[sdiy] Updating the Tau 1010 VCO

[lanterma at ece.gatech.edu]

Howdy SDIYers,

One of my students is going to try building this for her final project:

http://www.noniandjim.com/Jim/TAU1010/tauvco-b.gif

I sent her the note below; please let me know if I said anything that's off-base and might lead her astray...

- Aaron

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The CA3080 OTA is out of production, so you can use an LM13700 instead. The CA3080 is a single OTA, the LM13700 is a dual. If you get the VCO working, we can figure out something interesting to do with the other half of the LM13700, maybe make a tri-to-sin shaper (but save that for later).

You can use a TL082/084/072/074 instead of the given op amps (LM207 and HA2500).

Q1 and Q2 should best be a monolithic, matched PNP pair; I can bring in a matched pair chip for you. Instead of the 1.87K tempco resistor - that's an odd value isn't it? - just use a regular resistor somewhere close to 1.87K for now; once we get it oscillating then we'll worry about temperature stability.

I have no idea what the original transistors were; you should be able to use 3906 and 3904s where appropriate for Q4, Q5, and Q6.  For the JFET - I guess it's labled Q3? - try a J201 or MPF102 (if the stock room doesn't carry those let me know).

Q3 and Q4 form a buffer - I think they're using the FET Q3 for its relatively high input impedance and the BJT Q4 for its relatively low output impedance.

For right now, you can grab a random capacitor to play the role of the integrating cap hanging off of the 3080, but at some point want to swap this out for a higher quality polysterine or polypropeline cap.

- Aaron