VCO hacks, tweaks

[terry michaels]

Message text written by INTERNET:ijfritz at earthlink.net
>
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.
<

Ian:

Yeah, you're right.  If the Q81 has a room temperature (+25 deg C) tempco
of 3500ppm, then the tempco curve can't pass through the origin at 0
Kelvins, althouth it comes close.  My earlier statement was made after
examining a graph of resistance vs. temperature over the range of -55 deg C
to +275 deg C.  The resistance axis of the graph is stated to only two
decimal places, which doesn't give you much accuracy.  Also, we can only
assume the tempo can be extrapolated at the same slope through 0 Kelvins. 
This may not be true, it may deviate from a straight line response at low
temperatures.  The resistor is probably wound with KOVAR wire, which is a
nickel/steel alloy.  KOVAR is stated to have a tempco of 3500ppm at 25deg
C.  Maybe someone on the list has access to information on the resistance
of KOVAR  at low temps which can helps us with this.

Also, the Q81 is stated to have a tempco tolerance of + or - 300ppm, a
tolerance greater than the error we've been talking about.

I made a hasty assumption that the Q81 tempco is directly proportional to
temperature, that may not be exactly true, as it is based on less than
perfect data supplied by the manufacturer.

I think adding a zero tempco resistor to the Q81as you suggested is a good
plan, and should give a really close compensation of the expo transistor
tempco.   I'll guess that temperature stability will be more than adequate
for usage in "room temperature" situations, and further tweaking might not
accomplish anything useful.

 Hope I didn't thoroughly confuse the issue.

Terry