I was messing with my least favourite project in the office today. Something designed by a subby that I now need to get through EMC, well its power supply actually.
So the cabling we use has a shield, it's that industrial grade mains cable, it's steel. So I have taken advantage of this and connected it to the metal box the power supply is in. I made a mock up of the actual machine with a die cast box with resistors in it to load the supply with a "non reactive" load.
I used a near field probe and was surprised to find stuff coming off the shield at various frequencies, around 110MHz and various spots I had not noticed before as it is usually around 70MHz (I've been messing with it for a while). Then as I was doing some narrower band measurements my boss was changing the lights and I could see the signal literally going up and down to the banging he was doing (getting those horrible grills off office ceiling lights). Then I pointed the probe in the direction of the light fitting above me, I noticed some very slight bumps in exactly the same place.....
So it seems that i am picking up noise from the lights from cable screening on a power lead plugged into a socket.
This leads me to wonder as to what is the best way to separate myself from such earth interferences. Should I have a dedicated earth, or put some sort of filter in place? the spectrum analyser and the power supply were plugged into the same socket bar, so I seem pretty doomed.