[sdiy] Help understanding Serge 73 Oscillator

[Guy McCusker]

Many thanks!

Google translate does a good job of translating your very helpful page
for English-readers who are not also Japanese-readers.

Guy.




On Wed, Feb 21, 2018 at 2:47 PM,  <houshu at muj.biglobe.ne.jp> wrote:
> Hello Guy,
>
> Japanese explanation is available at:
> http://houshu.at.webry.info/200710/article_1.html
>
> I don't want to translate the whole article.  Sorry.
>
> Input current mirror of U1b has exponential response.
> Negative input terminal of U1a drifts with temperature,
> which compensates for the drift of the exponential response.
>
> Sam HOSHUYAMA
> Saitama, JAPAN.
>
>> 2018/02/21 6:27 Guy McCusker <guy.mccusker at gmail.com> wrote:
>>
>>
>> Hello list!
>>
>> I'm having a hard time figuring out how the exponential response is
>> generated in the Serge 73 oscillator design. The schematic is here:
>> http://electro-music.com/wiki/pmwiki.php?n=Schematics.OldSergeVCOUsingLM3900
>>
>> Like a lot of Serge designs there is heavy use of LM3900s which I do
>> not understand properly. It seems to me that the first stage (U1a) is
>> a more or less regular summing stage, the second (U1b) is an
>> integrator, and the third (U1c) is a comparator to reset the ramp via
>> the transistor across the integrating cap. So far so straightforward.
>>
>> But how (if at all) is an exponential CV response generated? The notes
>> on the schematic hint that somehow the transistors within the LM3900s
>> are being pressed into service as an expo converter but I can't wrap
>> my tiny brain around it.
>>
>> Can anyone help please?
>>
>> Cheers,
>> Guy.
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