Digital frequency generation in atomic frequency standards using digital phase shifting

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 submultiple frequencies of said source output frequency and an integrally-related modulation frequency, one of said integrally-related submultiple frequencies being frequency-modulated by the addition and subtraction of pulses at said integrally-related modulation frequency, and means for digitally mixing said plurality of integrally-related submultiple frequencies, including said frequency-modulated submultiple frequency, in synthesizing said modulated interrogation frequency.

In a method of digitally synthesizing a frequency-modulated microwave signal for exciting a physics package of an atomic frequency package, the improvement comprising the steps of:

• generating from said clock frequency a submultiple frequency with a period T;
• generating from said submultiple frequency N variously delayed signals, each of said N signals comprising the submultiple frequency delayed by a time T/N from each of the other N signals;
• providing each of said N variously delayed signals to a multiplexer;
• generating a multiplexer operating signal by generating a phase change frequency (DRC) pulse train signal by dividing the clock signal by a substantial number, and generating a frequency modulation signal by dividing the phase change frequency (DRC) pulse train signal by a further substantial number, and combining the phase change frequency (DRC) pulse train signal and the frequency modulation signal for operation of said multiplexer to provide an output frequency with one or more periods of T between the transition edges of the phase change frequency (DRC) pulse train signal and with one period of (T+T/N) at each edge of each pulse of the phase change frequency (DRC) pulse train signal during one half-cycle of the frequency modulation signal, and for operation said multiplexer or provide an output frequency with one or more periods of T between the transition edges of the phase change frequency (DRC) pulse train signal and with one period of (T-T/N) at each edge of each pulse of the phase change frequency (DRC) pulse train signal during the other half-cycle of the frequency modulation signal.

In an atomic frequency standard including means for generating from a clock frequency, a digital frequency-modulated microwave signal for application to a physics package the improvement comprising

• means for generating, from said clock frequency, a microwave submultiple frequency with a period T and for generating from said microwave submultiple frequency a plurality of N pulse trains at said submultiple frequency, said pulse trains being delayed from each other by a period T/N;
• and
• means for generating, from said plurality of N delayed pulse trains, a frequency-modulated digital signal comprising
• means for combining said plurality of delayed pulse trains, said combining means being operated by a modulation frequency and a phase change frequency (DRC) pulse train, said phase change frequency (DRC) pulse train operating said combining means to switch its output among the plurality delayed pulse trains to provide a pulse train output with an instantaneous frequency of 1/T for one or more periods between the transition edges of the phase change frequency (DRC) pulse train and with an instantaneous frequency of 1/(T+T/N) for one period at each pulse edge of the phase change frequency (DRC) pulse train during one half-cycle of the modulation frequency signal, and for operation of said multiplexer to provide an output instantaneous frequency of 1/T for one or more periods between the transition edges of the phase change frequency (DRC) pulse train and with an instantaneous frequency of 1/(T-T/N) for one period at each pulse edge of the phase change frequency (DRC) pulse train during the other half-cycle of the modulation frequency.

A method of controlling a frequency-modulated interrogation signal in an atomic frequency standard, comprising:

• generating, from a clock frequency of the atomic frequency standard, a frequency for modulation of the interrogation signal and a phase change frequency;
• using the phase change frequency to provide a different rate of phase change of the interrogation signal in alternate half-cycles of the modulation frequency;
• and
• controlling the rate of phase change in modulating the interrogation signal by varying the phase change frequency.