[sdiy] 3046 vco help?

[Oren Leavitt]

On 8/18/2011 11:34 AM, Dan Snazelle wrote:
> I have been messing around with the EFM 3046 vco
>
> http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/3046_VCO.pdf
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>
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> i would love to understand how the core circuit works.
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> (minus the square shaper)
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> I see 2 differential amps, a fet, a cap and a pnp....how does it work though?
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> I figure if i could get my head around a simple core like this transistor core, it might make understanding other discrete vcos easier to grasp.
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> Thanks for any help!

Amazing how much discussion this little 8 transistor VCO is generating :-)

On the subject of temperature drift, I'll bet the current sink 
transistor on pins 1-3 self-heats a little at the top end of the VCO's 
frequency range.
You might hear this as a portamento-like settling of pitch if you key 
the oscillator between a low frequency (say 200Hz) and a high frequency 
(say 5kHz).
Could be undesirable, or it could add to the character of that design - 
your call :-)

- Oren