[sdiy] Temperature compensation results

[René Schmitz]

Hi Ian,

> 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).

I would say there is always an "offset". Since Vbe isn't zero.
(Comparing to the use of the term offset in differential amps, where 
both transistors are kept at the same base potential.)

> 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 fail to see how that would be any different than adding an external CV 
to the CV summing node. (Or detuning via the panel tune pots.)
(I would assume that you would need a voltage that is PTAT for that to 
work. )

Could you please elaborate on the background of this?

> 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.

Awesome results!

> 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.

Do you think that individual oscillators would have to undergo this 
procedure, or do you think once you know the values you can skip the 
measurement and just use those obtained on another specimen?

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159