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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

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

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.

1

Ω

49

Ω

Coax cable (= 50

Ω)

to Spectrum Analyser

100

µF

OSCIN

OSCOUT

1

Ω