CP/PC364 Data Communications & Networks Laboratory

Frequency Modulation

PRELAB

Overview

Frequency Modulation (FM) is an electronic communication technique used to transmit information, usually via radio waves. FM works by varying the rate at which the signal is sent. As we saw in last week's lab, the result of an FM signal is that the amplitude of the signal remains the same, but the frequency (rate at which the pulses occur) of the signal varies. Radio technology today uses frequency modulation as a primary transmission mode (FM Radio). The sounds that we hear are being transmitted through the air by a change in frequency.

Today, instead of modulating voices, we will transmit a simple sine wave. Here, we can get a better look at exactly how the carrier frequency is modulated in relation to the signal. We will be using the XR-2207 chip to look at Frequency Modulation.

Datasheet: XR2207 [Copyright 1975 EXAR Corporation. Datasheet June 1997]

Objectives

  1. Examine frequency sweep from voltage levels
  2. Examine frequency modulation from an input signal
  3. Examine frequency demodulation to retrieve the original signal

Setup

Setup according to your prelab circuit diagram for frequency sweep operation (part 1).

Possible Excercise - Part 1 MATLAB

http://faculty.kfupm.edu.sa/EE/ajmal/0xx/ee370/EE370_Lab_Experiment_05.pdf

http://www.srmuniv.ac.in/downloads/lab6.pdf

Exercise - Part 1

  1. Frequency Control

    The VCO uses an RC circuit to vary both the frequency and duty cycle of an astable multivibrator with varying voltage levels. The frequency of operation is determined by the current drawn from the timing resistors. This current can be modulated by applying a voltage to these resistors. Changing this current using the voltage control (Vc) will modulate the frequency according the formula given beside your circuit in the datasheet.


    Test your circuit to ensure you are getting a 10 Hz carrier wave frequency. (Use either SWO or TWO output)
  2. Using the frequency generator create a 1 Hz input sine wave between +Vcc and -Vcc.
  3. Connect the sine wave from the frequency generator to pin determined in question 2 of your prelab. Connect the oscilloscope so that both the sine wave from the frequency generator and the output of the XR-2207 chip are visible on the screen.
    • Print the the screen with the input and output waveforms.
    • What is the relationship between the input frequency and the output waveform?
    • Save this circuit... you will need it for part 2!

Demonstrate and explain your results to the lab instructor

Part 2

  1. FM Demodulation

    Demodulation is a process that reverses the modulation of a signal. This process removes the carrier wave to reveal the signal which was sent. There are a few different types of FM demodulators, such as: quadrature detector, Foster-Seeley discriminator, phase-locked loop, digital signal processor, etc. We will be using the phase-locked loop process.

    In basic terms, a phase-locked loop compares two frequencies (in our case the FM signal vs the frequency created by the timing resistors and capacitors of the XR2212) and creates an output signal based on the error between the two signals. In order for these two circuits to be compatible they must both have the same carrier wave frequency.

    Datasheet: XR2212 [Copyright 1979 EXAR Corporation. Datasheet June 1997]


    Wire up your circuit from part 2 of your prelab.
    • Use the oscilloscope to view the FM signal (XR2212 input/2207 output) and the demodulated signal (XR2212 output).
    • Print the waveforms.
    • What is the relationship between the original input frequency (frequency generator) and the output waveform (demodulated signal)?

Demonstrate and explain your results to the lab instructor