Oscilloscopes

An oscilloscope is one of the handiest pieces of equipment in any laboratory. It provides a way of observing time-varying electrical signals (voltages). Since most detectors ultimately produce electrical signals, and most electrical signals vary in time on a scale from nanoseconds (10^-9 s) to seconds or more, an oscilloscope is very commonly used to measure time-varying signals, check the behavior of equipment, such as detectors, frequency counters, and multimeters. As seen in the picture, the oscilloscope trace shows whether your signal is a sine wave, a square wave, or some more complicated function of time.

Using an Oscilloscope

You supply a signal to the oscilloscope using a BNC cable. Some oscilloscope channels have an input impedance (resistance) of 50 W, while others have an impedance of 1 MW. Newer digital oscilloscopes typically allow the impedance of the channel to be changed between these two values. The impedance of the channel influences the speed of the channel. The smaller the impedance, the faster the channel can be. A 1-MW channel will typically not permit MHz signals to be observed. Use a 50-W channel for very fast signals.

Sweep Time

The time of a sweep may be adjusted from several nanoseconds to several seconds. The value on the dial sets the time per division on the oscilloscope screen, which typically has 10 divisions across the graticule.

Trigger

To display repetitive signals in a useful way, the oscilloscope needs to know when to begin each sweep. You tell it when to start by setting the trigger conditions. Usually, the signal itself is used to set the trigger condition. For example, you can ask the scope to sweep when the voltage of your signal rises above 0.25 V. In this case, set the trigger source to internal, the slope to positive, and the level to 0.25 V.

When you are looking around for signals, it can be useful to have the oscilloscope triggered more automatically. If you set the trigger source to line, then it will be triggered by the 60-Hz oscillations of the line voltage (coming from the electric company).

Mode

One more subtlety about triggering: you may set the trigger mode to Auto or Normal. In Normal mode, the scope sweeps only when the trigger conditions are met. In Auto mode, the scope waits for the trigger condition, but sweeps anyway if the trigger condition is not satisfied within a fraction of a second. I recommend setting the mode to auto unless it is causing you problems.

Updated 6/17/99 by Peter N. Saeta .