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Posted: 06:46:39 PM, 13/11/2013

Reigning in signals that drift too far

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In early 1975, I had a technician post at the Woods Hole Oceanographic Institution (WHOI), before starting grad school in the fall. I had taken an extra semester to get bachelor's degrees in EE and physics at MIT, and wanted a low-key position to recuperate. A friend worked in the oceanographic instrumentation department at WHOI and had just lost his technician at a critical time in the department's efforts.


The department was deploying ocean current-tracking floats using acoustic signals for ranging. The floats would send out chirps at precise times. More moans than a chirp, they swept from 250Hz to 251.5Hz over 80 seconds, and were detected using auto-correlating receivers. Signals in this frequency range propagate long distances in the Sound Fixing and Ranging (SOFAR) underwater acoustic channel. Multiple listening stations would time the arrival of the tones and track the floats at ranges over 1,000km.


In addition to the SOFAR floats, one big project was a portable listening station called KAREN (Keyboard and Reader Experimental Network), named after our secretary. She had bemoaned the fact that she never got to go to all the exotic places the engineers & scientists did, so the least we could do was take her namesake with us. KAREN was a mix of analogue and early CMOS logic ICs, with a paper-tape reader and strip chart recorder. Everything fit into a small metal suitcase for travel. I had spent the spring assembling and testing it, so I was reasonably familiar with its inner workings, even though I hadn't designed any of the actual circuitry.


To test KAREN, someone needed to go to Grand Turk, a 1 x 6-mile island 850 miles southwest of Bermuda. Grand Turk was largely defoliated in the 1700s to make it hot and dry for sea salt production. While sea salt was no longer a big seller, the hot and dry climate remained. There was a US Navy base on the north end of the island, and WHOI had a hydrophone array there. You need a security clearance to get on the base, but the usual fellow's wife was expecting a baby. Fortunately, I had worked as a tech for a defence contractor while in school and had a clearance. They loaded me up with KAREN, a freshly minted passport (my first), and a Sony Tektronix portable oscilloscope, then off I went. I was more than a bit nervous about the responsibility I had been entrusted with, but I wasn't about to admit it.


I took an early flight to Grand Turk after a night at a Miami airport hotel. The terminal was quiet, and I remember the security guy running the new X-ray machine inviting me to come around and look at the innards of the 'scope. I landed at Grand Turk and got the last of the two (!) rental cars on the islanda weathered '64 Mustang convertible. I checked in at the hotel, and managed to drive on the left side of the road out to the Navy base without mishap. WHOI's listening station was on a remote corner of the base, a few yards back from the beach, in a tiny shed with an anemic air conditioner.


I hooked KAREN up to the hydrophone array, but it refused to go through the start-up process. I didn't know if it had been damaged en route, or if something else was out of whack. I was scheduled to report in to WHOI. I drove back to town to call, which a process in itself. You went to the cable and wireless office and waited until it had a free line to the mainland. To save on pricey long-distance charges, the people there would call a military base in Florida, who patched the call to Otis Air Force Base on Cape Cod, who could then make a local call to WHOI. I reported my findings and we agreed to confer in four hours. I would open up the box and start chasing signals, while they poured over the design documents to see if they could guess what was going on.


I drove back to the shed, fired up the 'scope and started working through the schematics, scribbling timing diagrams as I went. Nothing appeared physically damaged. I found a spot where the edge of a logic signal had shifted just enough that it wasn't getting clocked into a register correctly, probably due to the heat. The catch was: How to fix it? There were no suitable unused gates, and I had no spare logic ICs. A quick search of the shed produced a soldering iron, a few basic tools, and in the bottom of a small drawer, a handful of mixed resistors and a few diodes.


With a background in analogue design, I don't recall if I had ever heard of a diode AND gate before, but I poked around and found four logic signals that could be combined to solve the timing problem. I rigged up my AND gate and everything worked perfectly! After a few tests, it was time to report back to WHOI. I returned to the cable and wireless office and waited for the call to go through. The designer had actually managed to deduce where the problem might be, and knowing what my limited resources were, had come up with the same fix! I was more than a little proud (and extremely relieved) to report that I had beaten him to it.


Testing with a SOFAR float lowered off a boat in Bermuda was delayed for a few days due to weather there, so I headed back to the hotel. The bar turned out to be the local watering hole, with a cast of characters right out of a James Bond novel. I ordered a gin & tonic while they cajoled me into a round of darts, and later introduced me to a game called liar's dice. But that's another story...


Doug White has worked at various research labs on a wide range of analogue, RF, and microwave circuits from VLF to W-band. After being informed, "Analogue is dead," by a digital engineering professor at M.I.T., he's managed to stay employed as a dinosaur since graduating in 1978.


He submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).


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