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characteristics under all temperatures. (The rubber mounts tend to harden at low temperatures.) The isolator trans-- missibility is designed to have a minimal resonance rise while providing a high attenuation of vibration input,$ at, high frequencies . Vibration-induced sidebands of the VCXO output, including those at the isolator resonance rise, are minimi, ed by a 6 fast VCXO control servo loop (bandwidth =I00 Hz).

Vibration Sensitivitv Design Considerations

Sources of vibration sensitivity in a conventional rubidium frequency standa d have shown to be both the VCXO and t h e physics package.? Since the output of a r~~bidiurn frequency standard is essentially that of a VCXO, vibrat ion-i nduccd sidebands of the VCXO appear directly in the Prequuncy stan- dard output at all, vibration f'requencies. In most frequenc~j standard applications, either as a clock or as a stable, low-noise frequency source, the vibration-induced sideband:< do not appear to be a serious drawback at high vibration frequencies. In order to improve the l o w freqilency vibra- tion sensitivity of the Litton TRFS over that of a conven- tional rubidium frequency standard, a large bandwi th !=I00 Hz) is implemented in the VCXO control servo loop. g Unlike a VCXO, the physics package is not intrinsically vibration sensitive. Vibration sensitivity of the physics package is generally considered as an engineering design challenge. In a typical mechanization of a rubidjum fre- quency standard such as the Litton TRFS, rubidium atomic resonance signal is in the form of a modulated (ac) light intensity riding on top of a large dc light background. The ac portion of the light, i.e., the resonance signal, is an extremely small fraction of the total 1 ight in tens it,^ detected by the photodetector. The ac signal is demodu- lated in a conventional phase sensitive detector, and pro- cessed further to generate dc control voltage for the VCXO.

Any spurious modulation in the detected dc light intensity in addition to the resonance signal, generates an erroneous control voltage. Such an effect is most significant when the spurious modulation frequency is close to the modula- tion frequency (fmOd) of the phase sensitive detector. The spurious modulation may be a result of, for example, periodic displacement of the lamp with respect to the photodetector under vibration. The TRFS physics package is designed for maximal mechanical integrity or the whole while allowing for adequate thermal isolation of the different components operated at different temperatures.

Vibration-induced sidebands of the VCXO also cause spur'ious light intensity modulation. The VCXO control servo loop is upset when the vibration frequency is close to 2xfmod. in order to reduce the vibration input at this frequency, the Litton TRFS incorporates a shock-mounted VCXO.

E l e c t r i c a l D e s i g n ----

One i m p o r t a n t a s p e c t o f t h e e l e c t r i c a l d e s i g n is t o m i n i - m i z e s p u r i o u s s i g n a l s w h i c h may u p s e t t h e V C X O c o n t r o l s e r v o l o o p r e s u l t i n g i n a n e r r o n e o u s o u t p u t f r e q u e n c y . Any s p u r i o u s s i g n a l a t f r e q u e n c i e s c l . o s e t o f m O d u p s e t s t h e V C X O c n n t , r o l l o o p a s a s p u r i o u s l i g h t i n e n s i t y m o d u l a t i o n d o e s . The i n t e r f e r e n c e e f f e c t s o b s e r v e d ' i n a r u b i d i u m f r e q u e n c y s t a n d a r d o p e r a t i n g i n t h e v i c i n i t y o f o t h e r s d e m o n s t r a t e s t h e e f f e c t o f t h e s p u r i o u s s i g n a l p i c k - u p . S p u r i o u s s i g n a l s may b e p i c k e d up from t h e v o l t a g e r i p p l e p r e s e n t i n t h e i n p u t p o w e r l i n e , i f n o t r e g u l a t e d a d c q u n t , e l y . The L i t t o n T R F S a c c e p t s two s e p a r a t e p o w e r i n p u t s : one f o r a l l h e a t e r s a n d t h e o t h e r f o r a l l e l e c t r o n i c s . B o t h p o w e r i n p u t s may b e 22V t o 3 3 V D C w i t h r i p p l e s a s l a r g e a s 3Vrrns a t a u d i o f r e q u e n c i e s , a n d may c o n t a i n h i g h v o l t a g e t r a n s i e n t s .

