by Dunstan Power, ByteSnap Design
Adopting pre-compliance EMC testing removes the risk of product failure and avoids costly re-testing after design.
Electronic products, such as home appliances, computers, tools and wireless devices, need to be tested for EMC (electromagnetic compatibility). Across the globe, regions and countries have their own standards, such as the CE certification in the European Union or the FCC’s standards in the U.S.
Testing is normally undertaken at the end of a project at a specialty lab. But when tests fail—what then? This is more common than you might think; it’s estimated as many as half of all projects fall at this final hurdle, with radiated emissions often cited as the main stumbling block.
For product designers, the cost of failing compliance testing is a nightmare scenario. 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 during the final EMC testing stage, the 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.
Design engineers can avoid this scenario by building pre-compliance testing into a project from day one. In the software testing industry, there is a move to shift testing 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.
What EMC testing actually entails
Complete EMC testing involves a number of different tests, described below:
Radiated emissions testing: This is carried out in a chamber or open field site. An EMC test receiver scans through frequencies up to a multiple of the highest clock frequency used in the device, recording the signal strength at each step. The test is designed to check for unintentional emissions that exceed a given pass band defined by the class of product.
Typical classes are for industrial and for residential/commercial. The emissions criteria are more stringent for the latter; the device needs to be quieter. These scans can be slow, particularly if the board has high frequency components on it, as the scan has to run up into the GHz frequency range with a dwell at each step. If you’re wondering where the time goes in EMC testing, this is one of the main areas.
Conducted emissions testing: Where the device has long cables attached or operates from the mains, those cables are passed through an LISN (line impedance stabilization network), RF current clamp or artificial mains network, and, again, frequencies scan to search for unintentional emissions. The frequency range is much shorter than for radiated emissions (say, up to 30 MHz).
Radiated immunity: This is the reverse of the radiated emissions test and is carried out in an EMC chamber. Here, the UUT (unit under test) is subjected to a radiated field, which, again, scans through the required frequency band, from 30 MHz up into the GHz range. The UUT needs to be operating so that the tester can see if there are any failures at any frequency. This is slow and may require a lot of concentration from the tester, depending on how a failure would be spotted. For instance, at the most basic, you could be staring at a monitor attached to a camera in the chamber. The industrial field strengths are higher than commercial, so if your device was required to be quieter (commercial), it also isn’t required to operate surrounded by the same level of interference, and so it is easier to pass the immunity testing.
Conducted immunity: If appropriate, the cables are subjected to conducted noise, scanned across the frequency band. As for the radiated immunity test, the device needs to be operating so that failures can be spotted.
ESD testing: ESD (electrostatic discharge) testing is usually carried out last, as it can be destructive. The UUT is subjected to air and direct discharges to any exposed contacts and to the case itself. The device should be in an operating state and should recover to that same state, or be unaffected, by the discharges.
Other tests: There are many categories of device and environment, such as commercial, industrial, military, medical, rail and automotive. Consequently, there are many specialized tests carried out that are just called up for specific device types. For instance, for mains power devices, and those with long cables, fast transient tests and surge tests, both of which can be destructive, can be carried out. They may be simulating lightning strikes or other mains transients. For automotive devices, there are also extended power tests, such as load dumps and high and low voltage tests, simulating such events as a battery being disconnected while the engine is running or the battery being reversed.
In our experience, the radiated and conducted emissions tests are the ones that fail most often, they are also time consuming tests, where if there is a failure, working around it and retesting is often a slow process. While we have seen failures in all of these categories, they are not all equal in terms of time to fix the issue. In addition to this, devices that perform well with emissions testing generally perform well at immunity testing also, and the reverse is the case, so by sorting out emissions problems, you are quite possibly also fixing potential immunity issues. To this end, the earlier you can catch these issues, the better.
There are a number of advantages of pre-compliance testing:
1. Project lifecycle accelerated through early error detection
The sooner product deficiencies are identified in the development process, the easier it is to rectify any shortcomings. It’s much more costly and time consuming to rectify issues after compliance testing than fixing them during the design stage. Pre-compliance testing allows you to focus on the areas that you have identified as potential causes for concern and find solutions for them early. Generally, the risk to a design failing is relative to how long you delay testing, so designers who schedule testing toward the end of a project are completely reliant on the design team’s skill and experience.
Early analysis of the electronics can also help steer system decisions. Apart from being about electronics, EMC is also about the system, and mechanical changes, such as adding EMI shields, coating boxes or adding EMC foam to fill any leaks/gaps in an enclosure, may be necessary.
Take the example of the design of an Android tablet. This will have multiple features such as LCD, touchscreen, USB, Wi-Fi, 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.
You would ideally test the core CPU with each section separately, and when a problem occurred, the source would be apparent. But the reality is that this is rarely a commercially viable approach. However, the use of development boards with pre-compliance testing can do some of this work at a lower cost. This highlights issues early on and allows for remedies to be implemented ahead of the final integration. This is not a substitute for the final testing, but may save some of the pain. In a similar vein, once the target hardware is available, partially populated boards can be used to isolate problem areas.
2. 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, you may be either over or under-testing, or applying the incorrect frequency range.
A spectrum analyzer and near field probe can be useful in finding emitters, once they have been identified as presenting radiation above the required limit, but less useful before a calibrated scan at a required distance. 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. 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.
In addition to 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.
4. Integrated testing means more agile project development
Stand-alone pre-compliance testing can be expensive, especially if a product doesn’t pass the 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’s good to be able to try it out quickly before committing to a re-spin of a circuit board.
5. Eliminate over-engineering for improved efficiencies
Early EMC testing can save money by reducing over-engineering, ensuring that a product can pass compliance tests easily. Before a product is tested, it’s not known where the problems might occur. This can lead to counter-measures being added where they are not required, counter-measures that will be 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 optimize the BOM (bill of material) cost.
As well as this having an electronic BOM cost impact, there is also an effect on the mechanical constraints (meaning the size of the board in all three dimensions). For a tight design, it’s crucial to optimize the EMC filtering, which can be large, at an early stage, as adding filtering in 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.
6. Additional uses of pre-compliance equipment
As well as EMC testing a product, “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 any change.
Similarly, tests can be carried out on comparative signal strengths of antenna configurations. For example, performance of one of our ZigBee products had markedly decreased on a recent batch, following the move to a new subcontract manufacturer. A new and old board were compared in an EMC chamber to determine the problem. It transpired that the stack-up had not been followed on the PCB, resulting in detuning and losses.
To minimize 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.