Linear Integrated-Circuit

LAB 2 – LINEAR INTEGRATED-CIRCUIT VOLTAGE-CONTROLLED OSCILLATOR

1.Introduction

A Voltage-controlled oscillator (VCO) is a free-running oscillator with a stable frequency of oscillation that depends on a timing capacitance, a timing resistance, and an external control Voltage. In this experiment, the operation of an XR-2207 monolithic linear integrated-circuit Voltage-controlled oscillator is examined. Its block diagram is shown in Figure 2. See the data sheet for a description of this circuit.

2.Materials Required

Equipment

1 - protoboard

1 - dual dc power supply (+6 V dc and -6 V dc)

1 - variable dc power supply (-5 V dc to +5 V dc)

1 - low-frequency function generator (1 kHz)

1 - standard oscilloscope (10 MHz)

1 - assortment of test leads and hookup wire

Figure 1. XR-2207 Functional Block Diagram.

Parts List

3.Pre-lab

Print the XR-2207data sheet from the ECET 314 CD-ROM and bring it to the lab session.

• Identify the section of the data sheet with the design formulas for binary keying inputs and record these formulas. You will use these with Circuit 1.
• Identify the section of the data sheet with the design formulas for frequency control (sweep and fm)and record these formulas. You will use these with Circuit 2.
• Identify the section of the data sheet with the design formulas for duty-cycle control (sweep and fm)and record these formulas. You will use these with Circuit 3.

4.Circuit 1 – Triangle and Square Wave Outputs

In this circuit, the XR-2207 Voltage-controlled oscillator is used for producing both triangle and square waves. The schematic diagram for the Voltage controlled oscillator circuit used in this section is shown in Figure 2. The Voltage-controlled oscillator circuit shown can simultaneously produce square and triangle wave outputs. The square wave output at pin 13 is an open collector stage capable of sinking up to 20 mA of load current. RLserves as a pull up resistor. The output at pin 14 is a triangle wave with a peak Voltage swing of approximately one-half of the total supply Voltage.

The VCO frequency is controlled by a timing capacitor connected between pins 2 and 3 and by external timing resistors connected to pins 4, 5, 6, and 7. Pins 8 and 9 are binary inputs that select which external timing resistorsare used to determine the VCO output frequency. See the data sheet for information on how the Voltages on these pins determine the VCO frequency.

Figure 2. XR-2207 Voltage-controlled Oscillator Circuit.

Procedure

1. Construct the Voltage-controlled oscillator circuit shown in Figure 2.

2. Calculate the four possible output frequencies for the four binary keying input conditions using theformulas in the data sheet.

3. Place the binary keying switches, S1 and S2, into their closed positions simulating a 00 binary keying input condition.

4. Record the output waveforms observed on pins 13 and 14.

5. Place the binary keying switches into each of the four possible conditions,measure their frequencies, and compare these to the frequencies calculated in step 2.

5.Circuit 2 – Voltage Control of Frequency

In this circuit, the operation of the XR-2207 as a Voltage-controlled oscillator is examined. The schematic diagram for the circuit used in this section is shown in Figure 3. The binary keying inputs are grounded, simulating a binary 00 condition. Consequently, the current from pin 6 controls the VCO frequency. The timing current can be changed by applying a control Voltage, VC, to the activated timing pin through a series resistor RC.In essence, a Voltage-controlled oscillator performs Voltage-to-frequency conversion. See the data sheet for information on calculating the VCO frequency for this circuit.

Figure 3. XR-2207 Sweep Frequency Operation.

Procedure

1.Construct the Voltage-controlled oscillator circuit shown in Figure 3.

2.Measure the VCO output frequency on pin 14 for control Voltages (VC) of-5 to +5 V, in 1-V steps.

3.Construct a graph showing the relationship between the dc control Voltage and the output frequency for the control Voltages given and frequencies measured in steps 2 and 3.

4.Plot the theoretical frequencies on the same graph. (It is convenient to use Excel for this. Plot the theoretical values as a continuous line and the measured data as discrete points.)

6.Circuit 3 – Duty Cycle Control

In this section, the duty cycle of the XR-2207 Voltage-controlled oscillator is examined. The duty cycle can be controlled by connecting one or both of the binary keying inputs (pin 8 and 9) to the square wave output at pin 13. The output waveforms can then be converted to positive or negative pulses and sawtooth waveforms. The schematic diagram for the Voltage-controlled oscillator circuit used in this section is shown in Figure 4. See the data sheet for information on finding the theoretical frequency and duty cycle for this circuit.

Figure 4. XR-2207 Sawtooth and Pulse Operation.

Procedure

1.Construct the Voltage-controlled oscillator circuit shown in Figure 4.

2.Calculate the output frequency and duty cycle using the formulas on page 15 of the data sheet.

3.Record the waveforms observed on pins 13 and 14, and measure the frequency and duty cycle of the waveform on pin 13.

4.Compare the measured and theoretical values.

7.Discussion Questions

1. In figure 2, when S1 is closed and S2 is open, which passive components affect the frequency?
2. Briefly, what is the purpose of the passive components in Figure 2 that do not affect the operating frequency?

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