
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): VU, DAVID HUNG
• Application Number:
  US08/358,832
• Time in Patent Office: 1,142 Days
• Field of Search: 331/3, 331/94.1, 331/2, 331/1 R, 331/1 A, 327/115, 327/113, 327/261
• US Class Current: 331/3
• CPC Class Codes: H03L 7/26 using energy levels of molecules, atoms, or subatomic particles as a frequency reference

Abstract

A physics package interrogation signal can be digitally synthesized for use in an atomic frequency standard with one or more sets of integrally-related sub-multiples of a clock frequency in which one of the integrally-related sub-multiple frequencies is varied to provide frequency modulation between integrally-adjacent sub-multiple frequencies through the use of preloadable counters, the controlled delay and the synchronization of digital pulse trains and operation in plural modes. The method and apparatus can reduce false lock possibilities, optimize the ability of the physics package to remain locked, and reduce phase noise in the output of an atomic frequency standard.

Claims

1. In an atomic frequency standard including a controllable frequency source for generating a source output frequency, a physics package for generating an atomic transition frequency through the excitation of an atomic gas;

means for generating, from the source output frequency, a modulated interrogation frequency;

means for injecting and mixing said modulated interrogation frequency with said atomic transition frequency;

means for generating a signal proportional to the difference between said atomic transition frequency and said modulated interrogation frequency; and

means for controlling said controllable frequency source from said signal and for locking said controllable frequency source to said atom transition frequency, the improvement wherein said means for generating, from the source output frequency, a modulated interrogation frequency comprises;

digital means for synchronously synthesizing a plurality of integrally-related sub-multiple frequencies of said source output frequency and an integrally-related modulation frequency, one of said integrally-related sub-multiple frequencies being frequency-modulated at said integrally-related modulation frequency; and

means for digitally mixing said plurality of integrally-related sub-multiple frequencies, including said frequency-modulated sub-multiple frequency, in synthesizing said modulated interrogation frequency.

2. The improvement of claim 1 wherein at least two of said plurality of integrally-related sub-multiple frequencies are synchronously synthesized by periodically resetting the digital means by which said at least two frequencies are synthesized.

3. The improvement of claim 1 wherein said digital means for synchronously synthesizing a plurality of integrally-related sub-multiple frequencies comprises a plurality of digital counting means, and at least two of said plurality of integrally-related sub-multiple frequencies are synchronously synthesized and periodically reset at a rate which is a multiple or sub-multiple of the modulation frequency.

4. The improvement of claim 1 wherein said digital means for synchronously synthesizing a plurality of integrally-related sub-multiple frequencies comprises common digital means used in the synthesis of at least two of said plurality of integrally-related sub-multiple frequencies.

5. The improvement of claim 1 wherein said means for digitally mixing said plurality of integrally-related sub-multiple frequencies includes a plurality of exclusive OR gates, and

wherein said plurality of integrally-related sub-multiple frequencies are mixed by said plurality of exclusive OR gates in synthesizing said modulated interrogation frequency, and at least two of said plurality of integrally-related sub-multiple frequencies are phased at the inputs of said plurality of exclusive OR gates to offset the transition times of said plurality of sub-multiple frequencies at said inputs.

6. The improvement of claim 1 wherein said frequency-modulated one of said integrally-related sub-multiples of said source output frequency is digitally varied on alternating half cycles of said modulation frequency to provide two integrally-adjacent sub-multiples of said source output frequency on alternating half cycles of said modulation frequency and to generate said frequency-modulated one of said integrally-adjacent sub-multiples being generated by a division factor d+E, where d is a whole number, and E equals a whole number h times ±

1/2, and said digital variation is effected by alternating E between a positive and a negative number on alternating half cycles of said modulation frequency.

7. The improvement of claim 6 wherein said frequency-modulated one of said integrally-related sub-multiple frequencies is further divided and further digitally varied on alternating half cycles of the modulation frequency in generating said modulation frequency, said digital variations being synchronized so that said source output frequency is divided by division factors of {d×

(d+E)} during each alternating half cycle of said modulation frequency to generate frequency modulation with a 50 percent duty cycle.

8. The improvement of claim 6 wherein said division factors generated by d+E, where E alternates between a positive and a negative number, provides a frequency-modulated lowest sub-multiple frequency with a maximum frequency variation approximately the bandwidth of the atomic transition frequency.

