AN901 Datasheet, PDF (9/14 Pages) STMicroelectronics – Electromagnetic compatibility

EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
3.4.1.2 Conducted EME Tests
The noise radiated by the microcontroller is caused by the supply current and the output
signal. So, the most significant conducted emission measurements consists of analysing
these signals with a spectrum analyser.
Two probes are used to extract the signal and to adapt the impedance to the spectrum ana-
lyser input.
Figure 1. Ground Current Probe
V_ss
Coax cable (= 50 Ω)
49 Ω
to Spectrum Analyser
1Ω
The 1-ohm resistor is inserted into the main GND wire, i.e. between the power supply, decou-
pling capacitor and pin load on one side and the IC GND and oscillator load on the other.
Figure 2. Output Signal Probe
OSCIN
OSCOUT
V_SS
1Ω
V_DD
100 µF
A good correlation can be found between radiated EME and ground current measurements.
The 1-ohm probe has very good high frequency (HF) characteristics up to 1 GHz. Due to low
signal levels, an amplifier is used.
Output Pin Probe
The HF resistance of wires on application boards is typically in the range of 100-300 ohms.
Therefore, the MCU can be seen as a noise generator connected to a 150-ohm antenna
system. These definitions are taken from standard IEC 1000-4-6. To convert the 150-ohm
board load to 50 ohms, a voltage divider is used.
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