[SI-LIST] Re: Decoupling for PLL
- From: zhangkun 29902 <zhang_kun@xxxxxxxxxx>
- To: Istvan NOVAK <istvan.novak@xxxxxxxxxxxxxxxx>
- Date: Sun, 29 Aug 2004 22:52:42 +0800
Istvan
The input of PLL is clock of 77MHz and the output is clock of 622MHz. I measure
the noise at 77MHz, 154MHz, 231MHz, ... , 622MHz, and so on.
We have found that there is some problem below 250KHz. My solution is to use
some beads of high impedance in low frequency domain. I have not check the
result.
Best Regards
Zhangkun
2004.8.29
----- Original Message -----
From: Istvan NOVAK <istvan.novak@xxxxxxxxxxxxxxxx>
Date: Sunday, August 29, 2004 9:23 pm
Subject: Re: [SI-LIST] Re: Decoupling for PLL
> 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
> frequencydomain. 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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