9. The improvement of claim 6 wherein said frequency-modulated one of said integrally-related sub-multiple frequencies is further divided by a division factor d_m sufficiently high to provide a modulation frequency whose departure from 50 percent duty cycle is small enough that frequency harmonics generated by its departure from a 50 percent duty cycle do not unacceptably affect the means for controlling the controllable frequency source and locking said source output frequency to said atomic transition frequency.

10. The improvement of claim 9 wherein said frequency-modulated one of said plurality of integrally-related sub-multiple frequencies is synchronized with another of the integrally-related sub-multiple frequencies with which it is digitally mixed.

11. The improvement of claim 1 wherein said source output frequency (f_CLK) is divided by a first counter with a division factor d₁ to provide an integrally-related sub-multiple frequency (f₁), and is also divided by a second counter with a division factor d₂ to provide an integrally-related sub-multiple frequency (f₂), and is also divided by a third counter with a division factor d₃ to provide an integrally-related sub-multiple frequency (f₃).

12. The improvement of claim 11 wherein d₁ <

d₂ ≦

d₃ ;

d₁, d₂, and d₃ are whole numbers, and d₂ /d₁ =2^e, d₃ /d₁ =2^f and d₃ /d₂ =2^g, where e, f and g are whole numbers and g can equal 0, and wherein Y (f₁ ±

f₂ ±

f₃)+Z f_CLK equals the atomic transition frequency, Y and Z being whole numbers.

13. The improvement of claim 12 wherein at least one of f₁, f₂ and f₃ is frequency-modulated with a 50 percent duty cycle.

14. The improvement of claim 12 wherein the crystal false lock frequency is greater than the frequency capture range of the control of the atomic frequency standard.

15. The improvement of claim 12 wherein said digital means of synchronously synthesizing a plurality of integrally-related sub-multiple frequencies of said source output frequency and an integrally-related modulation frequency operates in a non-lock mode providing a false oscillator lock frequency outside of the frequency adjustment range of the controllable frequency source and a lock mode optimizing the frequency modulation depth for the physics package.

16. The improvement of claim 15 wherein said second and third counters are preloadable counters, and are preloaded with a first set numbers for operation in the non-lock mode, and a second set of numbers for operation when said controllable frequency source is locked to said atomic transition frequency.

17. The improvement of claim 16

wherein f₁ equals 5 MHz, and

wherein during the non-lock mode the second counter is preloaded with a number providing f₂ equal to 156.25 kHz and the third counter is alternatingly preloaded with two different numbers in alternating half cycles of the modulation frequency to change f₃ between about 156.862 kHz and about 155.642 kHz in alternating half cycles of the modulation frequency, and

wherein during the lock mode the second counter is preloaded with a number providing f₂ equal to 250.000 kHz and the third counter is alternatingly preloaded with two different numbers in alternating half cycles of the modulation frequency to change f₃ between about 62.893 kHz and about 62.111 kHz in alternating half cycles of the modulation frequency.

18. The improvement of claim 11 wherein said third counter comprises said digital means used for synthesis of said one of said integrally-related sub-multiple frequencies being frequency-modulated and said integrally-related modulation frequency.

19. The improvement of claim 11 wherein said source output frequency is delayed by one-half cycle prior to each division cycle of said third counter to provide a third counter fractional division factor d₃ +1/2.

20. The improvement of claim 1 wherein said digital means for synthesizing, from said source output frequency, a digitally-formatted interrogation frequency comprises a first counter with a division factor for dividing said source output frequency to provide a highest sub-multiple frequency output, a second counter with a division factor for dividing said source output frequency to provide an intermediate sub-multiple frequency output, a third counter with a division factor for dividing said source output frequency to provide a lowest sub-multiple frequency output, delay means for at least two of said sub-multiple frequency outputs of said counters, a first exclusive OR mixer for mixing of two sub-multiple frequency counter outputs and providing a first mixed sub-multiple frequency output, at least one of said two sub-multiple frequency counter outputs being delayed, and a second exclusive OR mixer for mixing said first mixed sub-multiple frequency output with the remaining sub-multiple frequency counter output, said remaining sub-multiple frequency counter output being delayed.

21. The improvement of claim 20 wherein said digital means further comprises a half cycle delay generator for delaying said source output frequency by one-half cycle of said source output frequency and generating a half cycle delayed source output frequency, said half cycle delayed source output frequency being connected with the input of said third counter, said third counter operating to divide said half cycle delayed source output frequency during each half cycle of the lowest sub-multiple frequency.

