DIY EMC test cage - Page 1
Forum section: electronics / rf-microwave

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2022, 08:47:11 pm
content: As part of hunting a nasty EMC problem I embarked on trying to create a shielded environment that would allow me to make some kind of measurements on a device that fails the radiated emissions testing. There is just too much interference around to make a measurement without any form of shielding. A problem is that the emissions come mostly from the wires and these can't be shielded or filtered so I need quite a big setup to include the wiring as part of the measurement.I did find conductive cloth on Aliexpress but making a huge tent with it would be costly an no guarantees on whether it would work. I thought about different solutions and came up with the idea of using wire mesh that is used for fences or cages. At least this would be self-supporting and simplify the design because it shouldn't need a frame. So I went to the hardware store and bought some bird cage wire mesh and started folding it into a cage:This is the nearly finished 'product'; some initial testing looked promising enough to go forward. After I added the wooden edge frame I was quite content with how sturdy the cage felt. But that feeling didn't last long. I put some weight on top to keep the mesh for the lid flat and then it decided to collapse in on itself at the left side. Curses!I managed to straighten it out but it will need some reinforcement later on.The size is 60x80 by 90 cm high. This is a size I have a storage space for.I used aluminium tape (real aluminium so it is conductive) to get rid of the sharp points of the wire mesh. I laced the bottom and the side together with steel wire so there is a lot of contact between the mesh.The shielded chamber 101 says that the shield of signals coming in needs to be fully connected to the wall of the chamber so I made this with some bulkhead connectors and a piece of copper plate:For other signals a copper tube to feed them in is the recommended option so I added a piece of 22mm copper pipe. I choose 15cm as a reasonable length. Last but not least I soldered the copper plate into the mesh.Ofcourse I also needed some kind of antenna. I opted to make a simple dipole and found a website saying that making a dipole wide, makes it broadband. Just slap something together and see what it does. I found some phenol copper clad in my stash with PCB material and made an antenna which is nearly 90cm high. The plates are 10cm wide.Let's hook things up:Meanwhile I made a lid as well that fits nicely around the cage. I used a piece of PVC pipe to hang cables from. The mesh allows to tie cables down as well.The cage provides a shielding of around 25dB at relatively low frequencies:The yellow trace is with the lid open, the blue trace is with the lid closed. The blue trace is at the noise floor of the spectrum analyser in most places. The FM radio stations remain clearly visible though but for the measurements I wanted to make most urgently, these are not in the way.Working close to the noise floor of the spectrum analyser did cause some challenges. Even with the preamplifier on, I just missed a bit of dynamic range. So I bought a simple amplifier from Amazon.I used a USB cable and a ferrite core (with only the power wires wrapped around it) to power it.The amplifier itself is build around a SPF5189 which turns out to be a very low noise amplifier.Now this amplifier has a noise floor itself but it seems to give an extra 5dB to 10dB of dynamic range. Enough to pull the signals a bit more out of the noise.Same signal measured with (top trace) and without (bottom trace) amplifier:All in all I'm not dissapointed. Unfortunately the cage isn't big enough to do real far field measurements from a few meters away. And the antenna leaves much to be desired for sure but this cage did allow me to try a few modifications and gather some data to justify going to a real EMC lab or needing to do more work on the circuit.A future enhancement could be adding a finer mesh or conductive cloth to improve shielding. I have found a report that indicates a finer mesh leads to better shielding at lower frequencies. Or maybe I opt to make a large tent from conductive cloth which I can put up in the living room temporarily

author: wilhe_jo
author_rank: Regular Contributor
timestamp: September 02, 2022, 10:35:49 am
content: Ever thought of building a crawford cell?That's basically a big coax... similar to GTEM/TEM cells, but easier to build than GTEM cells and compared to normal TEM cells they benefit from the shielding.For the lower-end of the spectrum, they should be fine for you.A mesh is always a bit of a compromise. Sheet metal is the proper thing...73

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2022, 04:29:32 pm
content: I have been pondering on a Crawford / TEM cell. The problem is that I don't know how large it would need to be to include the device + a decent length of wires AND whether it is even worthwhile to embark on building it. Ideally I would like to do measurements that represent a far field situation on a setup that is 1 meter long. Looking at the specs for Tekbox open TEM cells, I estimate I would need to build one that is 2.5 to 3 meters long. Actually an open TEM cell with external shielding looks more practical to me because it is easier to access the DUT (on top of being simpler to build).

