Why IS my AC so bad?

I've been wondering just why my AC was as bad as it is, so I've been doing some investigation. The THD measurement is around 7% right now. If you look to the right, there's a picture of part of the AC wafeform. The white circle shows a small negative spike. This is caused by the fluorescent lamp on my desktop. The bigger problem however was the severe drop at the top and bottom of each cycle. It's circled in read.



After some experimentation, I discoverd that it is in fact being caused by a spike on the neutral coming from my laptop's power supply. The graph on the right is a graph of the neutral voltage at my desk. In an ideal world, it should be completely flat, and match the voltage at the ground pin but in my case every half cycle my laptop is putting a significant amount of current onto the neutral wire. Because the resistance between my desk and ground is not zero, the current flow causes a voltage spike to occur in the same direction as the incoming line voltage, with the effective result that the voltage between line and neutral dropping, and more noise and less effective power available to your equipment.

If I turn off the lamp and disconnect the laptop these neutral spikes disapear, the AC waveform becomes much cleaner looking and my THD measurements drop to under 3%.

All of this is great news, because it means I have at least two good ways to test the efficacy of any power conditioners that may come my way, especially those with special sockets for "digital" components.

Cheers!