Corrupt Data Filled the Air Waves

Think back to the 1980s when there was no Internet and the most advanced microprocessor for personal computers was the Intel 8088, which required a minimum of four clock cycles per instruction and used a 4MHz clock. Computers were slow!

The Massachusetts Institute of Technology got a contract to study the feasibility of dissemination of data using FM radio broadcast stations. It leased some of the spectrum used by the FM station owned by the Emerson Radio School in Boston. The data rate was 1,200 baud, and was encoded onto a 92kHz carrier by shifting the frequency a few kilohertz up and down to send a mark and space. The 92kHz carrier was mixed with the monaural audio, the stereo subcarrier, and the 67kHz subcarrier radio reading for the blind. The composite signal modulated the FM transmitter. The power dedicated to the transmission of the data was in the order of 25W or less.

MIT contracted my company to design and build the radio modems needed to conduct the tests. MIT students volunteered to use their computers to receive data, which consisted of national news. Since my office was about 30 miles from Boston, I could receive the signal, but it was not very strong. Since I already had a high-quality crystal controlled FM receiver, it was only necessary to design the 92kHz FSK demodulator board. The design was simple, but highly effective, complete with an RS-232 output.

I thoroughly tested the production prototype with my own equipment and then observed the signal from Boston. Both tests were excellent, and I was ready to send the prototype to MIT for final approval. It was only a few days before that I received a call from the MIT professor who said the modem put out only garbage! It was embarrassing. I checked a receiver in my office, and the data was clean.

A few days later, I visited the computer lab at MIT. There were computers everywhere. They were even experimenting with robotics. I asked to go to the men's room, and it was full of computers. Finally, the professor showed me the receiver that I had sent and the display on the oscilloscope. Yes, it was, in fact, garbage. I asked him if he was getting a strong signal. He pointed out the window to the tower on the other side of the Charles River. I explained that the power near the transmitter was not always as great as 10 or 20 miles away because of the gain of the antenna, but we both knew that was not a good explanation.

As I tried to solve the puzzle, I gazed around the room and realized that there were more than a dozen computers, all powered up and doing something or other. I asked the professor if he could turn off the computers. One by one, he turned them off. Soon, on the oscilloscope, a waveform started to appear. It got cleaner and cleaner until it was perfect.

Around 1980, computers were not regulated by the FCC, and no effort was made by the manufacturers to control the slew rate on the data lines, even those leaving the computer. The noise spectrum from some computers extended into the hundreds of MHz.

Shortly after my meeting, I got a confirmation to construct the balance of the order, and MIT successfully conducted its experiment.

This entry was submitted by Frank Karkota and edited by Rob Spiegel.

Frank Karkota worked with power transmitters in the range of less than 1MHz to 5GHz. He designed and built equipment for radio stations and eventually started a company that made commercial and consumer receivers that covered 500kHz to almost 1GHz.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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