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LF: Re: 500kHz and ADSL

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: LF: Re: 500kHz and ADSL
From: "James Moritz" <[email protected]>
Date: Thu, 25 Mar 2010 23:20:35 -0000
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Dear Andy, LF Group,
When I first installed my ADSL modem here, it would not "lock up" at all, until I did basically what you did to isolate the house extension wiring from the incoming wire to the master socket with the "microfilter" (basically a couple of chokes). I imagine the ADSL signal is a bit marginal here, being a fair way from the exchange, and having the MF broadcast noise to contend with. I find the modem sometimes goes into retrain mode when I start up the WSPR beacon here, but it seems to get over it quite quickly without affecting performance noticeably.
On receive, it is something of a problem on 500kHz - positioning a loop antenna within about 10m of the phone wire and modem reveals a high level of white noise-like QRM. Unplugging the modem makes this dissapear. winding the incoming phone wire onto a ferrite core to produce a common-mode choke yields about 10dB noise reduction. I used a noise-cancelling system, with a current transformer on the phone wire as the "noise antenna" with some success, but it was tricky to adjust due to the presence of further broadband noise apparently from the broadcast stations, requiring two sets of amplitude and phase balancing adjustments. I now have my RX antenna at the bottom of the garden, as far as possible from the phone wiring for this reason. However, since several other overhead phone lines pass about 10 - 15m from this point, I may be getting other people's ADSL noise too (and probably making their modems retrain... but I'd probably get in trouble if I disconnected them too!). 

Cheers, Jim Moritz
73 de M0BMU
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