I've been looking at a few power input circuits and quite a few of them suggest an inductive choke with a large bypass capacitor followed by smaller bypass capacitors.
In theory this is what I interpret the target effect is:
Bypass capacitors provide low impedance at high frequencies, and the inductor prevents current surges from entering/leaving.
Essentially, in using an inductor inline and a decoupling capacitor it's creating essentially a series LC circuit. Now one of the properties of LC circuits is they have a resonant frequency. I modeled a LC circuit with an attached purely resistive load to simulate what the circuit did under different kinds of loads:
VG1 is a voltage source to simulate noise and VM1 is a product of the simulation software for measuring voltages (I'm using TINA-TI).
Here is the resulting Bode plot for this circuit:
As expected high frequency noise is very effectively reduced (if I'm not mistaken, outside the peak region it should decay at 40 dB per decade). However, there is that ominous spike at around 20 kHz where noise is actually being very effectively amplified from the resonant frequency.
Is this typically not a factor when it comes to regulating power to a board? Why isn't the RC filter created using bypass capacitors enough? Is it because the capacitor size required for an effective RC filter is too high? Or am I missing something in my model?
The example recommended power filtering schematic comes from Atmel for their AVR micro controllers.