How many of you have beat down a harmonic at one end of the spectrum, only to have an otherwise low harmonic rise up above the limit at the higher end of the spectrum? This is often termed the “ballon effect”, where squeezing one end of a balloon makes it expand at the other end. This is usually due to board resonances within the PC board itself.
I recently received an interesting observation from fellow EMC consultant, Mike Farnet
, following an experiment he performed on reducing the emissions from a client’s embedded ARM processor board with Ethernet. There were strong 25 MHz harmonics from the PHY circuit, as is usual for these low-cost boards. The original harmonic was peaking strongly at 150 MHz. Here is his discussion:I use a 5407 EMCO GTEM and a Rigol DSA-815 TG+EMC spectrum analyzer. I use LabView to collect the data from my spectrum analyzer.
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.