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Design007-May2025

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MAY 2025 I DESIGN007 MAGAZINE 73 Figure 5: Unfiltered noise profile of DVHF2800S DC/DC. e 60-MHz peak has dropped from 67 dBμV to about 55 dBμV on the spectrum analyzer. Know- ing the transfer impedance of my current probe, that's a drop from 350 μA to 90 μA current at 60 MHz. at could easily be the difference between passing and failing an FCC radiated emissions test. …Or Filter It If we look back at the schematic from Figure 1, the 28 V to 12 V DC/DC converter has a switch- ing frequency of about 425 kHz, as seen in its data sheet (Figure 5). e 425 kHz fundamental and many even and odd harmonics up to at least 8 MHz (~the 19th har- monic) are clearly visible. In the data sheet, the manufac- turer recommends using an EMI filter with the converter, and you can see in Figure 1 the schematic follows that advice. It makes a significant difference as Figure 6, from the same data sheet, shows clearly. If you know you'll have high frequency noise in your circuit, it's not hard to mitigate it using board layout best practices, filter- ing, etc. But if you're looking at a board with a DC/DC conver- sion, it's easy to forget that you're introducing signals that are much higher frequency than the DC sig- nals that you want. SMT007 Karen Burnham is chief engineer and president of EMC United, an EMC consulting company based in Denver, Colorado. Figure 6: Filtered noise profile of DVHF2800S DC/DC.

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