Microprocessor controlled synchronisation loop for VCO - uses HF signal cycle count to measure phase errors between VCO and references passing error changes

摘要

The servo loop uses a microprocessor which is interrupt driven via an interrupt controller by a set of comparators, one for each external reference. A control signal is calculated by the processor and applied through a digital to analogue converter to a voltage controlled local oscillator. The load oscillator, in the region of 2048 MHz, and the reference signals are divided down to around 8 KHz and their phase differences, for each reference, measured by counting cycles of a 20480 MHz measurement oscillator between leading edges of the divided local oscillator and reference signals. Successive counts are compared and changes reported by interrupt to the microprocessor. An internal variable is then either incremented or decremented and if it reaches a preset level, the control voltage is altered by one step.