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How long is a piece of string?

I’m working on an application currently where trace impedance appears to make all the difference on performance as observed on a few unguided experimental revisions of the same board.

Everything is finger in the air at the moment and I’d like to acquire some conclusive measurements so my question is two fold:

  • Can LCR meters measure these minute inductances?
  • Can ~cheap~ LCR meters measure these?
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  • \$\begingroup\$ FYI, the piece of string is ballpark 0.5 nH per mm of length, give or take width, and height over substrate. For further reading, you may want to look into transmission line characteristics, particularly low-frequency equivalent parameters such as this. \$\endgroup\$
    – Tim Williams
    Commented Mar 13 at 13:35
  • \$\begingroup\$ Not tweezer or handheld meters from my experience with them, though I never went out of my way to measure traces with them. The ones I used were at least mid-range, not Amazon junk. I think you need a benchtop meter at least. Look at the specs based on Tim William's comment. \$\endgroup\$
    – DKNguyen
    Commented Mar 13 at 14:45
  • \$\begingroup\$ You're probably better improvising (or borrowing) a TDR if you need to characterize trace impedance. That said, measuring trace impedance is usually not needed since for a PCB geometry the factory should give you everything you need to calculate it. \$\endgroup\$
    – user1850479
    Commented Mar 13 at 18:50
  • \$\begingroup\$ You may want a network analyzer for this, they're usually the best way to measure very small reactances. You need to know how to use one though; it's not trivial and definitely not as foolproof as an LCR meter. \$\endgroup\$
    – Hearth
    Commented Mar 13 at 19:56

2 Answers 2

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Depends what you mean by "cheap". I don't think the $10 ATMega based ones will have a hope of measuring trace inductance, though they should be able to roughly measure a few pF of capacitance over a ground plane of a reasonable area trace. Cheap handheld ones are not much better, especially for measuring inductance.

The pricier handheld ones, with Kelvin probes, have a chance. Mine (an actual LCR bridge that measures in quadrature) has a 1nH resolution on the lowest range, and the reading seems stable.

You have not given much of a hint of the range of measurements required or what you are doing, but my guess is that it is a power application with fast switching edges. In that case you can infer the inductance by making measurements of the voltage and current slope (assuming the inductance isn't actually killing your circuit- in which case the main issues should be fairly plain to see).

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From this

Here is the capabilities of measurement of the Hioki IM3536

enter image description here

Low cost LCR meters should have "restricted" ranges.

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