Empathy List Archives

PK

Poul-Henning Kamp

Sun, Jun 20, 2010 10:18 PM

I am looking at the HP3458A SCAL procedure and for which I am
somewhat short on the required hardware, and have been pondering a
simpler way to perform the SCAL, and would like to hear your comments.

The basic requirement is to supply three AC voltages at two
or three frequencies:

3-10Vrms    @ 100kHz, 2MHz & 8MHz
0.3-1Vrms   @ 100kHz       & 8MHz
30-100mVrms @ 100kHz       & 8MHz

Forget the frequency precision for a moment, that is trivial.

As long as the voltages are within 0.2% (0.017dB?) for the the
various frequencies, the absolute value of the voltage is not
important.

Very few, if any, tone generators can meet that flatness spec, which
is why the cal-procedure specifies use of three hard to get thermal
converters.

I came up with this circuit:

  -----------+-------+--- 330R ----+-----+-------
                 |       |             |     |

SW1 and SW2 are either switches or, likely better: RF relays.

I built a birdnest version of this, using SMD resistors and
two standard relays from the junk-heap.

I tested it with my HP3577A, and the results are not discouraging:
The flatness from 100kHz to 2MHz is close to spec, but I loose
about 0.1dB from 2MHz to 8MHz.

The ideal mechanical construction would be a PCB with two 4mm
bananaplugs in one end, ready to plug into the HP3458A and a
BNC in the other end to connect the generator and a couple of
wires to drive the relays.

But I am far from sure I know how to design this PCB, nor
what kind of components are best (SMD/through-hole).

In fact, I am not even sure it is a good idea to have a ground
plane, since the right hand side has variable circuit impedance
depending on the switch positions.

The alternative is to change the circuit to have 50Z all the way
through, and a 50 Ohm terminator on the meter side.

In that case the voltage selection could be done by switching in a
10 and a 20dB attenuator.

Has anybody tried this ?

Any other ideas ?

--
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 am looking at the HP3458A SCAL procedure and for which I am somewhat short on the required hardware, and have been pondering a simpler way to perform the SCAL, and would like to hear your comments. The basic requirement is to supply three AC voltages at two or three frequencies: 3-10Vrms @ 100kHz, 2MHz & 8MHz 0.3-1Vrms @ 100kHz & 8MHz 30-100mVrms @ 100kHz & 8MHz Forget the frequency precision for a moment, that is trivial. As long as the voltages are within 0.2% (0.017dB?) for the the various frequencies, the absolute value of the voltage is not important. Very few, if any, tone generators can meet that flatness spec, which is why the cal-procedure specifies use of three hard to get thermal converters. I came up with this circuit: -----------+-------+--- 330R ----+-----+------- | | | | Generator optional | 130R 10R (HP33120A) thermal 51R | | HP3458A converter | SW1 SW2 | | | | -----------+-------+-------------+-----+------- SW1 and SW2 are either switches or, likely better: RF relays. I built a birdnest version of this, using SMD resistors and two standard relays from the junk-heap. I tested it with my HP3577A, and the results are not discouraging: The flatness from 100kHz to 2MHz is close to spec, but I loose about 0.1dB from 2MHz to 8MHz. The ideal mechanical construction would be a PCB with two 4mm bananaplugs in one end, ready to plug into the HP3458A and a BNC in the other end to connect the generator and a couple of wires to drive the relays. But I am far from sure I know how to design this PCB, nor what kind of components are best (SMD/through-hole). In fact, I am not even sure it is a good idea to have a ground plane, since the right hand side has variable circuit impedance depending on the switch positions. The alternative is to change the circuit to have 50Z all the way through, and a 50 Ohm terminator on the meter side. In that case the voltage selection could be done by switching in a 10 and a 20dB attenuator. Has anybody tried this ? Any other ideas ? -- 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.

RS

Randy Scott

Mon, Jun 21, 2010 6:32 PM

I tested it with my HP3577A, and the results are not discouraging:
The flatness from 100kHz to 2MHz is close to spec, but I loose
about 0.1dB from 2MHz to 8MHz.

Is this with or without the attenuation from your circuit?

Is is possible that the attenuator could add too much error? For example, I was looking at this SMA 10 dB attenuator:

http://www.crystek.com/microwave/spec-sheets/attenuator/CATTEN-0100.pdf

The difference in attenuation between 300 kHz and 10 MHz is 0.03 dB, nearly double the amount of error allowed.

Has anybody tried this?

I've been on the look-out for the thermal converters, but they seem quite difficult to find (for the prices I'm willing to pay, anyway).

