B2_Amplitude Modulation 2013.pdf (4661k)

(1)

SEE 3533

COMMUNICATION PRINCIPLES

Chapter II – Amplitude Modulation

(2)

2.0 Amplitude Modulation (AM)

Objective:

• To learn AM modulation and demodulation technique.

• To learn AM generation and detection.

AM modulation technique will be studied:

• Double Side Band Full Carrier (DSBFC)

• Double Side band Suppressed Carrier (DSBSC)

• Single Side Band (SSB)

• Single Side Band Suppressed Carrier (SSBSC)

(3)

3

2.1 Introduction

 Baseband: is a range of frequency signal to be transmitted. eg: Audio (0 - 4 kHz), Video (0 - 6 MHz).

 Baseband Communication

 Transmission without frequency shifting.

 Transmission through twisted pair cable, coaxial cable and fiber optic cable.

 Significant power for whole range of frequencies.

 Not suitable for radio/microwave and satellite communication.

 Carrier Communication

 Use technique of modulation to shift the frequency.

 Change the carrier signal characteristics (amplitude, frequency and phase) following the modulating signal amplitude.

(4)

2.2 DSB-FC – Full AM

• AM modulation is a fundamental modulation process in communication system.

• Carrier frequency signal >> than modulating frequency signal.

=> f_c >> f_m.

• Modulator is used to generate AM signal, am_DSB-FC(t). It is shown in block diagram below.



E

v

t



t

v

_AM

(

)



_c



_m

(

)

cos



_c

v

_m

(

t

)

v

_c

(

t

)

AM Modulator

Modulating signal

Carrier signal

(5)

5 • Let :

 Therefore, am_DSBFCsignal can be expressed:

 Given the modulation index :

 am_DSBFCcan be deduced to:

 From trigonometry identities:

 Therefore: c m

E

m





m

t



t

E

t

v

_AM

(

)



_c

1 

cos



_m

cos



_c



A

B





A

B



B

A





cos



2

1 cos

2

1 )

cos(

)

cos(





t

mE





t

mE

t

E

t

mE

t

E

t

v

m c c m c c c c m c c c c AM













cos

2 cos

cos

)

(

 







E

t



t

v

t

v

E

t

v

c m m c AM c m c AM



cos

)

(

cos

)

(









t

E

t

v

_c

(

)



_c

cos



_c

t

E

t

(6)

• Signal frequency spectrum ; am_DSBFC











t



mE

t

E

t

v

_AM



_c



_c



c

cos



_c





_m



cos



_c





_m

2 cos

)

(

Carrier signal Sidebands signal

) (V Amplitud

) (rads1



c



c m m

c 

 

0

c

E

2

c

mE

2

c

mE

m



2 2

m

c E

mE _

m

E

where

Jalur Sisi Bawah LSB

Jalur Sisi Atas USB Carrier band

(7)

7



0 

m



1 

c m

E

m



)

(

,

1 E

_m

E

_c

m



)

(

,

1 E

_m

E

_c

m



)

(

,

1 E

_m

E

_c

m



The modulation index is given by :

(8)

• The phase change for carrier signal when over-modulation

occurs and must be avoided.

• Modulation depth

greater than 100% must be

avoided,

(

m

> 1 > 100%)

(9)

9

2.2.2 Modulation Index,

m

min max min max min max min max               V V V V V V V V m p p p p p p p p V V V V m        min max min max where m c E E

V max  V min E_c E_m



 





 



m m c m c m c m c E E E E E E E E E E m         or dan Therefore m E m E m E m E



m t



E

Sampul  _c 1 cos_m



m t



E

Sampul   _c 1 cos_m

(10)

2.2.3 Power, AM

 In the modulation process signal has been converted to electrical signal in terms of current or voltage.

