Rich Adamson wrote: > ------------------------ > From: Nik Martin <[EMAIL PROTECTED]> > Subject: RE: [Asterisk-Users] FXO impedance matching > Date: Wed, 23 Jun 2004 11:02:00 -0500 > To: [EMAIL PROTECTED] > > >> Michael Welter wrote: >>> Jason A. Pattie wrote: >>>> Robert Hajime Lanning wrote: >>>> >>>>> Echo echo ech ech ec ec e e . . >>>>> >>>>> :) >>>>> >>>>> <quote who="[EMAIL PROTECTED]"> >>>>> >>>>>> 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. > > Nik, > > 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. > > Rich
http://www.digikey.com/scripts/DkSearch/dksus.dll?Criteria?Ref=25939&Site=US &Cat=33096753 I may have misspoken, but these are T1 specific matching/isolation transformers. Nik _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users