Repeat what works!

 Sherry Hess, VP of Marketing, AWR. Read profile >>

Those who know me know that I like to repeat what works and, preferably, strive to do it better. They also know that I am certainly grateful and ready to abandon doing what’s not working. In the context of the microwave and RF industry, I’d like to highlight two such activities that are working well—in my mind actually getting better—and are worthy of being repeated.

The first is MicroApps…

MicroApps is a session that was started seven+ years ago within the International Microwave Symposium (IMS). The MicroApps program gives exhibitors the opportunity to present targeted, in-depth talks on their products and technologies to a larger audience than can typically otherwise be assembled in their own booth space. I personally got involved with MicroApps three years ago when IMS was in Anaheim and J K McKinney was chairman. That year more than 50 papers were submitted for MicroApps, and they were recorded for later broadcast.

During my second year on the MicroApps committee at IMS 2011 in Baltimore, the concept of a midday panel session was launched and the standing room only nonlinear behavioral modeling topic (hosted by Microwave Journal) certainly was a success and still continues to provide talking points today as Larry Dunleavy of the University of South Florida recently shared with me that he asks his students to view the archive and do a comparative critique of the various approaches.

But even before my second year of volunteering for MicroApps, my enthusiasm for it piqued the interest of our European friends at European Microwave Week (EuMW) and they opted to give it a try. The first ever European MicroApps made its debut at EuMW in the UK in October 2011, and with 30+ papers, keynotes and respectable attendance, it was a solid start for this new facet of EuMW. The second year of planning and preparation is just beginning. Click here to learn more and submit your abstract for Amsterdam 2012.

The second activity worth repeating is the Women in Microwaves (WIM) reception…

I got involved with this program as well during IMS Anaheim. (I have JK to thank for volunteering me to take this on when I only asked what it was, since I had never heard of it before!) I thought outside the box a little bit and brought an outside venue and casual networking perspective to the event. Good turnout, good feedback and a new network of wonderful women in M&RF resulted. When this was repeated again in Baltimore with beyond-capacity turnout, our European friends (whom I first met at WIM Anaheim) again felt it was worth a try. Just recently, Dominique Schreurs of K.U. Leuven, Belgium, hosted the first annual WIM at EuMW. This was more than a reception; it was also a session with several women in the industry speaking – from recent graduates relatively new to the industry to veterans like Prof. Magdalena Salazar-Palma, IEEE AP-S President and Rebecca Brierley, RF filter engineer at Radiodesign, UK.

For IMS 2012, I’m not actively involved (yet) but hope to offer up some new ideas to keep this special networking activity for women going. As one of a small percentage of women in M&RF, just seeing and being able to identify the other women in this industry is a plus. And it certainly helped me to recognize by name the women that Microwave Journal recently featured on its November cover -- http://www.mwjournal-digital.com/mwjournal/201111#pg1

So, let’s keep working hard, working smarter and striving to do everything better if or when possible.  And, of course, that also means to stop doing what’s not working so that the time saved there can be better invested in other more progressive and positive-impact activities. See you all at IMS 2012 – around the MicroApps pavilion, the WIM event and, last but not least, AWR’s booth # 1514!

Are you a Do'er?

 Sherry Hess, VP of Marketing, AWR. Read profile >>

Last October I attended a three-day conference called Business of Software (BoS) in Boston. AWR’s CEO Dane Collins had attended it for the past two years and I wanted to as well, but my travel schedule always conflicted. So, by year three, I was determined to go and had long beforehand blacked out the dates on my calendar.

BoS is about how to run a software business and is largely targeted towards entrepreneurs, but I did meet some other folks from Nokia and big Fortune 100 firms who were there as well. As I began meeting and networking with fellow attendees, what soon began to resonate with me was that the group around me was a bunch of “do'ers.” You know, the people who get an idea and run with it. Typically, we call entrepreneurs do'ers because they get an idea to start a company and run with it, but you can find do'ers in any facet of work and life.

