VCO hacks, tweaks

[Ian Fritz]

Joacim --

Of course, temperature compensation with a linear tempco resistor can
only be exact at one temperature. I get different numbers from you,
though. The output current of a differential-pair converter (or
alternately, the oscillator frequency) varies as exp(-qV/kT). So for
compensation, V/T must be constant, which leads to (with just a bit of
algebra) dV/dT = V/T. So the tempco resistor should have (1/R)(dR/dT) =
(1/V)(dV/dT) = 1/T. So I get that 3500ppm/K corresponds to 286K, or a
chilly 13 deg C. At a cosy 300K the compensation should be 3333ppm/K. At
40 deg C, the compensation would be 1/(273+40) = 3200 ppm/K. The 174 Ohm
series resistance in my VCO mod gives an effective coefficient of
3500(2.00/2.17) = 3230ppm/K, agreeing with your idea that the best
compensation point might be a bit above room temperature.

  Ian



Joachim Verghese wrote:
> I fear this issue is even more complicated if you opt for really
> accurate compensation.
> 
> The problem is that the temp coefficient of the scale factor is a
> non-linear function of temperature. 3300ppm/K is accurate only around
> 30 deg(C) (303K), since
> 
> (303 + 0.5) / (303 - 0.5)  =  1.003306
> 
> At 40 deg(C) (313K), which perhaps is closer to the operating temperature
> of an enlosed circuit, the required compensation coefficient is
> 
> (313 + 0.5) / (313 - 0.5)  =  1.003200, i.e. 3200ppm/K
> 
> In other words, using a (linear) tempco resistor isn't the most accurate
> compensation method, although it works well enough for most purposes.
> 
> -joachim