Avoiding EMC issues: simple tests you can do yourself

This is a brief overview of EMC compliance with some practical tips on not getting caught out.

What is EMC and why should I care?

Imagine having an apparently working electronics product only to put it through compliance testing and have it fail. How frustrating! You got right up to the finish line and then suddenly you are swept backwards. There are ways to reduce that risk and this article looks at how electronics design teams can use good design process, and some basic measurements you can do yourself with modestly priced equipment to spot problems at the prototype stage.

The first thing to appreciate is that meeting the safety and EMC standards for the country you sell an electronics product in is a legal obligation. In Australia we have the Electrical Equipment Safety Scheme (EESS) which allows you to register yourself and your product, and also show your evidence of compliance. This allows you to put the Regulatory Compliance Mark (RCM) and your EESS vendor ID on the product.

Image credit: ACMA

If that sounds like a bunch of lawyers were involved, then that is partly true. The framework is legal but the compliance thresholds are technical. They are there to reduce the likelihood anyone gets injured or anything goes wrong because products are not designed well enough. We know we need this because in the past these types of problems happened, and it didn’t end well.

The other thing a compliance test house does is keep your test results on file and confirm to anyone asking that your test report is real. This is part of what you are buying when you pay for a compliance test report.

So that’s the background.

From here we focus on EMC or Electro-Magnetic Compatibility, which covers:

• Radiated emissions — we are acting like an unintentional radio
• Conducted emissions — high frequency noise on cables including mains
• Susceptibility — we are adversely affected by ESD or either of the above from other devices
   

How do we navigate this?

The first step is to determine what standards apply. The Australian Communications and Media Authority (ACMA) sets the standards and offers a list of applicable EMC standards on its website so you can determine the appropriate standard. It pays to be thorough here. Some examples are: ISM Equipment is covered by AS CISPR 11 and Multimedia is covered by AS/NZS CISPR 32.

How to avoid problems?

EMC Rule #1: You don’t need to solve a problem you didn’t create in the first place!

Let’s look at what is involved:

• Know what EMC standard applies — see above
• Read the data sheets for EMC guidance
• Impedance matching components in the right location
• Reduce dV/dt and dI/dt transients
• Reduce current loop area
• Filter at the edge
• Ferrites to energy
• PCB layer strategy
• Shielding if you need it
   

That is quite a list, but it is a useful reminder of all the areas that contribute to either having or not having a problem.

For the datasheet item, it is not unusual to find a note buried deep inside a power module data sheet saying, “To comply with EMC you will need to add …”. Seriously? But alas true. This leads to cheap products designed badly that appear to work but are not compliant. They could add those filter components to the module but that would make the module more expensive and potentially larger. To be fair, the nature of the required filter is sometimes dependent on the application. But I wish they were more up-front about it.

Matching components for impedance-controlled networks need to be correctly positioned to work correctly. The PCB layout must take that into account. You can’t just dump components where it makes your life easier doing the layout. Physics and RF behaviour are geometrically dependent when it comes to PCB layouts.

Reducing voltage and current rise-and-fall times (dV/dt and dI/dt) reduces the higher frequency signals they generate. This can be done by adding RC filters or setting bus controllers to soft drive.

Reducing current loop area reduces the radiating area. This is usually done by running the current return tracks back along the same path as the current feed. Or making them shorter. You are trying to make a smaller aerial for radiated emissions.

Filtering at the edge of a PCB will reduce the noise coming into the PCB, which improves its susceptibility results, and can prevent noise getting onto cables where it can radiate freely.

Use of energy-absorbing elements like ferrite beads can absorb RF energy. You can also use both filters and ferrites internally to constrain noise to specific regions of the PCB. The more you let it spread the harder it is to stop.

PCBs have layers. Choosing which ones are ground and how the layer stack works can also reduce EMC and susceptibility issues. If you have two ground layers in a four-layer PCB you can run noisy signals in between them so they act like shields. Make sure you allow for that when routing impedance-controlled tracks as the calculations give different results depending on whether tracks are on an outer layer or inner layer.

Shielding could be your final step. Most RF modules have small shields on the PCB because they want to limit unintentional radiation. It also helps with susceptibility. Some products need a fully shielded enclosure. You might also need shielded cables.

