[Asterisk-Users] Huge Echo

[canuck15]

Impedance is still impedance.  I am not sure how the telco world thinks
about it.  Based on basic circuit theory, impedance is the combination of
resistance and reactance (effective resistance to ac) at some frequency or
frequencies.  Collectively it is called impedance.  Long lengths of copper
will have some resistance but it should be very close to zero relative to
the ac component.  

You are simply trying to match the impedance of the source (the Telco CO).
This allows the maximum amount of signal transfer.  If there is a slight
mismatch then some of that signal will be reflected back.  Practically,
there will always be some tiny bit of resistance in the copper and the ac
impedance will vary by frequency so you will never get a perfect match.

-----Original Message-----
From: Rich Adamson [mailto:radamson at routers.com] 
Sent: Friday, September 09, 2005 1:07 PM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [Asterisk-Users] Huge Echo


> > Pull the clone card out of the system and look for the chipset 
> > numbers on the card. Go to the chip manufacturers web site and find 
> > the specs for that chip set. The specs will likely tell you the 
> > chipset was designed for the US 600 ohm impedance telephone network, 
> > and if your country's telco specs are different (which I'm very sure 
> > they are), through away the clone card. Without proper impedance 
> > matching there isn't anything your going to be do to fix the problem.
> 
> I can check the chip on monday, but local telco impedance requirements 
> are 600 ohms - just like US.
> 
> > >
> > > I'm thinking about playing around with increasing/decreasing 
> > > resistance by placing additional resistors in the circut. Messy, 
> > > but if it could help... What do you think?
> >
> > Adding resistance has nothing at all to with impedance matching. 
> > Resistance will impact the DC loop, but not the AC impendance. The 
> > AC impedance is a function of how the chipset was designed.
> >
> AFAIR, the impedance is not a simple factor, but a combination of 
> passive resistance plus reactance - which usually varies within 
> measured frequency range. Therefore channging the device resistance 
> WILL change its impedance. I know the result will not be perfect, but 
> at least i hope for better load match than it is now. I assume the 
> specified impedance is required within PSTN frequencies which will be
roughly 100-4khz, right?

That's not exactly true if you think about it. Since pstn lines are
typically copper based and have a varying resistance based on the length and
guage of copper, the copper resistance will vary from almost zero (close to
CO) to values over 1000 ohms. That certainly does _not_ change the AC
impedance of the line (regardless of its length); its still 600 ohms.

I don't recall for sure, but I'm fairly certain the impedance is relative to
1,000 hz tone.