HP5065A C-field current is temperature sensitive
I have been measuring the C-field current of my HP5065 for a couple
of days using my HP3458A.
To be more precise, I have measured the voltage across the two
parallel current sensing resistors A15R10 and A15R11.
I have not set up a precise temperature measurement for this
experiment, but eyeballing room monitoring, I find a close correlation
of approx 0.4 millivolt/C.
I also find that the phase-offset relative to a GPSDO correlates
significantely with the voltage measured.
If we assume that R10||R11 are not the cause of this, it corresponds
to 0.5 microamp/C sensitivity for the C-field current, and thus
C-field.
The total adjustment range of the C-field pot is 2e-9 which changes
the C-field current from 2.5 to 6 mA (ref: pg 8-48).
That means the C-field sensitivity is 5.7e-7/Ampere.
My 0.5 microamp/C therefore corresponds to 3e-13/C
...which is pretty much the MVAR floor in my measurements.
It seems plausible that a better C-field driver could improve the
stability.
Pretty much all the components in circuit could be causing this.
The ultimate reference for the C-field current is A15CR5, a 1N938
temperature compensated zener which is also the reference for the
+20V supply.
In the HP5065 the 1N938 is driven at approx 12mA, where later
datasheets indicates that 7.5mA is optimal for TempCo purposes.
Once the present measurement run is over, I'll do a run where I
measure the A15CR5 voltage directly, and if I can arrange it, also
with a spot temperature measurement.
If A15CR5 is the culprit, the next obvious step is to change A15R7
to 1500 Ohm, and see if that improves stability.
Another obvious experiment is to drive the C-field with a very
stable external supply, and see what that does for the MVAR.
--
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.
Dear Poul-Henning,
I have been suspecting this very mechanism to exist in the HP5065 among
others. I have not been overly impressed by the stability by which the
current is produced.
It would be interesting to see to what degree the surrounding
temperature as well as the mains supply voltage does. The mains supply
voltage will creep in two ways, one way for the raw rectified voltage
and second on the burning off the difference voltage causing the main
voltage to power to temperature variation.
It would be interesting to see how good damping the main voltage to
various internal volages there really is.
Cheers,
Magnus
On 08/18/2015 10:05 PM, Poul-Henning Kamp wrote:
I have been measuring the C-field current of my HP5065 for a couple
of days using my HP3458A.
To be more precise, I have measured the voltage across the two
parallel current sensing resistors A15R10 and A15R11.
I have not set up a precise temperature measurement for this
experiment, but eyeballing room monitoring, I find a close correlation
of approx 0.4 millivolt/C.
I also find that the phase-offset relative to a GPSDO correlates
significantely with the voltage measured.
If we assume that R10||R11 are not the cause of this, it corresponds
to 0.5 microamp/C sensitivity for the C-field current, and thus
C-field.
The total adjustment range of the C-field pot is 2e-9 which changes
the C-field current from 2.5 to 6 mA (ref: pg 8-48).
That means the C-field sensitivity is 5.7e-7/Ampere.
My 0.5 microamp/C therefore corresponds to 3e-13/C
...which is pretty much the MVAR floor in my measurements.
It seems plausible that a better C-field driver could improve the
stability.
Pretty much all the components in circuit could be causing this.
The ultimate reference for the C-field current is A15CR5, a 1N938
temperature compensated zener which is also the reference for the
+20V supply.
In the HP5065 the 1N938 is driven at approx 12mA, where later
datasheets indicates that 7.5mA is optimal for TempCo purposes.
Once the present measurement run is over, I'll do a run where I
measure the A15CR5 voltage directly, and if I can arrange it, also
with a spot temperature measurement.
If A15CR5 is the culprit, the next obvious step is to change A15R7
to 1500 Ohm, and see if that improves stability.
Another obvious experiment is to drive the C-field with a very
stable external supply, and see what that does for the MVAR.
Tonight I hooked my HP34972A DAQ up to the HP5065A and struck gold
right away: The reference zener has a tempco of 20PPM.
Possibly also of interest: The C-field driver is not very good
and noise on the +20V supply leaks straight through it.
Full details and plots here:
http://phk.freebsd.dk/hacks/hp5065a_temp/index.html
--
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.
I'm impressed with your great documentation ... but I wish to differ with some of your circuit analysis. I don't have an HP5065A and can only see the portion of the schematic you copied. Here is what I can surmise from your measurements (assuming you don't have any ground loops or measuring instrument or test lead thermoelectric issues):
The change of the 20 V power supply when you lock is roughly 400
uV (20 ppm).