I like to think of myself as a do'er. I started to blog way back in 2009 before it became popular for company execs to share their thoughts. David Vye of Microwave Journal and I were discussing social media and the idea blossomed. He suggested I write a guest blog, and I said, “OK, I’ll do it.” Now I have a guest column on MWJ and one on AWR’s website as well.

And then there was the IMS steering committee for Anaheim in 2011. In a discussion with a colleague at AWR, I was given a couple quick reasons I should join, and again I said, “Ok, I’ll do it.”  Once I got to the steering committee and found they had no one to run the Women in Microwaves (WIM) Reception, again, I said, “OK, I’ll do it.” Upon reflection, I realize I readily say, “Ok. I’ll do it.” Maybe I’m simply not good at saying no, but I think it is that do'ers like challenges -- if I like an idea, I’m willing to invest my own time and energy into it to see where it goes and make a success of it. 

In my mind, AWR customers are generally a collection of do'ers. They understand the advantages of cutting design times in half and the opportunities that can result from the time gained to take on additional challenges that add value. And who can argue against gaining back additional quality time in some facet of your own life, personal as well as professional? 

If you follow AWR (and I hope you do), our current ad campaign is entitled, “Stop Waiting and Start Designing.” I came up with this tagline a while ago when I was trying to figure out a way to get the attention not only of our current customers but also of potential customers. I wanted everyone to understand that our software cuts way back on the time you spend waiting for your simulations to finish, time you could use to be do more simulations to eek out extra performance and get a further leg up on the competition.

So here's my battle cry to all you do'ers out there... give AWR a try if you’ve never used it, or discover new ways it can help you cut time, even if it’s the 100^th time you’ve used it this year. Try out some new features or capabilities if you are a current customer or, for potential new users, try out our demo. If you never try it, you’ll never know if it is right for you. We sweetened the deal with a chance to win an iPad monthly and that certainly enticed a few of you out there (customer and prospects alike). Nice! Glad to see when ideas are validated by positive reaction.

AWR 2011 is ready for the “do'er” challenge.  Take it for a test drive to see what it can do for you.

The Wonderful World Of Electromagnetic Antenna Simulation

 Dr. John Dunn, AWR Corporation

It has become popular to simulate antennas in electromagnetic (EM) simulation software. After all, an antenna is by its nature an electromagnetic beast. Its whole purpose is to emit and/or receive electromagnetic waves as efficiently as possible. Even to more experienced circuit designers, antennas can be mysterious entities, not obeying the laws of normal circuit theory. For circuit design purposes, most designers focus on the input terminals of the antenna, modeled as a load impedance, which is to be driven with a certain amount of power. Hopefully, the power absorbed is going mostly into radiation. But, behind that load impedance is the world of the antenna designer. In this specialty, engineers try to synthesize the best possible antenna for the purpose at hand, making standard engineering compromises between the radiation pattern, and the efficiency, size, weight, and cost of the antenna.

Clearly, EM simulation and antennas are a natural fit. Because of the unique requirements of antennas, the electromagnetic simulator must support a number of critical features. There are a huge variety of antennas for a vast array of applications. Antennas are needed for technologies from commercial cell phones to military radar systems, with costs varying from inexpensive RFID applications to million dollar satellite systems, from low-tech wire antennas to high-tech phased arrays. Can only one EM simulation package be effectively used for all these applications? No. The physical requirements, and therefore the simulation needs, are too varied and vast.

Figure 1: Typical antennas as a function of electrical size and complexity of environment

Let's look at a few of the special problems that antenna software has to address. To aid in the discussion, Figure 1 shows typical antennas as a function of electrical size and complexity of environment. The size of the antenna compared to the wavelength at the frequency of operation is an important determinant of the computational method used. The antenna's size is depicted in the figure as the x-axis. On one extreme are reflector or dish antennas, sometimes hundreds of wavelengths long. On the other extreme are RFID antennas, a few hundredths of a wavelength long. Very different methods are required for these different-sized antennas Any method that works by subdividing the antenna into small sections compared to a wavelength, for example the finite element method, will not be practical for an electrically large structure. Instead, specialized codes exist for electrically large antennas that rely on high-frequency approximations. Methods like the geometrical theory of diffraction and the uniform theory of diffraction are used. Sounds like optical design, doesn't it? Basically, it's the same physics, and it's a long way from our familiar world of circuit design and simulation. The other extreme of electrically small antennas has its own challenges. Frequently, they incorporate special materials like ferrite cores with possible hysteresis effects, are inherently 3D in their geometries, and are in strange environments, such as under the skin of a cow!

