KVD for ACV?...

[Rax]

Hi all,
As I just acquired an AC voltage standard (Fluke 510A/1KHz) and having a KVD (ESI RV722), can I reliably divide the output (10VAC) with the divider, and output precise fractional AC voltages?

I think this is supposed to work very well with low DC voltages, which would allow the meter to look at this from its stealth 10G input impedance, but I'm not sure what the accuracy of the output would be with AC voltage.

Thank you!

[Vgkid]

You want an inductive decade divider.
Gertsch is a common maker , Eaton,  Palico,  esi

[srb1954]

You want an inductive decade divider.
Gertsch is a common maker , Eaton,  Palico,  esi

Sometimes also known as a ratio transformer. Gertsch is the best known brand but they are not cheap.

For AC signals ratio transformers have the advantage that their division ratios are inherently fixed and stable so they don't need periodic re-calibration like a KVD. However, they are somewhat limited in their usable frequency range and the maximum voltage that can be applied.

[Vgkid]

You want an inductive decade divider.
Gertsch is a common maker , Eaton,  Palico,  esi

Sometimes also known as a ratio transformer. Gertsch is the best known brand but they are not cheap.

For AC signals ratio transformers have the advantage that their division ratios are inherently fixed and stable so they don't need periodic re-calibration like a KVD. However, they are somewhat limited in their usable frequency range and the maximum voltage that can be applied.

Gertsch units are not that expensive, tegam/ eaton/esi units are.

[Rax]

So, a regular KVD is not appropriate for this - that's what I'm hearing.

I guess the Vac rating on my KVD gave me unfounded hopes... 

[Vgkid]

I believe that there used to be an ac response note in the esi rv-xxx manual.

[Martian Tech]

RTFM.

"The RV 722 is suited for use at low frequency ac as well as dc." [page 1-2]
"For ac measurements the detector is nearly always tuned and exhibits a fairly high Q..." [App Note 29, page 12]

https://www.ietlabs.com/pdf/application_notes/RV722_app_note.pdf

[Conrad Hoffman]

A KVD should work at low frequencies, but I don't know what the limit is. The price for ratio transformers varies widely as they aren't used as often as they once were. General Radio also made a couple that are quite excellent, but rarer than hen's teeth.

https://www.ietlabs.com/pdf/Manuals/GR/1493%20Precision%20Deacde%20Transformer.pdf

They also made a version with one or two less decades. I have one and a friend has the 1493, but those are the only two I've ever seen in the wild.

Henry Hall did a paper on precision AC ratios where he got down into the ppb range. Doesn't seem possible, but he did it.

[Rax]

RTFM.

"The RV 722 is suited for use at low frequency ac as well as dc." [page 1-2]
https://www.ietlabs.com/pdf/application_notes/RV722_app_note.pdf

Well, fine - and thank you - but it's interestingly tucked in the "Description" section (the very last sentence), and there's not a word about AC in the specs. Therefore, this tells me almost nothing. For instance, it'd be interesting to... disclose what exactly is "low frequency." Is it up to 1kHz, 10kHz, 20kHz (audio), is it 100kHz (typical "LF" in some quarters), or are we getting near the MHz'? Which I'm sure we're not.

"For ac measurements the detector is nearly always tuned and exhibits a fairly high Q..." [App Note 29, page 12]
https://www.ietlabs.com/pdf/application_notes/RV722_app_note.pdf

I don't think this is quite directly related, my question was specifically regarding a standard ACV source as divided by a KVD. I'd want to learn if I can output such divided ACVs into meters during calibration adjustment.

[bdunham7]

If the AC reference has a low source impedance and you ignore any parastic capacitance or inductance in the KVD, then the main remaining issue is the loading of the meter and the output impedance of the KVD at whatever setting you are using..  Assume you have 1M + 100pF for loading and you can then calculate what errors might be introduced.  For a 0.1 ratio on your KVD, I think the loading errors willl be more than you would like, probably more than 1%. 

[Rax]

If the AC reference has a low source impedance and you ignore any parastic capacitance or inductance in the KVD, then the main remaining issue is the loading of the meter and the output impedance of the KVD at whatever setting you are using..  Assume you have 1M + 100pF for loading and you can then calculate what errors might be introduced.  For a 0.1 ratio on your KVD, I think the loading errors willl be more than you would like, probably more than 1%.