The change of the C-coil current isn't really repeatable. I see a big
500 nA (111 ppm) drop at 1,000 seconds, but no such clear change earlier
in the test. But the actual voltage changes you are measuring seem to be
roughly correlated between the 20 V power supply and C-coil current
sense resistor changes. In the first 900 seconds I would guess that the
C-coil current sense voltage is rising at about 40% of the rate of the
20 V supply, then during the 1,000 second event the 20 V supply dropped
maybe 30 seconds before the C-coil current sense voltage dropped, and
the current sense voltage dropped about 80% of the power supply drop.
I'm guessing about these values, based on converting your current
numbers into voltage based on a perfect R10 || R11 parallel combination
of 691.6 ohms. I see that R10 has an * asterisk, and I wonder what is
shown for that note. R10 might be a selected value at manufacturer, or
it might have a specific temperature coefficient.
The temperature coefficient of the resistors may be much more important
than anything else, especially for an old product. I also wonder if
those old electrolytic capacitors (C4, C6, and C7 for example) are still
OK, or whether they are showing any changing leakage currents. You might
want to change them with new capacitors just in case.
So I'm not convinced that the time curve is showing a correlation based
on the 20 V power supply affecting the C-coil current. It looks to me
like the noisy jumps in these are not related to each other. It's
possible that measuring system errors (such as where you connected the
measurement system ground) might cause some of these changes. So you
might want to check your setup and the instrument and test lead
computed accuracy.
I also disagree with your estimate of the CR5 zener current. By your
report of the C-coil current and R10 || R11 sense resistance (691.6 ohms
if those stated resistance values were perfect), the current sense
voltage is 3.1266 V. So the voltage at the base of Q6A is also 3.1266 V.
I assume that Q6A/Q6B are a matched dual transistor in the same case, so
they are at very close to the same temperature. This means that the
current through R8 is (8.786 - 3.1266 V)/1333 = 4.2456 mA. The current
through R7 is (20.089 - 8.786 V)/925 = 12.2195 mA. So if there is
nothing drawing significant current in the wire extending left from C4,
the CR5 diode current must be (12.2195 - 4.2456 mA) = 7.9739 mA. That's
pretty close to the zero temperature coefficient current for the 1N938
you describe. So I do not recommend changing the diode current.
So I recommend changing all of the old electrolytic capacitors in that
area of the schematic for good measure (C4, C5, C6, and C7). Then
check the warmup voltage as you did before from terminal 1 of the C-
coil to the ground end of the current sense resistors R10 || R11. Be
sure to not use a different ground, since there may be significant
current and ground drop through the ground trace or plane. Be sure to
have the covers in place so the airflow and thermal characteristics
are as HP designed.
--
Bill Byrom N5BB
On Wed, Aug 19, 2015, at 04:45 PM, Poul-Henning Kamp wrote:
Tonight I hooked my HP34972A DAQ up to the HP5065A and struck gold
right away: The reference zener has a tempco of 20PPM.
Possibly also of interest: The C-field driver is not very good
and noise on the +20V supply leaks straight through it.
Full details and plots here:
http://phk.freebsd.dk/hacks/hp5065a_temp/index.html
--
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.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
In message 1440039768.738221.360877705.34D2F4EC@webmail.messagingengine.com,
Bill Byrom writes:
I don't have an HP5065A and can only see the portion of the schematic
you copied.
The manual is on K04BB if you want the full monty.
Here is
what I can surmise from your measurements (assuming you don't have
any ground loops or measuring instrument or test lead thermoelectric issues):
The measurement setup isn't optimal, I can only do one measurement
every five seconds on each point and the HP34972A is only a 6½ digit
instrument, but otherwise the setup is solid.
The change of the 20 V power supply when you lock is roughly 400
uV (20 ppm).
The change of the C-coil current isn't really repeatable. I see a big
500 nA (111 ppm) drop at 1,000 seconds, but no such clear change earlier
in the test.
It didn't actually lock until 1000 seconds, so one difference is that
the logic "continous operation" logic didn't reach final state and
its lamp didn't turn on in the previous attempts.
But the actual voltage changes you are measuring seem to be
roughly correlated between the 20 V power supply and C-coil current
sense resistor changes.
As they should be, because both are derived from the same A15CR5 zener.
I'm guessing about these values, based on converting your current
numbers into voltage based on a perfect R10 || R11 parallel combination
of 691.6 ohms. I see that R10 has an * asterisk, and I wonder what is
shown for that note. R10 might be a selected value at manufacturer, or
it might have a specific temperature coefficient.
The asterix means "selected". I have not been able to figure out
what criteria it is selected for.
The temperature coefficient of the resistors may be much more important
than anything else, especially for an old product.
