[volt-nuts] Checking an LCR meter

[Brooke Clarke]

Hi Dave:

The 4-terminal pair measurement method used on many of the HP (and other) LCR meters and impedance analyzers is much 
more accurate over many many decades of impedance value than network analyzers which are only accurate near 50 Ohms.  
This is explained in the impedance measurement handbook.  In my mind the key to an accurate measurement is to know which 
of a number of impedance measurement techniques to use.  The Z hdbk has contour maps showing the effect of frequency, 
impedance and accuracy for a number of different measurement techniques.
http://www.prc68.com/I/Z.shtml#KeyDocs

The other key idea is that LCR measurements are done on components without connectors and so there's the handling of the 
fixture parasitics.   This is far from trivial.  Again the Impedance Measurement Handbook is essential and the 
Measurement Accessories Selection Guide is very handy.

And finally by making impedance measurements over a broad frequency range you can fit a model to the data instead of 
utilizing the simple series or parallel 2 component model that's standard in an LCR meter.  This simple model may be 
fine for a lot of applications, but is not good for more complex cases.  For example the model for a crystal resonator 
that's built into the E4915, E4916 & E5100 is based on an S21 measurement and the use of a low-Z PI test fixture.  What 
all these instruments have in common is frequency sweep and DSP IF processing.  See my Crystal Equivalent Circuit web 
page for an example of a Z-transform (Z:T in upper left of screen shots) measurement.
http://www.prc68.com/I/Xec.shtml

Mail_Attachment --
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> 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
> manufacturers.
>
> 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
> equipment.
>
> 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
> approach.
>
> 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?
>
> Dave
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