Greetings All,
As L/C component value measurement appears to be a present topic of interest here I decided throw my 2 cents in by posting a couple of tables that show the measurements results from tests I made with my nanoVNA tweezers. The tables illustrate the level of accuracy I obtained using tweezers with my nanoVNA running under "nanoVNA Saver" PC app control. As mentioned in an earlier post my nanoVNA tweezers were described in the Nov/Dec/2021 issue of QEX magazine. The attached tables along with some descriptive text can be download from the ARRL's QEX article support files site via the following link:
The measurements were made over a frequency range of 50KHz to 150MHz (which is the practical upper limit for use of this method) for ranges of 1pF to 470pF, +/-1% (+/- 0.1pf and 0.25pF for the 1pF and 4.7pF values respectively) tolerance 0805 size SMD capacitors and 10nH to 4.7uH, +/-2%, 0805 inductors. They were installed in isolation on a low stray capacitance designed PC board. The reactance range for the capacitors tested was roughly 7 Ohms for 470pF at 50MHz to 1 megOhm for 1pF at 15MHz. The reactance range for the inductors tested was roughly 1 Ohm for 10nH@15MHz to 500 Ohms for 4.7uH@15MHz.
The measured capacitor values obtained were all within the manufacture specified tolerances. The lower value inductors also measured within the manufactures +/-2% tolerance specification. However at the lower frequency range the larger value 1 and 4.7uH inductors were found to be about 3% from specified values at some frequencies. (As one would expect better measurement accuracy at lower frequencies I wonder if these larger inductors happen to be in fact slightly out of tolerance???)
I find the tweezers to be a useful addition to my instrumentation inventory. For a case-in-point whilst experimenting with a class E, 14MHz 5W output power amplifier I needed to wind a pair of toroid inductors for the output network. The amplifier efficiency pivots on a proper output network. The optimum initial inductor values were calculated using a very useful class E amplifier design spreadsheet by WA0ITP that is based on specs from NM0S. The toroid inductors needed required less than 10 turns which means a single turn influenced the achievable inductance value by about 20%. After getting the L values as close as I could I entered the tweezer measured L values into the spreadsheet and re-calculated the optimum capacitor values. Out of the gate without any tuning I found the output power was just over 5 Watts with an efficiency of about 80%.
There is no doubt that a proper test jig used with an expensive industrial quality VNA would provide a more stable and accurate measurement method for L/C components across a relatively wide frequency spectrum. However I also believe a set of tweezers can be a useful and easy to use nanoVNA accessory for those experimenters wishing to make L/C component measurements at frequencies below 150MHz with quite good accuracy.
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Tom, VA7TA
Attachments:
TweezerResultsCapacitors.jpg (54170)
TweezerResultsInductors.jpg (68925)
