AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 32

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

AN2321/D
Performing the
radiated immunity test
Severity levels
Uniform field
requirements
Semi-anechoic
chamber construction
Hardware and
software requirements
for field generation
32
This test is usually the longest and most difficult to perform, requiring very
expensive capital equipment and considerable expertise. As with other
immunity testing, pass/fail criteria must be defined by the manufacturer and a
written test plan submitted to the test house. The EUT must be arranged for
normal operation and in the most sensitive mode, while subjecting it to radiated
fields.
Normal operation must be established within the test chamber while exposing
it to the leveled disturbance field as the frequency is swept over the required
frequency range of 80MHz to 1GHz. Some radiated immunity standards
commence at a frequency of 27MHz.
This standard normally requires immunity levels of 1V/m, 3V/m or 10V/m
however equipment specifications may have there own requirements at
particular problem (interference) frequencies. It is in the manufacturers interest
for the product to have an adequate level of immunity to radiated fields.
The new generic immunity standard EN50082-1:1997 calls up IEC/EN61000-4-3
which requires the establishment of a uniform test field over the area occupied
by the test sample. This is performed in an anechoic chamber lined with ferrite
absorber tiles, which serve to dampen reflections and resonances so that a
uniform test field can be established within the chamber. This overcomes the
deficiencies of conventional unlined chambers where reflections and field
gradients can cause sudden and often unrepeatable test failures. (The semi-
anechoic chambers are also ideal for accurate in-door precompliance “non
ambient” radiated emission measurements.)
The semi-anechoic chambers must accommodate the RF absorber on its walls
and ceiling. The mechanical and RF design specifications must accommodate
the very heavy ferrite tiles lining the chamber surfaces. The ferrite tiles are
mounted on a dielectric material and affixed to the chamber surfaces. In unlined
chambers, reflections from the metallic surfaces cause resonances and
standing waves which can produce peaks and troughs in the intensity of the
test field. Field gradients of up to 20 to 40 dB are common in unlined chambers
and this can induce sudden failure modes for what may appear to be a very low
field at the test sample. Chamber resonance results in poor test repeatability
and a high probability of “over testing”. (This may result in the over design of
the product.) These serious deficiencies are eliminated by the field
homogeneity requirements of the new immunity standard IEC1000-4-3.
High power broadband RF amplifiers are used over the frequency range of
26MHz to 2GHz to drive broadband transmitting antennas at a distance of 3
metres from the device under test. Fully automated tests and calibrations are
best performed under software control to allow greater flexibility in the testing
and full control of all key parameters such as sweep rate, frequency dwell time,
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA