I am working on a 25 MHz issue on an embedded ARM board with Ethernet. The strongest offending harmonic is at 150 MHz. See Figure 1.
Figure 1 – The harmonic profile before the capacitors were changed.
I was growing tired of waiting to collect 16000 data points for the 3 positions in the GTEM and was contemplating limiting the scan window to the 150 MHz target for faster debugging when the scan in Figure 2 told me “Bad Idea.”
Figure 2 – The harmonic profile after the capacitors were changed.