Using a real-time spectrum analyzer for EMI debug

Article By : Kenneth Wyatt

The SSA3075X-R real-time spectrum analyzer is equipped with 40 MHz RT bandwidth and EMI options.

edn hands-on projectsDigital circuitry can produce a wide range of harmonic frequencies and spectrum analyzers can show you this RF frequency spectrum (power or voltage), versus frequency. They are the one piece of gear that’s essential for EMC troubleshooting, but these instruments have traditionally been the most expensive item in anyone’s kit. However, several manufacturers are now making affordable quality instruments that are perfectly adequate for troubleshooting and pre-compliance work.

Both Rigol and Siglent Technologies released affordable swept spectrum analyzers several years ago and recently, both released real-time analyzers.

I had a chance to borrow Siglent’s new SSA3000X-R-series real-time spectrum analyzer (Figure 1) recently and was able to compare it to their SSA3032X swept analyzer. It uses the same compact form factor as their swept model. The model reviewed was their SSA3075X-R that can tune from 9 kHz to 7.5 GHz; other real-time models include top frequencies of 3.2 and 5 GHz. All now offer free vector network analyzer (VNA) and distance to fault (DTF) measurements. A preamplifier and tracking generator are standard, and the EMI bandwidths and quasi-peak detector functions are optional ($400), but recommended.

photo of the Siglent SSA3075X-R real-time spectrum analyzer on a benchFigure 1 Here is the Siglent SSA3075X-R real-time spectrum analyzer all ready for EMI debug.

Swept or real time?

We now have a choice to make: there are basically two types of measurement acquisition in spectrum analyzers; swept and real time. Swept analyzers are very similar to classic oscilloscopes in that the frequency span is “swept” between the start and stop frequencies, then analyzed and displayed in a series of events (sweep – analyze/measure – display), so there is some deadtime between sweeps while the system is measuring, processing the data, and displaying the results.

This delay can miss some pulsating or intermittent frequency captures. Real-time acquisition, on the other hand, captures and displays the frequency information at such a rapid process that it appears as if the display is showing captured data in “real time.” This fast acquisition depends on extremely fast A/D conversion, simultaneous FFT processing, and powerful computer processors. Some of the higher-priced real time analyzers can generally capture impulsive RF events down to nearly 1 μs (with 100% probability of intercept (POI)) and are very useful for displaying intermittent interference or digital modulations and their characteristics.

If your products include wireless, fast serial data streams, digital modulations, or otherwise include intermittent emission peaks, you might wish to consider one of these affordable real-time spectrum analyzers.

As mentioned, a real-time spectrum analyzer has the ability to capture brief intermittent signals and are perfect for capturing modulated wireless or digital signals, as well as general EMI troubleshooting. For example, within the 2.4 GHz ISM band, you’ll see the entire spread spectrum Wi-Fi signal, as well as the frequency-hopped Bluetooth signals very clearly. You can even observe multiple Wi-Fi access points on the same channel. This isn’t possible with normal swept-frequency spectrum analyzers. They also commonly include “waterfall” displays of frequency and amplitude versus time – a very powerful troubleshooting tool for intermittent EMI issues.

I wrote a free downloadable guide to real-time spectrum analyzers a few years ago that goes into much more detail on the differences between swept and real-time analyzers (Reference 2).

User interface: The UI is basically the same as their swept models, except parts are more refined and there is now the choice between normal swept and real-time modes. The ability to use a mouse or keyboard is a definite plus.

Display: The basic layout of the display is the same, but the resolution and fonts used are clearer. Soft keys are arranged on the right side and user modes on the left. Measurement settings are arranged along the bottom. The screen can be controlled using “multi-touch,” mouse, or keyboard.

Speed: The real-time model is fast! To be able to display acquired data in near real time, the internal processing speed must use the fastest processors available, and it shows. For example, setting up both the swept and RT models to the same parameters (span, RBW, etc.), the swept model showed a display speed of 57 ms while the RT model was 4 ms (Figure 2). That’s a 14× faster sweep rate.

photo of swept and real-time analyzers to compare sweep time Figure 2 A comparison of the swept and real-time analyzers (both in swept mode) shows the relatively faster sweep time of the SSA3075X-R (57 versus 4 ms sweeps).

RT bandwidth: The base model comes with a 25 MHz RT bandwidth, which is just OK for most EMI analysis. For more serious EMI analysis, they do have a 40 MHz RT bandwidth option for an extra $1,400, which I recommend.

