APPROACHES TO THE REALIZATION OF IMPROVED CLOCK PERFORMANCE IN CESIUM BASED DEVICES

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APPROACHES TO THE REALIZATION OF IMPROVED CLOCK PERFORMANCE IN CESIUM

BASED DEVICES

R. Garvey

To cite this version:

R. Garvey. APPROACHES TO THE REALIZATION OF IMPROVED CLOCK PERFORMANCE IN CESIUM BASED DEVICES. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-185-C8-188.

�10.1051/jphyscol:1981821�. �jpa-00221716�

JOURNAL DE P H Y S I Q U E

CoZZoque C8, suppZ6ment au n0Z2, Tome 42, de'cembre 1981 page C8-185

APPROACHES TO T H E R E A L I Z A T I O N OF IMPROVED CLOCK PERFORMANCE I N C E S I U M B A S E D D E V I C E S

R.M. Garvey

Frequency & Time Systems, Inc., Beverly, MA, U. S. A.

A b s t r a c t - The p e r t u r b a t i o n s imposed upon an atomic system a s a r e s u l t of t h e i n t e r r o g a t i o n process i n a cesium c l o c k a r e examined. Clock system l i m i t a - t i o n s which a r e derived from o r a t t r i b u t a b l e t o the servo process a r e a l s o explored. A microprocessor c o n t r o l l e d sys tern appears to o f f e r many a t t r a c - t i v e approaches which reduce o r otherwise c o n t r o l t h e s e p e r t u r b a t i o n s r e s u l t - ing i n improved c l o c k performance.

I n t r o d u c t i o n - The i d e a l of an unperturbed atomic system forms t h e b a s i s of a l l c u r r e n t approaches to p r e c i s i o n frequency s t a n d a r d s and c l o c k s . The r e a l i t i e s of p r a c t i c a l d e v i c e s a r e f a r from t h i s i d e a l however. The degree t o which o f f s e t s can b e measured and maintained e s t a b l i s h e s t h e l i m i t of accuracy and s t a b i l i t y of c m - m e r c i a l and l a b o r a t o r y atomic frequency s t a n d a r d s and clocks .

Discussion - A fundamental approach to t h e improvement of c l o c k performance could b e g i n with a s t r a t e g y t o e l i m i n a t e o f f s e t s a r i s i n g from t h e i n t e r r o g a t i o n p r o c e s s ( 1 ) . Atomic systems o t h e r than cesium present some i n t e r e s t i n g pros- p e c t s i n t h i s regard. The ion s t o r a g e work of t h e group a t NBS s u g g e s t s t h a t a b s o l u t e a c c u r a c i e s b e t t e r than one p a r t i n 1015 w i t h s t a b i l i t i e s of l e s s t h a n 10-l6 may be a t t a i n e d with a small ensemble of cooled ions i n a Penning t r a p ( 2 ) . I n v e s t i g a t i o n s by Vessot e t . a 1 . ( 3 ) i n d i c a t e t h a t c r y 0 e n i c hydrogen masers have t h e p o t e n t i a l t o e x h i b i t s t a b i l i t i e s of 10-18 t o 10-l8 f o r 100-1000 second averaging times. Considering t h e c m p l e x i t y of t h e s e approaches it i s reasonable t o assume t h a t f o r some time to c m e , o n l y labor- a t o r y r e a l i z a t i o n s w i l l e x i s t .

The cesium atom p r e s e n t l y forms t h e b a s i s f o r t h e i n t e r n a t i o n a l d e f i n i t i o n of time. The reasons f o r t h e s u p e r i o r i t y of cesium devices i s d i r e c t l y a t t r i b u t a b l e t o t h e small s i z e of t h e frequency o f f s e t s which a r e n e c e s s a r y t o r e a l i z e p r a c t i c a l d e v i c e s . There i s l i t t l e reason t o b e l i e v e t h a t t h e l i m i t a t i o n s to accuracy and s t a b i l i t y of commercial cesium beam s t a n d a r d s a r e fundamentally dififerent f r m t h o s e of t h e world's v a r i o u s primary s t a n d a r d s . The world's clock ensembles a r e composed predominately of commercial cesium i n s t r u m e n t s . Although c l o c k e v a l u a t i o n i s undeniably more t r a c t a b l e with a l a b o r a t o r y ( p r i m a r y s t a n d a r d ) i n s t r u m e n t , t h e l e s s e r l i n e Q and t h e sometimes v a r i a b l e environmental c o n d i t i o n s encountered by commercial d e v i c e s r e q u i r e more f i n e s s e i n t h e i n t e r r o g a t i o n p r o c e s s .

