Nuts Idea for RF filtering - Page 1
Forum section: electronics / projects

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 06, 2023, 11:46:48 am
content: When EMC norms became stricter some 20 years ago, it was quite common for a while for USB and monitor cables to have thick ferrite cores, and these are not particularly cheap if you have t buy them separately.Now I'm wondering, what would happen if you put the cable though stack of regular M8 nuts or steel washers?This regular hardware is quite cheap, and not as good for magnetic fields are silicon steel, but I recon it would generate eddy currents, and thus dampening quite easily.Maybe it would filter low frequencies too much, and thus influence the signal though the cable?It's also easy to experiment with, the reason I have not done it yet is for lack of a suitable signal source.Is this just silly, or worth looking into?What would be an suitable signal source for testing?I only have an old Rigol DS1052-E which has lousy TFT, but I can see noise upto 100MHz or so (Although attenuated at that frequency).Edit:I forgot to mention frequency range (see comment below)This is unlikely to work for monitor cables or USB.My main application is for microcontroller nodes on a network. The CAT-5 cable uses 24Vdc and 115k2 RS485, and I've noticed spurious resets due to the noise pickup of 20+ meters of cable.

author: moffy
author_rank: Super Contributor
timestamp: August 06, 2023, 12:31:04 pm
content: It would filter quite low frequencies relative to a ferrite, certainly not suitable for fast signals. Mains transformers 50/60Hz use silicon steel laminations to reduce eddy current losses, imagine a washer and you have a candidate for induction heating. Some higher frequency toroid cores use Powdered metal because there are lots of tiny air gaps which insulates the metal balls of the sintering material keeping the eddy currents modest while maintaining most of the magnetic properties of the metal, but they tend to be lower frequency, in the 10s to 100s of kHz.

author: Gyro
author_rank: Super Contributor
timestamp: August 06, 2023, 01:05:42 pm
content: Surely youwanteddy currents in this case. The objective is to dissipate the energy of common mode noise on the mains cable.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 06, 2023, 01:33:03 pm
content: For DC currents (automotive solar, etc) this probably works good.For (50Hz / 60Hz or even the 400Hz (boats, planes) I'm quite certain this also works. It's only a single winding, compared with a mains transformer, and inductance is quadratic with the number of windings, so for a single winding the loss / heating will still be small.And if your eddy currents absorb enough energy to get very hot, then you probably have bigger problems at hand.One of these days I wand to put RS485 drivers on an STM32. I have both 10MB/s and slew rate limited drivers. Seems like a good opportunity to test whether I can see any dampening effects.

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: August 06, 2023, 02:48:18 pm
content: FWIW steel wool is a very lossy material at RF.

author: Marco
author_rank: Super Contributor
timestamp: August 06, 2023, 03:08:08 pm
content: I wonder if common mode filtering by glueing a ferrite plate on serpentine coplanar differential waveguides makes sense. One plate can cover a lot of signals.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 05:19:27 pm
content: Would add a few ohms, not nearly enough to matter. Steel drops off quickly in the MHz: the skin depth is very shallow so the amount of inductance increase is minuscule.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2023, 05:35:39 pm
content: Note that the high-frequency loss of ferrites (beads or cores) around a wire is due to the large area inside the hysteresis loop, since they are not conductive. The energy required to go once around the loop is the area (integral) bounded by the curve ofBvs.H. Therefore, the power lost (transferred to heating the ferrite) therefore increases with frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2023, 08:31:52 pm
content: Quote from: TimFox on August 06, 2023, 05:35:39 pmNot quite; that's rather oversimplified. Two problems:1. If hysteresis is the end-all-be-all, then higher hysteresis would be "better", no? Why aren't we using speaker magnets (ballpark 0.4T * 300kA/m) for their stupendously high losses?2. Material resistivity may be high, but it's far from insignificant! Especially for MnZn ferrite, which is more than conductive enough to get a spark from, when energized with high voltage (even a few hundred V, really); it's in the ballpark of 1 ohm*m. (NiZn is more like 1Mohm/m, still not exactly an insulator, but high enough not to worry about until extremely high frequencies, and in significant thickness at that -- maybe even wavelengths.)I imagine I'm being unfair here (not mentioning other phenomena, does not mean denying other phenomena), but I take issue with the certainty with which it's been presented, which implies to readers that hysteresis is the exclusive/important effect.Hysteresis isn't very important, because, well, it's hysteretic: it only applies for signals large enough to cause motion around the curve. For soft ferrites, this doesn't actually take much (the coercivity and remenance are both fairly low), but losses do generally increase with signal level (expressed with the generalized Steinmetz approximation, the B^alpha exponent is typically greater than 2).But firstly it is a matter of matching: extremes mean nothing. Performance peaks somewhere inbetween. Large hysteresis means little or no effect on small signals, and more importantly, materials with high hysteresis tend to have low initial permeability, i.e. they don't do anything at all to small signals (looks like open air); the top/bottom of the hysteresis loop trends towards horizontal, the "harder" a material gets. Conversely, zero hysteresis would mean zero loss [by that mechanism], so it doesn't matter that it would be sensitive to arbitrarily small signals.But resistivity also plays a role. A typical figure is something like 10cm skin depth for average power ferrite at 100kHz. Doesn't seem very important -- but, there are indeed coresthat big, and they are in fact built with bricks glued together (much as laminated iron is, well, laminated). Granted, bricks are the only general-purpose form available in arbitrarily large size (glue up what you need!), so it's notjusteddy currents, but it happens to be convenient that that works out!Turn that figure up to 10MHz, and skin depth drops to more like 1cm. Or ~3mm at 100MHz. Starting to sound familiar -- what's the impedance peak frequency of a 1210 ferrite bead? Ever notice the impedance of a ferrite bead tends to go as sqrt(F) over a notable range (usually a couple decades)? Skin effect plays a big part in that!Which, connects with my previous post: you're notwrongto suspect steel nuts might do something -- they do indeed behave that way. But resistivity is, well, 10^4 or so smaller (and mu may be higher, though that varies widely), so the impedance range is proportionally lower, and/or the frequency 10^8 lower(!).Which, makes sense -- bulk steel is magnetically useful in the 1-100Hz sort of range. The skin effect of a solenoid (of typical design with a solid armature) is quite prominent, easily measured -- and dominates the electrical time constant of the component (i.e., somewhere in the 1-10s of ms; comparable to mechanical time constants, even!). At radio frequencies, the impedance is so low, you can't really even notice it. Now, suppose you had a material with middling resistivity -- like a ferrite, perhaps?Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2023, 08:36:28 pm
content: I was simply pointing out that ferrite beads, etc. induce increasing loss in the wiring as the frequency increases, due to their substantial hysteresis.Note that in the Rowland Ring measurement, with a core showing large hysteresis, as you increase step-wise the amplitude of the swing inH, due to higher swing in the current, you get a set of nested hysteresis curves/loops until you reach the excitation required to reach saturation, where the maximumBceases to increase with further step increases in maximumH.Of course, the wire through a core or bead also sees series inductance due to the permeability of the core and the flux linkage from the wire.

