AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 33

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

Pyramidal absorbers
Ferrite tile absorbers
Difficulties in radiated
immunity testing
Personnel safety
concerns —
non-ionizing radiation
MOTOROLA
modulation and field intensity. Software hooks allow synchronized monitoring
and stimulus of the EUT functionality. Interactive functions are desirable so that
real time changes in both the EMC test software and the EUT parameters can
be performed during the actual test. This user access feature allows rapid
logging of all test data for efficient evaluation and analysis of the EUT EMC
performance.
The traditional pyramidal (cones) absorbers are effective, however, the large
pyramid dimensions leave an unacceptably small usable space within the
chamber. The length of the pyramidal absorber should be in the order of 100cm
for a lower frequency of 80MHz, while lengths of over two meters are required
for operation at the lower frequency of 26MHz. Clearly, the usable space inside
the chamber is severely reduced. The pyramidal absorbers also have the
disadvantage that they are fragile, easily damaged by bumping and also highly
flammable. The use of these absorbers on the floor of the chamber is also
impractical. Field strengths of over 200V/m sustained for extended periods of
time result in a high risk of fire due to the heating of the pyramidal absorber.
Ferrite tiles are space efficient, however, they add a considerable weight to the
chamber roof, walls and doors and consequently, the mechanical structure of
the chamber becomes significant. They operate effectively at low frequencies
however they become relatively in-effective at frequencies above 1GHz.
Ferrite tiles are very compact (100mm by 100mm by 6mm thick) and withstand
field strengths of over 1,000V/m without the risk of fire hazards.
There are inherent difficulties in performing radiated susceptibility tests since
the support equipment used to operate the EUT, provide stimulus signals and
to monitor its performance must itself be immune to the susceptibility fields.
This can often present difficulties, particularly when the support equipment is
complex and requires many cables and interfaces that must connect to the EUT
via penetrations through the shielded test chamber. All cables penetrating the
test chamber must be shielded and/or filtered in order for them to be immune
to the radiated test fields and to prevent the degradation of the shielding
performance of the test chamber. Compromising the shielding performance of
the test chamber will result in the unintended radiation of the test fields into the
environment and this may cause interference problems to spectrum users. RF
filtering of data or signal lines is not always possible if there are high numbers
involved or when high speed data links are used. RF shielding of the test
equipment and interface cables can be difficult to achieve, even when using
shielded interface cables since the EUT configuration does not always end
itself to the maintenance of an effective RF shield.
Electromagnetic fields within the test chamber may exceed the recommended
safety limits for exposure of personnel so an operator cannot be in the test
chamber to monitor the status or performance of the EUT. One solution is to
use a remotely controlled EMI hardened closed circuit TV system.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
APPENDIX B: IMMUNITY MEASUREMENT STANDARDS
AN2321/D
33