Oscilloscopes are Heavy-Duty Signal Processing Machines


Henning Wriedt, US-Correspondent at Bodo’s Power Systems

VIP-Interview with Dr. Markus Herdin, Market Segment Manager Industry, Components and Research, Rohde & Schwarz, about the company’s expertise in oscilloscopes for power electronic designs.

 

Henning Wriedt: Dr. Herdin, which role does the oscilloscope play in modern electronic design and test?

 

Markus Herdin: Oscilloscopes are the Swiss Army Knife of modern electronics design. They act as a troubleshooting, verification, and production test tool. Due to their ability to capture almost any kind of waveforms and their post-processing capabilities, there are only a few things that, today, cannot be done with an oscilloscope.

 

Henning Wriedt: How did the oscilloscope evolve during the last ten years and how is your product portfolio currently structured?

 

Markus Herdin: The last ten years have brought many innovations to the oscilloscope business. From general-purpose electronics debug tools, oscilloscopes have evolved into heavy-duty signal processing machines that allow very fast and convenient analysis. One example is the acquisition speed modern oscilloscopes provide. It is possible to capture up to 1 million waveforms per second and display them on the screen.

This allows capturing of rare events, which would have been previously missed. Other examples are the fast Fourier transform (FFT) based spectrum analysis capabilities, which allow the user to perform EMI debugging tasks already at the engineer’s desk – and help save a lot of money due to less iterations with in-house testing. Not to forget new concepts like real-time de-embedding, which we introduced about a year ago, with the R&S®RTP oscilloscope (Figure 1).

 

Figure 1: RTP oscilloscope. Key facts: Bandwidth 4 GHz to 8 GHz; max. sample rate: 20 Gsample/s; max. memory depth: 2 Gsample; all trigger events at full bandwidth; up to 16-bit resolution; signal integrity in realtime.

Figure 1: RTP oscilloscope. Key facts: Bandwidth 4 GHz to 8 GHz; max. sample rate: 20 Gsample/s; max. memory depth: 2 Gsample; all trigger events at full bandwidth; up to 16-bit resolution; signal integrity in realtime.

 

Henning Wriedt: How important is a scope in automotive, communication, medical and IoT?

 

Markus Herdin: Automotive, communication, and medical products rely increasingly on high-performance computing systems. This makes digital design critical and verification of corresponding systems incredibly important. This is particularly true for automotive and medical applications because of the very high standards regarding product reliability they demand.

A big change happening in the automotive industry is the trend towards E-Mobility. Automotive companies and their suppliers are strongly investing into test and measurement solutions for power electronics design – and an oscilloscope with probes is the key instrument for this.

IoT is a different story. Companies developing IoT solutions are often small, innovative teams. For them, an oscilloscope has to cover as many use cases as possible. Modern oscilloscopes can cope with this pressure because of the trend that more and more analysis functions are constantly being added to them.

 

Henning Wriedt: Mobile electronic systems with and without battery are all around us with sometimes extreme low voltages and low power consumption. Can a modern oscilloscope cope with that situation?

 

Markus Herdin: Yes, but only with the right probing solutions. Actually, the dynamic range of an oscilloscope is limited by the A/D converter. Even though signal processing allows for the trade between bandwidth and resolution, there is a limit to it. This is where probing solutions come into play, which actually implement their own acquisition system.

The RT-ZVC probe is such an example. It is an 18-bit acquisition system that was developed particularly for applications where a high dynamic range for current measurements is required – like in IoT battery life debugging. The digital data is then transferred to the oscilloscope via digital ports, where the data is aligned with the signals captured by the oscilloscope and displayed on the screen. This is a unique solution that allows users to correlate power consumption with any other activity of the device under test. As a side effect, this probing solution also allows for measurements of extremely low voltages, lower than what any oscilloscope can do.

However, I do also want to mention power supplies in this context. Measuring power consumption requires particularly “clear” power supplies. If the power supply is noisy, it is very difficult to measure sleep-mode currents, as ripple on the power supply will directly translate into ripple in the current and therefore influence the current measurement. The R&S®NGL200 power supply is very well suited for this application due to its linear regulation and therefore very low ripple. In some applications, it is actually a great standalone solution for power consumption testing, as it features 6 ½ digit resolution voltage and current measurement.

 

Henning Wriedt: What kind of checklist would you recommend for a circuit designer, before he/she uses a scope for measurements and tests in power electronics (SiC/GaN semiconductors)?

