[sdiy] Was PTC/NTC intelectuall level of SDIY!

Bruce Duncan modcan at sympatico.ca
Thu Mar 20 19:40:57 CET 2008

At 08:32 AM 3/20/2008, you wrote:
>At 02:03 AM 3/20/2008, Bruce Duncan wrote:
>>Woo hoo..Rock'em Sock'em Synth DIYers battling out for supremacy.
>>If they were in class together would they be burning each other 
>>with their soldering irons ?
>Hi Bruce -- What is your take on the tempco issue?  What do you 
>think is "god enough"?  How much effort do you put into VCO stability?
>Just curious.

Hi Ian,

I haven't found a totally 100% satisfactory method of dealing with 
the tempco issue so far.
The easiest and most reliable method I have found for VCOs is to use 
a dual SMT transistor
DMMT3904W and a 1.8K 0805 3300ppm resisitor in the feedback loop of 
the coarse tune summing opamp.
( I would use 2K if they were available but Digikey only stocks 1.8K)
The Dual 3904s seem to be quite well matched and the low mass of both 
the transistors and 0805 T. resistor
means they will probably remain very close to the boards temperature.
I also add a big blob of epoxy over both components for thermal 
contact and they are very close to each other on the board.
With this method I get 10 octaves +/- 1 cent as long as the 
temperature doesn't change too much.
And I need very little high frequency compensation.
I find that VCOs really need to be calibrated in the environment they 
will be used in for best results
so I offer info in my user manual for customers who feel the need to 
adjust the tracking on-site.

In 10 years of selling modules I have never once had a VCO returned 
for calibration.
Well maybe one. But that had a blown IC. That may say more about my 
customers pitch
tuning sensitivity than anything but to me it says for the most part 
it is working acceptably.

I would like to find a better method but so far the ones I have seen 
use a lot of additional components
like the CA3280 method that Jim Patchell and others have advocated.


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