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

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 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 o u l d : – K n o w t h e b a s i c p r i n c i p l e s o f a n a l o g m o d u l a t i o n t e c h n i q u e s – U n d e r s t a n d h o w a m p l i t u d e m o d u l a t i o n i s p e r f o 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 o w f r e q u e n c y m o d u l a t i o n i s d o n e a n d h o w 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 o 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 o b r o 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 2

Wh e r e a r e w e i n t h e B I G P I C T U R E ? 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. Analog modulation/ Lecture relates to pages transmission 117–127 in textbook. techniques O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 3

A n a l o g m o d u l a t i o n – 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 4

Mo d u l a t i o n p r o p 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 5

E x a m p l e , a m o d 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 6

A MP L I T U D E MO D U L A T I O N O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 7

G e n e r a l a m p l i t u d e m o d u l a t i o n • The simplest form of AM is where the information can be found in the envelope of the bandpass signal • m AM is the so-called modulation index O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 8

C a r r i e r s u p r e s s i o n • The carrier signal contains no information and can be supressed Time domain Frequency domain Carrier AMPLITUDE MODULATION (WITH CARRIER) DOUBLE SIDE-BAND MODULATION UPPER SIDE-BAND MODULATION O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 9

D i ff e r e n t a m p l i t u d e m o l d u l a t i o n t e c h . AMPLITUDE MODULATION (WITH CARRIER) DOUBLE SIDE-BAND MODULATION UPPER SIDE-BAND MODULATION O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 0

F R E Q U E N C Y MO D U L A T I O N O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 1

F r e q u e n c y m o d u l a t i o n i n t r o d u c t i o 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 2

F M s i g n a l w i t h s i n u s o i d a l b a s e b a n d s i g . Frequency shift proportional to baseband signal (red) amplitude O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 3

F r e q u e n c y m o d u l a t i o n • Let the signal be • Where m FM is scaling constant and the instantaneous frequency is given by f 0 +m FM g ( t ) /2 p • The larger modulation index and baseband amplitude the larger is the frequency deviation D f = m FM g ( t ) /2 p • Modulation index: b = D f/f m Derived for information signal: O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 4

S p e c t r u m o f F M ( s i n u s o i d a l b a s e b a n d ) Larger modulation index β , larger bandwidth O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 5

S p e c t r u m o f F M ( s i n u s o i d a l b a s e b a n d ) • Approximate bandwidth by Carson’s rule O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 6

→ B a n d w i d t h e x p a n s i o 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 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 7

F M s t e r e o b r o a d c a s t i n g s i g n a l Sum-signal Difference-signal (Left+Right) (Left-Right) Mono receivers Stereo receivers But, how do you get proper stereo, listen to listen to with correct left and right? this part only this part too O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 8

S u m m a r y ● A n a l o g m o d u l a t i o n d o e s n o t u s e a n y “ d i g i t a l ” p a r t o 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 o n i s t y p i c a l l y d o n e b y i n fl u e n c i n g a m p l i t u d e o r f r e q u e n c y o 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 o d u l a t i o n c o m e s i n s e v e r a l f o r m s , w i t h a n d w i t h o u t c a r r i e r c o m p o n e n t , a n d t h e r a d i o 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 o d u l a t i o n e x p a n d s t h e b a n d w i d t h , c o m p a r e d t o 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 o 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 o F M b r o 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 ( o f l e f t / r i g h t ) , t o a l l o w f o r s i m p l e m o n o r e c e i v e r s . O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 2 ) 1 9