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2022, 05:13:51 pm
content: I kinda wanna build this, I recommend putting wood stain on the wood structure before assembly though to make it more durable and visually pleasing, and copper instead of aluminum tape, and a vanity garden fence under the wire mesh to make it better structurally, or perhaps over it, so that its harder to deform the mesh.

author: jonpaul
author_rank: Super Contributor
timestamp: September 02, 2022, 05:46:19 pm
content: Use copper screen, not galv mesh steel.Test with local noise RF sourcs on, off eg any electronic lLAMPS LED, CFL, HIDj

author: coppercone2
author_rank: Super Contributor
timestamp: September 02, 2022, 06:11:01 pm
content: I would wrap copper over the galvinized steel because its too easy to puncture a thin screen like that and totally destroy it

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 02, 2022, 11:12:25 pm
content: Navy had screened rooms, but for big stuff, a way to do approximate tests was to load the DUT and test gear along with a portable generator on a truck and drive to a natural gully with hills all around. I suppose about 4am would be the best time for HF. Usually would be lucky to get there by mid morning after getting organized and finding all the gear and cables , passes to get stuff out the gate etc.

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2022, 04:24:24 am
content: Quote from: mag_therm on September 02, 2022, 11:12:25 pmthe military can ruin anything

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2022, 04:26:52 am
content: seriously trying to find some stupid cable and getting papers signed at 4am, what a nightmarelet me guess you could not pre-package it the week before because it was in use right?

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 03, 2022, 06:06:43 am
content: I do recall a funny thing happened. Even though long time ago, better stay silent.I mentioned this as a possibility of time and place for nctNico, to drive out to low lying rural area and work at night.To maybe get a lower ambient, not knowing how quiet he needs.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: September 03, 2022, 07:16:07 am
content: Quote from: nctnico on September 02, 2022, 04:29:32 pmActually, a Crawford cell is a tem cell with shielding... However, your size requirement is kind of a problem.I have a 5m long GTEM cell... Max eut size is somewhere between 20 and 25cm (dimension of a cube).... But that thing is fine up to some GHz...On the other hand, the usual 3m distance in an anechoic chamber does not really give you far-field results in the lower frequency range either.Btw, I did some enquiry on EMC chambers in China. If you have the space and your're willing to assemble it yourself, you get a basic chamber for 100-150k...The next best thing would be to screen a room in a basement... I have one where ambient is right at the emission limits for industrial stuff. So the 20dB of your mesh would give me sufficient headroom even for residential testing.However, a big room without ferrites gives you quite some troubles with reflections...So all in all, radiated emission testing is nothing you can do without serious investments ( money, real estate and time)For large stuff, I tend to put the antenna as close as 1m. That way I get higher emission levels, but converting limits, re-calibrating antennas,... took quite some time.To "optimize" ambient noise, I turned my Yagi 360°, to find the worst direction, turned it 180° and moved the EUT accordingly. This helped a lot for my situation.73