Antennas are designed to radiate electromagnetic power to infinity. This means that any EM simulator must either be capable of including infinity, i.e. not be in a bounded environment, or do a good job of making it look like the antenna is not in a bounded box. To do this, the simulator uses special boundary conditions at the edge of the box. These boundary conditions have become very sophisticated over the past 20 years. Simple impedance boundary conditions used in the 1990s have been replaced by esoteric, specialized mathematical formalisms, designed to make the boundary look invisible to the antenna.

Finally, antenna software must model the physical geometry of the antenna and its environment. Some antennas are inherently 3D in appearance, meaning they cannot be simulated well with a planar simulator commonly used by circuit designers for planar, patch antennas. The environment surrounding the antenna might be complicated. For example, in biomedical applications, the surrounding bone, fat, and muscle all affect the radiation pattern of the antenna. 3D simulation software must be used. Finite element methods are an obvious choice, where the entire simulation space out to the boundary of the environment is meshed in tetrahedra. There are many variations to finite element methods. One of the most popular methods for antenna simulation actually meshes the metal surface of the antenna and uses what is known as a boundary element method. Its advantages are that it doesn't have a bounding box and can handle larger problems where it works. Hopefully, you can now see why there are such a vast and varied number of antenna programs. It's because there are vast and varied different types of antennas!

Figure 2: AWR's AXIEM 3D planar EM simulator is ideal for planar antenna analysis

There are many more simulator choices than for traditional EM simulators for circuits. The features and underlying numerical methods are constantly being improved. For example, fast solvers for planar arrays are now available. These methods make it possible to simulate arrays that were simply too complicated a few years ago. Parallel processing, in which the problem is distributed to more than one computer, is becoming a reality. Soon, we will all be using "the cloud." There even is software available for antenna synthesis now. You pick a design template for an antenna type, and the software predicts the performance for you based on design formulas. It then sends the layout to your EM simulator of choice.

So, next time you model that antenna in your circuit as a load impedance, please remember that it was brought to you by antenna designers using sophisticated, specialized, EM software.

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately held, growing company with thousands of users world-wide. Learn more:www.awrcorp.com

An exciting time for AWR and our customers!

 Dane Collins, CEO of AWR. Read profile >>

Over the last decade, AWR’s products have grown and matured to become a very powerful microwave design platform used throughout the high-frequency/wireless industry. With our success came a global expansion of our sales, support and development organization. Our customers world-wide know that AWR’s mission is to improve the design productivity of our customers (large and small) and now coupled to NI’s global organization, we are able to accelerate our pace to not only develop and deliver the excellent RF design tools you’ve come to know and trust from AWR but our proximity and ability to train, support, and deliver design services is also greatly enhanced.  

While change often causes a sense of uneasiness for most people, change in this case is exciting and beneficial and mostly transparent to our customers as AWR will continue to operate as an independent company under NI. Here are some details that I can share:

• First and foremost you, our customers are our number one priority. There will be no disruption in how we support you and respond to your needs.
  • The entire AWR sales channel remains in place as-is and all of our customers will continue to work with the AWR representatives and resellers they know and trust.
  • All of our support channels remain in place and all the people helping you yesterday will continue to help you tomorrow.
  • Some of you may however see quick expansion into yet-to-be-served AWR territories as NI’s vast network of offices allows AWR to deploy support organizations in regions simply not possible prior.
• The AWR vision and development plans stay their course and we’ll be excited to show you many new features and technologies in AWR 2011 at IMS2011 (Booth #1618)
• For our many customers whose business depends on our software, you should be assured of our longevity as this merger now brings to AWR, the backing of a multi-million dollar company who sees RF EDA as strategic to their long-term success.

