R&S MXO3 Oscilloscope for EMC measurements: part 1

Rohde & Schwarz recently announced the MXO3, a 1 GHz, 12-bit oscilloscope. The company sent a review unit. In this part, I found that it has some nice features for making EMC measurements, though it could use another.

R&S sent me a model MXO38, the eight-channel version with built-in 50 MHz single-channel AWG and two optional 8-bit digital channels. At just 15-in. wide x 9-in. tall x 6-in. deep, the MXO3-series is scaled down to fit most lab benches [1]. I’ve been using their larger MXO4 for a few months, so I was immediately familiar with the user interface, which is laid out intuitively and just the same. The MXO4-series is an inch wider and an inch taller with the same 6″ depth. The smaller MXO3 just seems more at home on my test bench. The base price for the four-channel model is $5,890, while the base price for the eight-channel model is $13,800.

As Figure 1 shows, R&S reduced the size when designing the MXO-series compared to the MXO4, yet kept most of the basic specs. The 1 mV low-noise vertical sensitivity, 12-bit resolution, 500 Msamples memory depth, and 21 ns trigger re-arm allow a terrific FFT spectrum display. The waveform capture is 4.5 million waveforms per second, providing real-time capture of up to 99%. From an EMC perspective, this provides a nearly real-time spectrum capture.

MXO3 and MXO4 oscilloscopes — Figure 1. The Rohde & Schwarz MXO3 and MXO4 side by side show the MXO3’s smaller size, which consumes less bench space. (Image: Ken Wyatt)

The 11.6-in. full HD touch screen provides plenty of room for multiple waveforms. For this EMC engineer, a big advantage is the ability to display up to four independent spectral displays simultaneously.

The MXO3 menu system is straightforward, and the keyboard is well laid out. Top to bottom, the front panel includes triggering, horizontal, vertical, and various modes: Autoset, Preset, cursor control, and screen capture buttons (Figure 2).

MXO3 oscilloscope — Figure 2. The front panel controls for the MXO3 are nearly identical to those of the MXO4 series and other Rohde & Schwarz oscilloscopes. (Image: Ken Wyatt)

In use, I found the default backlight was too bright for my office lab. This can be adjusted via Menu > Settings > Display > then touch the Backlight button and use the universal control knob (lower panel) to set the level.

Ports located on the rear include the usual communications connections, plus an HDMI port for connecting an external monitor (Figure 3). You’ll also find Trigger In and Trigger Out connectors, as well as the 50 MHz arbitrary waveform generator (AWG) RF output. The AWG is controlled from the front and can be set to several normal waveforms, including sample ECG and EEG.

Figure 4 shows the two 8-channel digital input ports. Being an EMC engineer, I did not test the digital ports. Attaching the cables could potentially pull on the connectors, causing reliability issues.

MXO3 oscilloscope digital ports — Figure 4. The MXO3 oscilloscope’s two 8-channel digital ports are located on the right side of the instrument. (Image: Ken Wyatt)

Setting up the MXO38 for EMC measurements

When I’m looking for clock harmonics on a PCB or cable, I’ll set the vertical sensitivity to the most sensitive: 1 mV/division. For larger signals such as power conversion circuits, I’ll use the Autoset button to get me in the general range, then adjust trigger level, horizontal, and vertical adjustments to obtain the best viewable display.

Next, I turn on the spectrum analyzer by either selecting Menu > Spectrum or simply pressing the “Spec” button in the vertical section of the front panel. You’ll obtain the menu selections shown in Figure 5.

MXO3 oscilloscope spectrum setup — Figure 5. The spectrum analyzer menu is easy to use. (Image: Ken Wyatt)

I usually prefer to set the Start and Stop frequencies manually, so press Start/Stop, then double-tap the Start and manually enter the desired start frequency. I usually leave this at the default “zero”. Repeat this by double-tapping the Stop button and manually setting the frequency. Sometimes, I will leave this at the default 1 GHz if I wish to see the whole spectrum.

Next, I like to turn off Auto resolution bandwidth (RBW), so I can manually set it to 100 kHz or 120 kHz (MIL or commercial, respectively), depending on what the appropriate standard requires for frequencies below 1 GHz.

I set the Window Type to the default Blackman-Harris setting. Note that there are four capture modes: Normal, Min Hold, Max Hold, and Average. For general probing, you should leave this set to Normal. For intermittent or broadband signals, I often use Max Hold to capture the signal envelopes for a few seconds.

The last steps include pressing Scale and choosing dBµV from the default dBm. Usually, I didn’t have to change the default scale factors, but you can change them as needed. This setup was far easier than I’ve seen on other manufacturers’ oscilloscopes.

