What is Low Pass Filter MATLAB?
A low pass filter in MATLAB is a filter that blocks the high-frequency signals and allows only the low-frequency signals to pass through it.
Table of Contents (click to navigate)
- Low pass filter in MATLAB
- Steps involved in designing a LPF in Simulink
Low pass filter in MATLAB:
A Low pass filter in MATLAB is a filter that blocks the high frequency signals and allows only the low frequency signals to pass through it.
When we deal in frequency domain in MATLAB, we need to filter some of the frequencies to acquire the desired data. When we say low pass filter or high pass filter, it means we’re defining some range of frequencies to pass through the filter and block the others.
For example, we want to pass the frequencies below 50kHz only, we use low pass filter for this purpose. This filter will pass those signals having frequency less than 50kHz and it will show 0V signal for higher frequencies.
In practical life, we use LRC circuits to design these filters. In MATLAB, we can design them in different ways. We can see the response of any filter by designing an LRC circuit in MATLAB Simulink, or we can just give a signal to a built-in filter in Simulink and use the scope to see it.
There are many ways to design a low pass filter in MATLAB but here we are discussing only one method of how to make a low pass filter on MATLAB Simulink Software.
Steps involved in designing a LPF in Simulink:
Here are the following steps to design a low pass filter in MATLAB:
Open MATLAB software and see the “Simulink Library” in the upper window.
Open the Simulink library and click on the highlighted button.
Search “sine” in the search box of Simulink library.
Drag this “sine wave” box into the window.
Select “continuous” from the Simulink library and drag a transfer function into the execution window.
Now we have to calculate the transfer function of that LRC filter so that we can put those values in this transfer function. Let’s take an example,
“Design a first order low pass filter in Simulink for the given specifications; test the designed model for a sinusoidal input signal at various frequencies”.
- Circuit is given here:
- Find the transfer function of this low pass RC filter.
H(s) = Vo(s) / Vi(s)
H(s) = (1/sC) / (R +1/sC)
H(s) = 1 / (RsC + 1)
H(s) = (1/RC) / (s+1/RC)
H(s) = (100) / (s+100)
- Now find the output voltage of this circuit. You can find it using voltage divider rule.
Xc = 1 / (2*pi*f*c)
Vo = Vin = R2 / R1+R2
Vo = Vin * Xc /R2+ Xc2
- Now take the input of Sinusoidal voltage from the simulink library.
- Draw a circuit in the same way as shown. And the requirements are given here. We have to check our low pass filter on these three settings.
- Voltage output a t a frequency of 1Hz. Assume Vin = 10V
- Voltage output a t a frequency of 10Hz. Assume Vin = 10V
- Voltage output a t a frequency of 100Hz. Assume Vin = 10V
Double click on sine wave and set your settings like this.
Double click on transfer function and make the settings as follows.
Go to the top window of your simulink screen and set the sample time to 1 .
- Start the execution by hitting the play button.
- You can see that scope has shown a graph of 10V at the output. It means filter has passed the voltage at 1 Hz frequency.
- Now change the frequency to 10Hz and see the result. The peak is less than 10V because now we are increasing the frequency. You will see that it will be zero on higher frequencies
- Again set the frequency to 100Hz and see the result. This time filter will pass a low voltage because of higher frequency.
- For the last time set the frequency to 200Hz. You will see that filter will block the voltage. Match you output voltage with the calculated voltage to see that at what frequency, filter will completely block the voltage and set it to zero.
Low pass filters will block higher frequencies and pass low frequency signals. In MATLAB, we have seen that if we design a low pass filter and insert its characteristic equation or transfer function into the filter block in MATLAB, we can use it to design the parameters for the desired frequencies. This is only one method to design a Low pass filter in MATLAB. There are many other ways to do it. And we will discuss it later.