Poul-Henning,
On 08/30/2015 09:54 PM, Poul-Henning Kamp wrote:
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In message <1698d85b-ebb6-45e3-9cb0-cbf780ce5...@n1k.org>, Bob Camp writes:
Since they needed to calibrate the voltage swing, there aren’t a lot of options
with the technology they were using at the time. Today there are a lot more
choices of how to get this sort of job done.
It's not just the swing, it's also the shape of the curve:
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
If it were just the range things would be a lot simpler.
Ehm, eh... that transistor pair you have there. How *tight* together is
really the transistors thermal connection? I bet not all that good.
The reason I got involved with counters and atomic references was
originally my interest in analog synthesizers, and there we use a
transistor pair for exponential conversion, which has scale and offset
issues and thermal issues. The use of a Q81 +3300 ppm/C resistor in the
division network helped to compensate the thermal properties of that
transistor pair, and you wanted stuff like MAT-01 where the two
transistors is thermally ties to each other and the put your tempco
resistor to that for good performance. All this requires good
measurement and good reference, so that what motivated me towards that
step. Anyway, the take-away is that you should look at that discrete
op-amp and see if it is not causing you the thermal dependence you are
trying to locate. Maybe replace it with a more modern op-amp like the
741 or something (irony may have been used).
Cheers,
Magnus
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