The spectrum analyzer menu includes a feature that automatically adds peak markers (Peak List; Figure 6) to spectral peaks. These labels include frequency and amplitude for each peak, as you’ll see later. You can add annotations (text, arrows, boxes, etc.) to the display. The oscilloscope stores screen captures in its internal memory or on a USB thumb drive, if installed. You can save and recall setups as well.

MXO3 oscilloscope spectrum peak list — Figure 6. The Peak List menu controls the number of labeled peaks according to threshold and excursion.

EMC measurements

Let’s try it out on some basic EMC characterization measurements on an Arduino Due Version R3 embedded processor, as there’s plenty to see. Let’s dive in!

This older model Arduino Due R3 is ideal for testing because it includes a conventional DC-DC converter with a switch inductor easily accessible (Figure 7). It is based on an Atmel ATSAM3X8E ARM Cortex 32-bit processor clocking at 84 MHz (12 MHz clock oscillator). The DC-DC converter runs at about 500 kHz and USB at 16 MHz. We’ll see all those signals clearly. I had to order mine from eBay, as the newest versions of the board do not include the same type of power conversion circuit.

Let’s make some real measurements. A proper characterization of a product for radiated emissions requires three steps. I discuss this in detail in volume 2 of my EMC Troubleshooting Trilogy books [2]. We’ll demonstrate the first step in this article.

Near-field probing to characterize the spectral response for all high-energy components
Characterization of high-frequency cable harmonic currents for all attached cables
Measurements a short distance away (~1 m) to confirm the harmonics that actually radiate

I used an R&S H-field probe (1 cm loop) for all the measurement points (Figure 8). Near-field probing of a PC board or cable should always be the first step. We want to identify each high-energy component and document its spectral characteristics.

Sensepeek magnetic holder — Figure 8. Ready to test. The Arduino board is mounted using a Sensepeek [4] magnetic holder. (Image: Ken Wyatt)

Later, when we start to measure harmonic currents in attached cables, we should be able to correlate which energy sources are coupling to which cables. Not all harmonic currents on cables will actually radiate efficiently, so we then use a close-spaced antenna to confirm.

A loop probe couples nicely to the switch inductor of the DC-DC buck converter, which allows us to observe many important waveforms and spectral displays (Figure 9).

oscilloscope H-field probe — Figure 9. The H-field loop is coupled by placing it flat against the switch inductor.

Using cursors and measuring the distance between switched waveforms confirms the converter is switching at a little over 500 kHz. There is ringing due to operating in discontinuous conduction mode (DCM). This ringing will manifest as a broad peak at the ring frequency, in this case, 4.2 MHz. You can observe this peak in Figure 10. I describe this concept in more detail in [3]. We can observe that each frequency spike occurs at 500 kHz intervals.

oscilloscope DC-DC converter measurements — Figure 10. Measurement of the DC-DC switch-mode converter shows the harmonic peaks. (Image: Ken Wyatt)

Now, let’s look at the 12-MHz crystal oscillator, shown in Figure 11. Probing near the oscillator confirms the 12-MHz harmonics. Here, we’ve increased the upper frequency limit to 500 MHz.

H-field probe oscilloscope — Figure 11. The H-filed probe is shown over the board’s 12 MHz processor clock. (Image: Ken Wyatt)

Note the use of Peak List to identify major harmonics. The adjustments, Max Results, Threshold, and Peak Excursion can be varied to automatically label just the most important peaks. Note that the delta difference in adjacent peaks in Figure 12 is exactly 12 MHz and that some harmonics extend out to 500 MHz, or more.

MXO3 oscilloscope frequency peaks — Figure 12. Spectral display shows the 12-MHz harmonics and each peak’s frequency. (Image: Ken Wyatt)

Next, let’s measure the 16-MHz USB clock.

Again, I coupled the probe to the USB IC and its 16-MHz clock oscillator in Figure 13. Note that the delta difference in adjacent peaks is exactly 12 MHz and that some harmonics extend out to 400 MHz, shown in Figure 14.

H-field oscilloscope probe — Figure 13. Probing the USB clock of 16 MHz with an H-field probe.

MXO3 oscilloscope frequency display — Figure 14. The resulting spectrum of the 16 MHz USB clock shows the fundamental frequency and harmonics. (Image: Ken Wyatt)

Summary

I found the MXO3 oscilloscope is very useful for general EMC debugging and troubleshooting. The ability to observe both the time domain and frequency domain with its fast capture is a real advantage. Up to eight waveforms may be stored for comparison to the measured waveform. This will greatly help in comparing “before and after” measurements as various mitigations are tried.

Part 2 of this series will discuss how to measure high-frequency currents on cables. Because cables are the most frequent contributor to radiated emissions, characterizing these harmonic currents is really one of the most important steps in mitigating emissions. We’ll also demonstrate how to connect a simple antenna to the scope to measure the relative strength of the actual emissions.