All the important ones are wire-wound, probably for exactly that reason.
those old electrolytic capacitors (C4, C6, and C7 for example) are still
OK, or whether they are showing any changing leakage currents. You might
want to change them with new capacitors just in case.
Good point.
So I'm not convinced that the time curve is showing a correlation based
on the 20 V power supply affecting the C-coil current.
No, the main correlation is via the common zener, but the step at 1000s
is not present in the zener voltage, which means that the current
generator has really bad supply sensitivity.
It's possible that measuring system errors (such as where you connected the
measurement system ground) might cause some of these changes. So you
might want to check your setup and the instrument and test lead
computed accuracy.
I think the way I've done it is OK. The 34972A has floating inputs
and I measure from a local GND for all six points. I'm not seing
any noise-artifacts.
I also disagree with your estimate of the CR5 zener current. [...]
[...]
the CR5 diode current must be (12.2195 - 4.2456 mA) = 7.9739 mA.
Good point.
That's
pretty close to the zero temperature coefficient current for the 1N938
you describe. So I do not recommend changing the diode current.
The 7.5 mA optimum is from a much later data-sheet, and may be for
a particular "high performance" variant of the 1N938, so there is
no guarantee that there even is a zero-tempco current for the one
in my HP5065.
Either way, fixing the zeners tempco is only half of the solution,
it looks to me like the "real" solution involves an entirely new
C-field current driver.
--
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.
I had this morning, and put it into action right away:
I disconnected the A15R8 resistor from the tempco afflicted A15CR5 zener diode,
and hooked it up to my Fluke 732A's 10V output (which is speced to 10mA).
I had expected to see a vast improvement in C-field drive stability, but
while the improvement is measurable, it is a lot less than I had expected.
Having eliminted that one, there are two influences left:
- Noise from the +20 V supply leaks through
The data evidently shows this to be the case, but the correlation does
not explain all the C-field current variability.
- Tempco in the constant current driver circuitry
I'm increasingly leaning this way, because even though A15R8 is fed
from the rock-stable Fluke, the voltage at the A15Q6A base still
varies quite a lot with temperature, but the short term jitter is
much better.
I don't have enough data yet to conclude if the MVAR floor is improved,
but it looks quite promising so far: The curve has very confidently
dipped below 1e-13 at 5000 seconds. It used to flatten at 3e-13.
Next experiment is going to be driving the C-field solenoid directly
from the Fluke 732A through a suitable low tempco current-limiting
series resistor.
Anybody want to bet what that will do to the MVAR floor ?
I'll write this up on my web-pages once I have more data.
--
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.
I have decided it is finally time to come clean about one of my
"trade secrets": I sometimes run voltages through MVAR.
Sometimes I also run temperatures, kilowatts and anything else
I happen to have a time-series of through the MVAR.
In this case, it tells us a lot about why the C-field current of
my HP5065 is unstable:
http://phk.freebsd.dk/hacks/HP5065A/20150822_mvar/index.html
I'm not done collecting data for the resulting effect on the HP5065
performance, but so far it looks like the MVAR floor is half of
what it used to be.
--
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.
Poul-Henning,
I abuse ADEV too! It really works well in some cases, doesn't it. Very clever of you to try it on a 5065A with voltage, current. etc. I assume you use the y-form (frequency) not the x-form (phase) in this case?
Notes:
-
The scale is more intuitive, I find, when sqrt is used -- so I'm curious why you chose MVAR instead of MDEV?
-
When plots mostly head down with a -1 slope, consider TDEV instead of MDEV, which effectively rotates by 45 degrees turning -1 slopes into 0 slopes. For some kinds of data a rise above a normal (zero) slope is more informative than a bend of a steep -1 line. Psychologically too, it removes the "things are working better and better as time goes on" impression that happens with a -1 slope, e.g., when ADEV is used on data from a locked loop.
-
Before you settle on MDEV, also try ADEV. There are cases where the massive averaging inside of MDEV ruins interesting noise periodics that would show up in ADEV. Then if you want to combine (2) and (3) you can compute ADEV(tau) * tau. Look at http://leapsecond.com/tools/adev5.c for the undocumented z flag. Like the real TDEV is to MDEV, this is a sort of TDEV for ADEV, but without the sqrt(3). It plots a zero slope where ADEV would plot a -1 slope.
/tvb
----- Original Message -----
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Sunday, August 23, 2015 11:16 AM
Subject: [time-nuts] Running voltages through MVAR (In re: HP5065)
I have decided it is finally time to come clean about one of my
"trade secrets": I sometimes run voltages through MVAR.
