“So, What Do You Do?”
By Dr. Eric Bogatin
www.BeTheSignal.com
December 2006
It’s the holiday season and with it comes the obligatory family gatherings and community parties. There’s that moment of dread you know you will face. You are standing by the eggnog bowl, you turn your back for just an instant, and your spouse disappears, leaving you defenseless. Along comes your spouse’s second cousin from Duluth and strikes up a conversation. After you exhaust the usual topics of weather, golf, and your house, there’s the two-second silence, followed by the inevitable question…. “So, what do you do?”
If you’re talking to another engineer, it’s easy. You simply say, “I’m a signal integrity engineer,” or “I design memory buses,” or “I design circuit boards.” But how do you explain what you do to your wife’s second cousin from Duluth, or to one of the parents of your children’s friends, or to some other mere mortal not in the high-tech industry.
I live in Kansas. When someone asked me the big question, I would say, “I teach signal integrity classes to hardware engineers.” After seeing too many glazed eyeballs (before I could even finish the answer), I gave up and now just say, “I’m an engineer.” This seems to be immediately satisfying, or people are afraid of the follow-up, because before I can add any details, the topic of conversation turns to kids. I can only envision that most people walk away thinking of me driving trains (we have a lot of trains in Kansas) or inspecting the structural integrity of buildings (we have a lot of new buildings in Kansas).
Here’s how some other engineers answer the question, “So, what do you do?”
Jeff Loyer, a signal integrity engineer at Intel, says, “I design computers.” However, he’s thinking of trying out, “I’m a microwave plumber.”
Bill Hargin, a product manager at Mentor Graphics says, “I help make (wheat) combines run fast and correctly by enabling computer chips to communicate with each other correctly. This speeds up the wheat harvest, putting bread, pasta and cereal on the tables and in the stomachs of children and workers worldwide—enabling them to live free, and ask questions like 'what do you do for a living?'"
Richard Schumacher, an engineer with HP says, "If computers were plumbing, I'd be the one who keeps the pipes from hammering."
Scott McMorrow, Director of Engineering at Teraspeed Consulting, says, “Electronics performance tuner and cat herder.”
Leonard Dieguez, a signal integrity engineer at Altera Corp does Scott one better. “I just avoid the question and tell them I raise cats. Then ask "Do you want one?"
Chris Padilla, an EMC/SI engineer with Cisco says, “"You remember way back when your mother used to vacuum while you tried to watch TV and the TV got all fuzzy and snowy? Well, I design products that don't allow that to happen. I make sure the vacuum cleaner doesn't affect the TV and I make sure the TV can handle any noise the vacuum cleaner makes."
Larry Smith, power integrity expert with Altera Corp says, “I try to get signals from here to there at light speed without loosing data. I bus around the same amount of DC current that it takes to start a car but this current has to work at a GHz.“
Bidyut Sen, Director of Packaging Technology at Sun Microsystems, says, “I used to say that I work in semiconductors. But in a party, in a response to my answer, somebody said, ‘Oh, you drive trucks?’ Since then I say that I work in hardware. Most of the time I just get a nod, they understand it is not software, and then don't ask anymore. Sometime I have said that I work with computers.”
Gus Panella, Manager of Advanced Product Development at Molex says, “Electrical Engineering... I do what it takes to get data from one place to another quickly. Then the pause as I wait for people to turn and run in order to get away. But I have a quick follow up. I also coach/play hockey on all my free moments. This usually gives everyone at least one general area to converse about.”
Steve Weir, signal integrity expert with Teraspeed Consulting, says, “"Make stuff that goes fast and still works reliably."
Meena Nagappan, signal integrity engineer with IDI says, “For high-speed signals, a signal integrity engineer is like St. Christopher—the patron saint of travelers. The patron saint protects the signals from any external influence and assures safe travel!”
My wife came up with the best response. She told me I need to say, “I teach chip designers how to turn a chorus of angry voices into a beautiful symphony of harmonious melodies.” Only she can carry it off with a straight face.
This and other signal integrity topics are covered in Eric’s public classes and online lectures, available from his website, www.BeTheSignal.com. Send your signal integrity technical questions to DoctorIsIn@BeTheSignal.com
Reader Q & A
If you have a question for the SI Doctor, please send it to DoctorIsIn@bethesignal.com.
Question from Jonathan in Rochester, MN: If you have a near-end crosstalk problem, doesn't xmt pre-emphasis exacerbate that problem?
Answer: Yes. A larger signal from the transmitter because of a pre-emphasis boost creates more near-end crosstalk in an adjacent line. However, an optimized design avoids situations where the transmit pair is adjacent to the receive pair, so this should never be a problem. If you use conservative design rules so the spacing between adjacent differential pairs is greater than at least three times the spacing between the two lines that make up the pair, the near-end differential crosstalk to the adjacent pair should be less than one percent. In this case, the crosstalk and the impact from pre-emphasis should have a negligible impact.
Question from Scott in San Jose, CA: What is the difference between an antipad and removing the nonfunctional pads?
Answer: In the quest to design a transparent via, there are two important design knobs to tweak. If you use conventional design rules, vias generally look capacitive compared with 50 or 100 ohms. To bring them up in impedance and match them to the impedance of the trace, you need to decrease their capacitance to the return path, which are usually the planes they pass through. There are two dominant sources of capacitance: the coupling between the barrel of the via to the plane it passes through and the capacitance between pads on intermediate layers and the planes on that or adjacent layers. To design a transparent via, it is usually necessary to increase the size of the clearance hole the via barrel passes through. This is the antipad clearance hole. Without a 3D field solver to help guide you, as a rough rule of thumb, the antipad hole should be made as large as practical. It is common practice to place a pad on each layer the via barrel passes through, whether there is a connection to that layer or not. This pad is called a nonfunction pad. It used to be thought this pad improved reliability. Today, there is evidence to suggest the nonfunction pad has no impact on reliability. Instead, it just increases the capacitance of the via. To move closer to a transparent via, you should eliminate all nonfunction pads.
Bio: Eric Bogatin is president of Bogatin Enterprises. Many of his papers are available on his website, www.BeTheSignal.com. He is the author of Signal Integrity - Simplified, published by Prentice Hall.
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