[RFI] 40m dsl modem update
Christopher E. Brown
cbrown at woods.net
Sat Feb 14 03:44:16 EST 2009
On Fri, 13 Feb 2009, W0MU Mike Fatchett wrote:
> It looks like I will need to build a filter. Just on a lark I found a Kcom
> RF1 telephone filter and it did stop the 40m signal from knocking the dsl
> off the internet. It also reduced the downstream speed from 7186 to about
> 168. I also put some ferrites on the cables but that had no noticeable
> Thanks for all the tips so far!
> Mike W0MU
ADSL2+ operates at up to 2.25Mhz, telephone RFI filters often cover down
into this range.
In my case, underground service but space limited (all antennas < 50 feet
from telephone cabling, primary < 30 feet).
I started out with mild interferance to the phones, and line errors or
even loss of sync depending on freq running 100w.
1) Added a proper ADSL2+ line splitter to the NID. This is a balanced
line splitter, audio is passed on to one set of ports, 10Khz and up to the
other. The provider stocks them, as they can "fix" problematic services,
by isolating the analog devices from the RF side, eliminating the issue
with pumping RF into the in-home wireing.
2) Fresh CAT5 plenum (100% teflon insulation) run from the DSL side port
to the DSL modem location.
3) Replaced all of the rj11 outlet plates with new units, with special
prep. I ran 5 turns of single twisted pair through a fair-rite 2631540002
type31 cylinder. This is small enough to be glued to the back of the
plate. I also added the same to the audio side feed in the NID.
4) On the rj11 plate for the DSL feed and on the DSL side of the splitter
in the NID I did similar but with 6 turns (should put the peak impedance <
Since I do happen to work for the provider of the DSL line, and have
alternate connectivity, I was able to monitor not just the minimal stats
available on the modem, but the much more useful stats on the DSLAM while
on the air.
Installation of the ADSL2+ rated splitter / moving the DSL modem onto a
dedicated feed upped the max sync rate for a 9db u/d SNR setting from ~
5300 to over ~ 9000kbps while maintaining the same very low error rate.
It also eliminated the disconnect issue, and lowered the error rate when I
was on the air. (I assume this is due to the RF isolation between the
extensive house wireing playing antenna and the DSL modems new feed).
Adding the toroids got rid of any trace of RFI on the phones, and the line
errors. Looking at the DSLAM stats, cannot tell if I am transmitting or
The DSL modem already had power and ethernet lines with multiple turns
through type31 snap ons to take care of RF coming from the device.
Total out of pocket, about $25 for the cable, ferrite and rj11 plates.
The splitter was provided, they are kept on hand for use one problematic
All that being said, I am not an expert, YMMV, every situation is
different and the hammer I chode just happened to fit the problem.
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