Henry Ott Consultants

• Clock rise/fall times that are less than 10% of the clock period (e.g., 1 ns for a 100 MHz clock).
• Apertures (slots, holes, seams, etc.) with a dimension greater than 1/20 wavelength (e.g., 6" @ 100 MHz).  See Frequency - Wavelength Chart.
• Isolated (split) ground planes. With a few exceptions, it is almost always preferable to have a single, low impedance, ground for a system.  See Grounding of Mixed Signal PCBs for more information.
• Slots in ground or power planes. If traces cross these slots, on adjacent layers, the return current path underneath the trace will be interrupted.  See Return Path Discontinuities for more information.
• Shielded cables with the shield terminated with a pigtail (the longer the pigtail, the greater the cable will radiate).
• PCB mounted connectors with metal backshells not directly making contact with the enclosure.
• Paint, or other non-conductive finish, used in the seams of a shielded enclosure.
• Multilayer PCB with signal layers that are not adjacent to a plane (ground or power).
• Multiple value decoupling capacitors (e.g., a 0.1 uf paralleled by a 470 pF capacitor) used to decouple digital logic.  This is not intended to suggest  that you should not also use large value bulk decoupling capacitors, typically 10 uF or greater,  in addition to  the 0.1 or 0.01 uF decoupling capacitors.  See Decoupling for more information.
• Ungrounded regions of copper (copper fill, copper flooding, thieving) on a PCB.
• Unshielded I/O cables without some form of common-mode filtering.
• A single point connection between circuit ground and chassis.

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