 The expression of AM signal components can be represented as follows:

) (V Amplitud

) (rads1



c



c m m

c 

 

0

c

E

2

c

mE

2

c

mE

2 2

m

c E

mE _ Di mana





t

mE





t

mE

t

E

t

v

_AM



_c



_c



c



_c





_m



c

cos



_c





_m

2 cos

)

(

(11)

11

R

E

m

R

E

m

R

E

R

mE

R

mE

R

E

R

V

R

V

R

V

P

c c c c c c USB LSB c USB LSB c T rms rms rms

8

2

2 2 2 2 2 2 2 2 2 2 2























































R

E

P

_c c

2



4

2 c USB LSB

P

m

P



where Therefore

2

4

2 2 2 c c c USB LSB SB

P

m

P

m

P

m

P



































2

1

2

2 2

m

P

m

P

c c c SB c T and

Therefore the relationship between the total power transmitted, P_T and the carrier signal power, P_c is as

(12)

















2

1

2

m

P

_T _AM _c

%

100 



T SB

P



Watt

Transmission efficiency, η for AM:

2 2 2

2 2

2

1

2

1

2 m

m

P

m

c







































where P_SBis the total sidebands signal power that contains information

If m = 1 (100% modulation), the average power, P_SB = 50% P_c= P_c/2

(13)

13

2 2

2 m

m







From:

The transmission efficiency with m = 1 is only 33.33% .

Therefore we can conclude that the transmitted power signal is mostly carrier power signal contributing of 66.67% from the total AM signal.

Whereas signal contains information in the LSB and USB transmitted is 33.33% from the total AM signal.

In practice, information signal is complex or non periodic signal,

eg: music, voice, image and etc. Its consists of many frequencies and harmonics components.

Its can be represented:

...

2 ;

2 ...

cos

)

(

2 2

1 1

2 2

1 1

m m

m

f

where

t

E

t

E

t

v













(14)

(15)

15

Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other.

Amplitude modulation is inefficient in terms of power usage and much of it is wasted. (66.67%)

At least two-thirds of the power is concentrated in the carrier signal, which

carries no useful information

The remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.

) (rads1



c



c m m

c 

 

0

c

E

2

c

mE

2

c

mE

2 2

m

c E

mE _ where

Amplitude(V)

(16)

2.3 DSBSC

• To increase transmitter efficiency, the carrier can be

removed (suppressed) from the AM signal.

• This produces a

reduced-carrier transmission

or

double-sideband suppressed carrier

(DSBSC) signal.

• A suppressed carrier amplitude modulation scheme

is

three times more power efficient

than traditional

DSBFC.

DSBSC Modulator

v

_DSBSC

(

t

)



v

_m

(

t

)

cos



_c

t

v_m(t)

(17)

17



A B





A B



B

A    cos 

2 1 cos

2 1 ) cos( )

cos(





t

E





t

E

t

E

t

v

m c

m m

c m

m c

m DSBSC









cos

2 cos

cos

)

(

t

E

t

v

_m

(

)



_m

cos



_m



t

E

t

v

_DSBSC

(

)



_m

cos



_m

cos



_c

Let :

Therefore

v

_DSBSC becomes :

From trigonometry identity :

(18)

• Frequency spectrum signal

am

_DSBSC

)

(

V

Amplitude

)

(

rads

1



c



_c





_m

m

c





0

2

m

E

2

m

E

m



2

m

E

Jalur Sisi Atas USB

Modulating band





t

E





t

E

t

v

_DSBSC



m



_c





_m



m

cos



_c





_m

2 cos

2 )

(

(19)

19

2.3.1.1 Power,

am

_DSBSC

 Components representation for am_DSBSC signal:

)

(

V

Amplitud

) (rads1



c



c





m

m c





0

2

m

E

2

m

E





t

E





t

E

t

am

_DSBSC



m



_c





_m



m

cos



_c





_m

2 cos

2 )

(

Isyarat LSB Isyarat USB

LSB

(20)

R

E

R

E

R

E

R

E

R

E

R

V

R

V

P

m

m m

USB LSB

T

rms rms

4

8

2

2 2











































In DSBSC, all the power transmitted is sidebands power.

If R = 1 ohm.

4

2

m T

E

P



SB

T

P



(21)

21

2.3.2 SSB

• Both in am_DSBFC and am_{DSBSC ,}the transmission bandwidth= 2 times the modulating signal bandwidth , v_m(t).

• Both techniques transmit 2 sidebands i.e LSB and USB, which contain identical information - the wastage of energy still occur.

• Another technique to reduce the transmitted power is am_SSB.

• In this technique of modulation only one sideband will be transmitted either LSB or USB signal.