Read Related Press Release >> 

 

Understanding Ports – What's The Fuss About De-Embedding?

 Dr. John Dunn, AWR Corporation

De-embedding is a process where a port's parasitics are removed. (As a byproduct, it also can set the reference plane where you want it.) All ports have parasitics. Another way of saying this is "no port is perfect." De-embedding attempts to make the port as perfect as possible in the imperfect world we live in. Sounds like a good idea, right? Well, yes and no. Unfortunately, de-embedding makes a number of assumptions that will lead to erroneous results if they are not understood, appreciated, and/or violated.

To understand de-embedding, I will again start with the analogy of measurements in a laboratory. I find that since EM simulation is supposed to model the real, physical world, it is useful to think of what we would do in that real world, i.e. the laboratory. A network analyzer is normally calibrated before it is used for measurements. What does this mean? In calibration, the analyzer measures some canonical structures, for example a perfect short, open, and load. (We won't get into the issue of how one gets a "perfect" load.) There will be errors in the measurements, due to the imperfections of the system. For example, the cables have loss, the connectors are not perfect, the isolation between the two ports is not infinite. The measured data on the test structures is then used to cancel out these errors.

Ports in EM simulators have imperfections, too. We can get rid of them in the same way as in the laboratory, by using the port to measure various canonical structures. A note on semantics: In the laboratory, the error removal process is called calibration. In most EM simulators, it is called de-embedding. Strictly speaking, the procedure is calibration (getting rid of the parasitics) and de-embedding (setting the reference plane). The details of the structures used depend on the specific port and the simulator. Sometimes simulations are carried out behind the scenes using simple lines. Sometimes lengths of line are attached to the port. But in all cases, the software is looking at some sort of canonical structure to get rid of the error.

The biggest user error in de-embedding is that designers forget the fundamental assumption that the calibration structures look the same as the port in the actual circuit. We are assuming that the port has no other metal near it other than the line to which it attaches. After all, the calibration structure is just a port and a line. I usually use the rule that there should be no extra metal within two substrates of the port for microstrip circuits if you are calibrating. Figure 1 shows an example of this. The left side of the figure shows a layout where port 1 is too close to another line. The calibration structure knows nothing of this line; the correction is for the wrong errors. The solution is then to either: 1) move the other line, 2) skip calibration, or 3) modify the calibration procedure using a custom or manual calibration.

Figure 1: Make sure no extraneous metal is too close to your port.

I'll leave you with an example I encountered while visiting a customer last year. The company makes high precision, distributed filters in microstrip technology. They could not get the simulated and measured results to agree below 50 dB, and had serious doubts about the accuracy of the simulator they were using. Theories were being voiced, ranging from the simulation needed to be in a box, to the loss formulas weren't right for the metal lines.

Figure 2 shows the solution to the problem. They had placed the ports too close to their filter, about one substrate height away. I moved the ports to be three substrates from the filter and the results agreed to over 80 dB! The confusion arose because the designers were de-embedding but violating the assumptions.

  

Figure 2: The ports were placed too close to the filter, as shown on the left side.

In conclusion, de-embed ports when possible. It reduces errors due to parasitics in the port, and of course that's a good thing. But be aware you aren't making the problem worse by violating the basic assumptions of the de-embedding procedure. As always, it never hurts to try out the calibration on a few simple structures before you go for the home run.

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately-held, growing company with thousands of users world-wide. Learn more:www.awrcorp.com

 

Stop waiting. Start surfing. – Part 2

 Sherry Hess, Vice President of Marketing, AWR. Read profile >>

So did you?  Stop waiting and start (fill in the blank) ?  The reason this part 2 blog took so long to write is that it took me this long to actually start surfing.  While I had great intentions to get going with this new adventure, I must confess that it took me much longer than I would have liked before I actually jumped in and did it.  Yes, I surfed.  I stopped waiting and started surfing.  (See my photo here for proof  )

Enough about me, now what about you?  Have you challenged yourself in 2011 yet?  I mean started something that you never thought you would ever try? For me, it was surfing. For some, it is skydiving, running a marathon, or climbing a mountain.

