[RFI] DSL Filter Update

Christopher E. Brown cbrown at woods.net
Mon Mar 9 12:40:12 PDT 2009

On Mon, 9 Mar 2009, Jim Brown wrote:

> On Sun, 8 Mar 2009 20:43:14 -0500 (CDT), Christopher E. Brown wrote:
>> Depends on the DSL gear, ADSL1 cuts off around 1.1, VHDSL uses much higher
>> freqs.
>> Common use these days in the US is ADSL2+, with ADSL1 hardware cycling out
>> of production.
>> Also, remember that RF can impact *BOTH* ends.  I have seen a couple cases
>> where enough signal made it into the line to impact both the CPE *and* the
>> DSLAM, as well as causing increased error rates for neighbors in the same
>> cable bundle.  A serious common mode choke at both the CPE and at the NID
>> could be needed in severe cases.  At my home I have chokes at both ends of
>> the line, only a single cylinder but my original DSL issues were not so
>> severe.
> Many thanks, Chris.
> For those of us not working in the telco world, please translate DSLAM,
> CPE, and NID. :)

DSLAM: Digital Subscriber Line Access Multiplexer (Terminates the upstream 
side of the DSL line, 24, 48 and 512 port chassis are common.  Back side 
of the DSLAM will normally be ethernet or ATM depending on the provider.)

CPE: Customer Premise Equipment (The DSL modem)

NID: Network Interface Device (In this case that little box on the house 
with the line supressors and the service test point)

> Your point about RF entering the cable at one point and traveling along the
> cable for some distance is well taken, although I would expect that to be
> most problematic below 4 MHz. I could also see the possibility data errors
> introduced by one subscriber modem throwing a monkey wrench into the overall
> data stream.

Depends, 75/80 are close and 160 is in the DSL range, but shorter 
wavelengths are more likely for find an optimal length of line to couple 
to.  The CPE gear is built to be light and cheap, easy enough to create a 
case of frontend overload, or more likely just a jump in the noise floor 
(as perceived by the CPE)

> Several thoughts.First, my measurements of common mode chokes that have
> historically been effective at protecting telephone lines show that they
> have VERY high choking impedances (10K Ohms - 80K Ohms). A single cylinder,
> even a rather large one, is typically a few tens of Ohms at HF -- pissing in
> the wind. It might be useful as part of a differential choke, but not as a
> common mode choke.

Fair-Rite 2631540002.  Published chart shows about 500 Ohms at 1Mhz and a 
peak of around 1900 Ohms at about 30Mhz with 3 turns.

I do not have a VNA or similar avail, so going by by SWAG, but 6 turns 
should give a peak < 4Mhz.  I would expect at least 4K Ohms if not more, 
and I am only dealing with 100w here.

> As for protecting the upstream side -- if there were equipment within
> several hundred meters of the ham transmitter, I could see protecting it
> with a common mode choke and a differential filter.

I have seen signals coupled strongly enough onto the line to make it 5 - 
10 thousand feet to the CO and still be strong enough to swamp the 
receiver in the DSLAM.

> It would also be quite helpful if the wiring in the vicinity of the
> transmitter was a very high quality twisted pair (like CAT5). It's too bad
> that virtually all commercially available RJ11 cables use parallel
> conductors. I suspect this is a major contributor to RFI to equipment on
> telco lines. Is anyone aware of RJ11 cables built with twisted pair?
> 73,
> Jim Brown K9YC

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