Using an Oscilloscope

[EricaS]

Hello

I'm trying to find a high impedance audio signal (in the megaohms) on a telephone line.

Can someone point out how to do this using an oscilloscope?

Regards.
E

 

[Audioguru]

A telephone line does not have any high impedance signals, it is about 600 ohms to 1000 ohms. It might measure megaohms if it is not connected to anything and its length is very short.
A telephone line is supposed to be powered by the central office and sometimes has a telephone that is off hook at its end.

Why do you think there is a high impedance audio signal on a telephone line?

 

[EricaS]

Why do you think there is a high impedance audio signal on a telephone line?

Its a short course project. There is a high impedance (megaohms) audio signal on a telephone line while the phone is on-hook. We need to find it using an oscilloscope.
Other equipment to assist is allowed.

I have no clue what to do.

 

[betwixt]

Telephone lines are normally 400 Ohm or 600 Ohm impedance, a high impedance signal applied across the line would be hugely attenuated and almost certainly lost in the background noise. I suspect you are not interpreting the project correctly, are you trying to measure a signal across an 'on hook' line such as might be sent for caller ID data before the handset is picked up? If that is the case, it's a low level signal but not a high impedance one. The impedance across a line that isn't in use is still low even though little DC current may be flowing through it.

Brian.

 

[Vbase]

Its a short course project. There is a high impedance (megaohms) audio signal on a telephone line while the phone is on-hook. We need to find it using an oscilloscope.
Other equipment to assist is allowed.

I have no clue what to do.

It is OK to call it high impedance line because the phone line is a DC current source modulated with AC. 50V 35mA current at dialing tone.
If you put 680 Ohm, 5%, 1/4W resistor across the line wires and hook your oscilloscope probe on the resistor you will see on the screen the audio signal.

 

[D.A.(Tony)Stewart]

Telephone uses current source in one direction and voltage source in other direction and thus a hybrid transformer is used.
Since current is high impedance signal terminated by line impedance, this is the one , that you may be looking for. Use differential probe method around hybrid or voice separator chip.

 

[Vbase]

Telephone uses current source in one direction and voltage source in other direction and thus a hybrid transformer is used.
Since current is high impedance signal terminated by line impedance, this is the one , that you may be looking for. Use differential probe method around hybrid or voice separator chip.

What will happen if the probe is earthed?

 

[Audioguru]

What will happen if the probe is earthed?

Then the telephone line will no longer be balanced and acts like an antenna to pickup a lot of mains hum. Aa balanced line cancels mains hum and other interference.

You can use two input channels on a 'scope as a differential amplifier that has a balanced input.