# How to measure differential signals using an Oscilloscope

#### Hesambook

##### Member level 3

Differential signal measurement in practice

To measure a differential signal, we have two options, one is using a differential probe and second is using a two channels oscilloscope. A differential probe is expensive but handles a better accuracy. Using two/four channels oscilloscope is the cheapest method which handles acceptable results.

Just you need to connect oscilloscope’s Channel-1 to one of the differential lines/wires and Channel-2 to another one. Then go to the math function and enable CH1-CH2 which means a difference. Then adjust the oscilloscope to observe the signal. You use the run/stop or a single shot button to freeze the signal on the screen and examine it.

I have examined the ADSL2+ signal on the telephone line using the above-mentioned differential measurement method.

Reference: http://bit.ly/2Z7sDTt

#### BigBoss

This method is a bit subjective and unstable.If the frequency is sufficiently high, phase shifts will be different by oscilloscope channels so the measurement may be a bit erroneous.
You presume that the oscilloscope has %100 identical channels and Phase Shifts are linear but this is not true in reality..

#### Hesambook

##### Member level 3
This method is a bit subjective and unstable.If the frequency is sufficiently high, phase shifts will be different by oscilloscope channels so the measurement may be a bit erroneous.
You presume that the oscilloscope has %100 identical channels and Phase Shifts are linear but this is not true in reality..
I am agree, as I mentioned in the article using the differential probe is the best, but not necessarily high voltage differential probes, because the main problem is to deal with high frequency pulses. This method at least gives a close picture of the final signal. The result on the screen is the result of the difference

#### schmitt trigger

Also....one has to make sure that both probes are properly and equally compensated.

A good way I have found is to compensate each probe individually (with the scope's CAL SIGNAL output).
Then attach both probes simultaneously to the same CAL output and perform the A-B comparison, and slightly tweak for maximum cancellation.

#### dick_freebird

Also....one has to make sure that both probes are properly and equally compensated.

A good way I have found is to compensate each probe individually (with the scope's CAL SIGNAL output).
Then attach both probes simultaneously to the same CAL output and perform the A-B comparison, and slightly tweak for maximum cancellation.
I like to use the "signal of interest" (or one side of the pair)
for this because the CAL signal on most 'scopes is way lower
frequency / edge rate than most of what I look at.

You would of course like to get identical model# probes to