Quiet Power

Column from: Istvan Novak

Istvan Novak is the principal signal and power integrity engineer at Samtec, with over 30 years of experience in high-speed digital, RF, and analog circuit and system design

January 18, 2024

Quiet Power: The Effect on SI and PI Board Performance

In signal and power integrity (SI and PI), we would ultimately like to see a reasonable agreement between the predicted or simulated and the measured performance of our circuits. Real-world measurements will always contain errors and usually show a distorted replica of the true behavior of the device under test (DUT). Real measurements always show more than just the behavior of the DUT. Even if we don’t consider random noise and random errors, in the measured data we have contributions from instrument, cable, and probe errors (just to name a few) that our calibration could not completely remove.
May 22, 2023

Quiet Power: 3D Effects in Power Distribution Networks

Signal integrity (SI) grew out of the electromagnetic compatibility discipline in the early 1990s and gradually became a hot topic in its own right. Although the term SI never appeared in the title, one of the first books on topic was Introduction to Electromagnetic Compatibility by Clayton Paul. About a decade later, power integrity (PI) became the new hot topic, creating a separate discipline on which a multitude of books and conference sessions were based.
November 07, 2022

Quiet Power: Noise Mitigation in Power Planes

Inductive kick has been a well-known phenomenon in the electronic industry from very early on. First associated with motors, AC-mains transformers and mechanical relays, people noticed large voltage spikes when the current-carrying circuit was opened. Later as more sophisticated electronic circuits emerged, the same thing was noticed any time when current was changing through an inductor, or for that matter, through any inductance, whether it was an intentionally placed discrete inductor piece or just the parasitic inductance associated with a current path. This phenomenon is captured by the third Maxwell equation, which describes Farady’s Law.
January 19, 2022

Quiet Power: Uncompensated DC Drop in Power Distribution Networks

One recurring question I get is how to factor the DC drop into the power distribution network design process. Whether you prefer time-domain based or frequency-domain based design approach, the DC drop on the distribution path must be taken into account. Professional tools can do a good job to simulate the DC voltage drop on power planes, vias and traces, so after completing the layout, it is always a good idea to check the DC drop to make sure that the design meets the requirements. Here, I will walk you through some of the important options and considerations.
July 12, 2021

Quiet Power: Ask the Experts—PDN Filters

In recent years I have been getting a lot of questions about PDN filters from my course participants and from friends, colleagues and even from strangers. Long gone are the days when the essence of power distribution design recommendation was “place a 0.1uF bypass capacitor next to each power pin.”
April 16, 2021

Quiet Power: Friends and Enemies in Power Distribution

In signal integrity, for high-speed signaling, high-frequency loss is usually considered a bad side effect that we want to minimize. The DC loss, on the other hand, matters much less, because in many high-speed signaling schemes we intentionally block the DC content of the signal.
August 27, 2020

Quiet Power: Be Aware of Default Values in Circuit Simulators

Simulators are very convenient for getting quick answers without lengthy, expensive, and time-consuming measurements. Istvan Novak explains how, sometimes, you can be surprised if you forget about the numerical limits and the limitations imposed by internal default values.
July 30, 2020

Quiet Power: Do You Really Need That Ferrite Bead in the PDN?

Many times, users have to rely on application notes from chip vendors to figure out how to design the PDN for the active device. Within this still vast area of application notes, Istvan Novak focuses on just one question that greatly divides even the experts: Is it okay, necessary, or harmful to use ferrite beads in the PDN?
February 15, 2020

Quiet Power: PCB Fixtures for Power Integrity

Power-integrity components—such as bypass capacitors, inductors, ferrite beads, or other small discrete components—can be characterized in fixtures. Istvan Novak discusses the wide range of PCB fixtures available for power integrity.
November 18, 2019

Quiet Power: How Much Signal Do We Lose Due to Reflections?