How about this for a collective challenge? AWR’s new ad campaign, “Stop waiting and start designing,” entices you to try our software or share your design successes with us. We even sweeten the deal with an incentive to win an Apple iPad 2…1 of 12 being given away during AWR’s fiscal year 2012 that starts April 1^st.

So, let’s make 2011 not only known as the year I actually surfed, or you ran that marathon, but let’s make it the year we all stop waiting to try AWR software and start designing with it. From April 2011 through March 2012, we will be giving away an iPad each month to one of you who take up our challenge.

Stop waiting and start designing. Get ready to jump in and visit www.awr-startdesigning.com.

 

EM Ports — A Ground Is Out There Somewhere

 Dr. John Dunn, AWR Corporation

As a designer, you certainly need to know where the ground of your circuit is. EM simulators work the same way. You should know the ground of each port so that you can properly interpret the results. If different ports have different grounds, don't be surprised if you see some non-intuitive results.

All ports used in EM simulators must have a notion of ground. Why? For argument's sake, let's agree it is because without it, you can't take your data and use it in a circuit simulator. Let's take a closer look at the issues.

As electrical engineers, we all know about ground. Along with voltage and current, it is one of the most fundamental concepts anchoring electrical engineering. But what you may not know is that ground is not used in Maxwell's equations, the physical equations that form the basis of electromagnetics. It is these equations that are numerically solved in EM simulators. Maxwell equations work with electric and magnetic fields, not voltage. Voltage is what is called a derived quantity — if you know the electric field for a problem, you can find the voltage between two points by integrating the electric field along a path between those two points.

Since EM simulators solve Maxwell's equations, and these equations don't use voltage and ground, why should we even worry about it? Indeed, we could create a simulator that calculates S-parameters using purely EM field concepts. First, we would set up our ports using wave concepts. (By the way, this is common in 3D simulators that use wave ports.) Then we could calculate the input and reflected power to the various ports by using the electric and magnetic fields at the port. Finally, we could take ratios of the various powers and determine our S-parameters. Unfortunately, we can't use these S-parameters in a circuit simulator. Circuit simulators work with current and voltage. In order to get our S-parameters into the circuit world, we need to have a notion of the port's impedance. There are many definitions for port impedance, but they all involve some kind of circuit concept: voltage or current or both. That's why we need to know what a port's ground is.

The easiest way to find a port's ground is to determine where the current coming out of the port originates. Every electrical engineer knows (I hope!) that current must flow in continuous loops. Current is flowing out of our port. Where is it coming from? This current is called the port's ground return current; i.e., it is the current flowing back into the port from the local ground. Sometimes this is fairly obvious. Figure 1 shows two types of port grounds for a planar EM simulator. (I am illustrating the concept using AXIEM from AWR.) Port 1 has a ground strap attached that lets current flow from the ground plane below to the port. Clearly, the port's ground reference is the ground plane below. A similar situation exists in EM simulators in a box. The line attaches to the perfectly conducting side wall of the box. The current return is out of the side wall, through the port, and down the line. The side wall is therefore the local ground for the port. However, look at port 2 in Figure 1. There is no ground strap. Where is the ground current return? The answer is that it is coming from infinity! Hopefully, as a practical engineer, this makes you feel uneasy. Infinity is a long way away. Fortunately, the ground plane in this case also goes out to infinity, so maybe that helps. I will discuss more of this concept in future articles.

Figure 1: Explicit and implicit grounding schemes in EM simulators

Figure 2 shows another common type of port. Ports 2 and 3 are edge ports, as we showed inFigure 1. Port 1 is an internal port. It goes by different names in different simulators: internal port, circuit port, series port. The idea is that you cut the line and place the port. The port has a port impedance; let's assume it's 50 ohms for simplicity. Now, where is port 1's ground? It is on the negative side of the port. The laboratory analogy is using a two-point probe. The red probe tip goes on the one side of the port; the black probe tip goes on the other. Local ground is the metal line the black probe tip touches.

