A recommended ADSL cable and Master socket Installation

Discussion in 'Interference Problems' started by David Mason, Aug 1, 2017.

  1. David Mason

    David Mason New Member

    Having an upgrade last year with a BT MK5 Router and A MK3 master socket I have started to have router lock out on 80m. I have fitted a screened twisted pair CAT5 cable with ferriites at both ends and fitted a 80m filter which stopped the router drop out. Went on holiday switched router off came back switched router on and now there is no problem so filter not required.
    The MK3 box has now been discontinued . So I would like to know if there is a recommended BT setup with router and master socket - is it now a MK5 master socket and what router cable and filters do I need in the face plate of a new master socket. Advice please
  2. Ken G3SDW

    Ken G3SDW Moderator

    David, i have asked the question for you to the EMCC and Dr David Lauder G0SNO is going to post the details later today.

    Hope that this helps.
  3. David G0SNO

    David G0SNO New Member

    David, I have some information that may be useful and and should have this ready to post later this evening.
    73, David, g0sno
  4. David G0SNO

    David G0SNO New Member

    I have been following discussions on VDSL with interest and I am working with John, m0jav EMC Committee Chair. I would like to offer my 2 cents worth on VDSL EMC.

    I have also included some introductory information for others reading this forum post. In xDSL terms, an RF immunity problem caused by a nearby radio transmitter is known as Repetitive Electrical Interference or REIN. Other sources of REIN include interference sources connected to the mains that get coupled onto the phone line via any equipment that is mains powered and also has a connection to the phone line. Other REIN sources include LED lighting with inadequate RF interference filtering, switching power supplies with inadequate RF interference filtering or faulty switching power supplies where the RF interference filtering capacitors or the main smoothing capacitor have failed open-circuit.

    The following information is based on my knowledge of ADSL and VDSL EMC but some of the measures mentioned below have not yet been tried so I would be interested to hear how they work in practice

    1. Linebox NTE5C master sockets

    The current 2017 version of the Openreach Linebox is called NTE5C. This has a curved case and is labelled "Master Socket 5C". For VDSL, it needs a Mk4 VDSL filter front cover that has "MK4" marked on the front. This is a separate item that fits onto the NTE5C back plate and provides separate telephone and data sockets. It includes centralised filter/splitter with RF filtering between the incoming phone line and the extension telephone wiring. This means that individual telephones do not need micro-filters and the extension telephone wiring shouldn't introduce RF unbalance onto the VDSL line.

    I haven't had a chance to test a Mk4 VDSL filter yet but it appears to have 4 chokes L1 - L4 and another inductive component T1. The VDSL line is believed to pass through one or more common-mode RF chokes. The Mk4 VDSL plate is physically different from the Mk3 (which is discontinued) but it may be similar electrically. In any cases of VDSL EMC problems involving emissions or immunity the first thing to do would be to get a Master Socket 5A with a Mk3 VDSL filter or a Master Socket 5C with a Mk4 VDSL filter fitted.

    Unfortunately, as the NTE5C replaces all previous versions, the Mk4 VDSL plate wouldn't fit onto a previous version of the NTE5 socket such as NTE5A. If anyone wants to have their master socket changed to NTE5C with Mk4 VDSL plate, Openreach would need to do it but an alternative is to keep an earlier type of master socket and add an additional NTE5C as part of the home extension wiring. This page shows how and it also gives other information about extension wiring, ringer wires and I-plates. http://www.kitz.co.uk/adsl/socket.htm

    A BT Openreach MK4 VDSL Plate & NTE5C Socket - Combined Unit are available from
    https://www.run-it-direct.co.uk/adsl-vdsl-faceplates/mk4btopenreachvdslsocket/
    The above web site also includes a photo of the Mk4 VDSL Plate.

    Another thing to check is that there is no BT 80A RF2 filter on the upstream side of the line between the master socket and the Openreach line. The BT 80A RF2 contains two separate RF chokes (see BT 80A RF2 photo on http://www.kitz.co.uk/adsl/btsockets.htm ) and some were fitted in areas where strong MW broadcast transmissions were breaking through onto electronic telephones with poor RF immunity. These must be removed from lines (by Openreach) before ADSL or VDSL can be used otherwise they cause very high loss to ADSL signals and may prevent VDSL from working at all. If it does work, high loss on the line will also make it less immune to REIN and prone tor dropping out. A friend who lives in Potters Bar, Herts had an ADSL service that was virtually unusable and her ISP was unable to find the problem. I found a BT 80A RF2 on the line, she asked her ISP to have it removed and the ADSL worked well after that.

    2. Notch filters

    An RF notch filter on the line would put in a high loss at a specific frequency so that when the VDSL line retrains, it doesn't use the frequencies in the notch where line loss is high. An RF notch filter would put in a 'per line' hardware notch to reduce emissions of VDSL interference in the notch frequency and it may help with RF immunity problems on a specific band if these are caused by differential mode signals.

