I'm currently working to quiet down the emissions from a stepper motor drive circuit.
The motor drive frequency is 22kHz, and there is significant 60MHz noise that comes ONLY from the 36V side as motors are driven or simply held in position. So far I've tried adding 1 Ohm gate resistors which caused some terrible shoot-through, and notably still left a big "hump" around 60MHz even though it reduced the 2.2MHz-spaced harmonic "spikes". I'm trying to approach the problem with an LC circuit at the output.
Unfortunately both sources suggest an inductor that's ~1/10 the motor's own inductance (about 160uH in this case), and the biggest I had laying around was 33uH. I used a 14pF cap for a cutoff frequency around around 220kHz, and was rather disappointed to see the spectrum change (fewer spikes) but still stay over the limit at 60MHz. I would have thought a cutoff frequency that far away from the problem area would have more of an impact.
This all leads into my question - how does the inductor factor into reducing the EMI from a motor? Simply by smoothing out the ripple current in the motor, and my inductor couldn't smooth it out enough? Are there more resources I can use to understand this better?