Radiated Immunity:

Radiated Immunity is a characteristic of a device to withstand external electromagnetic interference and continue to operate normally as intended.  A device can malfunction when subjected to Stray EMI radiations generated by nearby devices thus it must remain immune to such radiations to function normally. For example, a laptop connected to a local Wi-Fi should function smoothly in the presence of EMI radiations generated by a mobile phone during a phone call initiated and a speaker playing songs from the iPod connected via Bluetooth. 

What is Radiated Immunity Testing?

It is a test to assess the ability of an Equipment under Test (EUT) to withstand radio emissions from surrounding devices. It determines the radiated susceptibility threshold value of the device below which the device can operate safely. To elaborate, the test verifies if the EUT will operate as intended when subjected to a simulated RF noise level (equivalent to the actual operating environment), and evaluates the performance of the cables and internal circuitry of the EUT in an exposed electromagnetic environment. If the EUT operates as intended, then it passes the compliance test.

What is the Standard for Radiated Immunity Testing?

The IEC 61000-4-3 is an EMC standard by the International Electrotechnical Commission on the immunity requirements such as test procedures and test levels of electrical and electronic equipment to radiated electromagnetic energy. The tests are normally performed in the frequency ranges 800 MHz to 960 MHz and 1.4 GHz to 6 GHz. The frequencies or frequency bands to be selected for the test are only applicable where mobile radio telephones and other intentional RF emitting devices operate.

Test levels related to general purpose, digital radio telephones, and other RF emitting devices are given as follows:

“X” is an open test level and the associated field strength may be of any value. This level may be given in the product standard.

What is the Equipment Required for Radiated Immunity Testing?

Following are the test equipment required for the standard:

• Anechoic chamber
• EMI filters
• RF signal generator(s) capable of covering the frequency band of interest and of being amplitude modulated by a 1 kHz sine wave with a modulation depth of 80%. They shall have manual control (e.g., frequency, amplitude, modulation index) or, in the case of RF synthesizers, they shall be programmable with frequency-dependent step sizes and dwell times.
• The use of low-pass or band-pass filters may be necessary to avoid problems caused by harmonics.
• Power amplifiers
• The field generating antennas: Biconical, log periodic, horn, or any other linearly polarized antenna system capable of satisfying frequency requirements.
• An isotropic field sensor
• Associated equipment to record the power levels necessary for the required field strength.

What is the Test Procedure for Radiated Immunity Testing?

Radiated susceptibility testing analyzes the field levels in the environment up to which the device continues to operate normally, the radiated electromagnetic field coupling with cables, and the radiated electromagnetic field coupling with internal circuitry along with the effectiveness of the enclosures and EMI shielding in mitigating radiated electromagnetic energy.

The complete test is performed inside an anechoic chamber. A simulated electromagnetic field is transmitted to the EUT externally using an antenna. The currents induced in the simulated EMF under the externally induced electromagnetic environment are proportional to the radiated field strength. If the resultant test values of the EUT are greater than the acceptable limits, then the device receives a negative test result.

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