Thread: Franklin VFO ?
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Old 18th May 2019, 11:18 am   #46
G8HQP Dave
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,870
Default Re: Franklin VFO ?

Originally Posted by G6Tanuki
One thing I vaguely remember from the couple of hours of lectures about oscillators I underwent 40 years back (most of which was actually about generating ramps/staircases for video/radar stuff and multi-phase clocks for digital circuitry...) was that the noise-power contribution of an oscillator increases more-slowly than it's intended output as that output power is increased.
Yes, other things being equal, you get better noise performance from a higher power oscillator. Of course, you are also likely to get worse stability performance.

Originally Posted by kalee20
It is also remarkably difficult to find any text book which gives a method of predicting oscillator amplitude!
The amplitude will be such that whatever gain control mechanism is used sets the loop gain to be equal to 1.

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.
The amplitude build-up before you reach the steady-state condition depends on two things:
1. the bandwidth of the loop - which in many cases will be the bandwidth of the resonator
2. the excess gain - which will probably be varying as the amplitude builds up
I am not surprised that nobody could give you a satisfactory answer. A minor change to your circuit would give a different result; it might depend on temperature and supply rail voltage too. If you could produce a good mathematical model of how your circuit loop gain varied with amplitude then you could write down and (hopefully) solve a differential equation giving the time variation of the amplitude. It is essentially a non-linear feedback servo problem.
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