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2017

A high-quality device model captures device behavior across geometry (W/L), temperature (T), drive voltage (Vdd), among numerous other input conditions. How much time do we spend trying to achieve a good fit across all these axes? What if we could easily bring up graphs of all key device targets such Vth (Threshold Voltage), Idsat (Drive Current), GM (Transconductance), and their variations across these dimensions?  We could then monitor them in one place, and tune or optimize model parameters while observing the agreement between measured and modeled data.  With Keysight’s Model Builder Program (MBP) 2017, we can achieve this goal. In this note, we are going to go through a couple of examples to show how MBP Script makes it possible to monitor device targets across the entire matrix of input conditions.

 

First, let’s look at the Threshold Voltage (Vth) variation as we vary device width (W) and length (L) on 4 sides of the W/L matrix, as shown below. 

Figure 1. Four sides of the W/L matrix for evaluation of Vth variation

 

The 4 sides of W/L space define the boundaries of modeled device sizes provided by a given semiconductor technology. A customer may use a geometry outside of this space, but will then need to re-target the model parameters for this unusual device.

 

Vth is the key device parameter of any transistor device, be it BULK, SOI or FinFET.  It defines when a transistor transitions from off to on.  If the Vth has been well modeled along the 4 sides of the W/L space, then we have good confidence that the model will work over the entire W/L plane. Both short and narrow channel effects will be covered.

By placing Tasks in sequence, a user may define a real modeling flow to be executed automatically.  A “Task” is object to represent a step in a flow, and is represented by a blue icon in the flow panel, as shown below in figure 2.  With Tasks, a modeling engineer can group modeling building blocks to extract the next logical group of device parameters.  In MBP, we may create a new Task button, as shown below, called “Display_Vth_Scaling_4_sides.”  By  clicking on this button, we can define and display the related graphs.

 

Figure 2. Display Vth W/L scaling plots by Task button clicking

 

In figure 2, we plot Vth in a 2x2 layout, where each quadrant represents one side of the W and L scaling space.  The solid represents simulated data and the square represent measurements.  The MBP tool is now ready for model tuning or optimization.

 

To create this beautiful matrix of graphs, we need to create a new Task from MBP Script window and name it “Display_Vth_Scaling_4_sides,” as shown in figure 3.  In the newly created Task, in its Plot Select tab, click the “Add Plot” button to add these 4 plots.  We may populate the “Plot Name” using 2 of the predefined plot groups “vth_l_vbs” and “vth_w_vbs.” In the “filter define” field, we may filter the data based on desired input conditions, like “w=max(w)”.  The filter definition is highlighted in green for the first plot where vds=abs(min(vds)) && w=max(w). In other words, we filter the data for the absolute minimum Vds, and maximum W input conditions. 

 

 

Figure 3. Create a Task by grouping Vth W/L scaling plots

 

MBP provides the most commonly used device targets, including Vth, Gm, Idsat, Idlin, etc. as well as common plot templates like the scaling graphs over W, L, T and Vdd.

 

One powerful plot type shows how well the model has been fitted across input conditions like geometry.  This is called an “Error Data Grid” plot.  In figure 4, we present the RMS error of device targets Vth and Idsat in our 2-dimensional geometry plane. However, we can choose pairs of input conditions to study, for example:

  • W and L
  • W/L and T
  • W/L and VDD

 

Figure 4. RMS Error plots of Vth and Idsat across W/L plane.

 

When tuning model parameters, we can observe the RMS errors update on the fly. For example, slightly different model parameter settings lead to the following Error Data Grid plots:

Figure 5. another state of the RMS Error plot of Figure 4.

 

The numbers are color-coded for quick and easy reading.

 

In figure 6, we illustrate how to create these incredibly useful Error Data Grid plots. In MBP’s Script window, we choose our device target (Idsat), specify the X and Y axes as ‘w’ and ‘l’ below, coloring for various RMS error ranges, etc. We can even add more targets to our Error Data Grid plot.