Thank you, b, that's helpful. I assume the parasitic capacitance and inductance to be relatively sizable (relative to the frequency of the source, hence the "low frequency" requirement). But maybe this approach at least gives me a starting point to estimate under what conditions and which ranges I can actually trust the KVD to output ACVs that can be trusted.

[Vgkid]

While not helpful model wise, genrad, and esi do use mica card resistors, but of different winding techniques.
This should give you a guesstimate for frequency response.
Looking at the math, some accuracy can be had at low frequencies. Though I wonder how the inductance of the wiring plays into it.

[srb1954]

If the AC reference has a low source impedance and you ignore any parastic capacitance or inductance in the KVD, then the main remaining issue is the loading of the meter and the output impedance of the KVD at whatever setting you are using..  Assume you have 1M + 100pF for loading and you can then calculate what errors might be introduced.  For a 0.1 ratio on your KVD, I think the loading errors willl be more than you would like, probably more than 1%.

Thank you, b, that's helpful. I assume the parasitic capacitance and inductance to be relatively sizable (relative to the frequency of the source, hence the "low frequency" requirement). But maybe this approach at least gives me a starting point to estimate under what conditions and which ranges I can actually trust the KVD to output ACVs that can be trusted.

The calculation of the AC effect of loading is complicated by the considerable variation in the source resistance presented by the KVD as the settings are changed. There is a graph in the RV722 manual that shows the source resistance varying by a factor of about 100 as the KVD settings are adjusted over its range. You would have to take the worst case figure in that graph to determine whether your external capacitive/resistive loading causes excessive error at your frequency of interest.

Normally a KVD is used in a DC bridge circuit with a null detector so the load resistance presented at the null is practically infinite and the KVD source resistance doesn't have any effect on the setting accuracy.

 

[Kleinstein]

There are 2 problems with a KVD with AC. One is that AC voltmeters are usually not very high impedance, but more like 1 M or maybe 10 M and at least have some input capacitance. The 2nd is parasitic capacitance also inside the KVD. Because of the usually relatively large resitors parasitic inductivity is less of an issue.

With resistors in the 100 K range and parasitic capacitance in the 10s of pF the cross over from resistive to capacitve is in the 100 kHz range. For good accuracy one should be well (e.g. a fator 1000 for 0.1% range) below that. So in this case low frequency should mean about mains frequency with already a reduced accuracy.

[alm]

I'd look if you can find research articles from the time these dividers were common where they use them for AC or compare them to IVDs. What I could quickly find is this paper which in the introduction very briefly mentions that calibrating resistive dividers for AC voltage at high audio frequencies is problematic because of parasitic inductance and capacitance. This is even after calibration (correcting for frequency-dependence). So without any corrections I think Kleinstein's estimate would be reasonable.

There may well be more articles out there that may not use the words Kelvin-Varley or that are not very well indexed. For example, try skimming NIST special publication 300 volume 3, which is a collection of NIST papers from the sixties.

If you can't find any reference to people using them for AC, then that's probably not a sign that it works well

[Conrad Hoffman]

Not sure when/if I can get to this, but since I have multiple ratio transformers and KVDs, I should be able to do a comparison and see exactly what the KVDs are capable of. I need to clean up the bench, so this might be a good excuse. That GR manual I linked shows using a null meter and some parts for phase compensation, which I need to understand.

[Rax]

Awesome input, thank you all.

What about in joint venture with a null meter? I am thinking that possibly the parasitic effects may get canceled in a "zero reading" setup, with no (or infinitesimally small) currents flowing.

But along these lines, can anyone describe a practical application for ACV? I am not sure I fully grasp it in application (involving a null detector, that is). I'd essentially have an ACV standard (510A), a KVD (RV722), a calibrated meter for characterization, and a whole bunch of other things that may come in handy but can't imagine they do.

[Kleinstein]

An AC nullmeter it a bit tricky. Usually the null detectors for AC have to also look at the phase (e.g. as a lock in amplifier), so one knows which way to adjust. Otherwise one gets a more or less broad minimum.
Reading null helps with the input impedance of the meter, but not with capacitance to ground and internal capacitance, that is in parallel to parts of the divider.

A problem with the KVD is that it has a variable impedance of maybe some 50 - 100 K at the center and getting smaller to the ends, but not necessary in a simple way.
The big advantag of the transformer based dividers it that the outputs are low impedance, like 50 ohm or mybe even 10 ohm range. So parasitic capacitances are much less important.
Still they idelly use auxiliary dividers and driven shields / triax connectors if really high accuracy is aimed for.

