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Posted: 21 Feb 2010 ?? ?Print Version ?Bookmark and Share

Keywords:EMI? debugging? quasi-peak detector? pre-compliance? EMC?

Companies often find their electronic products failing the last hurdle, which is complying with the EMC requirement, before they can hit the shelves. This has led to the awareness that it is important to consider pre-compliance and EMI diagnosis in earlier design stages to minimise the impact of failing the test: cost of redesign and device recall, and delay in product launch. Having to wait until the end of the development cycle to find out whether a product passes the compliance can be a huge gamble as the cost involved rises exponentially with each progressing development stage.

Troubleshooting EMI from design investigation through pre-production prototype units, even as early as R&D phase, has thus increased in importance. The availability of preventive measures that can be experimented on the problematic boards does bring benefit.

The cost of correction.

Figure 1: The potential correction cost incurred goes up and the available measures to rectify emissions problems become limited as the products progress further in development stage.

Design engineers frequently discover that a new product design needs to be corrected many times until it passes the emission test limits. They neither possess the EMI diagnosis know-how nor has it been part of their responsibilities. Typically, the designed boards are passed to the EMC department or external labs for further test only when it is almost ready. However, it is challenging to make changes at this stage compared with doing so in the early designing phase where certain design aspects could have been better dealt.

The results of the EMC compliance is controllable by design, can be planned for and, therefore, the responsibility of the designer. Solving EMI is not a black magic that time-domain engineers should shun. You can get a better understanding of EMI problems as well as the effectiveness of the applied solutions by using an instrument you are familiar withthe oscilloscope.

Mask test

Figure 2: The mask is substituted by limit lines to determine whether the DUT is in compliance.

Equipping for pre-compliance

At this point, you'd ask: So how about me using EMI filters and quasi-peak (QP) detectors? That depends on whether you are testing for compliance, pre-compliance or simply performing EMI troubleshooting. EMI type RBW filters are narrower, rolling off at 6dB per decade rather than 3dB. The quasi-peak detector, which conforms to standards, detects weighted peak value depending on the signals peak value and repetition rate.

Using incorrect filters and detectors can influence both amplitude and frequency values returned by the equipment. In compliance test, these two are mandatory for complying with the standard. However, they are not critical for EMI diagnosis as the primary goal is to physically and electrically identify the emissions source. It is good enough for you to get a close estimation without the accuracy needed for actual compliance.

When identifying emission source, peak detectors are sufficient due to the fact that QP detectors will always be less than or equal to peak detectors. The results of QP detectors and peak detectors are related to the same signal's repetition rate, and you can formulate a math waveform or take this into consideration during troubleshooting. On the other hand EMI filters will only change the results slightly.

Unlike test receivers, oscilloscopes are not designed with built-in EMI compliance limit test. Using mask test, which is available on most oscilloscopes, or remote software, the EMI compliance limit can be defined on the oscilloscope to emulate EMI standards testing. You can then further setup additional masks to find problem areas that are of interest.

Mask test

Figure 2: The mask is substituted by limit lines to determine whether the DUT is in compliance.

Equipping for pre-compliance

At this point, you'd ask: So how about me using EMI filters and quasi-peak (QP) detectors? That depends on whether you are testing for compliance, pre-compliance or simply performing EMI troubleshooting. EMI type RBW filters are narrower, rolling off at 6dB per decade rather than 3dB. The quasi-peak detector, which conforms to standards, detects weighted peak value depending on the signals peak value and repetition rate.

Using incorrect filters and detectors can influence both amplitude and frequency values returned by the equipment. In compliance test, these two are mandatory for complying with the standard. However, they are not critical for EMI diagnosis as the primary goal is to physically and electrically identify the emissions source. It is good enough for you to get a close estimation without the accuracy needed for actual compliance.

When identifying emission source, peak detectors are sufficient due to the fact that QP detectors will always be less than or equal to peak detectors. The results of QP detectors and peak detectors are related to the same signal's repetition rate, and you can formulate a math waveform or take this into consideration during troubleshooting. On the other hand EMI filters will only change the results slightly.

Unlike test receivers, oscilloscopes are not designed with built-in EMI compliance limit test. Using mask test, which is available on most oscilloscopes, or remote software, the EMI compliance limit can be defined on the oscilloscope to emulate EMI standards testing. You can then further setup additional masks to find problem areas that are of interest.

EMI measurements require a different approach than the usual time-domain related measurements or other types of general RF tests. In EMI, engineers never quite know what signals may be present. Since each new DUT is different, choosing the correct tools for characterisation of EMI signals and identification of the emission source is of importance.

The spectral analysis functionality of oscilloscopes cannot replace traditional EMI test equipment, such as spectrum analysers or test receivers when it comes to pre-compliance and compliance test. Limited dynamic range, the lack of pre-selectors, preamplifiers and standard-compliance weighting detectors and bandwidths, restrict the application of oscilloscopes to EMI debugging purposes.

Oscilloscopes, spectrum analysers or test receivers tackle the EMI problem from different perspectives. Each requires a different approach to the test and offers different but complementary diagnosis techniques. Complementing EMI debugging with oscilloscope opens up a whole new realm and offers unprecedented debugging possibilities. Combining a pre-existing oscilloscope analysis tool in EMI debugging will help you quickly discover potential issues with your design.

What's next? Power is a major source of EMI noise and the scope's power harmonic analysis has a lot of potential in such debug work. We'll discuss that soon.

About the author


Alvin Ding
Business Development
Rohde & Schwarz
Alvin Ding has years of experience in the test & measurement industry, with extensive knowledge and focus on the latest and emerging technologies, such as high-speed serial data and RF. Alvin received his bachelor's degree with honours in electrical and electronics engineering (EEE) from Nanyang Technological University, Singapore. Alvin received the EEE Excellence award for his outstanding contributions and excellence in academic performance throughout his study.





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