V o l t a g e r e g u l a t i o n f o r e l e c t r o n i c s is a c c o m p l i s h e d by a s w i t c h i n g D C / D C c o n v e r t e r f o l l o w e d by a l i n e a r r e g u l a t i o n s t a g e . T h i s c o m b i n a t i o n p r o v i d e s a d e q u a t e r i p p l e r e g u 1 , z - t i o n a n d c o n s t a n t p o w e r d i s s i p a t i o n a t a l l i n p u t v o l t a g ~ ? c o n d i t i o n s . The f e a t u r e o f c o n s t a n t p o w e r d i s s i p a t i o n m i n i m i z e s i n p u t v o l t a g e d e p e n d e n t f r e q u e n c y s h i f t ( v i a t e m p e r a t , u r e dependence o f o u t p u t f r e q u e n c y ) . The 6 . 8 GHz r e s o n a n c e i n t e r r o g a t i o n f r e q u e n c y i s g e n e r a t e d by a s t e p r e c o v e r y d i o d e (SRD), w h i c h is d r i v e n by =40 MHz. Direct f ' r e q u e n c y s y n t h e s i s t e c h n i q u e is u s e d t o g e n e r a t e 40 MHz from t h e V C X O 10 MHz,

The r l l b i d i ~ l r n resonance s i g n a l i f 3 d e t e c t e d i n a c o n v e n t i o n a l way w i t h s i n u s o i d a l m o d u l a t i o n t e c h n i q u e . The m o d u l a t i o n w a v e s h a p e is g e n e r a t e d p i e c e w i s e d i g i t a l l y t h r o u g h d i v i d i n g t h e VCXO 10 MHz a n d a d d j n g t h e r e s u l t a n t s q u a r e waves w i t h p r o p e r a m p l i t u d e s .

T e m p e r a t u r e c o n t r o l c i r c u i t r y , w h i c h m a i n t a i n s t h e l a m p a n d t h e c a v i t y a t e l e v a t e d t e m p e r a t u r e s , i s o f h i g h g a i n , a n d u t i l i z e s b o t h p r o p o r t i o n a l a n d i n t e g r a l g a i n . C o m b i n a t i o n o f u n i q u e c i r c u i t d e s i g n a n d t h e s t r i p h e a t e r m i n i m i z e t h e s t r a y m a g n e t i c f i e l d g e n e r a t e d by h e a t e r c u r r e n t .

The e n t i r e c i r c u i t r y i s c o n t a i n e d i n f i v e c i r c u i t c a r d s , e a c h h a v i n g t h e size o f " 3 1 f x 3 f t . Each c a r d c o n t a i n s more t h a n o n e l a y e r o f g r o u n d p l a n e w h i c h a c t s a l s o a s h e a t p a t h s f o r e l e c t r o n i c c o m p o n e n t s , N e i t h e r h y b r i d s n o r L , S l l s a r e u s e d i n t h e e n t i r e d e s i g n ; a l l c o m p o n e n t s a r e d i s c r e t e d e v i c e s .

A1.1 m a t e r i a l , c o m p o n e n t s , p r o c e s s e s a n d c o n s t r u c t i o n m e t h o d s a r e i n a c c o r d a n c e w i t h t h e g o v e r n i n g m i l i t a r y s p e c i f i c a t i o n s a n d / o r w i t h t h e a c c e p t e d m i l i t a r y p r a c t i c e .

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p e r p e n d i c u l a r . tc tile Shsep::ri t . ~ ; 3 r - i ~ ~ ; r i : , ~ ~ f : . . ,.! -- -- I (3 f r e q u e n c y s h i f t wi t r ~ i n 5 2 ~ ; cj ~~ r1 : : l e~~ 3 : . , . .. ! I

f r e q u e n c i e s e x c e p t as not,c::j ? ? l o w .

A l a r g e u n e x p e c t e d f r e q u e n c y s h i f t was o b s e r v e d i n F i g u r e 3 b , when v i b r a t i o n f r e q u e n c y i s c l o s e t o f m O d . I n v e s t i g a - t i o n r e v e a l e d t h a t a f a s t e n i n g s c r e w was l o o s e d u r i n g t h e t e s t c a u a i n g t h e mic rowave c a v i t y r o c k e x c e s s i v e l y w i t h i n t h e p h y s i c s p a c k a g e .

