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

[msokolov@xxxxxxxxxxxxxxx (Michael Sokolov)]

Hello fellow gEDA users,

I have a telecom electronics design question - are there any folks here
who have done telecom stuff?

My question deals with DC power feeding on data-carrying copper loops:
ISDN, [S]HDSL, etc.  The typical circuit found in equipment interfacing
to such lines (either at the CO or at the remote terminal) looks like
this (ASCII art):

*-------------------3 | 3--------------------+
		    3 | 3                    |
                    3 |	3--+                 |
		    3 |    |		     +-----
To transceiver IC   3 |    = (1 uF or so)	    To copper loop
                    3 |    |		     +-----
		    3 | 3--+		     |
	            3 | 3		     |
*-------------------3 | 3--------------------+

I have observed the above circuit in SDSL chip datasheets as well as
inside most SDSL gear found on eBay, and I have faithfully reproduced it
on my OSDCU board - it works just fine so far.  But there are some
things I just simply fail to understand about this circuit, and I wonder
if anyone here might have some insight.

1. What is the benefit from having the DC blocking capacitor across the
   centre split in the transformer's primary winding?  What harm would
   there be if someone took that capacitor off the BOM and simply
   shorted those pins on the transformer to make the primary winding
   effectively non-split?

   While SDSL lines in North America are generally unpowered (I've never
   seen one with DC power on it), when someone does put DC power feed on
   a line of this kind (like they do with ISDN for example), they
   generally want the user-attached CPE to conduct some DC current (they
   call it sealing or wetting current, supposedly it somehow helps with
   contact corrosion), so my simple peasant mind thinks that a DC short
   ought to be closer to what they want than a DC open, yet they put
   that DC blocking cap in there to make the device present itself as an
   open in terms of DC termination.  Why?

2. Suppose that I do want my CO-side terminal unit to provide DC power
   feed on the line - say, to power a mid-span repeater.  Where would I
   need to connect my DC power source?  Would it need to be connected
   across the centre split of the line transformer's primary winding in
   parallel with the cap, or would it simply go across the copper pair
   instead?

   Some of those SDSL transformers (which are all custom parts) were
   apparently designed for fairly high DC current, so it seems like
   connecting the DC power source (or load) across the centre split is
   the way it's done.  But why?  Why can't one simply connect the DC
   power source across the copper pair in parallel with the SDSL box?
   If power feeding can be achieved by connecting the DC power source
   across the copper pair outside the main xDSL equipment, it could be a
   modularity advantage (design the main xDSL box without worrying about
   power feed and let those users who need it do it externally), whereas
   otherwise one needs to provide a special tap to allow the user to get
   to that centre split in the transformer winding.

TIA a lot for any insight.

MS


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