Pemodulat SSB

am

_SSB

t

v

_m

t



_c

t

v

_h

(

t

)

sin



_c

t

2

1 cos

)

(

2

1 )

(





v_m(t)

(22)

• To analyze, let v_m₍t) :

 and

 Therefore am_SSB :

 From trigonometry:

t

E

t

v

_m

(

)



_m

cos



_m































_

_









t

E

t

E

t

E

t

E

t

am

m c m m c m m c m m c m SSB



cos

4 cos

4 )

(

t

E

t

E

t

am

_SSB m



_m



_c m

sin



_m

sin



_c

2 cos

cos

2 )

(





t

E

t

E

t

v

_h _m



_m



_m

sin



_m

2 cos

)

(















_







A B



kos



A B



kos B

kos A

kos    

2 1 2 1 ) ( )

( A B  kos



AB



 kos



AB



(23)

23

• We can choose to transmit LSB or USB signal.

• Minus will have am_SSB-LSB and plus will have am_SSB-USB

   







































t



E t E t E t E t E t t E t kos t E t am m c m m c m m c m m c m m c m c m m c m m USB SSB                       _ _ _         cos 2 cos 4 cos 4 cos 4 cos 4 sin sin 2 cos 2 ) (

   





































(24)







t



E

t

am

_SSB

(

)



_m

cos



_c





_m

Isyarat

am

_SSB-LSB







t



E

t

(25)

25

• Frequency spectrum isyarat

am

_SSB

































t

E

t

E

t

am

m c

m

m c

m

SSB



cos

2 cos

2 )

(

Isyarat am_SSB-LSB

Isyarat am_SSB-USB

)

(

V

Amplitud

) (rads1



c



c





m

m c





0

2

m

A

2

m

A

m



m

A

Jalur Sisi Atas USB

(26)

2.3.2.1 Power

am

_SSB

 Mathematical representation for am_SSB signal components:

)

(

V

Amplitud

) (rads1



c



c





m

c





0

2

m

E

2

m

E

Isyarat LSB

Isyarat USB

LSB

V

USB

V

































t

E

t

E

t

am

m c

m

m c

m

SSB



cos

2 cos

2

(27)

27

R

E

R

E

R

E

R

E

R

E

R

E

R

V

R

V

P

m m

USB LSB

T

rms rms

4

8

2

2 2































We therefore reduced the

transmitting power by 50%

compared to am_DSB-SC. Assume, R

= 1 Ohm.

Therefore

4

2

m T

E

P



USB LSB

T

P



(28)

• Spektrum frekuensi isyarat am_SSB

















        t A t A t am m c m m c m SSB     cos 2 cos 2 ) (

Isyarat am_SSB-LSB

Isyarat am_SSB-USB













































m c m m c m m c m m c m SSB

f

A

f

A

f

A

f

A

f

AM



2

2 )

(

Spektrum am_SSB-LSB

Spektrum am_SSB-USB

) (Hz f ) (V Amplitud c

f fc  fm m

c f

f 

0 f_m

2

m

A

Jalur Memodulat

m

(29)

29

• VSB signal spectrum

( ) _c ( ) cos(2 _c ) _c '( ) sin(2 _c )

s t  A m t  f t A m t  f t

Vestigal Sideband (VSB)

Vestigial sideband (VSB) transmission : Modified AM transmission in which one sideband, the carrier, and only a portion of the other sideband are

transmitted

VSB Signal, Spectrum

• VSB signal waveform

( ) _c ( )cos(2 _c ) _c '( )sin(2 _c )

s t A m t  f t A m t  f t

where: '( ) : the output of ( ) passing a filter

( ) : full upper sideband, with a partial lower sideband. ( ) : full lower sideband, with a partial upper sideband.

m t m t

(30)

2.4 AM Generation (DSBFC)

2 methods – Direct and Indirect methods.

(i) Direct - Kaedah terus

+

E_c

cos_ct

v_s(t)cos_ct

_v

AM

(

t

)

Balanced modulator Isyarat asal

v_s(t)

mixer

 



E

v

t



t

v

t

E

t

v

c s

c

c s

c c

AM



cos









(31)

31

(ii) Indirect - Kaedah tidak terus

v

_s

(

t

)

v

_c

(

t

)

v

_i

_v

k

v

o

peranti tidak linar

Penapis Lulus Jalur

(BPF)

 

t

E

t

v

t

E

t

v

c c

c

s s

s



cos



 

t

v

   

t

v

t

v

_i



_s



_c

Input signal :

t

E

t

E

v

_i



_s

cos



_s



_c

cos



_c

...

