[Asterisk-Users] FXO impedance matching

Rich Adamson radamson at routers.com
Wed Jun 23 10:41:30 MST 2004

  From: Nik Martin <nmartin at radiancetech.com>
  Subject: RE: [Asterisk-Users] FXO impedance matching
  Date: Wed, 23 Jun 2004 11:02:00 -0500 
  To: asterisk-users at lists.digium.com

> Michael Welter wrote:
> > Jason A. Pattie wrote:
> >> Robert Hajime Lanning wrote:
> >> 
> >>> Echo echo ech ech ec ec e e . .
> >>> 
> >>> :)
> >>> 
> >>> <quote who="miguel at amplanet.com.br">
> >>> 
> >>>> What's the importance of the impedance matching in a FXO interface
> >>>> ? 
> >> 
> >>
> >> 
> > My experience is with excessive buzz and hum on the line.  When I
> > plug a vintage Western Electric phone into the line, there is no buzz
> > or hum because the phone has its own impedance matching circuitry. 
> > When I plug my AT&T 954 set into the line, I hear a lot of hum.  I'm
> > told the X100P does not have impedance matching.
> > 
> > Rich Adamson is the fellow to talk with about impedance.  Apparently
> > the hum on my lines is caused by a partial ground on either the tip
> > or ring (or both) wire.  If both leads have the same resistance to
> > ground (matched) then there is no hum.
> > 
> > I don't experience echo with the buzz and hum.  I've been told that
> > echo is caused when the circuit goes from four wire to two wire.
> > 
> > I'm trying to locate a schematic of an impedance matching circuit so I
> > can breadboard a device but haven't found one so far.  I anyone has
> > experience with this I invite him to reply.
> > 
> > Mike
> If you KNOW the impedances of the two lines, a simple impedance matching
> transformer available at any electronics distributer (Mouser, Digi-Key, etc)
> carries many differdnt types, that are just for this purpose.


Could you pass along something more specific then 'any electronics dist'?

The old Western Electric repeat coils (transformer) are no longer
available, and research with "several" manufacturers did not turn
up anything usable. An ordinary two-winding transformer won't work
as it does not pass the DC (supervision) component. If you know of a
four-winding 1:1 transformer, please enlighten us. Several are looking
for such a component.

The original posting was oriented towards the x100p card (which apparently
does not have any form of impedence matching support), while Mike's
posting relates to a channel bank where the manufacturer states the
fxo interface is high impedence (>1,000 ohms) unbalanced with no 
impedence matching on board.

In Mike's case building an impedence matching interface for the channel
bank is highly likely to improve the noise/hum/buzz that he's getting
on pstn lines.

Not sure the same is going to be true for the x100p though.


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