THE TRAVELLING WAVE TRANSDUCER

D. J. Gunton, M. F. L e w i s and E. G. S. Paige

Royal R a d a r Establishment, Malvern, Worcs., England.

ABSTRACT The f i n i t e v e l o c i t y o f a n e l e c t r o m a g n e t i c wave i n a n i n t e r d i g i t a l s t r u c t u r e l e a d s t o some i n t e r e s t i n g new e f f e c t s i n SAW devices . We d e s c r i b e t h e s e e f f e c t s and how t h e y may be enhanced by d e l i b e r a t e l y s lowing t h e e l e c t r o m a g n e t i c wave t o produce t h e T r a v e l l i n g Wave Transducer , w i t h its p o s s i b l e a p p l i c a t i o n s as a f i l t e r , as a d i s c r i m i n a t o r and as a n o s c i l l a t o r .

I n t r o d u c t i o n

The l a r g e r a t i o between t h e v e l o c i t y o f an e l e c t r o m a g n e t i c wave (EMW) and a s u r f a c e a c o u s t i c wave (SAW) makes it p o s s i b l e t o d e s i g n most SAW t r a n s d u c e r s under t h e assumption t h a t t h e EMW v e l o c i t y is i n f i n i t e . However, i f t h e e l e c t r o - magnet ic d e l a y down a l o n g t r a n s d u c e r approaches one p e r i o d (1 / f ) t h e f i n i t e EMW v e l o c i t y g i v e s

r i s e t o compl ica i ions") . d e s c r i b e how we e x p l o i t t h e s e e f f e c t s , d e l i b e r a t e l y r e d u c i n g t h e speed w i t h which t h e t r a n s d u c e r is e x c i t e d , t o form what we r e f e r t o as t h e T r a v e l l i n g Wave Transducer (TWT). F e a t u r e s o f t h i s t r a n s d u c e r i n c l u d e i ) u n i d i r - e c t i o n a l i t y ; i i ) h i g h convers ion e f f i c i e n c y ; i i i ) avoidance o f mechanica l ly induced m u l t i p l e r e f l e c t i o n s and i v ) t u n a b i l i t y a f t e r manufacture . The TWT h a s a narrow f r a c t i o n a l bandwidth, and its p o s s i b l e a p p l i c a t i o n s as a f i l t e r , as a d i s c r i m i n a t o r and as a n o s c i l l a t o r w i l l be d i s c u s s e d .

I n t h i s paper we

P r i n c i p l e s o f O p e r a t i o n

I f we conver t t h e c e n t r a l t r a n s d u c e r t o a normal one ( i e r e p l a c e t h e c o a x i a l c a b l e by a b u s b a r ) , s lowing o f t h e EMW w i l l s t i l l occur t o a reduced e x t e n t due t o t h e p e r m i t t i v i t y o f t h e s u b s t r a t e and t h e h i g h c a p a c i t y p e r u n i t l e n g t h of t h e s t r u c t u r e . '?le can d i s p l a y t h e e f f e c t d i s c u s s e d above on t h e w - k diagram; t h e r e l e v a n t branches a r e shown i n Fi[:. 2 . T h i s i l l u s t r a t e s t h e forward and r e v e r s e SAW and t h e forward EMW p r o p a g a t i n g and coupl ing through t h e p e r i o d i c l i n e (hence t h e f 27r/Ae o f f s e t o f k from z e r o ) . P e r t u r b i n g e f f e c t s due t o coupl ing have been o m i t t e d from t h e diagram. C l e a r l y , t h e phase matched c o n d i t i o n o c u r r s a t two f r e q u e n c i e s , f

and f

assuming i n f i n i t e EMU v e l o c i t y .

P s p l i t about f o , t h e synchronous f requency

aP'

The f requency s p l i t t i n ; : i s g iven by

where t = i a / V a and t = L e / V e . L and i a r e

t h e SAW and ZMW p a t h l e n g t h s , r e s p e c t i v e l y ; V _ a and V e a r e t h e SAW and EMU phase v e l o c i t i e s . I n Fig. 1 we show a t r a n s d u c e r arrangement

i n which a c e n t r a l t r a n s d u c e r may be e l e c t r i c a l l y e x c i t e d from one end, t h e e x c i t i n g wave t r a v e l l - i n g a l o n g a c o a x i a l c a b l e , s a y , and e x c i t i n g each f i n g e r p a i r i n sequence. ;io r e f l e c t e d e l e c t r o - magnet ic wave a p p e a r s i f t h e combined c a b l e - t r a n s d u c e r l i n e i s t e r m i n a t e d i n i t s c h a r a c t e r - i s t i c impedance. C l e a r l y , peak g e n e r a t i o n o c c u r s a t synchronis:.i which now becornes a phase matchini. c o n d i t i o n between EMW and SAW. decause a s u r f a c e wave can be launched e i t h e r p a r a l l e l o r a n t i - p a r a l l e l t o t h e EMW p r o p a g a t i o n d i r e c t i o n t h e r e a r e two i'eak f r e q u e n c i e s , f and f

