How to Calculate the Frequency from an Oscilloscope?
by Joost |Updated on: June 20, 2021
Oscilloscopes can measure and display the instantaneous voltage graphically but keep in mind that an oscilloscope and a graphic multimeter is not the same thing. It consists of a screen that has a graph shaped vertical and horizontal lines. An oscilloscope measures the voltage and plots it as a voltage vs. time graph on the screen.
It usually doesn’t show the frequency directly but we can get a closely related parameter from the graph. From there we can calculate the frequency. Some of the latest oscilloscopes these days can automatically calculate the frequency but here we are going to focus on how to calculate it ourselves.
To calculate the frequency, we need to connect it to a wire with a probe. After connecting, it will show a sine wave that can be adjusted with the controls and switches on the oscilloscope. So it is crucial to know about these control switches.
In the bottom line, you will have the place to connect your probe into the oscilloscope. Depending on what type of device you are using there can be one or more than one channel.
There is a horizontal and a vertical positional knob on the oscilloscope. When it shows the sine wave it’s not always in the center. You can rotate the vertical position knob to make the waveform in the center of the screen.
In the same way, sometimes the wave only takes a portion of the screen and the rest of the screen remains blank. You can rotate the horizontal positional knob to make the wave’s horizontal position better and fill the screen.
Volt/div and Time/div
These two knobs allow you to change the value per division of the graph. In an oscilloscope, the voltage is shown on the Y-axis and the time is shown on the X-axis.
Turn the volt/div and time/div knobs to adjust the value you want per division to show on the graph. This will also help you get a better picture of the graph.
The oscilloscope doesn’t always give a stable graph. Sometimes it can be distorted in some places. Here comes the importance of triggering of an oscilloscope. Trigger control allows you to get a clean graph on the screen. It is indicated as a yellow triangle on the right side of your screen.
Adjusting Oscillosocpe Graph and Calculating Frequency
Frequency is the number that indicates how many times a wave completes its cycle in every second. In an oscilloscope, you can’t measure the frequency. But you can measure the period. The period is a time it takes to form a full-wave cycle. This can be used to measure frequency. Here’s how you’ll do it.
Connecting the Probe
First, connect one side of the probe to the oscilloscope probe channel and the other side to the wire you want to measure. Make sure your wire is not earthed or else it will cause a short circuit that can be dangerous.
Using the Position Knobs
Positioning matters a lot as far as frequency is concerned. Recognizing the terminations of a wave cycle the key here.
After connecting the wire to the oscilloscope, it will give a sine wave reading. This wave is not always in the middle or takes the full screen. Turn the horizontal position knob clockwise if it isn’t taking the full screen. Turn it counterclockwise if you feel like it’s taking too much space on the screen.
Now that your sine wave is covering the entire screen, you have to make it centered. If the wave is on the upper side of the screen turn the knob clockwise to bring it down. If it is on the bottom of your screen then rotate it counterclockwise.
Trigger switch can be a knob or a switch. You will see a small yellow triangle on the right side of your screen. That is the trigger level. Adjust this trigger level if your shown wave has static in it or it is not clear.
Using Voltage/div and Time/div
Rotating these two knobs will result in changes in your calculation. No matter what settings these two knobs are, the result will be the same. Only the calculation is going to differ.
Rotating Voltage/div knobs will make your graph vertically tall or short and rotating the Time/div knob will make your graph horizontally long or short. For convenience use 1 volt/div and 1 time/div as long as you can see a full wave cycle.
If you can’t see a full wave cycle on these settings then you can change it according to your need and use those settings in your calculation.
Measuring Period and Calculating Frequency
Let’s say I used 0.5 volts on volt/div which means each division represents .5 voltages. Again 2ms on time/div which means each square is 2 milliseconds. Now if I want to calculate the period then I have to check how many divisions or squares it takes horizontally for a full wave cycle to form.
Say I found it takes 9 divisions to form a full cycle. Then the period is the multiplication of the time/div settings and the number of divisions. So in this case 2ms*9= 0.0018 seconds.
Now, according to the formula, F= 1/T. Here F is frequency and T is period. So the frequency, in this case, will be F=1/.0018= 555 Hz.
You can also calculate other stuff by using the formula F=C/λ, where λ is the wavelength and C is the speed of the wave which is the speed of light.
An oscilloscope is a very essential tool in the electrical field. An oscilloscope is used for looking at very fast changes in voltage over time. It is something multimeter can’t do. Where multimeter only shows you the voltage, an oscilloscope can be used to make it a graph.
From the graph, you can measure more than voltage, such as period, frequency, and wavelength. So it is essential to learn about the functions of an oscilloscope.