3 3 2

2 1

0





_i _i _i

k

E

m

v

m

v

m

v

(32)



















cos



....

cos

....

cos

2 cos

cos

....

cos

2 2 2 2 2 2 2 1 1 0 2 2 2 2 2 2 2 1 1 0 2 2 1 0

















t

E

m

t

E

m

t

E

m

t

E

m

t

E

m

E

t

E

m

t

E

m

t

E

m

t

E

m

t

E

m

E

t

E

t

E

m

t

E

t

E

m

E

v

s c s c c s c c s s c c s s c s c s c c s s c c s s c c s s c c s s k



Therefore:

Isyarat

v

_k

(t) was then filtered using band pass filter

(BPF)

tuned at

the resonance (salun) frequency,

f

_o

= f

_c

.













t

E

m

E

m

t

E

m

t

E

m

v

c s s c s c s c c s c c o



cos

2

1 cos

cos

1 2 1 2 1























(33)

33

Compare the output signal:

 

t

E



m

ω

t



ω

t

v

_AM



_c

1 

cos

_s

cos

_c

t

E

m

E

m

t

v

_o

(

)

_c

1

2

_s

cos



_s

cos



_c

1 2

1

















Didapati isyarat terhasil adalah sama kerana komponen frekuensi yang terhasil adalah sama walaupun berlainan amplitud.

Spektrum frekuensi sebelum penapis Spektrum frekuensi selepas penapis

v_k

0

sambutan frekuensi

penapis pembawa

f_s 2f_s (f_c-f_s) f_c (f_c+f_s) 2f_c f v_o

f LSB USB

pembawa

(34)

2.5 Generation of DSBSC

Direct – menggunakan pemodulat terimbang/balanced modulator.

Fungsinya seperti pendarab/multiplier – menghasilkan isyarat LSB dan USB sahaja.

Isyarat asal

v_s(t)

v_c(t)

v_s(t)cos_ct Pendarab

v

(a) Isyarat maklumat

t

t _v

DSBSC

t

(35)

35

Isyarat pembawa,

v

_c

diberi oleh siri Fourier sebagai ;

Mathematical Analysis

v

_c

(t)= { sin



_c

t

+ sin 3



_c

t

+ ………}

If

v

_s

(t)= E

_s

cos



_s

t

dan

k

is a multiplier sensitivity factor

Modulator output can be expressed:

   

































            _ _        _ _                   _     t t kE t t kE t t kE t t kE t t t kE t v t kv v s c s c s s c s c s c s c s s c s s c c s s c s o



























2 cos 2 cos 2 sin sin 2 sin sin 2 1 4 ... sin 4 cos 3 sin 3 1 sin 4 cos

(36)

Generation DSBSC – Indirect method

v

_s

(t))cos



_c

t

(Isyarat DSBSC)

E

_c

cos



_c

t

½

v

_s

(t)

-½

v

_s

(t)

(

E

_c

+½

v

_s

(t)

)

cos



_c

t

(

E

_c

-½

v

_s

(t)

)

cos



_c

t

+

-

AM

modulator AM

modulator

 Using 2 full AM modulator

 The input signal are the same with different polarity but the same carrier frequencies

(37)

37

2.6 Generation of SSBSC

Can be realized in two ways :

(i) Generate first DSBSC signal dan

(ii) Then filtered DSBSC signal with band pass filter (BPF)

v

_DSBSC

v

_SSBSC

Pemodulat terimbang

v

_s

(

t

)

cos



_c

t

BPF

BPF is a tuned circuit (litar tertala) that is very selective that will choose either LSB or USB to pass through.

Not important which sideband will be selected because both sidebands contain the same information.

(38)

Another method – using 2 balanced modulator that will produced 2 DSBSC signal with 180o _{phase difference (bezafasa)}_.

The circuit is called litar anjakan fasa (phase shifting circuit).