author: nctnico
author_rank: Super Contributor
timestamp: September 03, 2022, 12:09:36 pm
content: Quote from: wilhe_jo on September 03, 2022, 07:16:07 amNow you are entering into a grey areaLet me throw some thoughts out on the table... If I need to spend over >1k euro in materials and a considerable amount of time, then it is more efficient for me to go to an EMC lab. On top of that, I'm quite space constrained as well. The primary purpose to have some testing facilities in my own lab is to be able to make comparative measurements to assess the effectiveness of changes to a circuit. The shielded cage I made allowed me to do that because it has enough attenuation of outside noise for the particular case I had to work on. I'm the first to admit it is far from flawless though.A TEM cell looks like a good next step to take for a more permanent solution that also seems to be closer the kind of results I would get from testing at an EMC lab. With my mesh cage I have not been able to see the exact same peaks as during the radiated emissions pre-scan testing. What I did is modify the circuit so frequencies that are in the same band are suppressed and measurements in the mesh cage seem to confirm that but the confidence level of the measurements isn't high. If a TEM cell can get me higher confidence levels then it could be a worthwhile investment. Not only for fixing EMC problems but also as an extra service towards my customers. Typically I like to do EMC pre-scan tests (as good as I can) for devices that I develop to get some feeling whether it radiates a lot or not. Though a lot of devices I develop are never going through an official EMC test because the volume is too low. Another advantage of a TEM cell is that it can be used for immunity testing as well so that would be a plus. But at this point I'm not 100% sure of what a TEM cell would give me in terms absolutely measurement accuracy. If I can get within 6dB compared to a real EMC test, then I would be happy already. I see that Tekbox claims that using their TEM cell can give a good indication whether a device would pass or fail radiated emissions testing limits.Meanwhile I have found some interesting documents on TEM cells:A report co-written by Crawford which has some hands on formulas for building a TEM cell and some experimental results:https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote1013.pdfAnd a report for a DIY TEM cell (however I question the correctness of the formulas used):https://www.ewh.ieee.org/soc/emcs/acstrial/newsletters/summer08/pp2.pdfI have plugged some numbers & formulas into an Excel sheet in order to play with them a bit. It looks like a 2 meter long TEM cell (almost sigar shaped) would work for me. Probably it will need some shielding at the sides and / or partly closed sides to have mechanical strength. Due to the length the resonance frequency will probably be quite low though.Quote from: mag_therm on September 03, 2022, 06:06:43 amI'm in a country that has been shoved flat as a pancake by an ancient glacier so there aren't any valleys. Also due to dense population there are cell towers everywhere, wind turbines (which I assume have some power electronics on board) plastered allover the place and there is a huge AM transmitter station nearby.

author: radiolistener
author_rank: Super Contributor
timestamp: September 03, 2022, 10:08:13 pm
content: What kind of field you're want to reject? Electric or magnetic? Or electromagnetic plain wave?At low frequency the most complicated thing is to reject magnetic near field.I think you're needs to use thick layer of solid iron instead of metal mesh...And keep the distance between cage walls and test equipment as far as possible in order to reduce near field coupling...

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2022, 06:04:01 pm
content: Magnetic fields are not the problem. These die out pretty quick anyway. My problem are nearby transmitters and electrical noise that is typically present in a city. I did a scan up to 1GHz and LTE cell towers come in loud and clear inside the mesh cage. Not a surprise though.

author: max-bit
author_rank: Frequent Contributor
timestamp: September 05, 2022, 08:39:33 pm
content: A very average idea.It is not known to what extent this will be suppressed by the given frequencies.The cage itself may, for certain frequencies, show the phenomena of (internal) reflection resonances, so the parameters are very undefined.Building such a chamber at home is difficult and, for sure, expensive.It was better to do it from, for example, thin aluminum sheet.But there is still the question of internal reflections (and resonances, standing waves and such) there are special materials to dampen such hovers, unfortunately they are very expensive.

author: nctnico
author_rank: Super Contributor
timestamp: September 09, 2022, 12:42:54 am
content: Yuk... Still a no go at the EMC lab. I'm strongly suspecting there is a problem in the part of the test setup outside the chamber which ends up injecting noise into the chamber. If not then then I'm going to built a TEM cell that is large enough to hold the device under test and the cabling.

author: radiolistener
author_rank: Super Contributor
timestamp: September 10, 2022, 01:54:26 am
content: Quote from: nctnico on September 05, 2022, 06:04:01 pmRFI from nearby transmitters in GHz and sub-GHz band can be easy shielded with a metal box, because they are usually in a far field region. But there are a lot of noise from SMPS, LED lamps and other home electronics in the near field region, and it's not easy to fight with them because they are below 10 MHz. And even worse, they have RF leakage into mains, so you're needs to use mains filters against it.The same brand LED lamps can have RFI at different frequencies. But if they from the same part, their RFI peaks almost the same, but also have some frequency shift. So, if LED lamp produce RFI at unwanted frequency, you can try to replace it with another one. In such way you can put RFI outside spectrum band of interest. But if you want to clean noise floor, you're needs power off all LED lamps in your homeYou can catch some LED lamp which can produce very strong RFI noise. So, you're needs to test all your LED lamps.

