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HF SPEC’PRWlETERS FOR HETZRODYNE R E C E I V E R S

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D Buhland M.J . t.lurnma I n f r a r e da n dR a d i oA s t r o n o m yE r a n c h NASA G o d d a r dS p a c eF l i 5 h t Center G r e e n b e l t , MD. 2077 1

Spectrallinereceiversrequire a spectrometer to d i v i d e the detected s i g n a l i n t o a number o f h i g h r e s o l u t i o n c h a n n e l s f o r d i s p l a ya n da n a l y s i s o f a. m o l e c u l a rl i n ep r o f i l e .S e v e r a lt y p e s of s p e c t r o m e t e r sh a v eb e e n Hi? f i l t e r s , m u l t i p l e d e v e l o p e df o rr a d i oa s t r o n o m yr e c e i v e r sw h i c hu t i l i z e o s c i l l a t o r s a n dm i x e r s ,d i g i t a la u t o - c o r r e l s t o r sa n da . c o u s t o / o p t i cd e v i c e s . I nt h ei n f r a r e d t h e h e t e r o d y n cr e c e i v e rp l a c e ss e v e r er e q u i r c n e n t so n the s p e c t r o x e t e r ,p a r t i c u l a r l y i f i t i s t o beused with B balloon or space fliTht receiver. 30 y e a r s ago w i t h t h e d i s c o v e r y o f l i n e s H e t e r o d y n es p e c t r o s c o p yb q a n d u e t o h y d r o g e n ( H I ) a n dt h e nh y d r o x y l ( O H ) i nv z r i o u sy a l a c t i c gas c l o u d s . T n e s ee a r l yr e c e i v e r so p p r a t e da t L-bend (1-2 GHz) andusedmaserand t o a c h i e v eh i g hs e n s i t i v i t y .S u b s e q u e n th i g h e r p a r a m e t r i cs m p l i f i e r s a n u m b e ro fg e i a c t i cm o l e c u l us p e c i e s( f i g u r e f r e q u e n c yr e c e i v e r sr e v e a l e d 1 ) . A c t i v i t y was p a r t i c u l a r l y i n t e n s e i n t h e mm s p e c t r u m ,l e a d i n g to the d i s c o v e r yo fo v e r 50 i n t e r s t e l l z rm o l e c u l e s . The s p e c t r a ll i n e so ft h c s e m o l e c u l e sc o n t i n u ei n t o t h e f a r i n f r a r e d ( F I R ) where l i n e i n t e n s i t i e s s h o u l di n c r e a s e .I na d d i t i o nt o t h e r o t a t i o n a ll i n c s shown i nf i q u r e 1 t h e r e a r ev i b r a t i o n 2 1l i n e so fm o l e c u l e si n t h e i n f r a r e d . T h i s is i l l u s t r a t e di n t h e e n e r g yl e v e ld i a g r a mi n f i g u r e 2. R o t a t i o n 3 1l i n e so f S i 0 a r e shown a s b a r s w h i l e v i b r a t i o n a l l i n e s a r e shown a s a r r o w sm a r k e d 6 and 4~ T h e r o t a t i o n q ll e v c ls e p a r a t i o n s are e x a g g e r a t e d , b e i n g a p p r o x i m a t e l y 1000 times w e z k e rt h a n t h e v i b r a t i o n a ll e v e l s . The m o l e c u l a r 2 ) shows a w i d t h o f 50 km/s w i t n a s p e c t r a l spF.ctrum f o rO r i o n( F i g u r e d e t a i lr e q u i r i n g a r e s o l u t i o n o f < 1 km/s. Hence t h es p e c t r o m e t e ra u s t be c a p a b l e of p r o v i d i n gs e v e r a lh u n d r e dc h a n n e l s w i t h ;-? t o t a l b a n d w i d t h a p p r o a c h i n g 100 km/s. A b l o c kd i a g r a m of a t y p i c a lh e t e r o d y n er e c e i v e r i s shown i n f i g u r e 3 . For most r e c e i v e r s i n t h e i n f r ; r r e dt h ep h a s e l o c ks y s t e m is g e n e r a l l y a f r e q u e n c ys t a b i l i z e r . I F a m p l i f i e r sv a r y from b a n d w i d t h s of 100 PlHz t o s e v e r a l GHz d c p e n d i n qo n t h e l i n e w i d t h s e x p e c t e d .G e n e r a l l y there is a t r a d e o f f between low n o i s e and widebandwidth.The G a A s FET a m p l i f i e r is now w i d e l yu s e db e c a u s e of i t s lobi n o i s e , power 2nd w e i y h t

