[Asterisk-Users] FXO impedance matching

[Nik Martin]

Rich Adamson wrote:
> ------------------------
>   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.
> 
> 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