We know that in the signal integrity world, reflections are usually bad. In clock networks, reflection glitches may cause multiple and false clock triggering. In medium-speed digital signaling, reflections will reduce noise margin, and in high-speed serializer/deserializer (SerDes) signaling, reflections increase jitter and create vertical eye closure.
December 19, 2018

Quiet Power: Measurement-to-Simulation Correlation on Thin Laminate Test Boards

A year ago, I introduced causal and frequency-dependent simulation program with integrated circuit emphasis (SPICE) grid models for simulating power-ground plane impedance. The idea behind the solution was to calculate the actual R, L, G, and C parameters for each of the plane segments separately at every frequency point, run a single-point AC simulation, and then stitch the data together to get the frequency-dependent AC response. This month, I will demonstrate how that simple model correlates to measured data and simulation results from other tools.
December 13, 2017

Quiet Power: Causal Power Plane Models

Causal and frequency-dependent models and simulations are important for today’s high-speed signal integrity simulations. But are causal models also necessary for power integrity simulations? When we do signal integrity eye diagram simulations, we define the source signals, so if we use the correct causal models for the passive channel, we will get the correct waveforms and eye reduction due to distortions on the main path and noise contributions from the coupling paths. Istvan Novak explains.
May 11, 2016

Dynamic Models for Passive Components

A year ago, my Quiet Power column described the possible large loss of capacitance in multilayer ceramic capacitors (MLCC) when DC bias voltage is applied. However, DC bias effect is not the only way we can lose capacitance. Temperature, aging, and the magnitude of the AC voltage across the ceramic capacitor also can change its capacitance.
July 01, 2015

Avoid Overload in Gain-Phase Measurements

There is a well-established theory to design stable control loops, but in the case of power converters, we face a significant challenge: each application may require a different set of output capacitors coming with our loads. Since the regulation feedback loop goes through our bypass capacitors, our application-dependent set of capacitors now become part of the control feedback loop. Unfortunately, certain combination of output capacitors may cause the converter to become unstable, something we want to avoid. This raises the need to test, measure, or simulate the control-loop stability. Istvan Novak has more.
April 01, 2015

Effects of DC Bias on Ceramic Capacitors

The density of multilayer ceramic capacitors has increased tremendously over the years. While 15 years ago a state-of-the-art X5R 10V 0402 (EIA) size capacitor might have had a maximum capacitance of 0.1 uF, today the same size capacitor may be available with 10 uF capacitance. This huge increase in density unfortunately comes with a very ugly downside. Istvan Novak has more.
December 03, 2014

Quiet Power: Vertical Resonances in Ceramic Capacitors

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.
April 02, 2014

Quiet Power: Checking Cable Performance with VNA

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).
November 20, 2013

Quiet Power: Cable Quality Matters

In his August column Istvan Novak looked at the importance of properly terminating the cables that connect a measuring instrument to a device under test. He writes that we may be surprised to learn that even if the correct termination is used at the end of the cable, the measured waveform may depend on the quality of the cable used.
October 23, 2013

Quiet Power: Don't Forget to Terminate Cables

In high-speed signal integrity measurements, the first rule is to properly terminate traces and cables. However, many PDN measurements may be limited to lower frequencies, such as measuring the switching ripple of a DC-DC converter. Do you really need to terminate measurement cables if the signal you want to measure is the switching ripple of a converter running at 1 MHz?
August 07, 2013

Quiet Power: Do Not Measure PDN Noise Across Capacitors!

PDN noise can be measured in a variety of ways, but measuring across a capacitor will attenuate the high-frequency burst noise. Keep in mind that by measuring across a capacitor, the converter output ripple reading could be several times higher--or many times smaller--than the actual ripple across our loads.
January 15, 2013

Quiet Power: How to Read the ESR Curve

To use bypass capacitors properly, any designer must understand ESR (effective series resistance). A designer must understand what it means and how to read the ESR curve in measured or simulated plots.
August 15, 2012

Quiet Power: What's the Best Method for Probing a PDN?

Recently, one of Istvan Novak's friends asked him about the preferred method of probing a power distribution network: "Which probe should I use to measure power plane noise?" Although, as usual, the correct answer begins with "It depends," in this case the generic answer is more clear-cut: For many PDN measurements, a simple passive coaxial cable is better.
April 04, 2012

Quiet Power: Will Power Planes Disappear?

