I have a design for a DC-AC full H-bridge inverter implemented with IGBTs that is functioning quite well, but there is one problem that I am having trouble solving. I am seeing spikes in the FFT of the output noise floor that appear to be subharmonics of the PWM frequency. I am measuring the noise floor by setting the AC output of the inverter to 0V, and I calculate the FFT of the 0V output. See the image below for an example of my noise floor with apparant subharmonic spikes. The fundamental frequency of the PWM for this system is 18kHz. Notice in the image above that there are strong peaks at 18kHz, 36kHz, 54kHz, and 72kHz. These spikes are expected because they are the fundamental frequency and the harmonics. Also notice the peaks at 2.25kHz, 4.5kHz, 9kHz, and 13.5kHz. 18000/8 = 2250 It appears that some of these noise spikes could be subharmonics of the 18kHz fundamental, but they could also be regular harmonics of the 2.25kHz subharmonic. I have done a lot of testing and I am pretty confident that the problem is with the PCB I have designed or the problem is with the IGBT driver modules (mounted on this PCB). This PCB distributes the PWM signals to the IGBT driver modules, and it has a microprocessor and supporting circuitry to perform system monitoring and data collection. I still have not been able to determine the exact source of this noise. I also have not been able to determine how it is getting into my PCB. Any suggestions on how to clean up these subharmonic noise spikes would be appreciated. Any suggestions for how to determine what is transmitting this noise, or how the PCB is receiving this noise would also be helpful.