Early EMC testing: six tips for successful product development
01 August 2015
Dunstan Power urges product developers to get EMC testing as early as possible in their projects. Failing at the final hurdle can be costly and damaging to the successful outcome of a project, as well as slowing that all-important time-to-market.
To support CE marking in the EU, electromagnetic compatibility (EMC) testing is usually a prerequisite. There are also strict protocols outside Europe where it is mandatory; for example, for the Federal Communications Commission (FCC) in the US.
Generally, EMC testing is conducted at a specialist laboratory at the end of the project – and it is, of course, extremely frustrating when the tests fail. It is estimated as many as half of all projects stumble at this final hurdle, with radiated emissions often being the cause of failure.
For product designers, the knock-on financial effects of failing compliance testing are horrendous. The most traumatic phase comes after you have built a product that meets market demands; you are confident that it radiates little energy and is not susceptible to outside interference - yet at the final EMC testing stage, your product does not pass.
The cost of testing is already high, but re-testing slows down the entire project and frequently leads to budget overruns. Design engineers need to detect the source of the failure at a stage in the project when the integration of all the components can make this problematic. Scheduling pre-compliance testing into a project from day one is the best pre-emptive action to take to avoid EMC failure.
The electronics industry is now reflecting moves in the software industry to introduce testing earlier in the product development cycle. Investigating emissions from a device during each major development stage is a sound approach to avoiding expensive re-testing and high failure rates. There are a number of advantages to be gained from pre-compliance testing; here are six examples:
- Development accelerated by early error detection. Fixing problems after compliance testing is more expensive and time consuming than doing so during the design stage. The earlier product deficiencies are identified in the development process, the easier they are to rectify. Pre-compliance testing can be used to focus on any areas identified as potential causes for concern - enabling you to find solutions for them early. The risk of a design failing is often relative to the time taken to start testing, so designers leaving testing to the project end are totally reliant on the design team’s skill and experience.
To me, this is akin to the process of making a cake without carefully weighing the ingredients beforehand; but instead throwing it all together and baking it - and then discovering you didn’t add enough flour. System decisions can also be driven by early analysis of the electronics. EMC is not just about electronics – it also covers the system and mechanical changes that may be necessary. These could include adding EMI shields, coating boxes or adding EMC foam to fill any leaks/gaps in an enclosure.
- Test products early to compliance standards. Using an anechoic testing chamber before formal testing can determine whether or not a design will meet relevant compliance standards. Specific standards call up specific measurement limits, and these limits vary widely. If you are not testing to the standard that you will ultimately be judged against - EN55022, for example - you may be either over- or under-testing, applying the incorrect frequency range and so on.
A spectrum analyser and near-field probe can be very useful for sniffing out the location of emitters, once they have been identified as presenting radiation above the required limit, but less useful before a calibrated scan at a required distance, has been done. What may appear to be a problem at close range with a probe, can melt away in a chamber and of course the reverse is also true. Testing to a known standard early on focuses attention on real problems.
- Integrated testing = more agile projects. Stand-alone pre-compliance testing can prove costly - especially if a product doesn’t pass first time - as subsequent rounds of testing will be necessary after design alterations. When testing is integrated into development, however, a testing chamber and expert advice are available during the entire project lifecycle.
Design engineers offering EMC pre-compliance testing as part of their services will be continuously vigilant to areas of risk during product development. Testing during development with evaluation or strip boards, for instance, will provide the designer with the opportunity to include preventative measures in the form of additional circuitry, such as signal bead filters, to prevent potential issues.
- De-risk your electronics design project. Early EMC testing can de-risk a project by determining most non-compliance issues prior to submission for formal testing. The chances of the end design failing are greatly reduced, saving the resulting costs and delays associated with board re-spins and excess test house fees. As well as EMC, a chamber can be used to measure comparative signal strength for low power radios to check performance over time, or the effect of modifications.
- Savings generated through eliminating over-design. Early EMC testing can reduce design costs by avoiding over-engineering. Before a product is tested it is not known where the problems might occur. This can lead to unnecessary counter-measures being added; countermeasures that will present for the lifetime of the product. Henry Ford used to send engineers to examine Ford cars in scrap yards to understand which components still had plenty of wear in them due to over-engineering. This helped his engineers to downgrade the specification on these components to achieve a cost saving.
The equivalent can be done with EMC testing to optimise the bill of materials (BOM) cost. There is also a bearing on the mechanical constraints - the size of the board in all three dimensions - in addition to a BOM cost impact. For a very tight design, it is crucial to optimise EMC filtering, which can be large, at an early stage; adding filtering later on, once mechanical tooling is committed, may prove impossible. This is particularly the case with power line filtering using common mode chokes or Pi filters.
- Pre-compliance equipment: further uses ‘look-sees’ can be carried out as obsolete parts are replaced, or board layout changes. As CE marking is a self-certification process, this data can often be used to justify retention of the CE mark by reference to comparative measurements on the original unit. This would, of course, be dependent on the type and scope of any change.
In a similar vein, tests can be carried out on comparative signal strengths of antenna configurations. For example, we recently had an issue where the performance of one of our ZigBee products had markedly decreased on a recent batch, following the move to a new subcontract manufacturer. It transpired that the stack-up had not been followed on the PCB, resulting in detuning and losses. A new and old board were compared to prove the problem.
To minimise risk of EMC testing failure, ByteSnap Design has set up a testing chamber to support radiated emissions scans of customer’s products. This provides the ByteSnap team with additional ability to eliminate many of the problems prior to formal testing by extending our scope for agile design. Adopting a smarter approach to testing can remove a high degree of risk of product failure, as well as generate savings in the bottom line over the course of a project.
Dunstan Power is with ByteSnap Design
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