 

Figure 2: Port 1 is an internal port.

The S-parameters generated with this type of port can be confusing. For example, what is S11 at low frequencies for Figure 2, assuming all port impedances are 50 ohms. The answer is S₁₁ = 0.5. See if you can figure out why. For a real challenge, try figuring out S₂₁. It will make you appreciate why port ground and S-parameters can be tricky! Unfortunately, many designers don't appreciate the ports in Figure 2 have different grounds, and completely misuse the S-parameters.

In conclusion, take the time to understand the ports you use in EM simulation. Just like in real circuits, ground can be your friend or cause you grief. The easiest way to think of ground is current return. If you can predict where all the port currents are coming from, you are well on your way to avoiding problems with misinterpreting the results coming out of your EM simulator.

For a more detailed discussion of these issues, please see my white paper Understanding Grounding Concepts in EM Simulators.

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately-held, growing company with thousands of users world-wide. Learn more:www.awrcorp.com

 

DesignCon 2011 – Engineering Curiosity a Buzz!

 Sherry Hess, Vice President of Marketing, AWR. Read profile >>

DesignCon 2011 just ended, and it was the first year for AWR to exhibit. While a number of you use our software for high-speed SI design work, we have not allocated a significant portion of our tradeshow budget to this segment in the past. This year, however, we’ve changed it. We’ve expanded our presence at this show from largely tutorial in nature– thank you Dr. John Dunn for 3 years of top-notch talks on EM at this conference – to one that also incorporates a booth, partner demonstrations and on-line EDA Café interviews. (thanks Graham!)

Working a tradeshow booth is not usually a highlight for most people, but I can sincerely say that DesignCon was a real pleasure. This show was jam-packed with curious design engineers, and many proactively stopped by our booth and said, “I know AWR. What are you showing here?” The curiosity of the engineering community is contagious and the camaraderie extends to the aisles where questions and answers flowed freely. It was hard not to get excited by the buzz in the atmosphere and dynamics of all the attendees.

The show in many ways made me realize why AWR’s next ad campaign, “Stop Waiting and Start Designing” makes perfect sense. Our collective curiosity as engineers, designers, marketers, and managers keeps us pushing the limit of what’s possible. In the same way that AWR software makes designing the next big “GHz & beyond” wireless system or gadget possible, isn’t it time you got curious too and found out how and why?  Go ahead. Get curious. Learn more about AWR and how are software solutions are advancing the wireless revolution:
http://www.awr-startdesigning.com

 

Understand Ports: The Key To Mastering EM Simulation

 Dr. John Dunn, AWR Corporation

If you use an EM simulator, you use ports. No kidding! Ports are the way the EM simulator "talks" to the outside world and are the way you measure S-parameters — the "high-frequency" data that can be used in a circuit simulator. Analogy: Using probes in the laboratory; without them, you have no way of making a measurement.

The difficulty with ports is that they are often misunderstood, and therefore improperly used inside of EM simulators. To again make an analogy with the laboratory, imagine you use the wrong type of probe for your measurement of interest...

• A low-frequency probe cannot be expected to give a good result at microwave frequencies.
• An improperly calibrated network analyzer probe cannot be expected to give accurate answers to -70 dB.
• A two-port network analyzer is going to have limitations when looking at differential line issues. Here you can use transformers and get some useful information, but you had better understand their limitations.

 All ports (port types) have limitations, assumptions, and approximations. If you don't understand what they are doing, you may well fall victim to the topic I covered in my January article — the simulator gives an answer, but it's NOT the one you want. So, I'd like to devote the next couple of articles in this series to better understanding, appreciating, and using ports successfully within EM simulators. I guarantee that time spent understanding how ports work is time well spent. You will be rewarded with more accurate results and attain confidence when using your EM simulator of choice.

We'll get started by looking at two important concepts that all ports must address: grounding (next issue) and deembedding.

