how can i measure long distance cooper Line impedance ?

[ulaska]

measuring telephone line impedance

how can i measure long distance cooper Line impedance ?

i will measure the empedance of a telephone line which is cooper (0.9mm) and it is long distance 1km ~ 6 km .
there will be no any telephone signals, the 2 wires are reserved for this meauserement.

i want to use ADC and MCU pic16f876.

also, it will not be very sensitive , for example 30 ohm ~50 ohm and multiples.


**broken link removed**

not: of course not with Mulitmeter or ohmmeter

 

[rikie_rizza]

telephone line impedance measuring

Well you just shows the diagram...go ahead, build it. just force it with known current, read the voltage using ADC, and calculate the resistance using V=IR

 

[ulaska]

mulitmeter measure long distance

ok, must drive a voltage to the line and, read the return voltage. but what voltage i must drive to the line ? 12V ?

 

[TekUT]

how do i measure long distances

You dont't need to work with voltage, but instead use a current loop approach, inject a know current into the cable and measure the resulting voltage at the terminal where you're injecting the current. You can try with a commonly used integrated circuit for 4-20mA current loop or you can simply built your own current source by using the most simple way, a LM7805 voltage regulator and a resistor.
Here in attachement the circuit diagram (https://www.fairchildsemi.com/ds/LM/LM7805.pdf)
Of course, a first rougly estimation of the cable resistance can be useful to set the voltage amplfier gain in order to best fit the ADC input dynamic range.
This consideration apply, of course, at the DC component of the cable impedance (then the pure resistance components), if you like to measure the impedance (Z=R+jX) then the question is quite different and a little bit more complicated.

Cheers
F.F.

 

[rikie_rizza]

measure line impedance

Yea...use current loop...generate known current (Say 1 A for easy calc)...that would be the current force. Now, you must sense the voltage...."how to measure voltage while forcing current?", well you should read basic electronics book.

Anyway, let your ADC be the voltage reader, the wire is the Rload. If the 1 Km wire voltage reading is 0.4V, than the resistance would be 0,4ohm....see, its easy.

 

[jallem]

how to measure the impedance of a line

Use TDR....!!!!!!!!!!

 

[FvM]

how to measure telephone line

Use TDR....!!!!!!!!!!

Although the term impedance would normally designate the transmission line parameter, the DC resistance seems to be meant here, which is not accessible by TDR. However, for the intended accuracy, a value of 2x28 ohm/km could be assumed without any measurement, provided the length is known at all.