corresponding t o t h e two l a m c h d i r e c t i o n s . P aP

PORT 4

b k

F i g , 2 T r a v e l l i n g Wave Transducer r e s p o n s e s , f and f

P aP

Regard ins t h e TWT as a p a i r of cous led (EMW and SAW; t r a n s m i s s i o n l i n e s , t h e p o s s i b i l i t y of 100% energy convers ion and u n i d i r e c t i o n a l i t y becomes e v i d e n t . ,'o achieve t h i s it is n e c e s s a r y to use an e x c i t i n g f requency which o c c u r s a t a

+ r A + 4 r A + + r A - t

-PA - Fig. 1 Schematic diagram of the Travel l ing :lave

Transducer.

n u l l i n t h e ( s i n X / X ) response of t h e unwanted SAW and, of c o u r s e , t o have t h e c o r r e c t number of c o u p l i n c elements . an estimate based on coupled

"1975 Ultrasonics Symposium Proceedings, IEEE Cat # 75 CHO 994-SU" 422

.>unton, Lewis and Pa ige

mode t h e o r y ( 2 ) Xives t h e number of p-oups o f f i n s e r s , N I n e c e s s a r y t o g i v e 10% convers ion as n / ( 2 G ) , w h e r e r) is t h e enerzy convers ion e f f i c i e n c y o f t h e ;;roup. (The :;roup may c o n s i s t o f a f i n g e r p a i r , or appear as a number of f i n g e r p a i r s which may o r may n o t be tuned.)

One of t h e well-known problems o f l o n g , p e r i o d i c t r a n s d u c e r s t r u c t u r e s is m u l t i p l e r e f l - e c t i o n s o f t h e SAW between f i n g e r s which d i s t o r t s t h e p a s s band r e s p o n s e at t h e c e n t r e f requency. The TWT, by movine t h e c e n t r e f requency from f o

t o f or f d i m i n i s h e s t h e problem. P aP'

R e a l i s a t i o n o f t h e TWT and i ts performance

Because t h e v e l o c i t y o f t h e EMW is not i n f i n i t e , t h e TWT argument a p p l i e s t o any t r a n s - ducer . ducer s t r u c t u r e is about I O , so t h a t V = I O Va

and, from e q u a t i o n ( I ) , t h e f r a c t i o n a l s p l i t t i n g -4 is approximate ly 2.10 . Thus a t r a n s d u c e r of

about 5OGO wavelengths would be r e q u i r e d t o r e s o l v e t h e s p l i t t i n g . T h i s is abnormally l a r g e f o r a t r a n s d u c e r and is presumably why t h e s p l i t t i n g e f f e c t h a s gone unnoted. ( I t is not n e c e s s a r y t o have a p a i r o f o u t p u t t r :csducers as shown i f P i g . 1 ; t h e s p l i t t i n g w i l l appear i n an untermina ted t r a n s d u c e r because t h e E X W t r a v e l s i n both d i r e c t i o n s owing t o r e f l e c t i o n . )

The s lowing o f an EMW on a t y p i c a l t r a n s -

I I I I I

91 93 95 MHz

To enhance t h e s p l i t t i n g t e 1' ta may b e

i n c r e a s e d by v a r i o u s means: ( i j c o a x i a l c a b l e ; ( i i ) m i c r o s t r i p l i n e ; ( i i i i use o f a h i e h d i - e l e c t r i c s u b s t r a t e and ( i v j s u i t a b l e d e s i s n o f t h e t r a n s d u c e r u s i n g c o n v e n t i o n a l s u b s t r a t e s . The f i r s t two achieve t h e d e l a y by p h y s i c a l l y i n c r - e a s i n g t h e EMW p a t h l e n g t h . J?he t h i r d one i n v o l v e s making t h e t r a n s d u c e r s t r u c t u r e i t s e l f i n t o a slow-wave t r a n s m i s s i o n l i n e . 'The l a s t d e s c r i b e s a d e v i c e i n which t h e d e l a y l i n e is inc luded i n t h e t r a n s d u c e r d e s i g n and is i n t e g r a l w i t h i t , r e q u i r i n g one s u b s t r a t e and one e t c h i n g p r o c e s s t o produce t h e complete device . PIost o f t h e r e s u l t s p r e s e n t e d h e r e employed ( i ) f o r con- venience but similar r e s u l t s have been observed u s i n g (ii) and ( i v ) . A11 t h e observed Prequency s p l i t t i n g s a r e i n accordance w i t h e q u a t i o n (I).

