ELECTRONIC TEST EQUIPMENT

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on the front panel. A knob for each channel adjusts the volts/div (volts per division) scale of the graph, and turning it to “ 1” sets the horizontal gridlines 1V apart, going up and down. The baseline voltage is controlled by another knob, labeled Position or Vertical Position. This lets you move the trace up and down. You can use these knobs to look at more than one trace at a time, adjusting the displayed voltage ranges for each channel to superimpose them against one another.

Most scopes also have an input coupling switch that allows you to choose between AC, DC, or GND for each channel’s input. GND is for adjusting the oscilloscope; set your ground to 0V. DC mode displays the time-varying signal in absolute voltage, while AC shows the signal centered over the x-axis, adding or subtracting any constant voltage to show how the signal differs from a baseline average.

Trigger — Fire When Ready

Triggering is the hardest part for beginners to learn. One method is to read all about the trigger modes and how they work; the other is to “video-game it” and just keep knob-twiddling until you see the trace. Here are the most common types of trigger conditions, but there are many others:

Normal/single trigger — Triggers whenever the input voltage exceeds the knob-adjustable threshold. Auto — The scope guesses when to trigger, like the auto setting on a camera. TV trigger — Special triggering for television signals. External trigger — Trigger from an electrical signal via a cable plugged into the front panel. HF reject — Trigger when steadily rising voltage exceeds threshold, but not if it suddenly spikes. LF reject — Trigger on sudden spikes but not slowly varying voltages.

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Slope — Sets whether the trigger detects rising edges or falling edges.

PROBES

Oscilloscope probes connect your circuit to the oscilloscope electrical input. Most have a little hook that can grab onto the component’s leads on a circuit board. Unscrewing the hook reveals a sharp point that can dig into a circuit board’s copper traces directly. Probes also have a dangling alligator clip, which you connect to ground to provide a baseline 0V for your measurements. Poor grounding makes for bad measurements.

The most common scope probes are 10x probes, which reduce (attenuate) the input signal to one-tenth their original voltage, so a 10V signal will appear on the display as a 1V signal. (You might wonder why these are not called 10% probes.)

TRY IT! TEST AN AUDIO SOURCE

We heard distortion when we first hooked up an iPod mini to our big old stereo. Using a scope, we tracked down the problem. We wired the iPod headphone jack to the scope inputs, and played a test file of a pure 1kHz sine wave. With the iPod volume turned all the way up, we saw that the peaks of the wave were flat rather than round. This distortion is called “clipping.” Slight clipping makes sound a bit muddy, and more pronounced clipping makes it fuzzy and clearly distorted. The onset of clipping, where it’s slight, is much easier to see on an oscilloscope than it is to hear (Figure A).

Turning the iPod’s volume down to 90% eliminated the problem, and generated a nice clean sine wave. From this we learned that you should set your iPod to no more than 90% volume when hooking it up to a stereo (Figure B).