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Old 18th Jun 2021, 7:05 am   #2
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 18,197
Default Re: Levell TG200 Series Oscillators

Originally Posted by Chris55000 View Post
For oscillation to be sustained, this means the gain from J1/J2 and J3/J4) has to exceed the negative feedback fraction given by the 3k9 and 1k02 resistors in series from J10 emitter back to the frequency range switches!
In addition to this, there needs to be some mechanism to precisely hold the loop gain at unity, servo-controlling the amplitude in order to get a stable, fixed amplitude. Fixed gain will not do. A whisker too much and the amplitude increases until the output smashes int the limits imposed by the power supply and bias arrangements. A whisker too little and the oscillation decays and fades into the noise.

RC oscillators sometimes use thermistors to stabilise their amplitudes, sometimes detector diodes controlling an FET as a variable resistor. Some crude ones might let a valve or transistor drive itself into cut-off over a variable fraction of the cycle, but this introduces a lot of distortion. It's a common trick at RF where the tank circuit's stored energy acts as a flywheel/filter to give an adequate waveform.

So when 'reading' an oscillator circuit, the frequency control, amplification, and overall phase shift get covered first, but don't forget to look for the amplitude stabilisation mechanism. It isn't well covered in the literature, but it is still essential.

The uni labs had plenty of Levell oscillators and meters, but I don't remember ever looking at their manuals in the manuals rack. I was too interested in the Wavetek stuff.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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