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.

Reflections happen along an interconnect at any point where the impedance environment around the electromagnetic wave changes. Figure 1 illustrates this with a simple example using a uniform stretch of transmission line with Z01 characteristic impedance between Z0 reference impedance connections.

The formulas shown in Figure 1 for the G voltage reflection coefficient are generic and express the complex ratio of reflected and incident waves. We can apply the formula to steady-state impedances—something we could measure with a vector network analyzer—or to transient impedances, which would be the case when we use time-domain reflectometry. In general, the impedances that go into the formula—and as a result, the voltage reflection coefficient itself as well—are complex numbers with magnitude and phase or real and imaginary parts.

To read this entire column, which appeared in the October 2019 issue of Design007 Magazine, click here.

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2019

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

11-18-2019

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.

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2018

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

12-19-2018

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.

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2017

Quiet Power: Causal Power Plane Models

12-13-2017

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.

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2016

Dynamic Models for Passive Components

05-11-2016

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.

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2015

Avoid Overload in Gain-Phase Measurements

07-01-2015

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.

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Effects of DC Bias on Ceramic Capacitors

04-01-2015

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.

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2014

Avoid Overload in Gain-Phase Measurements

07-01-2015

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.

View Story

Effects of DC Bias on Ceramic Capacitors

04-01-2015

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.

View Story
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2013

Quiet Power: Cable Quality Matters

11-20-2013

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.

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Quiet Power: Don't Forget to Terminate Cables

10-23-2013

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?

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Quiet Power: Do Not Measure PDN Noise Across Capacitors!

08-07-2013

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.

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Quiet Power: How to Read the ESR Curve

01-15-2013

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.

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2012

Quiet Power: Cable Quality Matters

11-20-2013

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.

View Story

Quiet Power: Don't Forget to Terminate Cables

10-23-2013

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?

View Story

Quiet Power: Do Not Measure PDN Noise Across Capacitors!

08-07-2013

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.

View Story

Quiet Power: How to Read the ESR Curve

01-15-2013

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.

View Story
Back

2011

Quiet Power: Cable Quality Matters

11-20-2013

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.

View Story

Quiet Power: Don't Forget to Terminate Cables

10-23-2013

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?

View Story

Quiet Power: Do Not Measure PDN Noise Across Capacitors!

08-07-2013

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.

View Story

Quiet Power: How to Read the ESR Curve

01-15-2013

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.

View Story
Back

2010

Quiet Power: Cable Quality Matters

11-20-2013

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.

View Story

Quiet Power: Don't Forget to Terminate Cables

10-23-2013

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?

View Story

Quiet Power: Do Not Measure PDN Noise Across Capacitors!

08-07-2013

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.

View Story

Quiet Power: How to Read the ESR Curve

01-15-2013

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.

View Story
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