Thread: Franklin VFO ?
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Old 20th May 2019, 7:08 pm   #51
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: Franklin VFO ?

A good oscillator is a bit of a balancing act. I've spent a lot of years doing VCOs etc for synthesisers, once having designed a five-loop monster in the days before frac-N came along. The Franklin hasn't been one of my favourites and it seems to have fallen by the wayside as mainstream oscillator development bypassed it. But Granitehill seems to have decided that it's the one for him, and anyway, it's possible to do a fairly good job with almost any of the known circuits.

For oscillators in general: There has to be enough loop gain to get the thing to start in the first place, and this gain has to be achieved with only light coupling into the dominant resonator else the Q suffers and then the noise skirts widen. There has to be sufficient non-linearity to throttle back the gain to achieve a stable running amplitude but without the loading on the resonator worsening as the integral of gain over the full cycle backs off.

Achieving high enough levels for semiconductors to go non-linear to give governance of the amplitude without having tight coupling becomes difficult.

Chuck in the uncertainties of JFET parameters and the overall job is quite difficult.

Ulrich Rohde's synthesiser book (all versions... he kept revising it and adjusting the title to mention the then fashionable market areas) are usually an interesting read on types of oscillators. He goes into them with a low phase noise intent. I do get teased a bit about having bought a copy, though...

David
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