 

Figure 6. ErrorDataGrid plot creation

 

These newly created plots can be integrated in the Flow as another Task button, as shown below:

Figure 7. Display RMS Error plots by Task button clicking

 

Every Task object has a few properties of which “button mode” is one.  When the task is in button mode, then when you click it, it executes just that code to do an action, like extracting a group of parameters.  When the task is not in button mode, clicking on it has no immediate effect.  Rather, it marks where the flow may resume. 

 

By monitoring a model’s performance (RMS error) across all input conditions, MBP provides a high-level view of the model in real time.  Our modeling engineer may enjoy the peace of mind that the device model has been done thoroughly and accurately.

 

Please see the attachment below.  We have attached an example project that includes:

  1. demo data files
  2. demo model files, initial version and final version.
  3. script file that defines the flow Tasks and ErrorDataGrid plots

 

For more details about how to customize device targets and plots and how to customize the flow, please refer to the MBP Script tutorial.  https://devicemodelingworks.net/mbpst

 

ShuangCai

MALong

You have a waveform that was generated in MATLAB. How do you use that waveform as the source for a circuit in Genesys?

 

Keysight’s RF/Microwave Synthesis and Simulation Software, Genesys understands MATLAB. The full-featured MATLAB script debugger in Genesys enables you to develop error-free, fully compatible equations for data processing, simulation, and analysis. Genesys equation pages use MATLAB, so it is very easy to generate MATLAB waveforms in Genesys. You can paste MATLAB code directly onto a Genesys equation page to create any waveform.

 

Figure 1. You can easily import a MATLAB waveform into Genesys, and use that waveform as a source for a circuit.

 

Genesys Understands MATLAB

Running the MATLAB code in the equations page allows you to see and verify the waveform. Here, the waveform is stored and plotted in a variable called “PulseTrain”. The user will run the Equations first, and then the variable will be used by the source in the Genesys design.

 

Figure 2. The waveform is stored and plotted in a variable called “PulseTrain.” (Click image to zoom.)

 

How to apply the waveform to a circuit

In order to apply the waveform to a circuit, use a Custom Voltage source which allows us to specify a variable, PulseTrain, for the V parameter.

 

Figure 3. The variable PulseTrain is available for the source to use in the Genesys design.

 

From here, all that’s needed is to set up the Transient simulation for the desired time step and stop time. Run the simulation, and see the results below.

 

 

 

Summary

To use a waveform developed in MATLAB:

  1. Paste the MATLAB code that creates the waveform into an Equation page in Genesys.
  2. Assign the variable containing the waveform to a Custom Voltage source on the schematic.
  3. Run a Transient simulation.
  4. View the results.

 

Check out the related application note here.

 

 

 

 

Click here for a free trial of Genesys.

Having trouble getting started with your next circuit design? Keysight EEsof EDA’s YouTube channel is filled with “How To” videos and examples to help you with your complex designs. In each video, Keysight engineers walk you through the steps while also covering the fundamentals of each topic. At the end of each video, you can download the workspace to help get you started on your own projects. These five videos are a must-see for anyone working with Keysight ADS.

 

1. How to Design an RF Power Amplifier: The Basics

This video shows how power amplifier circuits work. If you are new to high-frequency power amplifier circuit design, this is the place to start. If you are an experienced designer, this video provides unique insights into the fundamentals that you may not have seen before. There is also an entire playlist dedicated to this topic on our YouTube channel.

 

Questions answered include:

  • What is AC power?
  • How is AC power generated and dissipated?
  • What topologies are convenient to use for power amplifier circuits?
  • What is a loadline and how can this be used to design a power amplifier?

 

 

2. How to Design for Power Integrity: Selecting a VRM

This video is one of a three-part series in Power Integrity tutorials. There are many factors to consider when selecting a VRM (Voltage Regulator Module), and this video covers two: Output Impedance and PSRR (Power Supply Rejection Ratio). This video uses measurement-based simulations to show that current mode topology is the best choice for flat impedance VRM.

 

Questions answered include:

  • What factors should I consider when selecting a VRM?
  • Why is flat impedance the PDN design goal?
  • Why is VRM selection not arbitrary?

