In the IEC 61000-4-6 CS test, test voltage strength levels such as 1V / 3V / 10V are defined. However, whether these voltage levels—i.e., immunity test voltages—are referenced at the "source" of the generator or at the input of the CDN (after the 6dB attenuator) is unclear. Both interpretations are reflected in the regulatory diagrams, leading to confusion. Here, we clarify and share insights to help EMC colleagues establish a correct understanding. It is recommended that this regulatory ambiguity be proposed for amendment at the annual IEC meetings.

Immunity Strength Requirements and Corresponding Voltage U₀ Table 1 below shows the immunity strength levels and corresponding voltages (U₀). In the standard, some diagrams mark the voltage U₀ at the generator's source, while others define it before the 6dB attenuator (as shown in Figures 1 and 2). This leads to a potential twofold discrepancy in the immunity test voltage.

Table 1

Figure 1

Figure 2

In Figure 2, markings similarly mislead engineers into thinking U₀ refers to the voltage before the 6dB attenuator.
On the other hand, some regulatory diagrams define U₀ after the 6dB attenuator, such as Figure 3 and Figure 4(Here T2 represents the 6dB attenuator.)

Figure 3

Figure 4

Interpretation of the Regulation Text and Conclusion

Analyzing the regulation text and formulas shown below, it is evident that the immunity test voltage is defined after the 6dB attenuator (because the formulas do not account for the 6dB attenuator). The 6dB attenuator is included in the test setup mainly to prevent damage to the generator from reflected waves. You can use a 6dB or even a 3dB attenuator：

Laboratory Validation Method

1. Directly select the internal calibration file for 150kHz/10V output (set to unmodulated mode), and measure the 150kHz VRMS using an oscilloscope before the 6dB attenuator to quickly confirm the source voltage. (If U₀ is 10 VRMS, you should measure 20 VRMS here.)

2. The standard voltage at this point should be 20 VRMS (±1.5dB).

3. Then select the internal calibration file for 200kHz/10V output (still unmodulated mode) and again measure the 200kHz VRMS before the 6dB attenuator. The voltage should still be 20 VRMS (±1.5dB).

4. Subsequently, you can verify measurements after the 6dB attenuator and after the CDN to understand the characteristics of both.

5. The same principle applies when using spectrum analyzers for measurement.