20
Switching power supplies
Displays
Outside the box:
Areas of the enclosure that are close to PCB trouble spots
Any gaps or openings in the enclosure; vents; areas near buttons, jacks
or displays; cables going through such openings
All cable connectors and all cords normally connected to the product.
In general, at frequencies below 200–300 MHz, undesirable emission is
produced by cables connected to or extended from the product. At higher
frequencies the sources typically lie inside the product. Often, excessive
emissions radiate from a cable even though the cable itself doesn’t generate
the signal—it just acts as an antenna. Pay special attention to how the EM
signals get to the cable.
During the Design Phase
This is the time to ensure agency compliance for your product at the lowest
cost and with minimum impact on your schedule. The 3M
™
ScanEM-C
Electromagnetic Near-Field Probe can help you do the following:
Check your first fully or even partially working PCB assembly for “hot”
EMI areas. Look at the schematic and the PC layout to see whether
anything can be done: shorten the traces, buffer some lines with resistors,
If nothing can be done to a PCB area in the way of prevention, an
EMI map of the board should help mechanical engineering design the
enclosure in such a way that it provides EM shielding right where it is
needed. This alone can save your company thousands of dollars in a
corrective redesign later.
