Earlier than we delve right into a step-by-step information to good engineering apply, two ideas to bear in mind. First, there are too many subpar take a look at reviews from engineers, who usually do glorious work however aren’t given sufficient time to doc it correctly. In consequence, when groups sit right down to evaluation the report weeks and even days later, essential particulars are lacking, and the take a look at engineers have merely forgotten a number of the extra delicate factors.
Second, there’s a lack of strong pointers for this course of. A fast search on Google doesn’t flip up a lot on the topic. These two elements led to the creation this text.
This text gained’t cowl methods to write an accredited EMC take a look at report, which requires detailed documentation of issues like take a look at circumstances, measurement uncertainty, and so forth. As a substitute, it’s geared toward design engineers and take a look at engineers who’re doing hands-on work in their very own labs.
In fact, managers are greater than welcome to learn alongside too. And should you’re a supervisor who finds this text useful, be happy to share it together with your group.
To kick issues off, let’s begin by taking a look at an instance of what to not do.
Determine 1 The screenshot exhibits a benchtop EMC report that the creator collected some time in the past. Supply: Min Zhang
Determine 1 exhibits a screenshot from a take a look at report. Discover something flawed?
First, there’s no setup image. Anybody aware of EMC/RF testing is aware of that even a minor setup error can result in considerably totally different outcomes. If a system is incorrectly examined as a failure, the consequence may result in pricey over-design. Alternatively, if a system is wrongly marked as a move, discovering out it fails on the accredited EMC lab is not going to solely come as a shock however can even add further prices and affect your product’s time to market.
What else? The take a look at circumstances should not clearly outlined. For instance, what does the engineer imply by “prolonged the defend with further foil”? It may need made sense on the time however belief me—when the identical engineer revisits the report a month later, they in all probability gained’t bear in mind the specifics. Furthermore, we’d wish to know if this “further foil” was terminated and, in that case, how. Sadly, no supporting picture was supplied.
Lastly, the engineer made an announcement with out providing a proof or supporting proof. When stating “noise was frequent mode somewhat than differential mode,” there have been no take a look at outcomes to again this up. In EMC/RF testing, assumptions should not sufficient. A easy RF present probe clamped on the cable bundle may have supplied the mandatory proof for this assertion.
So, how ought to we write a high-quality take a look at report? Based mostly on years of troubleshooting EMI points, right here is our step-by-step information to writing an efficient engineering report.
Step 1: Sharpening the instruments
Earlier than diving into the setup, it’s essential to make sure your gear is prepared for the duty—very like how a carpenter sharpens the instruments earlier than beginning a challenge. Whereas we’re not actually “sharpening” something, the thought is similar: earlier than any RF measurement, all the time examine in case your gear is as much as the job. Let me share an instance to emphasise the significance of this step.
In a single case, a consumer’s costly receiver had a broken RF front-end, they usually had been unaware of it. This can be a basic case of what I name “engineers’ bias”—the idea that high-priced gear is inherently dependable. Engineers usually place full confidence in pricey devices, however even these can fail.
In case your spectrum analyzer has a monitoring generator, you may simply examine for this difficulty. Merely join the TG output and the spectrum enter utilizing a coaxial cable and carry out a TG scan. One ought to see a flat, straight line throughout the entire frequency vary on the provided TG energy stage (usually between -20 dBm and 0 dBm). That is proven in Determine 2.
Determine 2 Here’s a option to examine the RF front-end of a spectrum analyzer. Supply: Min Zhang
The identical precept applies to different take a look at gear. As an illustration, if an RF present probe is by chance dropped, its switch impedance could also be affected. In such a case, you need to recalibrate the probe earlier than continuing with the take a look at. Likewise, line impedance stabilization networks (LISNs) needs to be often checked to make sure their impedance conforms to the related requirements.
It’s additionally essential to doc the final calibration, characterization, or inspection date for all gear used within the take a look at. If an incident happens (like dropping the present probe), make sure that to document it within the report. This ensures traceability. Whilst you can proceed with the take a look at, remember the fact that such occasions improve measurement uncertainty.
Step 2: Check set-up
You need to clearly present detailed images of the take a look at set-up. This could embrace an total view of the take a look at association, in addition to close-up photographs of particular particulars, such because the bonding wire, the way it’s bonded, and whether or not a continuity examine was carried out on the connections.
For performed emissions/immunities and transient assessments, embrace photographs exhibiting the bonding of the take a look at gear, cable structure, and particulars of the machine below take a look at (DUT) bonding, notably if a bonding wire is used to connect with the take a look at floor airplane.
For radiated emissions assessments, assuming you’re conducting them in your personal lab—since radiated immunity testing can intervene with the electromagnetic atmosphere—you shouldn’t carry out radiated immunity take a look at with no shielded tent. Be sure to incorporate footage that present the antenna set-up.
Do we have to embrace a system diagram of the take a look at setup?
The quick reply is sure, ideally. Whereas drawing a system diagram could take extra time than merely snapping a photograph, it’s nonetheless essential to incorporate a easy diagram. Common instruments for creating system diagrams embrace Microsoft Visio and PowerPoint. Determine 3 exhibits a system diagram utilizing Keynote on a MacBook. In the event you’re extra artistically inclined, be happy to make use of different drawing instruments—some engineers choose this strategy.
Determine 3 The system diagram exhibits a take a look at set-up for CISPR 25 performed emissions. Supply: Min Zhang
Determine 4 exhibits the precise take a look at set-up used for the performed emission take a look at, highlighting an total view of the take a look at association. You possibly can see how the take a look at gear is listed for the take a look at.
