[Asterisk-Users] FXO impedance matching

[Nik Martin]

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