[ntp:questions] Strange NTP problem on AMD Geode LX cards.

Brian Utterback brian.utterback at sun.com
Tue Oct 6 15:55:22 UTC 2009


Unruh wrote:

>> * The processor card uses a PCI clock generator capable of spread spectrum 
>> output, this is always enabled and not controllable from the BIOS - the chip 
> 
> No idea what "spread spectrum " means for a clock. 


Spread spectrum clock signals are a dirty trick to get by EMI 
regulations. When measuring the EMI generated by a piece of hardware, 
the interference signal is integrated over time and the frequency. 
Thus for a given amount of energy leaked, if the clock signal was 
completely steady all of the energy released would be at that 
frequency. By modulating the clock signal, the same energy can be 
leaked, but it is spread out over all of the frequencies, thus 
lowering the amount of energy leaked at any given frequency. So, the 
same energy is leaked, but the peak at any given frequency is lower. 
And since the EMI regulations regulate the peak EMI, a piece of 
hardware can pass that would not pass if the frequency were constant 
and steady.

It is very easy to implement. Just design your oscillator as a voltage 
  controlled oscillator and feed a sine wave into it.

Brian Utterback




More information about the questions mailing list