We n o t e t h a t no m e a s u r a b l e f r e q u e n c y s h i f t was o b s e r v e d a t v i b r a t i o n f r e q u e n c i e s c l o s e t o 2 x f m O d , and t h a t o u t p u t f r e q u e n c i e s a r e w e l l b e h a v e d a t low v i b r a t i o n f r e q u e n c i e s . No l o s s o f r e s o n a n c e l o c k w e r e s e e n d u r i n g t h e m e a s u r e m e n t .

Audio S u s c e ~ t i b i l i t v

Audio s u s c e p t , i b i l i t , y r e f e r s t o t h e T R F S o u t p u t f r e q u e n c y d e p e n d e n c e on t h e a u d i o f r e q u e n c y vo1 t )age r i p p l e p r e s e n t i n t h e i n p u t power. Tes t was c o n d u c t e d w i t h a 3Vrms r i p p l e a d d e d t o t h e n o m i n a l d c v o l t a g e s a p p l i e d t o t h e power i n p u t f o r e l e c t r o n i c s . The r i p p l e f r e q u e n c y was v a r i e d s l o w l y f rom 5 Hz t o 2K Hz d w e l l i n g f o r a n e x t e n d e d p e r i o d o f time a t r i p p l e f r e q u e n c i e s e q u a l t o 1 / 2 f r n o d , f m o d , and 2f, d . No f r e q u e n c y s h i f fOwas o b s e r v e d beyond t h e m e a s u r e m e n t p r e c i s i o n (12x10 ) .

Warm-up and Power Consumpt ion -

F i g u r e s 4a and 4b a r e t y p i c a l o u t p u t f r e q u e n c y b e h a v i o r d u r i n g warm-up a f t e r t e m p e r a t u r e s o a k a t room t e m p e r a t u r e ( n a t u r a l c o n v e c t i o n ) a n d a t -55OC ( f o r c e d a i r ) , r e s p e c - t i v e l y . Warm-up t e s t r e s u l t s a r e summar ized i n T a b l e I .

T a b l e 1 - L i - t t o n T R F S Warm-Up C h a r a c t e r i s t i c s

o Lamp S t a r t

Time After Power-On

Room Temp ------ -55OC ---

8 s e c 2 8 s e c

o R e s o n a n c e Lock 1 min 1 2 sec 2 min 22 sec

o warm-up t o 5 x 1 0 ~ ~ ~ 1 rnin 39 s e c 2 min 49 s e c f r o m t h e f i n a l f r e q .

o Total" S t e a d y S t a t e 10 ,4 watts 1 9 . 5 w a t t s Power

o T o t a l p e a k power 84 w a t t s 84 w a t t s d u r i n g warm-up f o r 20 sec f o r 42 s e c

T e m p e r a t u r e

No m e a s u r a b l e f r e q u e n c y s h f g t was o b s e r v e d beyond t h e mea-- s u r e m e n t p r e c i s i o n ( 5 2 x 1 0 - ) when t h e TRFS was s u b j e c t e d t o e n v i r o n m e n t a l t e m p e r a t u r e s t e p s r a n g i n g f r o m -55'C t o +71 O C .

O r i e n t a t i o n

A r u b i d i u m r e s o n a n c e f r e q u e n c y may b e o r i e n t a t i o n d e p e n d e n t u n d e r t h e i n f l u e n c e o f e a r t h m a g n e t i c f i e l d . D i s p l a c e m e n t of p h y s i c s e l e m e n t s w i t h r e s p e c t t o e a c h o t h e r u n d e r g r a v i - t a t i o n may a l s o b e a s o u r c e o f o r i e n t a t i o n d e p e n d e n c y . Tn a p r o p e r l y d e s i g n e d p h y s i c s p a c k a g e , b o t h o f t h e s e e f f e c t s a r e c o n s i d e r e d t o b e n e g l i g i b l e . P e r h a p s t h e most s i g n i f - i c a n t o r i e n t a t i o n d e p e n d e n c e o f t h e TRFS o u t p u t f r e q u e n c y a r i s e s f rom i t s t e m p e r a t u r e s e n s i t i v i t y . T h i s i s b e c a u s e t h e r a t e o f h e a t d i s s i p a t i o n , and t h e r e f o r e t h e b a s e p l a t e t e m p e r a t u r e o f t h e TRFS, d e p e n d on t h e o r i e n t a t i o n . $ t e m p e r a t u r e - r e l a t e d f r e q u e n c y s h i f t was r e p o r t e d by o t h e r s w h i l e m e a s u r i n g s t a t i c g - s e n s i t i v i t y o f a n o v e n - c o n t r o l l e d c r y s t a l o s c i l l a t o r .

