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LCR meter: what am I doing wrong?

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hboy007

Tue Nov 09 2010, 02:20PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

I'm currently using the HP 4284A meter to measure caps below 1pF. For 1pF, everything is within capacitor tolerances, but when I measure several 1pF caps in series, the value drops too much. I get a 10fF reading instead of something in the order of 100-200fF.

So what's wrong? I do "open" and "short" corrections and I use 4 coax cables that have their shields connected near the meter and near the DUT as suggested in the manual but still the readings are not plausible.

Can anyone point me in the right direction? Are 2m coax cables too long? Thanks in advance!

Steve Conner

Tue Nov 09 2010, 04:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Read the manual to see the specified accuracy. That may be normal behaviour. I find it surprising enough that the meter can read 1pF at the end of 2m of coax.

Also, when was it last calibrated?

hboy007

Tue Nov 09 2010, 04:52PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

The "last calibrated" sticker says 2009-02 so it can't be that bad, I suppose. There is a calibration option called "cable length compensation" that fixes phase errors to some extent. I will dig into the manual and see how to use it.

Luca

Tue Nov 09 2010, 05:24PM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

hboy007 wrote ...

The "last calibrated" sticker says 2009-02 so it can't be that bad, I suppose. There is a calibration option called "cable length compensation" that fixes phase errors to some extent. I will dig into the manual and see how to use it.

Try with cable length compensation...

Be careful with the "open" compensation. Keep the two probes far away while you perform such calibration.

Also, what's the frequecy you are using for that measurements? With such low capacitance I would use the highest possible frequency, in order to have the highest possible signal current.
Maybe you are using a frequency too low which gives an equivalent impedance so high that the instruments is no longer able to measure with precision.

Regards

Luca

radiotech

Tue Nov 09 2010, 06:52PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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The meter I use (Z meter) standardizes the test lead, shorted and
open prior to testing. fF-- does you meter read femptofarads?

hboy007

Tue Nov 09 2010, 11:56PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

Let me start with some quotes from the manual:

"The HP 4284A offers C-D measurements with a basic accuracy of +/- 0.05% (C), +/- 0.0005 (D) at all test frequencies with six digit resolution (the dissipation factor resolution is 0.000001) on every range."

Frequency range is 20 Hz - 1 MHz.

"When you select 4m (cable length), the four outer conductors of the HPOT, HCUR, LPOT, and LCUR test leads must be tied together at the end of the
HP 16048E 4m test leads. (I think this is where I mess things up!) In other words, the four-terminal pair conguration must
be terminated for the cable length selected. When an
HP 16048A/B/D/E test leads are used, use the furnished terminal
plate at the end of the cable for easy conguration."

Yes, the meter should be able to measure values down to the femtofarad range. Usually, the xxx.xxx pF range is used ( frequency is 1kHz and 10 kHz, depending on temperature and type of device to be tested). These measurements can also be reproduced ... well at least to within +/- "20fF" (whatever the true value of this reading may be)

Steve Conner

Wed Nov 10 2010, 09:36AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Luca wrote ...

Be careful with the "open" compensation. Keep the two probes far away while you perform such calibration.

That is a very good point. In order to tie the four shields together, you have to bring the four cables close at the DUT end. Maybe there is 0.1pF of stray capacitance between the exposed cores, and the meter compensates that out. That would explain the observed behaviour very well.

hboy007

Wed Nov 17 2010, 11:07PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

OK, I owe you a plot of the measurement I did recently:

The yellow points are as measured after changing the measurement setup (using 4 cables, connecting the shields, in short: as suggested by the manual / by some of you). The curve that drops down to zero was taken before any changes were done to the setup.

Influences of the ground plane and the copper fill around the caps on the top layer are not taken into account here so the yellow curve looks more or less okay to me. Unfortunately, the behaviour changes with frequency *sigh* ... it was worth the try, but in-situ measurements with long coax cables seem to be doomed.

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