[UK-CONTEST] RFI-ADSL Modem

[Dave Sergeant]

On 29 May 2007 at 14:00, justin at g4tsh.demon.co.uk wrote:

> I have noticed that after an upgrade to a faster ADSL+ modem my EMC
> problems returned.  Of course the faster modems use a wider chunk of
> the HF spectrum and the modem "front end" will be wider open and at
> some point external RF will get into the system and swamp the
> relatively weak DSL signals on the line.  Common mode chokes will go
> some way to curing this, but I suspect that some sort of low pass
> filter on the telephone line may be required to cure it(Anyone have a
> design for one of these?).  The side effect of this will be that you
> will get a slower connection.  My old 512 and 1 meg rate limited
> connections were OK, but now I have a so called "up-to-8meg"
> connection that barely gets me 2 meg of useable bandwidth and a new
> ADSL2+ modem.  The EMC problem is back.  I have even managed to kill
> it with 5 watts of RF on 40m!

The DSLMax system works by increasing your connection rate until it 
can go no further without it disconnecting. This means that it works 
typically with a SNR not that much higher than 6dB unless you are 
very close to the exchange, as against 25dB or so for a typical 2Mb 
system. What this means for amateurs of course is that it needs much 
less noise of any sort before it loses sync.

There are numerous horror stories with the Max system and at times 
one has to wonder whether it was wise to introduce it in the first 
place. There is huge commercial pressure to get people up to these 
higher rates, but for normal browsing etc 1-2Mb is perfectly fast 
enough. I am resisting all offers from BT to upgrade and have a 
perfectly stable 2Mb system. I did have a line problem last year 
which BT took ages to sort out (believed to be water ingress in their 
manhole covers but never found out exactly what). At that time I had 
all sorts of crackles on the phone as well, and 30m RF was found to 
disconnect it. If you have those sorts of problems persevere, they 
are incredibly hard to trace and it is not helped if you are forced 
to go through an Indian 'support' line.

Good luck Justin, if you persist they may offer to 'downgrade' you to 
your old and stable 1Mb....

73 Dave G3YMC

http://www.davesergeant.com