CHAPTER 14: Nonsinusoidal Oscillators pps

CHAPTER 14
Nonsinusoidal
Oscillators
Objectives
Describe and Analyze:
• Operation of the 555 IC
• Inverter oscillators
• Schmitt oscillators
• Wave-shaping
• Sawtooth oscillators
• Troubleshooting
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.
Square-Wave Oscillators






Square wave from a “free-running” 555 circuit.
The “Internals” of a 555

Frequency set by R
A
, R
B
, and C.
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 it’s 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.
555 Oscillator

f
OSC
= 1.44 / [(R
A
+ 2R
B
)  C]
555 One-Shot







t = 1.1RC
Inverter Oscillator







f
OSC
depends on the number of inverters (must be odd).
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 T
LONG
= 11 ns  2  3 = 66 ns
and T
SHORT
= 9 ns  2  3 = 54 ns
So f
LO
= 1 / 66 ns  15.2 MHz
and f
HI
= 1 / 54 ns  18.5 MHz
Crystal-Controlled
<insert figure 14-15 here>





Commonly used for microprocessor clock.
Hysteresis Oscillator






Schmitt trigger circuit on an op-amp.
Example Calculation

• For the circuit of the previous slide:
• Let R
1
= R
2
= R
3
= 10 k. Let C
1
= .01 μF
• Find the frequency of oscillation.
• [Hint: it takes about 1.1 time constants to get 67% voltage on
capacitor.]
• The 2:1 divider formed by R
2
& R
3
keeps the (+) input at
V
out
/ 2. C1 has to charge up to V
out
/ 2 to flip the compara-
tor. But it starts from –V
out
/ 2, which is equivalent to charging
from 0 to 2V / 3 with V applied. So, 1.1R
1
C
1

= 110 μs, but it
takes two “flips” for one cycle. So f = 1 / 220 μs  4.5 kHz.
Square to Triangle






Integrating a square wave makes a triangle wave.
Triangle to Sine







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






Also called a “ramp generator”, it can be used to
generate the horizontal sweep in a CRT circuit.
A Relaxation Oscillator

Shockley diode converts integrator into a “relaxation”
oscillator, so called because the diode periodically
relieves the capacitor’s “tension” (voltage)
Sample Calculation
• For the circuit of the previous slide, let the input
resistor R
i
= 100 k, the feedback capacitor C =
0.1 F, and let V
in
= –1 Volt. Calculate the frequency
if the Shockley diode “fires” at 10 Volts.
• I
in
= 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.
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.