CP/PC364 Data Communications & Networks Laboratory

Voltage Controlled Oscillators

PRELAB

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 (I ?????? = Ko*V/s).

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-2207 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 (SWO) and Triangle Wave Output (TWO), which we will discuss and compare. The second block is the binary keying inputs as shown at the bottom of Figure 1 of the datasheet. The binary inputs are used to select certain resistors to modify the frequency output. The XR-2207 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 binary keying inputs

Setup

Set up your circuit according to prelab specifications number 1.
Datasheet: XR2207 [Copyright 1975 EXAR Corporation. Datasheet June 1997]

Exercise

  1. Hook up the oscilloscope to the SWO output.
    • Record R3, Ct and the frequency.
    • Verify your frequency using Equation 1 below
    • Print the waveform
  2. Repeat step ????1 connecting the oscilloscope to the TWO 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 it has. Depending on how long it takes for the capacitor to fill with charge will vary the frequency of the pulse you are creating.

    Equation 1: Frequency
    f1 = 1/( Ct R3 )

    • 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 does the timing resistor 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 a combination of marks and spaces. In this case we should see four different frequencies as our output for each binary number that is input through pins 8 and 9. Each of the frequencies can be calculated using the formulas below.

    Frequency equations:
    f1 = 1/( Ct R3 )
    Δf1 = 1/( Ct R4 )
    f2 = 1/( Ct R2 )
    Δf2 = 1/( Ct R1 )

    • Setup your circuit for binary shift keying as determined in step 4 of your prelab. Start with pin 8 and 9 both to be 0. This is a "space" " space" transmission (binary 00).
    • 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 "space" "mark" transmission (binary 01).
    • Print the waveform and record the frequency.
    • Compare the frequency to what you found in your prelab.
    • Repeat steps 7 and 8 with pin 8 set high (1). These will be your "mark" "space" transmission (binary 10) and your "mark" "mark" transmission (binary 11).
    • How does R1 affect R2? How does R4 affect R3?

    Demonstrate and explain your results to the lab instructor.