[sdiy] Temperature compensation results

[Ian Fritz]

Hi All --

To start with, here's a trick that I haven't seen mentioned in any of the 
synth discusssions.

If there is a voltage offset in an expo converter, it will produce a 
temperature drift.  This is an "absolute" drift (all frequencies drift by 
the same ratio) as opposed to a "scale factor" drift (scales stretch or 
contract).

The "trick" is to deliberately add an adjustable offset and use it to 
cancel other sources of absolute drift, e.g., from temperature dependences 
in the oscillator core.  The relation between offset and drift is that 1 mV 
of offset in Vbe gives -129 ppm/K drift.

I added this to my dial-a-tempco circuit, so that there are now two 
adjustments: one for scale-factor drift and one for absolute drift.  After 
proper adjustment, the total drift is now down into the regime where it is 
quite difficult to measure, but it is definitely below 50 ppm/K.

There has been an objection that this method would be difficult to use 
because of the many temperature cycles it would take to dial in the 
corrections.  Actually it is quite easy, if you do it correctly.  The 
correct method is as follows:  First, set the compensation voltages to zero 
and carefully measure the scale-factor and absolute drifts.  Then use 
calculated equations to figure out what levels of compensation voltages (or 
pot turns) are needed for correct compensation.  Finally, just dial in 
these corrections using a DVM (or by counting turns).  This should bring 
you very close to exact compensation, but it may be necessary to iterate 
once to get right on.

To measure the temperature drift I used a very simple "oven".  The circuit 
is mounted inside an aluminum chassis box with standoffs.  The box is 
heated with a drug-store heating pad wrapped around it.  Temperature is 
measured with an IC sensor mounted right next to the converter.  This is a 
simple, quick, quiet and reproducible method.

Happy tuning!

   Ian