HP5065A C-field current is temperature sensitive
Bob,
Indeed. No bashing of the designer, it is an OK design for its age and
intended stability vs complexity. My point was that after the 702
design, many things happen and one thing they learned was to handle the
thermal balance. Already the 741 had taken a number of these steps. The
get-away might be that caring a bit about the thermal properties of the
operational amplifier could be where we can improve the design.
Cheers,
Magnus
On 08/31/2015 04:09 AM, Bob Camp wrote:
Hi
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.
Bob
On Aug 30, 2015, at 8:08 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
Poul-Henning,
On 08/30/2015 09:54 PM, Poul-Henning Kamp wrote:
In message 1698D85B-EBB6-45E3-9CB0-CBF780CE5550@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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In message 55E39AF2.2050708@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.
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.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
In message A1A54F1F-926F-42A9-8AEC-F83BCAF4E58D@n1k.org, Bob Camp writes:
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” [...]
I don't think chip op-amps appear until the 5065 is a toddler, later
revs use a couple of them.
The only thing I feel like beating them up about, is that appearantly
they didn't consider the tempco of the R10||R11 pair (or maybe it is
just R10 - not sure yet).
Either way: If any of you have a non-amazing HP5065, Measure the
A15CR5 voltage, multiply it by 76, and replace R10||R11 with something
of that value and better than 5PPM tempco.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hi Bob,
I haven't seen anyone bashing the designer, or the design of the 5065.
What I have seen a couple of enthusiasts that love the unit enough that
they are willing to spend countless hours bringing a couple of parts of
the design up to a more recent state of the art. Corby's modifications
to the optics added stability that no one anywhere has achieved in a Rb
standard. PHK's changes to the oven may achieve a similar improvement.
Between the two of them, the 5065 is looking pretty nice!
In my experience, there are quite a few pieces of equipment where people
are willing to take the relatively minor effort to repair them to match
their original specifications, but very few where people are impressed
enough with the foundation parts that they are willing to spend the
great amounts of time it takes to fully understand and modernize the
design.
I think that speaks well for the designers of the 5065.
-Chuck Harris
Bob Camp wrote:
Hi
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.
Bob
Poul-Henning,
On 08/31/2015 08:28 AM, Poul-Henning Kamp wrote:
In message 55E39AF2.2050708@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
Attached is a frequency plot of my 5065A vs. GPS (via PPS) over about 6
months. Note the changed freq and slope between about MJD 57205 and
57235. That correlates to the month we were out of town on vacation,
with the air conditioning set about 6 degrees higher than normal.
(There was probably less differential than that in the basement lab vs.
the thermostat upstairs, so don't try to do any absolute measurements
from this.)
On 08/31/2015 02:32 AM, Poul-Henning Kamp wrote:
In message A1A54F1F-926F-42A9-8AEC-F83BCAF4E58D@n1k.org, Bob Camp writes:
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” [...]
I don't think chip op-amps appear until the 5065 is a toddler, later
revs use a couple of them.
The only thing I feel like beating them up about, is that appearantly
they didn't consider the tempco of the R10||R11 pair (or maybe it is
just R10 - not sure yet).
Either way: If any of you have a non-amazing HP5065, Measure the
A15CR5 voltage, multiply it by 76, and replace R10||R11 with something
of that value and better than 5PPM tempco.
