Capacitance Measurements at 10 kHz and 100 kHz
a tech note under construction ...
The Agilent U1733C is a remarkable low cost handheld LCR meter. In the $400 price class range, it is far more accessible than a high end bench LCR meter typically costing several thousand dollars. For those of us who have been using ancient LCR bridges (e.g. hp 4260A, hp 4265B, ESI 260 series, etc.), it is a giant leap into the present!
Some have commented on odd displays while using the "Ai" auto-detection mode. Where the component is functioning as expected (e.g. a capacitor still has measurable capacitance at the desired test frequency), in some cases, manually setting a "L" or "C" range close to the component value, followed by stepping up or down a range can still achieve a desire measurement.
Occasionally the U1733C display simply shows "OL" and the U1733C refuses to produce any reasonable results, despite any number of attempts to select an appropriate manual range. One Post along these lines refers to a relatively large valued electrolytic capacitor. Hey, I got this meter, some might say, to see what my parts "look like" (RLC, etc.) at 10 kHz and 100 kHz and not many low cost meters can do that!
Unfortunately (re. 10 kHz and 100 kHz capacitance measurements), as usual, I am afraid that I bear some mildly bad news. However, before the bad news, I need to honestly say that I am extremely pleased with almost all aspects of the U1733C! Before we got our U1733C, our life was miserable when characterizing 20 mH air core inductors (using ancient bridges) for our FDM Proton Precession magnetometer. Now, using our new U1733C, that same job is an absolute joy! So despite what follows, we continue to strongly recommend this relatively low cost very versatile hand held LCR meter as a "must have".
Okay, the bad news. So you bought your U1733C hoping to have a good look at those relatively high value super audiophile Nichicon capacitors. After all, the U1733C measure capacitance parameters to 100 kHz, right?! Not so fast, look at the following capacitance specification chart from the page 77 of the operating manual:
Unfortunately, the dashes represent non-existent ranges. That is, the maximum Capacitance value that can be characterized at 1 kHz is 2,000 uF, not so bad. But, at 10 kHz, the maximum value is 200 uF, and at 100 kHz (sorry audiophiles), the maximum value is 20 uF.
It appears that beyond the maximum capacitance value, the U1733C is programmed to display "OL". So don't expect to get lucky with some especially well behaved electrolytic capacitor (i.e. capacitor parameters, especially at higher frequencies, of course include inductance and resistance).
BUT WHY?! Well, it turns out, with a little probing, the reason might be low signal levels. It appears that in the "dashed" ranges, there is not enough amplitude left to make a meaningful (Agilent quality) measurement. As you will see from the data that follows, the threshold appears to be somewhere below ~1 mV.
To make these measurements, I used a dedicated shielded RG-58 style BNC cable with a 1 uF series capacitor mounted on a dual banana plug. We connected that cable between the BNC input of a hp 3400A true RMS AC Voltmeter with full scales down to a 1 mV full scale range (BNC input) and the dual banana side plugs into the dual banana jacks of the U1733C. The capacitors under test had their leads pushed into the U1733C component lead compression plate jacks.
okay, so I began with a 0.1 uF high quality KEMET military style CK06 cased ceramic capacitor:
(* means digit varying)
100 Hz, 0.10723 uF, 615 mV (across C)
120 Hz, 0.10712 uF, 575 mV
1 kHz, 0.10602 uF, 602 mV
10 kHz, 0.10413 uF, 452 mV
100 kHz, 0.10170 uF, 9.9 mV
so far so good!
Next, I tried a high quality 1 uF WESCO 400V film capacitor:
100 Hz, 984.4 nF, 620 mV
120 Hz, 984.4 nF, 581 mV
1 kHz, 983.8 nF, 493 mV
10 kHz, 985.1 nF, 62 mV
100 kHz, 994.1 nF, 6 mV
Then a relatively small electrolytic Nichicon HE(M) 6.8 uF 100V 105c electrolytic capacitor:
100 Hz, 7.135 uF, 566 mV
120 Hz, 7.110 uF, 519 mV
1 kHz, 6.718 uF, 93.2 mV
10 kHz, 6.264 uF, 10.6 mV
100 kHz, 5.074 uF, 3.8 mV
still A'okay, but now:
Nichicon PF(M) 220 uF 50V 105c electrolytic capacitor:
100 Hz, 221.4 uF, 28.8 mV
120 Hz, 220.8 uF, 24.0 mV
1 kHz, 215.5 uF, 3.1 mV
10 kHz, "OL", 925 uV
100 kHz, "OL", 775 uV
Nichicon VX(M) 1,000 uF 63V 85c electrolytic capacitor:
100 Hz, 946.6 uF, 6.7 mV
120 Hz, 945.0 uF, 5.6 mV
1 kHz, 956.0 uF, 0.7 mV (700 uV!)
10 kHz, "OL", 0.3 mV (~280 uV)
100 kHz, -5.863 uF, 1.09 mV (okay, so this one is just a little screwed up; for some reason (a 1mV firmware threshold?) we don't get the expected "OL")
Afternote Feb, 2013: Dick Drawz suggested that there might be some significance to the negative C readings. He pointed out that axial electrolytics can have a significant series inductance. I tested a similar capacitor at 100 KHz on both the C and L scales (forcing the manual scale selection). The L of .66 uH and the C of 3.8 uH infact do both yield an equal reactance (XL and XC) of about 0.42 ohms.
Some follow up testing was performed with three small ferrite inductors, about 4 uH, 10 uH, and 100 uH tested at 10 kHz and 100 kHz on both the L and C scales and gave similar results with equal magnitude reactance results. The only odd result (albeit way out of the normal use envelope of the intrument) is that sometimes the C reading was negative (as would be expected for an L) on the nF scale, while when the C units changed to uF, the value was positive. Whether negative or positive, the reactances were all about the same. For example, at 10 kHz, the 10 uH L read 9.6 uH and +26 uF (both about 0.6 ohms X), however at 100 kHz, the same L read 9.5 uH (not surprising that it fell off with mu(f)) and -267 nF (both about 6 ohms X).
So, it does appear that "negative" C readings can have some odd (non-physical) relavance to the measurement. It is unclear why sometimes for "C" readings of an L they are negative and other times positive. Probably a user was never meant to see the "alternate" view of a "correct" measurement. Also, in most if not all cases, the Ai mode would auto-select the physically correct respresentation (e.g. L for an inductance).
hp manuals and related catalog pages are reproduced with Permission, Courtesy of Agilent Technologies, Inc.
COPYRIGHT © 2012, 2013 JOSEPH M. GELLER.