[SI-LIST] Re: Decoupling for PLL (continued)
- From: "Charles Grasso" <cgrassosprint1@xxxxxxxxxxxxx>
- To: <zhang_kun@xxxxxxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Sat, 4 Sep 2004 15:33:11 -0700
Hi Zhangkun,
Its very important to differentiate between an inductor and a ferrite bead.
A ferrite bead typically has minimal impedance in the kHz range and really
only
starts to kick in in the 10s of MHz range. An inductor, on the other hand,
can
exhibit LARGE reative impedance in the kHz range (depending on the value) -
but
will also exhibit resonances that need to be damped.
Chas
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zhangkun
Sent: Thursday, September 02, 2004 10:53 PM
To: si-list
Subject: [SI-LIST] Decoupling for PLL (continued)
Dear all
Recently, I have done some measurement. It is found that when the
bead(inductor) is used, the power ground noise at 333KHz from VRM is larger.
There is some noise near the clock frequency points.
For example, the output clock of PLL is 50MHz. At 50.333MHz and 49.667MHz,
there is noise, which is in proportion to the noise at 333KHz. Maybe the PLL
modulates the noise of VRM into the output clock.
Is there any SPICE model for PLL?
Best Regards
Zhangkun
2004.9.3
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