EMI/EMC Conducted immunity, using BCI probes

EN 61000-4-6 specifies stress levels in terms of voltage and calibrates the immunity set up using a 300 Ohm loop. Other standards specify stress levels in terms of injected current.

EN 61000-4-6 300 Ohm loop BCI calibration set up using EMCview PRO

• Test Level 1: 1V RMS, 1 kHz / 80% AM modulated
• Test Level 2: 3V RMS, 1 kHz / 80% AM modulated
• Test Level 3: 10V RMS, 1 kHz / 80% AM modulated
• Test Level X: special (unspecified)

The specified test levels are open circuit voltages at the output of the calibration fixture.

The voltages measured during calibration must be multiplied by 3 to allow for the 50 Ohm to 150 Ohm conversion and again by 2 to reach the open circuit voltage.

Hence, the measured voltage at the output of the 150 Ohm to 50 Ohm adapter is 1/6th the voltage of the desired stress level. In logarithmic figures, the measured voltage is 15.5 dB lower than the open circuit voltage.

Example: Test level 2 U0= 3V RMS open circuit voltage at the output of the calibration fixture = 3V/6 = 0.5V RMS at the output of the 100 Ohm to 50 Ohm adapter.

The required power at the output of the modulated amplifier is:

U0/6 [dBm] + BCI probe insertion loss + 6 dB (Attenuator) + 5.1 dB (80% AM RMS)

Assuming 16 dB insertion loss for the BCI probe in a 300 Ohm loop, the required RF power is:

7 dBm + 16 dB + 6 dB + 5.1 dB = 34.1 dBm = 2.6 Watt

During calibration, EMCview Pro will make a stepped sweep, for each frequency adjusting the tracking generator level in order to achieve 7 dBm -30 dB = -23 dBm at the spectrum analyzer input. The tracking generator level table will then be used to set levels during the actual immunity test.

EN 61000-4-6 immunity test set up using a BCI probe and EMCview PRO

ISO 11452-4 / MIL-STD-461 / DO 160A calibration set up using EMCview PRO

The automotive immunity standard ISO 11452-4 as well as immunity standards for defense and avionic products specify stress levels in terms of induced disturbance current.

Traditional calibration and test methods use current monitoring probes in order to create a control loop, thereby maintaining the required disturbance current. Levelling loops can be fooled by cable resonances. High impedance resonances cause the RF amplifiers output power to raise, in order to achieve the specified disturbance current. Similarly, low impedance resonances may significantly reduce the RF amplifiers output power. This situation is unrealistic of most real-life electromagnetic environments and could create a severe over-test or under-test situation.

In order to get around the over-test / under-test problems associated with the closed loop test method, the standards offer the substitution method (open loop), where the disturbance current is set by measuring current induced into a 100 Ohm loop. Forward and reflected power is measured using a directional coupler while setting the RF power to achieve the required disturbance current levels into the 50 Ohm load. During actual immunity test, the forward power level is set according to the value recorded during calibration.

Substitution calibration method set up for pre-compliance purpose:

Example: Test level I – 60 mA

A power of 22.6 dBm is required to achieve a current of 60 mA in a 50 Ohm load.

Assume 8dB insertion loss for a typical BCI probe in a 100 Ohm loop.

Assume 1 dB insertion loss for the directional coupler.

The required power at the output of the modulated amplifier is:

Test level [dBm] + BCI probe insertion loss + 6 dB (Attenuator) + 5.1 dB (80% AM RMS)

22.6 dBm + 8 dB + 1 dB + 5.1 dB = 36,7 dBm = 4.7 W

Example: Test level IV – 200 mA

A power of 33 dBm is required to achieve a current of 200 mA in a 50 Ohm load.

Assume 8dB insertion loss for a typical BCI probe in a 100 Ohm loop.

Assume 1 dB insertion loss for the directional coupler.

The required power at the output of the modulated amplifier is:

Test level [dBm] + BCI probe insertion loss + 6 dB (Attenuator) + 5.1 dB (80% AM RMS)

33 dBm + 8 dB + 1 dB + 5.1 dB = 47.1 dBm = 51.3 W

ISO 11452-4 / MIL-STD-461 / DO 160A immunity test set up using EMCview PRO

Refer to the standards for detailed drawings of the corresponding test set ups