[SI-LIST] Re: Decoupling for PLL
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <zhang_kun@xxxxxxxxxx>, <a.ingraham@xxxxxxxx>
- Date: Sun, 29 Aug 2004 09:23:28 -0400
Zhangkun,
When you say "harmonic frequency", do you mean the
output frequency of PLL? Most of the time the output
frequency is much higher than the PLL's filter bandwidth,
which is usually in the hundreds of kHz. PLLs tend to
be sensitive to noise on their analog supply pin at or
below the filter bandwidth. Have you also compared the
noise below 1MHz?
Regards,
Istvan
----- Original Message -----
From: "zhangkun 29902" <zhang_kun@xxxxxxxxxx>
To: <a.ingraham@xxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, August 29, 2004 5:34 AM
Subject: [SI-LIST] Re: Decoupling for PLL
> 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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