The design of the new small passive hydrogen maser prototype and some anticipated design changes between the version reported and the commercial versions are discussed. Specifications of the commercial hydrogen maser and performance data of the prototype are given

Dials, M. A.

Wert, L.

English

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COMMERCI 
. Dials and L. Wert 
E f  ratom 
Div is ion o f  Ba l l  Corporation 
I rv ine,  Ca l i fo rn ia  
ABSTRACT 
Efratom Systems Corporation has been par t i c ipa t ing  i n  an 
NBS Indus t r ia l  Research Associate Program since July, 
1981. One o f  the goals o f  t h i s  program i s  t o  produce new 
and improved smal 1 passive hydrogen masers. Efratom w i  11 
then develop a commercially avai lable f u l l  production 
hydrogen maser 
This paper describes the design o f  the new small passive 
hydrogen maser prototype and some ant ic ipated design 
changes between the version reported i n  t h i s  paper and 
the commerci a1 versions 
Specif icat ions o f  the commercial hydrogen maser and 
performance data o f  the prototype are given. 
INTRODUCTION 
This paper describes a program tha t  has been i n  progress f o r  about a year 
and a ha l f  t o  design, b u i l d  and t e s t  a Compact Passive Hydrogen Maser 
atomic frequency standard, i n  a form sui tab le f o r  quant i ty production, 
with the goal o f  making t h i s  instrument widely avai lable i n  the commercial 
market place. The design i s  derived from the extensive technology i n  
passive hydrogen masers, evolved over the past f i v e  years a t  the National 
Bureau o f  Standard by Dr.  Fred Walls and h i s  co-workers. 
Efratom Systems Corporation has entered i n t o  the Indus t r ia l  Research 
Program wi th  NBS t o  e f fec t  the t rans i t i on  o f  the laboratory accompl ish- 
ments t o  the leve l  o f  a p rac t ica l  and economically producible system. It 
i s  a major object ive o f  t h i s  program t o  achieve small s i t e  and high r e l i -  
ab i  1 i ty , whi 1 e preserving the extraordi nary performance a1 ready demon- 
s t ra ted by the NBS passive maser. 
System design 
Figure 1 shows the overal l  compact hydrogen maser block diagram. The 
descr ipt ion o f  operation o f  the passive hydrogen has been presented i n  
previous NBS publ icat ions [1,2,3,4], therefore only the changes i n  the new 
design w i l l  be described. 
409 
https://ntrs.nasa.gov/search.jsp?R=19830027099 2020-03-21T01:14:28+00:00Z
The primary change i n  the physics package (see f igure  2) i s  the use o f  a 
quadrapole magnet as the s ta te  selector. (The o l d  design uses a hexapole 
magnet.) The use o f  a quadrapole has two advantages. F i rs t ,  i t  permits a 
reduction i n  the length o f  the physics package from 36 t o  19.5 inches. 
Second, i t  increases the e f f i c iency  o f  s ta te select ion thereby al lowing a 
reduction i n  the f l u x  o f  molecular hydrogen required t o  achieve an accept- 
able signal t o  noise. The use o f  a quadrapole should allow a t  l eas t  a 
fac to r  o f  two reduction i n  the present H2,consumption rate. This l a t t e r  
advantage o f  the quadrapole i s  extremely important because i t  has a very 
benef ic ia l  impact on the requirements f o r  the maser vacuum system and H 
storage, al lowing a s ign i f i can t  decrease i n  the overa l l  size and weight g f  
the  maser wi thout sac r i f i c i ng  r e l i a b i l i t y .  
Frequency synthesizer 
A new synthesizer was designed f o r  the maser t o  provide f i n e r  output 
frequency adjustment resolut ion than was avai lable w i th  the commercial 
synthesizer used i n  t-%old design. 
output 5.0 MHz i n  2x10 
The new synth-%izer can adjust  the 
steps over a range o f  22x10 
Two synthesizers have been b u i l t ,  and spectral p u r i t y  and output i so la t i on  
measurements were made w i th  the fo l lowing resul ts:  
L(f)@ 5 MHz 
Iso la t ion  (Output t o  Output) 119 dB 
-116 dB @ 1 Hz 
-140 d8 @ 10 Hz 
-160 dB @ 100 HZ 
D i  ssoci ator 
Three dissociator c e l l  sizes were t r i ed ,  and it was found tha t  a smaller 
c e l l  i s  generally more e f f i c i e n t .  Also, i f  RF power i s  scaled i n  accor- 
dance wi th  c e l l  size, there i s  some ind ica t ion  tha t  a small c e l l  may be 
more re l iab le.  
As a resu l t  o f  these studies a c e l l  s ize o f  19 mm 0. D. by 46 mm long was 
chosen. Once fabricated, the pyrex c e l l  i s  annealed f o r  approximately 
e igh t  hours w i t h  hydrogen f lowing through it. 
The dissociator electronics i s  a single t rans is to r  sel f -exci ted osc i l l a -  
t o r ,  based on the design used i n  EFRATOM's rubidium osc i l la to r .  High 