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 07:08:20 am
content: Links to a couple of old threads:https://www.eevblog.com/forum/rf-microwave/diy-low-cost-rf-absorber-for-a-4-x-4-x-4-chamber/https://www.eevblog.com/forum/testgear/emc-chamber-build-log/

author: mawyatt
author_rank: Super Contributor
timestamp: September 15, 2022, 04:37:25 pm
content: Back in 2000 we had to build a "special room" that had limited access physical and electromagnetically. The "room" hosted a lab and conference room, and the usual cost for these types "rooms" was daunting!!We ended up using the insulating type drywall equivalent (8' by 4') which was a foam type with a thick aluminum outer covering for the walls, sub-ceiling and sub-floor. The 8' by 4' sheets were covered with another layer of thick aluminum foil and the seams were covered with a conductive aluminum tape, and everything was bonded with a conductive bonding epoxy and conductive screws. We saved a bundle which the executives took credit for and pocketed nice additional bonuses, typical of how engineers/scientist help enrich the company executives while suffering no real financial reward, other than a stinking plaqueAnyway, this material & technique might be useful for fabricating a DIY shielded enclosure/room on a budget.Best,

author: rhb
author_rank: Super Contributor
timestamp: September 15, 2022, 05:04:40 pm
content: I’d like to suggest you use galvanized HVAC duct metal. It’s cheap and effective. You’ll want to solder the seams or at least cover them with a wide piece of aluminum foil tape as used for HVAC duct joints.Isolating the feed thrus is seriously non-trivial,good luck. The people I know with many years of professional TEMPEST experience tell me you almost always encounter weird issues.

author: mawyatt
author_rank: Super Contributor
timestamp: September 15, 2022, 05:31:23 pm
content: We had all the "Electrical Feedthru" for the mains, Ethernet and Land Phone line (cell phones weren't allowed) handled by a special feedthru fixture specifically designed for such. The mechanical "Feedthru" was handled by other means.Best,

author: temperance
author_rank: Super Contributor
timestamp: September 24, 2022, 11:39:34 pm
content: I understand the idea of building you're own measurement setup. I've been toying with the idea myself but reverted to a different tactic because TEM cell's and other solutions are too large to handle in a small workplace. (they also take up space when not in use)You know what your problem looks like if radiation has been measured by an EMC lab. If so, the only thing you need is a way to do relative measurements in a repeatable way.My take on this has been to place the subject with cables and everything onto a piece of plywood. Fix cables in place with tie wraps and adhesive feet (don't know the name for those in English)-Measure radiation trough cables with a wide band current transformer. Mark places of interest with sticky tape or something.-Radiation from the PCB can be investigated with near field probes with the near field probe mounted onto a drill stand with an "extended" (a piece of broom stick) arm. Here you also mark the places of interest with sticky tape.Primitive but it works.

author: nctnico
author_rank: Super Contributor
timestamp: September 25, 2022, 07:49:43 pm
content: Quote from: temperance on September 24, 2022, 11:39:34 pmI get that and I have been using these methods myself for many years but sometimes 'primitive' methods just aren't good enough. The problem I'm hunting appears to consist of not one or two but three seperate problems! The basic problem are emissions from an ethernet cable that also carries power-over-ethernet. It turns out that the PoE injector my customer has been using during the tests by itself is causing enough emissions for a fail. The second problem is that the isolated PoE DC-DC converter on the board and the ethernet PHY are producing junk in the same frequency range. The junk gets onto the cable through the ethernet transformer. However, near field measurements don't cut it because you are always receiving one part of a differential signal stronger. I'm now trying to use a dipole antenna (placed several meters away) with the spectrum analyser set to a narrow band. The downside is that I can only look at very small parts (several 100's of kHz) of the frequency spectrum. Fortunately I have the results from the EMC lab that tell me the exact frequencies to look at.The test engineer from the EMC lab also brought up using a TEM cell to hunt this particular problem down. I agree the size of a TEM cell with a reasonable useable area is daunting but I see no way around it. For EMC testing you need at least 80cm of useable length to also accomodate the required length of wire. Then again, having a TEM cell could be a tool that is commercially interesting for me because -if it works well- I can offer better pre-compliance testing as a service to customers.