 

3. How to Use Fixture De-embedding to Match Signal Integrity Simulations to Measurements

This video provides a quick 4-step process to show how to de-embed a fixture from a measurement to validate a PCB channel model, and then how to embed the fixture with the model to do a direct compare of simulation to full-path measurement both in the frequency domain and time domain.

 

Questions answered include:

  • How can I match SI simulations to measurements?
  • How can S-parameters simplify the problem?
  • What is the best process to solve this problem?

 

4. How to Design Phased Array Systems

This video discusses the most important considerations for phased array system design, especially popular for 5G. It begins with the basics of phased array design, then covers 4 key parameters of phased array architecture. After watching this video, you will be able to download the simulation tool, SystemVue, to perform your own phased array modeling and simulations.

 

Questions answered include:

  • How does a phased array work?
  • What are the key elements of phased array architecture?
  • What factors influence the far field pattern?

 

 

 

5. How to Understand 5G: Beamforming

This video guides you through what kinds of multi-antenna system architectures are being researched for the next generation 5G standard. It provides examples with end-to-end link level simulation and demonstrates key technical issues of different multi-antenna beamforming system design under mmWave channel environments.

 

Questions answered include:

  • How does beamforming work?
  • How can I model and simulate for key multi-antenna systems architectures?
  • Is hybrid beamforming too good to be true?
  • What is the 3GPP channel model for mmWave frequency band?

 

 

 

These are just five of the growing library of “How to” videos from Keysight EEsof EDA. Apply for a free trial of ADS or SystemVue to get started on your own designs.

 

 

 

Click here to apply for a free trial of ADS.

 

Keysight EEsof EDA is introducing a new set of tools for 5G, starting with pre-5G modulation analysis. This new technology is geared toward innovators with 5G testing in mind.  The 89600 VSA software provides comprehensive analysis capabilities for pre-5G signals based on the Verizon 5G open trial specifications.  With the standardized pre-5G technical specifications, engineers are already taking steps toward signal analysis.

 

vsaFigure 1. 89600 VSA users can observe pre-5G uplink and downlink physics layer measurements based on the Verizon 5G specifications. This example shows a downlink signal with 8 component carriers.

 

5G networks promise faster speeds, higher data rate, easier connectivity, and better network performance. Pre-5G allows users to experience certain 5G network elements while ensuring compatibility with their current 4G and LTE platforms.

 

Keysight’s latest 89600 VSA software helps break through the complexity of pre-5G, and will do the same for 3GPP 5G New Radio (NR). 3GPP 5G NR is the emerging global 5G standard. It is expected to be included in Release 15 of the 3GPP standard, which is due out later this year. With Keysight’s new 89600 VSA software release, early adopters can design and verify performance to the draft specification now, before it is published. Its extensive set of tools for demodulation and vector signal analysis enable you to explore virtually every facet of a signal and optimize even the most advanced designs.  

 

Figure 2. The 89600 VSA software will unify and accelerate the development process by providing
frequency-, time-, and modulation-domain analysis results in a single measurement.

 

The 89600 VSA software will unify and accelerate the development process by providing frequency-, time-, and modulation-domain analysis results in a single measurement. Users can observe pre-5G uplink and downlink physical layer measurements based on the Verizon 5G specifications. The 89600 VSA software is a vital and effective tool that will help blaze the trail to the 5G frontier.

 

 

The latest 89600 VSA software has over 100 recorded demo signals available for trial users. Discover the fundamentals of pre-5G air interface parameters, physical channels and signals.

 

Read the technical overview for more information on pre-5G signal analysis.

 

See the vast capabilities of the 89600 VSA Pre-5G Modulation Analysis in this four-minute video tour.

 

 

You can now jump start your 5G development for Verizon 5G, and continue for upcoming 3GPP 5G NR with a free trial of the 89600 VSA software.

 

“The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them.”-Sir William Henry Bragg Inventor of X-ray spectrometer, Nobel Prize for Physics, 1915

Introduction

Much like Sir William Henry Bragg stated, often, the recipe for new discovery entails new light, so elements can be viewed in a fresh perspective.

