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

Bob Camp kb8tq at n1k.org
Sun Aug 30 22:09:09 EDT 2015


As we go joyously bashing the poor guys that designed this beast, it’s worth noting
just how old the design is. 741 op amps were indeed “modern” when they did much 
of this and quite possibly to modern to be trusted. Most of the design would have been
right at home in the late 1960’s at a conservative design house. As time has shown, in 
a lot of cases that mistrust of the early linear stuff was well founded ….The 741 only
was designed in 1968….The 5065 design dates to roughly that time. 


> On Aug 30, 2015, at 8:08 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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
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