22. The improvement of claim 21 wherein said third counter is a preloadable N counter, N being the maximum counter value, and is preloaded with different numbers less than N on alternating half cycles of said modulation frequency to provide differing sub-multiples d₃ '"'"' and d₃ " of said source frequency output and to generate frequency modulation of said lowest of said integrally-related sub-multiple frequencies.

23. The improvement of claim 22 wherein said third counter is preloaded with integrally-adjacent numbers M and M+1 on alternating half cycles of said modulation frequency and provides a frequency-modulated lowest sub-multiple of said source output frequency, said frequency-modulated lowest sub-multiple of said source output frequency comprising said source output frequency divided by division factors where d₃ '"'"' equals (N-M+1/2) and d₃ " equals (N-M-1/2) on alternating half cycles of said modulation frequency.

24. The improvement of claim 23 wherein the frequency-modulated lowest sub-multiple of said source output frequency is further divided by 2.

25. The improvement of claim 23 wherein said frequency-modulated lowest sub-multiple of said source output is further divided by a fourth preloadable N counter, N being the maximum counter value, said fourth preloadable N counter being preloaded with the number M on the modulation frequency half cycles during which the third counter is loaded with M+1, and preloaded with the number M+1 on the modulation frequency half cycles during which the third counter is loaded with M, the output of said fourth preloadable N counter comprising a digitally-formatted frequency with a 50 percent duty cycle.

26. The improvement of claim 25 wherein said digitally-formatted frequency with a 50 percent duty cycle is applied to a fifth counter and further divided to provide said modulation frequency.

27. The improvement of claim 25 wherein said source output frequency is 20 MHz and said digital means further includes a divide by two counter at the output of said third counter and a divide by eight counter at the output of said fourth counter, and said division factors d₁, d₂, d₃ '"'"' and d₃ " are respectively 4, 96, 191 and 193 respectively.

28. The improvement of claim 23 wherein said frequency-modulated lowest sub-multiple of said source output frequency is further divided by a high count counter providing a division factor d_m sufficiently high that the digitally-formatted modulation frequency output of the high count counter departs from a 50 percent duty cycle so slightly that said means for controlling said frequency source can operate acceptably.

29. The improvement of claim 28 wherein said source output frequency is 20 MHz and said division factors d₁, d₂, d₃ '"'"' and d₃ " are, respectively, 4, 96, 191 and 193, and the division factor d_m of said high count counter is 768.

30. The improvement of claim 22 wherein said source output frequency is 20 MHz and said division factors d₁, d₂, d₃ '"'"' and d₃ " are 4, 96, 191 and 193, respectively.

31. In an atomic frequency standard including a controllable frequency source for generating a source output frequency, a physics package for generating an atomic transition frequency through the excitation of an atomic gas;

means for generating, from the source output frequency, a modulated interrogation frequency;

means for injecting and mixing said modulated interrogation frequency with said atomic transition frequency;

means for generating a signal proportional to the difference between said atomic transition frequency and said modulated interrogation frequency; and

means for controlling said controllable frequency source from said signal and for locking said controllable frequency source to said atom transition frequency, the improvement wherein said means for synthesizing, from the source output frequency, a modulated interrogation frequency comprises;

digital means for synthesizing a plurality of integrally-related sub-multiple frequencies of said source output frequency and an integrally-related modulation frequency, one of said integrally-related sub-multiple frequencies being frequency-modulated at said integrally-related modulation frequency; and

means for digitally mixing said plurality of integrally-related sub-multiple frequencies, including said frequency-modulated sub-multiple frequency, in synthesizing said modulated interrogation frequency, said digital means for synthesizing, from said source output frequency, said plurality of integrally-related sub-multiple frequencies and said integrally-related modulation frequency operating in a non-lock mode that provides a false lock frequency outside of the frequency capture range of the means for controlling said controllable frequency source and in a lock mode that provides frequency modulation depth approximately the atomic bandwidth of the physics package.

32. The improvement of claim 31 wherein said digital means for synthesizing, from said source output frequency, said plurality of integrally-related sub-multiple frequencies of said source output frequency and said integrally-related modulation frequency comprises a plurality of recycling digital counters for providing, from said source output frequency, said plurality of integrally-related sub-multiple frequencies, one or more of said recycling digital counters being preloadable and preloaded with a first set of numbers for operation in the non-lock mode and with a second set of numbers for operation in the lock mode.