Generation of SSBSC

Pemodulat terimbang 1

Anjakan fasa 900

+

v

_SSBSC

Pemodulat Phase shift

900

t

ω

t

ω

mE

v

₁



_c

cos

_s

cos

_c

t

ω

E

t

v

_s

(

)



_s

cos

_s

t

ω

E

_s

sin

_s

t ω_c

cos

t ω_c

sin

mixer + +

Phase Shift Method pg.176, B.P.Lathi

- /2

Which delays the phase of every

spectral component

(39)

39

Output balanced modulator 1 :

Mathematical Analysis

t

mE

t

v

₁

(

)



_c

cos



_s

cos



_c

t

mE

t

v

₂

(

)



_c

sin



_s

sin



_c

Output balanced modulator 2 :

Hence output :

v

_SSBSC

=

v

₁

(t) + v

₂

(t)













cos





}

{cos

2 }

cos

{cos

2 sin

sin

cos

t

mE

t

mE

t

mE

t

mE

s c

c

s c

c

c s

c c

s c

















t

mE

(40)

• VSB signal spectrum

( ) c ( ) cos(2 c ) c '( ) sin(2 c )

s t  A m t  f t A m t  f t

2.7 Generation of VSB

v

_VSB



Penapis

VSB

v

_s

(

t

)

v

DSBSC

2cos



_c

t

f (Hz)

v

_VSB

LSB USB

carrier

(41)

41

2.8 Demodulation/Penyahmodulatan

Information

signal

Information

signal

Rx

Tx

Modulated signal

(AM/FM)

Process to get beack the modulating signal or original signal. Done at the receiver part.

Demodulation done by demodulator circuit also called detector circuit.

Simple and economic detector for AM wave is envelope detector (pengesan sampul).

(42)

2.8.1 Demodulation AM

_DSB-FC

(i) Envelope Detector Circuit – other name Rectifier

Detector

(Pengesan

Penerusuai)

or Diode Detector

(Pengesan Diod)

• Low cost

• Simple

• Does not need local carrier generator



Original signal

overlap with the envelope modulated

signal

+ C’

R’ R

a b c d

C

LPF

[E_c+ v_s(t)] cos _c t

(43)

43

(i)

Envelope Detector

IF RC is too large, voltage drop rate is slow and will cause diagonal clipping (perepangan pepenjuru) where parts of the peak input could not be detected at the output (Refer Rajah 2.18(a)) .

If RC is too small, output signal at the capacitor will have ripple (riak) and will cause distortion (wujud herotan) to the received information signal (Rujuk Rajah 2.18(b)) .

t

Rajah 2.18(a)

t

Rajah 2.18(b)

Discharge rate or voltage drop of the capacitor depends on RC time constant.

RC too

(44)

LPF Diode

Filter out DC component

R₂ R₁

C₁

C₂ R_s





m

ω

m

RC

2 / 1 2

1 



To avoid that problem make sure the value of :

m – modulation index and



_m– angle frequency of information signal

(45)

45 1

1  RsC



When s(t) > v_o(t)

Diode, D in forward biased condition Capacitor, C₁will charging up to

where

When s(t) < v_o(t)

Diode, D in reverse biased condition Capacitor, C₁ will discharge up to where ₂  R₁C₁



c

f

1

1 



BW f_c

1 1

2  



s(t) _v

o(t)

D

C₁ R₁ R_s

v_o(t)

s(t)

D

C₁ R₁ R_s

(46)

Selection effect of RC value

Kesan pemilihan nilai RC right value

(47)

47

Mathematical Analysis

Assuming the function of the diode as a switch, output of the diode, V_d:

 







E

v

t

cos

ω

t



k

(

t

)

V

_d



_c



_s _c

where k(t) is a representative of switching function of the diode

 

































_

_







cos

3 ...

3

1 cos

2

1 cos

t

v

E

V

_d _c _s _c



_c



_c





 









 





 





 

keranapemuat C menghalangarus terus melaluinya. 1 1 . pada salun frekuensi mempunyai LPF gi lebih ting frekuensi 1 ... 3 cos 3 1 cos 2 2 1 cos '









c s d titik s c c titik s s c c c c s c b titik d E t v V t v E V f t v E t t t t v E V V                     _ _     +

a b c _d

(48)

V pada titik a

t 0

[E_c+v_s(t)] cos _c t

E_c+v_s(t)

1/ [E_c+v_s(t)] V pada titik b

t

1/ [E_c+v_s(t)] V pada titik c

t

1/ v_s(t)

t V pada titik d

+

a b c _d

(49)

