# Low pass filter design and simulation using Pspice : tutorial 9

In this tutorial I will explain you the working of a low pass filter. The purpose of a low pass filter is to allow only the lower frequencies to pass through the circuit and block the higher frequencies. At the start a brief and concise introduction of filters specifically low pass filter along with them bode plots are provided with the explanation of the output they will show. After that the circuits is simulated using PSPICE and you are provided with the step by step approach to simulate a circuit and the result are compared with the theoretical discussion provided (which should be same). At the end of the tutorial you are provided with an exercise to do it by yourself, and in the next tutorials I will assume that you have done those exercises and I will not explain the concept regarding them

### Introduction to low pass filter

A low pass filter is such a filter which only allows frequencies with lower magnitude to pass through them and block the higher frequencies as the name suggests. The limit of the frequencies up to which the circuit allows is known as critical frequency and is given by the formula,

This frequency is present at -3db of the maximum magnitude or 0.707 or 70% of the maximum magnitude. The output plot in frequency domain or the bode plots of a simple low pass filter is given in the figure below,

Figure 1: Low pass filter

### How to design and simulate low pass filter in PSpice

• Lets’ design a simple circuit of a buck converter which is to be discussed in this tutorial and the boost converter with a few details provided is left for you as an exercise. Open the PSPICE design manager on your PC by typing design manager in the search bar. From the design manager click on the run schematic button to open a new blank schematic as shown in the figure below,

Figure 2: Opening schematic

• After opening the new schematic before jumping into designing first save the schematic by clicking on the file button at the top left corner and then selecting save as so that we can access it anytime in the future. Refer to the figure below,

Figure 3: Saving schematic

• Click on the get new part icon at the top bar of the schematic window in order to search for the components that are needed for circuit designing.

Figure 4: getting new part

• In the get new part window, type ‘Vac’ it will display an AC source available in PSPICE. From that list select a simple ac source as shown in the figure below,

Figure 5: Placing ac input

• Again open the get new part window and in the part name block type C, select the capacitor from the list given and then click on place & close as shown in the figure below

Figure 5: Placing capacitor

• Again open the get new part window and in the part name block type ‘R’, select the resistor from the list given and then click on place & close as shown in the figure below

Figure 6: Placing resistor

• Next step is to place a ground, do the same again and in the part name type Gnd and select the ground and then click on place & close as shown in the figure below,

Figure 7: Placing ground

• The placed components in the schematic window are shown in the figure below,

Figure 8: Placed components

• Click on the draw wire icon at the top bar of the schematic window in order to connect the already placed components for circuit designing, as shown in the figure below,

Figure 9: Draw wire

• Connect all the components to complete the circuit diagram as shown in the figure below,

Figure 10: Complete circuit diagram

• On the top of the schematic window, click on the Voltage/Level Marker button as shown in the figure below,

Figure 11: Voltage marker

• Place it at the output capacitor and at the input node as shown in the figure below,

Figure 12: Voltage marker placed

• Next step is to set the attributes of the input AC supply. Double click on the AC supply you connected in the circuit previously and set the magnitude of the voltage of the supply to 10V and the DC voltage to 0 as shown in the figure below,

Figure 13: Input source attributes

• The adjustable attributes in the AC supply is only the magnitude of the AC voltage and all other attributes should be left as it is. The DC attribute is used to adjust the offset of the ac voltage which is not need in this experiment hence left as 0.
• Set the value of the capacitor according to the requirement of your circuit by double clicking on the component and changing the value to 3u and then clicking save attr but I am leaving the value in this experiment as it is.
• Next step is to adjust the properties of the simulations in order to produce the graph of the voltage at the marker. Click on analysis and then click on Setup as shown in the figure below

Figure 14: Simulation setup

• A widow will appear, click on the AC Sweep block on the window and adjust the properties of the window according to your requirement, refer to the figure below

Figure 15: AC sweep properties

• Change the start and end frequency in the property window according the portion of the graph you want to see. If we are interested in checking the voltage on a specific wire in spite of checking it at a node, double click on the wire and inn the window that appear as a result, type the name of the wire you want to label it with, as shown in the figure below,

Figure 17: Labeling the wire

• Now comes the simulation part, click on the analysis at the top bar of the schematic window and then click on simulate as shown in the figure below,

Figure 18: Simulation

• The output voltage at the capacitor node is displayed in the figure below,

Figure 19: Output of simulation

• From the output it is obvious that the circuit simulated is a low pass filter. Above is shown the bode plot (frequency domain plot) of the circuit. At lower frequencies the circuit is allowing the voltage to pass through it and at higher frequencies it drops down to zero. Also the results are in accordance with the theoretical introduction.
• Connect another voltage marker at the input of the AC source to the see the AC sweep of the source along with that of the capacitor as shown in the figure below,

Figure 20: Voltage maker at input

• The output after connecting the voltage marker at the input too is shown in the figure below,

Figure 21: Output displayed with input

• The green line at the top represents the input AC sweep of the AC source connect. The straight line represents that the input source is constant (neither increasing nor decreasing with time) unlike that of the capacitor voltage. In the frequency domain we can say that the input source is allowing all the frequencies to pass hence straight.

Exercise:

• Try do the frequency domain analysis of any of the circuits we have done previously.

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