DESIGN CONSIDERATIONS AND PERFORMANCE OF NBS-6, THE NBS PRIMARY FREQUENCY STANDARD

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DESIGN CONSIDERATIONS AND PERFORMANCE OF NBS-6, THE NBS PRIMARY FREQUENCY

STANDARD

L. Lewis, F. Walls, D. Glaze

To cite this version:

L. Lewis, F. Walls, D. Glaze. DESIGN CONSIDERATIONS AND PERFORMANCE OF NBS-6, THE NBS PRIMARY FREQUENCY STANDARD. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-241-C8-246. �10.1051/jphyscol:1981829�. �jpa-00221724�

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Colloque C8, supplgment au nO1 2, Tome 42, de'cembre 1981 page C8-241

DESIGN CONSIDERATIONS AND PERFORMANCE OF NBS-6, THE NBS PRIMARY FREQUENCY STANDARD

L.L. Lewis, F.L. Walls and D.J. Glaze

Frequency and fime Standards Group, Time and Frequency Division, National Bureau o f Standards, Boulder, Colorado, U. S. A.

Abstract.

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

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For t h e l a s t several years, annual e v a l u a t i o n s o f NBS-6 have been made i n o r d e r t o p r o v i d e s t e e r i n g i n f o r m a t i o n t o t h e N a t i o n a l Bureau o f Standards (NBS) embodiment o f atomic time, AT(NBS). The standard has n o t been operated continu- o u s l y as a p a r t i c i p a n t i n t h e NBS t i m e scale. This philosophy has been supported by r e c e n t data g i v i n g t h e r e l a t i v e performance o f NBS-4 (which i s operated con- t i n u o u s l y as p a r t o f t h e NBS t i m e scale); t h e u n o f f i c i a l working t i m e s c a l e a t NBS, UTC(8/S); and a small passive hydrogen maser (SPHM). F i g u r e 1 shows t h e s t a b i l i t y obtained by comparing SPHM w i t h NBS-4 and w i t h UTC(8/S). NBS-4 i s normally a l a r g e c o n t r i b u t o r t o UTC(8/S). However, much o f t h i s p a r t i c u l a r data was taken when NBS-4 was t u r n e d off f o r maintenance. These curves i n d i c a t e n o t o n l y t h a t t h e j o i n t s t a b i l i t y o f SPHM vs. NBS-4 i s about 2 x t-' f o r

t < l o 5 s, b u t t h a t t h e long-term s t a b i l i t y o f UTC(8/S) i s about 2 x 1 0 - l 4 f o r

t < l o 6 s. An e f f o r t i s i n progress t o complete a d d i t i o n a l masers t o add t o t h e NBS t i m e scale. When these c l o c k s j o i n t h e scale, i t w i l l become even more unnecessary t o operate a primary standard continuously.

The short-term and long-term s t a b i l i t y o f NBS-6 were designed t o enable us t o achieve t h e accuracy l i m i t e d by c a v i t y phase s h i f t i n t h e clock. The accur- acy i s l i m i t e d by t h e degree o f r e t r a c e upon beam r e v e r s a l , which r e s t r i c t s t h e l e v e l o f c a n c e l l a t i o n o f c a v i t y phase s h i f t . The most u n c e r t a i n component o f t h e c a v i t y phase s h i f t i s t h e d i s t r i b u t e d s h i f t across t h e microwave c a v i t y windows.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981829

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- NBS4 PRIMARY CESIUM(b) 10)

F i g u r e 1.

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T h i s produces a f r a c t i o n a l u n c e r t a i n t y i n frequency o f about 1 0 - l 3 [I]. I n o r d e r t o measure t o t h i s l e v e l o f accuracy w i t h confidence, t h e short-term s t a b i l i t y must be adequate t o r e s o l v e

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Y t

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. T h i s i n t u r n r e s t r i c t s t h e beam c u r r e n t t o values g r e a t e r than about 2 PA.

I n a d d i t i o n , i n o r d e r t o make systematic checks, and t o complete a beam-reversal c y c l e , t h e long-term s t a b i l i t y o f t h e p r i m a r y standard should be b e t t e r than 10-l4 f o r several days. T h i s performance has been demonstrated by s t a b i l i t y measurements made between NBS-6 and NBS-4 [I] and between NBS-6 and a l a r g e passive hydrogen maser [2].

Construction. - A d e s c r i p t i o n o f t h e c o n s t r u c t i o n o f NBS-6 i s given i n Ref. 3.

For completeness, a summary i s presented here.

