All of this discussion around how to isolate PLLs from Vdd noise speak to a symptom, namely, too much noise on Vdd. True, it is possible to add all manner of networks such as have been discussed over the last few days, but these still only treat the symptom. It is a far better strategy to do a good job designing the power subsystem so that ripple is held to a minimum. This is necessary to satisfy noise budgets and to help make passing EMI tests easier anyway. The reason for these networks in the first place is that manufacturer's applications notes did not and still do not properly advise a user on how to design a good power subsystem. Once this is understood, and it isn't such a tough topic, ripple will be well within the tolerances of well designed PLLs. To support these statements, log onto www.procket.com. You will see a terabit router that has hundreds of PLLs in it as well as 5000+ 2.4 GB/S serial links. All of the PLLs are connected directly into the power plane from which they draw power. There are no ferrite beads or other networks in their power leads. All that was necessary was to design the decoupling, including plane capacitance, such that ripple was less than 20 millivolts to make them all happy. It would be far better if advice on how to achieve a maximum ripple amplitude were given in applications notes along with how much ripple a PLL can tolerate. This would allow predictable designs to be done. Among the reasons not to use ferrite beads in this manner are: they add extra components and they carry the risk of presenting the PLL with a high impedance source of power potentially making the PLL more unstable rather that more stable. In almost all cases I have seen, ferrite beads are band aids for designs and problems that are not well understood and carry the risk of making designs worse rather than better. When things were slower, the tended to do no harm, because the ICs involved were slow enough that the degradation in power supply impedance didn't much matter. That is no longer true. Lee W. Ritchey Speeding Edge P. O. Box 2194 Glen Ellen, CA 95442 Phone- 707-568-3983 FAX- 707-568-3504 I just used the energy it took to be angry to write some blues. Count Basie > [Original Message] > From: zhangkun 29902 <zhang_kun@xxxxxxxxxx> > To: Istvan NOVAK <istvan.novak@xxxxxxxxxxxxxxxx> > Cc: <a.ingraham@xxxxxxxx>; <si-list@xxxxxxxxxxxxx> > Date: 8/30/2004 12:00:11 AM > Subject: [SI-LIST] Re: Decoupling for PLL > > 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 > > > > > > > > > > > > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu