A quartz standard automatic frequency adjustment system

A system is presented that automatically adjusts the frequency of a quartz standard unit to the frequency of a reference unit. The quartz standard automatic frequency adjustment system (AFAS) has automatic adjustment which starts as soon as the signal to be frequency adjusted appears. The adjustment value is held between two successive adjustment procedures with minimum signal degradation. The adjustment is made through a change of the varicap voltage. A phase-locked loop with controlled self-oscillation is used. AFAS includes a phasemeter comparing phases between the reference signal and the signal to be frequency adjusted. The output of the phasemeter is shaped so as to trigger a specific numerical block which works as a without-zero integrator. The without-zero integrator also ensures the storage of adjustment as soon as frequencies are close enough. The end of the frequency adjustment procedure is determined by use of self-oscillation counting. Among other results, it is found that at 5 MHz the frequency difference range is Δf/f=2×10^-6, and the residual frequency difference after adjustment is 4×10^-12