[time-nuts] HP5065A C-field current is temperature sensitive
Magnus Danielson
magnus at rubidium.dyndns.org
Mon Aug 31 12:17:57 EDT 2015
Poul-Henning,
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.
Cheers,
Magnus
More information about the time-nuts
mailing list