A couple quick considerations to get you started. Pi filters (and even to some extent L filters) constructed from either inductors or ferrite beads have some non-obvious problems you need to consider. The problem stems from the fact that the filter is being driven from a very low impedance power source (in the milliohm to 10's of milliohm range most likely). This causes the typical filter to have a very peaky response instead of a nice low-pass roll-off. 6 to 10 dB peaking isn't uncommon. This means that whatever noise is on the input of the filter may actually be 6 to 10 dB higher at the output at the frequency of the filter peaking. Insertion of a small amount of resistance (say 1-3 ohms) in series with the inductor will usually tame the effect. (beware of the DC drop caused by the series resistance). Sometimes your inductor will have enough intrinsic resistance to make the filter well behaved. Ferrite beads sometimes have enough loss to squelch the peaking, but usually not (they have low DC resistance, but have lots of AC loss, but usually not at the frequencies the peaking taks place (in the kHz to 100's of kHz range). You need to model the filter using spice to do numerous what-if cases to decide which way to go. (this means you need decent spice models of your inductor and/or ferrite bead including both DC and AC loss as well as parasitics). Also note that if you are interested in the high-frequency bounce back characteristics you also need to include the parasitic inductance of your capacitors in the model as well (both cap inductance and mounting inductance.) You could also try to synthesize an unequal termination Butterworth filter (say Rin = 10 mOhms, Rload = 20 ohms) but you will probably find you need an inductor of several hundred mH and capacitors in near the Farad level (not to practical, but has a nice roll-off when simulated :) (the ratio of Rin to Rload as well as the desired low knee frequency is what kills you) Enjoy! -Ray Anderson Sun Microsystems Martin Euredjian wrote: > > Not sure if this is OT for this list... > > In isolating an analog (PLL, VCO, Amps, etc.) section from an otherwise > high-speed digital board I've encountered recommendations to build the usual > PI filter using an inductor (about 10uH) in one case and a ferrite bead (100 > to 300ohm) in the other. > > What are the merits of each approach? > > Thanks, > > =============================== > Martin Euredjian > eCinema Systems, Inc. > =============================== . ------------------------------------------------------------------ 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 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