“Let’s take that noise problem one step further,” said Pete, drawing me over to a nearby bench to examine a small PCB with a dissected Ethernet cable tied across its middle. “Say we run these alien crosstalk tests. We see errors. The next thing we want is to see what energy is getting coupled in there. So you get an oscilloscope or spectrum analyzer or some piece of equipment to measure the noise on that center channel while these other guys are sending stuff. You can mathematically model that noise and define it in terms of some type of more standard noise source.
In the standard, the IEEE has defined a white Gaussian noise—think of it as random noise—that is flat across the frequency range from 10 to 400 MHz with a specific noise power of 141.9 dBm/Hz.” I raised an eyebrow at the specificity, and he shrugged. “Hey, I live with this and don’t get out much. But anyway, the IEEE says you want to grab four independent noise sources, which we have here, and couple those into each pair of the wire. While you have that noise coupled in, see how well you do. It’s like somebody’s yelling at you and you’re still trying to communicate.
We’ve got, in this case, one fixture that we’ve designed ourselves using some very precision noise sources that are quite spendy. How much? Each one of these little things is $2000. This is $8000 of noise here. We tend to think of things in terms of equivalent values here. ‘This is like a used car. That thing over there is like a house.’ With truly random noise, you cannot mathematically cancel it. Instead, you just have to figure out ways to deal with it.”