Electronic Design

Digital Readout Scope

Digital readout of voltage and time, joystick positioning controls, and modular construction are but a few of the features which make the 35-mc, 50-mv/cm DuMont 425 a most unusual oscilloscope. Manufactured by Allen B. DuMont Laboratories of Clifton, N.J., the 425 provides unambiguous, accurate voltage and time readings from even the most complex waveforms. Of its many novel features, the most unusual is the digital readout.

To measure the peak-to-peak amplitude of a complex waveform, for example, the operator sets the Display Logic switch to the Read Out position. He then moves the waveform and the two dots which appear with it to a convenient position on the screen with the Pattern Positioning joystick. Using the Index Positioning joystick, he moves both dots till the indexing dot coincides with the negative level of the waveform. He maneuvers the voltage and time-scale thumbswitches so the scaling dot coincides with the positive level of the waveform. The upper left readout area provides a three digit voltage reading.

He can use a similar technique to measure the period of a waveform or time between points on the waveform. In this case, the upper right readout area gives a three-digit time readout, while a vernier can supply the fourth digit.

For accurate rise-time measurements, a push button under the voltage readout can automatically move the dots from the peaks of the waveform to the 10- and 90-percent voltage levels.

Electronic switches in the X,Y, and Z axes can provide unusual effects. For example, one can view the main sweep and the delaying sweep simultaneously, with one positioned over the other.

When the Dual Trace plug-in is used with the Delaying Sweep, the scope can display two completely independent voltages against two completely independent sweeps.

The bootstrap sweep circuits in the 425 main frame, with a range from 10 µsec/cm to 6 sec/cm provide sweep accuracy which is normally better than 3 per cent—always better than 5 per cent. The Delaying Sweep can provide delays from 0.5 µsec to 10 sec.

Available from stock, the 425 main frame costs $2750. The most common plug-in units range from $100 to $300. (electronic design, Feb. 3, 1960, p. 56)

Although this dot-moving technique may seem a little time-consuming, it represents another step along the way of turning the oscilloscope from a simple waveform display instrument into a true measurement tool.

See associated figure.

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