[sdiy] Thought experiment...

[Ian Fritz]

Ryan --

A couple of years ago I tried a converter similar to Jim's.  The main 
difference was that I used a commercia IPAT generator (LM234) instead of 
the second transistor pair.  On paper it should have worked fine, but I 
ended up with a bit of residual drift that I could never understand or get 
rid of.

That was when I decided to work on the idea of adjustable-passive 
compensation.  This works very well, but you have to be willing to make a 
setup to measure the drift in order to zero it out.
http://home.earthlink.net/~ijfritz/sy_cir7.htm
http://home.earthlink.net/~ijfritz/sy_cir9.htm

As far as which OTA to use, what matters is the matching between the two 
OTAs more than the performance of either.  A dc offset in the OTAs gives a 
"tuning drift" error, but doesn't affect the "scale factor" stability.

When testing for temperature effects, it is important to use heating rather 
than cooling.  Cooling always gives spurious results due to moisture 
condensation (unless your circuit is hermetically sealed).

   Ian



At 09:06 PM 3/22/2005, Ryan Williams wrote:

>The difficult part I think is the multiplier. I expect when I actually 
>build this I'll use the two OTA circuit (also used in Jim's circuit, Mike 
>Sims VCA in EDN magazine, and other places) although I've not yet measured 
>it's performance. Incase Jim or anyone else who is reading is familiar 
>with this circuit; is there any advantage to using the CA3280 in place of 
>the LM13700. I was under the impression that it's transistors were matched 
>well, better perhaps? What about a multiplier IC? If i remember correctly, 
>the two LM13700 VCA doesn't completely remove it's temperature error, but 
>only makes it much smaller. I don't really care about a couple of dollars 
>more if performance is better, atleast not for my purposes.