The best way to design a filter is to establish the performance criteria first.
Determine the source impedance Determine the load impedance Determine the attenuation required and at what frequency Determine the roll-off required Now you can determine what order filter you need and whether it is a butterworth or eliptical based on component count and cost. At this point you best use a filter design program such as FILSAP and then characterize it with SPICE. Don't forget that common mode rejection filters are a bit more tricky and require all lines to have a transfer impedance. Last but not least in importance is the topical placement of filters especially critical at high frequency. The closer they are located to the entry/exit point the better the performance. And, return path inductance and length, is not to be ignored. Hans Mellberg Director of Engineering Compliance Certification Services