The TRFS u n i t was p o s i t i o n e d a t v a r i o u s o r i e n t a t i o n s w h i l e i t s o u t p u t f r e q u e n c y was m o n i t o r e d c o n t i n u o u s l y . The u n i t was h e l d f i x e d a t e a c h o r i e n t a t i o n f o r a n e x t e n d e d p e r i o d o f time i n o r d e r t o o b s e r v e a n y t e m p e r a t u r e - r e l a t e d f r e - q u e n c y s h i f t . No m e a s u r a b l e f r e q u e n c y g $ i f t was o b s e r v e d a b o v e t h e m e a s u r e m e n t p r e c i s i o n ( 5 2 x 1 0 - 1 .

I n o u t V o l t a a e V a r i a t i o n

S i m i l a r a r g u m e n t p r e s e n t e d f o r t h e o r i e n t a t i o n d e p e n d e n c e a p p l i e s t o the i n p u t v o l t a g e d e p e n d e n c e , I f a u n i t d e p e n d s s o l e l y on a l i n e a r v o l t a g e r e g u l a t o r whose h e a t d i s s i p a t i o n d e p e n d s g r e a t l y on t h e i n p u t v o l t a g e l e v e l , t e m p e r a t u r e d e p e n d e n c e o f t h e o u t p u t f r e q u e n c y may b e m a n i f e s t e d a s i f i t w e r e t h e v o l t a g e d e p e n d e n c e . As d i s c u s s e d e a r l i e r , t h e L i t t o n TRFS i n c o r p o r a t e s a s w i t c h i n g DC/DC c o n v e r t e r f o l l o w e d by a l i n e a r r e g u l a t o r .

The L i t t o n u n i t was powered a t + 2 2 Vdc and t h e n t h e i n p u t , v o l t a g e was v a r i e d to 3 3 Vdc by s t e p s o f a few v o l t s . A t each v o l t a g e , t h e u n i t was o p e r a t e d f o r a n e x t e n d e d p e r i o d o f time t o o b s e r v e t e m p e r a t u r e - r e l a t e d f r e q u e n c y s h i f t . No m e a s u r a b l e f r e q u e n ~ y ~ s h i f t was o b s e r v e d w i t h measurement p r e c i s i o n of 52x10 . S h o r t - T e r m S t a b i l i t v

S h o r t - t e r m s t a b i l i t y o f 2 4 x 1 0-" a t 1 see a v e r a g i n g t i m e was m e a s u r e d w i t h p h a s e n o i s e S-70dB/Hz a t 1 Hz away f rom t h e 1 0 MHz c a r r i e r . Note t h a t t h e m e a s u r e d s t a b i l i t i e s a r e t h o s e o f p h y s i c s r e s o n a n c e - g f g n a l , n o t t h o s e o f V C X O . P h y s i c s s t a b i l i t y o f 52x10 1s o b t a i n a b l e , i f d e s i r e d , w i t h a m i n o r m o d i f i c a t i o n .

S U M M A R Y A N D ACKNOWLEDGEMENT

D e s i g n f e a t u r e s u n i q u e t o t h e L i t t o n TRFS a r e p r e s e n t e d b r i e f l y . P r e l i m i n a r y t e s t r e s u l t s a r e r e p o r t e d . D e m o n s t r a t e d p e r f o r m a n c e c h a r a c t e r i s t i c s o f t h e u n i t u n d e r v i b r a t i o n , e x c e l l e n t warm-up t i m e a n d t e m p e r a t u r e s e n s i t i v i t y b r e a k new g r o u n d s f o r t h e r u b i d i u m f r e q u e n c y s t a n d a r d t e c h n o l o g y t o b e b e n e f i c i a l t o many t a c t i c a l m i l i t a r y a p p l i c a t i o n s . We w i s h t o e x p r e s s o u r a p p r e c i - a t i o n f o r e n c o u r a g e m e n t s f r o m Mr. Richmond ( D i c k ) T e r r e l l , Dr. Emery Moore , M r . H a r r y D a u b e r t , M r . Alex H e r t z b e r g , a n d Mr. Dave D e u s e r . Much of o u r work h a s r e c e i v e d t e c h n i c a l s u p p o r t f r o m Mr. Chong C . Lee a n d Mr. Carl D a v i d s o n . T e c h n i c a l advices of Mr. Bruce G r o v e r , Mr. Howard Williams a n d Mr. Jim S t e e l e a r e a c k n o w l e d g e d .

