## Featured Engineer

### Doug Smith

Doug Smith - A Lab Rat's Experience in the Tech World

##### What are your favorite hardware tools that you use?

Home built probes and measurement tools. My favorite probes are square 1 inch loops built from stiff brass wire. I also build resistive voltage probes. I have made countless measurement devices for troubleshooting one being just a pair of tweezers with tape over the tip to find an ESD problem.

##### What are your favorite software tools that you use?

Mac OS X and Spice on OS X.

##### What is the hardest/trickiest bug you have ever fixed?

Not the hardest but certainly tricky, a case of latch-up of cells inside of a memory IC. Case was solved using one of my home built probes and a method I developed earlier after 3 hours of work. In this case, the problem was in the IC design, not the PCB. The problem had been causing field problems for over a year.

Another tricky case was of a gigantic diesel engine control. I was able to prove the problem was not in the hardware and the software problem was then found by the software engineers, a hardware engineer’s dream!

##### What is on your bookshelf?

My old college textbooks, various symposium proceedings, Henry Ott’s book, others, and of course my own book.

##### Do you have any tricks up your sleeve?

That is my specialty. I have developed hundreds of these and often invent new ones on current jobs. Many of these are described on my website and in detail on video at circuitadvisor.com. Some are also demonstrated briefly on my YouTube videos (search for dougcsmith).

##### Do you have any rules of thumb or quick formulas that you use regularly to make your life easier.

A path of reasonable sized conductor contributes about 20 nH/in to the larger loop. The value varies from less than this for small loops, 1 cm, to about double for larger loops but I use 20 nH/in as a “ballpark” number knowing that it varies substantially from case to case. This figure helps predict the effects of ESD for instance as Ldi/dt. One Amp of current rising in a nanosecond will generate 20 Volts across 20 nH. E = Ldi/dt is my favorite quick formula.

##### When solving an EMI issue what is your approach?

My approach is unique for every job but often involves measurements of currents and/or local fields as voltage induced into a square wire loop. The voltage induced in a small loop can predict if that local field can couple enough energy into nearby wires to cause an emissions problem or operational problems. A square loop can also measure voltage drops by mutual inductance across conductors or planes. Shielded loops do not work nearly as well as people think they do so I use unshielded loops. Shielded loops work well in the far field only. Search for “square” at my site to see why.

##### What has been your favorite project?

I have so many it is difficult to decide, but finding the cause of disruption of a gigabit lightwave system to be jingling change in people’s pockets is near the top. It was the worst case of the “Vice Presidenial” effect I have seen (where a prototype works great in the lab until the VPs come in for a demo). In this case the lightwave system went nuts, but only when the VPs were there for a demo. It worked great at other times. I tracked the source of the problem to the VPs jingling change in their pockets which created noise with a bandwidth of 20+ GHz. See my video on jingling change. The problem didn’t happen for the engineers because we were not wearing polyester suits.

##### Do you have any note-worthy engineering experiences?

One of my designs about 30 years ago caused a mushroom cloud in our factory. One of those experiences that can be funny after enough time passes. The problem was due to a combination of a design fault and powering PCBs in “burn-in” test from a power supply in the factory which much more power available than the actual system would have.

##### What are you currently working on?

Most of my work is proprietary for my clients so I can’t discuss details, but within any given year I work on analog, digital, EMC, ESD, and other problems. My work is a lot of fun and different every week. My normal involvement on a job is a few days but sometimes longer term projects come up.