CME136686LX Hardware Manual - RTD Embedded Technologies ...

EMI Management
Electromagnetic interference (EMI) is an undesired disturbance that affects an electrical circuit and is due to
radiation emitted from nearby electrical components or signals with rapidly changing level transitions. Effects
from a noisy source can sometimes be seen on signals that transition infrequently or on analog inputs where any
slight deviation on an input affects the quality of data read by the system. To improve both the levels of onboard
radiation and simplify the design of offboard filtering circuits, RTD has designed the CME136686LX cpuModule
with synchronized power supplies as well as components that support optional spread spectrum modulation.
Power Supply Switching Synchronization
In a system that has multiple high speed signals operating at different frequencies, unwanted frequencies called
beat frequencies can appear in the frequency domain. Beat frequencies are a mathematical combination of two
different frequencies. To reduce the impact of system-level beat frequencies, the cpuModule's power supplies
have been synchronized to one switching frequency. This reduces the possibility of noise during data acquisition
by having fewer combinations of different frequencies in the system.
Synchronizing the power supplies also inherently simplifies EMI filtering. A data acquisition system can
eliminate EMI by filtering known frequencies that are causing unwanted signal interference. Filtering the EMI
source in a system with more high frequencies is more challenging than filtering on a system with fewer high
frequencies.
Spread Spectrum
Another method to manage EMI is to reduce the peaks of onboard frequencies. One method of doing this is by
enabling spread spectrum. Components that support spread spectrum have the ability to reduce the intensity
of an onboard frequency by dithering, or lowering the frequency’s amplitude by modulating the frequency of a
signal over a wider frequency range.
The clock generator on the CME136686LX optionally supports spread spectrum. This feature can be enabled or
disabled in the BIOS. When enabled, the frequencies of the onboard PCI clocks are modulated.
Similarly, the onboard power supplies support spread spectrum, which may also be enabled or disabled in the
BIOS setup.
BDM-610000065 Rev C Chapter 4: Using the cpuModule 83