author: MartinL
author_rank: Regular Contributor
timestamp: September 25, 2022, 11:01:33 pm
content: Quote from: nctnico on September 25, 2022, 07:49:43 pmI have also had problems with PoE devices, they are challenging from an EMC point of view.However once you know that your emissions are coming from the cable, then there is no need to receive them at a distance with an antenna in order to measure the peaks concerned. You know that you must have common mode RF currents on the cable in order for it to radiate. So a current transformer wrapped around the cable should show you them. A couple dozen turns of magnet wire wound through a clip-on ferrite has done the job well enough in my experience, but you can buy a calibrated RF current probe if you want.You presumably already have common mode chokes on the TX and RX ethernet pairs, but do you have one on the power side, on the 48V lines where they come into the converter after the diode bridge? A lot of reference designs omit this but I found it made a big difference.Another big cause of problems in my case was overshoot and ringing on the switch waveform within the isolated flyback converter. A simple RC snubber worked wonders, and some improvements to the converter layout to reduce loop area and improve return paths helped too.TI app note SLUA469 has some good advice.

author: nctnico
author_rank: Super Contributor
timestamp: September 25, 2022, 11:36:41 pm
content: I have a current probe constructed from a clamp-on ferrite with several turns of wire around as well. It works well indeed. I've also constructed a simple two pair ISN.The problem however is that the current probe output shows a lot of spikes once the ethernet link goes up even with the PoE power supply disabled and the board powered from a clean lab PSU. When I disconnect the ethernet transformer at the PHY side (so the PoE part still works) there is barely any emission coming from the device when measuring at an EMC lab while being powered through PoE. I probably have overdone the filtering in the power side of the PoE part though. Also snubbers, MOSFET switching, real Shottky (without guard band and thus no reverse recovery) have been addressed resulting in great reductions in the near field emissions I measured in my own lab but appearantly also have a positive effect on the far field judging from the results at the EMC lab.The ethernet transformer does have integrated common-mode chokes but as a test I have added some extra common mode chokes on the board close to the PHY. It makes no difference at all. PHY power supply also looks clean.But the app note you mentioned (thanks) does show something interesting: Bob-smith termination on the center taps at the ethernet side of the transformer. I don't have that in my design and it is worth at least a try. My current working theory is that there is something wrong around the ethernet transformer. I should not be getting the amount of junk I see from the current transformer when the ethernet link is active. I also tested the previous version from the board which uses the same ethernet transformer but a different PHY and it shows the same amount of junk. So that rules out the ethernet PHY chip.

author: MartinL
author_rank: Regular Contributor
timestamp: September 26, 2022, 10:29:19 am
content: Bob Smith terminations may be helpful, I used a Wurth jack that had them built in.You didn't mention whether you have a common mode choke on the power side - i.e. L2 in the SLUA469 schematic?Even if your power converter is now clean by itself I think this may be helpful, as without it there is a path for noise to escape from the board onto the cable by going through the converter onto the power pairs, bypassing the common mode chokes in the TX and RX pairs.

author: MartinL
author_rank: Regular Contributor
timestamp: September 26, 2022, 12:10:14 pm
content: In fact, there's a test you could do to confirm that hypothesis by checking which pairs are radiating - cut off the outer insulation of the ethernet cable, separate the TX/RX pairs from the power ones, and put your current probe around each group in turn.

author: temperance
author_rank: Super Contributor
timestamp: September 26, 2022, 12:33:16 pm
content: What did you use instead of bob smith terminations? Because without those, common mode noise will be all over the place. Much like operating a switching power supply without a capacitor between sec 0V and primary 0V.

author: nctnico
author_rank: Super Contributor
timestamp: September 26, 2022, 02:57:56 pm
content: Quote from: MartinL on September 26, 2022, 10:29:19 amThere is. And extra series inductors as well.Quote from: temperance on September 26, 2022, 12:33:16 pmWell... uhhh.. nothing !QuoteThat sounds very plausible. I'm going to add the Bob Smith termination and see what that does.

author: temperance
author_rank: Super Contributor
timestamp: September 26, 2022, 03:30:02 pm
content: An extra inductor might create problems if not sufficiently damped.