e l e c t r i c  f i e l d s  are present before the discharge i s  ignited; these f i e l d s  
are s u f f i c i e n t  t o  i g n i t e  the discharge. Once lit, the f i e l d  i n tens i t y  i s  
substantial l y  reduced and coup1 i ng t o  the plasma i s evenly d is t r ibu ted  
over the c e l l  surface. The nominal D.C. operating power o f  t h i s  c i r c u i t r y  
i s  about 2.0 watts. This low power d iss ipat ion has several advantages: 
410 
act ive  cool i ng  i s  required 
l y  about a t h i r d  o f  t h i s  power (.7 watts) i s  d issipated i n  the 
bulb, therby keeping the temperature r i s e  low, which should help 
increase 1 i f e  
3) The r e l a t i v e l y  low energy plasma w i l l  keep de ter io ra t ion  o f  the 
bulb 's ins ide  wal l  t o  a minimum, thereby increasing l i f e  
Dissociat ion e f f i c i ency  was measured comparing input  power t o  
f o r  various d issoc iator  configurations. Measurements o f  atom f l u x  were 
performed using a kapton-pl a t i  num recombination detector i n  a balanced 
bridge. Some t e s t  resu l ts  are shown i n  f i gu re  4. For fu r the r  information 
see [SI. 
Performance Data 
Figures 3, 5 and 6 show data taken a t  NBS on the NBS small passive hydrogen 
maser. The primary dif ferences between the NBS maser and the new commer- 
c i a l  maser (CPHM) are: 
1) CPHM has a new microwave ampl i f ie r  t h a t  improves noise f i gu re  by 
about 3.5 dB. This should improve short  term performance by a fac to r  
o f  about .67. 
2) CPHM has the new frequency synthesizer 
3) CPHM uses a quadrapole s tate selector w i th  shorter beam opt ics  
4) CHPM has a new hydrogen pressure servo loop which w i l l  d i r e c t l y  
servo hydrogen atom densi ty i n  the storage bulb 
5) CPHM has magnetic sh ie ld ing around the source end t o  reduce the 
maser s e n s i t i v i t y  t o  changes i n  ambient magnetic f i e l d  
Figure 3 shows A l lan  variance data t h a t  was obtained by comparing the NBS 
passive hydrogen maser against three other clocks. The clocks were NBS-4 
(a stable laboratory cesium standard) and the two best commercial cesium 
clocks i n  the NBS ensemble. Three-cornered hat  analysis was performed on 
three sets o f  clocks, each se t  having the maser as one member. The RMS o f  
t h i s  data i s  shown i n  f igure  3. 
It should be noted t h a t  frequency d r i f t  i s  - not  removed from the data. 
Also other measurements o f  the maser versus the NBS ensemble have not 
shown a f l i c k e r  f l o o r  f o r  the maser, w i th  data t h a t  extends out t o  t h i r t y -  
two days. 
Figure 5 i s  a p l o t  o f  the f rac t iona l  frequency deviat ion o f  the NBS pas- 
s ive  maser versus NBS-4 f o r  one day averages. Only an average frequency 
o f f s e t  i s  removed from the data.-l# l i nea r  l eas t  squares curve f i t  t o  the 
data shows a d r i f t  o f  about 1x10 /day. (Note: the reported accuracy o f  
NBS-6, the primary frequency sta_@ard, impl ies an a b i l i t y  t o  measure 
absolute frequency d r i f t  o f  3x10 Given t h i s  
l i m i t ,  we have not  been able t o  f i n d  a measureable d r i f t  i n  the maser f o r  
data runs up t o  72 days i n  length. ) 
/day f o r  long data sets. 
411 
igure 6 i s  a p l o t  o f  the time erence between the maser a 
gain, only an average o f f s e t  fr cy has been removed. The 
deviat ion f o r  the p a i r  i s  10 ns over 38 days. 
Figures 7 and 8 are the prel iminary speci f icat ions f o r  the 
hydrogen maser. Some o f  the speci f icat ions are derived from d 
on the NBS small passive hydrogen maser. The speci f icat ions 
updated when more data become avai lable on the CPHM, and the device i s  
more f u l l y  characterized. 
Acknowledgements 
We would l i k e  t o  thank a l l  the people i n  the Time and Frequency D iv is ion  
a t  NBS f o r  t h e i r  help. We are especial ly gratefu l  t o  Dr .  Fred Walls f o r  
h i s  help i n  all  areas o f  maser technology, Dave Howe f o r  h i s  help on the 
microwave cavi ty,  Dave Al lan and J i m  Barnes f o r  t h e i r  help on data analy- 
s is ,  and D r .  Karl  Persson f o r  h i s  work on the dissociator. 
REFERENCES 
1. Walls, F. L., and Howe, D. A. Proc. 12th Annual Precise Time and 
2. Howe, D. A., Walls, F. L., Bel l ,  H. E., and Hellwig, H., Proc. 33rd 
3. 
4. Walls, F. L. , Proc. 8 t h  Annual Precise Time and Time In te rva l  Plan- 
5. 
Time In te rva l  Planning Meeting (1980). 
Annual Symposi um on Frequency Control (1979). 
Walls, F. L., and Hellwig, H., Proc. 30th Annual Symposium on Frequen- 
cy Control (1976). 
n i  ng Meeti ng (1976). 
Persson, K., and Walls, F. L., NBS Report " Invest igat ion o f  the Hydrogen 
Source f o r  Masers", t o  be published. 
41 2 
J
 
