Oscilloscope Background

Rohde & Schwarz oscilloscope used for EMI debuggingOscilloscopes featuring high sensitivity and dynamic range combined with powerful FFT capabilities are ideal tools for EMI debugging. Electromagnetic interference from electronic circuits and boards can be detected and analyzed with high speed and accuracy.

In the past, oscilloscopes were hardly the right choice for EMI debugging. They lacked adequate sensitivity for proper detection of electromagnetic interference. In addition, their fast Fourier transform (FFT) functions were not powerful enough to handle spectrum analysis, besides being complicated to use. Featuring sensitivity of 1 mV/div, bandwidth of up to 4 GHz and very low inherent noise, the R&S RTO digital oscilloscope from Rohde & Schwarz is an ideal choice for EMI detection and analysis using near-field probes. Based on the results of EMI compliance tests, the oscilloscope can be used in development labs to investigate electromagnetic emissions produced by electronic designs and identify their root causes. As a lower-cost alternative in such applications, there is the new R&S RTE oscilloscope with a bandwidth of up to 1 GHz.


FFT analysis with intuitive control like in a spectrum analyzer

Almost like a spectrum analyzer: user interface for the FFT functionFig.2: Almost like a spectrum analyzer: user interface for the FFT function.Almost like a spectrum analyzer: user interface for the FFT function.The FFT function is the key feature to EMI debugging with oscilloscopes. Conventional FFT implementations in oscilloscopes are inconvenient to use because the displayed frequency span and the resolution bandwidth are controlled by the time domain settings. This makes it difficult to navigate in the frequency domain and slows down signal analysis in the spectrum.

In the R&S RTO and R&S RTE, Rohde & Schwarz has developed an intuitive approach in which the FFT function is controlled in a similar way as in a spectrum analyzer. Typical parameters such as the start and stop frequency and the resolution bandwidth can be set directly (Fig. 2). The oscilloscope automatically makes the relevant time domain settings using its powerful signal processing capabilities and deep acquisition memory. This allows users to easily analyze EMI in the time and in the frequency domain, which greatly speeds up the process of identifying EMI sources.


Color-coded spectra for visualization of sporadic emissions

oscilloscope FFT functionFig. 3: The overlapping FFT function splits the acquired time domain signal into multiple segments to ensure high sensitivity for detecting sporadic EMI. The overlapping FFT function is a special feature of the FFT implementation in the R&S RTO and R&S RTE (Fig. 3). It provides high sensitivity for EMI detection while simultaneously allowing visualization of the spectral emissions over time. The oscilloscope divides the detected signal into many segments prior to FFT processing and computes a separate spectrum for each segment. This technique helps to reveal sporadically occurring, low-level signals in individual spectra. In the next step, the individual spectra are combined into an overall spectrum where the signals are color-coded by frequency of occurrence, i. e. sporadic signals have a different color compared to constant emissions. The color-coded overall spectrum provides an excellent overview of the nature and frequency of occurrence of the electromagnetic emissions that are present.


Correlation of sporadic emissions with events in the time domain

The gated FFT function allows the FFT to be restricted to a specific interval in the acquired time domain signal (Fig. 4). This time window (gate) can be moved across the entire acquisition period. This is useful for determining what segments of the time domain signal correlate with what events in the spectrum. For example, EMI from switched-mode power supplies can be correlated with overshoots in the switching transistor. Intermittent emissions produced by high-speed data buses with improperly routed lines can also be oscilloscope FFT displayFig. 4: The two time intervals that underwent FFT processing are highlighted in gray while the resulting spectra are displayed on the left and right below. Highlighted red: spectral component produced by an EMI pulse. Highlighted green: constant spectral component that is present in both spectra. clearly correlated with the relevant signal sequences using the gated FFT. After a problem is identified, the oscilloscope can be used to quickly assess the effectiveness of countermeasures such as blocking capacitors, additional shielding or changes to the routing of the bus signals.


Detection of sporadic emissions

Sporadic emissions rank among the most challenging EMI problems. First of all, such emissions are difficult to detect. Moreover, conventional measuring equipment is limited in its ability to analyze detected signals. Here, the mask function of the R&S RTO and R&S RTE offers a convenient solution (Fig. 5). It allows simple and flexible definition of frequency masks. If a signal violates a mask, it can be frozen with the “Stop On Violation” function. The ability to modify FFT parameters such as the frequency span being examined and the resolution bandwidth – even for previously acquired signals – makes this function very powerful. Even hard to detect electromagnetic emissions can be analyzed in detail.

The history function is also very useful for investigating EMI. It automatically saves the most recent signal acquisitions up to the maximum memory depth. In this manner, current and prior acquisitions can be compared and analyzed without limitations.


Summary: Oscilloscopes are useful tools in EMI debugging

Featurioscilloscope display mask functionFig. 5: The mask function allows detection and detailed analysis of sporadic emissions.ng powerful FFT signal processing, high input sensitivity and extensive acquisition and analysis capabilities, the R&S RTO and R&S RTE oscilloscopes are valuable tools for developers who need to perform EMI debugging on electronic circuits. Using overlapping FFT processing and color-coded spectral display, the oscilloscopes provide an overview of the frequency of occurrence of spectral components in the acquired signals, allowing fast identification of EMI sources. Since the FFT function is controlled similarly as in spectrum analyzers, users can easily navigate in the frequency domain without having to worry about the time domain settings.

Various add-ons such as the R&S HZ-15 compact, broadband near-field probe set round out the EMI T&M product portfolio. A new application note entitled “EMI debugging with the R&S RTO and R&S RTE oscilloscopes” provides useful tips and includes a practical example showing how to identify EMI trouble spots in electronic designs is available on the website of Rohde & Schwarz (here).

Author: Dr. Markus Herdin, Product Manager Oscilloscopes at Rohde & Schwarz