[sdiy] Analysis of Buchla 259 principal oscillator...

[Aaron Lanterman]

On Tue, 14 Feb 2006, mark verbos wrote:

>> P.S. I thought the 20.7K might be 26.7 or something, as it's very smudgy - 
>> but if you use that, you get, vtri = -2.8 for v- = 0, vtri = 3.1313 for v- 
>> = 5, and vtri = 2.2285 for v- = 4.29, again not really centered)
>> 
> are you looking at R154? because that is 28.7k. That should pull the signal 
> down.

Bingo! Thanks!

> Incidentally, I don't think the resistors at the integrator really 
> determine where the TRI wave is centered, the thresholds of the 
> comparator do that. If you were to build the integrator all alone, then 
> you may at some point feed it no input, THEN the + input would determine 
> where it settles. I don't see how that matters in this case. Am I wrong?

Let's say the current being pulled "into" the OTA (equivalently out of the 
cap) is i. Then

C s (vo - 7.5) = icur

          icur(s)
vo(s) = --------- + 7.5
            C s

Ah... I see... it's like an indefinite integral, where the 7.5 could be
any arbitrary constant... we really don't know necessarily where things 
are when the circuit fires up; taking inverse transforms it's really

vo(t) = int_0^t icur(tau) dtau + vo(0) where vo(0) is wherever it is when 
the circuit starts up.

So, Buchla is just wanting to, more or less, via virtual 7.5ness, hold the 
output of the OTA at 7.5 volts. Not sure why...

- Aaron

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