| How to Prevent Emission Problems
on Board Level |
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For many, the result of an EMC test comes as an unpleasant
surprise. Large amounts of money and time are spent in
re-designing the product, which causes heavy delays. The
main cause of these additional costs and delays can be
related to the designer's opportunities,
knowledge and desire
to perform an EMC test on the product as early as possible.
In this article, I will give some simple advices and examples
of test methods useful for avoiding undesired emission.
The final test is of course performed on the entire system
or product, but the EMC thinking must be applied already
from the concept stage! |
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Simple rules
To avoid unpleasant EMC surprises, these rules
must be followed:
- Start measuring as early as possible in the design
process.
- Find the source of the problem.
- Correct the problem on component level.
- Perform tests regularly during the design process.
- Perform tests according to existing EMC directives.
- Perform quality tests during manufacturing.
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| Emission pre-test |
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| Previously, designers have only used spectrum
analyzers and near-field probes when searching for emission
sources on board level. This measuring method has been
time-consuming and non-repeatable. Therefore it has caused
large uncertainties about where the source of the problem
is located and how different design modifications affect
the total emission. |
By connecting the spectrum
analyzer and near-field probe to an EMC scanner,
the designer can today build an ideal measuring
instrument for emission pre-test on board and equipment
level. This measuring method makes the pre-tests
simpler and cheaper since the designer himself can
perform the measurements and evaluate the results.
The emission sources can easily be studied graphically,
and the results of different design modifications
can be compared.
|
Emission (64MHz) coming from components on a PCB.
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Design rules
The board design has a fundamental importance
for giving the system or product good EMC characteristics.
The designer must follow these rules: |
- Don't use higher voltage or current than necessary.
- Don't use faster circuit devices than necessary.
- Use short connections on all levels.
- Use properly designed grounding, shielding and filtering.
Avoid large HF-current loops by using decoupling capacitors,
multiple voltage planes, a properly designed zero potential
plane, as well as twisted pairs to keep the signal and
return lines close together. |
| |
Board layout
A proper layout must minimize current loops
while minimizing the inductance of all parts used for
signals with fast rise and fall times. To avoid crosstalk,
sensitive signal conductors must be kept separated. The
power supply distribution must have low induction (multi-layout).
|
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Circuit board (100*160 mm) with a bad ground plane
(40MHz fields). |
Circuit board (100*160 mm) with improved ground
plane (40Mhz fields). |
|
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Logical circuits
When choosing logical circuits, follow these rules:
- Choose as "slow" a clock frequency as possible,
and duty cycles as close to 50% as possible.
- Force unused inputs and outputs to a predetermined
potential.
- Place signal and return lines as close together
as possible.
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Decoupling capacitors
The decoupling capacitor has to protect the circuit
from transients, and this is achieved by:
- Place the capacitor as close to the supply leads
as possible (minimize the current loop).
- Use a capacitor with a size as small as possible.
|
Filters
Use filters to minimize interference on inputs
and outputs to circuits and boards. These filters will
work as a "barrier" between two zones.
|
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Shielding
Shielding is used to "isolate" the emission
from a source by applying e.g. screen boxes and shielding
materials. This method is often expensive and therefore
used only when no other solution is working. However,
you must remember that shielding can cause problems with
temperature as well as with oxidation that might eventually
degrade the shielding effect. |
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The EMC directive
The product must of course be tested for compliance
with the existing EMC directive. What you often forget
after the CE approval is what will happen with the product
when it has been manufactured for some time. Often some
parts of the product are changed (new components or component
suppliers). This might cause the product getting EMC characteristics
different from those it was approved for. In this case,
the EMC scanner can also assist in performing emission
tests to find out if any changes of the EMC characteristics
have occurred.
. |
Conclusion
To make a product as inexpensive and good
as possible, you should consider EMC matters from the
very beginning of the design. Pre-tests should be performed
as early as possible, and problems should be corrected
on component and board level.
Perform EMC test according to existing directives.
Perform tests during manufacturing to ensure that the
EMC characteristics of the product are not degraded after
some time.
"See it before you CE it"
* All measurements and pictures made by EMC-system
DS642, Detectus AB.
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Jan Eriksson, Technical Director
Detectus AB, Sweden.
Telephone: +46-280-41122
Fax: +46-280-41169
E-mail: info@detectus.se
Homepage: http://www.detectus.se
|
Toronto
Toronto
info@interfaxsystems.com