A two-beam, f l o p - i n arrangement was chosen, where one beam corresponds t o s t a t e s e l e c t i o n o f t h e F = 4 h y p e r f i n e l e v e l , and t h e second corresponds t o s e l e c t i o n o f t h e F = 3 l e v e l . While o r i g i n a l l y two separate h o t - w i r e d e t e c t o r s were used 133, t h e l a c k o f s p a t i a l r e s o l u t i o n o f t h e two beams, coupled w i t h t h e b e t t e r n o i s e performance o f a s i n g l e d e t e c t o r , made t h e l a t t e r arrangementprefer- able. With an oven temperature o f 90 ^{O C ,} a beam c r o s s s e c t i o n o f about 2 mm x 9 mm, and a t o t a l p a t h l e n g t h of about 5.4 m, t h e detected beam c u r r e n t i s about 2 PA. The degradation o f t h e s i g n a l magnjtude from p r e v i o u s l y r e p o r t e d values i s

due t o a h i g h e r background pressure i n t h e vacuum system (about 2.7 x Pa o r 2 x t o r r a t t h e pump). The i n c r e a s e i n pressure i s due t o aging o f t h e 400 R/s i o n pumps, which a r e now being replaced a f t e r about seven years o f ser- v i c e . The Ramsey c a v i t y i s 3.74 m i n l e n g t h w i t h an average atomic v e l o c i t y o f l e s s than 200 m/s, which g i v e s a microwave resonance l i n e w i d t h o f about 30 Hz.

The beam passes through t h e microwave c a v i t y a half-wavelength from t h e s h o r t e d end. When o r i g i n a l l y constructed, t h e end-to-end c a v i t y phase s h i f t was l e s s t h a n 1 mrad.

A s l i d i n g c y l i n d e r w i t h b o t h an oven and a d e t e c t o r i s mounted a t each end o f t h e beam tube. Each oven i s separated from i t s companion d e t e c t o r b y a metal p a r t i t i o n , i n order t o reduce background from room-temperature cesium vapor.

Seals f o r t h e c y l i n d e r s a r e rubber O-rings coated w i t h s i l i c o n grease. I n o r d e r t o extend t h e l i f e t i m e o f a charge of cesium when t h e oven i s moved o u t o f t h e main vacuum system, an a u x i l i a r y i o n pump has r e c e n t l y been added t o t h e oven chamber. I n t h i s way, f a i l u r e s due t o o x i d a t i o n o f cesium a r e avoided when t h e d e t e c t o r chamber i s i n p l a c e f o r l o n g p e r i o d s o f time.

Magnetic s h i e l d i n g f o r NBS-6 c o n s i s t s o f two r e c t a n g u l a r moly permalloy s h i e l d s w i t h i n a c y l i n d r i c a l armco s h i e l d . C - f i e l d windings a r e placed c l o s e t o t h e i n s i d e s u r f a c e o f t h e innermost s h i e l d . A b a f f l e s h i e l d between t h e two ends o f t h e microwave c a v i t y serves t o reduce magnetic f i e l d d i s t o r t i o n s produced by t h e c a v i t y feed opening. O r i g i n a l l y , t h e magnetic f i e l d inhomogeneity was l e s s t h a n 5 x T peak t o peak a t an o p e r a t i n g C - f i e l d s t r e n g t h o f about 6 x T.

The s h i e l d s a r e degaussed whenever t h e C - f i e l d value i s changed, and g e n e r a l l y several days a r e necessary f o r t h e s h i e l d s t o r e l a x t o a f i x e d value a f t e r degaus- s i n g .

Microwave modulation i n NBS-6 i s s i n u s o i d a l phase madulation o f t h e 5.00688 MHz q u a r t z o s c i l l a t o r output, a t a frequency o f 18.75 Hz. The modulated 5.00688 MHz s i g n a l i s m u l t i p l i e d t o 9.19263177 GHz and a p p l i e d t o t h e microwave c a v i t y . T y p i c a l l y , a1 1 sidebands a r e l e s s than

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Beam c u r r e n t detected a t t h e h o t w i r e i s a m p l i f i e d by an FET source a m p l i f i e r w i t h an i n p u t r e s i s t a n c e o f about 1 0 l 0 ohms. The modulation component a t 18.75 Hz i s e x t r a c t e d by means o f a square-wave, phase-sensitive d e t e c t o r and i n t e g r a t e d t o form a c o r r e c t i o n s i g n a l which tunes t h e q u a r t z o s c i l l a t o r t o l i n e center.

