How to measure very small inductors/capacitors? - Page 1
Forum section: electronics / projects

author: uski
author_rank: Frequent Contributor
timestamp: November 03, 2017, 10:12:53 pm
content: Hi !I am designing a RF board which uses very small inductors and capacitors for antenna tuning purposes.I am running into unexpected issues with the antenna performance being really bad despite the antenna manufacturer tuning the matching network of the antenna with a VNA and reporting good performance on their side.I start to question the tuning components I have used. I suspect I may have got the wrong values or something like this.The problem is that these values are very small : 4.7-6.8nH for the inductors (they are 0805 wire-wound inductors) and 0.7-1.9pF for the capacitors (0603).I have a DER EE DE-5000 LCR meter and an eBay cheapo chinese 128x64 LCD meter. Both cannot read such low values (despite running the probe calibration for the DER EE unit).I'm starting to consider building a very small test fixture on a piece of PCB... any idea as to what would be the most effective way ?I believe some kind of oscillator could do the job, preferably with a frequency below 100MHz, not too many critical parts, and not too hard to calibrate (I could use some known-value parts for calibration).I would directly solder the part to test inside the circuit to prevent any influence of test leads and lead to a very reproductible test setup.Any suggestion ? I'm sure some of you have run into similar things before so before starting something by myself I wanted to see if someone already came up with a great solutionThanks !uski

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2017, 10:46:28 pm
content: Measuring the small caps is relatively simple, if one just needs the capacitance and not also series inductance. One can do that a really moderate frequencies, like in the kHz range. Apply an AC voltage to one end of the cap and measure the current at the other side with a kind of TIA or charge amplifier. In the pF range one might not even need phase sensitive detection. Just make sure the test fixture is not adding to much capacitance.The small inductors are more difficult. The best bet is likely a test fixture to use with the VNA. Be aware that the inductance is a property of a closed circuit, not that of a single small part only. In the nH range this gets important. So it depends on how the rest of the wirer are connected and where the ground plane is. Inductors also have a parallel capacitance and thus more than just an inductance to characterize. So the model fit will likely be more like 3 parameters (R,L,C).

author: Someone
author_rank: Super Contributor
timestamp: November 04, 2017, 02:36:26 am
content: Quote from: uski on November 03, 2017, 10:12:53 pmDid they measure the tuning with a VNA using your boards? Parasitics will be significant at such small capacitances and inductances.Quote from: Kleinstein on November 03, 2017, 10:46:28 pmSimilarly to the easy approach for capacitors you suggested, tiny inductors can be measured in a series LR filter excited by the fastest edge rates practically available. If no high speed scope is available to directly measure the rise time a comparator can be used to add enormous amounts of gain and a slow scope used to measure the time difference between edges calibrating to a short.

author: IDEngineer
author_rank: Super Contributor
timestamp: November 04, 2017, 04:17:30 am
content: Do a search on the Bob Pease video about testing ultra-low input capacitance on op amps. They came up with some clever tricks to address the problem of measuring values down into the femtofarad range. Perhaps you could apply similar techniques for measuring your capacitances and inductances... after all, you're at least three orders of magnitude larger!

author: TheSteve
author_rank: Unknown
timestamp: November 04, 2017, 04:21:48 am
content: I'd recommend a VNA with a tiny jig to hold the parts. I have used an SMA jack with a pogo pin soldered to it to measure 0603 sized parts in the value range you're interested in with good results.

author: tautech
author_rank: Super Contributor
timestamp: November 04, 2017, 04:31:25 am
content: I used to piss around with a precision RLC bridge until I got some SMD tweezers.http://www.smarttweezers.com/colibri-pro-specs/Those will easily meet your requirements.Sure the price initially hurts but after a year or so it's long forgotten.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 04, 2017, 05:35:35 am
content: TI makes some capacitance to digital converters that do picofarad sized capacitors really well, doing exactly what Kleinstein described. The FDC1004Q measures the range +/- 15 pF, and has the drivers for shields and guards you need to get a good measurement. The analog devices AD7745 works similarly with a +/- 4 pF range. Of course it is just a chip. You have to build a fixtures and do at least offset calibration.

author: macboy
author_rank: Super Contributor
timestamp: November 04, 2017, 12:30:48 pm
content: I never had a problem with low value C and the DE-5000. I recently measured 1.0 pF caps and it showed me 1.0x pF (can't remember the x).One key is careful calibration and measurement technique. You need to recognize that moving the clips or tweezers affects the parsitic capacitance created between the leads, and calibrate accordingly. Open calibration should be with the clips or tweezers in the measurement position. So for tweezers, do open calibration squeezed to the width of a e.g. 0603 component. For the gator clips, I do the short calibration using a piece of a component lead, rather than clipping them to each other. This simulates the ESR and ESL of the clip-to-lead connection that will always be present in measurements.Also for very small values, you usually want a high frequency, in order to push the reactive impedance into a more easily measurable range. 1 pF caps aren't used at 100 Hz, so don't try to measure them at 100 Hz.