Sometimes I also run temperatures, kilowatts and anything else
I happen to have a time-series of through the MVAR.
In this case, it tells us a lot about why the C-field current of
my HP5065 is unstable:
http://phk.freebsd.dk/hacks/HP5065A/20150822_mvar/index.html
I'm not done collecting data for the resulting effect on the HP5065
performance, but so far it looks like the MVAR floor is half of
what it used to be.
--
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.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
In message 85E3D5A82F314700BCA6493EEF245CCF@pc52, "Tom Van Baak" writes:
- The scale is more intuitive, I find, when sqrt is used -- so I'm curious why you chose MVAR instead of MDEV?
Actually the plot is MDEV now that I think of it...
I've added a footnote.
- When plots mostly head down with a -1 slope, consider TDEV
instead of MDEV, which effectively rotates by 45 degrees turning
-1 slopes into 0 slopes. For some kinds of data a rise above a
normal (zero) slope is more informative than a bend of a steep -1
line. Psychologically too, it removes the
"things are working better and better as time goes on" impression
that happens with a -1 slope, e.g., when ADEV is used on data from
a locked loop.
Good point.
- Before you settle on MDEV, also try ADEV. There are cases where
the massive averaging inside of MDEV ruins interesting noise periodics
I generally hunt periodics with FFTs, but yes, ADEV is useful for the
sort of "almost has a stable frequency" like HVAC's turning on/off etc.
--
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.
Poul-Henning,
Using ADEV and MDEV, and indeed TDEV as TVB points out, for analyzing
other physical signals than phase/frequency is maybe not very common,
but if Kocher if you know your field well enough.
David Allan and I discuss this every once in a while. ADEV/MDEV and
friends is really the statistical tools to analyze and separate noises
of different slopes. Together with the bias functions you can use it to
verify you have the white noise you assume, and only then you can
rightfully make use of that property to meet the intention of GUM.
Flicker noise is indeed one such noise-form that does require better
tooling.
Don't forget to use FFT as well as analyzing non-repetitive systematic
trends. I'd love to see the raw data more closely.
So keep up the good work.
Cheers,
Magnus
On 08/23/2015 08:16 PM, Poul-Henning Kamp wrote:
I have decided it is finally time to come clean about one of my
"trade secrets": I sometimes run voltages through MVAR.
Sometimes I also run temperatures, kilowatts and anything else
I happen to have a time-series of through the MVAR.
In this case, it tells us a lot about why the C-field current of
my HP5065 is unstable:
http://phk.freebsd.dk/hacks/HP5065A/20150822_mvar/index.html
I'm not done collecting data for the resulting effect on the HP5065
performance, but so far it looks like the MVAR floor is half of
what it used to be.
Using the Fluke 732A as voltage reference improved HP5065A stability,
but not very much, so I looked closer at the constant-current
generator and the C-field pots "calibration":
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
... and found out that the current-sensing resistors R10||R11 have
a very high temp-co.
Getting closer...
--
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.
Keep in mind that potentiometer R6 is one of the key components of the
C-field circuit.
Together with R8 and R12, it defines the reference voltage to Q6A/B.
The tempco of that wire-wound potentiometer can certainly not be neglected.
I would suggest to check the C-field temperature dependency with an
external precision current source feeding the C-coil directly.
Adrian
Poul-Henning Kamp schrieb:
Using the Fluke 732A as voltage reference improved HP5065A stability,
but not very much, so I looked closer at the constant-current
generator and the C-field pots "calibration":
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
... and found out that the current-sensing resistors R10||R11 have
a very high temp-co.
Getting closer...
In message 55E1ADA8.2000102@arcor.de, Adrian writes:
Keep in mind that potentiometer R6 is one of the key components of the
C-field circuit.
Together with R8 and R12, it defines the reference voltage to Q6A/B.
The tempco of that wire-wound potentiometer can certainly not be neglected.
Absolutely not, it's clearly part of the picture, but something has
10x the tempco of that one right now.
I would suggest to check the C-field temperature dependency with an
external precision current source feeding the C-coil directly.
That's what I'm doing right now: C-field driven by Fluke 732A
through 2.5k resistor.
--
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
Most potentiometers have very good TC when used in a “ratio mode”.
You can quickly mess that up by putting resistance in series with
the pot. It’s a bit surprising how may times you see this inherent stability
messed up that way.
Bob
On Aug 29, 2015, at 9:03 AM, Adrian rfnuts@arcor.de wrote:
Keep in mind that potentiometer R6 is one of the key components of the
C-field circuit.
Together with R8 and R12, it defines the reference voltage to Q6A/B.
