Fundamentals of EMI Pre-Compliance Testing to ensure Compliance Success
Download slide presentation here.
The ability to determine likely success or failure prior to expensive compliance testing can significantly reduce product development cycle times and reduce rework costs associated with compliance failures. Access to an affordable pre-compliance test package allows the development team to complete radiated emissions, conducted emissions and Immunity testing during sub-assembly development and prior to full compliance verification. Cost effective Spectrum Analyzers, RF Signal Generators and compliance software enable design teams to find EMC problems sooner and ensure compliance test success.
Who Should Attend?
Hardware Product Designers and Pre-compliance testing technicians
Speaker: Jason Chonko
Jason Chonko is an Applications Engineer at Rigol Technologies with a focus on EMC and EMI applications. For over twelve years, he has been helping engineers get the most out of their gear. Jason has a B.S. in Physics from Kent State University in Kent, OH.
The following are questions presented to the speaker by the attendees during the webinar, along with answers to each.
Mardiguian’s EMI Troubleshooting Techniques 3.2.3 advises that test equipment be bonded to GRP
Answer: That was an error in my slides. I need to add that to the slides next time. Thank you for the information.
What cheap antennas can one use for such a scan?
Answer: You can use rabbit ears, a bowtie antenna, or a log-periodic PCB antenna for many scans. They each have advantages and disadvantages, but are low-cost. An internet search should yield a number of suitable antenna manufacturers.
You can also find reasonably priced antennas from commercial outlets like Com-Power or Aaronia.
Is the field probe close loop size relevant?
Answer: The diameter of the loop determines the spatial resolution and flux capturing capabilities of the near field probe. A larger loop will capture more magnetic flux (more sensitive to small fields) with a given magnetic field density but won’t be as good at determining the exact position of the source of the EMI. A smaller loop will give you a higher degree of spatial resolution, but won’t be as sensitive to the same magnetic flux.
You can start your scan with a large loop (3-5cm diameter) to find the rough area of the EMI and then use a smaller loop (1cm) to gain more spatial precision.
For conducted emission, are both Qausi peak detector and average detector necessary?
Answer: Full compliance testing requires special detectors such as Quasi-Peak or averaging detectors, like EMI Average or CISPR Average.
Pre-compliance tests, like those described during our presentation, can provide useful information that can help you solve EMI issues without using any special detectors.
Quasi-peak and averaging detectors will never exceed data collected using the positive peak detector. For pre-compliance testing, we are usually looking for “worst case”. Therefore, positive peak is acceptable.
But, if your DUT design has intermittent RF (bursts of communications/digital/square wave/pulses), then the Quasi-Peak or Averaging detector information can prevent you from over-engineering a solution.
For radiated emission test, the frequency range should be from 30/80 MHz to about 1 GHz? not from 150 KHz
Answer: An error on my slides. Yes. FCC subpart 15 for unintentional radiators does start at 30MHz. Thank you.
When you add the attenuator to see if the scan peaks are real, shouldn’t the peaks be less than before?
Answer: If it is an external attenuator, you are correct. Adding external attenuation should lower the peaks and the noise floor by the amount listed on the attenuator. Peaks that drop by values greater than the attenuator are likely to be mixing products, not real peaks.
If it is an internal attenuator (part of some spectrum analyzer designs), then the noise floor will decrease.. but the peak amplitudes will remain at the same amplitude level as the spectrum analyzer will adjust for the setting of the internal attenuator.
Do you have information available about susceptibility testing? Can you maybe give a quick summary?
Answer: Susceptibility testing involves delivering RF power to a design and observing the DUT performance. Typical susceptibility tests are conducted (inject RF power on cables, connectors, and wires going into a product) and radiated. Susceptibility testing uses RF sources, antennas, and current clamps. We have some useful notes on susceptibility testing here.
Where do you put the antenna in an EMI tent?
Answer: Ideally, you want the DUT and Antenna to be separated by the distance matching the guidelines for the compliance test that you are emulating. In most cases, this would lead to a large tent and is probably not practical.
If the testing is not in a semi- or full anechoic chamber, there will be reflections and errors in the measurement due to environment. The important part for useful pre-compliance measurements is to have an environment (including the tent, positioning of the DUT, antenna, etc..) that is consistent. Experimentation with your pre-compliance measurement system should be the start of your testing. You could move the antenna around the tent to at different heights, positions, angles.. possibly over successive times of day. You can then have more confidence in the repeatability of your measurements.