i recently noticed that with the scope running with the leads attached but not connected to anything, that i was picking up a regular, repeated a/c signal. the scope was setup for an o2 sensor with a frequency measurement on which was reading 50hertz. our mains voltage supply is 240volts at 50 hertz.
moving the laptop trolley to the centre of the workshop stops the interference. is this normal or have i a problem with my leads or the workshop voltage supply?
regards jez.

Hi Jez,

This is normal. What you have there is your lead is acting as antenna since they aren't referenced to any other voltage potential. Once connected, you should not see this any longer. Be aware though, this 1 meg-ohm impedance does lightly load the O2 circuitry within the ECU. It is a subtle shift that most wouldn't notice.

I've attached an example of what you are referring to. This is a little different than what you're accustomed to seeing I'm sure, but it should get the point across. This is a phosphor emulation mode. What you'll notice is that there are two cursors placed on the capture. The time between them is 16.6 mS. Now, since we know that 1000mS = 1S, we just simply take 1000 and divide that by 16.6. That will give us the frequency that is coming through the front end. 60 hertz is what we here in the US should expect to see if that is the source. You can see that 60.24 is very close to that. So, in conclusion, we know that an A/C field produced by transmission lines is causing what we are seeing on the scope when the leads aren't referenced to an alternative source.  

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