 

Markus Herdin: It is important for them to understand their real measurement needs. Silicon carbide (SiC) and gallium nitride (GaN) semiconductors allow for very fast switching speeds. Theoretically, this requires very expensive probing solutions in the order of 4-5 times greater than a regular oscilloscope with significant drawbacks regarding analog signal quality and usability.

However, looking at the real demand, one can see that customers choose to operate these semiconductors at only moderate speeds in order to avoid excessive radiated or conducted emissions, and expensive shielding solutions.

Therefore, traditional probing concepts work very well. We recommend that our customers look at their real needs and then choose their probes accordingly - in order to save their money.

 

Markus Herdin: I see EMI debugging as an increasingly important application area for oscilloscopes. While, in the past, their FFT capabilities were limited and usability was not great, this has changed a lot.

Today, they provide us with the capability for efficient debugging of EMI problems, both for conducted as well as radiated emissions, especially for power electronics engineers – and they are convenient to use. Well, and since engineers, anyway, always have an oscilloscope available on their desk then no other equipment is needed for these additional tasks.

The scope performs this task by calculating the FFT in order to determine the spectrum; this has been the case for a long time. The innovation, however, comes in when this is done in an intelligent way, by making use of powerful signal processing hardware and algorithms.

The big advantage for conducted emissions and debugging in the case of power supplies is that the scope can analyze the effect of EMI filters on the emission and at the same time look at the stability using the Bode plot feature, for example, with the R&S®RTM3000 (Figure 2). This is very helpful as the EMI filter influences the stability of switched mode power supplies; hence, optimizing EMI filters also requires verifying control loop stability. So, one gets all those things in just one instrument.

 

Figure 2: RTM3000 oscilloscope. Key facts: Bandwidth: 100 MHz to 1 GHz; Sample rate: up to 5 Gsample/s; Memory depth: up to 80 Msample; ADC resolution: 10-bit; Display: 10.1’’ capacitive touchscreen.

Figure 2: RTM3000 oscilloscope. Key facts: Bandwidth: 100 MHz to 1 GHz; Sample rate: up to 5 Gsample/s; Memory depth: up to 80 Msample; ADC resolution: 10-bit; Display: 10.1’’ capacitive touchscreen.

 

For debugging radiated emissions, the scope captures the emission with a near-field probe. Here, sensitivity and speed of the FFT are essential, as the customer has to move the near-field probe over the device under test to understand where the noise is coming from. The R&S®RTO2000 (Figure 3) is a great instrument for these tasks; many of our customers love it because it is so simple to use.

 

Figure 3: RTO2000 oscilloscope. Key facts: Bandwidth 600 MHz to 6 GHz; max. sample rate: 20 Gsample/s; max. memory depth: 2 Gsample; up to 16-bit vertical resolution; MSO: 16 digital channels.

Figure 3: RTO2000 oscilloscope. Key facts: Bandwidth 600 MHz to 6 GHz; max. sample rate: 20 Gsample/s; max. memory depth: 2 Gsample; up to 16-bit vertical resolution; MSO: 16 digital channels.

 

Henning Wriedt: Can I reconfigure a scope for different applications without compromising its performance?

 

Markus Herdin: Yes, you can actually; oscilloscopes use the latest field-programmable gate array (FPGA) technology like dynamic reconfiguration. This means that the signal processing chain can be reprogrammed dynamically according to the application running on the oscilloscope.

A typical example is the protocol decode operation. There, the corresponding protocol decoder is only loaded into the FPGA if it is used, which saves FPGA resources and, therefore, costs. This translates for the customer into more functionality at the same price.

 

Henning Wriedt: How can an engineer detect sporadic emissions and single events? And run tests under hazardous conditions?

 

Markus Herdin: The trigger functionality of oscilloscopes is the essential principle of capturing specific, but also sporadic or single events. This is a feature as old as the oscilloscopes – but only available in the time domain.

The novelty is that this principle can also be applied to signals that are calculated out of the captured signal. One example is the FFT where, for example, spectral masks can be set in order to capture just the signal that contains a specified frequency content. Another example is a trigger event derived from the real-time protocol decoder, which allows capturing only events where a specific digital signal was present.

A user can join this capability with something called history mode, which is, basically, an automatic memory for consecutive acquisitions. Because of this, the user can “move back in time” to review thousands of acquisitions that have happened.

This is very useful in combination with dedicated trigger conditions. A customer can capture all events that contain the specified signal content and later review when they happened and how they looked like.Henning Wriedt: What advantages has a FFT analysis for a circuit designer?

