Jim,
A low frequency cutoff strategy can negatively impact overall service
stability. The best strategy is to keep the transmitter RF from getting
into the cabling and as you said, good cable, properly installed is a key
starting point. Bonding and grounding of the cabling is another good step,
as is reducing the proximity of the antenna system to the service drop and
other cabling. Line isolators are another good tool for the service line
and other cabling to the DSL RG/modem. Chokes only seem to be effective
when applied in front of STBs and not on the service line to the DSL
RG/modem.
Thanks & 73,
Gordon Beattie, W2TTT
201.314.6964
-----Original Message-----
From: rfi-bounces@contesting.com [mailto:rfi-bounces@contesting.com] On
Behalf Of Jim Brown
Sent: Sunday, February 27, 2011 9:48 PM
Cc: rfi@contesting.com
Subject: Re: [RFI] RFI U-verse
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.
http://www.tonnesoftware.com/
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
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