Frequency o f f s e t s may be g e n e r a l l y c a t e g o r i z e d i n t o two a r e a s : those r e l a t - ing t o imperfect i n t e r r o g a t i o n and those r e l a t i n g t o imperfect u t i l i z a t i o n of t h e i n f o r m a t i o n gleaned from t h e i n t e r r o g a t i o n p r o c e s s . I n t h e former category a r e , f o r example, t h e c a v i t y phase s h i f t and second o r d e r Doppler c o r r e c t i o n s , magnetic f i e l d s t a b i l i t y and homogenity. I n t h e l a t t e r catagory might be included servo o f f s e t s o r inadequate servo g a i n .

The c a v i t y phase s h i f t e r r o r a r i s e s from an end-to-end phase d i f f e r e n c e between t h e two i n t e r r o g a t i n g regions of t h e beam a p p a r a t u s . I n l a b o r a t o r y devices beam r e v e r s a l provides a measure of t h i s e r r o r . The technique s u f f e r s however f r m

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981821

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an i m p r e c i s i o n i n t h e geometrical accuracy of t h e r e v e r s a l coupled with a d i s t r i b u - t i o n of phase a c r o s s t h e r f c a v i t i e s . Several techniques f o r reducing o r eliminat- ing t h i s e f f e c t have been advanced. ~ e c k e r ( ~ ) s u g g e s t s t h a t p a r t of the e f f e c t may be analysed by modulation a t m u l t i p l e s of the l i n e w i d t h . An i n t e r r o g a - t i o n technique u t i l i z i n g two i n t e r r o g a t i n g frequencies n e a r the cesium resonant frequenc has been advanced as a technique to e l i m i n a t e t h e end t o end phase s h i f t e f f e c t ( 5 7 . To t h e e x t e n t t h a t beam v e l o c i t y d i s t r i b u t i o n s a r e t r a j e c t o r y independent the d i s t r i b u t e d c a v i t y phase s h i f t i s eliminated as w e l l .

U t i l i z a t i o n of a superconducting Ramse c a v i t y s t r u c t u r e has a l s o been suggested a s a s o l u t i o n t o t h i s problem(6y. O p t i c a l s t a t e s e l e c t i o n and d e t e c t i o n of the atomic beam may provide a technique to produce beams without the i n h e r e n t v e l o c i t y d i s p e r s i v e t r a j e c t o r i e s of magnetic-gradient s t a t e s e l e c t i o n ( 5 ) . This has the p o t e n t i a l to make beam r e v e r s a l a m r e a c c u r a t e technique as well as providing m r e homogeneous v e l o c i t y d i s t r i b u t i o n s which can make raore t r a c t a b l e t h e c a l c u l a t i o n of second o r d e r Doppler c o r r e c t i o n s .

Apart from t h e i s s u e s of i n t e r r o g a t i o n a r e q u e s t i o n s of accuracy and s t a b i l - i t y i n t h e servo e l e c t r o n i c s . Servo o f f s e t s may a r i s e from leakage of m d u l a t i o n s i g n a l s i n t o the s i g n a l channel. Assymetries i n t h e demodulation process l i k e w i s e can produce frequency o f f s e t s . F o r t u n a t e l y t h e s e types of e r r o r s a r e r e l a t i v e l y e a s y t o d e t e c t v i a servo gain v s frequency experiments.

The magnitude and frequency response of t h e servo g a i n determine t h e e x t e n t t o which i n s t a b i l i t i e s i n the system flywheel o s c i l l a t o r a r e removed with r e f e r e n c e t o t h e atomic resonance. Optimization of system performance a t the loop u n i t y g a i n frequency i s c.ne c r i t e r i o n f o r t h e s e l e c t i o n of loop g a i n . I f t h e frequency stab- i l i t y of t h e system o s c i l l a t o r changes as a r e s u l t o f , f o r example, environ- mental p a r a n e t e r s optimum performance i s no longer a s s u r e d . Changes i n o t h e r system para- meters ( a v a i l a b l e beam s i g n a l , o s c i l l a t o r tuning s e n s i t i v i t y , e t c .) can produce changes i n loop g a i n . It i s t h i s p o s s i b i l i t y which makes it i m p r a c t i c a l i n cmmer- c i a 1 cesium beam instruments to i n c r e a s e dc loop g a i n by t h e a d d i t i o n of a second s e r v o i n t e g r a t o r : a decrease i n loop g a i n r e s u l t s i n servo i n s t a b i l i t i e s .

With these thoughts i n mind, we, have pursued concepts f o r a new cesium beam frequency standard which focus on t h e following:

1. Provide improved c o n t r o l of t h e cesium beam i n t e r r o g a t i o n . 2 . Sense and adapt to t h e e x t e r n a l environment.

3 . Make optimum use of a v a i l a b l e i n f o r m a t i o n .

