Common Mode Radiation; The Vx Ripples With/Without Gate Resistors - Intel Quark SoC X1000 Design Manual

Figure 57. The Vx Ripples with/without Gate Resistors (Left: without gate resistor/
Right: with gate resistor)
Besides the 50 to 300MHz EMI noise, the hundreds kHz switching noise can result in
signal integrity issues if the noise is coupled to IO nets in proximity. It is critical to trap
the noise within the input loop and minimize the noise propagation. Several design
methods can be tried to reduce the switching noise (ripple): increase the total input
capacitance value, mix the power and Vss via at the phase node, and minimize the size
of the phase noise.
17.3.3

Common Mode Radiation

Common mode radiation happens when current travels through an IO cable or when
signals are coupled to a heat sink (heat spreader) or power/ground planes. Cable,
metal planes, and heat sink then become antennas that radiate. If the antenna is
electrically small, it can be an electric dipole (for example IO cable or emission below
300 MHz). Its radiation intensity is proportional to the length of the antenna and signal
frequency. However, the intensity becomes the maximum when the signal frequency
reaches the antenna resonant frequency. Heat sink radiation and power/ground plane
resonances are examples. Heat sink radiation and power/ground plane resonances are
catching more attention recently because of the increasing signal speed (frequency).
Another example of common mode radiation is due to the signal timing skew on a
differential line. The skew causes the unbalance of the currents traveling on the traces.
As a result, a common mode current is induced and the emission becomes much
higher. Besides the timing skew, any unbalance designs in the PCB including the non-
homogeneous PCB material such as FR-4 can result in common mode current on a
differential pair. Therefore, differential routings, especially USB2, should follow the EMC
design rules mentioned in the following sections.
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Intel Quark™ SoC X1000
PDG
112
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Intel Quark™ SoC X1000—Electromagnetic Interference
June 2014
Order Number: 330258-002US