• Grounding. As electrical engineers, we work with the notion of ground everyday. Ground gives a circuit its voltage reference. Without it, we can't even talk about voltage in a sensible way. What isn't as well known is that every port has its own ground reference. The S-parameters you get at a given port use the "local" ground. If you don't have it properly defined, you can get completely confusing results. I like to remind engineers that not understanding ground can be hazardous to your health! Think of someone taking the back off of a tube TV (yes, they are still around) and measuring the voltage with a two-point probe. They get a few volts. Then, they reach their hand in and find out about not understanding ground! Fortunately, not understanding ground in your simulator will not land you in the hospital, but it certainly can mess up your simulation results.
• Deembedding. This is the process of removing the parasitics associated with the port — for example, its parasitic capacitance and inductance. Incidentally, not all ports can be deembedded. Does that make them useless? Not necessarily. To again refer to the laboratory, should you not use hand held probes because they can't be calibrated? Of course not, but you had better understand their limitations.

I invite you to join me over the coming months and issues as we explore the wonderful world of ports in EM simulators, with all their assumptions, pitfalls, and limitations.

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately-held, growing company with thousands of users world-wide. Learn more:www.awrcorp.com

 

The Trouble With EM — It Always Gives An Answer!

 Dr. John Dunn, AWR Corporation

Now I know what you are thinking. How can this be a problem? As an overworked, overloaded, and overcommitted designer, I'm sure you have bigger issues to worry about than a CAD tool that (almost) always gives you an answer. The question is: does it give the right answer?

EM simulation tools follow the standard engineering paradigm: GIGO (Garbage In, Garbage Out). An EM simulator works by meshing up the physical geometry of the problem it has been given. Then, it solves for the S-parameters at the various ports that have been placed in the geometry. Ultimately, there are only two ways the simulator won't give an answer: if the setup of the geometry is so messed up that the simulator has no idea what to do (e.g. there are no ports), or if the problem is so large numerically that it will never solve it in a reasonable amount of time, and the job is eventually aborted. In all other cases, you will get an answer.

I've seen many naïve designers happily take the S-parameters from an EM simulator and rely on them as a definitive result. Nonetheless, I assume most designers will at least question a result of S11 = 1 when they are studying an interconnect line, but even so, the problem is much more subtle. For example, you get: |S11| = - 43.4 dB at the center of your filter's pass band. Do you believe it? Why? Unfortunately, many designers never question the assumptions that went into that result: the mesh size, the boundary conditions, and the ground return assumptions, to name a few of the top culprits.

So what is a designer to do? Be skeptical. Try some simple cases you know are right first. When confronted with a large, complicated problem, the experienced EM tool user will first try simpler problems that give insight into the more important and complex issues. As a starting point, change your mesh settings to see if it makes a difference. If you have to evaluate the layout of a 16-line bus feed, begin with two lines to see what mesh is reasonable to get what you need. Try to get it close to known results by comparing answers from standard models. Simplified problems tend to run quickly, and you can build your intuition. Don't make the mistake of not taking a few hours to gain experience on problems you understand before you tackle that two-day simulation monster.

EM simulators are wonderful tools, but please remember they almost always give an answer. It's your task to ensure the answer is a sensible one. And when in doubt, don't believe it unless you can prove it to your satisfaction!

About The Author
Dr. John Dunn, a recognized expert in EM modeling and simulation for high-frequency and high-speed circuit applications, is a senior applications engineer at AWR and develops and presents AWR training material to customers world-wide. Before joining AWR, he was head of the interconnect modeling group at Tektronix and a professor of electrical engineering at the University of Colorado, Boulder, where he led a research group in EM simulation and modeling. Dr. Dunn received his Ph.D. and M.S. degrees in applied physics from Harvard University and is a senior member of IEEE. You can contact him at jdunn@awrcorp.com.

About AWR
AWR is the innovation leader in high-frequency EDA. Its software solutions quicken the pace at which high-tech products like cell phones and satellite systems are developed. When AWR software is part of the design process, engineers can deliver cutting edge, affordable products faster, more reliably, and at a lower cost. Headquartered in El Segundo, CA, AWR is a privately-held, growing company with thousands of users world-wide. Learn more:www.awrcorp.com