Miscellaneous Radio and Electronics E-Blog

VDSL and EMC

For some time I have been in the habit of operating at the same time every day on 40m with 100W CW. Chatting to a neighbour I found that they were thinking of moving ISP because the ISP could not fix an intermittent VDSL problem that occurred around the time of my transmissions.

A simple test revealed that true enough, 40M RF caused loss of VDSL connection. An examination of the telephone wiring revelaed the reason why. It was old, very old – there was no NTE-5 of any age just an ancient punch down line jack. A rats nest of extension wiring was connected directly to the line. The VDSL filter was actually an ADSL filter and it was in line with the only telephone in use in the house, a DECT phone.

The current ISP had sent out the modem for the end-user to connect when in fact they should have sent an Openreach engineer to do an install. No Openreach engineer would have permitted installation of VDSL without installing the correct NTE-5 and filters.

Our neighbours are great folk and rather than just doing the minimum I offered to re-install all the phone wiring and bring it up to scratch. They would benefit from an increse in speed, a higher level of reliability and importantly to them I would sort out some very ugly wiring with loops of cable and badly installed wall warts. From my perspective I would have peace of mind knowing that I wasn’t likely to be a source of an EMC problem, even though obviously, since a VDSL shouldn’t respond to environmental RF, I couldn’t actually be held responsible.

I don’t advise others to take this action unless they are qualified to do so. I am pleased to be a professional chartered engineer with many years telecomms industry experience to prop me up and I felt qualified to undertake this role. I also had assessed the level of risk to house infrastructure and I wouldn’t be driling holes, just using surface conduit, removing defunct wiring and attaching a brand new NTE-5 to an existing back box.

To be sure of getting the best results for my neighbour I made a small balanced line rf sniffer and examined the telephone line signal with my Spectrum Analyser, a Siglent SVA1000X series. I could see the VDSL signal clearly at 7Mhz. It was quite a poor level, presumably becasue of the faulty installation and when I put my 40m TX on an autosender I could see the VDSL signal was poor in comparison with 7Mhz RF. A notch filter was designed and made up using a T32-6 toroid and this was tested on the SA and tuned to 7.1Mhz.

The day for the work came around and all surplus wiring was disconnected and tied back, out of sight. The small 40m notch filter was placed across the line on the consumer side of the NTE5. Toroidal cores were used for common mode chokes on the router power line, the VDSL line just prior to entry in the router and on some direct connect Ethernet cables attached to the router. All telephone cabling was reterminated and the system brought back up again.

Tests revealed that the synced VDSL speed had increased significantly and that no amount of RF that I could generate had any impact. An excellent result and no drama.

01 July 2021


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