The performance of a p r a c t i c a l d e v i c e is i l l u s t r a t e d i n F ig . 3 ( a ) - (d) . The d e v i c e c o n s i s t e d of a 95 MHz c e n t r e f requency d e l a y l i n e w i t h a wi3e-band t r a n s d u c e r o f l e n g t h 601 and a s i n g l e narrow-band t r a n s d u c e r w i t h 20 g o u p s of 3 f i n g e r p a i r s c e n t r e d 20% a p a r t . Its r e s p o n s e when e x c i t e d from a s o l i d busbar is shown i u Fig. 3 ( a ) ; t h e peak o c c u r s a t f . F i g s . j ( b ) and 3 ( c )

show t h e r e s p o n s e s a t f and f o b t a i n e d when

t h e r e w a s i n t r o d u c e d a d e l a y between s u c c e s s i v e groups o f f i n g e r p a i r s of approximate ly 0.6 nS by means o f c o a x i a l cab le . The combined r e s p o n s e , w i t h peaks a t f and f s i m u l t a n e o u s l y , is i n

F ig . 3 ( d ) and was produced by o p e n - c i r c u i t i n g t h e

P aP

P aP

I I I I I

Fig. 3 The d e t e c t e d frequency r e s p o n s e s of a TWT s t r u c t u r e . a ) Busbar e x c i t a t i o n , fo

b ) Forward SAW o u t p u t , f

c ) Reverse SAW o u t p u t , f

d) Combined o u t p u t , f and f

P

aP

P aP

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Gunton, Lewis and Pa ige

d e l a y l i n e a t one end. The s p l i t t i n g i s about 0.5 HIlz, or a f r a c t i o n a l s e p a r a t i o n from t h e o r i g i n a l c e n t r e f requency o f about f I p a r t i n 400. C a r e f u l examinat ion o f t h e main peaks r e v e a l s t h a t t h e d i s t o r t i o n p r e s e n t i n t h e c o n v e n t i o n a l t r a n s - ducer response and a t t r i b u t e d t o m u l t i p l e r e f l e c t - i o n s is e l i m i n a t e d i n t h e d i s p l a c e d responses .

An i l l u s t r a t i o n of t h e post-manufacture t u n i n g c a p a b i l i t y is given i n Fig. 4 ( a ) i n which t h e r e v e r s e SAW r e s p o n s e of a d e v i c e des igned t o have a va lue o f t h e c e n t r e f requency f o of 60.7 MHz h a s been s h i f t e d as shown by i n t r o d u c t i o n o f c o a x i a l c a b l e d e l a y l i n e s o f t h e l e n g t h s shown on the f i g u r e .

A p p l i c a t i o n s o f t h e TWT

Without e x c e s s i v e d e l a y o f t h e (which i n t h e l i m i t matches t h e a c o u s t i c d e l a y ? Y ) t h e TWT c h a r a c t e r i s t i c s d e s c r i b e d h e r e a r e o n l y a p p a r e n t i n narrow bandwidth t r a n s d u c e r s . Thus a t r a n s - ducer w i t h narrow nd f requency o p e r a t i o n , eg f o r MFSK requirements'", may be r e n d e r e d e f f i c i e n t and may be p r e c i s e l y tuned through t h e f i n e c o n t r o l o f f e r e d by a s low wave c i r c u i t . F u r t h e r , t h e number o f f i l t e r s r e q u i r e d is reduced by a f a c t o r o f two by u s i n g both f and f

aP' P

a t i o n o f a narrow band f requency d i s c r i m i n a t o r . A r e d u c t i o n i n t h e number o f f i n g e r p a i r s o u p s from 20 t o 8 i n t h e d e v i c e d i s c u s s e d i n r e l a t i o n t o Fig. 3 broadened t h e bandwidth o f each response s u f f i c i e n t l y t o g i v e a s u i t a b l e o v e r l a p and t h e o u t p u t from a d i f f e r e n t i a l a m p l i f i e r when t h e

Another a p p l i c a t i o n of t h e TWT is t h e form-

Fig. 4 a )

b )

17cm

36 cm

63cm

88crn

$0 61 MHz

93 95 MHz

Demonstrat ion o f t h e t u n i n g c a p a b i l i t y o f t h e TWT. The l e n g t h o f c o a x i a l c a b l e between a d j a c e n t groups o f f i n g e r p a i r s t o produce t h e EMW d e l a y a p p r o p r i a t e t o each curve is shown on t h e f i g u r e .