 

This week on Tim’s Blackboard, I will start with the motivation for Fourier to introduce his series, follow by his unintentionally visit to the frequency domain, and end with how the new frequency domain view helps us understand the root cause of eye closure. 

 

Fourier and “The Analytic Theory of Heat”

Whenever frequency domain is in a conversation, there is no escape from mentioning the name of this famous mathematician and physicist: Joseph Fourier.

 

Fig. 1: Jean-Baptiste Joseph Fourier. Image credit: https://commons.wikimedia.org

 

Although well-known for Fourier Series and Transform, in his 1800’s publications, the French-born scientist in Fig. 1 was originally analyzing heat flow.  

 

To solve the heat equation in a metal plate, Fourier had the idea to decompose a complicated heat source as a linear combination of simple sine and cosine waves, and to write the solution as a superposition of the corresponding eigen-solutions. Nowadays, this superposition or linear combination is known as the Fourier Series [1].

 

Fourier Series: Unfamiliar Yet Familiar

Although trying to represent a complicated function with linear combinations of sine and cosines might sound foreign, the decomposition of a complicated element into simpler sub-elements is a familiar idea.

 

In his lecture on Fourier Series, MIT Professor Dennis Freeman cleverly illustrates the similarity between Fourier Series and the Cartesian representation of an arbitrary vector in 3D-space [2].

 

Fig. 2: An arbitrary vector in 3D-space.

 

Shown in Fig. 2 is an arbitrary vector in 3D-space. Without additional coordinate information, our view of the vector is a geometric one: a line. However, as soon as we place the vector in a coordinate system, the vector geometry translates to vector magnitudes and directions. In a Cartesian system, there are three different components: one in x-direction, one in y-direction and one in the z-direction, as demonstrated in Fig. 3.

 

Fig. 3: Representation of an arbitrary vector in 3D-space in Cartesian coordinates. The original vector is separated into three components of various magnitudes in different directions.

 

The concept of Fourier Series is extremely similar. In Fourier Series, one deconstructs a periodic function into sines and cosines of different frequencies. The different frequencies of cosines and sines are analogous to the different directions in the Cartesian coordinates.

 

Take a classic ideal square wave for example. Fig. 4 shows the comparison between representing a vector in 3D-space and expressing a square wave with Fourier Series.

 

Fig. 4: Comparison between a vector in 3D-space and an ideal square wave expressed in Fourier Series. The sine waves of different frequencies correspond to the different directions in Cartesian coordinate system. 

 

It is important to note in Fig. 4, the “…” in the Fourier Series expression indicates an infinite sum of sines with only odd harmonics. Mathematically, we write

 

 

Unlike the vector in 3D-space, where only three magnitudes and directions are needed to recreate the vector, we need infinite number of magnitude and directions to truthfully represent the ideal square wave in Fourier Series.

 

But Tim, what if instead of infinite number of odd harmonics, I only have the first 10?

 

In ADS, there is a Vf_Square source that lets you experiment with the number of harmonics you desire to be in the Fourier Series. The result of the simulation is in Fig. 5.

 

Fig. 5: ADS simulation result of including only the first 10 odd harmonics in the square wave.

 

Stepping into Frequency Domain

Writing a function in the form of Fourier Series gives us a fresh perspective. Specifically, by looking at the Fourier Series construction of a function, we are able to visualize the frequency components present in the function and the strength of each frequency component.

 

Let’s revisit the ideal square wave expression, the Fourier Series shown below has both “direction” and “magnitude.”

 

 

Because the multiplication factor in front of ω0 indicates the frequency of the sine wave, we plot the factor, n, on the x-axis. For each nth harmonic, there is a specific magnitude that goes on the y-axis. Fig. 6 illustrates the parameters we are plotting.

Fig. 6: Illustration of what goes on a frequency domain plot. On the x-axis, we plot the harmonics, and we plot the magnitude on the y-axis.

 

Fig. 7 displays the log-log plot of frequency domain spectrum up to the 100th harmonic of the sine wave component that makes up the ideal square wave. The 1/n relationship of the magnitude and harmonic is made clear in a log-log plot.  

