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

Dave Baxter spam at goes.nowhere.com
Tue Oct 13 13:24:34 UTC 2009


In article <1254844523.604551 at news1nwk>, brian.utterback at sun.com says...
> 
> 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

That, and most things that leak like that, can also be susceptable to 
incoming EMI.  It's a bean counter bodge job, as above to "fool" EMI 
measuring receivers, so they don't have to pay to do a propper job in 
the first place.  Real sloppy design, if you have to use spread-spectrum 
clocks to pass the EMC tests.

Such cut price techniques have also been found to foul up other systems 
that genuinely use spread spectrum communications, clock osc's have 
harmonics that extend up into the low microwave area.  Systems such as 
BlueTooth, and WiFi, can do odd things as a result that are difficult to 
track down.

Best get it right the first time, it's cheaper in the long run too, but 
heck, how many carreer bean counters stay anywere for more than a year 
anyway.  I know some so called "engineers" like that too.  Get it to 
production asap, then leave.

Regards.

Dave B, who strangely works in EMC!




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