Download - Electronic Oscillators
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Oscillators
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Objectives State the function of the oscillator.
Describe positive feedback used in the oscillator
Explain the Barkhausen criterion
List the three requirements for proper oscillatoroperation.
Describe the operating characteristics of thephase-shift and Wien-bridge RC oscillators.
Describe the operation of the Colpitts and Hartley
oscillators. Describe the operating principles of crystals and
crystal-controlled oscillators.
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Positive feedback.
Av
v
RL
vin
vf
Amplifier(=180)
Feedback network(=180)
voutIn-phase
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How to generate a signal.
Regenerative
feedback
Av
v
vin
vf
vout
Av
v
vin
vf
vout
Amplifier
Oscillator
1v vA
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Effects ofvAv on oscillator
operation.1v vA Barkhausen criterion:
1v vA
1v vA
1v vA
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Phase-shift oscillator.
IfR1 = R2 = R3 = Rand C1 = C2 = C3 = C, thenosc
1 1and
292 6vf
RC
R1
R2
R3
C1
C2
C3
Feedback network
Av
(Av
< 0)
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Wien-bridge oscillator.
R
R
C
C
Rf
Rh
osc
1 1and
2 3vf
RC
1 3
2
f
v
h
f
h
RAR
R
R
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Wien-Bridge Oscillator (Lab.)
741
R
R
C
C
R2
R1
vo
RST
vi1
vi2
R = 10k
C= 15nF
R1 = 8.2 kR2 = 3.9 k
RST
= 120 k
2x1N4148
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Colpitts oscillator (CEconfiguration.)
1 2
1 2
T
C CC
C C
1
2r
T
fLC
2 1
1 2
Cv
C
X C
X C
R1
R2
RE
RFC
C1
C2
L
VCC
vout
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ExampleR
1
R2
RE
RFC
C1
3.3nFC
2
0.1F
L
47H
VCC
vout
1 2
1 2
3.3nF 0.1F3.19nF
3.3nF 0.1FT
C CC
C C
1 1411kHz
2 2 47H 3.19nF
r
T
f
LC
2 1
1 2
3.3nF
0.1F
0.033
C
vC
X C
X C
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R1
R2
RE
RFC
C1
LP
VCC
vout
R1
R2
RE
RFC
C1
L1
VCC
vout
L2
Hartley oscillator.
1 2
1
1
2
T
r
T
L L L
fL C
1
1
2r
P
fL C
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Other Oscillator Circuits Clapp oscillator
Armstrong oscillator
Etc
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Quartz crystal.x
x
y
y
z
z
CC L R
CM
XL
XC
fs
fp
f
Symbol
Equivalent circuitFrequency response
s p
f f
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Crystal-controlled Colpittsoscillators.
RE
RFCR1
R2
C2
C1
vout
Y1
VCC
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Pierce oscillator.
XTAL
CF CCv
out
RG
RFC
+VDD
C1
C2
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Typical Crystal Oscillator Circuits usedin a Microprocessor-Based System
Rf
C1 C2
R1
vout
C1 = C2 = 22 pF or 30 pF
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Overtone Mode Normal crystal frequency 25 MHz.
Higher frequency is obtained by tuning the
frequency to crystal harmonics (overtones).
Used in digital and communicationselectronics.
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Summary Oscillator principle
Phase-shift oscillators
Wien-bridge oscillator Colpitts oscillator
Other LC oscillators
Crystal-controlled oscillators