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It is just an amplifier gone wrong

Pointers on crystal oscillator design for digital engineers. By Barry Priestley.

Although not all engineers are dismissive of oscillator design, it is certainly true that the subject was poorly covered when electronics was all analogue and is further squeezed by digital techniques. This leads to trouble when a real oscillator has to be produced in a hurry.

Simple theory

The basis of continuous wave oscillator design is an amplifier with a selective feedback network coupled to it loosely. If the loop gain – that is, the amplifier gain less the coupling loss – exceeds unity, then noise impulses will start sinewaves at the resonant frequency. These will increase exponentially in amplitude until limiting reduces the loop gain. Notwithstanding what some books might indicate, the noise does NOT disappear once sinewave amplitude has built up; rather, it causes phase jitter compared with the ideal sinewave. Whilst this is usually at too low a level to be visible as time jitter, it may still be important if the oscillator provides an a/d conversion clock or is also used for rf circuits.

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Graham Pitcher

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