[volt-nuts] Checking an LCR meter
acbern at gmx.de
acbern at gmx.de
Sat Feb 7 07:26:43 EST 2015
I have a smiliar issue, so interesting to read this. Mine is related to older gear like 4192a and 4275A meters which I am using. All these old meters, if properly adjusted an calibrated, are very good meters, you can repair them yourself, and are very cost efficient (what did you pay for our 4284 if I may ask, I find them still pretty expensive). Newer gear, as you stated, did not add much higher percision if at all (I was told by professional seller of used gear that this is why they are very much demanded, he cennot get enough of them).
I too do not want to spend the money for getting these meters calibrated on a regular basis, it would cost a fortune over time. The same issue applies to my other gear, so I have decided I have a few basic standards calibrated externally on a regular basis, and the rest is calibrated from there:
My standards are voltage (10V 732a/4910), resistance (10k, SR104), rf level (nrp z 55, power sensor dc to 40ghz), and lower frequency thermal converter. The rest is then calibrated by trsnsfer measurements (in a traceable manner). The 3458A e.g. is always good for ratio measurements because of its excellent linearity, so with a stable voltage source, one can calibrate resistors from 1ohm to 1mohm easily and precisely. Just as an example. On a ide not e.g. I have my 732a 10V calibrated only (to < 0,5ppm acc.), and do the 1V calibration with the 3458A. One can do that more often, as this output is less stable, using a 3458A, and saves money externally too.
To your question: today the capacitance is derived in national standard labs from the quantum ohm by quadrature bridges. There also is a calculable capacitor model being used (earlier), but this requires precise machining, and I have not further considered this. Quadrature bridges as used in standard labs are accurate to <1ppm. These are lab setups with cables flying arround, built up by a set of special transformers, dds generators, lock-in amp detector amplifiers and so on, so these are no special all-in-one meters. Others have shown/reported that pretty basic lab bridges using standard stuff (7 digit ratio transformers, standard dds generators, precision ac level meters) are still good to 10e-5, which is pretty precise. Dont get me wrong, this still requires expensive and precision gear, but nothig very special (e.g. no special custom made transformers and stuff).
So I am currently preparing a setup of a coaxial quadrature bridge. A 1000pf coaxial capacitor (gr type), these are reasonably cheap and very stable, and as standard a 10kohm coaxial resistor will be used. This can be build using rf cases and a good, low ppm/k resistor. I am still trying to figure out the math re. error propagation (e.g., how does a potential phase error of the dds generator needed to generate the quadrature signal translate into a mesurement tolerance). There is some data online, some with a download fee, and also books.
So with this setup, one can precisely calibrate a 1000pf capacitor. The gear needed may sound a little expensive, but I have it anyway, amongst other reasons also to calibrate my other stuff (e.g the ratio transformer is used for ac meter and calibrator cal.), so all I needed in addition is the capacitors (actually I bought gr types from 1pf to 1000pf) and the coaxial cabling setup which is a litte special related to the ratio transformer, rest is just bnc cabling. These gr capacitors have very little drift, they are air capacitors built from sheet metal with temp elong. coefficient of essentially 0. This allows that they are calibrated say every 2 years or more (cal labs do that too).
I did also buy some time ago russian hermetic glas mica capacitors, which I had calibrated externally, so it will be interesting to see how they aged. (These should be fairly stable, if you want one let me know). I installed them into a rf box with 4 bnc connectors (similar to the ones used as standard cal devices by Agilent). For the gr capacitors, you need an adapter between the gr874 connectors and the 4 BNCs. You also need to build this.
Thats all then for the 1000pF. The other capacitors can then be calibrated with a similar setup using again the ratio transformer in a capacitor bridge setup. And one needs the math behind to derive the accuracies of the measurements. As you plan to do an initial cal of your meter externally, you could vaildate the bridge tolerance calculations. (Actually we could later also exchange capacitors to validate results, I have done that with a voltnut here in Germany on some other stuff some time ago.)
Then you also need shorts and opens and resistors in shielded cases, this is straigtforward though, I had built them up some time ago already for my meters.
> Gesendet: Freitag, 06. Februar 2015 um 23:10 Uhr
> Von: "Dr. David Kirkby (Kirkby Microwave Ltd)" <drkirkby at kirkbymicrowave.co.uk>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: [volt-nuts] Checking an LCR meter
> I have bought an HP 4284A precision LCR meter. This is an old model with a
> basic accuracy of 0.05% and covers 20 Hz to 1 MHz.
> Converting the specifications into determining the uncertainty of a
> measurement is nontrivial, but I think it reasonable to assume the
> uncertainty will always be >0.05%.
> Surprisingly the current precision LCR meter from Kesight, the E4980A (20
> Hz to 2 MHz) offers the same basic accuracy. So while fairly old, the 4284A
> doesn't seem to be miles behind the current crop LCR meters from the top
> The recommended calibration period on the 4284A is 6 months, which would
> get rather expensive - on the current E4980A the calibration period is a
> more respectable 12 months.
> I am looking for suggestions on how I can get "reasonable" confidence in
> the instrument at "reasonable" cost, without returning it to Keysight every
> 6 months.
> I have a 3457A DVM, but mot much else in the way of precision low frequency
> It has 4 BNC connectors for Kelvin probes.
> I suspect that getting precision resistors and keeping them for a house
> standard might be worthwhile, but are looking for suggestions on the best
> I will send it to Keysight once when it arrives to ensure that there are no
> faults on it, but I don't currently feel I can justify getting it
> calibrated every 6 months.
> Maybe I can make some stable "standards", then measure them soon after the
> LCR meter calibrated been calibrated and periodically measure their values.
> Any suggestions about how to approach that?
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