Whither The 1.21 Jigowatts?
Those who have seen Back to the Future no doubt recall Doc and his revelation about the DeLorean’s “1.21 jigowatt” power requirements. Thankfully, we had neither plutonium nor lightning strikes present on our tour, but we did have a flux capacitor. That’s the cylindrical capacitor housed within the bisected metal block you see above. The data cable sets inside the center conductor, and it literally is in flux—hence the name. In electromagnetic field theory, there are lines of flux, so X-Lab engineers joke about it being the infamous “flux capacitor.” In reality, all capacitors are flux capacitors. Gotta love that geek humor. Anyway, the inserted cable has noise coupled onto all four of its wire pairs simultaneously.
Techs examine several signal types through the flux capacitor test to assess their effect on differential signaling, which assists with noise cancelation.
“Differential signaling is a very brute force method of noise cancellation,” explained Pete. “We split the signal, invert one half, run this split signal into a difference amplifier, and look at the resulting signal. If you have a signal that is common to both halves—meaning the same phase, polarity, and amplitude—coming into the receiver, the signals should cancel. Well, that’s great in math, but in the real world, one pair of wire will probably be a little longer or shorter than another, so you have an offset in time. They’ll probably be manufactured slightly differently, so the amplitude or power will be different. You subtract two things that are not identical but close. That’s why you get a little noise left over at the end.”
I asked if this was the same principle behind active noise cancelation in headphones.
“That’s a good way to think about it,” he said. “By the way, our alien crosstalk test uses a differential noise source. It comes in and stays there. For common-mode noise, we’re hoping that the system will work really well, the noise will balance, and be eliminated. But in the real world, it doesn’t always happen.”
(Note: For those who question the spelling of “jigowatts,” see http://www.imsdb.com/scripts/Back-to-the-Future.pdf at the famous shot 67.)