[RFI] RFI U-verse
martin.s.ewing at gmail.com
Sun Feb 27 15:31:02 PST 2011
AT&T's U-Verse service is based on VDSL2, which uses frequencies from around
100 kHz up to 8.4 MHz in carrier channels of about 4.3 kHz bandwidth.
Typical phone lines at typical distances limit the maximum usable frequency
because of excess attenuation. My line, about 2100 ft from the U-Verse
fiber node, won't carry channels above about 5.7 MHz. (More at
I suppose I could improve things on 40 M by inserting a ~6 MHz low-pass
filter at the input of the U-Verse RG, since I already know I get nothing
useful above 5.7 MHz. (I would have to be careful about the passband phase
response.) But then AT&T would certainly decline to guarantee my service.
It would not help on 80 or 160, of course.
On a recent visit, an AT&T tech decided that I'd be better off with "RG-6"
coax feed instead of Cat5. They installed a balun at the outdoor NID and
ran coax to the RG. That may have helped my ham interference issues, but it
seemed to add attenuation at high DSL freqs, so that my maximum bit rate was
reduced by 5 or 10%. (FWIW, a good filter for coax is probably a little
easier to build, considering connectors and shielding.)
The VDSL2 system is pretty good about detecting and swapping out bad
channels, i.e., channels that have bad SNR due to interference. Available
software diagostics (see my site above) let you monitor the channel
assignments, called "bit loading". In my case, I can now operate at 100W on
80 M. I get a burst of errors at the start of a transmission (possibly
causing a short TV freeze), but the system adapts after a few seconds. With
higher power levels, the RG seems to panic and do a full retrain that means
a TV/Internet/telephone outage of about 2 minutes -- a real bummer for all
I enjoy a technical challenge, and we're not especially addicted to TV, so
we battle on. Other hams might want to carefully consider the options
before selecting their new digital service.
I have found AT&T pretty responsive to my ham radio issues and to other
non-ham U-verse problems we have.
On Sun, Feb 27, 2011 at 4:48 PM, Jim Brown <jim at audiosystemsgroup.com>wrote:
> On 2/27/2011 12:11 PM, BEATTIE, GORDON (ATTSI) wrote:
> > The key thing is to not just reduce off portions of the spectrum through
> > the incorrect application of chokes.
> One problem with DSL is that the signal on the hard wired phone line
> extends from low audio up to nearly 4 MHz. As I understand it, these
> systems will limit their bandwidth (which lowers the max data rate) if
> they see a loss of bandwidth in the circuit or see noise in the higher
> part of the spectrum. My advice with respect to differential filtering
> of these circuits is to use a good filter design program to design for
> the widest possible bandwidth that does not include the transmitter
> fundamental, and for a characteristic impedance of 100 ohms. This IS
> the Zo of CAT5 cable, and most closely spaced pairs are in this range or
> just below it. There are some decent filter design programs are
> distributed free with the ARRL Handbook, and they are also available for
> free download on the author's website. The one I'm thinking of is called
> Elsie. Jim Tonne is a contributor to the ARRL Handbook.
> If the interfering transmitter is on 160M and the coupling is
> differential mode, I would first install the best CAT5/6/7 cable I could
> find for the wired portion of the link and get serious with common mode
> ferrite chokes. If this were not sufficient, I would add the
> differential filter noted above, and, if still more was needed, lower
> the cutoff frequency to about 1.5 MHz. This would reduce the data rate,
> but as I understand it, most DSL systems would continue to work. Would
> this be a reasonable approach with UVerse?
> 73, Jim K9YC
> RFI mailing list
> RFI at contesting.com
martin.s.ewing at gmail.com
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