[SI-LIST] Re: Decoupling for PLL
- From: Zhangkun <zhang_kun@xxxxxxxxxx>
- To: steve weir <weirsp@xxxxxxxxxx>, reanderson@xxxxxxxxxxxxx
- Date: Fri, 27 Aug 2004 17:21:19 +0800
steve
I have measured the power ground noise by spectrum analyzer. There is no
obvious difference.
At some frequency, when the inductor(bead) is changed to resistor, the power
ground noise become larger. I am confusing.
Best Regards
Zhangkun
2004.8.27
----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "zhangkun 29902" <zhang_kun@xxxxxxxxxx>; <reanderson@xxxxxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 27, 2004 4:01 PM
Subject: Re: [SI-LIST] Re: Decoupling for PLL
> Zhangkun, if you have a DSO that exports ASCII, then you can process the
> data to obtain the FFT. FFT's are great for seeing what is going
> on. There is also some commercial software for that purpose.
>
> Steve.
> At 07:46 AM 8/27/2004 +0800, zhangkun 29902 wrote:
> >ray and steve
> >
> >Thank you for your good advice. I have measured the power ground noise at
> >the power pin of PLL. I do not see great difference between using
> >inductor(bead) and resistor. This is my problem. The difference is about
> >3db, which could be ignored.
> >
> >I do not use scope by spectrum analyzer.
> >
> >It is obvious that the RC filter has lower Q than LC filter. Here I use
> >resistor of 0 ohm. This should have some effect on power ground noise. I
> >want to get some different measurement about power ground noise.
> >
> >Best Regards
> >
> >Zhangkun
> >2004.8.27
> >
> >----- Original Message -----
> >From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
> >Date: Friday, August 27, 2004 3:21 am
> >Subject: [SI-LIST] Re: Decoupling for PLL
> >
> > > steve weir wrote:
> > >
> > > > Ray, zhang_kun, any linear filter book will show you your
> > > options.
> > > > Basically, you should start with the required impedance of the
> > > filter
> > > > at DC, use that to determine your resistor, and then derive L
> > > and C
> > > > based on the cut-off frequency required. To contain the
> > > ringing, set
> > > > the damping coefficient to a reasonably high value. Circuit 3A
> > > used
> > > > to be very popular with voltage mode switching power supplies.
> > > >
> > > > Steve.
> > >
> > >
> > >
> > > Yes, you could design an impedance matched filter by untilizing
> > > any of
> > > the tried and true methods for synthesizing RF filters, BUT, I
> > > think you
> > > will find that when you specify the source resistance to be very
> > > low
> > > (say on the order of milliohms as PS supply outputs are) and the
> > > load
> > > resistance to be that of a typical analog ckt like a PLL (in the
> > > 100's
> > > of ohms) and that your desired roll-off frequency to be in the kHz
> > > to
> > > 100's of kHz range you will find that the L and C values that fall
> > > out
> > > of the synthesis procedure are such that they are unrealizable on
> > > most
> > > PCB designs.
> > >
> > > -Ray
> > >
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