[sdiy] drift followup

[jhaible at debitel.net]

Thanks Ian!

Even though I'll use a linearised input, this one
will go to my personal archive. 

JH.


> Hi gang --
> 
> Last week a question came up concerning temperature drift in a 
> current-switching tri VCO using an unsaturated current switch, e.g. a 
> CA3080 driven at 60 mV, such as in my old ultrasonic VCO (Electronotes #
> 112).
> 
> I've now worked out the math to calculate the (tuning) drift of an OTA with 
> finite voltage drive.  The calculation only takes a couple of lines if you 
> work with (yuk) hyperbolic functions.  Here is the result:
> 
> Define "a" as half the ratio of the drive voltage to the thermal voltage:
> 
> a = 0.5 Vin / Vth,  where Vth = 26 mV,
> 
> and define T0 as room temperature (300 K).
> 
> Then the drift D is given by
> 
> D = -a (sech a) (csch a)  / T0
> 
> It is easy to verify that for small "a",  D reduces to  -1/T0, i.e. the 
> familiar result of -3550 ppm/K.
> 
> For a 60 mV drive I find a = 1.15 and D = -1550 ppm/K.  This is consistent 
> with the fairly large amount of tuning drift I found in the ultrasonic VCO, 
> which was compensated as described in a previous post.  Be sure to note 
> that this is a tuning drift -- the scale factor is not affected.
> 
>    Ian
> 
> 
> 




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