author: moffy
author_rank: Super Contributor
timestamp: August 06, 2023, 10:46:35 pm
content: Quote from: Gyro on August 06, 2023, 01:05:42 pmYou want eddy currents/losses at the right frequencies otherwise you effect the signal you are trying to clean up. I didn't notice where the OP said it was only for mains cables, he stated:"Maybe it would filter low frequencies too much, and thus influence the signal though the cable?"P.S. The eddy current losses are proportional to f^2, so there is a frequency selective component. Hysteresis losses are proportional to f.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 07, 2023, 12:39:20 am
content: Quote from: moffy on August 06, 2023, 10:46:35 pmOops, my bad. I made an edit to my first post to add that.I also did a simply hysteresis test on a few ferrite cores (meant for RFI suppresion).1. Stick it to a permanent magnet.2. Stick it to a piece of steel (it sticks).3. Remove the magnet carefully (in an attempt to keep remanent magnetism)4. the ferrite core does not stick to the steel anymore.So hysteresis (at DC) is not a simple factor.I'm guessing that the hysteresis of mild steel may be a problem, in that maybe it does not filter at all until there is enough (RF) energy to overcome the hysteresis.But I'm having some trouble distinguishing between hysteresis and coercivity.https://en.wikipedia.org/wiki/CoercivityI do have a function generator (JDS6600) that goes up to a few MHz.Which prompted me to do a simple measurement.1. Output to 10k resistor to GND.2. Adjust FG to 10MHz and 12Vpp on my Rigol scope.3. Add washers or ferrite core between FG output and resistor.I find the results surprising / confusing.With the washers (15 "Carosserie" washers M6, OD 30mm) I see no difference.With one ferrite core I measure 12.4V and with two ferrite cores I measure 12.6V on the Rigol.I tried some unknown ferrite cores, of the "long and skinny type" that are made to be put around cables.I also did it with a 100Ohm series resistor to put some more current through the wire, but it also does not show a significant results.I also tried with a few different toroidal inductors.Putting the wire though the toroid does not show a significant signal change.Only when putting the inductor in series with the FG output, I see some significant dampening.I would have expected to see some significant signal attenuation in all cases.At 10MHz a 10k resistor should be equivalent to:L = Z/W = Z/(2*pi*f) = 1e4/ (2*pi*1e7) = 160uHThat mostly explains it.I see (sort of) expected attenuations when I put a toroidal inductor in series, but those things have 20 to 50 windings to get to that inductance. which means that a wire though it once is just 1/400 to 1/2500 of that, and this suggests that a 10MHz test signal is just too low in frequency.Last, I looked up some SMT resistor sized ferrite bead datasheets. They specify an impedance of 10 to 1000 Ohm at a 100MHz test frequency. So they won't show much at my measly 10MHz either.

