Design Con 2015

Time-tested technical tools and techniques

-July 16, 2013

The other day, I happened across an interesting blog (these used to be called a “column”) at the EE Times Test & Measurement Designline site. It showcased using your oscilloscope screen as a “written” clock display by tracing Lissajous figures on screen (see Turn Your Old Oscilloscope Into a Clock). Whoa, I stumbled... that term "Lissajous" really took me into the "way-back" time machine!

Here's why: back in the day, before microprocessors ruled the land, when vacuum tubes were the only amplifier choice, and when "digital" was mostly a relay-based reality, Lissajous figures were the only viable technique for frequency comparison.

In brief, you would feed your reference standard from a crystal or other oscillator into the horizontal-axis input, and the frequency source to be assessed into the vertical input. If they were both of the same frequency, the resulting on-screen ellipse would be "static"; if they differed, the ellipse would rotate about its z-axis. With some practice and experience, you could even get a sense of the magnitude of the difference between them, by the rate of rotation. [Be warned: if you weren't careful, you could also get hypnotized or mesmerized by the rotating patterns!]

Using the Lissajous-pattern technique, you could also check for basic ×2, ×3, and ×4 frequency relationships, as well as other small-integer ratios (i.e., 3:2), by seeing if the on-screen pattern rotated or not. A 2:1 ratio yielded a “figure 8,” for example. (You can read more about Lissajous figures, or Google it for more references.)

But using Lissajous figures for frequency checking has gone the way of the selenium rectifier, and with good reason. Even a low-cost, basic frequency meter is much more versatile, accurate, precise, and convenient for just about every application. Still, you might use the Lissajous-figure technique or an updated version when checking your frequency source against an atomic clock, where you are looking for slight differences and the frequency meter is not good enough - somewhat like you use a bridge (Wheatstone) circuit to compare two signals when their actual value is not critical, but their closeness to each other is the parameter of interest. But that's an unusual situation, of course.

The fact is that many old-time techniques and tools were born of necessity and ingenuity, of course, but they have been superseded or made obsolete by today's instrumentation.

Thinking about it some more, though, I realized that there is one "ancient" tool that is still in widespread use: the Smith chart, from the 1920s (click on the link to read a short, well-written article about it). Sure, we often use it on-screen instead of on special circular graph paper, but it is still the handiest visual tool for assessing and matching impedances in RF applications. That's a design world where power transfer and impedance matching are usually more important parameters than conventional voltage or current in many cases, so the Smith chart's versatility and insight are still quite helpful, and often essential.

Are there any other relatively old engineering tools and techniques that are still in fairly widespread use? Is there perhaps a nomogram that you use on a regular basis, for example? Give the question some thought - you may surprise yourself, and the rest of the readers.

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