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I n f o r m a t i o n T r a n s m i s s i o n - - PowerPoint PPT Presentation

Sep 07, 2023 310 likes •526 views

I n f o r m a t i o n T r a n s m i s s i o n C h a p t e r 4 , A n a l o g m o d u l a t i o n OVE EDFORS Electrical and information technology L e a r n i n g o u t c o m e s A

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SLIDE 1

I n f

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m a t i

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T r a n s m i s s i

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C h a p t e r 4 , A n a l

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m

  • d

u l a t i

  • n

OVE EDFORS Electrical and information technology

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SLIDE 2

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 2

L e a r n i n g

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t c

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e s

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f t e r t h i s l e c t u r e t h e s t u d e n t s h

  • u

l d :

– K

n

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t h e b a s i c p r i n c i p l e s

  • f

a n a l

  • g

m

  • d

u l a t i

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t e c h n i q u e s

– U

n d e r s t a n d h

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a m p l i t u d e m

  • d

u l a t i

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i s p e r f

  • r

m e d a n d w h i c h b a s i c v a r i a n t s t h e r e a r e

– U

n d e r s t a n d h

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f r e q u e n c y m

  • d

u l a t i

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i s d

  • n

e a n d h

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a n i n c r e a s e d b a n d w i t h l e a d s t

  • g

a i n s i n q u a l i t y ( S N R )

– U

n d e r s t a n d w h y F M r a d i

  • b

r

  • a

d c a s t i n g i s t r a n s m i t t e d t h e w a y i t i s

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SLIDE 3

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 3

Wh e r e a r e w e i n t h e B I G P I C T U R E ?

Analog modulation/ transmission techniques Lecture relates to pages 117–127 in textbook.

In analog modulation there are no digital parts in the transmitter and receiver. The analog information (baseband) signals are connected directly to the analog transmitter and picked up directly from the analog receiver.

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 4

A n a l

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m

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u l a t i

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– b a s i c p r i n c i p l e s

  • Shift the frequency to an appropriate frequency for transmission
  • Vary the amplitude or phase to represent the information

– Phase slope (time derivative of phase) = frequency shift

  • The original signal A(t) is often called the baseband signal
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SLIDE 5

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 5

Mo d u l a t i

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p r

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e r t y

  • Shifting the frequency does not modify the information content
  • There are two replicas, one at positive frequencies and one at negative
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SLIDE 6

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 6

E x a m p l e , a m

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u l a t e d b a n d p a s s s i g n a l

  • A 5 kHz bandpass signal modulated with a 50 kHz carrier
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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 7

A MP L I T U D E MO D U L A T I O N

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 8

G e n e r a l a m p l i t u d e m

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u l a t i

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  • The simplest form of AM is where the information can be found in the envelope of the

bandpass signal

  • mAM is the so-called modulation index
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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 9

C a r r i e r s u p r e s s i

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  • The carrier signal contains no information and can be supressed

Frequency domain

Carrier

Time domain AMPLITUDE MODULATION (WITH CARRIER) DOUBLE SIDE-BAND MODULATION UPPER SIDE-BAND MODULATION

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1

D i ff e r e n t a m p l i t u d e m

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d u l a t i

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t e c h .

AMPLITUDE MODULATION (WITH CARRIER) DOUBLE SIDE-BAND MODULATION UPPER SIDE-BAND MODULATION

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 1

F R E Q U E N C Y MO D U L A T I O N

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 2

F r e q u e n c y m

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u l a t i

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i n t r

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u c t i

  • n
  • Shift the frequency to an appropriate frequency for transmission
  • Phase slope (derivative) = frequency shift
  • Let the baseband signal change the frequency of the bandpass signal

– High amplitude (baseband signal) – high frequency – Low amplitude (baseband signal) – low frequency

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SLIDE 13

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 3

F M s i g n a l w i t h s i n u s

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d a l b a s e b a n d s i g .

Frequency shift proportional to baseband signal (red) amplitude

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 4

F r e q u e n c y m

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u l a t i

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  • Let the signal be
  • Where mFM is scaling constant and the instantaneous frequency is given by f0+mFM

g(t)/2p

  • The larger modulation index and baseband amplitude the larger is the frequency

deviation Df = mFM g(t)/2p

  • Modulation index: b=Df/fm

Derived for information signal:

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 5

S p e c t r u m

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F M ( s i n u s

  • i

d a l b a s e b a n d )

Larger modulation index β, larger bandwidth

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SLIDE 16

O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 6

S p e c t r u m

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F M ( s i n u s

  • i

d a l b a s e b a n d )

  • Approximate bandwidth by Carson’s rule
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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 7

B a n d w i d t h e x p a n s i

  • n

G a i n i n S N R →

  • The SNR after demodulation is determined by the modulation index
  • We can trade bandwidth with SNR
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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 8

F M s t e r e

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r

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d c a s t i n g s i g n a l

Sum-signal (Left+Right) Difference-signal (Left-Right) Mono receivers listen to this part only Stereo receivers listen to this part too But, how do you get proper stereo, with correct left and right?

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O v e E d f

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s E I T A 3

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h a p t e r 4 ( P a r t 2 ) 1 9

S u m m a r y

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n a l

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m

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u l a t i

  • n

d

  • e

s n

  • t

u s e a n y “ d i g i t a l ” p a r t

  • f

t h e t r a n s m i t t e r / r e c e i v e r c h a i n s

  • Mo

d u l a t i

  • n

i s t y p i c a l l y d

  • n

e b y i n fl u e n c i n g a m p l i t u d e

  • r

f r e q u e n c y

  • f

a c a r r i e r s i g n a l

  • A

m p l i t u d e m

  • d

u l a t i

  • n

c

  • m

e s i n s e v e r a l f

  • r

m s , w i t h a n d w i t h

  • u

t c a r r i e r c

  • m

p

  • n

e n t , a n d t h e r a d i

  • b

a n d w i d t h i s “ t h e s a m e ” a s t h e b a s e b a n d s i g n a l b a n d w i d t h

  • F

r e q u e n c y m

  • d

u l a t i

  • n

e x p a n d s t h e b a n d w i d t h , c

  • m

p a r e d t

  • t

h e b a s e b a n d s i g n a l , a n d t h e l a r g e r t h e e x p a n s i

  • n

t h e b e t t e r t h e q u a l i t y ( t r a d i n g b a n d w i d t h a n d S N R )

  • S

t e r e

  • F

M b r

  • a

d c a s t i n g t r a n s m i t t h e s u m a n d d i f f e r e n c e s i g n a l s (

  • f

l e f t / r i g h t ) , t

  • a

l l

  • w

f

  • r

s i m p l e m

  • n
  • r

e c e i v e r s .

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SLIDE 20