[Skywise]

(03-31-2015, 11:19 PM)Roger Hunter Wrote: Duffy;

This may be a stupid question but why run on batteries?

Roger

Not stupid at all. There's two aspects to this. One, mains voltage is surprisingly noisy. It's easier to run the laptops on batteries rather than invest in very expensive power filtering. By the time he bought a filter good enough for what he's doing, he could buy more laptops.

Second, the noise is even more problematic because of the frequencies he's measuring. Mains is either 60hz of 50hz depending where you are in the world. The problem is harmonics, multiples of the mains frequency. Without getting into AC signal theory, anything that is not a pure sine wave will have harmonics. And AC power is not a pure sine wave. So you get harmonics.

In broad generalities, VLF frequencies are below 30kHz. That would be the 500th harmonic of 60Hz. Typically you might get a few dozen harmonics. But it's also the other noises on the mains that cause problems. Someone turning on or off even a room light causes a spike or dip and the harmonics of that go off the charts.

Running on batteries eliminates that noise. And noise is exactly what he's trying to measure.

Brian