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A How2Power Design Video Series*:
Troubleshooting Distributed Power Systems

Stability, noise, and EMI problems occur in all types of electronic systems, both large and small. As electronics technology continues to evolve, many of the conventional techniques used to assess stability, noise, and EMI issues are proving inadequate. Building block components such as op amps, voltage references, linear regulators, and switching regulators are inadequately characterized in vendor datasheets. Even worse, IC datasheets often advise use of external components that degrade component performance, creating stability or noise problems in the user’s application. These issues make it imperative that circuit designers verify the performance of their building block analog and power components on the bench, preferably in system. Unfortunately, many conventional test techniques for assessing component stability and noise performance in-system fall flat because many components do not allow access to internal control loops, or do not lend themselves to in-system testing, or do not adequately characterize component performance. In some cases, conventional test instruments and test setups lack the bandwidth, selectivity, or sensitivity to adequately assess component performance in or out of system.

In this 8-part video series, Steve Sandler of AEi Systems and Picotest discusses essential concepts, relationships and test methods needed to identify and correct stability, noise and EMI issues in electronic components and systems. Relationships between stability, noise and impedance are discussed, and methods for assessing stability and noise issues (both in and out of system) using impedance measurements are described at length. Many of the test methods described rely on the use of vector network analyzers or VNAs (available from multiple instrument vendors) and specialized adapters (available from the presenter’s company. Picotest). The use of VNAs represents a departure from conventional approaches using frequency response analyzers (FRAs) or oscilloscopes, but promises higher fidelity and the ability to make measurements that the other instruments cannot. However, as Sandler discusses in this video series, to fully address stability, noise and EMI issues, designers need to make measurements in multiple measurement domains—time, frequency, spectrum, and impedance. To that end, he presents test methods using VNAs, spectrum analyzers, and oscilloscopes, explaining how to optimize the test setups and interpret measurement results. Summaries and links for the individual videos in this series appear below along with links to the references cited in the videos. All of these videos are posted on How2Power’s YouTube channel where you’ll also find comments on the videos in this series. Viewers are encouraged to post their comments on the videos as well or to email their comments to the editor.

* This video series was originally presented in the May 2013 through January 2014 issues of the HOW2POWER TODAY newsletter.

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Troubleshooting Distributed Power Systems (Part 1): Why Stability Matters

Control-loop stability impacts power supply performance in multiple ways. Even if the control loop is not oscillating, poor stability—as evidenced by low phase margin—in voltage regulator and reference circuits can lead to problems such as poor PSRR or reverse transfer performance. What’s more, many system-level problems such as clock jitter, noise-induced degradation of circuit performance, and EMI can be traced back to poor control-loop stability in the power supply. In this 5-minute video presentation, Steve Sandler discusses how stability impacts circuit performance, offering examples that demonstrate the effects of stability problems on both power supply and system performance.

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Materials referenced in this video:

  1. "Making The Connection Key To Accurate Test Data." (Read the How2Power Design Guide summary of this article.)
  2. "When Bode Plots Fail Us." (Read the How2Power Design Guide summary of this article.)
  3. "Tale of the Bode Plot Failure." (Read the How2Power Design Guide summary of this article.)

Troubleshooting Distributed Power Systems (Part 2): Impedance Is the Critical Measurement

Whether your goal is to optimize system performance or to troubleshoot issues in distributed power systems, impedance measurement is an indispensable tool. That's because there is a direct correlation between impedance, which is a highly observable characteristic, and two key measures of system performance: noise and stability. In this 10 minute video, Sandler discusses the value of impedance measurements and demonstrates their usefulness with two examples: one using vendor-supplied data for a voltage reference and another using ADS-generated data for a second-order control loop. As Sandler explains, analysis based on impedance measurements can more thoroughly assess control loop stability than Bode plot measurements, and can do so more conveniently for control loops that are hard-to-access in system, and for devices (like some voltage regulators) that do not allow access to the feedback path.

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Troubleshooting Distributed Power Systems (Part 3): Measuring Impedance Using VNAs (1-Port)

Vector network analyzers (VNAs) have always had measurement capabilities that looked like they would be particularly useful in assessing the performance of analog and power components and circuits. Unfortunately, the input range of these instruments did not extend low enough in frequency for many power and analog applications. However, two of the more recently introduced VNAs—Omicron Lab’s Bode 100 and Agilent’s E5061B—have changed this situation, making it possible for VNAs to measure the impedance of many types of power components and circuits as Sandler explains in this video. This video focuses on single-port measurements, describing how they can be applied to measure the impedance of low power circuits such as linear regulators, voltage references and op amps as well as semiconductors, capacitors, and inductors. Test set up requirements are discussed and example measurements are presented.

