[sdiy] tempco stuff

[Seb Francis]

IIRC, there's a pretty straightforward, but thorough explaination of VCO 
temperature compensation in this PDF:
http://www.milton.arachsys.com/nj71/pdf/MSM/msm-vco.pdf

Seb


ryan williams wrote:

> hi all,
>
> I'm writing a paper for an engineering communications class. class 
> about speaking & writing, not communication systems (already had that 
> one). We have to write a technical paper on something that interests 
> us. I chose, temperature compensation in musical instruments. I will 
> not have time to actually build things and take measurments. I would 
> like to, but there isn't much chance I can blow off my EE projets for 
> that.
>
> Anyhow, I had some questions. For tempco resistors, often I see people 
> use resitive dividers. It seems to me that this can allow pretty good 
> compensation even with non 3353ppm resistors (number I calculated as 
> ideal assuming 25degC is room temperature).  What I am trying to find 
> a good resistor ratio for the divider. I am picking values at the 
> endpoints of some temperature range I think the device will work in. I 
> chose 77F to 100F (25C to 38C) then solve for a resistor so that the 
> ratio of Vin*Divider/Vt is the same for both temperatures. I have not 
> yet included the tempco of the normal resistor. I get almost exactly 
> 24Kohm with a 1K 3500ppm tempco. Any other scaling of the input to get 
> 1V/octave would be in an opamp before the resitive divider. Is this a 
> reasonable method? Any other ways to adjust the tempco using other 
> circuits?
>
> for electronic compensation. Does anyone have any comments on a Vt 
> reference that I could use for scaling the input voltage. I have seen 
> this design, called the Brokaw Cell (pg 299 of opamp applications 
> handbook). And I have thought of another similar circuit using two 
> opamps and a feedback loop on each to keep two transistors of a diff. 
> pair's collector currents constant. then the base voltage on one is 0V 
> and the other is controlled by one of the opamps. The emitters are 
> controlled by the other opamp. The output would be the base voltage as 
> some constant ln(Ic1/Ic2) multiplied by Vt. I'm wondering about other 
> possible references. I have seen Jim Patchell's but do not know enough 
> to compare it with other circuits. Anyone know of something written on 
> this subject?
>
> I have lot of plots to make before this thing is over. In the end, I 
> plan to put it on my web page because it might be helpful for people 
> who know <= myself.
>
> -ryan
>
>