Tutorial Electronic Workbench 3

The oscilloscope

An oscilloscope is a far more powerful instrument than a bargraph indicator or even a voltmeter. It can show you the time dependence of the signals in your circuit. The EWB oscilloscope provides a fairly close approximation of a real one. It has two independent input channels, A and B, an input for an external trigger and a ground connection.

Figure 3. The EWB oscilloscope icon with its terminals.

To look at the output of your signal generator you can add anoscilloscope to the circuit you just made.

1. Drag the oscilloscope onto the circuit window, and double-click on it.The oscilloscope has four terminals, for two independent input channels, a trigger input and a ground connection. The input channels sense voltages with respect to ground! As long as there is at least one ground terminal attached to your circuit, it is not necessary to connect the oscilloscope ground. We will discuss the issue of how the oscilloscope is triggered in class. At this point, leave the triggering

on auto.

2. Connect channel A to the "+" output of the function generator, and

activate the circuit. You should now have a sine wave on your oscilloscope screen.

3. Make drastic changes in the signal amplitude and frequency, andadjust the sensitivity and time base settings such that you still maintain an easily interpretable picture of the wave form on the oscilloscope screen. It may be necessary to occasionally reactivate the simulation.

Figure 4. Using the oscilloscope to investigate the signals from the

function.

4. Change the offset on the function generator to a value of the order of

the amplitude. This adds a constant voltage to the signal. You will see

the trace on the oscilloscope move up (or down). You have two options

to move it back to center.

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5. Change the "y position" such that the trace comes back on center.

This can always been done as long as the offset is not too large. (Most

oscilloscopes cannot produce an internal offset that is much larger

than the full scale display range.)

6. Change the "y-position" back to zero, and select "AC" as input

coupling mode. In this mode the DC component of the signal is

removed. The EWB oscilloscope is very good at this, but real

instruments have a difficulty distinguishing between DC and very

slowly oscillating signals. In practice, avoid the AC input mode for

signal frequencies less than 100 Hz.

To get a larger image of the oscilloscope, try the expand button. On the

expanded display you will find two vertical line cursors. By moving these

around you can measure time and amplitude of points on the displayed

traces.