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Troubleshooting Distributed Power Systems (Part 4): Measuring Impedance Using VNAs (2-Port)

In the previous installment in this video series, Sandler discussed the benefits of using VNAs to measure the impedance of devices encountered in distributed power systems, providing details on how to perform and interpret one-port measurements. In this video, the focus shifts to making and interpreting two-port impedance measurements, particularly those in which the device under test is connected “in shunt through” with the VNA ports. Shunt through, wideband measurements can be made from approximately 100 microohms to a few ohms. This is the measurement performed by designers for the power distribution network (PDN) assessment of VRMs and POLs. This technique may also be used to measure the impedances of batteries, dc-dc converters, EMI filters, and other functions. Setup requirements such as 4-wire connections, a common-mode transformer, dc blockers, and ac versus dc coupling are explained in this video. Also discussed is the use of a preamp to measure impedances below 1 milliohm. In addition, the video presents impedance measurement examples such as a POL output, a motherboard PDN and a 250-microohm resistor.

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Troubleshooting Distributed Power Systems (Part 5): Using Current Injectors

Previous videos in this series discussed the use of vector network analyzers (VNAs) to measure impedance using one- or two-port configurations. This video discusses another method of measuring impedance--the current injector method. The current injector is a voltage-controlled current source. In the impedance measurement, the current injector is modulated by the VNA, which also measures the resulting voltage at the device under test as well as the modulated current. The VNA calculates voltage/current to obtain impedance. Although not quite as accurate as the two-port VNA impedance measurement, the current injection technique has advantages including wide range (approx. 1 milli-ohm to thousands of ohms), the ability to measure in-system, and a suitability for measuring low-power devices such as op amps, voltage references and voltage regulators. In addition, current injectors can be used to carry out other types of tests such as non-invasively measuring PSRR, determining power integrity and signal integrity sensitivity, and in generating high-speed load steps. In this video, Steve Sandler discusses each of these current injector measurement capabilities and presents test examples that illustrate how designers can perform and interpret these measurements. Along the way, Sandler offers many test tips to help the engineer obtain the most accurate results.

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Troubleshooting Distributed Power Systems (Part 6): The Switch Node

System and power converter issues are frequently related to a converter’s switching characteristics, which are most easily observed at the switching node. In this 10-minute video, Steve Sandler discusses the measurement and interpretation of switch-node waveforms as observed in point-of-load regulators or POLs. He discusses the instrumentation requirements for measuring switch-node waveforms, why switch-node waveforms should be viewed using different time scales, and the impact of scope probes on measurements. With those measurement requirements as background, Sandler examines how switching frequency and duty cycle affect power supply stability as well as EMI. Measurement examples demonstrate the influence of the switching frequency on the zero-order hold, which in turn impacts stability margin; the impact of stability on switching frequency jitter; and the impact of frequency jitter on EMI and output ripple and noise.

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Troubleshooting Distributed Power Systems (Part 7): Measuring Ripple

Circuit designers, particularly power supply designers, are frequently required to measure power supply ripple. Nevertheless, many engineers struggle with this measurement as sensitivity, selectivity, and bandwidth limitations degrade the accuracy of oscilloscope results. But as Steve Sandler explains in this 7-minute video, specialized probes and adapters can improve the results obtained from the conventional time-domain approach to measuring ripple, while other approaches to measuring ripple—involving the spectrum and impedance domains—can yield more-accurate and more-insightful results. Various test setups and measurement techniques are described in this video and example results obtained from testing different point of load regulators are discussed. Sandler also provides tips to help designers avoid common measurement pitfalls.

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Troubleshooting Distributed Power Systems (Part 8): Making Time Domain Measurements

There are inherent challenges in measuring power supply waveforms in the time domain. Limitations in oscilloscopes and probes, difficulties imposed by linear time and amplitude scales, and even seemingly helpful scope features like trace averaging can either degrade measurement fidelity or hide “glitches” and waveform anomalies that can wreak havoc with system performance. In this last installment of the video series, Steve Sandler delves further into the subject of how to make time-domain measurements of power supply signals, providing further discussion on the sources of measurement error and many examples of noise and stability problems that engineers may be missing. Sandler dispenses techniques and tips on how to observe hard-to-spot problems and how to make higher-fidelity measurements in general. Even experienced power supply designers may be disappointed to hear that their oscilloscopes are underpowered with respect to bandwidth and sampling speeds. As with the earlier videos, this one is not just for power supply designers, but for any circuit designers concerned with power integrity in their system designs

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About the presenter:

For more information on power-related test and measurement topics, see the Advanced Search page of How2Power Design Guide, and select 'Test and Measurement" under "Design area."

Steven Sandler is the founder and chief engineer of AEi Systems, where he leads development of high-fidelity simulation models for all types of simulators as well as the design and analysis of both power and RF systems. Sandler has over 30 years of experience in engineering and is a recognized author, educator and entrepreneur in the areas of power, RF and instrumentation.

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