A few thoughts on heat tests and heaters

[Rene Schmitz]

Hi, 

with the recent discussion on the heater stabilization, 
and on how to test it, I have some comments:

A few months ago I did wan't to test a thermo compensation circuit.
(the circuit has been discussed here before, so all that is in the archives)

I did the heat gun, and freeze spray tests and after I had tried it, I had
a few concerns about the results.
There is a temperature stable point in every VCOs expo convertor, when
dVbe=0 then there is nothing to drift with temperature. This depends of
course on the chosen frequency at which the test is performed. To make any
use of the measurement, one would have at least have to measure two
frequencies, one below and one above the temperature stable point. 

Then obviously there is the problem of temperature gradients, when using
the iron to heat up the chip, nothing makes shure that the heat is
distributed evenly on the package and the chip. 
The second problem I encountered was when using freeze spray, because there
will be water condensing on the circuit which causes leakage currents.
(I now use it only in TV repairs, and hope there won't be any sparks :) 

Finally a few thoughts on the ovenized convertors (and why I don't use these):

It is often repeated that this circuit will draw much current. Me too ;)
But one thing that isn't often mentioned is that some transistor parameters
degrade with increasing temperature. For example the leakage current!
The leakage is interesting only in VCOs that have the cap to the supply
because 
then the sinking transistor will have a rather large voltage across it. (In
VCOs where the current flows from a virtual ground this is not an issue.)

Another thing makes this even worse: the heater can't make the chip cooler
than it is. So if you choose your heater temperature too low, the
regulation won't work at temperatures above that, and if you make it too
high the performance of the transistors may degrade.

Bye
 Rene


             ,   : (uzs159 at uni-bonn.de)
|") [" |\ | ["   : (http://www.uni-bonn.de/~uzs159)
|"\ [_ | \| [_   : (http://members.xoom.com/Rene_Schmitz)