2 5 m h z t h e l o w p a s s f i l t e r p e rf o r m

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Unformatted text preview: lo s e d b y t h e b a s e b a n d c o n tr o l l e r . T h e I F A G C c o n tr o l v o l t a g e is a p p li e d a t th e A G C PWM p in. Th e g a in c a n b e r e d u c e d o v e r 5 3 d B (t y p ) a n d e x h i b i t s a l o g -li n e a r c h a ra c t e ri s t i c . T h e b a c k e n d a l s o c o n t a i n s i n d i v i d u a l l o w p a s s filt e rs o n e a c h c h a n n e l . T h e l o w p a s s -f i l t e r b a n d w i d t h i s t h e u s e f u l S D A R S d o w n c o n v e rt e d b a n d w i d t h ( 6 . 2 5 M H z ) . T h e l o w p a s s -f i l t e r p e rf o r m a n c e i s f a c t o r y tri m m e d . T h e b i t I O T s w i t c h e s b e t w e e n t h e f a c t o r y -tri m m e d s e t a n d t h e c o n tro l t hro u g h t h e I2 C - c o m p a ti b l e b u s u s i n g b it s B 4 – B 1 . E v e n w h e n u s i n g t h e f a c t o r y -tri m m e d s e t , t h e u s e r c a n s t i l l s l i g h t l y m o d i f y t h e c u t o f f fr e q u e n c y ( b y ± 2 5 0 k H z ) b y v a ry i n g b it s L P 1/L P 0 . H i g h p a s s f i l t e r s a r e a l s o i n s e rt e d i n t h e b a c k - e n d s i g n a l p a t h s . T h e ir p u r p o s e i s t o r e m o v e t h e D C o f f s e t . T h e y a r e d e s i g n e d f o r a l o w c o r n e r fr e q u e n c y s o a s n o t t o d e g r a d e t h e S D A R S c o n t e n t . T h e ir e x a c t c u t o f f fr e q u e nc y i s s e t b y t h e e x t e rn a l c a p a c it ors c o n n e c t e d b e t w e e n I F 2 a c c e s s p i n s , g i v e n b y t h e f o ll o w i n g e q u a ti o n : f c u t o f f = 1/(2 x π x R x C ) [ H z ] wh ere R = 8000Ω, C = e xt ern a l c a p a c itor to b e c onn e c t e d . F i n a ll y , t h e H P F b it a ll o w s a n i n c r e a s e t o t h e b a c k- e n d g a i n b y 4 d B a t t h e s li g h t e x p e n s e o f a d e g r a d e d i nb a n d l i n e a ri t y . Fre qu e n c y G e n era tion A n o n- c h i p V C O a n d a l o w-s t e p fr a c ti o n a l- N P L L e n s ur e t h e n e c e s s a ry fr e q u e n c y g e n e r a t i o n . T h e 1 s t m i x e r’ s L O i s a t t h e V C O f r e q u e n c y i t s e l f , w h i l e t h e 2 n d m i x e r’ s L O i s t h e V C O f r e q u e n c y d i v i d e d b y 4 o r b y 8 ( b i t D 4 8 ) . H e n c e , t h e t w o p o s s i b l e I F fr e q u e n c i e s f o r S D A R S a r e 4 6 7 M H z a n d 2 5 9 M H z . T y p i c a l a p p li c a ti o n s a r e b a s e d o n 2 5 9 M H z I F fr e q u e n c y . T h e r e f e r e n c e d iv i d e r p a t h in t h e P L L c a n e it h e r u s e a n e x t e rn a l c ry s t a l a n d t h e o n- c h i p c ry s t a l o s c ill a t or or a n e x t e r n a l T C X O t h a t c a n o v e r d ri v e t h e o n - c h i p c r y s t a l o s c ill a t or. A r e f e r e n c e d i v i s i o n r a ti o o f 1 or 2 i s s e t b y t h e R E F b it . T h e c ry s t a l o s c ill a t or (or T C X O ) s i g n a l i s a v a il a b l e a t p in R E F O U T . T h e o u t p u t is e it h e r a t t h e s a m e fr e q u e n c y a s t h e r e f e r e n c e s i g n a l , o r d i v i d e d b y t w o , b a s e d o n t h e s e ttin g o f b it R F D . T h e V C O m a i n d i v i s i o n r a t i o i s s e t b y b i t s N 6 – N 0 (f o r t h e i n t e g e r p a rt) a n d b i t s F 1 9– F 0 0 (f or t h e fr a c t i o n a l p a rt) . T h e m i n i m u m s t e p i s b e l o w 3 0 H z , s m a l l e n o u g h f or e f f e c ti v e A F C t o b e i m p l e m e n t e d b y t h e b a s e b a n d . T h e c h a r g e - p u m p ( p i n C P O U T) i s t o b e c o n n e c t e d t o t h e V C O t u n i n g i n p u t ( p i n V T U N E ) t h r o u g h a n a p p r o p ria t e l o o p filt e r. O v e r c urr e n t Pro t e c tio n T h i s D C f u n c t i o n a l l o w s e x t e r n a l c ir c u i t r y c o n s u m i n g u p to 1 5 0 m A a n d c o n n e c t e d to th e p in V O U T A N T to sin k c urr e n t fro m a V C C li n e ( p i n VI N A N T) t hro u g h o v e r c urr e n t - p r o t e c t i o n c ir c u i t r y . W h e n n o o v e r c urr e n t i s p r e s e n t , a l o w d ro p o u t v o l t a g e e x i s t s b e t w e e n p i n s V I N A N T a n d V O U T A N T . I n o v e rc u rr e n t c o n d i t i o n s (i n c l u d i n g s h o rt- c ir c u i t fr o m V O U T A N T t o G N D ) , t h e c u rr e n t i s l i m i t e d t o a p p r o x i m a t e ly 3 0 0 m A a n d b it A C P in th e R E A D b yt e st a tu s g o e s hi g h. T h i s c ir c u i t a l s o s e n s e s i f t h e c urr e n t d r a w n a t t h e p i n V O U T A N T i s t y p i c a ll y l a r g e r t h a n 2 0 m A , i n w h i c h c a s e t h e b i t A N D fr o m t h e R E A D b y t e s t a t u s g o e s h i g h (t h e p u r p o s e i s t o i n f o r m t h e b a s e b a n d c o n tr o l l e r i f t h e r e i s a n y d e v i c e d r a w i n g c urr e n t fro m V O U T A N T). _______________________________________________________________________________________ 9 MA X2140 D e t a il e d D e s c rip tio n (M A X 2 1 4 0 E V k i t , c urr e n t d r a w n a t V O U T A N T , IV O U T A N T = 1 5 0 m A m a x , V C C = 3 . 1 V t o 3 . 6 V , VI N A N T = 3 . 1 V t o 5 . 3 V , f R F = 2 3 2...
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This test prep was uploaded on 03/13/2014 for the course ECE 453 taught by Professor Staff during the Spring '08 term at University of Illinois, Urbana Champaign.

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