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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Aug 31 12:17:57 EDT 2015


On 08/31/2015 08:28 AM, Poul-Henning Kamp wrote:
> --------
> In message <55E39AF2.2050708 at rubidium.dyndns.org>, Magnus Danielson writes:
>> Ehm, eh... that transistor pair you have there. How *tight* together is
>> really the transistors thermal connection? I bet not all that good.
> They're in the same TO5, but I don't know if they're the same die.
> I have about 45-50 microvolt difference between their bases.

Monitor that over time to see if there is a correlation.
The current will vary more than the voltage would illustrate, due to the 
exponential properties.

>> 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).
> Yes, that is absolutely the plan.
> The integrator and AC-amplifier in the HP5065 gets upgraded to
> chip op-amps in later revs, but this one does not.

Indeed. Modern low-offset op-amps has temperature-gradient input stages, 
and is a marvel of stability compared to what the original 5065 
designers could do easily. Seems like their transistor pair choice 
solved quite a bit of the issues, but as we look to shave of 
disturbances it is worth looking at.

Tempted to mod up my 5065s now.


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