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Controlled Impedance: A Real-World Look at the PCB Side

DAN BEAULIEU: When I was a very young man, way back in the taped artwork days of the 1970s, I was a program coordinator (a fancy name for expeditor) at Rockwell’s Maine Electronics in Lisbon, Maine. One of my programs was the Burroughs Scientific Processor (BSP) created by Burroughs Corporation, based in Paoli, Pennsylvania. We were building very high-tech 14- and 16-layer boards for this program. We were told that these BSP computers were so powerful that the first one was already “running” the airport in Narita, Japan.

Now, this was 1975, mind you. The only computer I had ever seen was being installed in a glass room at the company. I was walking around tracking my PCBs with a pen and clipboard and I had no idea what “running an airport with a computer” even meant. By the way, I did get a chance to see a BSP system being built. It was quite an impressive sight, especially since they had slimmed it down to three refrigerator/freezer-sized units. I remember being amazed that something that small could run an entire airport!

And this was the first time I had ever heard of impedance. All I knew about it was that these two guys, Bob McQuiston from Burroughs and Andy Yenco from Maine Electronics, spent weeks building literally hundreds of boards, inventing ways to measure this impedance thing and then throwing out most of them. It would break my heart to carry all of these great looking boards to the scrap heap day after day.

Finally, one day I heard Andy let out a whoop and slap Bob on the back (there was no man-hugging back then). They had done it! They had finally built and measured some boards with the right controlled impedance. That was 40 years ago, so I’m thinking that maybe I saw the first controlled impedance boards being built and measured. Now, can somebody please tell me what controlled impedance is? Bob Tarzwell, can you help me out here?

BOB TARZWELL: Sure, Dan. And to make this really useful (I’m never sure if you really understand my answers to your eternal questions anyway), I’ll direct my answer to those guys who really care: the PCB designers.

As a designer, your project may require a specific impedance of, say, 52 ohms, plus or minus 7%. The big question is: Does the fabricator give you what you ask for? Well, maybe, and maybe not. Here is why.

The standardized coupon the PCB manufacturer inserts into the panel is designed to reproduce the same impedance effects your circuit should see, but because it is not inside the circuit it’s only a close approximation test. When PCB manufacturers set up the panel, of course they use as much of the panel as possible, which means the impedance coupon will be on the outside of the production 18x24” panel. The question would be, is there any difference between the center of the circuit board and the very outside where the impedance coupon is located? Unfortunately many times the answer is yes. 

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