Randy.

> I tested it with my HP3577A, and the results are not discouraging: > The flatness from 100kHz to 2MHz is close to spec, but I loose > about 0.1dB from 2MHz to 8MHz. Is this with or without the attenuation from your circuit? Is is possible that the attenuator could add too much error? For example, I was looking at this SMA 10 dB attenuator: http://www.crystek.com/microwave/spec-sheets/attenuator/CATTEN-0100.pdf The difference in attenuation between 300 kHz and 10 MHz is 0.03 dB, nearly double the amount of error allowed. > Has anybody tried this? I've been on the look-out for the thermal converters, but they seem quite difficult to find (for the prices I'm willing to pay, anyway). Randy.

PK

Poul-Henning Kamp

Mon, Jun 21, 2010 7:21 PM

In message 52474.15137.qm@web57705.mail.re3.yahoo.com, Randy Scott writes:

I tested it with my HP3577A, and the results are not discouraging:
The flatness from 100kHz to 2MHz is close to spec, but I loose
about 0.1dB from 2MHz to 8MHz.

Is this with or without the attenuation from your circuit?

Pretty much the same in all three states (3v,1v,0.1v)

The difference in attenuation between 300 kHz and 10 MHz is 0.03
dB, nearly double the amount of error allowed.

Yes, after starting to play with this problem, I have become quite
suspicious of any dB number with two decimals :-)

I am not trying to meet the spec here, I think that is pretty much
impossibly by any means other than thermal converters. I am just
trying to see how good I can do this, with moderate means.

I have been told that even with thermal converters, people soon learn
to put a box over the TC to stabilize airflow, in order to meet the
0.2% spec.

I've been on the look-out for the thermal converters, but they
seem quite difficult to find (for the prices I'm willing to pay,
anyway).

Indeed. I have been lucky and picked up a 10V and 2V Fluke A55
a couple of weeks ago, and have been having a lot of fun with them
since.

I can't help wondering how HP did this in production, quite a lot of
their RF relays don't meed the 0.2% spec for insertion loss...

Poul-Henning

--
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 <52474.15137.qm@web57705.mail.re3.yahoo.com>, Randy Scott writes: >> I tested it with my HP3577A, and the results are not discouraging: >> The flatness from 100kHz to 2MHz is close to spec, but I loose >> about 0.1dB from 2MHz to 8MHz. > >Is this with or without the attenuation from your circuit? Pretty much the same in all three states (3v,1v,0.1v) >The difference in attenuation between 300 kHz and 10 MHz is 0.03 >dB, nearly double the amount of error allowed. Yes, after starting to play with this problem, I have become quite suspicious of any dB number with two decimals :-) I am not trying to meet the spec here, I think that is pretty much impossibly by any means other than thermal converters. I am just trying to see how good I can do this, with moderate means. I have been told that even with thermal converters, people soon learn to put a box over the TC to stabilize airflow, in order to meet the 0.2% spec. >I've been on the look-out for the thermal converters, but they >seem quite difficult to find (for the prices I'm willing to pay, >anyway). Indeed. I have been lucky and picked up a 10V and 2V Fluke A55 a couple of weeks ago, and have been having a lot of fun with them since. I can't help wondering how HP did this in production, quite a lot of their RF relays don't meed the 0.2% spec for insertion loss... Poul-Henning -- 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.

GB

Greg Burnett

Mon, Jun 21, 2010 9:07 PM

The key to making this work is to develop a stable, repeatable process, and
to have a way to characterize your system flatness at the far end of your
cable. Any flatness errors (including those of your relays and attenuator)
can be accounted for during characterization if your system is stable and
repeatable.

I would expect your attenuator(s) to have imperfect flatness characteristics
from 100kHz to 8MHz. This imperfection is OK, if you have a way to
characterize it. But this brings us back to the use of thermal converters
for characterization - and for that reason I'd want to keep my
source-attenuator-load at 50 ohms to match my 50 ohm thermal converters.

It's possible to improve your attenuator flatness by adding frequency
compensation (e.g. capacitance across series leg of your attenuator).
However, you'd still need thermal converters to characterize your result.
Also note that your system flatness error (e.g., at 8MHz relative to 100kHz)
could be any number - even very large error - as long as you've
characterized the error and you are accounting for it in your process.

In your 3458A Calibration Manual, page 3-8, I recommend adding the following
steps:

...Before Step 1...
Step 0: Execute the ACAL DC command. Then confirm 10VDC and 1VDC are in
tolerance.