[Rax]

An AC nullmeter it a bit tricky. Usually the null detectors for AC have to also look at the phase (e.g. as a lock in amplifier), so one knows which way to adjust. Otherwise one gets a more or less broad minimum.

The article mentioned by Martian Tech may offer some clues:

"For ac measurements the detector is nearly always tuned and exhibits a fairly high Q..." [App Note 29, page 12]
https://www.ietlabs.com/pdf/application_notes/RV722_app_note.pdf

[alm]

Not sure when/if I can get to this, but since I have multiple ratio transformers and KVDs, I should be able to do a comparison and see exactly what the KVDs are capable of. I need to clean up the bench, so this might be a good excuse. That GR manual I linked shows using a null meter and some parts for phase compensation, which I need to understand.

The ESI DT72A manual also describes using a R with a variable C circuit to make the phase shift of two dividers the same. I'd think a generic AC millivoltmeter might be good enough after you adjust the phase shift for minimum reading between the two dividers, although you may have to readjust every time you adjust a divider substantially.

I was also considering comparing a resistive KVD (Fluke 720A) with an IVD (ESI DT72A) , but I was planning to use absolute readings of a DMM to compare, which won't be 1 ppm accurate, but then I don't expect great performance from the KVD. Meter input impedance, which would be 1 MOhm in parallel with some dozens of pF best case, and even lower for some AC measurement standards (Wavetek 4920) will certainly be a concern. But the closest I have to an AC null meter is the Fluke 8920, which I'm not sure is up to the task.

But along these lines, can anyone describe a practical application for ACV? I am not sure I fully grasp it in application (involving a null detector, that is). I'd essentially have an ACV standard (510A), a KVD (RV722), a calibrated meter for characterization, and a whole bunch of other things that may come in handy but can't imagine they do.

See for example the application section of ESI DT72A (and probably manuals for other ratio transformers). The main use would be using the AC null detector to compare the divided output to that of another divider.

[guenthert]

You want an AC null detector.  A lock-in amplifier can act as a (high impedance) sensitive AC null detector.  As a bonus, they typically have a reference signal output, which can be used as an AC voltage supply for the bridge (you probably will need an AC amplifier though).

[alm]

I looked at the Fluke 720A manual, and the only way I can see the load impedance of a DMM in ACV range not destroy any accuracy, even at very low frequencies, is to use the divider at ratios like \$2\cdot10^{-4}\$ and lower. That way the output impedance is low enough that variations in the DMMs input impedance will not swamp the uncertainty budget, but uncertainty due to the KVD linearity of 0.1 ppm of input will be 0.05%, and you will still need to correct for the loading impedance. The lower the ratio, the higher the linearity error and the lower the loading error. So I'd argue KVDs like this are not useful for AC unless you can increase the load impedance using either potentiometric technique (null meter against another divider) or some sort of buffer amplifier.

If the only usable way to use a KVD for ACV is using a null meter because the loading due to the input impedance of a DMM is too high, then I'd argue that fact alone is enough to severely limit their usability for ACV. Since you'd probably need to determine correction coefficients for the KVD for ACV using another divider, what would be the point of subsequently using that KVD to compare it to yet a third divider?

I don't see how an AC null meter would work with another non phase locked source like adjusting an ACV calibrator or stable function generator to a Fluke 510A.

[Alex Nikitin]

The ancient KVD I have in my home lab is specified for AC work up to 4kHz, with 0.001% error (same as at DC) up to 400Hz.

Cheers

Alex

[Rax]

Not sure when/if I can get to this, but since I have multiple ratio transformers and KVDs, I should be able to do a comparison and see exactly what the KVDs are capable of. I need to clean up the bench, so this might be a good excuse. That GR manual I linked shows using a null meter and some parts for phase compensation, which I need to understand.

Looking forward, Conrad! And thanks!!

[Conrad Hoffman]

GR appears to use the common 1232 as a null meter. Maybe OK with the ratio trans, but very low input impedance for a resistive divider. They sold the 1232-P2 preamp, which seems to be 100 meg input and a gain of 0.7, which would improve things, but not as much as desired. Also has high shunt capacitance. It's also single ended, which makes me a bit wary. (I don't have one) Seems like an opamp buffer would be easy enough.