Jack, The often mis-used concept of service radius of capacitors dates back to the days when there were hardly any planes on the PDN, most chips were fed with wide traces and capacitors were placed as close to the device pins as possible. To answer the question "How close the capacitor should be?" under these circumstances, the service area concept might give some useful insight. With planes and multiple capacitors I find this concept often very misleading. It might be more useful to think in terms of extreme-terminated transmission lines. Imagine that you have a dead short at the location of the capacitor and you connect this short to the observation point (pin of chip) with a transmission line, formed by the trace or the plane shape making the connection. The pin of the chip will see the input impedance of the shorted transmission line, which, if the line is electrically shorter than quarter wave (or odd-integer multiples, but we dont want to go there unless our PDN has to service known comb-line noise spectrum), will be inductive. The inductive input impedance is proportional to the electrical distance through the tangent function, so the farther we place the capacitor, the higher the inductance becomes. Dependent on where you may loose effectiveness of bypassing, you can then draw a conclusion of the 'service area' or how far you can place the capacitor to still do something useful. Instead of using ideal short, you can also modify the transmission line expressions to include the realistic impedance of the capacitor. Though it discusses laminates, the same reasoning can be found in more detail in http://www.electrical-integrity.com/Quietpower_files/Quietpower-16.pdf Regards, Istvan Novak Oracle On 8/27/2013 3:52 AM, Jack Si wrote: > Hi experts, > I read from an application note that the effective radius of the capacitance > if 0.005*lamda. lamda is the actual wavelength of the capacitor's resonance > frequency.i.e, 2pi*vp*sqrt(LC). where vp is the propagation velocity. From > this equation, i infer that the inductance(ESL) and capacitance are directly > proportional in square root. i.e, radius increase with the increase of ESL or > C. > > > But in paper "Effective Decoupling Radius of Decoupling Capacitor" i found it > is inversely proportional. Please suggest me where i miss the way. > > > Thanks and Regards, > > Jack > > ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu