I think we have more of a terminology issue here. Common mode
isolators/chokes, or as you have called them filters, are of value. I just
don't think of them as filters as they do not have bandpass or reject
qualities. Small point of difference, but I think we agree. As you
suggested, common mode isolators do all the good things that you suggested.
We have seen cases where people do put bandwidth-limiting devices ahead of
the DSL modem in an effort to keep specific frequencies out of their modem.
The result may be more permissive for Amateur Radio operation, but can
reduce the overall bandwidth available for the DSL service, which can make
the service more vulnerable to other noise or line performance-driven
On your other points, you are "spot on" as well, as those cleanup measures
do make a significant operational difference when seeking to improve the
bandwidth and the robustness of any DSL service.
Thanks & 73,
Gordon Beattie, W2TTT
From: Christopher E. Brown [mailto:firstname.lastname@example.org]
Sent: Sunday, February 27, 2011 11:54 PM
To: J.Gordon Beattie, Jr., W2TTT
Cc: email@example.com; 'Gordon Beattie'; firstname.lastname@example.org
Subject: Re: [RFI] RFI U-verse
On Sun, 27 Feb 2011, J.Gordon Beattie, Jr., W2TTT wrote:
> 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
> 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
> Thanks & 73,
> Gordon Beattie, W2TTT
While I agree with most of this, I have to strongly disagree with you
comments about a common mode filter on the DSL side of the line.
When you are saying is the _exact opposite_ of what I have seen many
I have _never_ seen a proper common mode choke reduce the performance of
any DSL variant, symmetric or asymm, legacy through VHDSL.
By proper, I mean (as one example) a high twist rate 22 - 28ga pair,
wrapped through a 2.4" Type 31 toroid, turns count approp to the
Someone doing a scramble wrap of untwisted, or worse yet a split
winding will cause issues but this is not a proper setup.
I have never seen this cause issues or reduce sync rate.
It often increases sync rate, due to enforcing balance and blocking
"common mode" RF pickup from the environment that then becomes
differential somewhere between the pre and the CO.
This is my personal experience, where in my last three homes (all <
6kft from CO) initial ADSL2+ sync rates were < 5Mbit and any HF use would
cause loss of sync or errors depending on band. Fixing the house cabling
(ADSL2+ rated splitter at the NID, general cleanup and dedicated run from
the splitter to the modem) brought sync rate up to 10+ Mbit, with only
minor error rate increases on some bands. Placing properly wound chokes
at both ends of the NID to modem run increased sync rates to 15+ Mbit and
eliminated any change in error rates when on HF.
This is also my experience at work. I have been a packet data networking
guy for 17 years now, near 8 years in the engineering dept of of one of
the 3 largest communications providers in the region. My normal workflow
centers around things much larger than a single customer drop, but the
department is the final level of technical escalation for issues. We use
alot of ADSL and various multipair symmetric Ethernet over Copper
The first thing to check is proper cabling, then equipment grounding and
bonding. After that... I have had chokes installed in a number of
cases, on simple ADSL lines and one per pair on multipair EoC drops in
high noise environs (commercial locations with lots of large VFDs, etc,
repeater sites, broadcast toper locations, etc).
It does not always help, but I have never seen a circuit degraded by
properly built and applied chokes. About half the time, we see a increase
in sync rate and lower error rates...
On the other front (degrading the line), I have seen chokes like
2 separate 28ga wires, wound in 2 "lumps" about an inch apart on a 2.4"
one wire loose/messy wound over one third of the toroid, the other
loose/messy over the top of the first with ~ 50% overlap
A very very clean winding, but split on opposite sides of the core, and
with one side reversed during install
All of these pretty much took the circuit offline.
I normally take 24ga insulated, twist to about 4 turns per inch (4 full
360, not 4 half turns) and wrap anywhere from 10 - 32 turns on a 2.4" #31
toroid. There are 2 inline with the ADSL2+ annex M line I am using right
now. Line is 5.5kft, set 9db/9db for channel threshold levels and syncs
about 2Mbit faster in the downstream with these in place.
And yes, I have tested at work with "perfect" circuits, 1kft of quality
cable in the CO for ADSL1, ADSL2+, eGSHDSL and several VHDSL platforms
with 2 inline found no negative impact on a line already running at max
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