A KIND OF SMALL HYDROGEN MASER FOR TIME-KEEPING

Z, C, Z h a i , C. F. L in , J. W. He, H. X. Huang, J . F. Lu Shanghai Observatory

Academia S i a i c a

ABSTRACT

A kind of small h,ydmgen maser standard far timekeeping ia being cleuelaped d Shanghai Observatory-The rmrrrer employe a cylindrical capocirively l d e d cwity codruetion, r&g in significanl rrize and weighs redudan compared la a trad- t i o d hydrogen mcraer.The Q of the compact caoily is electronidly enhanced by n s u W e & feedbaek into the e d y to enable mstained m r clecillotion.The b n g term Bsubiliry of the maser is improved b y a cwity f repuency stabilization servo sy-m. Thia pper dencriben &he &gn and JFuebpmenL o[ lhe maser, as weU au photographs of the new nureer eyatern during the construction phaee.

After the succeee of the first Chinese h y d r o ~ e n Maaer made at Shanghai Observatory in 1973, 6 more hydrogen lasers wi tb several improvements to the maaer design rere made a t S h ~ n ~ h a i Observatory for VLBI obserrationa and for time-keepiny.Theae hydrogen Yaaere are coareatioaal laboratory ataadarde,

To equip chinese VLBI network, a new tenerat ion of hydrogen maser, a integrated, rabged and easily tranaportable maaer, rra developed succeaafally in 1987. C a * a 3 And so far 6 this kind of hydrogen masera have been pat to aee in chinese VLBI netvork and in mili- tary ''' 'I'

At present,Shughai Obaerratory meanwhile is developing the third en era ti on of hydrogen maeer, a Q-enhanced maser, nh'ich emp logs a compact car i ty dea ibn, reaul t ing in signif icant aite and weight redact ioae compared to a conrent ional maeer, The long- term stability for this kind of maser ia improved by a cavity frequency atabilizstion servo eyetem, This paper deecribee the desipn and development of the Q-enhanced hydrofen maser, ae well ae photographa during the construction phaee,

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3. DATES COVERED 00-00-1991 to 00-00-1991

4. TITLE AND SUBTITLE A Kind of Small Hydrogen Maser for Time-Keeping

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Shanghai Observatory,Academia Sinaica,Shanghai 200030, China,

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13. SUPPLEMENTARY NOTES See also ADA255837. 23rd Annual Precise Time and Time Interval (PTTI) Applications and PlanningMeeting, Pasadena, CA, 3-5 Dec 1991

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OPERATION PRINCIPLE

9 e operation principle of a Q-enhanced aaser oscillatsr ij shown in Figure 1. 3iaiiar to a conventional maser, a state-aelected beam of hydroben atom^ is focused into a stoiage b u l b $laced inside a c u m ~ a c t cavity tuned to i h c hyperfine transition frequency of !round-rtate hydro~en atoms The interaction of the atoms with thr cavity electroma~aetic field causes the atoms to radiate.The luesea in the compact cavity are such that the maser oscillation condition is difficult to aatigfy. Thir limitation i g overcome by positive feedback, ss shown in Fig I A portion of ihe externally amplified maeer output ie fed back into the cavitg, The attenuation in the feedback loop determinee the amount of feedback, while the ph a s e ~ h ifter adjaets propa~ation delays to ensure that the feedback signal ia in phnae w i ~ h the electromagnetic field in the cavity. Thue, the cavity l o e ~ e e are effec- t iielg reduced 3r cavity a-eahanced and sustained maser osci l laiion can be obtained,

Eowerer, t h e cavity in a Q-enhanced oscillator is not an isolated component as in a tonreat ional aaset. The cavity resonance frequency is sengi t ire to phaie ehif t in the feed- back loop, In fact, the cavity and the feedback loop form a reaonant system that ia euscepti- b l e to environmental perturbations. It is therefore essential to have a cavity frequency stabilization servo system,

