KVD for ACV?...

[Vgkid]

The 1232a preamp is just a fet input 2n3457

[Rax]

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

That's a useful data point, and it kind of makes sense to me. I should probably not expect the ppm performance (in DC) my unit has (.1ppm) in AC under any scenario.

[alm]

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.

Does the input impedance matter if you're measuring between two dividers that are matched in amplitude and phase? The DT72A manual shows a recipe where one of the divider outputs is grounded to work with grounded detectors.

A buffer amplifier could make it actually useful for real loads instead of purely potentiometric measurements, though.

[Kleinstein]

If measuring zero the input impedance of the meter would not matter, but the capacitance to ground can matter.  Chances are a meter will have some capacitance to ground too, especially if mains powered. So it would at least load down one side.
Similar just the cables will have capacitance (e.g. some 1 pF per meter) and this may need active driven shields to avoid loading. E.g. an auxiliary divider or directly drive the source via a transformer to get the output about at ground.
AC bridge circuits can be quite a bit more complicated than a simple DC circuit.

[alm]

If you ground one side of the meter (the output of one of the dividers), and adjust the other divider so amplitude and phase are matched, would capacitive loading of the meter still be a problem?

[Kleinstein]

With briging the divider output to a virtual ground one can avoid much of the capacitive loading. However this needs additional effort, usually some sort of transformer drive to than drive both ends of the dividers and maybe extra adjustment (may need capacitive and resistive trim) to get really close to zero.
This is nothing new but old standard, especially in times when semsitive AC meters often needed mains power or at least large batteries.
https://nvlpubs.nist.gov/nistpubs/jres/40/jresv40n3p245_a1b.pdf

[alm]

See figure 2.1c (page 13) of the ESI DT72A manual for an example of this.

[alm]

I just came across the "DC and Low Frequency AC Ratio Measurements" application note in the already linked https://www.ietlabs.com/pdf/application_notes/RV722_app_note.pdf (starts on page 34). It mentions using resistive dividers for DC and low frequency AC, and inductive dividers for higher frequencies. But unfortunately its not very specific about what low frequency means.

Since this app note is from the sixties, it only talks about bridge configurations and not about lower impedance DMMs as loads. Rax, do you have the 10kOhm or 100 kOhm input impedance version of the RV722? According to the manual the RV722 existed in both versions. The 10 kOhm would probably be more usable with a 1 MOhm ACV DMM as load.

[Kleinstein]

The 1 M DMM input impedance could at least be compensated to a certain degree. If the error is small, one could do a test with additional series resistance/capacitance so that the output impedance about doubles and use the 2 readings than to correct. Without any corretion 1 M load may be a problem already.

[alm]

I'm worried about the stability of this 1 MOhm impedance, so that's why in my suggestion about what setting to use on the KVD (2e-4 or lower assuming a 100 kOhm KVD), I picked a setting where the uncertainty due to load impedance was attenuated about a factor of 100 compared to the uncertainty due to KVD linearity. So the that if the load was 1 MOhm +/- 10 kOhm (1%), its contribution to uncertainty would be roughly the same as the KVDs linearity's contribution (which is attenuated by a factor 1/x where x is the setting of the KVD, like 2e-4). Then you could calibrate the setup by building a bridge from a KVD and IVD with a 1 MOhm load across the KVD (wiper to low).

But I admit that I never measured the stability of input impedance in ACV mode, and that obviously over frequency range this will become more complicated. Is the DMM more or less resistive than a 'resistive' voltage divider?

[Rax]

Rax, do you have the 10kOhm or 100 kOhm input impedance version of the RV722?

I have the 100k version.

[1audio]

FWIW I have an RV722 a 7 decade Gertsh ratiotran (and an HF ratio tran) and a Fluke 931B AC differential meter. If I can find time and bench space I'll try comparing them. (I also have the Fluke 8060A and 8922a) It will be interesting.

However chasing PPM accuracy for AC is a fools errand. Its really hard to do and the number of error sources are enormous. Even though the Gertsch is 7 digits I'm not sure what instument can reliably detect that last digits change. Just determining flatness of your source is really difficult. I have several thermal voltage converters but their accuracy vs frequency degrades a lot as frequency goes up. References to .0001% are more like wishful thinking in our world. Maybe more real at NIST with the JJ based DAC (I should offer one to the audiophile market- $500K per channel and you need a staff physicist to keep it running).
What I suggest would be to compare the KVD to the IVD using the differential meter (1 meg/8 pF ) by just comparing the output. (Its meaning of differential is different from a differential input.) Nulling AC strikes me as really questionable if you do not have a differential detector. The stray cap to everything is the leakage issues at DC amplified. The nice part of the IVD is that its output impedance is lower than the input.

