Quote:
Originally Posted by Radio Wrangler
The amplitude governing method in an oscillator relies on either driving an active device into cut-off, or into significant compression. It is remarkably difficult to make an oscillator which runs at low levels - especially one which can be relied on to start.
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It is also remarkably difficult to find any text book which gives a method of predicting oscillator amplitude! There is a lot of info out there about calculating frequency and what affects it - but try finding anything about calculating amplitude or how to design for a given amplitude and you'll find zilch. F Langford-Smith says the most when he mentions 'the necessary experimental work.' And that's it!
Years ago I built a Wien-bridge oscillator at 50Hz. I also needed to ensure that oscillation had established before something else (a high voltage power supply) was enabled, so I put a time delay to inhibit the power supply. But how long for? I couldn't calculate how long the Wien bridge oscillator would take to build up from noise. I sent a letter to Wireless World (no UKVRR then!) and it seemed nobody else could either. So it came to experimenting with a slow-time base 'scope.
I'm not an oscillator expert, but I am familiar in principle with the Franklin circuit, basically it just has one connection (plus ground) to the tuned circuit. The overall non-inverting amplifier with feedback from its output to its input just works as a negative impedance in parallel with the tuned circuit. So for a good, high-Q LC circuit the coupling thereto can, as stated, be extremely loose.
Isolating the active devices as much as possible from the LC circuit means that any variation in said devices will have correspondingly little effect. So stability, phase noise, harmonic content etc can be expected to be really low. The down side as I see it is that trying to couple a load to the tuned circuit will immediately degrade things. Taking the output instead from the middle of the amplifier will add the amplifier noise to the output - long-term frequency stability will be as good as the tuned circuit (obviously) but short-term phase noise will exist. How much, again, probably comes down to experiment, just as RW suggests!