[sdiy] expo converter

[tomg@efm]

Hi Scott, Yes it can be modified, extensively if you wish but that
would just confuse the issue. The problem here is builder error 
not the vco in question. Roughly 20-30 people have done this 
one with success.  It's one ic 2 transistors one cap and 9 resistors
for the core. I can't make it any easier. Ben is just going to have to 
suffer through it.  Start over a few times maybe try something else
like the vco8-3046 for example. It's about the same parts count.

If you would like to see a super modified version you'll have to 
swear to secrecy and I'll send it to you. In this version I am 
using the ota tempco. I have completely solved the ota-tc thing
and can incorporate it into every expo-converter I am aware
of. A $0.58 ota beats a $2.50 tempco every time and seems to
work just as well. I am not posting it to the open web for obvious
reasons. 

Tom




> I think the VCO could have a wider range (ie, from subaudio thru audio)
> by increasing the charging cap to 3300pF, and changing the 470K to 22K.
> A max charging current of 500-600uA for the top freq range is OK, and
> you can get much lower freq response (down to 0.1Hz at ~3nA charging
> current).
> Any comments, TomG? This is your circuit.