Designing Ideas

In the late 1930s, the first Great Depression still had its hooks sunk firmly in the average amateur-radio operator's wallet, and hams had to make do with materials on hand. For higher-power operation, an enterprising ham could remove the octal base from a metal-encased 6L6 vacuum tube rated for 19W plate dissipation, fan out the leads to increase breakdown voltage, invert the tube in a coffee can filled with transformer oil, and run the 19W tubes at 75 to 150W input—well over the nominal rating. As one grizzled old-time ham remarked, "It worked fine as long as you sent short dashes and didn't chat much."

I don't recommend routinely overloading solid-state components, because burning epoxy stinks up the lab, but using components in unorthodox ways can solve problems and possibly save time and money. And besides, it's fun.

The leadoff Design Idea in this batch describes how you can divert an unsuspecting SMPS controller IC to a life of toil and suffering in the test lab, or to put it in formal terms, you can Use a switching-regulator controller to generate fast pulses. Note that you'll have to pay attention to bypassing, circuit layout, and proper output termination to get good results.

In contrast to the adventurous and sometimes brief careers lived by microprocessors and exotically applied (or misapplied) ICs, the shift registers employed in our second Design Idea, Shift registers and resistors deliver multiphase sine waves, most likely will die from boredom as they shuffle ones and zeros into resistors to produce pretty good sine waves. Most likely, this application will join the fast-pulse generator in a lab somewhere and pass its time watching the oscilloscopes play poker.

Of course, any instrument that passes as "intelligent" should be smart enough to switch itself off when nothing of interest is going on. And "intelligence" doesn't necessarily require the presence of a microprocessor—all it takes is some logic that's a little brighter than a handful of TTL chips. A Design Idea entitled, CPLD automatically powers itself off shows the way. Note that the artwork accompanying the Design Idea shows the CPLD in question in very general terms. Keep the CPLD's automatic-shutoff core and build your application around it.

Our fourth and last Design Idea, Amplifier removes common-mode noise on RGB differential-video-transmission line, addresses the chronic problem of common-mode noise on video signals. While you can use an isolation transformer or a bulky common-mode choke (bulky because adequate 60-Hz rejection requires a large toroidal core for best results), you may prefer an all-electronic solution—this Design Idea offers an alternative. You'll have to provide power for the amplifier, though, and unless it's well isolated the power supply may introduce its own common-mode path. Based on a hasty analysis, I'd guess that a pair of 9V alkaline batteries would power the circuit for 50 hours or thereabouts.

While we're on the subject of electronic-device abuse, you might want to visit "Mike's Electric Stuff," an online virtual museum of assorted electron tubes, glow and Geissler lamps, Nixie™ displays, and a bunch of other glass-encased electronic components including a glassware-intensive analog-to-digital converter. Highly recommended, and good for a half-hour of slack time.

Until next time, 73,

—Brad