    Home-brew filters such as L-C series traps should not be connected to the phone line, even on the customer side of the master socket but it seems less likely that there would be any objection to a band-stop or notch filter that uses a quarter wavelength open-circuit stub made out of telephone extension cable with an RJ-11 plug. A stub can be connected in parallel with the line using an RJ11 to 2 RJ11 socket doubler like this:
    http://www.maplin.co.uk/p/rj11-to-2-rj11-adapter-pu97f
    or
    https://www.run-it-direct.co.uk/rj11-couplers-adaptors/RJ11doubleadaptor/
    This should work at the RJ11 VDSL output of the master socket or at the RJ11 VDSL socket at the back of the router. The back of the router may be the best location. It should work whether it is coiled up or straight.

    If a transmission line is open-circuit at one end, it looks like a short-circuit at the other end at frequencies where the length of the line is an electrical quarter wavelength or an odd number of quarter wavelengths, 3/4 or 5/4 wavelengths, etc. The wavelength in cable is not the same as the wavelength in free space and in round figures, the velocity factor would be around 0.66 so at 10.125 MHz for example, a quarter wavelength would be approx. 0.66 x (29.7/4) metres or 4.9 m (your mileage may vary).
    We have tried a quarter wave stub at 10.1 MHz and if carefully trimmed to length, it puts in a notch about 150 kHz wide centred on 10.125 MHz. We haven't tried it for the 80 m band but for a quarter wave stub on 3.7 MHz (81 metres), the stub would be approx 13.37 metres long. It would also put in other unwanted notches when it's an odd number of half wavelengths long, e.g. 3/4 lambda (11.1 MHz). The width of the notch may only be about 50 kHz

    To tune the stub, cut the cable a bit too long then connect a small coupling loop at one end and tune it up with a dip oscillator or put the stub in parallel with the output of an RF sig.gen (OK, so it's a balanced cable on an unbalanced output), tune to find the minimum then trim to the required frequency. The length would need to be correct within +/-0.5% or better.

    If a stub is required for 1.9 MHz, it stub would be approx 26.04 metres long and the notch would probably be very narrow, maybe less than 50 kHz. It would also put in other unwanted notches e.g. 3/4. 5/4, 7/4 lambda (5.7 MHz, 9.5 MHz and 13.3 MHz) so it would make a few holes in the VDSL bands.
    An open-circuit stub would put in a 'per line' hardware notch to reduce emissions of VDSL interference notch and it may help with RF immunity problems on a specific band if these are caused by differential mode signals.

    3. Common-mode chokes

    Not all RF immunity problems are caused by differential mode signals however. It appears that some DSL modems have better immunity to common-mode signals than others. That would probably require ferrite common-mode chokes as well as or instead of a stub.

    If an immunity problem is caused by common-mode signals, a common-mode choke between the master socket and the router should improve RF immunity. The practical difficulty is that the input impedance of the router for common-mode signals is likely to be quite high so the common-mode choke needs to introduce a sufficiently high impedance to be effective. It may need about 3k ohms so it needs a suitable grade of ferrite and enough turns.
    The Mk3 and Mk4 VDSL filters already contain common-mode chokes in the xDSL path but according to https://www.run-it-direct.co.uk/bt-junction-boxes/BT80BRF3/ a BT 80B RF3 Junction Box may still be required to reduce Repetitive Electrical Interference (REIN) (see photo on http://www.kitz.co.uk/adsl/btsockets.htm ) The BT 80B RF3 has 3-way IDC to 3 way screw Connectors and it is intended to be fitted on the upstream side of the master socket and this would need to be fitted by Openreach. The only way to fit one as a DIY job would be on the customer side of the master socket.

    I think the common-mode chokes in the BT 80B RF3 and the Mk3 and Mk4 VDSL filters may be optimised for MF but the performance falls off at HF. They are probably quite good at 3.5 MHz but for best results over a wide range of frequencies such as 1.8MHz to 28MHz, you could try my RJ11 filter published in April 2016 Radcom EMC Column. This has two different grades of ferrite, Fair-rite grade 31 and 43.

    4. Screened cables

    Assuming that the Openreach line comes via an overhead dropwire, this will pick up common-mode RF from nearby radio transmitters. Using possibly 2 metres or more of shielded twisted pair cable from the master socket to the router isn't likely to reduce the common-mode pickup by much by itself but nevertheless, it may be useful in conjunction with a common-mode choke between the master socket and the shielded section of cable. This is because the shielded twisted pair cable has a relatively low impedance to ground for common-mode signals so it should make a common-mode filter more effective. Whether it works in practice however would depend on how the cable screen is grounded. An earth spike near the master socket with a short wire should work but it may not be practical in most cases. Another way might be to ground the cable screen at the router to the negative side of the DC power supply to the router.
  5. David G0SNO

    David G0SNO New Member

    Regarding an earth spike near the master socket, it should be noted that if you are on a UK PME supply then you would need also to read leaflet EMC07 v3 Earthing and the Radio Amateur, which is available on http://rsgb.org/main/technical/emc/emc-publications-and-leaflets/

    A possible way to connect to the negative side of the DC power supply to the router could be to make a short plug to socket DC power cable extension lead for the router and bring out a wire connected to the negative side.

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