[SI-LIST] Re: Decoupling for PLL
- From: steve weir <weirsp@xxxxxxxxxx>
- To: tom@xxxxxxxxxxxxx, <cgrassosprint1@xxxxxxxxxxxxx>, <zhang_kun@xxxxxxxxxx>, <a.ingraham@xxxxxxxx>
- Date: Sun, 29 Aug 2004 20:09:09 -0700
Tom, given Zhangkun's observation that substitution of an R for the L in
the power feed "fixes" the PLL, I think it is safe to say that the problem
is power related.
Regards,
Steve.
At 03:09 PM 8/29/2004 -0700, Tom Dagostino wrote:
>No where in this discussion has anybody said, "this PLL is failing because".
>Until we know why the PLL is failing nobody can fix it, no matter how clean
>the power supply is.
>
>Tom Dagostino
>Teraspeed Consulting Group LLC
>503-430-1065
>tom@xxxxxxxxxxxxx
>www.teraspeed.com
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Charles Grasso
>Sent: Sunday, August 29, 2004 4:03 PM
>To: weirsp@xxxxxxxxxx; zhang_kun@xxxxxxxxxx; a.ingraham@xxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Decoupling for PLL
>
>
>Steve - I completley agree. I have tried to measure a power bus with a SA -
>only to find that the 50 ohm termination swamped the reading. (This was on a
>two layer board where the Power impedance is pretty high to begin with)
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
>Sent: Sunday, August 29, 2004 3:21 AM
>To: zhang_kun@xxxxxxxxxx; a.ingraham@xxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Decoupling for PLL
>
>
>Zhangkun, if I understand correctly, you said that you have a bead and a
>big electrolytic capacitor and two or three ceramic capacitors. If you are
>looking at this with a 50ohm spectrum analyzer load, you are very likely
>damping the network. Try a 50 ohm resistor across the network without the
>spectrum analyzer and see if it works as it does with the series
>resistor. You should make up an attenuator so that the spectrum analyzer
>does not load the filter.
>
>Steve.
>At 05:34 PM 8/29/2004 +0800, zhangkun 29902 wrote:
> >Andy
> >
> >I use spectrum analyzer to measure the power ground noise in frequency
> >domain. At almost all the harmonic frequency, the amplitude with 0 ohm
> >resistor is less of 3db than that with bead.
> >
> >The pre-filter power is of plane pair.
> >
> >Best Regards
> >
> >Zhangkun
> >2004.8.29
> >
> >----- Original Message -----
> >From: Andrew Ingraham <a.ingraham@xxxxxxxx>
> >Date: Saturday, August 28, 2004 9:48 pm
> >Subject: [SI-LIST] Re: Decoupling for PLL
> >
> > > Zhangkun,
> > >
> > > Some PLLs can be very sensitive to tiny amounts of noise. They
> > > are, after
> > > all, analog devices.
> > >
> > > When you say the noise difference is only 3dB, (1) that might be
> > > just enough
> > > of a difference to cause a problem, if your circuits were just on
> > > the edge
> > > of misbehaving; and (2) if you are measuring and comparing
> > > broadband noise,
> > > it is a meaningless comparison. The PLL is particularly sensitive
> > > to noise
> > > at particular frequencies, and you may need to look specifically
> > > for those
> > > frequencies. A broadband noise measurement could mask the
> > > frequencies that
> > > are causing the problem. Even a spectrum analyzer might, if you
> > > don't know
> > > what frequencies to look for.
> > >
> > > Another thought: does connecting the instruments to measure the noise,
> > > significantly affect the circuit? Maybe simply connecting probes
> > > reducesthe noise enough to stop that PLL from misbehaving (while
> > > the other two PLLs
> > > continue to misbehave).
> > >
> > > How clean is the pre-filtered power? Is it a plane?
> > >
> > > Is there anything else of an analog nature on this board?
> > >
> > > Regards,
> > > Andy
> > >
> > >
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