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February 2018
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Matlab Example – Bode Plots of a Low Pass Filter (Part 2)

The Matlab tutorial article continues with providing an example of a low pass filter (LPF) to graph its bode plot in Matlab.  The low pass filter was implemented using a simple RC-circuit.

To review, bode plots are basically logarithmic graphs of a linear time-invariant system as a function of frequency. Bode plots are useful tools since they plot the magnitude and phase of a transfer function.

One of the bode plots graphs the magnitude of the system . The graph describes how much of the input signal will be amplified or attenuated by the system as the frequency of the signal changes. The above video describes the system as a low pass filter and shows how bode plots are generated using Matlab. The magnitude axis of the Bode plot, expressed as decibels of power 20 times the common logarithm of the amplitude gain.

The other bode plot is the phase and it describes how much of the input signal will lag or gets delayed as it passes through the system. Two examples were shown in the video. At the cutoff frequency the bode plots show a 45 degree phase shift for a single or first order low pass filter and the second plot show a 90 degree phase shift for a second order low pass filter using a simple RC filter.

One Matlab graphical user interface (GUI) to graph bode plots can be found at Swathmore College.

BodePlotGui is a graphical user interface written in the MATLAB® programming language. It split its transfer function into elements to draw the piecewise linear asymptotic approximation for each element. The program appears to be a versatile program for teaching and learning the construction of Bode diagrams from piecewise linear approximations.

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