Nonsinusoidal

1
CHAPTER 14

• Nonsinusoidal
• Oscillators

2
Objectives

• Describe and Analyze
• Operation of the 555 IC
• Inverter oscillators
• Schmitt oscillators
• Wave-shaping
• Sawtooth oscillators
• Troubleshooting

3
Introduction

• There are other ways to make an oscillator
  besides phase-shifters and resonators.
• The term astable covers a group of oscillator
  circuits, many based on hysteresis in one form or
  another. It also covers chips designed for the
  purpose, such as the 555.
• The old term multivibrator is also used to name
  these circuits. It goes back to vacuum tube days
  when they actually used electromechanical
  vibrators in circuits.

4
Square-Wave Oscillators

• Square wave from a free-running 555 circuit.

5
The Internals of a 555

• Frequency set by RA, RB, and C.

6
Functions of the 555

• The 555 is still popular after all these years
  because it is easy to use. It performs two
  functions
• Square-wave oscillator (astable)
• One-shot (monostable)
• Strictly speaking, a square-wave has a 50 duty
  cycle. But unless the duty cycle is low, astables
  are called square-wave oscillators even if its
  not 50.
• A one-shot produces a fixed-width output pulse
  every time it is triggered by a rising or
  falling edge at its input.

7
555 Oscillator

• fOSC 1.44 / (RA 2RB) ? C

8
555 One-Shot

• ?t 1.1RC

9
Inverter Oscillator

• fOSC depends on the number of inverters (must be
  odd).

10
A Calculation

• For the circuit of the previous slide, find the
  frequency range if each inverter has a delay of
  10 ns ? 1 ns.
• Period T delay ? 2 ? of inverters,
• so TLONG 11 ns ? 2 ? 3 66 ns
• and TSHORT 9 ns ? 2 ? 3 54 ns
• So fLO 1 / 66 ns ? 15.2 MHz
• and fHI 1 / 54 ns ? 18.5 MHz

11
Crystal-Controlled

• ltinsert figure 14-15 heregt
• Commonly used for microprocessor clock.

12
Hysteresis Oscillator

• Schmitt trigger circuit on an op-amp.

13
Example Calculation

• For the circuit of the previous slide
• Let R1 R2 R3 10 k?. Let C1 .01 µF
• Find the frequency of oscillation.
• Hint it takes about 1.1 time constants to get
  67 voltage on capacitor.
• The 21 divider formed by R2 R3 keeps the ()
  input at ?Vout / 2. C1 has to charge up to
  Vout / 2 to flip the compara-tor. But it starts
  from Vout / 2, which is equivalent to charging
  from 0 to 2V / 3 with V applied. So, 1.1R1C1
  110 µs, but it takes two flips for one cycle.
  So f 1 / 220 µs ? 4.5 kHz.

14
Square to Triangle

• Integrating a square wave makes a triangle wave.

15
Triangle to Sine

• With enough diodes, the signal is very close to a
  sine.

16
Sawtooth Oscillator

• Also called a ramp generator, it can be used to
  generate the horizontal sweep in a CRT circuit.

17
A Relaxation Oscillator

• Shockley diode converts integrator into a
  relaxation oscillator, so called because the
  diode periodically relieves the capacitors
  tension (voltage)

18
Sample Calculation

• For the circuit of the previous slide, let the
  input resistor Ri 100 k, the feedback capacitor
  C 0.1 ?F, and let Vin 1 Volt.
  Calculate the frequency if the Shockley diode
  fires at 10 Volts.
• Iin 1V / 100 k 10 ?A, and charging a
  capacitor with a constant current means the
  voltage ramps up linearly at a rate of ?V / ?t
  I / C. So ?t (C / I) ?V.
• The period T (0.1 ?F / 10 ?A) ? 10 Volts 0.1
  sec.
• So f 1 / T 10 Hertz.

19
Troubleshooting

• As always, check all DC voltages.
• Typically, these oscillators either work or they
  do not they do not tend to drift.
• Frequencies are not precise (except for crystal
  stabilized) so oscilloscope measurements are OK.
• Though not often used, if an aluminum
  electrolytic is the timing capacitor, it is a
  suspect.
• If a potentiometer is used to adjust an RC time
  constant, check if it has been tweaked.
• Look for physical damage to components.