Oscilloscope 101

The purpose of this lab was to give us some practice with an oscilloscope.

Our first exercise goal was to display and measure a sinusoidal wave. First, we powered up the function generator and hooked it up to the scope. In the FG options, we set it to display a sinusoid with a frequency of 5kHz and peak-to-peak amplitude of 10V.

Next, we got everything on the oscilloscope dialed-in per the instructions (auto-scale, channel, horizontal control, vertical scale) to verify the FG's output. Based on our settings, here was our scale:

x-axis (time): 50μS

y-axis (voltage): 2V

Viewing the graph on the screen, we counted the boxes according to the scale. Our results confirmed our FG's settings; the peak-to-peak amplitude was 10V (5 boxes) , and the period was 180μS (1/180μS ≃ 5kHz).

We also calculated an anticipated RMS value, using the definition of Vrms=Vmax/sqrt(2). With a Vmax of 5V, we should get a Vrms of 3.54V. Verifying with a voltmeter, we measured a DC voltage of 0V and an AC voltage of 3.32V (≃3.54V), as expected.

Exercise 2 involved displaying a DC offset. We added a +2.5V offset on the FG, keeping all other settings the same on the FG and oscilloscope.

The result: the DC sinusoid was shifted up by 2.5V, but the AC sinusoid was not:

The voltmeter gave the same results: our VDC moved up to +2.51V, but VAC stayed at 3.32V.


This is because our voltmeter and oscilloscope distinguish between DC and AC voltage in a mixed DC/AC signal and ignore the constant value being added to the AC sinusoid.

For exercise 3, we set the FG to display a square wave with the same offset from exercise 2. The voltmeter measured a VDC of 2.51V, and a VAC of 5.24V.

We can use this to calculate our expected Vrms:

We calculated the area under the AC sinusoid (with an amplitude of 5.24V) over a period from 0 to π. We then divided the area of the sin wave by the area of the box, giving us 3.34V. This matches our VAC from exercise 2.

 Our last exercise involved guessing the characteristics of a "mystery signal" generated by Professor Mason, based on what showed up on our oscilloscope. Professor Mason tweaked the settings until two different signals were coming from channels 1 and 2. For the channel 1 signal, we used the scale on our oscilloscope to determine that the zero-to-peak DC voltage was 1V (2V peak-to-peak), with no offset (it was symmetrical WRT the y-axis). Our period was 4 boxes. At 10μS/box, the frequency was 1/(4*10μS) = 25kHz.


The channel 2 signal had a 0.1V zero-to-peak DC voltage (0.2V peak-to-peak) with a +0.3V offset. Our period was 4 boxes at 50μS/box. 1/200μS = 50kHz.

Tweaking scales to determine the characteristics of our mystery signal