About the test
This standard is part of a set of basic EMC standards that outline test procedures that must be successfully accomplished before products can be sold. The object of this particular standard is to establish a common reference of a product or device to radiated RF immunity cause by any emitting source. Products must be designed and tested to insure that they are immune to both intentional transmitters, such as walkie talkies and cell phones, and unintentional RF emitting devices like electric motors and welders. There are 4 basic test levels which a product must withstand in order to pass this test:
|Test Level||Field Strength|
This standard does not suggest that a single test level is applicable over the entire frequency range, and should be tested for the appropriate test level for each frequency range. Testing for general purpose RF emissions covers the 80MHz to 1000MHz frequency range, and should be performed without any gaps. For digital radio telephones and other higher frequency devices, tests should be performed in the 800MHz to 960MHz and 1.4GHz to 6.0GHz frequency ranges. Tests in these ranges do not need to be applied continuously over the entire range, and the ranges may be limited to specific frequencies for compliance with specific operating bands in the country the product will be sold in.
There are four major changes that were made to this standard in the third version to better measure immunity at higher frequency ranges which are now more commonly present. First and most importantly, the new harmonic distortion requirement of the test setup must be better than -6dBc. This is to minimize distortion at higher frequency ranges. Also, a linearity check must be performed before testing to ensure that the RF amplifier used is not operating in compression. Third, the frequency range was extended up to 6GHz as a result of increased use of these bands by newer electronic devices. Lastly, there is a change to the test table material requirement. This was done because materials like wood can become reflective when exposed to higher frequencies.
Test Equipment Needed
Anechoic Chamber – The chamber must be big enough to house a uniform field with sufficient dimensions to cover the entire EUT.
EMI Filters – Though not always necessary, use of filters should not add any additional resonance effects during the test
RF Signal generator(s) – The use of a signal generator must be able to cover the frequency being tested, and multiple generators may be needed if multiple frequencies are being tested. It must also be amplitude modulated by a 1 kHz sine wave with a modulation depth of 80%. Also, the generator must have a manual control or be programmable with frequency dependent step sizes or dwell times.
Power Amplifiers – this is used to amplify the signal and provide antenna drive to the necessary field level. Harmonics must be at least 6dB below the fundamental frequency
Field Generating Antennas – These can be either bi-conical, log periodic, horn or any other linearly polarized antenna system
Isotropic Field Sensor – This must have adequate immunity to the field strength being measured and use a fiber optic link to an indicator outside of the chamber
Equipment to record power levels – usually a computer system
The tests performed to conform to this standard must be done in a shielded enclosure. This is because many countries have laws prohibiting interference to radio communications, and this can arise if not properly shielded. The facility must also have a barrier between the EUT and the test instruments. This will minimize distortion and limit errors in results. Interconnecting wiring that can penetrate the shielded enclosure is used, and this wiring must properly attenuate conducted and radiated emission as well as preserve the integrity of the EUT signal and power responses. It should be noted that anechoic chambers are less effective at lower frequencies.
Before any tests are performed, the chamber and equipment must be calibrated to make sure they can properly perform the required tests. Calibration will ensure uniformity of the field over the test sample is sufficient to confirm the validity of the results. When a calibration is performed, it is valid as long as the test setup used remains unchanged for testing and should be recalibrated every time the setup is changed. If the setup remains unchanged, calibration should be performed annually. During calibration, the field sensor should be 1 to 3 meters from the antenna, and 16 points of measurement must be performed to validate a uniform field area with a minimum size of 0.5m by 0.5m. The actual UFA should be large enough to cover the EUT. An alternative to this way of calibration is through the use of the constant power calibration method.
Testing should be performed in a configuration which is as close as possible to actual conditions which the EUT will be used. A metallic grounding plane is not required, but the EUT should be placed on a non-metallic, non-conductive material. Low dielectric constant materials such as polystyrene are recommended. Manufacturers wiring and connectors should be used for the tests. If the required wiring length required for the EUT is less than 3m, then the specified length should be used. If the required length is longer, a minimum of 1m of cable should be exposed to the RF field, and excess cables should be bundled in the center of the cable in lengths of 30-40cm.
All testing must be performed under normal operation conditions. The test plan should specify:
• Operating conditions of the EUT
• Whether table top or floor standing
• Type of test facility and position of antenna
• Type of antenna
• Frequency range, dwell time and frequency steps
• Size and shape of the UFA
• Whether partial illumination is used
• Test level to be applied
• Type and number of interconnecting wires
• Performance criteria
To begin the test, the EUT is initially placed with one face coincident with the calibration plane. The frequency ranges that are going be used are swept with signal modulated pausing to adjust the RF signal level or switch antennas as necessary. Step sizes during the test should not exceed 1% of the preceding frequency value. Dwell time should not be less than the time necessary for the EUT to be exercised and respond, and may not be less than 0.5s. Testing should be repeated with the antenna facing each side of the EUT. Also, polarization of the field generated by the antenna means that each side needs to be tested twice, once with the antenna vertically and once horizontally. It is important to make sure that the EUT is fully exercised during testing.
Results should be classified in under one of following definitions:
• Temporary loss of function or degradation of performance which ceases after disturbance ends
• Temporary loss of function or degradation of performance which requires operator intervention for correction
• Non-recoverable loss of performance or degradation