Capacitance Measurement

WORK IN PROGRESS……..

I wanted a quick and easy test to check for small capacitance in the pF range, and inductors in the nH range for that matter.  My current equipment consists of a NanoVNA H4 and an off brand LC200A LC meter.  The NanoVNA is a great tool but requires calibration, software, fixtures, etc.  Accurate but not fast.   The LC200A was $30 (at the time) and I had my doubts on accuracy in the lower ranges.  Searching around I found an LCR tester around $70 (Amazon) from FNIRSI called the LC1020E.  This seemed like it could be a nice LCR meter and it even included Kelvin clips to help with accuracy.  Unlike the LC200 the FNIRSI looks and feels well built for years of use and abuse.

The NanoVNA, in my opinion, is by far the most accurate.  The draw back is having to calibrate and using the NanoVNA Saver program, which I prefer on the eyes.  One thing I did notice is you can read capacitance on the NanoVNA itself but it is off several pF from the NanoVNA saver program(???).

[test calibrations!]

The LC200A has worked great for me in the past and claims a resolution down to .01pF.  The accuracy for a small capacitance, like 18pF, reads OK but over time it will drift and periodically have to re-zero the unit.  For example in our 18pF experiments below leaving the capacitor in the fixture will result in a ready over 22pF after several minutes.  Zeroing out the meter and leaving for several minutes will result in an open reading of xxpF

Enter the LC1020E….. Will it do the job?  I guess we will see in the results below.  After we get set up and calibrated we’ll check out an 18pF capacitor then go down to 1.0pF, .82pF. and finally .68pF.

Trying to compare apples to oranges……..

The LC200A uses alligator leads for measurement, which can vary small capacitance quite a bit down in the (sub) pF levels as you move around.  The NanoVNA will need a fixture of some sort, and I’m skeptical how good a 24″ long Kelvin leads will be on a couple pF or less capacitance on that LC1020E.  In the past I made up an SMA to alligator leads for the NanoVNA which was OK but I could see capacitance changes as the leads were moved around and wanted something as little more stable.  So let’s build some fixtures!

The fixtures consists of banana jacks, terminal blocks, and a piece of prototyping board.  Stack-able banana jacks were used and easily removed from the plastic housing.  M3 screws, washers, and locking nuts were used to secure the jack to the perf board as well as a point to attach a small piece of wire going to the terminal blocks.  The jacks came from Tayda (A-1824 & A-1825) but can also be found on Amazon, Ebay, etc.  The terminal blocks were 5mm DG300 style (A-666) which happen to be spares in the junk box.  For the NanoVNA a male SMA PCB connector was in the junk box.

 

Set up and Calibration

Now that all 3 units have their fixtures in place the units need to be calibrated:

• The LC200A was easy by pressing the zero button with nothing in the fixture.
• The LC1020E was open/short calibrated per the manual.  One thing I noticed was the meter was finicky reading small capacitance in auto mode.  The solution was to place it in capacitance mode, setting the frequency to 100KHz, range to 100K ohms, speed to Slow, and the second measurement as X (impedance).
• For the NanoVNA I referenced a YouTube video from Electronics for the Inquisitive Experimenter – https://www.youtube.com/watch?v=Pti8Erw_Kkg  The frequency range used was in the area of 100KHz – similar to the LC1020E frequency.

18pF test

We’ll start with something easy with an 18pF +/-2% NP0 100V ceramic capacitor.  We took 3 capacitors from the box and ran them through each meter.  Each capacitor should read between 17.64 and 18.36pF

	LC200A	LC1020E (and X calculated)	NanoVNA
1	xx.xxx	xx.x   xx.xxx	xx.xxx
2	xx.xxx	xx.x  xx.xxx	xx.xxx
3	xx.xxx	xx.x  xx.xxx	xx.xxx

A bit disappointing is the FNIRSI only has a display down to .1pF.  However, using the second measurement (X) and calculating with the formula C = 1/(2*pi*F*C) gives us a more precise reading.  Maybe FNIRSI will add a couple more digits in a firmware upgrade?

[FNIRSI Z-picture here]

STAY TUNED FOR MORE…….