Up to what frequency does radiated emissions need to be tested (CISPR 11 / EN55011)?

In the recently updated EN55011:2025 standard for industrial, scientific, and medical (ISM) equipment, there is a significant change regarding the frequency range for radiated emissions testing.

Whereas before, you often could get away with testing up to just 1 GHz, the new standard requires that you test radiated emissions up to a frequency dependent on the Highest internal frequency of the device under test.

• For devices with a highest internal frequency up to 108 MHz, testing is required up to 1 GHz
• For devices with a highest internal frequency between 108 MHz and 500 MHz, testing is required up to 2 GHz
• For devices with a highest internal frequency between 500 MHz and 1 GHz, testing is required up to 5 GHz
• For devices with a highest internal frequency above 1 GHz, testing is required up to $5f$ up to a maximum of 6 GHz. Examples:
  • For f=1GHz, testing up to 5 GHz
  • For f=1.1GHz, testing up to 5.5 GHz
  • For f>=1.2GHz, testing up to 6 GHz
• If the highest internal frequency is unknown, testing is required up to 6 GHz

Also you need to consider that your device may contain PLL clocks or other frequency multipliers that generate higher internal frequencies than the main clock frequency of your microcontroller or FPGA. These higher frequencies must be taken into account when determining the highest internal frequency of your device.

Source: Table 10 of EN IEC 55011:2025

Definition of Highest internal frequency

The Highest internal frequency is defined in EN IEC 55011:2025, section 3.1.17 as the highest fundamental frequency generated within the equipment under test, including frequencies used only within an integrated circuits.

This definition clearly also includes frequencies that are generated only within an integrated circuit, such as PLL output frequencies that are not available on any external pin of the IC.

Typically one would determine the highest internal frequency by examining the technical reference manual of the relevant integrated circuits used in the device under test, such as microcontrollers, FPGAs, clock generators, etc.

Harmonics do not need to be considered, only fundamental frequencies.

If you are unsure about the highest internal frequency of your device, you can try to verify the frequency content of the device using a spectrum analyzer and an E-field near field probe near to the die of the relevant integrated circuits. However, this will also pick up harmonics, so you need to be careful to identify only fundamental frequencies, and correlate the results with the documentation.