[SI-LIST] Re: Decoupling for PLL
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 28 Aug 2004 09:48:20 -0400
Zhangkun,
Some PLLs can be very sensitive to tiny amounts of noise. They are, after
all, analog devices.
When you say the noise difference is only 3dB, (1) that might be just enough
of a difference to cause a problem, if your circuits were just on the edge
of misbehaving; and (2) if you are measuring and comparing broadband noise,
it is a meaningless comparison. The PLL is particularly sensitive to noise
at particular frequencies, and you may need to look specifically for those
frequencies. A broadband noise measurement could mask the frequencies that
are causing the problem. Even a spectrum analyzer might, if you don't know
what frequencies to look for.
Another thought: does connecting the instruments to measure the noise,
significantly affect the circuit? Maybe simply connecting probes reduces
the noise enough to stop that PLL from misbehaving (while the other two PLLs
continue to misbehave).
How clean is the pre-filtered power? Is it a plane?
Is there anything else of an analog nature on this board?
Regards,
Andy
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