[SI-LIST] Re: Decoupling for PLL
- From: Clifford van Dyk <clifford@xxxxxxxxxxxxx>
- To: zhang_kun@xxxxxxxxxx
- Date: Fri, 27 Aug 2004 08:27:21 +0200
Hello Zhangkun
You may want to verify that the DC voltage level at you supply pin to
your PLL is within spec. Often inductors have a reasonably large DC
resistance (even in the order of ohms), that causes significant voltage
drop if your PLL draws significant current from the supply. This would
explain why your resistor (0R) works fine.
Kind regards,
Clifford
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.
>
>I have measured the power ground noise. The power ground noise in the two
>situation is different of 3db, which could be ignored. The power ground noise
>is very small. The power ground is about -50dbm at the harmonic frequency of
>PLL.
>
>It is obvious that the key element is the decoupling circuit. However, when I
>measure power ground noise, I could not find the obvious difference. Please
>give me some advice. Is there anything which could not be measured at PCB
>level?
>
>Best Regards
>
>Zhangkun
>2004.8.27
>
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