[UK-CONTEST] ADSL Modem RFI

[Dave Sergeant]

I suffered a couple of disconnections yesterday when transmitting on 
160m (QRP!). So I have just re-read this thread.

Firstly a comment on the OZ7C filter. This is a high pass filter which 
cuts off according to his plots around 1.2MHz. Suitable for normal ADSL 
but totally unsuitable for those of us on ADSL2+ where the ADSL signal 
extends to above 2MHz. Hence totally unsuitable here and I expect for 
many others. Note that BT wholesale is rapidly installing ADSL2+ across 
the country and virtually all LLU services use it. And of course top 
band is right in the ADSL2+ spectrum...

My situation is that I have just recently moved to O2 on their LRU 
service. Works well, and I normally synch just below 10Mbs. My previous 
BT service using initially the Voyager 205 on ADSL and then their Home 
Hub on ADSL2+ was pretty stable, no knowledge of dropouts anywhere from 
RF - both those routers would stay synched for weeks on end.

The O2 router, their Wireless Box 2, is of Thomson origin and does seem 
less stable - the longest time between synchs I have had is around a 
week. And it seems a bit more susceptible to RF. It is well known that 
it is a bit cheap and nasty... but basically works.

All the telephone lines are underground here, no drop wires to pick up 
nasties. The master socket is by the front door and it is not 
convenient to have the router there, it is fed from an extension in the 
upstairs shack via a fairly short telephone cable extension. Openreach 
fitted their new style master socket earlier in the year which has the 
bell wire filter - but the bell wire is still wired. There may be some 
advantage in fitting an ADSL Nation splitter faceplate, but I would 
have to extend the wiring to the shack.

On the subject of ring wire filtering, this seems normally to be 
concerned in getting rid of noise induced before the master socket. 
With underground cabling this is less of a problem, but surely if you 
only filter or cut the ring wire at the master socket end and leave the 
rest in, that will still pick up RF locally and couple it into your 
router. I would have thought it best to disconnect it at both ends.

Anyway my problem is minor compared with what some of you are getting..

73 Dave G3YMC


On 16 Nov 2009 at 21:25, Steve Wilson, G3VMW wrote:

> The real improvement came when I built the OZ7C filter as recommended by
> Jan G0IVZ and Andy ZC4VJ. I didn't have any 6.8uH chokes, so I wound
> some (35 turns on a T50-2 toroid). Putting this filter in line with the
> router near to the telephone socket did the business. No more router
> disconnection at full power on any band. There is a slight degradation
> of S/N ratio on 160m and 40m (but not 80m), but the router stays
> synchronised and the connection holds. There is about 1dB of insertion
> loss attributable to the filter, but I'd rather have slightly slower
> ADSL sync speeds than frequent disconnects.
> 
> There's some more ideas about the OZ7C filter here (in French):
> 
> http://ed64.ref-union.org/technique_fichiers/Filtre_ADSL.pdf


http://www.davesergeant.com