Analog Circuits and Systems Prof. K Radhakrishna Rao

Lecture 35: Non-Sinusoidal Oscillators

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Review

�  Regenerative Comparator Schmitt Trigger �  Pulse Width Modulation �  Duty Cycle Generator �  Inverting and Non-inverting Schmitt Trigger �  Hysteresis �  FM and FSK Generation

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Astable multi-vibrator/Function Generator

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( ) ( )( )Given

T 2RCs sV 1 1 e 2 V

1 1T 2RCln ;f1 T

−+ β − = β

+ β= =− β

Simulation 1

�  R1=R2=R=1k�; C=1�F;T=2RCln3=2.2msec; f=454Hz

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Simulation 2

�  Single Supply based Astable Multivibrator �  R1=R2=R=1k�; C=1�F;T=2RCln3=2.2msec; f=454Hz

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On-Off Temperature Controller

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Monostable multivibrator

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( )

( )

LM 555 Timer; Cost = $0.48

T2RC

r s s r

r s

V V 1 e V V

1 V VT RCln

1

−⎛ ⎞⎜ ⎟⎝ ⎠

⎛ ⎞⎜ ⎟+ − = β +⎜ ⎟⎝ ⎠

+=

− β

Simulation- Timer Circuit

�  R1=R2=R=1k�; C=1�F;T=0.693msec; f=100Hz; Square Wave; 7V

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Function Generator

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Waveforms

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s 1s

2

1

2

2

1

V 2RT VRC 2 R

RT 4RCRR1f

T 4RCR

=

=

= =

Simulation

�  R=R1=1k �; R2=2.2k�; C=1�F; T=1.82msec; f0=550Hz

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Saw-tooth Waveform Generator

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Saw-tooth Waveform Generator (contd.,)

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s a 11 s

2

s11

2 s a

s12

2 s a

1 2

2s1

2 22 s a

V V 2RT VRC R

VRT 2RCR V V

VRT 2RCR V V

T T T

VR4RCR V V

−⎛ ⎞ =⎜ ⎟⎝ ⎠

=−

=+

= +

⎛ ⎞= ⎜ ⎟−⎝ ⎠

Simulation- Function generator with asymmetry

�  R=R1=1k �; R2=2.2k�; C=1�F; Va=5V

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Simulation- Function generator with asymmetry (contd.,)

�  R=R1=1k �; R2=2.2k�; C=1�F; Va=-5V

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Function generator with offset

�  R=R1=1k �; R2=2.2k�; C=1�F; Voff=1V

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Function Generator and VCO

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Sensitivity of the VCO:

c 2

1

2vco

c 1 c

V Rf40RC R

Rf fK Hz / VV 40RCR V

=

∂= = =∂

• FM (Frequency Modulation) generator • FSK (Frequency Shift Keying) generators

Function Generator and VCO (contd.,)

�  VCO is an important building block in a Phase Locked Loop. �  A function generator is an oscillator which produces square wave,

triangular wave, rectangular wave with specific duty cycle and saw tooth.

�  It can also output sine wave by converting the triangular wave to sine wave using diode function generator.

�  It is the VCO circuit that was discussed earlier XR2206, function generator IC manufactured by EXAR that is popularly used in all test oscillators in laboratories today.

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Simulation – FSK Generation

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Simulation –FM Generation

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XR2206 – Features

�  Low-Sine Wave Distortion, 0.5%, Typical �  Excellent Temperature Stability, 20ppm/°C, Typ. �  Wide Sweep Range, 2000:1, Typical �  Low-Supply Sensitivity, 0.01%V, Typ. �  Linear Amplitude Modulation �  TTL Compatible FSK Controls �  Wide Supply Range, 10V to 26V �  Adjustable Duty Cycle, 1% to 99%

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XR2206 – Datasheet

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Min. Typ. Max. Units

Max. Operating Frequency

Lowest Practical Frequency

Frequency Accuracy

Temperature Stability

Frequency Sine Wave

Amplitude Stability2 Supply

Sensitivity Sweep Range

0.5

1000:1

1

0.01

+1

+10

4800

0.01

2000:1

+4

+50

0.1

MHz

Hz

%

ppm/

�C

ppm/

�C

%/V

fH=fL

LM 566 - Features

�  Wide supply voltage range: 10V to 24V �  Very linear modulation characteristics �  High temperature stability �  Excellent supply voltage rejection �  10 to 1 frequency range with fixed capacitor �  Frequency programmable by means of current, voltage, resistor or

capacitor

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LM566- Data Sheet

Min. Typical Max. Units Maximum Operating Frequency 0.5 1 MHz VCO Free running Frequency -30 0 30 % VCO Sensitivity 6.0 6.6 7.2 kHz/V

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Conclusion

�  Limitations due to comparator characteristics ◦  Rise time ◦  Fall time

�  Limitations due to opamp characteristics ◦  Maximum frequency of oscillation

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Conclusion

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