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 > > ------------------------------------------------------------------ > 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