Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
(OP)
Hi all,
I have the need to look at some wave forms on incoming phone lines for amplitude and power influence. This is simply measuring the balanced phone line up to about 3 KC. As this is not a common occurrence for our shop we do not want to spend a mint on a differential probe. We just use a simple phosphor scope for most other measurements similar to this.
Would it completely skew our measurements to simply use a GFCI on the power supply and lift the ground lead in the power supply and leave the case of the scope floating? Is there a source for a cheap differential probe instead?
I have the need to look at some wave forms on incoming phone lines for amplitude and power influence. This is simply measuring the balanced phone line up to about 3 KC. As this is not a common occurrence for our shop we do not want to spend a mint on a differential probe. We just use a simple phosphor scope for most other measurements similar to this.
Would it completely skew our measurements to simply use a GFCI on the power supply and lift the ground lead in the power supply and leave the case of the scope floating? Is there a source for a cheap differential probe instead?





RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
If you don't have that possibility, you can hook up an instrument amplifier (a beefed-up differential amplifier) and use it as a "balun". For the Telephone frequency band, that is fully satisfactory. Have a look at
http://www.analog.com/static/imported-files/design...
where the fun begins with Picture 2-1. Or use a monolithic INA (chapter 3). They have a good CMRR your frequency range as long as you use gain=1. At higher gains, the CMRR dives. Frequency response in your frequency range is good at any gain.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
I guess the answer to your question would depend on where the signals of interest lie with respect to the overall signal to noise range of the phone line.
Good phone lines do exhibit a high degree of longitudinal balance (electrical symmetry to ground) in order to attain the signal to noise ratio that they do. Assuming the signal under analyis is on a live phone line of some distance, any asymmetry introduced might bring the noise floor up significantly. Your ungrounded scope would certainly not be electrically symmetrical (e.g. difference in capacitive coupling to ground of the probe tip, compared to the other input, being the floating scope and ground lead). At some point down the s/n curve that would become an issue.
RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
Thanks for the help.
RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
back in the day when peripheral operations were the thing...how things have changed
RE: Unbalanced Oscilliscope measurements of balanced phone lines about -10 dBm
Below are a couple links to short Youtube videos demonstrating the concept referenced above for using a dual channel oscilloscope in "add inverted channel mode" to view a balanced differential signal. The videos use Modbus RTU over RS-485 as the example, but the principle is the same.
Two parts
Modbus pulses on oscilloscope part 1: https://www.youtube.com/watch?v=yNtuVDKkOnk
Modbus pulses on oscilloscope part 2: https://www.youtube.com/watch?v=qERWxdZLWhg