VCO hacks, tweaks

[terry michaels]

Message text written by Joachim Verghese
>
> I'm glad you caught the 3300ppm/K vs. 3500ppm/K problem, I missed that
one
> completely.

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


<

Hi Joachim:

I have to dispute your math.  The scale factor for the exponential function
of a silicon transistor is given as delta Vbe = kT/q log delta Ic.  (ref:
National Semiconductor Application Note 30).  K = absolute temperature,
which means the scale factor varies directly with absolute temperature, and
is a straight line relationship which reaches zero at zero temperature. 
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.

Terry