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A u s t r i a .West Germany. A b s t r a c t I n h i g h performance SAW T V -I F f i l t e r s m u l t i s t r i p couplers a r e used t o suppress unwanted spurious b u l k mode transmission. Small maximum f i n g e r overl a p i s chosen t o keep f i l t e r c o s t low due t o small s u b s t r a t e square dimensions. Because o f t h e small apertures d i f f r a c t i o n becomes severe on 128' r ot a t e d Y-cut l i t h i u m -n i o b a t e . As r e c e n t l y published scanning e l e c t r o n microscope i n v e s t i g a t i o n s show, p a r t s o f t h e waves launched by t h e apertures o f the i n p u t transducer a r e spread d i r e c t l y i n t o the o u tp u t transducer. Experiment shows t h a t these detect e d spurious waves are o n l y about 40 t o 50 dB down compared t o midband transmission. T h i s i s p e r f e c t l y v e r i f i e d by c a l c u l a t i o n s u s i n g t h e p a r a b o l i c ani s o t r o p i c d i f f r a c t i o n formulas. Using a simple basic theory, d i f f r a c t i o n e ff e c t s i n f i l t e r s using m u l t i s t r i p couplers a r e taken i n t o c o n s i d e r a t i o n i n f i l t e r a n a l y s i s . Experimental r e s u l t s a r e i n good agreement w i t h t heo ry . 1. I n t r o d u c t i o n M u l t i s t r i p couplers (MSC) a r e w i d e l y used i n surface a c o u s t i c wave (SAW) band-pass f i l t e r s on l i t h i u m -n i o b a t e t o suppress b u l k mode transmission, e s p e c i a l l y when h i g h stop-band r e j e c t i o n i s req u i r e d . Because o f i t s l a r g e a p e r t u r e t h e MSC i s a l s o assumed t o reduce d i f f r a c t i o n e f f e c t s , i f i t i s s i t u a t e d near t o t h e apodized transducer. However, t h e r e i s another spurious s i g n a l caused by d i f f r a c t i o n , which,at l e a s t as f a r as we know,has n o t been mentioned i n l i t e r a t u r e . This spurious s i g n a l i s demonstrated by v i s u a l i z a t i o n o f t h e propagating SAW by means o f t h e scanning e l e c t r o n microscope technique r e c e n t l y published by Feuerbaum / I / , Eberharter /Z/, and V e i t h /3/. Several s t u d i e s have been made t o d e s c r i b e SAW propagation i n MSC's. Marshall /4/ used a model o f two modes, a symmetric one and an antisymmetric one, propagating a t d i f f e r e n t wave v e l o c i t i e s , whereas I n g e b r i g t s e n /5/ and F a r n e l l /6/ used a more d e t a i l e d t h e o r y accounting f o r a f i n i t e spect r u m o f several d i s c r e t e eigenmodes. Experimental i n v e s t i g a t i o n s have been made by Mason /7/, who 'This work was supported by t h e Fonds zur Fijrderung d e r wissenschaftlichen Forschung, Vienna, p r o j e c t S-22/11. Laboratories, Siemens Component D i v i s i o n , D-8000 Munich, Nest Germany. s t a t e s t h a t an extremely h i g h degree o f a n i s o t r o p y i s caused by t h e m u l t i p l e m e t a l l i c s t r i p s o f t h e MSC deposited on t h e surface o f t h e substrate. f a s t computation o ...

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