Test and Measurement Forum

I'm trying to trace a very intermittent fault on my PC/telecoms network. If I want to connect my Picoscope across the incoming phone line to monitor for transients what's the best way to do this?

I already know about the -48V line level and the 90V ring signal, the main concerns are which way round (does scope ground go to tip or ring) and is any isolation required?

Peter Gant

I know this is a little late, but I'm new here and hope I can help if you have not found a resolution to your issue.

I do work for a telecommunications company working in trouble isolation and higher level technicial support.

I would first ask, what kind of trouble are you having? If you're still having trouble

On pots lines, the ring voltage is 90-120VAC so, dont stick your tongue on it, -- dont ask.

There are three ways to measure the quality of the copper. Tip to ground, ring to ground and ring to tip. Ring to ground is the "ringing" side, and you can measure that to ground and should see 90-120 vac. You should also be able to measure ring to tip across the pair with a load on the circuit to get the voltage reading. To measure if there is correct current being delivered to the set you would need to put a load on the circuit, shunt it, and measure the current. Normal sealing current is 23ma with -48VDC comming out of the central office. Ringing current should not exceed 220ma at no less than 40VAC measured ring to ground.

You can also check the capacitance across the pairs on each lead (if you can get the central office technician to remove the lightning protection coils) to get the length of each conductor and check to see if you have any load spacing issues. Loads on telcom circuits are typically 88mh inductive loads to balance the transmission line, and if spaced correctly should help not hinder, but if not done correctly can mess up a circuit.

Hope this helps... I know its not an exact answer, but I hope there's enough to help.


Travis,