To a l l o w implementation of t h e s e concepts, a microprocessor based system appears t o b e t h e b e s t approach. The s o p h i s t i c a t i o n of such a system allows optimum system c o n t r o l w e n during c l o c k turn-on and warm-up. The a b i l i t y to c o n t i n u a l l y a s s e s s t h e s t a t u s of i n t e r n a l system parameters as well as e x t e r n a l environmental para- meters allows one t o implement a d a p t i v e servos which can r e a c t i n such a f a s h i o n a s

t o provide b e t t e r timekeeping a b i l i t y . Several examples can s e r v e t o support t h i s i d e a .

A fundamental problem of unattended c l o c k o p e r a t i o n i s t h a t of lock a c q u i s i - t i o n a t turn-on. The somewhat u n p r e d i c t a b l e warm-up behavior of c r y s t a l o s c i l - l a t o r s may be accomodated by an a c q u i s i t i o n r o u t i n e which m n i t o r s o s c i l l a t o r oven s e r v o performance, which scans the o s c i l l a t o r tuning range examining e r r o r s i g n a l c h a r a c t e r i s t i c s and which a d j u s t s t h e servo loop time c o n s t a n t to minimize o s c i l - l a t o r d r i f t derived o f f s e t s .

Our approach t o t h e implementation of a microprocessor based servo i s shown i n block form i n F i g u r e 1. The e r r o r s i g n a l from t h e cesium i n t e r r o g a t i o n process i s sampled synchronously with t h e modulation and converted to d i g i t a l form.

rd---

I

El

1 9192 MHz 1 I DIGITAL TO ANALOG CONVERSION I Figure 1: Block Diagram of Microprocessor Controlled Cesium Beam Clock

C8-188 JOURNAL DE PHYSIQUE

Demodulation, s c a l i n g and i n t e g r a t i o n a r e implemented i n system s o f t w a r e . A f i n a l c o n v e r s i o n back t o an analog v o l t a g e p r o v i d e s a c o r r e c t i o n v o l t a g e t o t h e system o s c i l l a t o r . Such a d i g i t a l servo p r o v i d e s t r u e i n t e g r a t i o n without t h e u s u a l per- formance l i m i t s imposed by t h e l i m i t e d dc g a i n of analog i n t e g r a t o r s . Implementa- t i o n of more s o p h i s t i c a t e d and a d a p t i v e s e r v o s becomes r e a l i z a b l e .

S i m u l t a n e o u s time m u l t i p l e x e d e x a m i n a t i o n of o t h e r system parameters r e l a t i n g t o environment and t o t h e cesium i n t e r r o g a t i o n p r o c e s s a l l o w adjustment of £re- quency s e r v o p a r a m e t e r s o r implementation of o t h e r system s e r v o s so a s t o o p t i m i z e t o t a 1 system performance.

Conclusion - Many of t h e acknowledged s t a b i l i t y l i m i t i n g parameters a s s o c i a t e d w i t h cesium beam d e v i c e s can be b e t t e r c o n t r o l l e d using c o n t i n u o u s system i n t e r r o g a t i o n and c o n t r o l . Monitoring of system p a r a m e t e r s l i k e w i s e p r o v i d e s improved e s t i m a t e s o f c l o c k performance a s w e l l a s i n d i c a t i o n of i n c i p i e n t f a i l u r e . Such an approach s h o u l d l e a d t o improved performance of cesium based d e v i c e s not o n l y i n l a b o r a t o r y e n v i r o n m e n t s b u t e s p e c i a l l y i n f i e l d a p p l i c a t i o n s .

A r e d u c t i o n i n environmental s e n s i t i v i t y of ensemble member c l o c k s w i l l produce enhanced s t a b i l i t y of TAI and o t h e r time s c a l e s p a r t i c u l a r l y w i t h r e s p e c t t o long term ( e . g . s e a s o n a l ) e f f e c t s . T h i s r e s u l t s i n i n s t r u m e n t a t i o n which becomes l i m i t e d by t h e S/N o f t h e r e s o n a t o r r a t h e r t h a n by t h e f l u c t u a t i o n s of t h e e n v i r o n m e n t .

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3 . VESSOT, R. F. C . , E. M. MATTISON and E. L. BLOMBERG, P r o c . Thirty-Third Frequency C o n t r o l Symposium (1979) p . 511.

4 . BECKER, G . , M e t r o l o g i a _l2 (1977) 99.

5 . GARVEY, R. M., H. HELLWIG, S. JARVIS, J r . , and D . J . WINELAND, IEEE T r a n s . Instrum. Meas., IM-27, ( 1 9 7 8 ) , 349.

6 . WINELAND, D . J . , M e t r o l o g i a _ l 2 (1977) 121.