REFERENCES

1 . Rome Air Deve lopment C e n t e r o f t h e A i r F o r c e Systems Command, C o n t r a c t No, F19628-83-C-0174 .

2 T . M . Kwon, H . W i l l i a m s and T . M c C l e l l a n d , P r o c . 3 7 t h Ann. F r e q . C o n t r o l Symp. ( 1 9 8 3 ) p . 1 2

3 M . T e t u , e t . a l . , Proc. 3 8 t h Ann. F r e q . Con t , ro l Symp. ( 1 9 8 4 ) ; i n p r i n t

4 . B . G r o v e r and T. Kwon, P r o c . 1 5 t h PTTI M e e t i n g ( 1 9 5 3 ) , p . 677

5 . P u r c h a s e d f rom F r e q u e n c y E l e c t r o n i c s I n c . , M i t c h e l F i e l d , N . Y .

6 . T . M . Kwon a n d T. Hahn, P r o c . 3 7 t h Ann. F r e q . C o n t r o l Symp. ( 1 9 8 3 1 , p . 1 8

7. T . Kwon, B . G r o v e r a n d H . W i l l i a m s , F i n a l T e c h n i c a l R e p o r t p r e p a r e d f o r RADC/USAF S y s t e m s Command u n d e r C o n t r a c t No. F19628-82-C-0042 ( R A D C - T R - ~ ~ - ~ ~ O March, 1 9 8 3 ) ; W i l l b e a v a i l a b l e t h r o u g h NTIS

8 . Norman H o u l d i n g , P r o c . 1 3 t h PTTI M e e t i n g ( 1 9 8 1 ) , p . 5 0 5

9. P r i v a t e c o m m u n i c a t i o n w i t h D r . J . Vig and D r . E . F i l l e r , US Army E H A D C O M , N . J .

QUESTIONS AND ANSWERS

ALAN JENDLY, OSCILLOQUARTZ: I h a v e a q u e s t i o n r e g a r d i r l g t h e g- s e n s i t i v i t y , t h e s t a t i c g - s e n s i t i v i t y .

M R . K W O N : A l l r i g h t .

M R . JENDLY: You m e n t i o n e d t h a t y o u a r e u s i n g a n o s c i l l a t o r h ' l i ich t y p i c a l l y h a s a s t a t i c g - s e n s i t i v i t y o f f o u r p a r t s i n t e n t o t h e m i n u s t e n p e r g .

M R . KWON: T h a t ' s c o r r e c t .

MR.JENDLY: Y o u a l s o m e n t i o n e d t h a t y o u a r e u s i n g i t a t a b a n d - w i d t h o f a b o u t 1 0 0 H z , I b e l i e v e . T h e n y o u s a y t h a t t h i s device, t h e c o m p l e t e d e v i c e , i s t o t a l l y i n s e n s i t i v e tu o r i e n t a t i o n .

My q u e s t i o n i s : I f y o u t a k e t h i s d e v i c e a n d s l o w l y r o t a t e i t b y 1 8 0 d e g r e e s , a n d 9 0 d e g r e e s w i t h i n t h e t h r e e a x e s , d o n ' t y o u m e a s u r e t h o s e f o u r p a r t s i n t e n t o t h e t e n ?

M R . K W O N : T h e a n s w e r t o t h a t q u e s t i o n i s n o , b e c a u s e we h a v e a v e r y f a s t s e r v o l o o p . S o , t h e o r i e n t a t i o n d e p e n d e n c e o f t h e V C X O i s s e r v o e d o u t by t h e p h y s i c s p a c k a g e .