VCO hacks, tweaks

[Ian Fritz]

Terry --

Hmmm... I'm confused. I haven't seen the Q81 spec sheet. But if the
resistance were directly proportional to temperature, then we would have
R = AT and (1/R)(dR/dT) = 1/T, so the room temperature coefficient would
*have* to be 1/298 or about 3300ppm/K. Then no correction for the
specified 3500ppm coefficient would be needed.

I'm just guessing here, but I've been assuming that the form of the
resistance is approximately R = R_0 * (1 + (T-298)/286). This has a
tempco of 1/286 = 3500ppm/K, a value of R_0 at room temperature, and
comes close to going through the origin (but misses by a factor of
1-298/286, or 4% of R_0).

  Ian

terry michaels wrote:
> 
> The Tel Labs, Inc. datasheet on the Q81 resistor has a graph which shows a
> straight line resistance vs. temperature change between -55C and +275C
> which, when extrapolated, passes through zero.  Therefore, the Q81 has a
> linear tempco, proportional to absolute temperature.  The Q81 will give a
> first order compensation for the scale factor of a silicon transistor, and
> can be trimmed to the exact value needed by adding a small value zero TC
> resistor in series with the Q81, as suggested by Ian Fritz.
> 
> BTW, the value of 3500 ppm given by Tel Labs is the tempco between -55C and
> +275C, averaged to room temperature.  The value is misleading if you don't
> know that the manufacturer calculated this way.