[SI-LIST] Re: Decoupling for PLL
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 26 Aug 2004 11:26:10 -0700
As Steve mentioned, the problem most likely stems from peaking in your
supposed low pass filter network.
The typical low pass filter that is realized with a random inductor or
ferrite bead and a random capacitor (as they are usually made (not
designed), isn't an impedance matched filter and exhibits peaking
(typically 10 to 20 dB) somewhere near the roll-off frequency. This is
due in part to the very low impedance of the P/S driving the filter.
There are several ways to fix the problem. The simplest is to put a
small resistor in series with the inductor.
The value needs to be small enough to not introduce excessive DC drop,
but large enough to de-Q the circuit and eliminate the objectionable
peaking. See http://si-list.org/files/tech_files/power_filt.pdf for a
short document I put together a while back that describes the problem
and simulation of same. For a more elegant solution, see the paper by
Erickson ( http://ece-www.colorado.edu/~rwe/papers/APEC99.pdf ) and
implement one of the filter topologies described in Figure 3 on page 2.
I recommend the topology of figure 3a. which uses a shunt connected
series RC compensation network (component value calculation described in
section V.) . This circuit doesn't have the DC drop issue that the
simple series R solution has.
-Ray Anderson
steve weir wrote:
>At 12:04 AM 8/27/2004 +0800, zhangkun 29902 wrote:
>
>
>>Dear all
>>
>>I have a question about decoupling for PLL. In one of our PCB, there are
>>three PLL. PI decoupling circuit is used. When the inductor is used, the
>>PCB does not work. When resistor of 0 ohm is used, the PCB work well.
>>
>>
>
>This is due to the inductor peaking. You can capture your clock output and
>put it through an FFT, and also capture the PS voltage and put it through
>an FFT. The problem frequency(s) should correlate. PLLs can be sensitive
>to rather low frequency noise, some all the way down to the KHz.
>
>Steve.
>
>
>
.
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