I can not stress strongly enough that the ability to see the spectrum is incredibly valuable. The "signature" of the spectrum dispay was incredibly valuable in the positive identiification of the RFI source. This would have been an impossibility without the spectrum display. In this neighborhood the spacing between homes was 10-20 feet and we would not have been able to pinpoint the VDSL2 users in a higher population density area.
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We were able to determine with 100% accuracy whether a home had Century Link VDSL2 service or not. Later after the "management folks" had left we drove through the neighborhood with a Century Link engineer in the vehicle with us and played a little game of "VDSL2 or not". After the repair work we could still see the VDSL2 RFI on the spectrum display but it was only a few dB above the base noise level. The on premise wiring is owned by the customer and may contain stubs and other wiring issues which contributes hugely to the problem of radiated interference.
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The craftperson went to the interferring house and found several wiring issues and was able to reduce the interference significantly which also improved the signal to noise ratio (SNR) as seen by the modem. Century Link readily agreed that this was interference from VDSL2. Without the ability to see the frequency spectrum before and after we would have had a difficult time convincing them that they were the source of the interference over such a wide frequency range. We convinced them to temporarily turn off the VDSL2 service to the home that we were sure was the source of the interference. They were adamant that VDSL2 was built with non-radiating technology and the interference was not due to their equipment. This is believed to be associated with modem optimization for the individual telephone lines.Ĭentury Link was contacted and we had a meeting at the site of the interfence with Century Link management, engineers, and craft people. Driving past other houses showed similar interference spectrums but the shape of the amplitude envelope in the 3.8MHz to 5.15MHz range varied significantly. The abrupt noise increase at 3.8MHz was about 15dB. The graph is a screen capture from the HDSDR program controlling an SDR Play radio which had an external filter to eliminate AM broadcast overload.
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The graph below shows the upstream characteristics starting at 3.8MHz and stopping at 5.15MHz. Please look at the link on this web site at "Active Monopole Amplifier" for the hardware specifics.Īfter building the initial system and driving around the neighborhood we discovered several sources of RFI that we were reasonably sure was VDSL2 upstream data. I had been thinking about building a broadband tracking system for other purposes and this provided the impetus to get moving with this project.
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Some research by Tom, W0IVJ convinced him thay the interference might be one of the upstream data frequency bands for the VDSL2 service that Century Link uses to provide data and their Prism TV service based on the frequency of the interference. Early attempts to track this noise down via conventional loop antennas was not very successful. The interference, as heard with a radio demodulating an SSB signal, appeared similar to wideband noise with little in the way of distinguishing characteristics. His neighborhood, in southwest Denver,CO, was built about 45 years ago and has overhead telephony wiring. A couple of years ago I got involved with helping Dale, W0DGD who had a big interference problem on the upper end of 80m.