...Add the following instruction to steps 3 through 9...

Before taking your reading, wait 2 minutes for temperature equilibrium
between your cable, connector/adapter and thermal converter.

Incidentally, due to the 3458A's input impedance imperfections as a function
of high-frequency, I really can't much imagine people using the 3458A AC
mode for measurements above 100kHz anyway. The 3458A's AC "claim-to-fame" is
at frequencies below 100kHz, especially below 20kHz (ACSYNC mode, sinewave
sources). But for frequencies such as 2 or 8 MHz, the measurement error is
large and measurement uncertainty is large. It's one thing to adjust and
verify the 3458A at 2 and 8 MHz (with its input terminated by a 50 ohm
load), but it's another thing in the real world to make measurements at
those kinds of frequencies with "who-knows-what" load (e.g. just the 3458A's
input impedance characteristic alone).

Best,
Greg

----- Original Message -----
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
To: "Discussion of precise voltage measurement" volt-nuts@febo.com
Sent: Monday, June 21, 2010 1:21 PM
Subject: Re: [volt-nuts] HP3458A SCAL hardware

In message 52474.15137.qm@web57705.mail.re3.yahoo.com, Randy Scott writes:

I tested it with my HP3577A, and the results are not discouraging:
The flatness from 100kHz to 2MHz is close to spec, but I loose
about 0.1dB from 2MHz to 8MHz.

Is this with or without the attenuation from your circuit?

Pretty much the same in all three states (3v,1v,0.1v)

The difference in attenuation between 300 kHz and 10 MHz is 0.03
dB, nearly double the amount of error allowed.

Yes, after starting to play with this problem, I have become quite
suspicious of any dB number with two decimals :-)

I am not trying to meet the spec here, I think that is pretty much
impossibly by any means other than thermal converters. I am just
trying to see how good I can do this, with moderate means.

I have been told that even with thermal converters, people soon learn
to put a box over the TC to stabilize airflow, in order to meet the
0.2% spec.

I've been on the look-out for the thermal converters, but they
seem quite difficult to find (for the prices I'm willing to pay,
anyway).

Indeed. I have been lucky and picked up a 10V and 2V Fluke A55
a couple of weeks ago, and have been having a lot of fun with them
since.

I can't help wondering how HP did this in production, quite a lot of
their RF relays don't meed the 0.2% spec for insertion loss...

Poul-Henning

--
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.

volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

The key to making this work is to develop a stable, repeatable process, and to have a way to characterize your system flatness at the far end of your cable. Any flatness errors (including those of your relays and attenuator) can be accounted for during characterization *if* your system is stable and repeatable. I would expect your attenuator(s) to have imperfect flatness characteristics from 100kHz to 8MHz. This imperfection is OK, *if* you have a way to characterize it. But this brings us back to the use of thermal converters for characterization - and for that reason I'd want to keep my source-attenuator-load at 50 ohms to match my 50 ohm thermal converters. It's possible to improve your attenuator flatness by adding frequency compensation (e.g. capacitance across series leg of your attenuator). However, you'd still need thermal converters to characterize your result. Also note that your system flatness error (e.g., at 8MHz relative to 100kHz) could be *any* number - even very large error - as long as you've characterized the error and you are accounting for it in your process. In your 3458A Calibration Manual, page 3-8, I recommend adding the following steps: ...Before Step 1... Step 0: Execute the ACAL DC command. Then confirm 10VDC and 1VDC are in tolerance. ...Add the following instruction to steps 3 through 9... * Before taking your reading, wait 2 minutes for temperature equilibrium between your cable, connector/adapter and thermal converter. Incidentally, due to the 3458A's input impedance imperfections as a function of high-frequency, I really can't much imagine people using the 3458A AC mode for measurements above 100kHz anyway. The 3458A's AC "claim-to-fame" is at frequencies below 100kHz, especially below 20kHz (ACSYNC mode, sinewave sources). But for frequencies such as 2 or 8 MHz, the measurement error is large and measurement uncertainty is large. It's one thing to adjust and verify the 3458A at 2 and 8 MHz (with its input terminated by a 50 ohm load), but it's another thing in the real world to make measurements at those kinds of frequencies with "who-knows-what" load (e.g. just the 3458A's input impedance characteristic alone). Best, Greg ----- Original Message ----- From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: "Discussion of precise voltage measurement" <volt-nuts@febo.com> Sent: Monday, June 21, 2010 1:21 PM Subject: Re: [volt-nuts] HP3458A SCAL hardware In message <52474.15137.qm@web57705.mail.re3.yahoo.com>, Randy Scott writes: >> I tested it with my HP3577A, and the results are not discouraging: >> The flatness from 100kHz to 2MHz is close to spec, but I loose >> about 0.1dB from 2MHz to 8MHz. > >Is this with or without the attenuation from your circuit? Pretty much the same in all three states (3v,1v,0.1v) >The difference in attenuation between 300 kHz and 10 MHz is 0.03 >dB, nearly double the amount of error allowed. Yes, after starting to play with this problem, I have become quite suspicious of any dB number with two decimals :-) I am not trying to meet the spec here, I think that is pretty much impossibly by any means other than thermal converters. I am just trying to see how good I can do this, with moderate means. I have been told that even with thermal converters, people soon learn to put a box over the TC to stabilize airflow, in order to meet the 0.2% spec. >I've been on the look-out for the thermal converters, but they >seem quite difficult to find (for the prices I'm willing to pay, >anyway). Indeed. I have been lucky and picked up a 10V and 2V Fluke A55 a couple of weeks ago, and have been having a lot of fun with them since. I can't help wondering how HP did this in production, quite a lot of their RF relays don't meed the 0.2% spec for insertion loss... Poul-Henning -- 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. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

