Lab #5: Oscillators SAIT- ENT-DIGI-260

Lab #5: Oscillators

Updated January 2014

Objectives:

  1. Observe and analyze the operation of a Schmitt-Triggered Oscillator.
  2. Observe and analyze the operation of a Crystal Oscillator.

Pre-Lab Preparation:

  1. Attend lectures and review the theory of operation of Schmitt-Triggering, Schmitt-Triggered Oscillators and crystal oscillators.

Equipment Required:

  1. Experimenter’s board with 5V power supply
  2. Function Generator
  3. Oscilloscope
  4. PC with access to specification sheets
  5. Parts kit and misc. resistor and capacitor values
  6. Misc parts supplied in lab

Additional Notes:

  1. Marks will be instantly deducted from your lab if you fail to offset the function generator voltage.
  2. No EWB work is required for this lab.

Procedure 1:

Part 1: Observe the operation of a Schmitt-Triggered Inverter

  1. Look up the device specification sheet and record the Vt+ and VT- ranges in Table #1.
  2. Using a VCC of 5 volts, connect a 0 - 5 volt triangular waveform to the input of a 4093 Schmitt trigger(configured as an inverter.).Set the frequency at about 1 kHz.
  3. Using a dual channel oscilloscope, measure the upper transition point (UTP, or Vt+) and lower transition point (LTP, or Vt-) by referencing the output waveform to the input waveform(use the appropriate trigger settings on the oscilloscope to trigger on the triangular waveform for both channels). Record your measured values in Table #1, ensuring they are within specifications.

Part 2: Design and observe the operation of a Schmitt-Triggered Oscillator

1.Utilizing the gate from Procedure 1, Part 1, design the circuit shown in Figure #1 below. Use C: 0.01µF, and R: 10 kΩ.

2.Use the measured Vt+, Vt-, R and C to calculate the anticipated tH, tL, Duty Cycle and Frequency of the oscillator. Write the values in the space providedin Table #2.

3.Construct the circuit and record in Table #2 the measured tH, tL, Duty Cycle and Frequency.

4.Change the value of C to 0.1µF and repeat the procedure. Record the observed frequency and duty cycle in Table #2.

5.Answer the questions and the end of the lab, and demonstrate this circuit to your instructor.

Figure #1: Schmitt-Triggered Oscillator

Procedure 2:

Build and analyze a crystal-based oscillator circuit.

  1. Construct the circuit shown in figure 2. The values are as follows:
  2. Potentiometer: 10 kΩ
  3. Resistor: 100 Ω
  4. Capacitor: 330pF (may need to modify based on circuit response)
  5. Crystal: 3 to 4 MHz
  6. Gate: 74LS14

Figure 2: Crystal-based Oscillator

  1. Use an Oscilloscope to observe and record the output frequency of the oscillator as the potentiometer is adjusted through its full range. Answer the question at the end of the lab.
  1. Adjust the oscillator until the output is at the crystal’s rated frequency. Demonstrate this circuit to your instructor and answer the question at the end of the lab.

Table#1

Specification / Value / Measured / Value / Within Specs?
Vt+ max / Vt- max
Vt+ min / Vt- min
C (0.01μF)
C (0.1μF)
R (10kΩ)

Table #1: Schmitt-Triggered Device Transition Voltages, R and C.

Calculation (Schmitt-Triggered Astable Multivibrator):

Use measured values for Vt+, Vt-, R and C

Values: R=10kΩ, C=0.01µF, E=5V, Vt+= ______, Vt-= ______

Specification / Predicted Frequency / Predicted Duty Cycle / Measured Frequency / Measured Duty Cycle
D.C.
f(C=0.01µF, R =1 kΩ)
f(C=0.1µF, R =1 kΩ)

Table #2: Schmitt-Triggered Oscillator output

Questions:

  1. In Procedure #1, Part 2 you measured the output frequency and duty cycle of the Schmitt-Triggered oscillator.
  1. Why is this circuit considered an astable multivibrator?
  1. By what factor did the output frequency change with the 10 times change of the capacitor value? Explain.
  1. How is the Duty Cycle controlled for this circuit?
  1. In procedure 2 you observed the output of a crystal-based oscillator.
  1. What was the frequency range of the oscillator as you adjusted the potentiometer?
  1. As you adjusted the potentiometer, what did you observe as you reached the crystal’s rated frequency?

Signatures:

Procedure 1 Calculations, circuit, and questions: ______

Procedure 2 Circuit and questions: ______

Student Name: ______Date: ______

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