49

am_DSB-FC(t)

C

y(t)

Litar Kuasa Dua y =x2

Penapis Lulus Rendah

x(t)

                         

 A A m t m t A m t A m t kos t

t kos t m A t m A t kos t m t m t kos A A t kos t m A t kos t m t kos A t kos t m A t am t x c c c c c c c c c c c c c c c c c c c FC DSB



2 ] ) ( ) ( 5 . 0 5 . 0 [ ) ( 5 . 0 ) ( 5 . 0 2 ) ( ) ( 2 ) ( 5 . 0 ) ( 5 . 0 2 5 . 0 5 . 0 ) ( 2 ) ( } )] ( {[ )} ( { ) ( 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2                   _ DC component Information signal Harmonic signal

)

(

)

(

t

A

m

t

y



_c

After passing through LPF the output is the required information signal ie:

(ii) Square Detector (Pengesan Kuasa Dua)

2.8.1 Demodulation AM

_DSB-FC

(50)

2.9 Demodulation AM

_DSB-SC

• Due to the envelope modulated signal is not in the

same form of modulating signal,

m(t)

so the

envelope detection technique could not be used.

• The detection of modulated signal could be done

using

pengesan segerak (

synchronous detector

)

also

(51)

51

2.9.1 Pengesan Segerak (Synchronous)

• Litar segerak, memerlukan sebuah penjana pembawa tempatan.

• Di mana isyarat penjana ini perlu disegerakkan dengan isyarat pembawa maklumat yang digunakan pada pemancar.

)

(

cos

)

(

)

cos(

)

cos(

)

(

)

cos(

)

(

)

(

2

t

m

t

m

t

am

t

x

c

c c

c SC

DSB





_

X

Penapis

Lulus Rendah

Penjana Pembawa Tempatan (LO)

c(t)=cos(ω_ct)

am_DSB-SC(t) x(t) y(t)

Multiplier

Analisa matematik :

(52)

Analisa Matematik

• Keluaran pendarab adalah

• Identiti trigonometri

• Maka

• Selepas melalui LPF isyarat keluaran adalah isyarat maklumat asal



1 (

2 )



2

1 )

(

2

u

kos

u

kos









)

2 (

)

(

2

1 )

(

2

1 )

2 (

1 )

(

2

1 )

(

t

kos

t

m

t

m

t

kos

t

m

t

x

c c











)

(

2

1 )

(

t

m

t

y



)

(

)

(

)

(

t

m

t

kos

2

t

(53)

53

2.9.1.1 Kesan Ralat Frekuensi Pembawa

• Masalah ini akan menyebabkan herotan berlaku di dalam proses penyahmodulatan isyarat am_DSB-SC.

X

Penapis

Lulus Rendah

Penjana Pembawa Tempatan (LO)

c(t)=kos[(ω_c+Δω)t]

am_DSB-SC(t) x(t) y(t)

Multiplier

(54)

Analisa Matematik Ralat Frekuensi

• Keluaran pendarab adalah

• Identiti trigonometri :

• Maka

• Dengan melalukan isyarat x(t) ke dalam penapis lulus rendah, isyarat maklumat dapat diperolehi semula.



(

)

(

)



2

1 )

(

)

(

A

kos

B

kos

A

B

kos

A

B

kos











)

2 (

)

(

2

1 )

(

)

(

2

1 )

2 (

)

(

)

(

2

1 )

(



















t

kos

t

m

kos

t

m

t

kos

t

m

t

x

c c

)

(

)

(

2

1 )

(

t

m

t

kos



y



]

)

[(

)]

(

)

(

[

)

(

t

m

t

kos

t

kos

t

(55)

55

Implikasi Ralat Fasa Pembawa

• Kesan ralat fasa ini akan mewujudkan herotan, oleh yang

demikian penalaan isyarat pembawa tempatan perlulah tetap.

• Keluaran pada LPF mempunyai faktor kos(φ).

• Di mana jika

) ( )

( 2 1 )

(t m t kos



y 

0 



2 





) ( 2 1 )

(t m t y 

0 )

(

t



y

Untuk memastikan pengayun tempatan (LO) ditetapkan fasanya keada isyarat masukan supaya keluarannya isyarat maklumat/asal maksima, Gelung Costas / PLL digunakan.