[SI-LIST] Re: Decoupling for PLL

Andy

I use spectrum analyzer to measure the power ground noise in frequency domain. 
At almost all the harmonic frequency, the amplitude with 0 ohm resistor is less 
of 3db than that with bead.

The pre-filter power is of plane pair.

Best Regards

Zhangkun
2004.8.29

----- Original Message -----
From: Andrew Ingraham <a.ingraham@xxxxxxxx>
Date: Saturday, August 28, 2004 9:48 pm
Subject: [SI-LIST] Re: Decoupling for PLL

> 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 
> reducesthe 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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