The r e s p o n s e o f a TWT based f requency d i s c r i m i n a t o r .

d e t e c t e d TWT r e s p o n s e s were used as i n p u t s is shown i n F ig . 4 ( b ) : t h e device oGera tes ass& f requency d i s c r i m i n a t o r over t h e range 94,75 t b ) 95.25 MHz.

We have a l s o used t h e d i s c r i m i n a t o r p r i n c i p l e t o demonstrate t h e TWT e f f e c t i n a normal l a d d e r t r a n s d u c e r v i t h convent iona l b u s b a r s , and t o de te rmine t h e s p l i t t i n g d f = f v e l o c i t y i n t h i s t r a n s d u c e r s t r ) u c t u r e f o r which A f was t o o small t o measure d i r e c t l y . The d i s c r i m i n - a t o r was f e d from t h e two ends o f t h e l a d d e r s t r u c t u r e i n a s i m i l a r manner t o t h a t f o r F ig . b(b). I f t h e o u t p u t o f t h e d i s c r i m i n a t o r is V when t h e

d e t e c t e d r e s p o n s e c e n t r e d on f is t h e o n l y i n p u t ,

and is V when f i s t h e o n l y i n p u t , t h e

d i s c r i m i n a t o r o u t p u t when both i n p u t s a r e used is

- fap and t h e EMU

P P

aP aP

A V = V - V P aP

where X = Nn( f - f o ) / fo . (N is t h e e f f e c t i v e number o f a c o u s t i c wavelengths i n t h e l a d d e r t ransducer . ) The approximate e x p r e s s i o n is v a l i d f o r small s p l i t t i n g and square- l a w d e t e c t o r s . I n t h e s e c i rcumstances t h e s e p a r a t i o n of t h e p o s i t i v e and n e g a t i v e peaks i n AV is independent o f d f bu t t h e ampl i tude o f AV is d i r e c t l y p r o p o r t i o n a l t o A f , as shown i n e q u a t i o n (2 ) . I n t h e p a r t i c u l a r l a d d e r t r a n s - ducer i n v e s t i g a t e d we deduced a f r a c t i o n a l s l l i t t i n g of 8.10-5 and a r e d u c t i o n i n t h e EMW v e l o c i t y by a f a c t o r o f 4.1 from its f r e e s p a c e va lue .

A SAW o s c i l l a t o r h a s been made i n which t h e t u n a b i l i t y and t h e i n h e r e n t low i n s e r t i o n l o s s o f t h e TWT are e x p l o i t e d . An i n t e r e s t i n g p o i n t concerns t h e mode s e l e c t i o n requi rement ; s t r i c t l y t h i s can o n l y be met by a d j u s t i n g t h e i n p u t and o u t p u t t r a n s d u c e r e q u a l l y , b u t i n p r a c t i c e a use- f u l r a n g e can b e covered by trimming j u s t one t r a n s d u c e r .

Conclusion

We have d e s c r i b e d a d e v i c e which e n a b l e s u s t o e x p l o i t t h e wave n a t u r e o f t h e e l e c t r o m a g n e t i c feed t o a SAW t r a n s d u c e r . We have demonstrated a SAW t r a n s d u c e r w i t h a f i n e l y c o n t r o l l a b l e f requency r e s p o n s e and w i t h reduced m u l t i p l e r e f l e c t i o n e f f e c t s . Although low convers ion e f f i c i e n c y h a s n o t y e t been achieved , u n i d i r e c t i o n - a l i t y is a p p a r e n t i n Figs . 3 ( b ) and j ( c j . I n p r e s e n t i n g t h e r e s u l t s of t h e TWT we have g i v e n a c l e a r demonst ra t ion o f t h e e f f e c t s o f t h e f i n i t e v e l o c i t y o f t h e EMW f e e d i n g a t r a n s d u c e r . As SAW d e v i c e s become more s o p h i s t i c a t e d and t h e s p e c i f i c a t i o n t o be met becomes more demanding we a n t i c i p a t e t h a t t h e e f f e c t s demonstrated h e r e w i l l become i n c r e a s i n g l y r e l e v a n t i n t h e i r des ign .

Acknowledgments

The a u t h o r s wish t o thank A.S. Young and D.J. S n e l l f o r t h e i r a s s i s t a n c e w i t h t h e p r o d u c t i o n of t h e exper imenta l SAW devices .

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