Radio Days - Embedded.com

Radio Days

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In 1998 an anti-shoplifting device reset a man's pacemaker. He collapsed, but a nearby nurse's quick application of CPR saved his life.

Recently various sources reported that a malfunctioning flat-screen TV emitted radiation on the 121.5MHz band, one reserved for emergency radio beacons. Law enforcement zeroed in on the location of the false distress call and descended on the college student's room. He was ordered to leave the set off.

Old timers will remember AM radio transmissions being obliterated every time a car with a poorly-maintained ignition system drove by.

Last month the Federal Communications Commission (FCC) deregulated broadband over power lines (BPL), a fascinating technology that promises to put high-speed Internet access at every household outlet. Unfortunately the licenses permit operation between 2 and 80MHz, a huge swath of spectrum currently occupied by numerous other services. Ham radio and cellular advocates are up in arms, concerned that BPL will generate tremendous amounts of interference. It seems to me that BPL violates the basic tenet of good RF engineering: constrain unwanted radiation using twisted pair or coax. Power lines do flip over each other once or twice per mile, but only to limit the 5 million meter wavelength of 60Hz radiation. At HF frequencies the wires are very effective antennas.

This NTIA chart dramatically illustrates how jam-packed the spectrum is.

The anti-BPL community complains that there's really no need for this technology. DSL and broadband-over-cable are everywhere today. Here in Baltimore I consistently get 3MB/sec downloads via Comcast's cable network, for a reasonable (considering just how much I use this service) $70 or so per month.

For ethical reasons we engineers have a responsibility to limit unwanted RF emissions. When electronic products interfere with each other everyone loses. Hospitals, for instance, are radio wastelands where too many pagers, cell phones, and products with embedded systems all emit so much noxious RF that many systems just don't work properly.

The FCC and European Union's Conformit Europenne (CE) regulations mandate certain limits for radio interference. Design a product that breaks the rules and you'll face heavy fines. The law requires that an independent lab certifies requires most products.

Long ago I merely scraped through the two electromagnetics courses all EEs were required to take. The teacher was quite proud of the “elegant” del and curl notation of Maxwell's laws. Though the equations sure were pretty to look at, they never made much sense to me. But I was planning to be a digital engineer so figured electromagnetics would be as useful to my career as celestial mechanics.

Then I got a job calculating satellite orbits on a 360/95. And computer speeds climbed from 1MHz to nearly infinity. Today's sub-nanosecond switching speeds means even a 4MHz microcontroller spews excessive levels of RFI.

As concerned citizens we have a responsibility to conserve the environment — in this case the scarce natural resource of the radio spectrum. We embedded developers are custodians of this precious resource. Follow the FCC and CE rules, of course. But go a step further. Some CPUs have programmable slew rates for outputs. Use the slowest transitions your system will allow. Keep clock rates as low as possible. Use multilayer PCBs to control emissions. Enter sleep mode rather than an idle loop. Use fiber rather than long copper cables.

We all want the benefits of wireless electronics. Carelessly designed spectrum-polluting embedded systems will make that dream much harder to achieve.

As will poor FCC decisions.

Jack G. Ganssle is a lecturer and consultant on embedded development issues. He is conducting a seminar building better firmware faster in Las Vegas on Dec 10. Contact him at . His website is .

Reader Response



You forgot to mention that satellite TV also offers internet access. This means that there are already 3 viable sources toconnect to the internet. Why do we need another source? And, why one with all of the issues that BPL brings. I think thisis another case where money talks, and the promoters use the high priced lobbyists to get what they want. NTIA and the ARRLhave conducted scientific investigations of the interference that BPL creates, and it is a very serious problem.

Unfortunately, the FCC does not want to hear of this, and have continued to follow the money rather than the technicalmerits. The FCC needs to have a few more engineers and a few less lawyers. And, they need to become immune to the desiresof the lobbyists.

– Howard Smith


As a concerned ham, I wrote to my congressman about RF polution and BPL. I mentioned my concern and asked him to be aware of this and to do something if he could. I got a nice letter back (probably written by an aid) that explained how this was all being looked at and, if I remember correctly, told me that essentially, it was all going to be OK. Of course, where BPL is being deployed, many hams cannot operate on HF because of the interference. Who will BPL have to bother before they finally take a real (engineering) look at this?

Also, a few years ago I was working on issues for power companys and human health. Magnetic fields at 60 Hz do not have the energy needed to conclusively cause problems with biological systems; or so the research went. Even transient fields at 50 Khz were deemed no longer a concern. But we know well the problems RF pose for biological systems yet, where are the same groups who so violently opposed transmission and distribution line expansion and sought mitigation of their high magnetic fields?

– Tom Sullivan

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