Conducted Emissions Test Equipment and Noise Reduction Guidelines

One of my classes in college was so difficult that the professor would give us the test questions a week in advance. Even knowing exactly what to study for the exam, lots of students still failed. Conducted emissions analysis for electromagnetic compatibility (EMC) is similar. Devices need to be checked to see if they’re putting too much noise back into the grid through their power supply. Otherwise, the FCC will think you’re disrupting utility power. It’s not difficult to pre-test devices for EMI conducted back into the power source through the power supply. However, lots of products still fail when it comes time for the final check. Failure at the last stage costs a lot in both time and money. All that can be avoided by preparing yourself with the right equipment and doing some pre-compliance testing. It’s also a good idea to look at your PCB design and power supply to try and root out any conduction problems at the source. Sometimes the simple act of switching power supply, can assist. Another thing to consider is the mode of your components (common mode or differential mode).

Benefits of Pre-Compliance Testing

We also had several classes back then with examinations that were open book. Lots of students wouldn’t study beforehand because they thought if they had their books the test would be easy. They were wrong, and lots of them failed. Lots of people assume that the conducted emissions and reduction part of EMC is simple compared to a radiated emission, but they cause just as many failures.

If you have a conducted emission test failure, you’ll have to do it again, which can cost thousands of dollars. Failing a college exam is bad, but a blunder in these tests is like failing a whole class. Pre-compliance equipment is expensive, but not as costly as repeat certification testing. Failing an EMC assessment can also delay getting your product to market. If you have major fixes that need to be done it could significantly delay your project. Better to catch problems at the beginning when they’re relatively easy to fix.

And you thought college tests were hard.

Pre-Compliance Equipment

The equipment you’ll need for the conducted emission is fairly similar to that required for radiated emissions. A kit like this will cost you several thousand dollars.

• Spectrum (Required) - A spectrum is the backbone of pre-compliance testing. This will allow you to analyze any electromagnetic interference (EMI) coming out of your board. This will likely be the most expensive piece, costing upwards of $1,000.

• Software (Required) - An open book test isn’t helpful if you don’t know how to read, just like a spectrum without software. Some spectrum analyzers come with software included. If yours doesn’t, make sure you choose one that is compatible with free programs.

• Line Impedance Stabilization Network (LISN) - This is one thing you need for conducted emissions that you don’t for radiated emissions. A LISN isolates your device from any noise in the power source and matches impedance so that your spectrum analyzer can operate accurately. If you want your tests to be really accurate, you might need two of these.

• Pre-Amplifier (Optional) - A pre-amp will boost any frequency signals coming out of your device, making them easier to read. It’s not necessarily required, but might be a good idea.

Careful routing and grounding will help you avoid conducted EMI.

Possible Problems

Now that you have all your equipment it’s time to test your product. If everything turns out to be fine, you might resent me for getting you to do pre-compliance, but I’m okay with that. Alternatively, if there’s a problem, it’s likely that the source of it is in your power supply or PCB. Things like crosstalk and ground loops can amplify noise in your designs, which can then be conducted out through your power supply.

While your chosen power supply may supposedly already be compliant, the reality can be different. A specific supply may have come from a bad batch. Power supplies are also sometimes only tested with DC current frequency, and AC noise coming from your can force them out of compliance. Regardless of the reason, if your supply is non-compliant, your device probably will be as well. Pre-compliance testing can help you determine if you chose the right supply, or if you need to make a change before finalizing your designs.

Crosstalk happens when signals on your board go where they’re not supposed to. There are many causes of crosstalk including: mixed AC/DC signals, poor differential pair routing, and poor high speed routing. Generally you’ll want to keep radiated EMI signals far away from other circuits. You should also keep traces short to keep them from radiating EMI like antennas. Looking into an EMI filter, may be the way to go.

Ground and power planes can also be sources of trouble for your PCB. Generally ground and power planes help attenuate EMI, but if you create ground or power loops in them you’ll have a problem on your hands. To avoid doing that you should carefully design your ground planes.

You should always try to pass EMC certification tests the first time. Repeat assessments will cost you time and money and are easily avoided. Performing tests yourself and catching problems early is the way to go. Even better, you can further prevent issues by designing a PCB for EMI reduction. Try to reduce crosstalk and eliminate ground loops so that your circuit passes the initial test.

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