33. The improvement of claim 32 wherein at least one preloadable counter is preloaded with two different numbers on alternating half cycles of the modulation frequency to change said one frequency-modulated frequency of said plurality of integrally-related sub-multiple frequencies during alternating half cycles of the modulation frequency.

34. A method of synthesizing a frequency-modulated frequency from an output frequency f_CLK of a controllable frequency source of an atomic frequency standard for interrogating its physics package and locking said controllable frequency source to an atomic resonance frequency, comprising

synthesizing said frequency-modulated interrogation frequency from a plurality of integrally-related frequencies derived from said output frequency,

providing a first set of plural, integrally-related frequencies for operating said atomic frequency standard in a first mode, and

providing a second set of plural, integrally-related frequencies for operating said atomic frequency standard in a second mode,

said first set of plural, integrally-related frequencies being provided when said controllable frequency source is not locked to said atomic resonance frequency, and said second set of plural, integrally-related frequencies being provided when said controllable frequency is locked to said atomic resonance frequency.

35. The method of claim 34 wherein said first set of plural, integrally-related frequencies provides operation of said atomic frequency standard without generating a false lock frequency within the frequency adjustment range of the controllable frequency source, and said second set of plural, integrally-related frequencies provides operation of said atomic frequency standard with a frequency modulation depth selected for good performance of said physics package.

36. The method of claim 35 wherein said second set of plural, integrally-related frequencies provides a frequency modulation depth approximating the maximum gain signal amplitude characteristic of the physics package.

37. The method of claim 34

wherein said first and second sets of plural, integrally-related frequencies each include at least three integrally-related sub-multiple frequencies derived from said output frequency f_CLK of said controllable frequency source, said first frequency f₁ of each said set being derived by dividing the output frequency f_CLK by a factor d₁, said second frequency f₂ of each said set being derived by dividing said output frequency f_CLK by a factor d₂, and said third frequency f₃ of each said set being derived by dividing said output frequency f_CLK by a factor d₃ ;

wherein, for each said set, division factor d₁ is less than division factor d₂, which is less than or equal to division factor d₃, where d₁, d₂ and d₃ are whole numbers, and d₂ ÷

d₁ is equal to 2^e, d₃ ÷

d₁ is equal to 2^f, and d₃ ÷

d₂ is equal to 2^g, e and f being whole numbers and g being 0 or a whole number, and

wherein, for each said set, Y times the sum of f₁, f₂ and f₃ plus Z times the output frequency f_CLK equals the atomic transition frequency, Y and Z being whole numbers.

38. The method of claim 37 wherein at least one of the frequencies f₂ and f₃ of each said set is frequency-modulated with a 50 percent duty cycle.

39. The method of claim 37 wherein f₃ of said first set of plural integrally-related frequencies provides a false lock frequency greater than the frequency capture range of the controllable frequency device.

40. The method of claim 37 wherein at least one of the frequencies f₁, f₂ and f₃ of said second set of plural integrally-related frequencies is frequency-modulated, and the modulation depth of said one frequency is in the range of about 600 to about 2000 Hz, peak to peak.

41. The method of claim 40 wherein the modulation depth of said one frequency is about 800 Hz, peak to peak.

42. The method of claim 40 wherein said at least one of the frequencies f₁, f₂ and f₃ is derived by dividing the output frequency f_CLK by one of the division factors d₁, d₂ or d₃ plus a number E, where E equals a whole number h times ±

1/2 and where E alternates between a positive and a negative number on alternate half cycles of said modulation frequency to provide frequency modulation of said one frequency.

43. The method of claim 42 wherein f₃ of said first set of plural, integrally-related frequencies provides a false lock frequency greater than the frequency capture range of the controllable frequency source.

44. The method of claim 43

wherein the atomic resonance frequency is for rubidium;

the output frequency f_CLK equals 20 MHz and f₁ of said first and second sets of plural integrally-related sub-multiple frequencies equals 5 MHz;

wherein f₂ and f₃ of said first set of said plurality of integrally-related sub-multiple frequencies is 156.250 kHz, f₃ being frequency-modulated between about 156.862 kHz and about 155.642 kHz; and

where f₂ of said second set of plural integrally-related sub-multiple frequencies is 250.000 kHz and f₃ of said second set of plural integrally-related sub-multiple frequencies is about 62.500 kHz frequency-modulated between about 62.893 of kHz and about 62.111 kHz.

