CP/PC364 Data Communications and Networks Laboratory

Frequency Shift Keying Lab

Overview

Creating a Carrier Wave with a Voltage Controlled Oscillator

In telecommunications the carrier wave is a waveform (usually sinusoidal) that is modulated by an input signal to transfer information. The carrier wave is usually a very high frequency as it needs to be faster than the input signal. For example, when listening to a radio you must tune the dail to a "station". This "station" is actually the center frequency of the waveform that is carrying the music/voices/etc (eg. 91.5 The Beat broadcasts at 91.5MHz). This is the carrier frequency. To create this carrier wave and examine how it can send a signal we will use a Voltage Controlled Oscillator (VCO).

VCOs are electronic oscillators whose output frequency is linearly related to the control voltage (f = Ko*V, where Ko is a constant). If the phase of the VCO output square wave is considered as the output variable, the VCO can be modeled as an integrator.

Before adjusting the output frequency using voltage we must first determine how to manipulate the circuit to give us our primary carrier frequency. Similar to most multivibrators, the carrier frequency is set by the timing capacitor and timing resistor.

The XR-2206 is composed of two primary function blocks (Figure 1 of the datasheet). First we will look at the top portion, the voltage controlled oscillator (VCO). It has two outputs, Square Wave Output (SYNCO) and Triangle or Sine Wave Output (STO), which we will discuss and compare. The second block is the current switch inputs as shown at the left of Figure 1 of the datasheet. The current switch inputs are used to select certain resistors to modify the frequency output. The XR-2206 can be configured for frequency sweep operation, frequency shift keying (FSK) and for duty cycle.

Objectives

1. To understand the operation of a VCO
2. To understand the function of the timing resistor and timing capacitor
3. Examine frequency shift keying using the FSK input
4. Demodulate the signal to recover the original waveform

Setup

Set up your circuit according to prelab specifications number 1.
Datasheet: XR2206 [Copyright 1972 EXAR Corporation. Datasheet June 1997]

Task

1. Hook up the oscilloscope to the SYNCO output.
   • Record R₁, R₂,C and the frequency with the FSK input at zero volts.
   • Verify your frequency using the appropriate equation below.
   • Print the waveform.
2. Repeat step 1 connecting the oscilloscope to the STO output.
   Demonstrate and explain your results to the lab instructor.
   
   

3. Timing

   Resistors and capacitors are used together to determine the frequency of an oscillator. Capacitors are used in timing circuits because of the charging properties that they have. Varying how long it takes for the capacitor to charge varies the frequency of the created pulse.

   Frequency equations:
   f₁ = 1/(R₁C)
   f₂ = 1/(R₂C)

   • Using the values from step 2 of your prelab verify the frequencies you calculated using your circuit.
   • Plot your results in your lab book.
   • How does the timing capacitor vary the output?
4. Using the values from step 3 of your prelab verify the frequencies you calculated using your circuit.
   • Plot your results in your lab book.
   • How do the timing resistors vary the output?
   Demonstrate and explain your results to the lab instructor.
   
   

5. Frequency Shift Keying (FSK)

   Frequency shift keying is a process that varies the frequency of a carrier signal. Both the amplitude and phase remain constant during this process. The most basic form of FSK is Binary Frequency Shift Keying. BFSK uses two discrete frequencies to transmit 0s ("spaces") and 1s ("marks"). BFSK can be used to transmit a 2 bit binary number. This would result in a combination of marks and spaces. Each of the frequencies can be calculated using the formulas.

   • Setup your circuit for binary shift keying as determined in step 4 of your prelab. Start with pin 9 to be 0. This is a 'space' transmission (binary 0).
   • Print the waveform and record the frequency.
   • Compare the frequency to what you found in your prelab.
   • Change pin 9 to high (1). This is a 'mark' transmission (binary 1).
   • Print the waveform and record the frequency.
   • Compare the frequency to what you found in your prelab.
   • View the demodulated signal in the same way as in the Frequency Modulation lab.
   • Print the waveform and see how it compares to the original input.
   Demonstrate and explain your results to the lab instructor.