413 
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z
 
W
 
5
. 
0
 
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cn 
414 
PASSIVE HYDROGEN MASER STABILITY 
Z B  
2 8  
2 4  
2 1  
2 0  
1. a 
1. (I 
1.4 
1.1 
1. a 
FIGURE 3 
DISSOCIATOR PERFORMANCE 
415 
J PASSIVE HYDROGEN MASER VS NBS 
AVERAGE FREWENCY OF 5.8310-9 REMOVED 
I 
PASSIVE HYDROGEN MASER VS NBS4 
AVERAGE RIEDUBNCI O f  S.5lcO REMOVE0 
M A R Y  SPEC I FICA 
o u t p u t  
Isolation(0utput to  O u t p u t )  
Signal -to-Noise Ratio 
L ( f )  a t  5 M H t  
Short-Term Stabi 1 i t y  
Long Tern Drift 
Max. Frequency Change 
Over Life of U n i t  
Retrace (Turn Off, T u r n  On) 
Temperature Coefficient 
Magnetic Ffeld Sensitivity 
Input Power (Steady State) 
Input Vol tages 
Warmup Time 
Opera ti ng Temperature Range 
Size 
Neight  
An t i  ci  pa ted Life 
5.0 MHz Sine wave, 1 Vrms, 50Q , 2 buffered outouts 
,110 dB 
-113 dB a t  1 Hz 
-135 d8 a t  10 Hr 
-155 d8 a t  ?lo0 Hz 
cry = 2x10'12 T + for  1s c T e ~ O , O O O S  
clxlO"J/day (Note 1) 
< , + l X l O '  l 2  
~5x10"' 
~5x10' '/OC 
1 ~ 1 0 ' ' ~  Jf / f  f o r  A ext. f ie ld  of 21 gauss (Note 2)  
(70 watts 
+28 VDC (Note 3) 
218 VDC 
+ 9 VDC 
%24 hours to lock 
+looc to  + 35'C (Note 4 )  
19" rack, % 14" h i g h  x 24" deep 
60 1bS - 80 1bS 
>5 years 
FIGURE 7 
417 
I 
Avsrrglng Tim, 1 (scc.) 
FIGURE 8 
418 
MR, SAM WARD, JPL 
Have you any idea as t o  what i s  causing the d r i f t ?  
MR. DIALS: 
Basical ly, l i k e  I said, we can ' t  f i n d  any l e f t  i n  t h u s  pa r t i cu la r  un i t .  
MR. SAM STEIN, NBS 
I j u s t  wanted t o  say t h a t  work has been going on a t  NBS passive masers 
since about 1973, and t h a t  work was supported the whole time by the 
Naval Research Laboratory. 
AUDIENCE : 
What improvement d i d  you f i n d  by replacing the hexapole magnet by the 
quadrupole magnet? Did you improve the e f f i c i ency  o f  the source? Have 
you explained how t h a t  happens. 
MR, DIALS: 
We haven't measured t h a t  ye t .  That p a r t i c u l a r  u n i t  w i t h  the quadrupole; 
we j u s t  s ta r ted  looking for  the hydrogen l i n e  a couple o f  weeks ago. 
The reason we did, we expect t o  improve the overa l l  size, the over- 
a l l  l i nk ,  and secondly, we expect t o  improve the e f f i c i ency  and we think,  
based on t a l k s  w i th  Harry Peters and some other papers, t h a t  we ' l l  buy 
something on the order of a fac to r  o f  two t o  four  improvement on the 
hydrogen l i n e  Q. But we have not measured t h a t  yet .  
DR. COATES: 
Thank you. 
41  9 


A commercial hydrogen maser: Progress report

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