Those that are familiar with Digital Multimeters (DMMs) know that they are great for measuring the voltage of a circuit at one specific moment. As soon as the signals in our circuits start varying with time, the data becomes less useful, appearing as an assortment of quickly changing numbers. Most electronics circuits are not simple enough to justify measuring a single point of reference. Either the signal varies with time, or you might have some components with non-linear behavior like capacitors. However, if you spent time recording each point that your DMM read in a graphical table, you could get a very useful idea of what is happening in the circuit, but waste a lot of time in the process. An Oscilloscope presents an alternative to this time-consuming process by mapping out this data for you.
As an example of where an Oscilloscope might be useful, we will go through characterizing an RC circuit (a simple circuit comprised of a resistor and capacitor). In the most basic sense, capacitors slowly collect charge before slowly discharging over time. But what does that mean in terms of signals? Let's find out!
I have created a basic RC circuit and included the schematic below so that you can reference its values:
We will begin by connecting the positive side of oscilloscope channel 1 (orange wire) to the second resistor (R2), the waveform generator (yellow wire) to the second resistor (R2), and ground (black wire) to the other node where the resistor and capacitor connect. You can see how your project should appear in the image below:
Make sure your settings match the settings shown above. Time base of 10ms/div. Trigger on rising edge at 100mV. Oscilloscope Channel 1 on with 200mV/div. Waveform Generator set to a square wave with 9Hz frequency and 3Vpp Amplitude.
Click Run and you should see the following signal:
In electrical engineering terms, the first half of the signal (with the green trigger on) it is the forced response and the second half is the natural response. More simply, the first section shows the response of the circuit when you suddenly apply a voltage, i.e. the charging of the capacitor. The second section shows the response of the circuit when you remove that voltage, i.e. the discharging of the capacitor. It can be useful to measure how fast either the discharging or charging happens, and with an Oscilloscope you can easily see the signal and take these types of measurements.
There are of course lots of equations and theory that govern the charging and discharging of capacitors, but I won’t get into that. If you are interested in learning more, the free Real Analog coursework provides a great deal of information on the subject.
A Note on Triggering
Having completed the circuit from the previous exercise, let’s take a moment to understand triggers. Triggers define a condition that is to be met before the acquisition is started. But what does that mean in practice?
You may have noticed that when we got the OpenScope up and running, the section labeled “Trigger” had a symbol with an arrow pointing up automatically selected. Let’s try clicking on the “OFF” button in the trigger setting to turn the trigger off. You should now see the signal bounce all around the window. Clicking the button with the arrow symbol pointing up should stabilize the signal. This demonstrates the purpose of triggers, which is to stabilize the part of the waveform that you care about and quickly analyze the signal.
Now we will explore how the trigger settings change the viewpoint on the signal. The settings we had previously should appear with the arrow symbol pointing up and 100mV.
The symbol that you select defines where the trigger is on the rising edge or on the falling edge. For periodic signals (e.g., a signal that repeats, think sine wave) there will be multiple points where the signal crosses a certain value. This requires a specified trigger where the signal is rising or falling. The value that you enter is the value that the signal will trigger on. You can visualize the trigger in WaveForms Live (shown below) with the green line. Dragging this back and forth will allow you reposition the signal on your screen.
Now change the value to 5.2V in the sidebar. You will see the signal move so the trigger is in part of the signal that begins to flatten out. You may also notice that the signal starts to move around a bit. This demonstrates part of the importance of selecting the trigger value.
First, the value must be within the signal, or it will never trigger. Next, it should be within a more stable part of the signal, otherwise, the small voltage changes caused by noise will move the trigger point around. This results in a less than stable signal.
Now change the value to 500mV, and select the symbol with the arrow pointing down (i.e. the falling edge trigger). This will move the trigger to the falling edge, which also happens to be the most stable part of the signal. You should see the signal stabilize enough to take measurements and drop cursors to gain valuable data!
More About RC Circuits: Frequency Domain
In the first example of an RC circuit we looked at how the circuit responds to a square wave input at a fixed frequency. This is helpful for determining what will happen when we turn the circuit’s power source on and off. But what if we want to know what happens when we put signals at varying frequencies through our circuit?
For signal processing, it is often helpful to build a circuit that can filter out certain frequencies or noise. With a BODE plot we can determine what frequency signals will go through our circuit and at what amplitude. By plotting all the responses on the same graph, we can quickly build, test and adjust our circuit.
Let’s begin by building a simple RC circuit as shown below:
Connect the Waveform Generator (yellow wire) to the resistor (R1), and the Oscilloscope channel 1 (orange wire) to the node with the resistor and capacitor. Finally, connect your ground (black wire) to the capacitor. The setup should imitate the image below:
Make sure the Oscilloscope in WaveForms Live is stopped before clicking the button with the squiggly line on the right edge of the plot window (see image below).
This will open the BODE plot, also referred to as the amplitude response. Check that the settings match the plot below, then click “Run”.
Each of the dots represents a frequency that ran through the circuit and was compared to the output. The dot’s “X” value is the frequency and the “Y” value is the ratio of input amplitude to output amplitude in dB.
This circuit shows a low-pass behavior since the lower frequencies have a value closer to 0 dB, or a ratio of 1. To learn more about Bode Plots and their cutoff frequencies check out the Real Analog course.
Find additional information about introductory circuits on the Digilent Wiki.
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