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Integrated Oscilloscopes Tackle a Changing Landscape

Parent Category: 2014 HFE

By Varun Merchant 

Digital systems are becoming faster, more capable and of course more complex. In today’s world, new electronic system have to sip power, connect to Wi-Fi and Bluetooth networks, and be stable as a rock. But how is this impacting the test and measurement tools engineers need when working in a mixed domain environment that includes everything from DC to RF, analog and digital signals, and serial and parallel buses?

To find out, we asked engineers what bench instruments they used most frequently in addition to an oscilloscope. Here is what they reported:

  • Digital voltmeter         87%
  • Function generator      68%
  • Spectrum analyzer      45%
  • Logic analyzer            33%
  • Protocol analyzer        15%

Assembling a bench full of instruments capable of performing these tasks can be a formidable and costly undertaking. More often than not, engineers will need to share instruments with colleagues across the lab, which means there will be a lot of “hurry up and wait” scenarios. And then there’s the challenge of trying to master the interface of multiple instruments often from different manufacturers. 

To address this reality, test and measurement suppliers are now starting to offer mixed domain oscilloscopes that combine support for time and frequency domains and multiple other functions in a single box. In addition to all the capabilities of a digital oscilloscope, other functions found in these integrated instruments include spectrum analyzer, logic analyzer, protocol analyzer, arbitrary function generator and digital voltmeter (DVM).  

But how does such an instrument work in practice? Can it really replace multiple standalone instruments? While there may be occasions where a specialized instrument will be needed, an integrated scope can in fact perform the majority of everyday measurement tasks an engineer is likely to encounter. In this article, we’ll first tackle a typical power measurement task using an integrated scope and then look at how a built-in spectrum analyzer in a scope compares to the more typical FFT capability. We then take on a real-world mixed domain use case. 

Validating a Switching Power Supply Design

Oscilloscope-based power measurements enable any user to quickly get the same accurate and repeatable results as a power supply expert, even if they rarely deal with power measurements. This example shows common power measurements and how they’re done with an integrated oscilloscope using automatic power measurements, an integrated DVM, and differential and current probes.

In this example, the input voltage (yellow) and current (blue) from an AC-to-DC converter is shown in Figure 1. The integrated 4-digit DVM was then turned on to monitor the DC output voltage. The measurement statistics at the right side of the DVM display indicate that the output voltage is very stable, and the graphical readout provides a visual indication of voltage variations. Power measurement applications were then used to take input power quality measurements including power, crest factor, and power factor to characterize the effects of the power supply on the AC power source. From there, current harmonics measurements were used to provide a frequency-domain analysis of the input current, in both graphical and tabular formats.

1410 HFE Oscilloscope01

Figure 1 • Monitoring the DC output voltage with the DVM. The AC input voltage waveform is shown in yellow and the current waveform in blue. 

 

Another key power measurement is switching loss in the switching device, a major limitation to the efficiency of a power supply. In this case, the differential voltage across the MOSFET (yellow waveform) was measured, as was the current flowing through the switching device (blue waveform). Then the instantaneous power waveform was generated (red waveform in Figure 2) and switching loss power and energy measurements were displayed.

1410 HFE Oscilloscope02

Figure 2 • Switching loss power and energy measurements are displayed. 

Finally, the safe operating area measurement allows automatic monitoring and pass/fail testing of switching behavior over various input and load conditions. By comparing the switching device’s voltage, current, and instantaneous power levels relative to the device’s maximum ratings, this measurement is used to assure that device reliability will not be compromised by exceeding specifications.

Mixed Domain Use Cases

Recent research shows over 40 percent of embedded design projects include some form of wireless capability. This fact alone makes a compelling case for the integration of time domain and frequency domain analysis in a single instrument.  A particular challenge is that modern wireless communications such as 802.11ac, are using wide bandwidth modulation schemes to provide greater data throughput. In order to effectively measure modern wireless technologies, it is often necessary to capture the entire channel bandwidth at a single point in time.

What’s more, the use cases for a spectrum analyzer are not limited to just pure RF applications. Even for designs without wireless technologies, engineers are facing EMI, crosstalk, and noise-related issues that are easier to diagnose in the frequency domain than in the time domain. Understanding the emissions profile of a product under test at all test frequencies often requires the ability to see those frequencies at the same time, to avoid missing a critical event.  

Scope-based FFT

Most oscilloscopes have the capability of calculating and displaying a Fast Fourier Transform (FFT) of the acquired time domain signal. On the surface, this would seem to provide adequate frequency domain analysis capabilities for many users. However, the typical oscilloscope, even with FFT capability, is a poor substitute at best for a spectrum analyzer when it comes to looking at spectral information.

In order to make spectral measurements, an input capable of measuring high frequency signals is required. Many modern communications signals operate in the ISM bands at 2.4 and 5.8 GHz. Even making measurements on a relatively low frequency 900 MHz system requires an input frequency range of 2.7 GHz to examine the third harmonic.

While oscilloscopes are available with bandwidths that can measure these signals, the spectrum is limited to the bandwidth of the oscilloscope—potentially forcing purchase of a more expensive scope than might otherwise be required for time domain analysis of analog or digital signals. Additionally, since signal amplitude gradually rolls off to -3dB at the oscilloscope’s rated bandwidth, RF measurements made anywhere near the rated bandwidth of the oscilloscope are attenuated significantly.

