Re: gEDA-user: OT: DC power feed on xDSL circuits

[msokolov@xxxxxxxxxxxxxxx (Michael Sokolov)]

Hello again,

I have figured out the answers to some of my questions regarding DC on
xDSL by talking to a support engineer at Midcom, one of the vendors for
the custom transformer parts used for xDSL.  (Trying to get support from
the SDSL transceiver chip vendor would have been hopeless as it's
Mindspeed, but I hadn't previously thought of trying the transformer
vendor instead.)  Here is what I've got:

* The most pimped-out design for an ISDN/IDSL/SDSL/SHDSL subscriber
  terminal (the kind that would be made by someone who likes to cover
  every possible corner case and has the parts and PCB real estate to
  spare) uses a fancy DC termination scheme.  DC through the line
  transformer's winding is blocked with a cap, but another device is
  connected in parallel across the line which provides the fancy DC
  termination.

  Lucent's LH1465 ISDN DC termination IC is the example I am familiar
  with.  The DC termination is fancy in that the device presents a high
  DC resistance until the voltage reaches a certain threshold; then it
  "switches on" and the DC resistance becomes quite low; it then stays
  "on" for as long as the current flowing through it remains above
  another threshold.  There is yet another threshold for the maximum
  allowed current; when that one is reached, the device increases its DC
  resistance to limit that current.  Finally, the presence or absence of
  DC current is indicated to the ISDN device's microprocessor via an
  optocoupler.  Fancy indeed.

  The stuff above is what I already knew prior to my original inquiry.

* The subject of my inquiry was devices made for SDSL and IDSL (rather
  than ISDN or SHDSL) which do away with the expensive fanciness
  described above on the presumption/realisation that it would be a
  waste of effort and parts because the SDSL/IDSL lines in North America
  don't have intentional DC on them.

  In the absence of intentional DC on the line one does away with the
  LH1465 or equivalent, but there remains the question of what to do
  with the DC blocking capacitor: leave it in or take it out?  The
  choice made in the design of dominant SDSL CPE devices from Copper
  Mountain, Netopia and Inefficient Networks has been to leave the cap
  in, and that is what I have reproduced on my OSDCU.

  The Midcom support engineer didn't have a clear answer to this
  question either except to confirm that their HDSL/SDSL transformers
  have been designed with the provision for DC flowing through them and
  that they are guaranteed to perform within specs with up to 160 mA
  flowing through the primary winding.  (160 mA is a lot, all the specs
  I've seen for over-the-line powering of remote terminals and repeaters
  say the limit is 60 mA.)

  I have decided to leave the DC blocking cap on the OSDCU board in
  place in any future revisions because:

  a) In the CPE application I simply "do like the others" and that's
     what I'll tell to anyone who asks :-)

  b) In the CO application with DC power feeding we do need an open
     rather a short in there as explained below.

* Now moving on to the question of interest to me: how would one provide
  DC power on the line if such a thing was needed.  I've asked the
  Midcom support engineer the same question I had asked on this list: is
  it better to apply the DC power to the centre split or outside the
  transformer?  The Midcom guy agreed with my reasoning that either way
  ought to work, and the discussion then turned to the question of why
  they have designed their transformers to work with up to 160 mA of DC
  when the requirement could have been avoided if one simply opted to
  apply the DC feed in parallel with the DC-blocked winding.

  The Midcom guy explained to me that the core needs to be gapped in
  order to make the transformer DC-tolerant, but since they also need to
  gap the core in order to provide the required primary inductance (the
  SDSL transceiver chip requires the custom transformer to dual-function
  as a high-pass filter with a precisely specified primary inductance),
  it appears that making the transformers DC-tolerant didn't cost them
  much extra.

  While the only authoritative answer was that Midcom had designed their
  transformers with the specs that they have (including the 160 mA DC
  current) because that's what the SDSL chip vendor had asked them to
  make, by analysing the situation rationally we came to the conclusion
  that applying the DC source or load to the centre split and letting
  the feeding current flow through the winding results in a smaller part
  count.  If one were to apply DC in parallel with the DC-blocked
  winding, the DC power supply would at the minimum need a big inductor
  at its output in order to provide a high impedance at the data signal
  frequencies, whereas if one applies DC to the centre split, the
  transformer itself takes care of that.

* The conclusion I have reached for any SDSL designs that we may create
  in the open source community is as follows:

  + On the OSDCU we can keep things simple by leaving the DC blocking
    cap where it is and not providing any extra taps beyond the main
    SDSL jack.  The vast majority of users would use it as a CPE device
    or perhaps a non-power-feeding CO terminal, and it would simply work
    as is.  Those very special users who would like to use it as a CO
    terminal with DC power feeding would be able to connect an external
    power supply (including the big inductors or whatever is needed for
    high impedance at the data signal frequencies) in parallel to the
    SDSL pair coming out of the OSDCU.

    Leaving the DC blocking cap inside the OSDCU as it is would be quite
    important for this application scenario, as otherwise any attached
    DC supply would be shorted locally!

  + If we ever design an SDSL repeater or some other device that needs
    to draw power from the line as an inherent feature, we can make use
    of the high DC current capability of Midcom's transformers and
    connect the DC load to the centre split without much extra effort.

* Regarding future revisions of the OSDCU board, there is no need to
  bring out taps for access to the line transformer's centre split.
  I would however like to do something better with the transient voltage
  suppressor's earth ground pin than what I'm doing currently.

  On the current board this pin is wired to a via that is supposed to
  accommodate a single header post, but I now realise that what I have
  is not terribly useful.  I am reluctant to connect it to the chassis
  ground, and I think that the principle of "first, do no harm" would be
  better fulfilled by bringing this pin out to some kind of screw
  terminal post on the rear panel.  The user can then either leave it
  unconnected (resulting in a situation identical to the mainstream SDSL
  CPE devices which use 2-terminal TVSs with no Earth ground connection)
  or connect it to a really good Earth ground with a dedicated green
  wire.

  What I need to find then is a single-circuit screw terminal block that
  would solder onto the PCB and protrude on the edge like a connector,
  and hopefully be as compact as possible so that it can be squeezed
  into the rear panel of the OSDCU.  Any suggestions?

MS


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