MASER OSCILLATOR SYSTEM

The physics unit of the Q-enhanced maaer i a shorn in Pig 2. The mechanical structure except for the cavity ia similar to a conventioaal maser, The vacuum chamber is made of aluminum and it connected to the source manifold bg a titanium tube which provides mech- anical support rith lo* thermal leakage to the cavity interaction region, Vacuum maintenance is provided by 70 1 1 s ion pump, as shorn in Fig 3,The source of the state-#elected atomic hydroten beam,conaisting of an r f diesociator and a hexapole magnet, ia fabricated aa the coaventional deai#n.The solenoid ia made of multi-lager printed circuit desi~n, allons very complete cancellation of epurious magnetic field and provides a rugged,cloee-fitting, and simple coil lystew which ie equipped with two end field gradient correction toile driven from the sane current source as the main coi1,Magnetic shielding is provided by four layers of concentric cylindrical shield8 nith conical end capa as e h m n in Fig I,

The cavity consiats of a aection of 1Ecm 0.3. "Smm w a l l ~ l 6 c m long aluminum tubing rith aluminum end plates. Input and output coupling loope as well as a varactor reactance tuaer are mounted on the bottom end plate, For tuning the frequency of the cavi tg, a big mechanical pieton inside the cavity is mounted on the top end plate.The storage bulb, iOcm O,D. 'f llcm long, provides mechanical support for the cglindrical loading capacitor. The

c a ~ a c i t o r coneiete of four equally spaced electrodes fabricated from 0.5m thick x 6 c m ride'? llcm long copper shims and attached to the b u l b by epoxy, The bulb is coated on the inside surface w i t h P46 Tef loa b y standard techniques. The loaded car ity is about 6000. Fig 5 s b o w ~ the cavity-sto:age 5 d 1 5 s ~ b a s s e a b l y

Seoaiati tea~eiature controls a i e provided a t the done, cyinder, and base sectione of t h c v a c n m chsrb~: $ 3 w ~ l l a s at ostoven alsminnm cvlinder located the outside of the eecond Iiteer oi' the atlfnetic shields T u minimize DC strap fields, double bifilar heater ricdings a d DC heater c ~ r r e n t s a r e u s e d Fig 5 shows under assembling of the maser asc i l lator

ELECTRONICS SYSTIZI!

A s ~ e n t ioce; above, the cavity and the feedback loop form a resonant system that is ~ u s c e p t i b l e to environmental perturbation4 I t i a therefore eseential to have a cavity i r e i l d e n c y etabilitation servo system, ds shown in Fig 7, if the deeired cavity frequency ie io, thea two test s i g n a l 9 of equal amplitude at ftequencies f, and f o symmetrically situated with resoect to fo and at half-power poiate of the cavity reeponse will be alter- aately injected i n t o t h e cavity by square rare modulating the teet signal source. I f the cavity response is represented bg the solid carve in Fig 7, then the rectified test aignals have tie same amplitude and there would be n o error signal at the modullating frequency,On the other hand, if the cavity baa drifted so that the reeponee is represented by the dotted curve,caviky traasmissiona at frequency f l and f a are quite differeat.The rectified test signals produce a square wave at the modulating freqnency,Thie error signal is synchron- ously detected and additional ~ a i n ie provided by a smoothing integrator, the oubput of which i e used to biae the varactor reactance tuner so that the cavity response ie slewed back to the desired solid curve,