So being proactive and following guidelines like these can help you not have problems. But how do you know for sure?

Measure EMC in-house

A compliance test will typically cost between $7000 and $20,000 depending on the product category and the number of countries you want to sell your product in. That is a sizable expense, so you want to make sure you pass!

There are two ways to do this:

Pay by the hour for use of a compliance test house and they will do tests and show you informal test results. Typical rates for this are $400 per hour. You must fit into their schedule.

Have some equipment and do initial tests in-house to spot obvious problems and fix them before going to a compliance test house. This has the advantage that you can check for improvements without having to go back to the test house each time.

We have chosen the second option. We do this all the time and it quickly pays for itself. The sort of equipment we selected is:

• Low noise test area
• Spectrum analysers
• Antennas
• Near field probes
• LISN
• TEM cell
   

A quiet place to test is the best place to start. Not strictly a piece of equipment but definitely part of the test set-up. You then use the equipment with less concern about what signals are from the device under test, DUT and what is just there from the rest of the world.

This is easiest to show with a real case study.

This is a test house-provided mains conducted emissions test report to CISPR 11.

Image credit: EMC Technologies Bayswater

The black triangles are the problem areas. But what is causing this?

Next, we tested in-house using a spectrum analyser fed by a line impedance stabilisation network (LISN) to separate the RF portion from the regular mains portion so we could look at just the RF portion.

Image credit: Successful Endeavours

Peak 5 on the above correlates to Peak 1 from the test house. Now we have our first big bonus from in-house testing. The four peaks below are not on the test house report because they are not looking below 150 KHz for conducted emissions. Based on their separation and ~25 KHz minimum spacing we are confident that this is coming from the RCM compliant mains power supply module we are using as the primary power supply. A power supply can pass a compliance test with a simple load, like a resistor, but fail if the load is more complex, or in this case, we have a bridge rectifier upstream.

Now we zoom in and use some near-field probes to find where these specific frequencies originate. Below is a picture of the sort of probes available and you can also get preamplifiers for locating faint signals. The one with the pointier end is for electric field tracing and the other three are for magnetic field tracing. Both probes pointed to the same problem area.

Image credit: TEKBOX

We determined the bridge rectifier was too slow and allowed switching noise through when changing current direction. Add a suitable X class capacitor on the power feed side and retest and the problem was solved, including all the higher frequency peaks that were generated. We would not have spotted that root cause as easily from just the test house scan.

Radiated emissions are more complicated. You will need antennas or a transient electro-magnetic (TEM) cell which is like a small, screened room. Lots of other things contribute to locally detected radiated emissions because, unlike a cable, every radio in range, including cell towers, is contributing to the scene. The TEM cell is a good solution for small products and encloses the product and collects all the emissions, separating them from outside emissions sources. The spectrum analyser connects to the TEM cell so you can see what is happening.

Image credit: TEKBOX

If the product is larger and can’t fit into the TEM cell, then we typically use smaller log periodic antennas because they are compact and cover a wide frequency range. We usually look at relative results to demonstrate an improvement has been made. Below is a typical set of log periodic antennas. Even if you need to do a pre-compliance scan at a test house, having the in-house gear and knowing the level of improvement you need allows you to do that faster and with higher confidence. You iterate without incurring the higher costs of the test house.

Image credit: Successful Endeavours

Conclusion

EMC compliance is a legal obligation for products sold in the Australian market as well as overseas markets. While compliance test houses can provide you with comprehensive test reports and are necessary for formal proof of compliance, a mixture of good design practice and basic in-house pre-compliance test capability can save you a lot of headaches when you are ready to go and get that formal compliance test certificate.

There are many options for in-house pre-compliance test equipment; below is a list of what was used in the examples above. These are not the only options:

• Rigol DSA-815 Spectrum Analyser
• TEKBOX TBPS01-40dB near field probes with 40 dB RF amplifier
• TEKBOX TBTC2 TEM cell
• Kent Electronics Log Periodic Antennas
• Mains AC LISN — we built our own for low-current testing

Top image credit: iStock.com/audriusmerfeldas