Industry expert William Kimmel of Kimmel Gerke Associates shares a few techniques and best practices of troubleshooting common EMI problems in today's digital designs including common causes of EMI and detecting root cause. The MDO4000 Series mixed domain oscilloscope is featured as the tool of choice in this insightful Digital Designer's Handbook.
Choosing the test tool
Though spectrum analysers are key elements in EMI emission testing, the data collected during testing is not well suited for diagnosing the causes of emission problems. Three factors considered very useful for diagnosing problems are precise frequency measurements, bandwidth control and span control.
Mixed domain oscilloscopes (MDO) such as the Tektronix MDO4000 Series are relatively new test options that combine the functionality of an oscilloscope (time domain) and a spectrum analyser (frequency domain) in one instrument. This dual capability is particularly helpful when troubleshooting EMI problems, which are often specified in the frequency domain.
The MDO’s time-correlated display is a potentially useful feature for EMI troubleshooting. A trigger is set up on one channel (analogue, digital or RF) and when the trigger event occurs, data from all channels is acquired, ensuring the displayed signals are time-correlated. This provides insight on the cause and effect relationships between EMI problems in both the frequency domain and the time domain.
Identify offending frequency sources
Although readily available during testing, and generally logged as part of the test process, the EMI test report does not always show the exact frequency contributions. Crystal oscillators and clocks are very accurate frequency sources. The possible problem frequencies on an EMI report will be the actual clock frequency or a harmonic of the clock frequency. Clock frequencies are often divided into submultiples of the clock frequency, and harmonics of those will show up from data and address buses.
EMI emission troubleshooting begins with identifying the specific frequency source. Using a spreadsheet, one can quickly generate a list of possible harmonic frequencies, given the actual clock frequencies in use. Combined with the test data, the offending source can be quickly identified.
Looking for resonance
Radiated emissions are the result of two factors, namely hidden transmitters and hidden antennas. The former are usually harmonics of clocks or other highly repetitive signals, and the latter are usually cables and circuit board traces.
Like real antennas, exciting a resonance with a harmonic can cause the emissions to peak, which is why emissions fail at higher frequencies, when lower frequencies are in compliance.
Furthermore, most systems have multiple hidden antennas that are resonant at multiple frequencies. Thus, fixing one problem frequency may result in problems at other frequencies. Troubleshooting therefore, should be done by choosing a wider frequency span.
Spectrum analysers can be set to display a start frequency and a stop frequency, also referred to as the frequency span, or just the span. The MDO, and most quality spectrum analysers allow any frequencies (within the range of the instrument) to be used. However, some lower cost analysers have limited range selection.
For test purposes, especially automated testing, the span setting is not critical. For manual use, the span can be set so that the amplitude and frequency are easily readable.
Checking emissions from clock and data buses
While computer clocks are the principle concern in high frequency emissions, simple subdivisions also appear such as 1/4, 1/8, etc. All other factors being equal, each time one divides the frequency to half, the emission level goes down 6 dB. So subdivision reduces the emission levels quite quickly, which is why the fundamental clock is often the biggest contributor.
Checking for broadband noise
By definition, a signal is broadband if the fundamental frequency is lower than the bandwidth of the receiver. Thus, a switching power source of, say, 80 kHz looks broadband if the receiver or spectrum analyser is set at 100 kHz to 120 kHz (the two common bandwidths cited for radiated emissions). Random noise, such as that might come from a brush type motor will seem like broadband noise, no matter what the receiver bandwidth is set to.
Making the measurements – an example circuit
A wide span setting on the spectrum analyser or mixed domain oscilloscope usually makes it easy to identify harmonic sources. Spectrum analysers and mixed domain oscilloscopes can be more useful for EMI diagnostics by manipulating frequency span and bandwidth. Wide frequency span shows a good overall picture of harmonic generation, which is particularly useful for identifying resonances that may be LC or wavelength related.
Narrow span and receiver bandwidth help identify problem frequencies. Broadband noise on screen is proportional to receiver bandwidth, and can be lowered by reducing bandwidth. If the receiver bandwidth is then set narrower, the separate harmonic frequencies will become visible.
MDO4000 Series mixed domain oscilloscope
Featuring a built-in spectrum analyser, Tektronix’s MDO4000 Series mixed domain oscilloscopes help capture time-correlated analogue, digital and RF signals for a complete system view of the device. From seeing both the time and frequency domain in a single glance, and viewing the RF spectrum at any point in time to see how it changes with time or device state, to solving complicated design issues, quickly and efficiently, Tektronix’s MDO4000 Series transforms the way testing is done.
Tektronix’s MDO4000 Series mixed domain oscilloscopes are available from TekMark Australia