A functional block diagram of a-enhanced maser signal-precessing system is shown ia Pit 8.The cavity Q enhancement and frequency ~tabilization servo are located,respectivelg, in the central portion and in the left s i d e of the diagram The front end microwave electr- onics is common to both systems After the first conversion, the eignal ie divided into two channels by a power-divider, In one channel, a carrow crystal band pass filter (3W=3RnZ) passea maser oscillation qignal to t h e clock rignal proceseing circuite, In the other channel, the e i ~ n a l e are rectifed and srachronously detected for the cavity stabiliaation servo system To minimize interference with radiating atoms due to switching sidebande, the teat eifaal synthesizer is witched to qeuerate al ternateiy the two teat frequeaciee at a relatively low rate of iHZ,The spacing of the frequencies f,and f , is selected to be 30KAz since s t r o a ~ maser oscillation could be obtained with an enhanced cavity width of that rna4ni t ude,

lha t we should mention i s that except f o r the front-end components, the eifnal- ;*?cessicg electronics is housed in a rack sepe~ste from the physics unit, A thermal r ~ ~ n r i o l ' la i r i e used t o r r ~ u l a i e the'tzmperature of an aluminum box in which the front-end ;;:.a ~ s v ? ? I ? c t - o n i c c o r n ~ o n e n t s a r e rcountzd T h e s e components iaclude the feedback loopa and t s e ~ c t ~ g ! ~ t f i r t h t t r i v ~ s t h e reactance tuner The box location is chosen to minimize the i L c g t i 3. ii2 t:ansiniiiion line i s e d in maser inplt-outtoing couplinl, Fig 9 shows the maser n l b c t r a n l c s s y s t e m includiab the cavity stabilization servo. '''

THE LAS'I' WORD

:n t h e section6 above, we described the desitn and develosment of a small oscillatiag covgact hydrogen maser a t Shanghai Observatory, as well as soae photographs during t h e c o a ~lruc t ion onaae At preaenc, the maser it under assembl ing and teeting. Hopefully we can ; S t a i r s:,ne ? s t a a t t h 2 beginning u f t h e ~ e r t j e a r .

' I 1 Z C. Zhai, Shanghai Observatorg' a hydrofen maaere, JIETE Vol 27, 11, 1981. :,"I . Z. C. Z h a i t t c , A kind of applied hydrogen maser, "Progreea in Astronomyw , Vol, 6,

No. 3, 1998. 3 1 . Z. C, Zhai, C, F. Lin etc, Performance evaluation of the Shanbhai Observatory's new

11-maser, 'Electromagnetic Metrology", International Academia publishers, 1989, : d l . L C . Zhai, C,P. Lin etc, A aer El-maser time and Frequency standard at Sheahan

VLSI s t a t ion of Shanghai Obaervatory, proc of 2lst PTTI meeting, 1989, . 5 1 . J , Y . Liao, and 2 . C. Zhai, A report on the hydrogen maser to be used for GPS time

comparison, proc of PTTD, 1990. i 63 8. T, #, Vang, An osci l let ing compact hydroten maser, proc of 44 th ilnnna symposium

on Frequency coatrol, 1980. : 71 C. F, Lin J. W. Ile etc, A hydrogen maser ri t h cavity auto- tuner for time keeping, to

be publiehed in the proc of 23rd PTTI meeting, 1991,

STORAGE BULB AMPLIFIER

ATTENUATOR

J~ w I

STATE SELECTOR

I COMPACT PHASE CAVITY SHlFTER

Pit, I Schematic of a 4-ennaaced maser o s c i l l a t o r

P ig . 2 The photograph of a 4-enhanced Pi!, 3 The photograph of a small ~ a s e r osci llator s y s t e m s i t e Ion pump

Pi! 4 The photograph of gagnetic s h i e l d i n ! r i t h c o n i c a l e n d c a p s ,

P i g 6 The maser o g c i l l a tor t y g t e m 11nder aasembl in !

Pig, 7 c o o r e p i for cavily f r e ~ o e n c y stabilization rerro ayatea

STORAGE BULB

VARACTOR MICROWAVE CAVITY DIODE '

DETECTOR x2 4

405751Hz SYNTH.

t

GENERATOR COMBINER

1 OUTPUT

F I ~ 8 Funciiuaal b l o c k d i a g r a m ot' 4 - ~ n b a a c e d maser oscillator with c a r i t p stabilization servo s y s t e m

Fig, 9 The photograph of Q-enhanced maser electronics sjstem