NEW - Bode Plots are Overrated
Control loop stability is critically important. But whether your focus is on power supply design, power integrity, or mixed-signal power system design, the Bode plot probably isn’t going to provide you with reliable or optimum design guidance. The five major reasons why Bode plots are no longer sufficient is detailed in this article.

NEW - Killing the Bode Plot
Many new linear devices, including voltage references, voltage regulators and opamps have internal feedback loops, prohibiting traditional bode plots. Yet, the assessment of stability is critical, since it directly impacts system performance, including clock jitter and system noise. This session takes a look at a non-invasive stability assessment. The theory behind the measurements, how accurate it is and measurement demonstrations for a variety of devices and setups.

NEW - Fix Poor Capacitor, Inductor, and DC/DC Converter Impedance Measurements
Measuring VRMs, PDNs (power distribution networks), capacitors, and inductors, requires measurement accuracy in the milliOhms range - whether it’s inductor DCR, VRM output impedance, or capacitor ESR. This requires you to use the 2-port shunt measurement to achieve this accuracy. Yet, often the DUT impedance range is large, spanning many orders of magnitude beyond what the 2-port measurement can handle. This article discusses the limits of this popular measurement setup and how to extend them in order to measure larger impedance ranges than would otherwise be possible with the 2-port measurement alone.

NEW - NISM VS. BODE PLOTS - A PowerPoint presentation comparing Non-Invasive Stability Measurement (‘NISM’) with traditional Bode Plots.
Companies are focused on Bode plots as the only stability criteria. Yet there are cases where this data is not accessible and/or where it does not provide a relative stability assessment. In these cases, NISM may be the only option and in other cases it provides a better overall assessment than a Bode plot. This presentation provide proof of the accuracy of the NISM testing technique by way of example.

NEW - How to measure the world's fastest power switch
Gallium Nitride (GaN) FETS are poised to replace silicon power devices in voltage regulators and DC-DC power supplies. Their switching speeds are significantly faster and their RDS(on) is lower than silicon MOSFETS. That can lead to higher power efficiency power sources, which is good for all of us. If you're designing power circuits with GaN devices, you need a grasp of the device's switching speed. To measure that, you're oscilloscope, probes, and interconnects must be fast enough to minimize their effect on the measurements.

Friday Quiz: Power Circuits & Measurements
With today's high-speed, low power circuits showing up everywhere, making power measurements has become a critical exercise for any circuit designer. One upon a time, you measured voltage and current with a DMM and looked at power waveforms with an oscilloscope. Today's high-speed switching circuits, especially those using gallium nitride technology, can switch power much faster than other semiconductor processes. Questions for this week's quiz come from Steve Sandler' book, Power Integrity.

Cable tips for your next measurement
Choosing the right instrument and using the proper measurement techniques are essential to getting the precise results. However, one common limitation lays directly between the device under test (DUT) and the test instrument: the cable.

Impedance measurements stabilize op-amp buffers
The unity-gain opamp buffer circuit is routinely used to transform high-impedance inputs into low-impedance outputs. When implemented well, the results work out nicely. More frequently than not, however, opamps suffers from degraded stability caused by output capacitive loading. To improve stability, start by measuring the op amp's impedance.

NEW - Voltage References Behaving Badly: Output Caps Are Key Source Of Poor Stability
The purpose of a voltage reference is to provide a very precise dc voltage level. In general the goal is to provide a very precise, low-noise voltage source. By its very nature, a voltage reference is quite susceptible to control loop issues. This is due in part to the low dc output resistance of the reference and its low power circuitry, which generally results in a relatively high effective output inductance. Despite this susceptibility, many voltage reference manufacturers recommend the use of an output capacitor. The following measurements highlight a few of the issues resulting from the addition of the output capacitor to the voltage reference circuit.

NEW - Measuring Stability Key To High Performance
The ability to maintain stable control loop performance is important in achieving high performance in circuits in point-of-load regulators, voltage references, op-amps, and other electronic circuits.

NEW - Simple trick to measure plane impedance with a VNA
The question of time vs. frequency has long been a controversial topic; in some cases, it leads to rather heated discussions. The argument in favor of a vector network analyzer (VNA), a frequency domain instrument, is that the dynamic range and signal to noise ratio (SNR) of a VNA are much better than they are for a time domain reflectometer (TDR). The argument in favor of TDR measurements is that they tend to be lower cost and are taken from a direct reading, so there is little to interpret. Fortunately, most new TDRs can also transform measurements to S-parameters (much like a VNA) and most new VNAs can also transform to time (providing TDR equivalent data).

Match Impedances When Making Measurements
Oscilloscope probes are handy tools for making measurements, but they have limitations. For example, they have rather limited bandwidth and that ground clip can create a loop that can pick up noise. You might think of using a coax cable. You can, but make sure impedance mismatches don't ruin your day.

