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

Yehuday, Sean,

I disagree.  I doubt that the VCO won't start.  I believe the loop is not 
locking.

The LC filter is flat well into the MHz, whereas these types of PLL's are 
typically sensitive to noise from 10 KHz up to several MHz.  The other 
danger of an LC network that has been mentioned here before is that with 
the light load of the PLL, the bead may have a substantial Q making a nice 
resonant tank out of the LC network.

Regards,

At 09:41 AM 3/31/2003 +0300, Yehuda Yizraeli wrote:

>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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