A predetermined bias voltage is applied to the input of a summing integrator in a frequency lock loop to provide fine tuning control of a voltage controlled crystal oscillator. In this arrangement, the predetermined bias voltage is summed and integrated with a phase error signal to alter the output frequency of the voltage controlled crystal oscillator. Since the servo loop operates to null the net voltage at the input of the integrator, the circuit induces a frequency error which just compensates for the intentional voltage offset bias applied to the integrator. The offset bias arrangement produces incremental offsets within a total range of about three parts in ten to the eleventh power and permits the output frequency of an atomic frequency standard to be accurately and stably adjusted in small increments to agree closely with the frequency of a national standard or other system reference.
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1. A frequency adjustable stabilized frequency standard comprising an atomic frequency resonator for providing a signal at a frequency corresponding to a quantum mechanical transition resonance frequency, a variable frequency oscillator for generating a frequency standard signal at a first selected frequency, a low frequency oscillator whose frequency of oscillation is relatively low compared to the output frequency of the variable frequency oscillator, a phase modulator coupled to receive the variable frequency oscillator signal and the low frequency oscillator signal and operable to generate a phase modulated output signal, frequency multiplier means coupled to receive said modulated output signal and to generate therefrom a hyperfine transition inducing signal whose frequency is a multiple of the output frequency of said variable frequency oscillator, means for comparing the phase of the output signal produced by the atomic frequency resonator in response to the application of said hyperfine transition inducing signal with the phase of said low frequency oscillator signal to generate an error signal representative of their phase difference, a summing integrator for producing in response to said error signal a smoothed correction signal, said integrator having an input and an output, the input of said integrator being coupled to the phase comparing means for receiving said phase error signal, and the output of said integrator coupling the smoothed correction signal to the input of said variable frequency oscillator, and bias means coupled to the input of said summing integrator for offsetting said error signal with a fixed bias signal.
2. In a stabilized frequency standard of the type including an atomic frequency resonator for providing a signal at a frequency corresponding to a quantum mechanical transition resonance frequency, a variable frequency oscillator for generating a frequency standard signal at a selected frequency, a low frequency oscillator for generating a relatively low frequency modulation signal, a phase modulator for phase modulating the variable frequency oscillator output signal with said relatively low frequency modulation signal, frequency transforming means coupled to receive said phase modulated signal for producing a hyperfine transition inducing signal for application to said atomic frequency resonator, phase detector means coupled to the output signals of said atomic resonator and said low frequency oscillator for detecting a difference in phase between the two signals and for producing an error signal proportional thereto, and including a summing integrator coupled to receive said error signal and producing in response thereto a smoothed correction signal for altering the output frequency of the variable frequency oscillator, the improvement comprising bias means coupled to the input of said summing integrator for offsetting said error signal with a fixed bias signal.
3. A method for providing fine tuning control of an output signal produced by a voltage controlled oscillator of the type biased by reference to the output signal of a primary frequency standard, whereby the frequency of the voltage controlled oscillator can be adjusted to agree with a standard system reference, comprising the steps of modulating the output of the voltage controlled oscillator by a low frequency reference signal; synthesizing the phase modulated oscillator output signal to produce a transition inducing signal; applying the transition inducing signal to the input of a primary resonator; comparing the phase of the output signal produced by the primary resonator with the phase of the low frequency reference signal; deriving an error signal which is proportional to said phase difference; summing said error signal with a bias signal; integrating the sum of the error signal and bias signal to produce a correction signal; and, applying the correction signal to the input of the voltage controlled oscillator.