ka7oei

Ethernet filters

ka7oei

March 2019

I noticed the DXE devices when they first appeared - and have wondered about them. I did notice that they didn't support POE (hence the claim of galvanic isolation) which means that they would be of… (more) little help when it comes to quieting the oft-noisy IP cameras that are becoming increasingly common, nor would they help in preventing the conduction and radiation of power supply noise from a POE-powered wireless link radios (e.g. for Internet or "Mesh" operation) - or from the power inserters/Ethernet switches that power these devices, which are often bad or worse than the device being powered in terms of RFI.

This makes me suspect that the DXE devices are simply back-to-back Ethernet transformers - with some care taken in the design to allow operation at Gig-E speeds.

For quieting Ethernet connections I have, for some time, just used the "flat" Ethernet cable (often called "Cat-6" or "Cat-7", regardless of whether they really are...) wrapped around large-ish ferrite toroids. It would seem that one can wrap this "flat" cable rather tightly without upsetting its geometry as it will still work fine at Gig-E speeds when "spliced" inline with a Gig-E circuit, provided that one uses a Gig-E rated double-female RJ-45 connector.

In testing, I've verified the following in a fixture that allowed the insertion of an Ethernet cable/device and inputted a signal on one side with a spectrum analyzer on the other side, terminating in 50 ohms:

Using 10-12 turns on the following cores (approx. 33mm O.D., 23mm I.D., except where noted):

- Mix 77: >=20dB across AM BCB and onto 160 meters (Fair Rite 5977003801)
- Mix 31: >=20dB from 160 meters through 30 meters (Larger core - Fair Rite 2631803802 - I haven't found a convenient source for the 33mm O.D. in this mix)
- Mix 43: >=20dB from 40 meters through 10 meters (Fair Rite 5943002701)

For a particularly troublesome situation, three cores with 10-12 turns each - two Mix 43 and one Mix 31(or Mix 77, which works almost as well at 40 meters ) in series, but each separated by about 1.5 inches (3-4cm) to prevent cross-coupling seemed to work well from 160-10 meters with well over 30dB isolation (>50dB measured at some frequencies) and about 20dB to 6 meters. I've used a similar scheme with a Mix 77 core to keep my 630 meter and 2200 meter transmissions from getting into devices.

Because these are simply common-mode chokes, they have no effect on (properly made) signals on the Ethernet cables themselves - and can be used to pass DC (as in POE).

For higher frequencies (VHF, UHF) it gets increasingly difficult to effectively quash radiation on cable, but more common split ferrites designed for the frequency range, placed as close to the offending device(s) as possible, are the best bet.

73,
Clint
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Ethernet filters

WSPR, who is running it?

Seeed Metal case and GPIO connector