[sdiy] Buchla/Bergfotron modifications

[Aaron Lanterman]

Another of my students (Manu) is working on building a Buchla-style 
triangle core VCO.

I did an evil thing on my midterm quiz, which is I gave them the Buchla 
259 exponential converter schematic to analyze - in Buchla's original 
handwriting. Uhm... this didn't go over so well, and required a lot of 
during-exam clarification. Not a good idea on my part. I still feel guilty 
about it.

So, I didn't really want to throw the Buchla 259 schematic at him to work 
from, lest he immediately try to set it on fire. I suggested he try 
hacking at the Bergfotron triangle-core (if nothing else since it's a 1 
V/oct converter):

http://hem.bredband.net/bersyn/VCO/VCO%20Buchla%20core.gif

Part of the way my final project is structured is that their final design 
can't be purely an existing design - they must modify it in some at least 
vaguely interesting way.

Manu has the original Bergotron up on the breadboard. It's not working 
yet, but I'm sure he'll get it working, right Manu? :)

Anyway... I'm less picky about fancy modifications here, since VCOs are 
inherently such finniky beasts.

Anyone have a favorite modification the've wanted to try but haven't had 
time to?

I was thinking of (not sure if any of this would change the performance or 
not)

* Use the allegedly superior LM319 (it's what Paul uses in his VCOs) 
instead of the LM311.

* Use the higher-input-impedance CA3140 instead of the TL072 as the 
integrating op amp

* Both of the above...

I want to keep the four-BJT-OTA (vs. replacing it with a LM13700 or 3080, 
as Mark did), since I want to be able to use an NPN pair in the expo 
converter.

I gave him a CA3083 for the expo converter (Ian often recommends it), and 
was thinking the spare transistors could be set up as a "heater" as in Q3, 
Q4, and A2 of Figure 1 of the AN-299 ap note:

http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/opamp/AN-299.pdf

Any other assorted ideas of easy but interesting modifications?

- Aaron

P.S. That CA3083 - I remember Harry, I think, telling me to tell students 
put the substrate pin the negative rail, or the transistors would act 
weird - am I remembering that right

P.P.S. I love my job!

-----------------------------------------------------------------------------

Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
College of Electrical and Comp. Eng.   Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: Centergy 5212