How to measure a SMD ferrite bead? - Page 1
Forum section: electronics / beginners

author: somuchtyler
author_rank: Contributor
timestamp: January 09, 2021, 11:42:42 am
content: I am reverse engineering some PCBs and I am trying to properly identify all the components. I am running into a bit of a problem properly identifying SMD ferrite beads. As far as I am aware the manufacturer specifies the resistance in ohms at 100mhz.Am I right in thinking I will need some type of signal generator along with an oscilloscope to figure out the resistance at 100mhz? I was hoping there was some type of measuring device built specifically for this purpose like an LCR meter that can just tell me a rough measurement of the ferrite bead.I was looking at this threadhttps://www.eevblog.com/forum/beginners/random-unknown-ferrite-bead-calculation/and they mention "100MHz oscillator. Set it up as a voltage divider: osc output -- FB -- 50 ohm terminated cable (e.g. scope/speccy/RF voltmeter input)."I was wondering if there is a low cost solution to measure a ferrite bead. I was looking at the nanoVNA which looks cheap enough on ebay, but I guess I will need a arbitrary waveform generator that can generate a 100mhz waveform which apparently costs $,$$$.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2021, 11:48:33 am
content: nanoVNA is all you need.What would you use the generator for?

author: somuchtyler
author_rank: Contributor
timestamp: January 09, 2021, 11:56:04 am
content: Quote from: Yansi on January 09, 2021, 11:48:33 amWouldn't I need to generate a 100mhz signal to figure out how much resistance the ferrite bead is providing?

author: somuchtyler
author_rank: Contributor
timestamp: January 09, 2021, 12:05:24 pm
content: just to reiterate, I am trying to measure the resistance in ohms at 100mhz of the following part:

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2021, 12:15:13 pm
content: Quote from: somuchtyler on January 09, 2021, 11:56:04 amAgain: If you have a network analyzer, what for?Maybe you should study up how a VNA works. It has a built in generator. Or two. (sometimes even 4, if you have a four port network analyzer).

author: somuchtyler
author_rank: Contributor
timestamp: January 09, 2021, 01:39:12 pm
content: Quote from: Yansi on January 09, 2021, 12:15:13 pmHow does one use a VNA to measure resistance of a ferrite bead at 100mhz?

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2021, 01:47:41 pm
content: https://www.google.com/search?client=firefox-b-d&q=how+to+measure+impedance+of+component+nanoVNAJust shove the damn component at the transmit port of nanoVNA, that has the measurement plane calibrated till the measurement end where the DUT is.And what you want to measure, is impedance, not just resistance. The DC resistance of the bead is some low ohms.

author: cdev
author_rank: Super Contributor
timestamp: January 10, 2021, 01:50:36 pm
content: Its easy to do this with a VNA but the way I understand it where its used in your intended use is important. The simplest way is to measure it as I think Mechanotrommer shows in his test fixture across the dut plane.You could make a fixture for ferrite bead sized SMT components that would probably work fine for VHF using veroboard and pogo pins.As I understand it, it may matter-depending on your use it may matter quite a bit where it is in the circuit, (series or parallel) and whether the impedance is much more capacitive or inductive. (In the case of a ferrite bead like any inductor its going to be more inductive until it reaches a certain frequency the self-resonant frequency, and then its more capacitive). As Blueskull points out there may be added resistance too, added to dampen Q.Hopefully the graphics help. I would start simply by getting something like the mechanotrommer board the "DUT" one on the wood block, calibrating your VNA to that, and then putting your device across it at the exact calibration plane. That will be a starting measurement.There is a "VNWA testboard kit" for the (DG8SAQ) VNWA which has been copied and is widely available.Its best suited for throughhole components(It provides a sort of matrix of possibilities and it uses machined sockets so its relatively low loss.But it also has (just) two signal paths set up with pads for soldering smt components. (and it comes with some resistors which you can use to make a "load" and shorts and I suppose an open too (two empty header pins)There are extra header pins. But you will really mostly want to use the ones at the ends in the appropriate manner, depending on whether you want to measure it in shunt or series circuit.One trick that's useful for measuring shunt impedance is to make a piece of coax where its stripped in the middle and the braid and center conductor are divided and then there is a section of solder wick where you place your component between the two. The transitions are smooth. You can put one or two capacitors in there (this is esp. useful for testing bypass caps)You pull the ends apart to grab the DUT(s). Am I making sense? This is a very simple jig for testing shunt capacitors or inductors.Series, you either solder it in, (best) or grab it somehow in a way that approximates where its going to be used.There are lots of dev boards that can be used for VNA test fixtures. WA5VJB sells some LNA boards that would make very good VNA test boards (or bias-tees, as the bias tee part of the LNA is done right, with the inductors placed in a zig zag fashion.) and they are cheap. He has the pads placed in a way that would make it easy to have either series or shunt.Thisis a good video. He also shows how you'd use Simsmith.