Is there any additional calculation required to determine the -3dB point for a passive Butterworth filter? (I'm not strong on the heavy formula stuff). I've been using the online calculators to do a 6 pole 400MHz filter (and modelled it in PSpice as well), but when I get the values and proto it up, it's coming up at 336MHz at -3dB. I thought it might be a change in the inductances as the parts were listed as being at that inductance at 100MHz, so I designed and built a 100MHz version, but that came out at 94MHz at -3dB. So am I missing something? Or is there something specific I'm unaware of (I'm new to filters) in regard to needing to use extremely low impedance inductors or something that would cause this issue? Been using 5% parts for both L and C