Engineering Specification
ES-3U5T-1B257-AA
EMC Design Guide for Printed Circuit Boards
Frame 26 of 78
Rev. A
10/01/2002
Printed copies are uncontrolled
Table 34. Sample RE Data
Frequency
(MHz)
Current
Electric field
(µV/m)
Electric field
(dBµV/m)
10 6.34
mA 40.7 32.2
30 2.04
mA 45.5 33.2
50 1.15
mA 52.1 34.3
70 0.737
mA 47.5 33.5
90 0.494
mA 52.6 34.4
110 0.331
mA 52.7 34.4
130 0.214
mA 47.6 33.5
150 0.127
mA 37.6 31.5
170 63.7
µA 24.2 27.7
190 17.6
µA 8.4 18.4
Recall from Table 21 in Section 2.3 that the Ford RE limit from 1.8 MHz to 200 MHz is
10 dBµV per meter. Table 34 shows this PCB circuit would fail the RE limit set by ES-
XW7T-1A278-AB spec at all of the spectral frequencies.
The predominant contribution to radiated emissions is due to the so-called Common-
Mode (CM) current flowing in cables attached to an electronic device, and acting as
efficient antennas in the frequency range which is considered (up to 2.5 GHz). The CM
current is simply the net current in the cable. Ideally, this net current should vanish,
because each current that enters the electronic device through the cable, also leaves it
through the cable. Due to parasitic effects, this balance is disturbed and a CM current
results. This CM current determines the amount of radiation because in the balanced case,
the radiated field of each of the different wires in the cable almost cancel each other.
Since only the net current in the cable is important, the cable may be considered as one
single wire carrying this CM current. In automotive electronic devices several hundred
different signals contribute to the overall CM current on attached cables. In order to
estimate the contribution of the different nets, the basic CM current generation principle
has to be understood and two basic mechanisms, i.e. current-driven and voltage-driven
current excitation, need to be considered.
The current-driven mechanism is due to the partial inductance of the return currents in
the ground plane, which produces a voltage drop across the ground plane, and injects the
CM current into the attached cable.
The second mechanism is voltage-driven, because the signal voltage directly drives the
CM current through the parasitic antenna capacitance.
Summary :
Sample RE Data Frequency (MHz) Current Electric field (µV/m) Electric field (dBµV/m) 10 6.34 mA 40.7 32.2 30 2.04 mA 45.5 33.2 50 1.15 mA 52.1 34.3 70 0.737 mA 47.5 33.5 90 0.494 mA 52.6 34.4 110 0.331 mA 52.7 34.4 130 0.214 mA 47.6 33.5 150 0.127 mA 37.6 31.5 170 63.7 µA 24.2 27.7 190 17.6 µA 8.4 18.4 Recall from Table 21 in Section 2.3 that the Ford RE limit from 1.8 MHz to 200 MHz is 10 dBµV per meter.
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