Speaking as an EMI specialist... Spread spectrum clocking has gradually phased into most modern PC architectures. Many vendors sell a clock driver that, given the master clock input, outputs a spread spectrum clock for the big nasty components in the computer (CPU, memory, etc.) and a separate, non-spread stable clock for the slower I/O components such as USB and ethernet. Typically the I/O clocks are much slower in speed and don't have high currents like the main system clocks. I/O is typically more stringent regarding "jitter" which is basically what you're doing when you use spread spectrum clocking.
Spread spectrum started more in the low-end computing products, like printers, since cost margins prevent you from using big expensive sheets of metal to shield the unit. Using spread spectrum allows you to get away with plastic enclosures. I recall Lexmark had in fact pioneered (and patented?) an early spread spectrum waveform.
And if you think suppressing EM interference from a PC is bad, try working on a gigabit switch router! We have to verify the system is EMI 'clean' up to a couple GHz, and with increasing speeds I see EMI requirements climbing up to 10GHz probably by years end.
Slashdot Mirror
Speaking as an EMI specialist...
Spread spectrum clocking has gradually phased into most modern PC architectures. Many vendors sell a clock driver that, given the master clock input, outputs a spread spectrum clock for the big nasty components in the computer (CPU, memory, etc.) and a separate, non-spread stable clock for the slower I/O components such as USB and ethernet. Typically the I/O clocks are much slower in speed and don't have high currents like the main system clocks. I/O is typically more stringent regarding "jitter" which is basically what you're doing when you use spread spectrum clocking.
Spread spectrum started more in the low-end computing products, like printers, since cost margins prevent you from using big expensive sheets of metal to shield the unit. Using spread spectrum allows you to get away with plastic enclosures. I recall Lexmark had in fact pioneered (and patented?) an early spread spectrum waveform.
And if you think suppressing EM interference from a PC is bad, try working on a gigabit switch router! We have to verify the system is EMI 'clean' up to a couple GHz, and with increasing speeds I see EMI requirements climbing up to 10GHz probably by years end.
Gary