Temperature Compensated VCO follow up

[Ian Fritz]

Jim et al. --

Well, 390ppm/K isn't a bad result at all! Especially for just a couple days
of tweaking!!  Hal Chamberlin's improved version of Terry Michael's saw
oscillator only did about 1000ppm/K.

Out of curiosity, I wonder if you can say anything yet about the freq
dependence of the drift. Remember that the expo converter gives a T
dependence like exp[q(dVbe)/kT], where dVbe is the differential base-emitter
voltage. If the compensation isn't perfect, then the drift in ppm/K is zero
when dVbe=0 and increases -- in opposite directions -- on either side of
zero.

Did the change from 80 to 100mV affect the tracking accuracy? I always seem
to find the best tracking around 60-70mV.

By an amazing coincidence, just today I finally got all the drift out of my
differential-integrator tri osc. I've been fighting this one for a couple of
months. I'll post that tale separately.

  Ian


----- Original Message -----
From: "patchell" <patchell at silcom.com>


>     I managed to get the tempco down to 390ppM/C.  This is a triangle
> VCO using an NPN/PNP pair to switch the current.  I was driving it at
> about plus and minus 80mV before and changed that to plus and minus
> 100mV to get the improvement.  But, it looks like I will be forced to
> try a saw tooth oscilator now.  Somehow, I suspect the current switch
> (or OTA if that is how you want to look at it) is causing a lot of the
> temp drift.
>
>     Anybody have any experience concerning OTA's in triangle oscilators
> and any tempco problems they might cause?
>