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

[Ian Fritz]

Hi Bruce --

Many thanks for the detailed reply.  The dual SMT transistors have been 
mentioned here several times, but you may be the first person using them 
seriously. I wonder if their series emitter resistance is different from 
the usual thru-hole parts.  Those require a fair amount of compensation.

I have spent some time with the SMT tempco's.  I measured them in a special 
bridge/heater setup I made a while back.  The temp coeff was very close to 
optimal.  But I had trouble getting steady readings.  I found the same 
thing recently using one in an oscillating filter.  Whenever the CV is 
changed, the pitch takes several seconds to stabilize.  I think I have been 
seeing a bit of self heating.  Maybe your potting method and the good 
coupling betwen the devices  alleviates this.

   Ian


At 12:40 PM 3/20/2008, Bruce Duncan wrote:

>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.
>
>
>Bruce
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