The s t a b i l i t y o f t h e q u a r t z o s c i l l a t o r alone i s b e t t e r than 1 0 - l 2 f o r times l e s s t h a n 100 s, which a l l o w s t i m e s o f t h e order o f seconds t o l o c k t o t h e microwave resonance. T h i s servo system should r e s o l v e l i n e c e n t e r t o b e t t e r than one p a r t i n l o 6 , which i n t u r n would l i m i t f r a c t i o n a l r e s o l u t i o n o f t h e microwave frequency t o about 3 x 10-15.

Eva1 uation.

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C8-244 JOURNAL DE PHYSIQUE

u n c e r t a i n t i e s o b t a i n e d i n t h e 1980 e v a l u a t i o n . As many u n c e r t a i n t i e s as p o s s i b l e a r e determined a t t h e t i m e o f e v a l u a t i o n , b u t t h e measurement o f t h e magnetic f i e l d inhomogeneity was p o s s i b l e o n l y a t t h e time o f o r i g i n a l c o n s t r u c t i o n . Even t h i s u n c e r t a i n t y i s b e l i e v e d t o have n o t changed, since t h e symmetry o f t h e Ramsey f e a t u r e s associated w i t h each Rabi microwave resonance i s s t i 11 excel 1 ent.

TABLE I. NBS-6 U n c e r t a i n t i e s , 1980 Evaluation.

Source o f U n c e r t a i n t y Bias (u x 1013) U n c e r t a i n t y ( x 1013) Y

1. (a) C a v i t y Phase S h i f t ( f o r 3.3 one d i r e c t i o n ) ( r e s i d u a l ( t y p i c a l ) f i r s t - o r d e r Doppler S h i f t )

(b) Second-order Doppler S h i f t -2.8 0.10

( t y p i c a l )

2. P u l l i n g by neighboring t r a n s i t i o n s +O. 3 0.20 3. Magnetic F i e l d E f f e c t s

(a) O f f s e t due t o f i n i t e f i e l d +I767 ( t y p i c a l )

(b) Magnetic f i e l d inhomgeneity +O. 02 0.02

(c) Majorana t r a n s i t i o n s

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4. Servo System O f f s e t s

(a) A m p l i f i e r o f f s e t s 0 0 . 1

(b) Second harmonic d i s t o r t i o n 0 0.2

5. RF Spectrum 0 0 . 1

6. C a v i t y P u l l i n g 0 0.01

...

RSS e r r o r due t o systematic

frequency biases 0.87

Random U n c e r t a i n t y 0.15

1. (a) As mentioned above, t h e l a r g e s t source o f u n c e r t a i n t y i s t h e c a v i t y phase s h i f t . The value o f t h i s frequency s h i f t i s obtained by comparing NBS-6 frequencies f o r o p p o s i t e l y d i r e c t e d cesium beams. It i s assumed t h a t a l l o t h e r sources o f frequency b i a s a r e e i t h e r measured o r are i n v a r i a n t upon beam r e v e r s a l . Since t h e 1976 e v a l u a t i o n , t h e f r a c t i o n a l frequency b i a s i n t r o d u c e d by c a v i t y phase s h i f t has s t e a d i l y increased from 0.25 x 1 0 - l 3 t o t h e p r e s e n t value o f about 3.3 x 10-13. The u n c e r t a i n t y i n t h i s number was determined by measuring t h e p o s i t i o n

dependence o f t h e frequency. A more complete s e t o f measurements o f t h e change o f frequency w i t h beam p o s i t i o n i s now i n progress w i t h t h e 1981 e v a l u a t i o n o f NBS-6.

(b) The second-order Doppler s h i f t i s determined from an a n a l y s i s o f t h e Ramsey lineshapes a t d i f f e r e n t microwave powers. The u n c e r t a i n t y i n t h i s b i a s (Ay

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2. Since t h e s t a t e - s e l e c t i o n magnets i n NBS-6 d e f l e c t atoms w i t h d i f f e r e n t M values d i f f e r e n t l y , t h e magnitudes o f t h e seven Rabi resonance peaks a r e n o t symmetric about t h e c e n t r a l peak. A t low C - f i e l d values, t h i s i n t r o d u c e s an asymmetry a t t h e c e n t r a l Ramsey resonance associated w i t h t h e unequal slopes o f t h e wings o f t h e adjacent Rabi l i n e s . The s i z e o f t h i s e f f e c t i s estimated by measuring t h e d i f f e r e n c e i n s i g n a l s i z e a t equal frequency steps on each s i d e o f t h e Ramsey peak. I n a d d i t i o n , t h e C - f i e l d i s chosen t o g i v e a Zeeman s p l i t t i n g o f these l i n e s which reduces t h e e f f e c t t o an acceptable l e v e l .

