Power line voltage out of phase with ambient 60Hz electrical noise - why?

I just bought my first oscilloscope and have been tinkering with the measurement of different signals just to learn the ropes and familiarize myself with the scope.

One of the first things I noticed was the ambient 60Hz "noise" that is picked up when a probe is connected to the scope. It is obviously originating from the electrical lines in the building, and I can verify this by holding the probe in my hand and placing my other hand nearer or farther from an electrically powered object or cable - the amplitude of the noise increases or decreases respectively.

Then I probed the mains voltage to see how clean the incoming power is. Next, I viewed the aforementioned 'noise' signal on one channel and the mains power on the other channel.

What I saw was surprising and I don't understand the reason: The mains voltage shows a clean 60Hz sine wave. The 'noise' signal is a jagged sine wave, still obviously 60Hz but with a lot of static. However, the two signals are out of phase, by what appears to be approximately 90 degrees if my measurement is correct. I would have expected the phase of the mains power to match the phase of the noise. I was concerned my scope had some sort of delay between sampling the two channels, so I switched the inputs and still saw the exact same phase difference. Attached is a screen capture showing the two signals (red = mains, yellow = noise).

A little more research shows that transformers can cause phase shift between the input and output sides. (TI pdf) Is this what I'm observing? or is something else in play?

If I'm missing a basic point, please help me understand it or at least guide me in the right direction.

• I personally don't trust these scopes enough to probe mains with them and I suggest that you do not either. The AC trigger line is in phase with the AC signal coming into the PSU. Here are the screenshots for me: first,second The phase difference does seem to be 90 degrees to me. Dec 8, 2013 at 20:27
• I agree with @AndrejaKo, probing mains with an oscilloscope can be very tricky, even for the experienced EE. Better to use a transformer. Dec 8, 2013 at 20:49
• @RyanGriggs on my scope this mode is called 'X-DEFL' (deflection), sometimes it is called X-Y mode and probably other names are used too. Instead of using a time base for the X-deflection, you use channel B for X-deflection and channel A for the regular Y-deflection. It is very handy to check phase difference or frequency difference between the two channels. If the circle slowly changes shape to a line the frequencies differ. Check Lissajous on Wikipedia. This is a nice image: en.wikipedia.org/wiki/File:Circular_Lissajous.gif Dec 9, 2013 at 7:16
• @Ryan Griggs You got the background from jippie and I'll tell you how to get it in practice: Press the display button and after that the H3 button. Above H3, the XY mode setting should change from OFF to ON. Same procedure turns it off. Dec 9, 2013 at 7:19
• @Ryan Griggs About the trust side: A bit of both, since my scope had some measurement problem, but the main issue is safety. In theory the scope says it's CAT II rated for 400 V and that in theory might be OK for connecting it to power plug. Due to problems I already mentioned, I had to take it apart completely few times and after seeing internals I wouldn't trust it with mains power at all. Also I don't know what probes you're using, but if they're the one that come with the scope... Well they aren't the safest either. If you're going to probe mains, do it from behind a small transformer. Dec 9, 2013 at 7:25