Three Stability Assessment Methods Every Engineer Should Know About
Many engineers are familiar with the Bode plot as an effective stability assessment method. However, some authors suggest and even teach that the Bode plot is the only method needed. This article shows why this thinking is short-sighted. A single, low cost instrument that can produce Bode plots, as well as two other stability assessment methods is discussed providing a more comprehensive stability assessment set of guidelines.

NEW - When Bode Plots Fail Us
It may come as a surprise to some that a control loop’s Bode plot does not always depict the loop stability and that the output impedance measurement, obtained non-invasively, always does. Challengers of the non-invasive phase margin measurement approach believe that the method is not always an accurate representation of loop phase margin. While this can be true for transfer functions that are more than first order, the Bode plot does not always accurately depict the control loop stability in these cases either, while the non-invasive output impedance measurement does. The phase gain margins do not necessarily provide an accurate assessment of the control loop stability. This has ramifications in WCCA where guidelines call for PM and GM as the compliance criteria for loop stability. This article looks at two cases where the Bode plot doesn’t get the stability assessment right.

How to Measure PSRR
How to measure PSRR of a linear regulator circuit that powers an oscillator for an ADC. How can I make this measurement using my oscilloscope?

Common Oscilloscope Usage Mistakes
Avoiding these common pitfalls will get you better data with minimum effort. Steve Sandler takes a look at four common scope mistakes and gives details on how to avoid them.

Assessing Point-Of-Load Regulators Using Non-Invasive Techniques
Evaluating the stability of a Point-Of-Load (“POL”) regulator can be performed using both traditional Bode plot and output impedance measurement techniques.

Measure small impedances with Rogowski current probes
The VNA can be used with the Rogowski probe to measure impedance, such as the input and output impedance of a power supply or passive device.

How To Measure Ultra-Low Impedances
High-speed digital circuits need clean power, but can stress power systems in ways that make it difficult for the system to deliver what’s needed. For example, the current drawn by high-speed processors and FPGAs can change by tens of amperes in a nanosecond. The power system is expected to remain stable when this occurs, without ringing, over-shooting or under-shooting, or responding in any way that might cause the circuitry to misbehave. The power distribution network (PDN)—which, together with the power supply itself, form the power system—must maintain a low source impedance, with minimal resistance and well-controlled reactive elements.

Transformer Parameter Extraction
This paper shows how to use the network analyzer to extract the transformer characteristics, including the magnitude and the location of the leakage inductance terms, the mutual inductance, and the capacitance terms.

Debunking Transformer Performance Myths
In this article we intend to dispel three common myths regarding transformers, and more generally, magnetic coupling.

Why Network Analyzer Signal Levels Affect Measurement Results
When measuring a circuit or device with our network analyzers, engineers generally inject a signal and believe that the measured results reflect that of a small-signal AC measurement. Many of us have been taught to keep the signals "very small". This raises the questions, "how small is very small" and "is this an issue with my network analyzer"? SPICE based circuit simulators use partial derivatives to perform small-signal analysis, and therefore, are not sensitive to the signal level; regardless of the AC magnitude the results are linearized and scaled.

Apply an AWG for Power Design
A combination function generator and arbitrary waveform generator provides a low-cost means of exercising different design options when considering a particular power source.

New Techniques for Fault Diagnosis and Isolation...
This paper describes new software techniques to perform analysis, diagnosis, and isolation of failures in analog and mixed-signal circuits including switched mode power supplies.

Powering RF Systems
As design consultants specializing in circuit and systems reliability analysis, we evaluate many power supply designs used to power microwave and RF applications. Unfortunately, many of these power designs simply don't work as well as they need to; from the simplest linear regulators, to the switching converters, all the way up to the complete distributed power systems.

Deconstructing Step Load Responses Reveal a Wealth of Info
When analyzing power circuits, engineers routinely evaluate the stability using gain-phase measurements also known as bode plots. Many manufacturers of voltage and low-dropout regulators are now doing away with the bode assessment in favor of only the step load response. This article will show what can be learned from the step load response.

Non-Invasively Assess Your Multiple-Loop LDO’s Stability
Some low-dropout regulators (LDOs) use multiple feedback loops to improve output voltage transient response. In many of these cases, some or all of the loops are inside the IC. Designers can assess the stability of these regulators by using the non-invasive stability measurement capability of the OMICRON Lab Bode 100 vector network analyzer (VNA).

Stability Assessment of Fixed Regulators
There are cases where a Bode plot cannot be obtained from a control loop. For instance, a three terminal regulator or hybrid may not have its control loop exposed. With production hardware, it may not be possible to break into the control loop and inject a signal.

Five Things Every Engineer Should Know About Bode Plots
The Bode plot is recognized for its simplicity and the simple asymptotic approach makes the method useful for both design and assessment of power system control loops. However, Bode Plots do not always give an accurate representation of stability. While there have been several articles discussing when and why Bode plot potentially give misleading results, this article presents an excellent case study on the pitfalls, as well as a simpler alternative.

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