A Bode plot is one of the most useful tools we have for understanding how a circuit or system responds to different frequencies, which makes it especially helpful when we work with filters, amplifiers, or control systems. With the Analog Discovery 3, we can generate and observe Bode plots directly from our own circuits, allowing us to connect theoretical concepts to real, measurable behavior. This hands-on approach helps us see how component values, design choices, and real-world imperfections shape a system’s frequency response, something that is difficult to appreciate from equations alone. Not only that, but generating bode plots may even be required for some courses, like Introduction to Electronics for example.
For this demonstration, we will be using a simple RC circuit with a sinusoidal input of amplitude 1. For a high pass filter, we will measure the voltage across the capacitor as the output. Lets say we want to design a high pass filter with a cutoff frequency of 10kHz. Using the equation below, we can rearrange it in order to find the capacitor value we should use given a certain resistance.
$$ f_c = \frac{1}{2\pi RC} \to C = \frac{1}{2\pi Rf_c} $$
Since 1k resistors are quire common, we will use this value for the resistor in the circuit. Solving for the capacitor value given the desired cutoff frequency of 10kHz, we will find the appropriate capacitor has a capacitance of about 15.9nF, which we round down to 15nF for accessibility purposes. A 15nF capacitor can be found in the Mercer X Lab by asking a staff member. We can then build the circuit in LTSpice.
In order to run the simulation, we will press the black gear icon to initialize the ac analysis. Enter the following information below:
Once that has been filled, click ok and start the simulation (press the green button if the simulation does not start automatically). Once started, press on the junction indicated by the red arrow to observe the output.
We can now see the frequency response of our primative RC circuit. In order to find the exact location of the cutoff frequency, we need to find the point in which the output drops 3 decibels from the start. Right click on the graph and select “Place cursor on Active Trace”.