[Conrad Hoffman]

So, I cleaned up the bench and did a test. Turns out I have a RV622A (100 kohm input), not a 722. Used that against a North Atlantic Industries RB-504 ratio box (ratio transformer). Various "null" detectors were tried: HP3478A DMM, Tektronix 1A7A differential plug-in and a floating GR 1232A AC bridge null meter. Signal was from a Wavetek 185 at 50-400 Hz.

The short answer is this was a dismal failure. The phase shift problem is so severe that one can only null about 3 digits, not exactly metrology lab precision. Just being read with a DMM, either box divides as expected. The easiest thing to do would be to simply calculate the error on on the RV unit due to loading and factor that in. The impedance of the transformer box is low enough that it can be ignored in many/most cases.

I'm going to try the phase compensator shown in the manuals and see how well I can do with that. There's still the matter of capacitive loading of the RV box, so one has to factor that in, depending on frequency. Next test will just use the KVD and see how bad the loading problem really is, and if it's easy to compensate.

There are some GR articles, one in the May 1968 Experimenter, talking about how they could achieve 200 ppb with the ratio transformers. I think I've got a ways to go yet.

[Alex Nikitin]

I suspect that one of the problems might be a (relatively) high harmonics content of the Wavetek 185 output.

Cheers

Alex

[Conrad Hoffman]

No question it's a rounded off triangle wave, but the THD is pretty low and the problem is so bad I don't think I'm anywhere near that limit. I'm going to do something more practical and see how big the errors are just using the KVD to calibrate a meter, since that's all the OPs got right now. It does raise the question of what to use as a generator for AC calibrations. I have a few, but all have some drawback. The 185 is extremely amplitude stable over time and frequency, but not flawlessly clean. My various GR generators will do much higher voltages, but aren't stable or clean. I have an HP with similar issues, plus a broken one! I think Thaler made an AC chip with much better performance, but I'm guessing that's been gone for a long time. I suppose I could run the sine wave from my THD analyzer through an amp, but that's more effort than I'm up to. FWIW, when I do my own meters I use an old Fluke 540B to compare with a DC reference. I don't have the calibration constant for it, but assume it's close to unity.

[Rax]

using the KVD to calibrate a meter, since that's all the OPs got right now. It does raise the question of what to use as a generator for AC calibrations.

Just to help clarify, what I'd use with this is a 10V AC standard sine from a 510A. My Audio Precision box need not apply due to dB-realm accuracy.

I've also started to look at Gertsch units. Would an RT5 serve the purpose? Which would be to scale standard AC voltages for different calibration points.

Arguably, I guess I may currently essentially be piecing together a multifunction calibrator (just doing it with 25 different components).

[Conrad Hoffman]

This is crude at best, but shows what you're up against. The signal generator was an ancient GR BFO, since that's the only thing on the bench that goes to 10 Vrms. The input voltage was monitored with an HP3455A. The target meter was a HP3478a because it was handy. The 3478A was just plugged into the ratio transformer, and then the KVD, at the same settings, noting the reading for each. Nothing was very stable, but the results are still clear.

[1audio]

The Gertsch ratio transformers are all OK if you understand their limitations. Basically 50 Hz to 10 KHz and don't overdrive them. There are versions with 4 5 6 7 and even 8 decades. They are intrinsically accurate. Calibration would be to check for failure (bad switch).

For AC calibration your 510 is a good start. It needs to be calibrated as a baseline.

Here is some info on the 510 mods I did to get lower distortion. https://www.diyaudio.com/archive/blogs/comments/comment3920.html 

Unfortunately for AC there are several gotchas- distortion affects accuracy. 1% distortion can degrade the accuracy significantly.  Frequency response is not an easy one to check. Very few oscillators are really flat. Many will have errors at higher frequencies and even cabling can be an issue. It will keep you very busy.

[Conrad Hoffman]

The 510 is interesting. Does it really manage to hold 10 ppm (typical) over a 0-10 mA load? That seems very good.

[Rax]

The 510 is interesting. Does it really manage to hold 10 ppm (typical) over a 0-10 mA load? That seems very good.

If you mean the 24hr accuracy, I think it's more like 100ppm (.01%).

[Rax]

Here is some info on the 510 mods I did to get lower distortion. https://www.diyaudio.com/archive/blogs/comments/comment3920.html 

Thanks a lot for this. One quick question - I can't find an R51 on the schematic if my life depended on it. But are you sure that's not a typo, and instead you refer to R61? I think a CCS would make sense there.