Measure an inductor in circuit - Page 1
Forum section: electronics / beginners

author: allenc
author_rank: Contributor
timestamp: July 17, 2021, 04:41:57 pm
content: Given this LPF circuit, assuming J1 and J2 are NOT connected to anything, can I accurately measure inductance across L1 in circuit using an LCR meter?This is supposed to be a 400 MHz LPF found on a DEEPELEC NanoVNA test board (section 11)https://www.amazon.com/dp/B08RNLDSL4. When I measure the frequency response using a NanoVNA-F the 3db point is 355 Mhz. I suspect that the difference is due to component tolerance; I think the inductor is 48 nH when it should be 40 nH. I think this because an optimal inductor for such a 400 MHz LPF would be 39.7 nH, so the manufacturer probably used a low cost, low precision 40 nH 20% device, and my board happens to have an inductor that is on the upper end of that tolerance (48 nH.) When I compute the 3db point with a 48 nH inductor using Elsie I get exactly the result that the NanoVNA-F reports: 355 Mhz.So I'd like to verify this by measuring L1 with a BK 880 LCR meter. My instinct is that I can measure it in circuit while C1 and C2 float, but I'd like to know for sure.

author: rstofer
author_rank: Super Contributor
timestamp: July 17, 2021, 04:54:06 pm
content: You would still have the two capacitors in series then in parallel with the inductor.I would probably remove the inductor from the PCB...

author: allenc
author_rank: Contributor
timestamp: July 17, 2021, 04:58:13 pm
content: Yes, I see. The caps are still in-circuit through ground, i.e. not floating. Totally obvious. Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: July 17, 2021, 05:20:17 pm
content: I’m not familiar with the B&K unit: does it have a guard terminal? If so, it might work if your circuit, as drawn, can be put on a plastic box or similar to float its ground, and then connect the LCR meter’s guard to that “ground”, leaving only the L in-circuit.Even then, the inherent self resonance of L (due to its self capacitance) may affect the circuit at 400 MHz more than you can measure with that meter at lower frequencies.

author: allenc
author_rank: Contributor
timestamp: July 17, 2021, 05:30:48 pm
content: Why yes, the 880 does have a guard terminal. They make two probes (one a tweezer type) to do this. It's the only handheld LCR they make that has this feature.Thanks for pointing this out.

author: Faringdon
author_rank: Super Contributor
timestamp: July 17, 2021, 07:40:16 pm
content: bk880https://bkpmedia.s3.amazonaws.com/downloads/datasheets/en-us/880_datasheet.pdf..i dont think this LCR meter is any good for your requirement, and considering the high frequency at which your inductor will be used.I think in your case, some kind of on-the-bench resonance measurement technique is needed, and with the inductor out of that circuit.Perhaps a VNA can do it though..but they cost a bomb

author: ledtester
author_rank: Super Contributor
timestamp: July 18, 2021, 06:58:21 am
content: Quote from: Faringdon on July 17, 2021, 07:40:16 pmThe "NanoVNA" is very inexpensive - search aliexpress/ebay.A video:- nanoVNA - Measuring Inductors and Capacitors (Vers. 3)https://youtu.be/iJ1qKE5O0bY