Methods and apparatus for digital frequency generation in atomic frequency standards
AssigneeEfratom Time and Frequency Products, Inc.
InventorsGerold L. Skoczen, William F. Cashin, Jr., Jaral Del Pasin
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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.
Claims59
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.
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.
A method of synthesizing a frequency-modulated frequency from an output frequency f.sub.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.
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.sub.CLK) including a first frequency (f.sub.1) equal to the clock frequency divided by a division factor d.sub.1, a second frequency (f.sub.2) equal to the clock frequency divided by a division factor d.sub.2, and a third frequency (f.sub.3) divided by a division factor d.sub.3,
- wherein d.sub.1 <d.sub.2 .ltoreq.d.sub.3, and d.sub.1, d.sub.2 and d.sub.3 are whole numbers, wherein d.sub.2 /d.sub.1 =2.sup.e, d.sub.3 /d.sub.1 =2.sup.f and d.sub.3 /d.sub.2 =2.sup.g, and e and f are whole numbers and g is zero or a whole number, and wherein Y (f.sub.1 .+-.f.sub.2 .+-.f.sub.3)+z(f.sub.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.sub.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.
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.
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.
Dependencies are inferred from the claim text.