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PHASE LOCKED LOOP

MODULE-5

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PHASE LOCKED LOOP

Evolution of PLL began in early 1930s
Fundamental building block in electronic technology
PLL are used in
Frequency Modulation (FM) Stereo decoders
Motor Speed Controls
Tracking Filters
Frequency synthesized transmitters and receivers
PLL is available as a single package ex: SE/NE560 series

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BLOCK DIAGRAM OF PLL

PHASE DETECTOR

FILTER

VOLTAGE

CONTROLLED

OSCILLATOR

AMPLIFIER

PHASE DETECTOR

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PLL

A Phase Locked Loop accepts an input voltage waveform (Vi) and generates another wave (Vo) that is phase locked to the input.
The frequency of the generated wave (fo) is exactly equal to the frequency of the input wave (fi) and is generally phase shifted from the input by a fixed angle.
Feedback from the output prevents frequency of the generated wave from drifting away from the equality with the input.

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Vi	Vo	Vd
L	L	L
H	H	L
L	H	H
H	L	H

Vi

Vo

Vd

0

gnd

Vc

L

H

L

H

L

H

L

H

L

H

L

H

L

H

Phase Detector

Vi

Vo

Vd

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Vi

Vo

Vd

Vc

H

L

H

L

H

L

H

L

H

Phase Difference=90 degrees

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Vi

Vo

Vd

Vc

H

L

H

L

H

L

H

L

H

Phase Difference > 90 degrees

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Vi

Vo

Vd

Vc

H

L

H

L

H

L

H

L

H

Phase Difference< 90 degrees

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Phase-Lock Mode- When the output frequency becomes equal to the input frequency, PLL is said to be in phase lock mode. The feedback loop maintains the lock when the output frequency becomes equal to input frequency.

Frequency in hz

Capture Range

Tracking Range

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Performance Parameters of PLL

Loop Gain
Tracking Range
Capture Range

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Loop Gain of PLL

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Tracking Range (Hold-in Range)

Tracking Range

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��A PLL system with 105kHz input has a VCO with a 100kHz free-running frequency and a 3.3kHz/V sensitivity. The phase detector sensitivity is 0.68V/rad and the amplifier gain is 5. Calculate the loop gain, input/output phase difference, the static error voltage and the VCO control voltage.�

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Phase Detector

Vi	Vo	Vd
L	L	L
H	H	L
L	H	H
H	L	H

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Characteristics of XOR-Gate

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Phase Detector

S	R	Q
1	0	1
0	1	0
0	0	No Chang

L

Output

L

H

L

H

L

H

L

H

L

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FREQUENCY DETETCTOR

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Calculate the tracking range, maximum error voltage and maximum amplifier output voltage for the PLL system.�

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Capture Range

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LOW PASS FILTER

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VOLTAGE CONTROLLED OSCILLATOR