author: moffy
author_rank: Super Contributor
timestamp: August 07, 2023, 01:15:34 am
content: When you added your ferrites and washers was it over the output and return path of the cable, they will tend to cancel out their magnetic fields, or just the output path?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2023, 02:54:02 am
content: Quote from: Doctorandus_P on August 07, 2023, 12:39:20 amYeah, this iswaytoo high for EMI purposes. Consider a cable or harness tied to a chassis, or cabling laying in a tray or duct, or routed along steel studs or beams in a building: the cable overall (regardless of how many conductors are inside it; imagine them shorted together for now!) looks like a round-wire-over-plane transmission line geometry. Thus we can use such a calculator to estimate its impedance. For typical values, you will find figures of 50 to 200 ohms; with the higher values being the most dubious, as a large part of that will be radiation resistance (the loop between ground plane and conductor becomes large).In several EMC tests, cables are elevated above a thick copper ground plane by 50mm on low-k supports (usually foam blocks). This gives closer to 120-150 ohms, and if cables are laid back and forth with say >150mm spacing between adjacent loops or other cables, the coupling between adjacent paths will be fairly small and the cables behave pretty much like transmission lines.This is still a fairly high impedance in the grand scheme of things, so it either takes a LOT of inductance to have effective filtering against it (~mH!), or grounding/shielding, or bypass (think 'Y' caps), is necessary to get substantial attenuation and filtering. In the common mode -- this is all a common-mode model, considering the sum currents flowing in a cable as a whole, ignoring differential currents between wires within it.Since ferrite beads are typically in the 30-300 ohm range, we expect very little attenuation or filtering from them, as-is -- i.e. an impedance divider of 300R into 150R is only 10dB attenuation. The main advantage comes when we consider that these cables -- being generally unterminated lines flying through space -- support standing waves, which can therefore have impedances much higher, and much lower, than Zo. Which in turn modifies how much coupling those waves get, through connectors, shields, etc. The purpose of the ferrite bead, is to introduce series impedance near a low-impedance node, substantially cutting the Q factor of those resonances, and this function can yield 20dB+ improvement at select frequencies (i.e. at cable resonances).Quote from: moffy on August 07, 2023, 01:15:34 amThis is common mode, so the problem is indeed when the currents aren't perfectly canceled. Whether that's due to external fields (immunity) or internal fields coupling out (emission); by reciprocity, it's the same thing.Tim

author: moffy
author_rank: Super Contributor
timestamp: August 07, 2023, 03:19:10 am
content: Quote from: T3sl4co1l on August 07, 2023, 02:54:02 amWhether common mode or not it is easier to measure the effect with a greater field as a comparative measure between the ferrites and the nuts. It is easier to measure a large difference rather than a small one.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 07, 2023, 08:00:23 am
content: @moffyI put a single wire through the washers / ferrite core. Only the wire between the function generator and the resistor. So it was not a "common mode" filter, but just a series filtering element for testing.And I also wrote I repeated the measurements with a 100 Ohm resistor in series.@T3sl4co1l 50 to 200 Ohm is also in the range commonly quoted for cable impedances. some time ago I put a square wave into a roll of CAT5 cable and experimented with termination resistors. Whenever it got far from 120 Ohm, reflections started appearing.I also had a look at a datasheet of ferrite beads:https://www.mouser.com/datasheet/2/447/WC706-2904245.pdfAnd they appear to have a fairly small effective frequency range. At the 10MHz I measured, they do not do much, and I guess that @1GHz, the signal just passes around the things.Most of the problems I had with my microcontroller nodes, were build on perf board. I guess that part of the solution is to use bypass capacitors at the incoming RJ45 connector, just as with all the IC's and layout the wiring for minimum inductance, just as with all bypass capacitors. Proper PCB's with a solid GND plane helps a lot for preventing spurious resets.The tests I did were years ago, and fairly simple. By switching either a fluorescent light or a halogen desk light (with old fashioned iron core transformer) I could generate a reset on my uC quite often.And again, I've (re-)learned that just throwing some components at the problem does not work well.To get a good Idea of what the stack of washers will do is beyond my measuring capabilities. They do "nothing" at 10MHz and I can't measure far beyond that.

author: jonpaul
author_rank: Super Contributor
timestamp: August 07, 2023, 11:50:09 am
content: Iron and steel hardware will not filter anything.Ferrite beads and cores desigend for EMI use are very cheap.We use stock CM and NM chokes and Fair Rite brand beads.The split core ferrites are easy to change if needed without breaking the cable.See the many books and sites on CM/DM noise and filters,j

author: moffy
author_rank: Super Contributor
timestamp: August 08, 2023, 03:56:28 am
content: As a mildly humorous experiment I tied running a cable through 1.5m of copper tubing, plumbing that I had lying around. The picture shows the setup and the traces are blue for the drive signal and yellow for the return across a 47R resistor. I ran the test twice, once with the copper tubing and once without, trying to keep the enclosed area of the loop the same/similar. The tubing result shows a noticeable drop in peak value, the signal is supposed to be a square wave but looks more like a distorted sine at 5MHz. The copper tube was floating. That's a lot of copper for the result.

author: coppercone2
author_rank: Super Contributor
timestamp: August 08, 2023, 04:27:05 am
content: make a filter out of an I-beam and/or a section of railroad track, then go back in time to 2005 and post it online