Determine 4 The precise take a look at set setup lists all of the gear. Supply: Min Zhang
Different key info
Your report also needs to embrace particulars in regards to the energy provide settings, similar to voltage and present. If there’s any supporting gear for the DUT, make sure that to seize this in each the images and the system diagram whereas documenting the operational standing within the report.
Environmental circumstances similar to room temperature and humidity are usually not required for in-house assessments, however should you’re conducting electrostatic discharge (ESD) investigations, it’s essential to doc these elements, as humidity can have an effect on the take a look at outcomes.
It is suggested to all the time take a look at and measure ambient EM noise earlier than beginning any benchtop EMC take a look at, and these outcomes needs to be completely documented within the take a look at report. Usually, a benchtop energy provide can introduce inner noise, which can be picked up by the LISN throughout performed emission assessments.
Moreover, LED lights and close by gear usually generate EM noise, which may simply couple to the DUT’s cable leads and affect the emission readings. When working with no shielded atmosphere—which is commonly the case for design engineers testing and troubleshooting on the bench—the most effective apply is to benchmark the ambient noise. This may be executed utilizing the spectrum analyzer itself or by utilizing software program to save lots of the ambient noise knowledge for comparability in future research.
Step 3: Acquiring take a look at outcomes
It’s all the time a good suggestion to save lots of outcomes instantly from the gear or via a linked pc (assuming the mandatory software program is put in), somewhat than counting on a photograph of the display. This strategy presents a number of benefits.
Initially, trendy gear software program usually offers much more info than what’s seen on the display, such because the date, time, and sampling price (for an oscilloscope, for instance). Moreover, saving knowledge digitally avoids potential points like reflections that will happen in images.
One other profit is that a number of traces can later be processed for comparability functions. Some software program even permits you to doc in depth particulars, similar to take a look at circumstances and operation modes, making the report extra complete and traceable.
An instance is proven in Determine 5.
Determine 5 Right here is an instance of take a look at outcomes generated by software program Tekbox EMCview. Supply: Min Zhang
Step 4: Analyzing take a look at outcomes
For a junior engineer, analyzing take a look at outcomes can appear daunting, however we encourage you to offer it your greatest effort. To start, give attention to figuring out the failure mode—typically it could be a resonance within the spectrum scan, or a narrowband sign failure. It’s essential to supply some type of clarification. A very good instance is proven in Determine 6.
Determine 6 It’s essential to supply some type of clarification whereas analysing the take a look at outcomes. Supply: Min Zhang
On this radiated emission consequence, two points are evident. First, there’s a broadband noise profile within the 50 to 80 MHz frequency vary, and second, there are narrowband noise traits between 100 and 200 MHz. Moreover, a single narrowband spurious sign seems at 222 MHz.
On this case, we highlighted these areas of curiosity and supplied explanations for every. As all the time, should you suspect a particular perpetrator is inflicting the noise, show it by offering additional outcomes—this considerably enhances the worth of your evaluation, as demonstrated in Determine 6.
What should you don’t absolutely perceive what’s taking place? On the take a look at stage, on the very least, provide a couple of potential explanations. You possibly can say, “We consider it might be one of many following causes,” and record some potentialities. You may as well point out that additional testing or simulation could also be wanted to pinpoint the basis trigger. That is essential as a result of when a group of engineers opinions the report collectively, different group members usually contribute helpful insights and ideas.
Step 5: Troubleshooting and fixing
If the take a look at report contains troubleshooting and fixes, the options should be clearly acknowledged and supported with enough proof. This could embrace images, take a look at outcomes, and a transparent rationale for the repair. For instance, an engineer may say, “The ability cable linked to the motor proved to be the primary radiating mechanism, and a ferrite sleeve on the mains cable solved the issue.”
Nevertheless, this strategy is problematic for the explanations we’ve mentioned earlier. A simpler assertion can be:
“The motor energy cable was recognized as the primary supply of radiated emissions, as disconnecting the cable considerably decreased the noise between 50 and 80 MHz. We then utilized XXX (half quantity) ferrite cores to the motor cable, putting it close to the motor connector, and ensured the ferrite cores had been near the car chassis (the situation is essential). As proven in Determine 7, this resulted in improved efficiency. See the comparability of the earlier than and after ends in Determine 7.”
Determine 7 That is how the troubleshooting half in a report appears like. Supply: Min Zhang
By stating the troubleshooting outcomes on this method, you present much more confidence within the resolution.
Step 6: Abstract and conclusion
We consider report also needs to embrace ideas, suggestions, or actions that should be taken. Engineers could suggest design modifications, however it’s essential to record the potential dangers related to these modifications. This highlights that EMC engineering usually includes compromise. Whereas engineers could make strong ideas, they have to additionally contemplate different elements similar to thermal or mechanical design, which could complicate implementation.
It’s additionally important to contemplate different fixes. Throughout troubleshooting, you might be usually restricted by the instruments at hand, and the answer you discover will not be probably the most cost-effective. That is particularly related for quantity producers, the place even small value variations can have a major affect.
By this level, we’ve supplied readers with a strong information to writing a benchtop EMC take a look at report. The rules outlined listed here are relevant throughout many areas of engineering. We welcome your ideas and suggestions.
Dr. Min Zhang is the founder and principal EMC marketing consultant at Mach One Design, a UK-based engineering agency specializing in EMC consulting, troubleshooting, and coaching. He at present chairs the IEEE EMC Chapter for the UK and Eire department. Zhang may be reached at min.zhang@mach1design.co.uk.
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- Discover EMI sources with an oscilloscope
- EMC design contest to give attention to sign looking
- An introduction to troubleshooting EMI issues
- EMI emissions testing: peak, quasi-peak, and common measurements
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