The tempco of that wire-wound potentiometer can certainly not be neglected.
I would suggest to check the C-field temperature dependency with an
external precision current source feeding the C-coil directly.
Adrian
Poul-Henning Kamp schrieb:
Using the Fluke 732A as voltage reference improved HP5065A stability,
but not very much, so I looked closer at the constant-current
generator and the C-field pots "calibration":
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
... and found out that the current-sensing resistors R10||R11 have
a very high temp-co.
Getting closer...
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
In message 8BE4F2B1-171B-4117-88A5-A6DC64D9890D@n1k.org, Bob Camp writes:
Most potentiometers have very good TC when used in a “ratio mode”.
You can quickly mess that up by putting resistance in series with
the pot. It’s a bit surprising how may times you see this inherent stability
messed up that way.
Yes, but they do have a good reason for it here: They want the
C-field pot to be "calibrated" in frequency so they need the
resulting voltage to follow the quadratic C-field sensitivity
decently well.
I still havn't figured out what component is responsible for the
huge tempco, but it is not the C-field pot. There's a few PPM
there but not that many.
--
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.
Bob,
agreed. But in the 5065A C-field circuit, the pot is wired as a variable
resistor.
The Bourns data sheets specifies +/- 50 ppm / deg C max.
Together with the two series resistors, the effective TC of the pot
drops to about 20 ppm.
That is indeed just 2 times more than the effective temperature
coefficient of the reference diode (1N938).
Btw. the 1N938 is running at about 7.5 mA +/- 1 mA, not at 12 mA as
mentioned earlier.
Some 3.5 to 5.5 mA are going through R8, R6, R12.
To get the correct diode current, this current has to be subtractrd from
the total current through R7 (Q5, Q6 base currents neglected).
Adrian
Bob Camp schrieb:
Hi
Most potentiometers have very good TC when used in a “ratio mode”.
You can quickly mess that up by putting resistance in series with
the pot. It’s a bit surprising how may times you see this inherent stability
messed up that way.
Bob
On Aug 29, 2015, at 9:03 AM, Adrian rfnuts@arcor.de wrote:
Keep in mind that potentiometer R6 is one of the key components of the
C-field circuit.
Together with R8 and R12, it defines the reference voltage to Q6A/B.
The tempco of that wire-wound potentiometer can certainly not be neglected.
I would suggest to check the C-field temperature dependency with an
external precision current source feeding the C-coil directly.
Adrian
Poul-Henning Kamp schrieb:
Using the Fluke 732A as voltage reference improved HP5065A stability,
but not very much, so I looked closer at the constant-current
generator and the C-field pots "calibration":
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
... and found out that the current-sensing resistors R10||R11 have
a very high temp-co.
Getting closer...
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
Another counter argument against the “ratio mode” is that with zero current
through the wiper, odd things can happen. There is always something
else to consider in any real design.
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.
Bob
On Aug 30, 2015, at 8:05 AM, Adrian rfnuts@arcor.de wrote:
Bob,
agreed. But in the 5065A C-field circuit, the pot is wired as a variable
resistor.
The Bourns data sheets specifies +/- 50 ppm / deg C max.
Together with the two series resistors, the effective TC of the pot
drops to about 20 ppm.
That is indeed just 2 times more than the effective temperature
coefficient of the reference diode (1N938).
Btw. the 1N938 is running at about 7.5 mA +/- 1 mA, not at 12 mA as
mentioned earlier.
Some 3.5 to 5.5 mA are going through R8, R6, R12.
To get the correct diode current, this current has to be subtractrd from
the total current through R7 (Q5, Q6 base currents neglected).
Adrian
Bob Camp schrieb:
Hi
Most potentiometers have very good TC when used in a “ratio mode”.
You can quickly mess that up by putting resistance in series with
the pot. It’s a bit surprising how may times you see this inherent stability
messed up that way.
Bob
On Aug 29, 2015, at 9:03 AM, Adrian rfnuts@arcor.de wrote:
Keep in mind that potentiometer R6 is one of the key components of the
C-field circuit.
Together with R8 and R12, it defines the reference voltage to Q6A/B.
The tempco of that wire-wound potentiometer can certainly not be neglected.
I would suggest to check the C-field temperature dependency with an
external precision current source feeding the C-coil directly.
Adrian
Poul-Henning Kamp schrieb:
Using the Fluke 732A as voltage reference improved HP5065A stability,
but not very much, so I looked closer at the constant-current
generator and the C-field pots "calibration":
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
... and found out that the current-sensing resistors R10||R11 have
a very high temp-co.
Getting closer...
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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.
--
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.
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
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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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.