 

Fig. 7: Frequency spectrum of an ideal square wave up to the 100th harmonic. The magnitude of the harmonics is inversely proportional to the order of each harmonic. 

 

Extension of Fourier Series

Indeed, Fourier Series is very useful when it comes to representing a periodic waveform. Nonetheless, one major limitation of Fourier Series is the assumption of periodic waveform.

 

Let’s take the impulse response of a channel for example. Fig. 8 is the waveform of a channel we investigated in Dirac Delta Misnomer. The impulse response is NOT a periodic function. If I am interested in the impact of the channel on different frequency components, I would need a way to transform the aperiodic time domain response to the frequency domain.

Fig. 8: Time domain impulse response of a channel is not a periodic function.

 

In the next post, I will show that with the help of Fourier transform, an extension of Fourier Series, I can convert the time domain impulse response to the frequency domain insertion loss, as shown in Fig. 9.

 

Fig. 9: Frequency domain representation of the time domain impulse, also known as the insertion loss.

 

Because the insertion loss plot gives us valuable information on how each frequency component is affected by the channel, we can then identify the root cause of eye closure.

 

Conclusion

As Sir William Bragg points out, new discovery requires a new point of view. There is no doubt Fourier’s approach to the heat equation is a novel one.

 

By using Fourier Series, we examine the ideal square wave through the frequency domain looking glass. In the next weeks, we will see how we apply Fourier transform to understand the root cause of eye closure.    

 

That's this week's Tim's Blackboard. See you in two weeks!

 

Experiment with the square wave source: 

References

[1]

Wikipedia contributors, "Fourier series," 9 August 2017. [Online]. Available: https://en.wikipedia.org/w/index.php?title=Fourier_series&oldid=786176863.

[2]

D. Freeman, "6.003 Signals and Systems," Massachusetts Institute of Technology: MIT OpenCourseWare, Fall 2011. [Online]. Available: https://ocw.mit.edu.

 

Demands for faster data speeds and more reliable services are at an all-time high. 5G is predicted to meet these demands, and much more. Although 5G is currently still in the planning stages, researchers are uncovering solutions that were previously thought impossible. Until recently, mm-waves have been viewed as unsuitable for mobile communication. However, new research has shown that propagation issues can be overcome, with help from mm-wave small-circuit designs.

 

Plextek RFI is a leading company in 5G design, specializing in the design and development of RFICs, MMICs and microwave/mm-wave modules. The designers at Plextek RFI manage their own RF On Wafer (RFOW) test facility, providing several leading foundries highly developed design services. Their designs are used in a wide range of applications from test instrumentation to infrastructure equipment and the latest mm-wave 5G systems.

 

Plextek RFI also has a growing archive of video tutorials on various design topics in a wide range of applications from test instrumentation to infrastructure equipment and the latest mm-wave 5G systems.

The most recent video tutorial provides a visual demonstration of the design, layout, and performance of a Dual-Band 5G Power Amp using Keysight ADS.

 

dual band layout

Figure 1. Full layout of a mm-wave 5G dual-band power amplifier.

 

Dual-band power amps are vastly popular in proposed 5G designs due to the wide range of frequencies used in 5G applications. They perform almost as well as two single-band PAs combined, as they are capable of electronically switching their operating band between the 26GHz and 32GHz 5G bands. They are small, inexpensive, and with ADS, easy to design.

 

ADS schematic of dual-band PA

Figure 2: ADS schematic of three-stage dual-band power amplifier.

 

s-parameters pf PHEMT switch transistor

Figure 3: S-parameters vs. frequency of a pHEMT switch transistor, used to alter effects of transmission lines in low and high bands.

 

 

Watch the 20-minute tutorial video to dive further into each stage of the power amp design, and to see the effects of the pHEMT switch by simulating the S-parameters at high and low band.

 

For more information on Plextek RFI, go to here.

 

See more tutorials by Plextek RFI here.

 

 

If you are interested in learning more about 5G design using ADS, an upcoming webcast will cover the “Circuit Design Phase” of a 5G system done all in ADS.

Click here to watch the webcast.

 

apply for free trial

Apply for a free trial of ADS.

