View Full Version : ADSL2 Filters

Terry Porritt
02-04-2006, 11:49 AM
In anticipation of the arrival of ADSL2 (don't laugh), I'm considering installing a filter in the incoming phone line, and running dedicated CAT5 cable to a socket, as the current phone wiring is circa 1968, (though there are currently no problems with 2Mbps ADSL).

The filter will most likely be a modified line filter, not the Telecom MM3200 'splitter'.

Does anyone know whether the specs are/will be the same for the higher frequencies involved, or will new filters be required for ADSL2?

02-04-2006, 11:58 AM
The existing filters should be fine. All they are is low-pass filters, which allow the voice frequencies (around 300Hz to 3400Hz) through and block off the higher frequencies. As ADSL2 is higher in freq, then there should be no difference.
The ADSL line socket of the filter is connected directly to the phone line.

Terry Porritt
02-04-2006, 12:19 PM
Thanks pheonix, I thought maybe that would be the case.

Graham L
02-04-2006, 01:27 PM
The filter is just a choke in each line (on the same ferrrite core) with a capacitor between the lines on the low frequency (phone) side, Terry. So they aren't symmetrical. ;)

Terry Porritt
02-04-2006, 02:26 PM
Right Graham. I intend to modify a Dynalink 'Microfilter', which is quite a nice unit, 2 halves screwed together, with flying leads to the phone line in at the junction box, and flying leads to connect to the existing phone wiring. Then CAT5 cable to an RJ45 socket, pins 4 & 5, which corresponds to the 2 centre pins used on the modem RJ11 cable. (I think :) )

Graham L
02-04-2006, 02:47 PM
The telephone "RJ" series have the pair pins arranged so you can use 6 pin plugs in 8 (or 10) pin sockets. The locking key keeps it central (though it's a good idea to use the little plastic adaptor to stop it wobbling).

1 2 3 4 5 6 7 8
1 2 3 4 5 6So the 4-5 and 3-6 pairs in 8 pin line up with 3-4 and 2-5 of the 6 pin. (1-2 and 7-8 don't ... but there used to be an old US pinning which did .. 1-8, 2-7, 3-6, 4-5.)

I'd be inclined to look for a 6 pin socket: I've found them at demolition places, but you should be able to get them new too: they are often used in PABX systems, to avoid people plugging in non-digital phones with BT plugs. It's best to use 6 pin sockets with the 6 pin plugs. They fit in an 8, but it's not ideal.

Graham L
02-04-2006, 03:35 PM
The 6-way plug and the 8-way socket are designed to be compatible and the component tolerances are carefully specified to ensure a reasonable fit. Also, all three types use the centre pins for terminating "line 1". Nevertheless, these required tolerances are not always achieved and the smaller plugs often fit only "more or less" into the larger sockets. As a result, contacts may not prove very reliable because of misalignment problems. There is also considerable risk of the plastic moulding of these smaller plugs over-stressing the outer contact springs in the socket so that when the correct matching plug is fitted the outer springs do not make reliable contact.

All in all, it is strongly recommended that only the matching plug should be inserted into a modular socket and customers in buildings with generic or integrated cabling to AS/NZS 3080, AS/NZS 3086, or EIA/TIA 568A, should be warned not to insert 6-way plugs into their RJ45 wall sockets.
Telepermit newsletter 100 (www.telepermit.co.nz/nl100.html) might be of interest.

Terry Porritt
02-04-2006, 04:16 PM
Well there are two ways to go, either use a modular phone socket (easiest), or use an RJ45 ethernet socket, and use ethernet twisted pair CAT cable to connect from modem to wall socket, with an RJ11 one end and an RJ45 the other.

It is also interesting that the as-supplied modem cable with RJ11 plugs has parallel wires and it is quite long. I have no idea what would be supplied with an ADSL2 modem, or whether parallel wires would be a problem, maybe not if the spacing and diameter gave about the right sort of characteristic impedance ?

Graham L
02-04-2006, 04:29 PM
That flat parallel wire is the standard telephone cord. I think the Americans call, it "silver satin". It's commonly available in 4, 6, and 8 conductor form. It has the advantage that the crimp tools usually have a stripper which fits it, and the conductors can be just pushed straight into the plug, without thinking. Twisted pair cable has to be stripped, then have the individual conductors sorted out and pushed into the appropriate positions. Saves a lot of time and it's very hard to get wrong. (If you put the plugs in the same orientation on the cable, you get a pin order reversal between the ends, but that's not usually a problem). You will note that there's a slight ridge on one side so you can identify the sides, if it matters.

It's designed to work with telephones. (300-3400Hz). :D You will get unwanted coupling between the pairs because they aren't twisted, and 2A and 1A, and 2B and 1B, are adjacent. Usually there's only one pair being used in one of these cables, so I suppose that doesn't matter much. But putting 2+ MHz for ADSL through it, I'd say "short is best". There'd be quite a high capacitance in it.