[SI-LIST] Re: Decoupling for PLL

Steve

I measured from 100Hz t0 1MHz and from 1MHz to 1GHz. When I measured, the 
decoupling cap is soldered off. Therefore, the power ground noise is not 
"colored".

If there is ringing problem inside the chip, how to measure?

Best Regards

Zhangkun
2004.8.27
----- Original Message ----- 
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "Zhangkun" <zhang_kun@xxxxxxxxxx>; <reanderson@xxxxxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 27, 2004 6:08 PM
Subject: [SI-LIST] Re: Decoupling for PLL


> Zhangkun, is the spectrum analyzer set to read down to at least the LC 
> corner frequency?  Were you able to get a good coaxial connection across 
> the capacitor back to the spectrum analyzer?  If so, then the only other 
> thing that comes to mind is that the ringing problem is inside the chip.
> 
> In any event, the circuit will not be well-behaved without a resistor.  So, 
> if your layout is committed and you only have pads that can be used by a 
> resistor, or an inductor,  select a resistor that satisfies the DC 
> requirements, and if necessary adjust the capacitor for the low frequency 
> insertion loss required.
> 
> Steve.
> At 05:21 PM 8/27/2004 +0800, Zhangkun wrote:
> >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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