[Gllug] Advice on ADSL line attenuation and noise margin

[Christopher Hunter]

On Thu, 2010-03-11 at 21:37 +0000, t.clarke wrote:
> The 'master socket' my brother bought came from broadbandhelp.co.uk (or
> something like that) amd he thinks its the adslnation one.
> It could well be the same one that solwise sell  - he checked the other day
> and the one he bought is now apparently out of stock til April!
> He also bought a 'high quality modem cable' for two quid - the cable that
> sits between the wall socket and the router itself;  whether or not this
> contributes anything noticeable or not is a matter of conjecture!
> 
> I will be getting one for our ADSL connection at the office to try as well -
> we only get a crap 2Mbps due to distance from the exchange, so any improvement
> will be worth having.  Will report further on that in due course.
> 
> Tim

I've found that some pieces of gear connected to a phone line affect the
level of the ADSL signal more than others.  

Everything connected to a phone line is supposed to be "approved" and
each item is meant to appear (electrically) as  "telephone instrument".
Years ago, when I designed answering machines, fax machines, cordless
phones and the like, I had to take great pains to get the complex
impedance presented to line to "look" just like a 746 telephone!

These days, the manufacturers seem less interested in that kind of
accuracy, and the approval bodies are much less stringent.

The "Sky Box" is a case in point, and attenuates ADSL quite markedly.  I
looked at this be the simple expedient of connecting my 'scope across
the phone line, and going 'round the house connecting and disconnecting
all the various bits of telephonic gear to see what effect each had.

If you have spare microfilters, it's often worth using two in series
with some gear.  It sounds peculiar, but the attenuation due to the item
is halved, and I found that (for example) connecting two microfilters in
series before the Sky receiver meant almost no change in received ADSL
level, less reflection (so a better quality ADSL signal) and thereby a
significantly faster connection.

I also have a Panasonic Digital Cordless Phone (one of my designs from
the late 80s) which has no effect on the ADSL level when connected
through a microfilter, but a more modern model was almost as bad as the
Sky receiver!

The ideal is, I suppose, to put the filter at the incoming "master"
socket, and have all the telephone equipment fed entirely separately
from the feed to the modem.  Again, it might prove worthwhile to use two
filters - I'm going to try this at the weekend.  

I have a "Master" socket-type filter which I can easily fit, then run
some Cat 5 cable with the Western Electric plugs crimped on to it for
the modem, and use the existing house phone wiring for the instruments.
I also have a spare microfilter or two, so I'll try adding one after the
master socket, and see what happens.  I'll let you all know.

Chris

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