[SI-LIST] Re: a problem about PLL bypass

Hi Sean Lu,

The resistor cause phase shift from zero frequency while the ferrite bead is
high frequency element. While trying to boot, the power supply inject
antiphase voltage spikes into the oscillator and it stops. The use of the
resistor cause the phase shift (at any frequency!!), thus moving the startup
and the stop noise from each-other.

Hope it helps.

BTW, checking oscillator current should demonstrate very low power which
means no drastic IR on it.


Regards, yehuda





-----Original Message-----
From: lu Haizhao [mailto:luhaizhao@xxxxxxxxxx] 
Sent: Monday, March 31, 2003 9:31 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] a problem about PLL bypass



Hi,all

   In the Motolola's telecom controller XPC8260-200MHz's handbook,the PLL
bypass circuit is the following:

       VDD          _____
       -------------|____|-----------------------VCCSYN
                      R      |            |
                            ___          ___
                            ___ 10uF     ___ 0.1uF
                             |            |
                             |____________|
                                    |
                                    |
                                   GND

    the R is 10 Ohm.
    In the board,I use a ferrite bead instead of a resistance.This ferrite
bead has a resistance of 0.01 Ohm in steady current,and 60 Ohm in 100MHz.
    the problem is when the board is under -5=A1=E6 to =
-30=A1=E6=A3=ACsometime the 8260 can not boot up normally or can not boot
up.and if you use a resistance,not a ferrite bead,the problem will
disappear.
    Can someone tell me the cause? Thanks!

Regards,

Sean Lu
Huawei Technologies co.,Ltd=20
Shenzhen China=20

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