Whoever is saying such things, hasn't been saying them to me. I think you should pose your question to them. If your concern is noise transmission through the PDN, this varies substantially depending on where you look and what excites the PDN. From a power delivery standpoint only, you care about PDN noise up to the cut-off frequency of any given IC package as mounted on your PCB. This can be as low as a couple of MHz, or as high as hundreds of MHz depending on the power supply and IC of interest. A core logic power connection may well be oblivous to anything above 10 or 20MHz, while I/O can be sensitive to 100's of MHz. It all depends on the specifics of the IC. Many power supply manufacturers specify noise and ripple as measured through a 20MHz single pole LPF. It's convenient for them, but as Istvan Novak has pointed-out, allows some really ugly supply outputs to advertise much cleaner looking specifications. If this is a concern, and you are not in a position to extract / impose a wideband specification from your suppliers then you can either: periodically test and qualify incoming product, or incorporate a 20MHz LPF into your design. The former approach has the disadvantage of what to do with the nonconforming supplies. As far as the vendor is concerned the supplies work to spec and they have no reason to take them back. You also have to worry about what you are going to use to get product out the door. The latter approach has the advantage that you can rely on the manufacturer specs as-is. Often this filter function can be realized with a single low inductance capacitor like an X2Y(r) mounted close to the power supply connection to the PCB, and attention to local etch. A further advantage of this type of approach is that the filter is bidirectional. The filter attenuates high frequency load noise feeding through back to the power supply input where in many cases it is a potential EMC issue. Steve icer world wrote: > hello all: > When test the ripple of the power of a board ,It's said that I should test > it below 20MHz.But I found that it's much defferent when I test the ripple > in the full band width frequency.As I think we should test the ripple without > any frequency limit ,or we can't observe the noise produced by other devices > which can couple into the power system .I hope someone will give me some > !advice,thanks > > > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.net > > 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 > > > > -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (866) 675-4630 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------------------------------------ Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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