A frequency synthesiser locks a voltage-controlled oscillator (VCO) to a crystal…
PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Feedback, operational amplifiers and analogue signal processing
Electronics for Measurement, Ch Active filters, oscillators and timers
Problem
A frequency synthesiser locks a voltage-controlled oscillator (VCO) to a 25kHz crystal reference through a phase-locked loop with a divide-by-480 in the feedback path and a loop bandwidth of 200Hz. The crystal reference is specified to ±1 part per million. The VCO, run open-loop (unlocked), drifts by about ±3kHz over the temperature range the instrument experiences, but this drift happens slowly, over minutes, as the enclosure warms.
((a)) Estimate the loop's locked output frequency.
((b)) For frequency wander occurring slowly, at rates below the 200Hz loop bandwidth, estimate the output frequency's wander in hertz, and say which part of the loop is responsible for it.
((c)) For fluctuations occurring faster than 200Hz, say which part of the loop sets the output's wander, and explain in one sentence why the loop cannot suppress it.