10 May 2015

Six Benefits of Pre-Compliance EMC Testing

Dunstan Power, ByteSnap Design looks at the benefits of early EMC testing that is performed on equipment during its development.

We all know that electromagnetic compatibility (EMC) testing is recommended and normally required to support CE marking in the European Union and mandatory when meeting other strict protocols, such as for the FCC in the US. Typically it is conducted at a specialist lab at the end of the project. But what happens when the tests fail? This will hardly come as a surprise, as many projects trip up at this last hurdle, as many as 50% by some estimates, with radiated emissions often being the top reason.

The cost of failing compliance testing is a nightmare scenario for most of us product designers. 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 upon final EMC testing stage, your product fails.

The cost of testing was already high, but re-testing will stretch the planned budget and slow down the entire project. Now the design engineers will need to investigate where the problem is coming from, at a stage in the project when the integration of all the components can make this difficult.

Pre-compliance EMC chamber arrives

Pre-compliance EMC chamber arrives on site

Design engineers that avoid this scenario build pre-compliance testing into a project from day one. In the software testing industry, there is a move to shift testing left and introduce it earlier in the product development lifecycle. Likewise, this thinking is moving to the electronics industry - investigating emissions from your device during each major development stage is the best way to avoid costly re-testing and high failure rates.

There are a number of advantages of pre-compliance testing:

1. Detect errors early error detection to speed up project lifecycle

The earlier product deficiencies are identified in the development process, the easier it is to rectify any shortcomings. Fixing problems after compliance testing is much more expensive and time consuming than fixing them during the design stage. Pre-compliance testing can focus on the areas that you have identified as potential causes for concern and find solutions for them early.

The risk to a design failing is usually relative to how long you delay testing, so designers that leave testing to the project end are completely reliant on the design team’s skill and experience. An analogy I like to use is the process of baking a cake without checking the taste at different phases of the process, but instead throwing it all together; baking it and then finding out you forgot to add enough sugar.

Early analysis of the electronics can also drive system decisions. EMC is not just about electronics, it’s about the system and mechanical changes may be required, such as adding EMI shields, coating boxes or adding EMC foam to fill any leaks/gaps in an enclosure.

Take the example of the design of an Android tablet. This will have multiple features such as LCD, touchscreen, USB, WiFi, Bluetooth and camera. These are positioned around the architecture of the core CPU and memory. Each of these sub sections will emit some form of unintended electromagnetic energy but hopefully all at acceptable levels.

Ideally you would test the core CPU with each section separately, and when a problem occurred, the source would be apparent. In reality, this approach is rarely commercially viable. However, the use of development boards with pre-compliance testing, can do some of this work at a lower cost to allow issues to be highlighted early on and remedial steps taken ahead of the final integration. This is not a substitute for the final testing, but may save some of the pain.

Similarly partially populated boards can be used to isolate problem areas once the target hardware is available.

2. Test products early to compliance standards

Using an anechoic testing chamber before formal testing can determine whether a design will meet relevant compliance standards. The ability to test to EN55022, EN61000 and EN61000-3-2, as well as MIL-STD-461 for emissions provides confidence in your design.

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.

3. Integrate testing into development for more agile projects

Stand-alone pre-compliance testing can be expensive especially if a product doesn’t pass first time, as subsequent rounds of testing will be required after design alterations. However, when testing is integrated into development, a testing chamber and expert advice is available during the entire project lifecycle.

Design engineers that offer EMC pre-compliance testing as part of their services will be continuously on the lookout for areas of risk during product development. For instance, testing during development with evaluation or strip boards will provide the designer with the opportunity to add in preventative measures in the form of additional circuitry, such as signal bead filters, to prevent potential issues.

Test houses typically charge time in half day blocks, but often only a single RF emissions sweep is required to see if a problem is still there or is probably fixed. The maxim “if you haven’t fixed it, it isn’t fixed” can apply to EMC testing as much as software and hardware design. When you come up with a fix, it is good to be able to try it out quickly before committing to a re-spin of a circuit board.

4. De-risk your electronics design project

Early EMC testing can de-risk a project by determining many, if not all, non-compliance issues prior to submitting for formal testing. This is one of the ways the time taken on pre-testing pays back over the course of the project. The end design is much less likely to fail, saving the resulting costs and delays associated with board re-spins and excess test house charges.

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. Pre-compliance testing makes certification an overall less stressful experience.

5. Savings can be made by eliminating over-design

Early EMC testing can save money by reducing over-engineering, so ensuring that a product can pass compliance tests easily. Before a product is tested it is not known where the problems might occur. This can lead to counter-measures being added where they are not required. Countermeasures that will present for the lifetime of the product.

As an analogy, Henry Ford used to send engineers to examine Ford cars in scrap yards to understand which components still had lots of life 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 BOM cost.

As well as this having an electronic BOM cost impact, there is also an impact on the mechanical constraints i.e. the size of the board in all three dimensions. For a very tight design, it is crucial to optimise the EMC filtering, which can be large, at an early stage, as adding filtering in late 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.

6. Other uses of pre-compliance equipment

As well as EMC testing a product when it is first produced, “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. Clearly this depends on the scope and type of the change.

Similarly, 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, at ByteSnap Design, we decided to set up our own testing chamber to allow us to make radiated emissions scans of customer’s products. This provides us with additional ability to eliminate many of the problems prior to formal testing by extending our scope for agile design.

After all, adopting a smarter approach to testing can remove a high degree of risk of product failure, not to mention the savings in the bottom line over the course of a project.

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About the author

Dunstan Power is Director at ByteSnap Design. he is a chartered electronics engineer providing design, production and support in electronics to all of ByteSnap Design's clients. Having graduated with a degree in engineering from Cambridge University, Dunstan has been working in the electronics industry since 1992 and in 2004 founded Diglis Design Ltd, an electronic design consultancy, where he developed many successful electronic board and FPGA designs. In 2008, Dunstan teamed up with his former colleague Graeme Wintle to establish a company that would supply its clients with integrated software development and embedded design services, and ByteSnap Design was born.

ByteSnap Design is a specialist in innovative embedded systems development encompassing hardware and software design with an international client list. The company is a Zigbee Alliance member and Windows Embedded Silver Partner. The team’s experience ranges from electronic design to iOS, Android and Windows mobile app development. ByteSnap Design won Design Team of the Year in 2013 at the British Engineering Excellence Awards (BEEA) and was a finalist for Consultancy of the Year 2011 for its design work on electric vehicle charging posts for the London 2012 Olympic Games, and was highly commended the following year, at BEEA 2012. The company also won European Design Team of the Year 2011 in the Elektra awards. The consultancy also has experience in electronic circuit and microcontroller design, Linux and embedded software development, designing hardware products such as PDAs and smart meters, and software projects such as developing Windows CE board support packages (BSPs) and signal processing applications.

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