Seamless switch-over frequency synthesizer

A timekeeping system adapted to utilize at least two atomic frequency standard inputs and produce a clock output that tracks the phase of one of the atomic frequency standard inputs and which exhibits seamless switch-over upon a transfer in atomic frequency standard inputs, the timekeeping system comprising:

• an atomic frequency standard selector switch adapted for receiving as inputs a plurality of atomic frequency standard inputs and providing an output of at least two atomic frequency standards, one of which is a primary atomic frequency standard input;
• a phase comparison subsystem coupled to the selector switch and adapted for computing the phase difference between the primary atomic frequency standard input signal and a secondary atomic frequency standard input signal and for computing the phase difference between the primary atomic frequency standard input and a timekeeping system output and for estimating the phase error and frequency offset of the atomic frequency standard inputs;
• a digital phase lock loop subsystem coupled to the phase comparison subsystem for phase locking the timekeeping system output to the primary atomic frequency standard input based on the computed phase differences and the estimated phase errors and frequency offsets;
• a digital to analog converter coupled to the digital phase lock loop subsystem for converting a digital phase loop output to a corresponding analog control signal;
• and
• a voltage controlled crystal oscillator coupled to the digital to analog converter, the voltage controlled crystal oscillator adapted to receive the analog control signal and provide the timekeeping system clock output in response thereto.

In a timekeeping system, a method of switching from a primary atomic frequency standard input to a secondary atomic frequency standard input without affecting the phase and frequency of the timekeeping system output, the method comprising the steps of:

• (a) measuring the phase difference between the primary and secondary atomic frequency standard inputs;
• (b) estimating a phase offset and a frequency offset between the primary atomic frequency standard input and the secondary atomic frequency standard input;
• (c) phase locking the timekeeping system output to the primary atomic frequency standard input based on a measured phase offset and a measured frequency offset between the timekeeping system output and the primary atomic frequency standard input prior to switching from a primary atomic frequency standard input to a secondary atomic frequency standard input;
• and
• (d) phase locking the timekeeping system output to the primary atomic frequency standard input based in part on an estimated phase offset and estimated frequency offset between the primary atomic frequency standard input and secondary atomic frequency standard input after switching from a primary atomic frequency standard input to a secondary atomic frequency standard input.