45. A method of synthesizing a frequency-modulated frequency from a clock frequency derived from an output of a controllable frequency source of an atomic frequency standard for interrogating its physics package and locking said controllable frequency source to an atomic resonance frequency, comprising

selecting a plurality of frequencies from a set of integrally-related frequencies,

each of said selected integrally-related frequencies of said set being a sub-multiple of said clock frequency (f_CLK) including a first frequency (f₁) equal to the clock frequency divided by a division factor d₁, a second frequency (f₂) equal to the clock frequency divided by a division factor d₂, and a third frequency (f₃) divided by a division factor d₃,

wherein d₁ <

d₂ ≦

d₃, and d₁, d₂ and d₃ are whole numbers, wherein d₂ /d₁ =2^e, d₃ /d₁ =2^f and d₃ /d₂ =2^g, and e and f are whole numbers and g is zero or a whole number, and wherein Y (f₁ ±

f₂ ±

f₃)+z(f_CLK)=the atomic resonance frequency, Y and Z being whole numbers, and wherein one of said second and third frequencies is selected for frequency modulation and is frequency-modulated by dividing said clock frequency f_CLK by a division factor that is alternatingly incremented with a number E, where E equals a number h times ±

1/2, the resulting positive and negative E being the numbers by which the frequency modulation division factor is alternatingly incremented, h being any whole number except 2; and

synthesizing said frequency-modulated frequency from said plurality of selected integrally-related frequencies.

46. In an atomic frequency standard in which an interrogation frequency for a physics package is generated from a plurality of integrally-related sub-multiple frequencies derived from an output frequency of the atomic frequency standard, the improvement comprising digital means for synchronously synthesizing at least two of said plurality of integrally-related sub-multiple frequencies, including a frequency-modulated sub-multiple frequency and a modulation frequency of the frequency-modulated frequency.

47. The improvement of claim 46 wherein at least two of said plurality of integrally-related sub-multiple frequencies are synchronously synthesized and periodically reset to obtain and maintain a phase relationship between the digital means by which said at least two frequencies are synthesized.

48. The improvement of claim 46 wherein said plurality of integrally-related sub-multiple frequencies are synthesized by a plurality of digital counting means, and at least two of said plurality of integrally-related sub-multiple frequencies are synchronously synthesized by periodically resetting the digital counting means to obtain and maintain a phase relationship between the digital counting means by which at least two frequencies are synthesized.

49. The improvement of claim 48 wherein at least two of said plurality of counting means are periodically reset to obtain and maintain a phase relationship between the digital counting means at a rate equal to the modulation frequency or a multiple or sub-multiple of the modulation frequency.

50. The improvement of claim 49 wherein said at least two counting means are periodically reset by a reset pulse generator triggered at the modulation frequency.

51. The improvement of claim 46 wherein at least two of said plurality of integrally-related sub-multiple frequencies are synchronously synthesized by common digital means used in their synthesis.

52. The improvement of claim 46 wherein said means for digitally mixing said plurality of integrally-related sub-multiple frequencies includes a plurality of exclusive OR gates, and

wherein said plurality of integrally-related sub-multiple frequencies are mixed by said plurality of exclusive OR gates in synthesizing said modulated interrogation frequency, and at least two of said plurality of integrally-related sub-multiple frequencies are phased at the inputs of said plurality of exclusive OR gates to offset the transition times of said plurality of sub-multiple frequencies at said inputs.

53. The improvement of claim 52 wherein said plurality of integrally-related sub-multiple frequencies are phased at the inputs of the plurality of exclusive OR gates by delay means at one input of each of said plurality of exclusive OR gates.

54. In a method of generating an interrogation frequency for a physics of an atomic frequency standard from a plurality of integrally-related sub-multiple frequencies derived from an output frequency of the atomic frequency standard, the improvement comprising synchronously synthesizing at least two of the plurality of frequencies, including a frequency-modulated frequency and a modulation frequency of the frequency-modulated frequency.

55. The improvement of claim 54 wherein said at least two frequencies are synthesized by dividing the output frequency with at least two independent counting means, and said at least two frequencies are synthesized synchronously by periodically resetting both said two independent counting means for synchronization with said output frequency.

56. The improvement of claim 55 wherein said two independent counting means are periodically reset at the modulation frequency or a multiple or sub-multiple of the modulation frequency.

57. The improvement of claim 55 wherein said two independent counting means are reset at the modulation frequency synthesized from said output frequency.

58. The improvement of claim 54 wherein said at least two frequencies are synthesized with common digital means.

59. The improvement of claim 58 wherein said at least two frequencies are synthesized by dividing said output frequency using said common digital means.