This Case Study highlights work published in a recent paper in IEEE Power Electronics Magazine entitled Utilizing Modern Design Methodologies for Wide-Bandgap Power Electronics by my colleague here at Keysight EEsof EDA, Chris Mueth, and by Rakesh K. Lal of Transphorm, Inc. The high di/dt and dV/dt edges in switched-mode power supplies (SMPSs) combined with layout parasitics can create unwanted voltage spikes. The authors demonstrate that EM-circuit co-simulation can predict these effects. With the insights from this predictive tool in hand, they rapidly explore the design space and mitigate the impairment. The time and money spent on board spins is reduced and the time to market improved.

The Challenge

Since GaN switches are intrinsically very fast, one can have a very high change in voltage versus a given change in time (dv/dt) (>300 V/ns) and a change in current versus a given change in time (di/dt) (>5 A/ns). So, designers need to use good design practice for high-frequency layouts. Three cardinal rules that apply are:

  • Minimize capacitances to ground or other nodes at high dv/dt nodes to minimize Ispike = C dV/dt
  • Minimize parasitic inductance in high di/dt branches to minimize Vspike = L di/dt
  • Guard or shield high-impedance signal nodes, such as the gate of a drive transistor with appropriate guard rings and shields.

Physical prototypes are costly and time consuming to build and don't give insight into details like current crowding (which is indicative of excess inductance). But virtual prototyping in a tool like ADS (which allows EM circuit co-simulation) do exactly this. 

 

The Solution

The authors used ADS to gain insight into design weaknesses. The key point here is that ADS has a built in electromagnetic (EM) field solver allows you to extract an EM-based model of the layout parasitics. You can co-simulate regular SPICE-like lumped elements along with the effects of the layout. You can plot the voltage spikes and do "what if..." design space exploration, such as using a ground plane for the return current, to minimize their effects.

 

The Results

 

The Transphorm reference design analysis used the Momentum method of moments EM field solver. The EM-model extraction took roughly one hour. The tool automatically creates the components representing the layout from the port-to-port network parameters generated by the EM field solver. The analysis tool then simulated the circuit schematic including the extracted model in the time domain.

 

After experimenting with various "virtual prototypes", the final reference design utilized two power planes, which were poured onto two different PCB layers. This provided the best possible reduction in power plane inductive parasitics. In addition, the power planes were placed close together and provided an additional capacitance benefit. As a best practice, the ground layer was located under the main trace routing layer to provide additional capacitance to help reduce the stray inductance of these traces. 

 

The reference design produced an efficiency of 98.5% for the buck half-bridge configuration. This correlates well with the 98.3% efficiency seen in the simulation. The gate-driver waveforms also correlated well.

 

Are you working on switched-mode power converters? Do you face the same challenge? Or something else? Please log in and leave a comment and/or "like" this posting!

 

Best regards,

-- Colin

 

PS Here's the link to request an ADS evaluation license if you want to try it.

The trend in switched-mode power supplies is to use wide band gap devices because these enable a higher switching frequency and higher edge speeds (the “di/dt” of the switched loop). These two in turn enable a smaller, lighter, cheaper power supply because the energy storage capacitors and magnetics can be smaller if you top them off more frequently. The higher edge speeds enable higher efficiency because there’s less heat dissipated when you have lower switching losses because the transistors spend less time in the dissipative cross over region.

 

These high slew rates come with a dark side, in particular the large spike voltage and noise generated by the layout parasitics, particularly inductance, of the PCB layout traces. This phenomenon is often called conductive electromagnetic interference (conducted EMI).

 

My colleague, Andy Howard, talked about how to deal with this a while back in his video entitled “How to Design DC-to-DC Power Converters”, but a frequently asked question was “When should I start to worry about layout parasitics inductance? Is there a quick rule of thumb that says kind of ‘Caution: Further investigation’s needed?’” The answer is "Yes!" and this follow up video is about how to make these estimates. Here's the link:

 

How to Estimate Voltage Spikes from Layout Parasitic Inductance in Switched-Mode Power Supplies

 

Layout parasitics cause spike voltages in the switched loop of a switched-mode power supply