How to Prevent Emission Problems on Board Level
Posted on behalf of CNRood, active in the BeNeLux & Sweden
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, the author will give some simple advice 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!
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.
Emission pre-test
Previously, designers have only used spectrum analysers 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 analyser (Tektronix RSA306A series real time spectrum analysers) 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.
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).
Logical circuits
When choosing logical circuits, follow these rules:
- Do not use faster digital frequency than necessary. Avoid fast rise time and fall time.
- Force unused inputs and outputs to a predetermined potential.
- Place signal and return lines as close together as possible.
Decoupling capacitors
Example of emission coming from a PCB with different decoupling capacitors.
The decoupling was placed at the microprocessor and the measured frequency was 480 MHz. Lower left corner no decoupling, lower right corner 100 nF.
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 (minimise the current loop).
- Use a capacitor with a size as small as possible.
Filters
Use filters to minimise interference on inputs and outputs to circuits and boards. These filters will work as a “barrier” between two zones.
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.
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 RSE642, Detectus AB.