PK

Poul-Henning Kamp

Mon, Jun 21, 2010 9:25 PM

In message A4745DFDC9CB48A0931823828C04651E@gb02, "Greg Burnett" writes:

Incidentally, due to the 3458A's input impedance imperfections as a function
of high-frequency, I really can't much imagine people using the 3458A AC
mode for measurements above 100kHz anyway.

See, this sort of gets to the heart of my effort: I don't care
about performance above 100kHz, but I have not been able to figure
out from the manual, what impact, if any, the 2MHz and 8MHz
calibrations have at lower frequencies.

If there is no relevant impact, My attenuator method will certainly
work for my purposes.

--
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 <A4745DFDC9CB48A0931823828C04651E@gb02>, "Greg Burnett" writes: >Incidentally, due to the 3458A's input impedance imperfections as a function >of high-frequency, I really can't much imagine people using the 3458A AC >mode for measurements above 100kHz anyway. See, this sort of gets to the heart of my effort: I don't care about performance above 100kHz, but I have not been able to figure out from the manual, what impact, if any, the 2MHz and 8MHz calibrations have at lower frequencies. If there is no relevant impact, My attenuator method will certainly work for my purposes. -- 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.

GB

Greg Burnett

Mon, Jun 21, 2010 10:02 PM

Right. SCAL at 2 and 8 MHz will not affect performance below 100kHz. (The
box self-adjusts its input attenuator audio frequency response during ACAL
AC by adjusting for most-flat response of an internally generated
squarewave.)

----- Original Message -----
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
To: "Discussion of precise voltage measurement" volt-nuts@febo.com
Sent: Monday, June 21, 2010 3:25 PM
Subject: Re: [volt-nuts] HP3458A SCAL hardware

In message A4745DFDC9CB48A0931823828C04651E@gb02, "Greg Burnett" writes:

Incidentally, due to the 3458A's input impedance imperfections as a
function
of high-frequency, I really can't much imagine people using the 3458A AC
mode for measurements above 100kHz anyway.

See, this sort of gets to the heart of my effort: I don't care
about performance above 100kHz, but I have not been able to figure
out from the manual, what impact, if any, the 2MHz and 8MHz
calibrations have at lower frequencies.

If there is no relevant impact, My attenuator method will certainly
work for my purposes.

--
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.

volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

Right. SCAL at 2 and 8 MHz will not affect performance below 100kHz. (The box self-adjusts its input attenuator audio frequency response during ACAL AC by adjusting for most-flat response of an internally generated squarewave.) ----- Original Message ----- From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: "Discussion of precise voltage measurement" <volt-nuts@febo.com> Sent: Monday, June 21, 2010 3:25 PM Subject: Re: [volt-nuts] HP3458A SCAL hardware In message <A4745DFDC9CB48A0931823828C04651E@gb02>, "Greg Burnett" writes: >Incidentally, due to the 3458A's input impedance imperfections as a >function >of high-frequency, I really can't much imagine people using the 3458A AC >mode for measurements above 100kHz anyway. See, this sort of gets to the heart of my effort: I don't care about performance above 100kHz, but I have not been able to figure out from the manual, what impact, if any, the 2MHz and 8MHz calibrations have at lower frequencies. If there is no relevant impact, My attenuator method will certainly work for my purposes. -- 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. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

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HP3458A SCAL hardware