[SI-LIST] Re: Decoupling for PLL (continued)
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 3 Sep 2004 08:41:03 -0400
Zhangkun,
While you might want to look for generic PLL SPICE models, don't bother.
Generic models won't help you at all. You would need models for your
particular PLL components, especially the loop filter and VCO, and maybe the
phase detector. They would need to be detailed enough to include the
mechanisms you think are happening. Generic models don't do this.
It's not surprising that the 333 kHz is modulating the VCO. It could be
either phase (frequency) or amplitude modulation, or both. Either process
generates sidebands that are 333 kHz to either side of the main VCO (PLL)
output frequency.
That's why many PLLs have power filters: because the VCO or analog stages
are sensitive to such noise which gets modulated onto the VCO.
Another question you need to ask is, is the 333 kHz coming from the VRM, or
from the PLLs and the circuits they drive? You might have a low level
sustained oscillation due to feedback.
Regards,
Andy
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