[SI-LIST] Re: Decoupling for PLL
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Zhangkun <zhang_kun@xxxxxxxxxx>, reanderson@xxxxxxxxxxxxx
- Date: Fri, 27 Aug 2004 03:08:57 -0700
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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