[SI-LIST] Re: Decoupling for PLL
- From: steve weir <weirsp@xxxxxxxxxx>
- To: zhang_kun@xxxxxxxxxx, a.ingraham@xxxxxxxx
- Date: Sun, 29 Aug 2004 03:21:10 -0700
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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