The following is a technical explanation as to why high-bandwidth “Gigabit” modems and routers can create high amounts of electromagnetic interference (EMI) that is transmitted to your computer. This is from an electrical engineer who is trained in electromagnetic compatibility (EMC) engineering, which is an EE specialty that minimizes EMI problems in electrical equipment.
“Driving GHz frequency data on long cables requires a lot of power to overcome the capacitance and inductance of the cables, which tend to limit the rise/fall times of any signal. Typically, a “peaked” drive scheme is used, intentionally generating a wicked higher differential voltage spike at the start of a transition (from 0-1 or 1-0), to get it moving in the right direction faster (optical fiber drivers have the same problem and use the same technique, which can be a serious EMI problem). The problem of the “wicked spike” is that it’s frequency and current is necessarily several times higher than the data rate, so much higher in emissions.
In EMC, there is the principle known as “conservation of bandwidth”. To reduce emissions, you always want to use the lowest frequency/bandwidth possible, since this effectively eliminates most emissions more than 10x the square wave bandwidth. Slower is better, as long as it is adequate to meet your needs. Getting rid of square waves, and smoothing signals to be “just fast enough” is also a significant part of EMC design.
A square wave has very strong higher harmonics to 10x the frequency of the square wave, and significant harmonics to 100x the frequency. There is a design trade-off for speed – it causes much more high frequency emissions.
The wired Ethernet cables will radiate the fundamental frequency of the Ethernet data rate, plus harmonics. The cables are designed to minimize the current loop area (all emissions are frequency x current loop area x current) by using twisted pairs, and by driving them with equal and opposite signals (AKA differentially). That helps, but the common mode EMI on the cable does radiate as an antenna, and as wavelength gets shorter, the cable length becomes more and more effective at radiating it.
The common mode (beyond SMPS) happens because of the difficulty of making perfectly balanced differential drive of the two wires of each pair over long distances. At 100Mhz, it is a challenge, but at GHz frequencies, you’re in trouble – the cable is highly effective as an antenna, and at those frequencies every slight variation in inductance and capacitance will increase the uncancelled common mode emissions.”