 

Markus Herdin: The FFT analysis provides a different view of the signals under test. While in EMI debugging applications, the spectrum itself is what the customer wants to see; for general-purpose signal analysis, the FFT allows identifying signal components that are otherwise hidden in the time-domain waveform. A good example are power integrity measurements. One of our customers had difficulties with the signal integrity of his digital design but he did not know the reason behind it.

We approached the problem by looking at the power rail with a dedicated probing solution: a power rail probe. This enabled us to capture the problem: noise that was coupled onto the power rail. Going into the frequency domain with the FFT function revealed the root cause - a Wi-Fi RF signal got coupled onto the power rail. This, of course, deteriorated the quality of the power supply and led to signal integrity problems of his digital design.The other advantage is that, today, using an FFT is no longer complicated.

An oscilloscope allows us to directly set the essential parameters like start and stop frequencies, as well as resolution bandwidth, so the customer does not have to think about the exact relationship between time domain and frequency domain. The oscilloscope does that in the background.

 

Henning Wriedt: The probe between the scope and the DUT must not only meet safety standards but also ensure precise test results and shouldn’t interfere with the DUT. In general: What is your take on probes?

 

Markus Herdin: Probes are the most important element when it comes to measurements. As they pick up the signal from the device under test, they directly influence the quality of the measurement itself. Often, standard passive probes are used to perform high-quality measurements, which can lead to wrong conclusions and a lot of extra work.

A good example of this are power integrity probes. They are optimized for power integrity measurements, and using a standard passive probe for such a measurement can result in way too high peak to peak voltage measurements and wrong conclusions about the quality of the power rail.

 

Henning Wriedt: Which probes are especially helpful in fast switching power electronics?

 

Markus Herdin: High-voltage differential probes are the right solution to choose. They have a very good common-mode rejection ratio and allow for safe and convenient measurements.

 

Henning Wriedt: Which are the benefits of Power Integrity Testing?

 

Markus Herdin: Power integrity testing becomes increasingly important as high-speed electronic designs find their way into many applications. As the designs become more sensitive to noise, the requirements for DC power supplies also become more stringent. The real problem is that noisy DC power rails do not necessarily lead to an immediate product failure, but increase the likelihood of a failure in the, not-so-well controlled, real world where the products are actually being used.

The noise level of a DC power rail has a direct influence on e.g. the jitter of digital clocks, which increases the likelihood of bit errors in digital data transmissions inside of a high-speed design. Another regular issue is that the IoT devices show coupling of RF antenna signals to power rails, which leads to product failures as well. These things are taken into consideration when testing for power integrity.

 

Henning Wriedt: What are the main advantages of a modern Power Analyzer?

 

Markus Herdin: Power analyzers are typically used if particularly high measurement accuracy is needed, or conformance to specific standards has to be tested. In contrast to oscilloscopes, they use a high-resolution AD converter running at much lower speeds and integrate limit testing according to pre-defined standards. An example is energy label testing. For consumer products, it is required to show the energy consumption according to the energy label standards. This is a typical measurement of the R&S®HMC8015 (Figure 4) power analyzer by Rohde & Schwarz.

 

Figure 4: HMC8015 Power Analyzer. Key Facts: Power measurement range: 50 μW to 12 kW, analog bandwidth: DC to 100 kHz; Sampling rate: 500 ksample/s; 16 bit resolution for current and voltage; basic accuracy: 0.05 %.

 

Henning Wriedt: Circuit designers need sophisticated and reliable power supplies. What can they expect from R&S?

 

Markus Herdin: Rohde & Schwarz offers a wide portfolio of power supplies to meet the requirements of different applications, reaching from basic benchtop to performance and specialty power supplies. For example, the power supplies for educational purposes focus on stability and simple operation, while other models are tailored to specific applications and have advanced features. Our portfolio is constantly growing and you can expect new and interesting product introduction by Rohde & Schwarz in the product category of power supplies.

 

Markus Herdin (PhD), Market Segment Manager Industry, Components and Research, Rohde & Schwarz. He is a seasoned market development professional at Rohde & Schwarz with a focus on power electronics test & measurement applications. His previous experience at Rohde & Schwarz includes roles in product management, product development and corporate business development. Herdin holds a PhD in Electrical Engineering as well as an MBA from the University of Chicago Booth.

 

More information: Bodo’s Power Systems    Source: Bodo's Power Systems, May 2019