author: nctnico
author_rank: Super Contributor
timestamp: September 26, 2022, 05:00:07 pm
content: That has been addressed as well by adding shunt capacitors. The design is a bit tricky because there is a limited amount of capacitance that may be directly at the connector of a PoE port. The inductors also have quite a bit of series resistance to have extra dampening.

author: nctnico
author_rank: Super Contributor
timestamp: September 27, 2022, 11:35:17 pm
content: Meanwhile I have tried the Bob Smith termination on the center taps of the transformer at the cable side but that makes no difference at all. I also tried different ethernet transformers with no real improvement. I messed something up in the ethernet circuitry. There are some series resistors and TVS diodes as well. I'm going to take those out tomorrow (it is late already) in order to strip the ethernet circuitry back to the bare minimum and go from there. I'm seeing a hard spike at a fixed frequency. I doubt it is the PoE power supply but I'm going to verify that again. Also, adding ferrite clamps over the ethernet cable does nothing at all.

author: MartinL
author_rank: Regular Contributor
timestamp: September 28, 2022, 09:02:17 am
content: Quote from: nctnico on September 27, 2022, 11:35:17 pmWhat frequency?Assuming 100Mbps ethernet, the symbol rate on on the wire is 125MHz so you'll see peaks at multiples of this, in my experience 250MHz is the main one. If that's what you see then look at the circuitry between the PHY and the jack.If you see 25, 50 or 100MHz peaks then those are coming from the MII interface on the other side of the PHY, so focus your efforts there.Anything that's not a multiple of 25MHz, it's coming from somewhere else on the board.

author: nctnico
author_rank: Super Contributor
timestamp: September 28, 2022, 11:51:30 am
content: The spike I'm using as a 'reference' is a strong one at 43.52 MHz (according to the spectrum analyser which may have a frequency error) but there is a whole band of noise popping up. And it is only there when the ethernet link is up so it is not coming from the PoE power supply. At the worst peak the difference between ethernet active / inactive is about 25dBuV (where the inactive state hits the noise floor of the EMC lab's test setup) so it is not just a small spike. I'm going to look at the TVS diodes as a next step. It could be these are rectifying part of the signal causing intermodulation products. I'm not ruling out other power supplies but all rails towards the PHY have filtering (ferrite bead + >10uf decoupling in various sizes & values).I know about the high frequency products that come from some ethernet PHYs which is why I specifically choose a PHY chip which has low EMI by design (at a slightly higher price). This was a problem with the previous version of the product that used a 'cheap' PHY from SMSC / Microchip (the extra filter components needed made it expensive).Update: it is not the TVS diodes, signal filtering or PoE supply. There is something else going on.

author: MartinL
author_rank: Regular Contributor
timestamp: September 29, 2022, 11:07:49 pm
content: I doubt any spectrum analyzer will be off by more than a few tens of ppm, so I expect that 43.52MHz is accurate.Interesting number. Exactly 17 times 2.56MHz, which is useful since it's 2^8 x 10kHz. I doubt that's a coincidence.I don't know what else you have on your board, and what it else it does when the ethernet link is up, but I feel like there's a clue there.

author: nctnico
author_rank: Super Contributor
timestamp: September 29, 2022, 11:49:55 pm
content: Meanwhile I think I have found the problem. There seems to be nothing wrong with the ethernet interface on the device itself. I measured the signals and impedance (TDR) and that looks just fine. Also a second board measures identical.Though, the device is connected to only two pairs out of four so two pairs are open (it is not possible to connect the open pairs to the device; a design choice). With the unused pairs terminated at one end, the signal at the offending frequencies drops significantly. Ballpark 10 dB according to what I have been able to measure with a current probe (a few turns on a ferrite clamp) and a DIY ISN using a relatively short UTP cable (3 meters). Cable length does seem to matter with two pairs open. During the EMC test I used a 15m cable. During the most recent EMC test I left the unused pairs open as part of trying to rid of the noise from the PoE injector. All in all it looks like an error in the test setup.Addition: nice catch on the frequency. It probably has to do with the symbol rate in the ethernet idle pattern.

author: nctnico
author_rank: Super Contributor
timestamp: October 21, 2022, 01:52:27 pm
content: Update: with the other end of the ethernet cable terminated on all pairs, it passed but not with a large margin. It is surprising how much of the ethernet signal leaks out despite the use of twisted pairs.