3.(a) The q u a d r a t i c magnetic f i e l d dependence o f t h e M = 0 -+ M = 0 t r a n s i t i o n i n t r o d u c e s a r a t h e r l a r g e frequency s h i f t . T h i s frequency o f f s e t i s determined by measuring t h e Zeeman s p l i t t i n g s o f t h e M = il+ M = il Ramsey l i n e s , which p r o v i d e s a very accurate measure o f t h e average magnetic f i e l d . The u n c e r t a i n t y i n t h i s b i a s i s g i v e n by t h e accuracy o f t h e Zeeman frequency measurement, which can be made t o almost a r b i - t r a r y p r e c i s i on.

(c) The e f f e c t o f Majorana t r a n s i t i o n s on frequency i n p r i m a r y Cs c l o c k s i s unknown [4]. Some authors have suggested t h a t such e f f e c t s may produce f r a c t i o n a l s h i f t s as l a r g e as 10-12 [4,5]. I n t h e case o f NBS-6, i t should be noted t h a t g r e a t care has been taken t o a v o i d sharp changes i n magnetic f i e l d from t h e s t a t e - s e l e c t i o n magnet regions. T h i s has been accomplished i n l a r g e p a r t w i t h t h e use o f s h i e l d s i n t h e t r a n s i - t i o n regions. The p r e s e n t e v a l u a t i o n o f NBS-6 i n c l u d e s a b r i e f study o f t h e frequency s h i f t w i t h C - f i e l d change, which may shed some l i g h t on t h i s subject.

4. (a) Based upon servo-system design c a l c u l a t i o n s , i t should be p o s s i b l e t o o b t a i n t h e microwave resonance l i n e c e n t e r t o b e t t e r than I n a d d i t i o n , measurement o f frequency s h i f t s upon r e d u c t i o n o f loop g a i n i n d i c a t e t h a t amp1 i f i e r o f f s e t s c o n t r i b u t e l e s s t h a n 1 0 - l 4 t o t h e f r a c t i o n a l u n c e r t a i n t y . I t i s i n t e r e s t i n g t o note t h a t r e c e n t l y t h e

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beam current in NBS-4 (which has an identical servo system to NBS-6) dropped from 10 pA to 0.1 pA as the cesium oven became depleted. The standard remai ned 1 ocked to the microwave resonance, with a fractional frequency shift of less than 2 x 10-13.

(b) It very difficult to measure the actual second harmonic distortion in the modulated microwave signal which drives the Ramsey cavity. Instead, since frequency shifts that arise from such distortion are closely related to the modulation amplitude, the modulation amplitude is varied.

This procedure gives an upper 1 imit to such shifts of about 2 x 10-14.

5. As described earlier, sidebands on the microwave spectrum are symmetric, producing offsets of less than 10-14.

6. The uncertainty in offsets due to cavity pulling is estimated from a combination of the uncertainty in our ability to set the cavity to the cesium frequency and our ability to set the microwave power to optimum.

It is not a problem in NBS-6, entering at the 10-15 level.

The RSS error due to these systematic frequency biases is 8.7 x 10-14, with an additional random uncertainty of 1.5 x 10-14. Consequently, the total uncer- tainty (rms) in NBS-6 is less than 9 x 10-14.

An additional correction in the frequency of primary cesium standards has recently come to light [Itano, W. M., Lewis, L. L., and Wineland, D. J., to be pub1 ished]. Room temperature blackbody radiation should produce a differential ac Stark shift of the ground-state hyperfine levels of cesium. The size of the fractional correction is about-1.7 x 10-l4 at 300 K. This correction has not been included in previous evaluations of NBS-6, but will be included in the 1981 evaluation.

References.

1. Wineland, D. J., Allan, D. W . , Glaze, D. J., Hellwig, H. W., andJarvis, S.

Jr., IEEE Trans. Instrum. Meas IM-25 (1976) 453.

2. Walls, F. L., and Howe, D. A., Proc. 32nd Annual Symp. on Frequency Control (1978) 492.

3. Glaze, D. J., Hellwig, H. W . , Allan, 0. W . , and Jarvis, S. Jr. , Metrologia 13 (1977) 17.

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4. Allan, D. W., Hellwig, H., Jarvis, S. Jr., Howe, 0. A., and Garvey, R. M., Proc. 31st Annual Symp. on Frequency Control (1977) 555; Becker, G., IEEE Trans. Instrum. Meas. IM-27 (1978) 319.

5. Urabe, S., Nakagiri, K . , Ohta, Y., Kabayashi, M., and Saburi, Y., IEEE Trans. Instrum. Meas. IM-29, (1980) 304.