ANALOG ELECTRONICS

PHASE LOCKED LOOP

PREET|patel1513101090322nd B.E. ElectricalAIIE

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CONTENTS• Introduction• Block diagram of PLL• Phase detector• Low pass filter• Voltage controlled oscillator• Pin diagram of PLL• Characteristic of 565 PLL• Application of PLL• PLL as a frequency synthesizer• AM detection using PLL

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INTRODUCTION

• Phase locked loop(PLL) is basically a closed loop system. Its application is to lock the output frequency and phase of input signal.

• PLL is also used for communication circuits in two different ways:• For demodulation application• Synchronization of signal

• PLL principle is used in application such as FM demodulation, FSK demodulation, motor speed control, frequency multiplication and division.

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BLOCK DIAGRAM of PLL

PHASE DETECTOR LOW PASS FILTER

VOLTAGE CONTROLLED OSCILLATOR

(VCO)

Input Output

Ve Vc

Vo , foVi , fi

Feedback Path

A BASIC PHASE LOCKED LOOP

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PHASE DETECTOR

• The two inputs to a phase detector or comparator are the input voltage Vi, at frequency fi and the feedback voltage from a Voltage controlled oscillator (VCO) at frequency fo.

• The phase detector compares these two signals and produces a dc voltage Ve,  which is proportional to the phase difference between fi and fo. The output voltage Ve of the phase detector is called as “error voltage”. This error voltage is then applied to a low pass filter. 

PHASE DETECTOR• ANALOG

• DIGITAL

• FLIP-FLOP

Types

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LOW PASS FILTER

• The low pass filter removes the high frequency noise present in the phase detector output and produces a ripple-free dc level. 

• This dc level is amplified to an adequate level and applied to a voltage controlled oscillator.  The dc amplifier output voltage is called as the control voltage Vc .

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VOLTAGE CONTROLLED OSCILLATOR

• The control voltage Vc is applied at the input of a VCO.  The output frequency of VCO is directly proportional to the dc control voltage Vc .

• The VCO frequency fO is compared with the input frequency fi  by the phase detector and it (VCO frequency)  is adjusted continuously until it is equal to the input frequency fi . i.e

fO = fi

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PLL IC 565Pin Configuration

1

2

3

4

5

6

7

14

13

12

11

10

9

8

SE/NE 565

-V

Input

Input

VCO Output

Phase Comparator VCO Output

Reference Output

Demodulated Output

External Resistor for VCO

External Capacitor for VCO

+V

NC

NC

NC

NC

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CHARACHTERISITIC of 565 PLLSr

No.CHARACTERISTIC RANGE/VALUE

1 Operating frequency range 0.001Hz to 500KHz2 Operating voltage range ±6 to ±123 Input level required for tracking 10mV rms minimum to 3Vp-p maximum4 Input impedance 10kΩ typically5 Output sink current 1mA typically6 Output source current 10mA typically7 Drift in VCO center frequency fo with

temperature 300 ppm/°C typically

8 Drift in VCO center frequency fo with supply voltage

1.5% / V maximum

9 Triangular wave amplitude Typically 2.4 Vp-p at ±6 supply voltage10 Triangular wave amplitude Typically 5.4 Vp-p at ±6 supply voltage11 Bandwidth adjustment range < ± 1 to > to ± 60%

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APPLICATION OF PLL• PLL can be used as:

• Frequency Multiplication

• Frequency Translation• AM Detection• FM Detection• FSK Demodulation

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Crystal Oscillator

Divide by M

network

Phase detector

Low pass filter

Error amplifier

VCODivide by N network

PLL as a FREQUENCY SYNTHESIZER

Output

fvco =

𝑓 𝑜𝑠𝑐𝑁𝑀

Input

𝑓 𝑜𝑠𝑐 PHASE LOCKED LOOP

BLOCK DIAGRAM

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OPEARATION of PLL as a FREQUENCY SYNTEHSIZER• The frequency synthesizer is supposed to produce an output signal, the

frequency of which can be precisely adjusted to any value in a prescribed range. The output of a synthesizer should be stable.

• In order to ensure the stability of output frequency, a crystal oscillator of frequency fosc is used.

• The output frequency of crystal oscillator is divided by M with the help of M network. Thus the input frequency to PLL is (fosc/M) as shown in following figure.

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• The PLL will compare this frequency with the frequency at the output of divide by N network, and will try to adjust this frequency equal to .

• In order to obtain the same frequency i.e. (fosc/M) at the output of divide by N network, the VCO frequency should be adjusted to:

fvco =

AM DETECTION USING PLL

Phase shift at

90o

Phase detector

Low Pass filter

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PLLVCO output

Dem

odul

ated

ou

tputAM Input

BLOCK DIAGRAM

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• The AM signal to be demodulated is applied to the 90o phase shifting network as well as the PLL.• The PLL is locked to the carrier frequency of the AM

signal. Therefore the VCO output frequency is same as the unmodulated carrier. • When the VCO output is locked with AM signal perfectly, the

phase shift between the two is 90o

OPEARATION of PLL as a AM DETECTOR

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• In order to nullify this phase shift, the AM signal is phase shifted by 90°.  This will bring both the inputs to the phase detector in phase. 

• The phase detector circuit is basically a multiplier which will produce both sum and difference components of frequencies at its output. • The low pass filter will allow only those frequency components

which are close to carrier and lower than it.• The advantage of using the AM detector using PLL is its high noise

immunity which is not possible to obtain using the conventional peak detector type AM detector FM Detection using PLL.

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REFERENCE

• WIKIPEDIA

• TECH-MAX, J.S. KATRE

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