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'(ot) measuring phone'
2000\03\25@085238 by Jane Ifurung

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Does anyone of you knows how to measure the phone line
impedance?

I have a voice multiplexer project and I interface to
Private Automatic Branch Exchange. I can't balance the
transmit and receive gain of my system. I suspect it
depends on the phone line length and impedance.

I'm hoping for your answers.

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Jane Ifurung
UP Diliman, Quezon City
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2000\03\25@094929 by gzkwan

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I have user the SUNPLUS PCR01 CPU to Do it!
the MIC input pin to measure the phone line singal!


Jane Ifurung wrote:

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2000\03\25@111555 by David VanHorn

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>> Does anyone of you knows how to measure the phone line
>> impedance?

Sure, build an impedance bridge.  Two known resistances, precisely matched,
a variable, and the unknown. Feed a tone in across the bridge, measure the
imbalance, tune the variable to minimize the imbalance, measure the
variable, that's your impedance.

Doing this on a live line presents some special considerations. You might
be better to get the spec, and design to that.

>> I have a voice multiplexer project and I interface to
>> Private Automatic Branch Exchange. I can't balance the
>> transmit and receive gain of my system. I suspect it
>> depends on the phone line length and impedance.

It probably does. Or more precisely, your return loss depends on how well
you match your impedance to the line.

I'm guessing the PI phone system uses US standards, which would be 600 ohms
nominal, with some reactance, but I forget. It's been a few years since I
dealt with this.

If you have a flexible hybrid design, a microcontroller, and a way to
measure signal levels, then you can have the system calibrate itself by
injecting a tone on the transmit side of the hybrid, and measuring the
level on the receive side of the hybrid, then tune a digital pot for
maximum return loss. You should only have to do this once. It's certainly
not something you want to do on every call!

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2000\03\25@181811 by Graeme Zimmer

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> Does anyone of you knows how to measure the phone line
> impedance?

DC impeadance is easy...

Put a variable resistance across the line and vary it untill you see half of
the open circuit voltage. The resistance then equals the line resistance.

For AC impeadance you will need to make up a simple resistive bridge (one
resistor and a pot) and couple it to the line via capacitance. Then feed the
required AC test tone to the bridge via an audio amp and measure the nulll
with a floating CRO or AC meter.

Draw a graph of the different value null for different frequencies.

............... Zim

2000\03\25@182648 by Graeme Zimmer

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> Draw a graph of the different value null for different frequencies.

Oh, and to get a really tight null you will need to use compex values to
balance the bridge..

E.g. it will require a certain amount of inductance and/or capacitance as
well as resistance.

.............. Zim

2000\03\26@223330 by Plunkett, Dennis

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Ah finally a topic for me,
This is not as easy as it sounds the AC impedance can vary depending on lots
of things these days.
You may find that the PABX comes complete with adaptive ballance, so the
length of the line is not important. I fear that your case is more complex
due to you working on the 2 wire side. In this case you have to have a
terminal ballance return loss of better than 30dB. As for send and receive
stuff it is quite simple, I have posted this information many times before.
The impedance that you are looking into may be complex (Will for new PABX,
not for old) but a nominal TN6 (600R) will do this. But then I am getting
off track here. It would seem that your are attempting to go into the 2 wire
side? I suspect that this is a Ring out Loop in line ie provides battery and
ring and detects off hook etc. If this is the case, you have done all the
hard work and the impedance should be a walk in the park. Ok so here is the
deal:-

       PABX sends to customer -6dBr
       PABX  receives from customer 0dBr

Where 0dBr is the reference point in the MDF and is a nominal voice power
of -15dBmO (600ohms)

In the above it is assumed that the line will have a loss of 3dB, in any
case the PABX like any nominal (Note that as all are not the same) will have
a switching loss of 6dBr from the receive to the send, and that is all that
you need.
Oh yes overhead response, your circuit must operate up to 3.14dB above the
nominal maximum levelm thus at 600ohms you need to be able to receive
+3.14dBm


Dennis





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