IEEE Std 299: Measuring the Effectiveness of Electromagnetic Shielding Enclosures
IEE 299 provides uniform measurement procedures for determining the effectiveness of electromagnetic (EM) shielding enclosures. The effectiveness is tested at frequencies from 9 kHz to 18 GHz. This range can be extendable down to 50 Hz and up to 100 GHz.
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As previously mentioned, IEEE determines the effectiveness of devices tested at frequencies from 9 kHz to 18 GHz. The standard states that the owner of the shielding enclosure should provide the frequencies at which the shield should be tested. They should also provide the frequencies and shielding effectiveness (SE) limits for pass/fail.
This standard suggests a range of test frequencies that would provide very high confidence in the effectiveness of the shield. The IEEE electromagnetic shielding standard dose not define SE limits for pass or fail
Ultimately, IEEE-299 has three primary objectives. First, this standard provides a standard procedure for the measurement of the effectiveness of shielded enclosures, in a broad range of radio frequencies. This includes a minimum set of recommended frequencies.
Second, this shielding effectiveness EMC test standard provides Identical procedures applicable to frequencies other than the standard sets. Lastly, IEEE299 provides an optional measurement technique to detect the nonlinear behavior of high-permeability ferromagnetic enclosures.
The measurement procedures provided in this standard apply to any enclosure having a smallest linear dimension greater than or equal to 2.0 meters. Separate methods should be used for enclosures with any dimension smaller than 2.0 m. In the case of enclosures that are to be used in anechoic or semianechoic applications, this procedure applies prior to the installation of any RF absorber materials.
To understand the requirements of this standard, an engineer must understand the terminology. Included in the following paragraphs are definitions to some of the commonly used terms in this shielding effectiveness testing standard.
An accessible test location is a location that can be reached by a test antenna or probe without modifying a parent structure. A dynamic range (DR) is the range of amplitudes over which the receive system operates linearly. For a measurement, the DR is the difference between the reference level and the minimum discernable signal above the noise floor.
The minimum discernable signal is defined as one with an amplitude of 3 dB or more above the test system noise floor. This is what should be verified during the DR validation step of the SE procedures. This represents the maximum SE measurable at that frequency with that particular equipment and settings.
The local source is an emitter located close enough to a shielding enclosure for its EM energy to illuminate only a localized portion of a shielding face. The effect is assessed by choosing the poorest performance in the set of measured locations. A parent structure is a permanent enclosure or outside housing that contains the shielding enclosure.
Shielding effectiveness (SE) is defined as the ratio of the signal received from a transmitter without the shield, to the signal received inside the shield when the shield is placed between the transmitting antenna and the receiving antenna.
A shielding enclosure is a structure that protects its interior from the effect of an exterior electric or magnetic field. Conversely, it also protects the surrounding environment from the effect of an interior electric or magnetic field.
A high-performance shielding enclosure is generally capable of reducing the effects of both electric and magnetic field strengths by one to seven orders of magnitude depending upon frequency. An enclosure is normally constructed of metal with provisions for continuous electrical contact between adjoining panels, including doors.
As experts in shielding effectiveness compliance testing, Keystone Compliance can meet any testing need. Our EMC and shielding experts provide guidance throughout the test program. Their consultative approach differentiates us from other test labs in the country. This one of the reasons we are regarded as the best shielding effectiveness test lab.
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