Of equal importance when making RF measurements is signal fidelity. The most important measure of fidelity in a spectrum analyzer is Spurious Free Dynamic Range, (SFDR). This multi-faceted specification indicates the ability for a spectrum analyzer to detect and measure small signals in the presence of large signals. Because of their general purpose nature, oscilloscopes provide ~45dBc SFDR, much worse than the ~60dBc provided by spectrum analyzers. Low noise performance is important for measuring low level signals, and out-of-band emissions for transmitters.

Another downside of using an FFT on an oscilloscope for making frequency domain measurements is that the oscilloscope’s user interface is understandably optimized for time domain measurements. This makes it quite difficult to make typical spectrum analyzer adjustments, such as center frequency, span, and RBW. Adjusting the display typically involves manual calculations of the time domain parameters of sample rate, record length and FFT window shape. It is also often impossible to get exactly the desired settings. 

Spectral analysis with a scope FFT is limited compared to a spectrum analyzer. Manual cursors are typically required to identify the frequency and amplitude of peaks in the spectrum. Typical spectrum analyzer trace types such as Max Hold, Min Hold and Average are not available.  Likewise, typical spectral measurements like Channel Power or Occupied Bandwidth are not available. 

Integrated Spectrum Analyzer

A mixed domain oscilloscope avoids the limitations of scope FFT by incorporating a true spectrum analyzer to address the problems discussed above. At the same time, MDOs leverage oscilloscope acquisition technology to provide wide capture bandwidth, a performance advantage over entry-level spectrum analyzers.

The integrated spectrum analyzer of an MDO provides a dedicated pathway and input for the spectrum analyzer as shown in Figure 3, providing the required performance for typical RF signals without requiring the traditional oscilloscope channels to equal that performance. Thus, adequate performance levels are achieved on both analog and RF channels while keeping the price of the instrument in line with a bench oscilloscope. The RF channel in the MDO has flat frequency response across the entire frequency range leading to more accurate measurements and an SFDR rating of up to -65dBc (typical).

1410 HFE Oscilloscope03

Figure 3 • An MDO has a distinct signal path optimized for RF performance. Some instruments also have a dedicated A/D converter to allow simultaneous time and frequency domain acquisitions. 

From a usability perspective, an MDO provides an experience similar to that of a spectrum analyzer. The MDO automatically optimizes acquisition parameters for the frequency domain when making spectral measurements. It also offers a complete set of spectrum analyzer controls for most common adjustments including Center Frequency, Span, Reference Level, RBW and Markers. 

An MDO also provides a more complete set of analysis capabilities when compared to an oscilloscope FFT. Typical spectrum analyzer trace types are supported including Normal, Max Hold, Min Hold, and Average as well as typical spectrum analyzer detection methods including +Peak, -Peak, Average, and Sample. A range of automated measurements are also available including Channel Power, Adjacent Channel Power Ratio, and Occupied Bandwidth.

But an MDO is not exactly the same as a standalone spectrum analyzer – it takes advantage of the wide bandwidth architecture of an oscilloscope to offer notable improvements.  This means that an MDO is able to acquire the entire spectrum of interest with up to 3.75 GHz capture bandwidth. 

Searching For a Noise Source 

A common debug task is tracking down the source of noise in a design. An integrated spectrum analyzer enables mixed-domain debug using a single instrument. In this example, a very high-frequency signal riding on one of the low-frequency signals was discovered while probing around the circuit board. Using a cursor measurement in the time-domain display, the dominant noise was seen at about 900 MHz.

Switching to an integrated spectrum analyzer, a near-field probe was used to capture radiated signals. The spectrum analyzer’s center frequency was set to 900 MHz and the span set to 2 MHz. A dedicated front panel keypad is available for setting these and other RF parameters. Then the near-field EMI loop antenna was slowly moved over the circuit board looking for the highest signal level at 900 MHz. The strongest signal was found at the output of a clock generator circuit in an FPGA as shown in Figure 4.

1410 HFE Oscilloscope04

Figure 4 • Strong 900 MHz radiation was detected at the FPGA.

 

For further analysis, a spectrogram display could be used to monitor variations over time. In this case, the signal appeared to be fairly stable. After examining the FPGA layout, it was determined that the signal corresponded to the ninth harmonic of the 100 MHz Ethernet clock – a poor circuit board layout resulted in magnetic coupling to other signals in the design.

The Future of Scopes?

Is an integrated oscilloscope right for you? As always, your mileage may vary according to your needs and requirements – be sure to take a close look at spec sheets in comparison to your intended applications. But with prices coming down to match “standard” digital oscilloscopes and wireless becoming commonplace in embedded systems, it’s safe to say that integrated oscilloscopes are here to stay and represent the future of where oscilloscopes are headed. 

1410 HFE Oscilloscope05About the Author

As Mainstream Technical Marketing Manager for Tektronix, Varun Merchant supports mid-range wireless and power conversion product offerings. Before joining Tektronix, he worked for several year in product development and design in the semiconductor industry. He holds an MSEE from the University of California, Santa Barbara and an MBA from the Stern School of Business, NYU. 

 

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