After being fed specifications in a piecemeal fashion, an engineer figures out how to meet a test spec by thinking inside the box
By Radcliffe Cutshaw, Contributing Writer
The project was to design a device to segment the upstream band used by cable modems. Cable modems are collision moderated: When two or more modems try to send at the same time, a collision is detected and both wait a random time before sending again. When large numbers of modems are using the network, the network slows considerably. One way to increase the speed of the network is to segment it; break the network into smaller segments.
The device we were working on would pass one segment, (5 to 45 MHz) through and frequency-convert three other segments (also 5 to 45 MHz) to different frequencies, combine the segments together, and use the combination to modulate an analog laser. The three frequency-converted segments would be converted back to 5 to 45 MHz at the cable network head end to be demodulated.
The entire project was designed to be modular: frequency sources, mixer boards, and filters were mounted on separate boards and could be used in the both up converter and down converter side–with the up converter section being in box on a pole and the down converter section being in a rack mount.
Our main problems stemmed from being fed the specifications piecemeal–sound familiar? We knew a customer supplied the mother board and a mounting box would be used, but initially all we had was a definition of a plug-in connector. When we finally received the printed circuit layout of the motherboard, we found that a pin that we had planned on using was unavailable. Things like this happened throughout the project.
A major problem arose when we obtained the environmental specifications. The one specification that we did not know how to test for was for heat. The box had to sit on a telephone pole any place in the U.S., even in the Arizona desert at 120F.
The low-temperature specification could easily be met by the components that we were using. The high-temperature spec was a whole other issue: One component that was critical was the analog laser, which had a maximum working heat specification of 147F, at which temperature the laser would become unstable.
The problem was how to test the temperature inside the box given an ambient temperature of 120F. We had a temperature oven and temperature probes, but we were not even near the hardware stage with the design, so we could not put a working unit in the box.
I first became aware of the problem by overhearing the project mechanical engineer talking over the problem with the company owner on the telephone. When the engineer hung up, I walked over to his desk and said that I might have a solution to the problem.
“First of all, the entire unit uses 95 watts of power. An incandescence light bulb is about 100% efficient….,” I explained. He started to interrupt. I held up my hand and said, “Hear me out before you say anything!” And I continued the sentence: “…..at producing heat. If you put a 100 watt light bulb in the box, it will produce slightly more heat than the converter unit. If you measure the temperature inside the box, it would probably be only about a degree higher.” He leaned back, smiled, picked up his phone, and began dialing. He gave me credit for the idea when he talked to the owner.
He did the measurement, yielding a temperature of 127F, a 20-degree safety margin. Today, hundreds of thousands of these boxes are on telephone poles throughout the nation.
Sometimes it pays to think inside the box.
Mr. Cutshaw, a serial entrepreneur, is currently a private consultant specializing in RF and analog design and development. He has been involved in many areas of engineering throughout his career.