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The HF s p e c t r o m e t e rh 2 sb e e nd e v e l o p e da t GSFC t o p r o v i d e v i d e b a n d w i d t h s ( > 1 GHz) a s well a s h i g hr e s o l u t i o n ( 5 HHz). T h i s i s a c c o m p l i s h e db yd i v i d i n gt h e 128 c h a n n e l f i l t e r b m k i n t o h i g h and l o w r e s o l u t i o ns e c t i o n s . Thc h i q hr e s o l u t i o ns e c t i o n is L u n e a b l eb yp r o v i d i n g a s e c o n dm i x e ra h e a d of t h e f i l t e r bank. T h i s i s n e c e s s a r yb e c a u s e qas lasers a s l o c a l o s c i l l a t o r s a r e o n l y i n f r a r e dr e c e i v e r sw h i c hu s e

373

t u n e a b l e t o s p e c i f i c l a s e r f r e q u e n c i e s . To c o m p e n s a t e for a s t r o n o m i c z . 1 a s e c o n dl o c a l o s c i l l a t o r d o p p l e r s h i f t s and m o l e c u l e f r e q u e n c y d i f f e r e n c e s a n dn i x e r is n e e d e d . A d i a g r a m of t h e RF s e c t i o n of t h e f i l t e r bank i s shown i n f i g u r e 4. l’his RF s p e c t r o m e t e r was d e s i g n e d f o r a lop CO l a s e rr e c e i v e r . The mixer i s a HgCdTe d i o d e madeby D. S p e a r s( L i n c o l n fab). Output of t h e m i x e r is u s e a b l e t o 1 . 5 GHz and so t h e low r e s o l u t i o n s e c t i o n o f t h e f i l t e r b a n kh a s a b a n d w i d t h of 1 . 6 GHz and a r e s o l u t i o n of 2 5 I4Hz. The h i p h a b a n d w i d t h of 320 MHz and a r e s o l u t i o n o f 5 KHz. r e s o l u t i o ns e c t i o nh a s T u n e a b i l i t y i s p r o v i d e db y a b a l a n c e dm i x e r2 n d o s c i l l e t o r s w h i c ht u n e from 1 . 6 t o 3 . 2 GHz. T h i s allows the h i g h r e s o l u t i o ns e c t i o n t o be s e t at; a n y p o i n tw i t h i n t h e l o wr e s o l u t i o ns e c t i o n where t h e s p e c t r a l f e a t u r e is l o c a t e d .T h i s i s i l l u s t r a t e di nf i y u r e 5 w h i c h shows a n atmosphgric a b s o r p t i o nl i n e of Ozone as s e e na q a i n s tt h e s o l a r continuum.The left handspectrum is t h e low r e s o l u t i o ns e c t i o nr u n n i n g from 0 t o 1 . 6 GHz w i t h r e s p e c t t o t h e CO l a s e r f r e q u e n c y . The h i g hr e s o l u t i o ns e c t i o n on t h e r i g h t i s s e t on t2e wiqs of t h e l i n e p r o f i l e and shows a 320 MHz d e t a i l o f t h e w i n gs h a p e .I n terms of v e l o c i t y f o r a na s t r o n o m i c a ls o u r c e , 1 km/s is e q u a lt o1 0 0 MHz a t a m v e l e n q t ho f lop. Hence t h e f i l t e r bankbandwidth km/s a n dt h er e s o l u t i o n t o 0 . 2 5 k n / sa n d c o r r e s p o n d s t o 5 1 6 km/s a n d2 3 . 2 0.05 km/s f o r t h c two s e c t i o n s . The RF d i a g r a m i n f i g u r e 4 s t a r t s a t t h e o u t p u t of t h e HqCdTe m i x e r where t h e RF s i g n a l goes t h r u a b i a s T and a low n o i s ep r e a m pl o c a t e dc l o s e t o t h em i x e rd e w a r . A v a r i a b l ea t t e n u a t o r i s a t t h ei n p u tt o the filter bank rack a n dp r o v i d e s a l e v e l s e t a s well a s a r e i n o t e l yc o n t r o l zero f u n c t i o n f o r r e c o r d i n gt h e DC c h a n n e l o f f s e t s . The s i g n a l i s t h e ns p l i ti n h a l f f o r t h e 0-1and 1-2 Gh‘z a m p l i f i e rc h a i n s . After b o o s t i n gt n es i z n a l l e v e l t o 100 mw i t i s s p l i t by power d i v i d e r s and f e d t o t h e i n d i v i d u a l f i l t e r t r a y sw h i c hc o n t a i n 8 f i l t e r s e a c h .I na d d i t i o nt h e 0.1-2 GHz s i g n a l i s s p l i tt od r i v et h eb a l a n c e dn i x e ra n d t h e high resolution f i l t e r s a r e p r e s e n ti n t h e h i g h a m p l i f i e rc h a i n s .V a r i o u sb a n d p a s s r e s o l u t i o ns e c t i o nt op r e v e n ta m p l i f i e rs a t u r a t i o n . The RF s i g n r lf r o m e a c h f i l t e r i s p a s a e dt h r o u g h a s q u a r e l a w d e t e c t o r so t h a t t h e v o l t a s e A v a r i a b l eg a i n op amp b o o s t s o u t p u t is p r o p o r t i o n a l . t o t h e p o w e ri n p u t . t h e s i g n a ll e v e la n dt h e n t h e i n d i v i d u a lc h a n n e lo u t p u t s are s e n t t o an t h e s i g n a l st o1 2 bit i n t e g r a t o r / m u l t i p l e x e rw h i c hi n t e g r a t e sa n dc o n v e r t s numberswhich a r e a c c u m u l a t e ds e p a r a t e l ya ss i g n a l ar.d r e f e r e n c e i n a. 32 b i t m m o r y .F u r t h e rs y n c h r o n o u sd e t e c t i o na n ds p e c t r a lp r o c e s s i r , ? are done by a n LSI-11 c o m p u t e r o r HP 9830 c a l c u l a t o rc o n n e c t e d t o a T e k t r o n i x t e r m i n a l . T h i s p r o v i d e so nl i n es p e c t r a la n a l y s i sc a p a b i l i t y f o r immediate r e t r i e v a l of m o l e c u l a r l i n e p r o f i l e s .

