CP/PC364 Data Communications & Networks Laboratory

Amplitude Modulation

Overview

Amplitude modulation (AM) is an electronic communication technique used to transmit information, usually via radio waves. AM works by varying the strength at which the signal is sent. The result of an AM signal is that the frequency of the signal remains the same, but the amplitude (height/strength) of the signal varies.

We will be using a sinusoidal wave for the control voltage to modify the signal. We will be using the XR-2206 chip to develop the carrier frequency as a sine wave, and to look at Amplitude Modulation.

Datasheet: XR2206 [Copyright 1972 EXAR Corporation. Datasheet June 1997]

Objectives

1. Develop a sinusoidal wave with minimum distortion
2. Examine amplitude modulation

Setup

Set up according to your prelab circuit diagram for sine wave generation with minimum distortion. Record and use the values given below in Table 1 to begin the lab. Ensure you have a sinusoidal wave displayed on your oscilloscope.

Exercise - AM modulation

1. Sinusoidal Wave

   A sine wave is a continuous wave with no sudden changes or angular movements. The sine wave has the unique property that it keeps its shape even when it's added together with another sinusoidal waveform of a different phase. This is very important in physics, math, signal processing, audition, electrical engineering, and many other fields. A sine wave can be used to create other types of waveforms, such as the square wave, triangle wave and even the irregular sound waves made by human speech, using a process called Fourier analysis. The frequency of a sine wave is obtained by measuring the length of time between the middle of the rising slope of one pulse to the middle of the rising slope of the next pulse. The amplitude is measured from the center of the waveform to the peak, or bottom peak to top peak divided by 2.

   
   Set R_B at the midpoint and adjust R_A for minimum distortion. With R_A set, adjust R_B to further reduce distortion.
   • Ensure you are getting a 100 Hz wave. This is the carrier.
2. Using the frequency generator create the signal you will be sending.
   • Connect the frequency generator to the oscilloscope to see the waveform you are creating.
   • Modify the frequency to give a 10 Hz sine wave.
   • Modify the amplitude and offset to get a wave with amplitude between -V_cc/2 and V_cc/2.
     You want to achieve a modulation of between 50% and 100%.
3. Connect the sine wave, (i.e. the signal), to the amplitude modulating signal input.
4. Connect the oscilloscope so that both the sine wave from the frequency generator and the output of the XR-2206 chip are visible on the screen.
   • Print the waveforms (input and output together) and paste it in your lab notebook.
   • What do you notice about the carrier wave amplitude at -V_cc/2? 0V? V_cc/2?
   • How does the signal differ above and below 0V?
   • What is the relationship between the input frequency and the output waveform?