Digital and Analog Communications Paper

Digital and Analog Communications Paper

Instructions/Descriptions

Digital and Analog Communications Paper

ELEC 331 Digital and Analog Communications

M1A2 Lab: Filter Review

Introduction

Filters are critical elements in electronic communication systems as they have the ability to reject signals of frequencies outside the desired band. The goal of this laboratory exercise is to use MultiSim to review the concepts of filters and their responses. We will pay a special attention to the roll-off or attenuation, which is measured in dB/decade or dB/octave. The order of a specific filter determines its roll-off.

Procedure

1. Simple Filter

We will begin our study by building in MultiSim the circuit in the figure below. Note that this circuit uses the Virtual Op Amp rather than a commercial unit.

Figure 1: Filter circuit using Virtual Op Amp

There are different methods that we can use to characterize this filter. The simplest method is to use the ‘Bode Plotter’ instrument available in the same location as the other instruments. This instrument generates the Bode plot of the transfer function between the output and the input. For this purpose, connect the IN to the input AC source and the OUT to the output of the Operational Amplifier. Don’t forget to connect the grounds appropriately.

Before we can proceed with characterizing the filter, we need to configure the Bode Plotter correctly. Open the dialog box by double clicking on the instrument. You can change the size by moving the cursor to the lower right corner of the instrument. By default, the Bode Plotter has a black background. You can change it to white to clicking on the ‘Reverse’ button under Controls.

Configure the Blode Plotter as follows:

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Frequency range: 100 Hz to 500 kHz Magnitude range: – 40 dB to 5 dB

Refer to the figure below in case you have problems with configuring the instrument.

Figure 2: Filter roll-off characteristics using the Bode Plotter

Run the simulation and calculate:

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• Cut off frequency of the filter. Remember that this will be the frequency at which the magnitude of the filter is 3 dB below the value in the pass band.
• Roll-off in dB per decade: Measure the gain at one frequency, the gain at a frequency ten times higher and calculate the difference. Remember to do this while the filter is attenuating, not in the pass band
• After doing these calculations, what can you conclude about the order of the filter?

If you have difficulties measuring these values, change the configuration in the Bode Plotter.

2. Cascaded Filter

We can increase the order of the filter by connecting the output of the initial filter to the input of a similar circuit. Build another filter with the same exact values which is connected to the output of the first filter. Refer to the figure below if needed.

Figure 3: Two-stage cascaded filter circuit

Connect the input to the Bode Plotter to the function generator and the output to the output of the second operational amplifier.

Following the same process se used before, calculate:

• Cut off frequency of the new (cascaded) filter
• Roll-off (dB/decade) of the new filter
• Estimate the order of the new filter

(You may have to modify the parameters of the Bode Plotter as needed.)

Lab Report

In general, for each lab you do, you will be asked to setup certain circuits, simulate them, record the results, verify the results are correct by using circuit analysis techniques, and then discuss the solution. In preparing your lab report, you should follow the Lab Report Format, viz., it should contain an abstract summarizing the results of your experiments, introduction/background, experimental methods used, results of your experiments, followed by a one page, single spaced discussion of your experiments, accuracy and precision of your results, errors if any, what went right for you, what you had difficulty with, what you learned from the experiment, how it applies to our coursework, and any other comment you can think of. In addition, you should include screen shots from the MultiSim software and any other figures, tables, or diagrams as necessary.

Your lab report should be a professional, high-quality technical report to warrant full grade for your lab work. This effort is an important piece in improving your written communication skills. In addition to having the correct technical content, your report should be free from English and grammar errors, should display clarity and continuous flow of content (with citations if necessary), and should convince the reader about your mastery of the technical topic of the lab assignment. Your citations should be in APA or IEEE style, and should not have mix-up of the styles.

ELEC 331 Laboratory Module 1: Review of Filters page 4

Digital and Analog Communications Paper

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