A 6 4 c h a n n e l f i l t e r b a n k wasdeveloped f o r u s ew i t hl o pa n d 5OOp r e c e i v e r s . The l o p c o n f i g u r a t i o n i s s i m i l a r t o t h ef i l t e r b a n k d e s c r i b e d p r e v i o u s l y . The 5 0 0 p s y s t e mr e q u i r e sn a r r o w e rb a n d w i d t h ss i n c e the doppler s h i f t , is 2 Mliz f o r 1 km/s. The b a n kc o n t a i n s a 5 MHz low r e s o l u t i o n a 1 MHz h i u hr e s o l u t i o ns e c t i o nw h i c h are b o t ht u n e a b l e . The s e c t i o na n d RF s e c t i o n of t h e s p e c t r o m e t e r is shown i nf i g u r e6 . A t the input to the f i l t e r bank i s a mixer d r i v e nb y two t r a n s i s t o r o s c i l l a t , o r s whichcan b e s e l e c t e d t o a c q u i r e ar,y RF f r e q u e n c yu p t o 5 GHz. F o l l o w i n g this t h e r e i s 374

an o p t i o m l a m p l i f i e r a n d a v a r i a b l e a t t e n u a t o r t o p r o v i d et h e c o r r e c t g a i n o r a t t e n u a t i o n t o s e t t h e l e v e l i n t h e f i l t e r c h a n n e l s . Two a m p l i f i e r sa n d a f i l t e r b o o s ta n db n n d l i m i t t h e s i g n a lb e f o r e it i s s p l i t i n t o t h e h i g h is s e n t a n d low r e s o l u t i o n p a t h s . A t t h i s p o i n tt h eh i g hr e s o l u t i o ns i g n a l t h r ua n o t h e r mixer t o down c o n v e r t i t t o 165 MHz. F i n a lp o w e ra m p l i f i e r s f i l t e r t m y s t h r o u g h a p o w e rs p l i t t e r .T h e rest t h e nd r i v et h ei n d i v i d u a l of t h es y s t e r n is similar t o t h c 128 c h a n n e l f i l t e r b a n kT . he 64 c h a n n e l E lop d i o d e l a s e r receivei' ( r e f . 1 ) . s p e c t r o m e t e rh a sb e e nu s e dw i t h F i g u r e 7 s h o w s some 32 c h a n n e l s p e c t r n of e x c e s s n o i s e it? t h e m i x e rc a u s e d b yn o i s es i d e b a n d sa n dm u l t i p l em o d e si nt h ed i o d e laser local o s c i l l a t o r . The f i r s t t h r e e( a , ba n dc ) show b r o a d b a n dn o i s e w i t h d i f f e r e n ts p e c t r a l two ( d a n d e ) show b e a t s b e t w e e nd i f f e r e n t modes of t h e s h a p e s .T h en e x t l a s e r . T h en u m b e r si n s i d e t h e f i g u r e si n d i c a t et h er e d u c t i o n factors which were u s e do nt h es p e c t r a( a , da n d e > . The l a s t s p e c t r as h o w st h e noise of t h ep r e a m p w h i c h h a s been s u b t r a c t e d from t h e o t h e r s p e c t r a . T h e s e f i q u r e si l l u s t r a t et h ep r o b l c m of d e s i a n i n q a s p e c t r o m e t e rw h i c h i s a b l e t o cope w i t h t h e l a r g e d y n a m i cr a n g ei n t h e n o i s ea n d its chanT;e i n s p e c t r a l s h a p e ,w h i c h i s d e t e r m i n e db y t h e m i x e r RF r e s p o n s ea n d t h e n o i s ep r o f i l e of t h e local o s c i l l a t o r . The 5 / 1 KHz c o n f i g u r a t i o n of t h e f i l t e r b a n k i s s c h e d u l e d t o b eu s e d w i t h a 5 0 0 r~e c e i v e r ( r e f . 