Istvan Novak takes a look at an award-winning paper presented at DesignCon 2012, and he discusses the apparent disappearance of power planes from PCBs. In the future, the need for power planes may diminish or go away altogether. The change is already under way, and power planes, full-layer planes in particular, are disappearing fast from printed circuit boards.
February 01, 2012

Do Bypass Capacitors Change Plane Resonances?

My friend Greg recently asked me, "If I add surface-mount capacitors to a bare pair of planes, I am told that the resonant frequency will drop. On the other hand, someone with expertise is telling me that this is not the case. What would you expect to see?" As happens many times, both observations have elements of the truth in them, and a third scenario is not out of the question.
November 16, 2011

Be Careful with Transmission Lines in Plane Models

Last month, we learned how we can determine the grid equivalent circuit parameters for a plane pair. You may wonder: Is it better to use LC lumped components in the SPICE netlist or to make use of SPICE's built-in transmission line models? In short, we can use either of them, but we need to set up our models and expectations correctly.
October 12, 2011

Quiet Power: Simulating Planes with SPICE

There are several excellent commercial tools available for simulating power distribution planes. However, you don't need a commercial tool to do simple plane analysis. You can, for instance, write your SPICE input file and use the free Berkeley SPICE engine to get result. If you want to do your own plane simulations, there are a couple of simple choice.
August 16, 2011

Quiet Power: Does Dk Matter for Power Distribution?

We know that in signal integrity, the relative dielectric constant (Dk) of the laminate is important. Dk sets the delay of traces, the characteristic impedance of interconnects and also scales the static capacitance of structures. Is the same true for power distribution? The answer is yes, but for power distribution all this matters much less.
November 03, 2010

Do Not Perforate Planes Unnecessarily

For this column, I will take a quick detour from the series on the inductance of bypass capacitors. I will devote this column to a few comments about via placement and its potentially detrimental impact on signal and power integrity when antipads heavily perforate planes.
August 18, 2010

Inductance of Bypass Capacitors, Part III

In Part III of a series, we'll take a look at loop or mounted inductance. Loop inductance is important, for instance, when we need a reasonably accurate estimate for the Series Resonance Frequency (SRF), or for the anti-resonance peaking between two different-valued capacitors or between the capacitor's inductance and the static capacitance of the power/ground planes it connects to.
July 21, 2010

Quiet Power: Inductance of Bypass Capacitors, Part II

We finished the last Quiet Power column with a few questions about the inductance of bypass capacitors: Why do different vendors sometimes report different inductance values for nominally the same capacitor? Start by asking the vendors how they obtained these inductance values.
May 19, 2010

Why PI Design is More Difficult Than SI

Why is power integrity design more difficult than signal integrity design? Reasons abound, and unlike SI, we've only begun to study PI. Collective wisdom and experience gained over the coming years will help to alleviate the pain somewhat, but we should expect the challenge to stay with us for some time.
April 14, 2010

Why S11 VNA Measurements Don't Work for PDN Measurements

In this edition of Quiet Power, Istvan Novak continues to examine one-port and two-port vector network analyzer set-ups for PDN measurements, and other tricks and techniques for measuring impedance values below 5 milliohms.
February 24, 2010

PDN Measurements: Reducing Cable-Braid Loop Error

At low and mid frequencies, where the self-impedance of a DUT may reach milliohm values, a fundamental challenge in measurement is the connection to the DUT. Unless we measure a single component in a well-constructed fixture, the homemade connections from the instrument to the DUT will introduce too much error. What's the solution? By Istvan Novak.
January 27, 2010

Quiet Power: Calculating Basic Resonances in the PDN

In my last column, I showed that the piecewise linear Bode plots of various PDN components can create peaking at some interim frequencies. Today, I must cover peaking in more detail, because, even today, certain articles, books and CAD tools provide the wrong answers to this problem.
January 08, 2014

Comparing Cable Shields

In his last column, Istvan Novak looked at the importance of properly terminating cables even at low frequencies and also showed how much detail can be lost in PDN measurements when bad-quality cables are used. This month, he analyzes a step further the shield in cables.
April 02, 2014

Checking Cable Performance with VNA

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).
December 03, 2014

Vertical Resonances in Ceramic Capacitors

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.
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