[time-nuts] HP5065A C-field current is temperature sensitive

[Magnus Danielson]

Poul-Henning,

On 08/30/2015 09:54 PM, Poul-Henning Kamp wrote:
> --------
> In message <1698D85B-EBB6-45E3-9CB0-CBF780CE5550 at 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