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

Hi Yehuda,
        Thanks for your kindly reply.
         Something in your mail I can not understand well.Can you tell
me the detail?
         You say"antiphase voltage spikes",is that when power up,the
power will drop because of capacitor?
         And the oscillator,do you mean the VCO in the PLL or the
oscillator ourside that provide clock signal to XPC8260?
        Thank you very much!

Regards,

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

-----=D3=CA=BC=FE=D4=AD=BC=FE-----
=B7=A2=BC=FE=C8=CB: Yehuda Yizraeli [mailto:yehuday@xxxxxxxxxxxx]=20
=B7=A2=CB=CD=CA=B1=BC=E4: 2003=C4=EA3=D4=C231=C8=D5 14:42
=CA=D5=BC=FE=C8=CB: 'luhaizhao@xxxxxxxxxx'; si-list@xxxxxxxxxxxxx
=D6=F7=CC=E2: RE: [SI-LIST] 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]=20
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=3DA1=3DE6 to =3D
-30=3DA1=3DE6=3DA3=3DACsometime 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=3D20
Shenzhen China=3D20

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