General performance: Both the swept and RT analyzers appeared to have the same sensitivity. Advertised display average noise level (DANL) is -165 dBm/Hz. Phase noise is pegged at less than -98 dBc/Hz, and resolution bandwidths (RBW) can be set between 1 Hz and 3 MHz. Amplitude accuracy is less than 0.7 dB. Real-time capture is advertised to be less than 7.2 μs for a 100% POI.

Example measurements

Test 1: Embedded processor data

In this test, I’m simply measuring the typical digital processor bus activity using a medium-sized Beehive Electronics H-field probe, which is my favorite size for general PCB characterizations (Figure 3). The analyzer is looking from 10 to 50 MHz (40 MHz real-time span) and we can see a combination of narrow band and broadband EMI. Note that the broadband bus noise is nearly covering up all the narrow band clock harmonics.

photo of measuring the typical digital processor bus activity using a H-field probe and the spectrum analyzerFigure 3 Use the real-time mode to capture digital processor data bus EMI.

spectrum analyzer screen capture of the processor bus EMI measurementFigure 4 In this screen capture of the processor bus EMI, the thin blue line is the equivalent of “max hold” in a swept analyzer and indicates the maximum amplitude of the measurement.

Figure 4 shows a clearer picture of the narrow band clock harmonics and superimposed digital bus noise. The thin blue line is an indication of the maximum broadband amplitude, which is pulsing up and down according to the firmware programming. Note that in real-time mode, you can clearly observe both the narrow band and broadband emissions.

Test 2: Spurious oscillation

Real-time spectrum analyzers are quite useful for identifying spurious or intermittent emissions. In this case, we’ve zoomed in on a broad spurious oscillation from an op-amp running “open loop,” due to a poorly-matched output load (Figure 5).

spectrum analyzer screen capture of an op-amp spurious oscillationFigure 5 This is an example of a spurious oscillation of a poorly-terminated op-amp at it’s open-loop frequency of about 103 MHz.

In this example, spurious oscillation wouldn’t be that distinct when using a swept analyzer. You’d also need to wait several seconds in max hold mode to see the envelope. These spurious oscillations are normally unstable in frequency and can be moved in frequency merely by touching the circuitry.

Test 3: Motor noise

For this test, I’ll use an RF current probe to measure the harmonic currents on some cables attached to a demo board that includes a 100-MHz clock, along with some digital processing. I’m also adding a digitally-controlled DC brush motor with PWM controller cabling through the same current probe. This will simulate a multi-function circuit and will display both narrow band and broadband EMI (Figure 6).

photo of using the spectrum analyzer and a current probe to measure harmonic cable currents from a demo boardFigure 6 Now we’re using the current probe to measure harmonic cable currents from a demo board. I’ve added a PWM motor controller that will add some broadband EMI to the narrow band harmonic spikes.

Figure 7 shows a closeup of the combined narrow band EMI and broadband motor noise superimposed. Note that the broadband EMI completely covers some of the lower narrow band harmonics, but because we’re in real-time mode, we can clearly see these superimposed signals.

spectrum analyzer screen capture of the combined narrow band harmonics and broad band motor noiseFigure 7 This screen capture shows the combined narrow band harmonics and broadband motor noise.

Note that motor noise – especially brush motors – can generate high amounts of broadband EMI up through 1 GHz. Broadband EMI can rarely be measured accurately using swept analyzers, due to the dead time between measurements. Max hold mode on the swept analyzer can capture this broadband noise, but you generally have to wait a couple minutes for the envelope to completely fill in and this ends up obscuring any narrow band emissions. Sometimes, this broadband noise can cover up the narrow band harmonic spikes completely.

If you’re attempting to perform radiated emissions testing at a compliance test lab and their swept spectrum analyzer happened to record a burst of broadband EMI, it could indicate a failure. By using the real-time analyzer back at your work bench, you’d have known the issue right away and could have resolved it ahead of time through normal filtering or shielding.

The SSA3075X-R real-time analyzer (7.5 GHz) equipped with 40 MHz RT bandwidth and EMI options is roughly $12,200 versus the swept SSA3075X (7.5 GHz) with EMI option at $7,400. There are still some minor firmware bugs that should be cleared up by the time this is published. For example, Trace #1 (yellow on the screen) displays as violet in the saved screen capture. If you’re having to deal with intermittent EMI or digital modulations, then the 25 MHz RT bandwidth may work out as a minimum, however the 40 MHz RT bandwidth is recommended as best for all-around EMI debug.

This article was originally published on EDN.

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.

References

  1. Siglent Technologies
  2. Wyatt, Real-Time Spectrum Analyzers, Interference Technology, 2016.

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