2 , 3 a n d 4 ) . F o ra na s t r o n o m i c a ls p e c t r o s c o p y e x p e r i m e n ta t t h e MASA ?in t e l e s c o p e on FIauna Kea i n ;.lay, 19d0. D e s p i t et h e l a r q e a t m o s p h e r i ca b s o r p t i o n ( > 10 d b )s e v e r a li n t e r s t e l l a r or planetary m o l e c u l e ss h o u l db ed e t e c t e d T . his is t h e f i r s t major a s t r o n o m i c a l t h e 500 m i c r o nr e q i o na ns h o u l dq i v ea n s p e c t r o s c o p ye x p e r i m e n ti n o f f a r i n f r z r e dm o l e c u l a r i n d i c a t i o n of t h e i n t e n s i t y ,w i d t ha n dc o m p l e x i t y p r o f i l e s o n whicn t o base f u t u r e s p e c t r o m e t e r d e s i z n s . T h e RF s p e c t r o m e t e r will c o n t i n u e t o p r o v i d e t h e b e s t me?ns of a c h i w i n gu l t r a - w i d eb a n d w i d t h s f o r i n f r a r e dh c t e r o t i y n er e c e i v e r s . For h i g h r e s o l u t i o nw i t h a l a r s e n u m b e ro fc h a n n p l s , thP acousto/optical s p e c t r o m e t e r will be t h e p r i n c i p l ei n s t r u r n s n t ,p a r t i c u l a r l y for balloon or sp.ace f l i g h ta p p l i c a t i o n s ( r e f . 5 ) .

375

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REFERENCES K o s t i u k , T . ; J e n n i n g s , D. E . ; and Mumma, M. J . : Development and Performance of a Laser Heterodyne Spectrometer Using Tunable Semiconductor Lasers as LocalOscillators.HeterodyneSystemsand Technology, NASA CP-2138 , 1980. (Paper 1 5 o f t h i s c o m p i l a t i o n . )

1. G l e n a r , D . ;

2. Tannenwald,P. E.: FarInfraredHeterodyneSystems.HeterodyneSystems andTechnology, NASA CP-2138, 1 9 8 0 .( P a p e r2 5o ft h i sc o m p i l a t i o n . )

3. Koepf,Gerhard A.: Sub m L a s e rL o c a lO s c i l l a t o r s :D e s i g n Criteria and Results.HeterodyneSystemsandTechnology, NASA CP-2138, 1980. ( P a p e r 32 o f t h i s c o m p i l a t i o n . )

4. McAvoy,

N.: S u b m i l l i m e t e r Wave HeterodyneRadiometry. NASA paper p r e s e n t e d a t theHeterodyneSystemsandTechnologyConference (Williamsburg, V a . ) , Mar. 25-27, 1980.

5 . Chin,Gordon;

B u h l , David;andFlorez,Jose M.: Acousto-OpticSpectrometer f o rR a d i o Astronomy.HeterodyneSystemsandTechnology, NASA CP-2138, 1 9 8 0 (. P a p e r2 9o ft h i sc o m p i l a t i o n . )

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