|
The manufacturer must supply the test house with a well documented EMC
Test Plan. This will expedite testing, enhance repeatability and minimise
testing costs. This is usually prepared in consultation with the test house.
CONTENTS
Some devices have more susceptible failure modes for particular
disturbances and some operating modes are more susceptible than others. For
example, the testing of a cable (connected to a signal port of a device) for
susceptibility to transients may not identify a susceptibility if that signal
port is not active during the immunity test. The test plan therefore, must
ensure that the device being tested is operated in a such a way that all
critical operating modes are tested for immunity. This will often necessitate
the use of support equipment to allow the device under test to operate as
intended in the test lab. For some devices, it is often impossible to flag or
identify momentary disturbances during the actual tests. In these cases,
special EMC exerciser routines will need to be developed to allow the device to
operate under test conditions and to report or flag disturbances to its normal
operation. Purpose built monitor equipment is sometimes required.
CONTENTS
The EMC Test Plan must also provide a way of speeding up the operation where
the device operates over long cycle times. For example, when performing
radiated susceptibility testing of an alarm system that has a 2 second cycle
time, there is a high probability that some operating modes will be missed (not
tested) if the frequency is scanned at the prescribed maximum rate of 0.0015
decades per second. The chance of detection of a single failure mode is even
lower for ESD event which involves single discharges lasting less than 30 ns.
CONTENTS
There are many important criteria to be considered when deciding on the
test configuration. A thorough EMC test plan should be prepared in consultation
with the test house. There a many factors that can impact on the emission
levels and they include hardware configuration, software configuration, clock
speed, types of interfaces, peripheral devices, bus activity and many others.
* Verification of Normal Operation
Exerciser software, test instrumentation, dummy loads, simulators and
special indicators are often necessary.
* Clock Speed
Higher operating speeds generally result in higher emission levels and
reduced immunity.
* Operating Modes
The different operating modes should be investigated, all parts of the EUT
must be exercised. All input/output ports, displays, peripherals and interfaces
should also be exercised.
* Software/Firmware
A special EMC Exerciser software routine that accesses all ports,
peripherals, memory areas, busses etc can increase the reliability of the
results and reduce the total test time.
* Hardware Configuration
All slots, daughter boards, peripherals are likely to effect the EMC
characteristics, and should be included in the test.
* Interface Connectors
Many products have different types and multiple numbers of interface
connectors so it is important that those that are normally used are connected
for the test.
* Interface Cables
Cables are a major cause of EMC failure so it is essential that they
are fitted for the testing. The use of shielded cables is recommended since
this will greatly increase the compliance rate and the margins. The cable
lengths should be typical of normal usage.
* Cabinets/Cases
The case should be properly assembled and checked for missing screws,
paint overspray and other factors that could effect the shielding or ground
plane requirements.
* EUT Grounding
The method of grounding the EUT can impact on the EMC characteristics
so careful attention is required in the selection of the grounding and earthing
arrangement. Earth connections are made in a manner consistent with the normal
application of the equipment.
| 
