NBS TECHNICAL NOTE 662

U. S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

A Review of Precision Oscillators
NATIONAL BUREAU OF STANDARDS

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na! Bureau ian#

MY 5 1975

*<% A Review of Precision Oscillators

H. Hellwig

Time and Frequency Division

Institute for Basic Standards

l/&x National Bureau of Standards

Boulder, Colorado 80302

1.

ST

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director Issued February 1975
Library of Congress Catalog No. 75-600006

National Bureau of Standards Technical Note 662

Nat. Bur. Stand. (U.S.), Tech Note 662, 24 pages (Feb. 1975)

CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC. 20402

(Order by SD Catalog No. C13. 46:662) $0.70
A REVIEW OF PRECISION OSCILLATORS

ABSTRACT

Precision oscillators used in PTTI applications

include quartz crystal, rubidium gas cell, cesium

beam, and hydrogen maser oscillators. A general

characterization and comparison of these devices

is given including accuracy, stability, environmental sensitivity, size, weight, power consumption, availability and cost. Areas of special

concern in practical applications are identified

and a projection of future performance specifications is given. An attempt is made to predict

physical and performance characteristics of new

designs potentially available in the near future.

Key Words: Accuracy; atomic clocks; clocks; crystal

oscillator; frequency standards; stability; survey of

clocks

.

INTRODUCTION

Very recently, the author published a survey of atomic frequency standards [1,2]. This survey covered in an exhaustive way the presently available atomic standards, the manufacturers of these standards, as well as laboratories which

are active in this field. The survey also included all

known and published principles which are leading or may

lead to new or improved frequency standards in the future.

The reader is encouraged to study this and other recent

surveys [3-8] . This paper does not duplicate these published results but rather expands them to include precision

oscillators other than atomic oscillators and quantitative

data on operational parameters such as warmup, retrace and

several environmental effects. Also, this paper attempts

to predict the performance of some new concepts which have

been developed and which appear to pose no technical difficulties in their realization as frequency standards available in the near future. For these concepts stability,

operational, and environmental, parameters are predicted.
In this paper we will refrain from any discussion of new

concepts or principles which, though promising, cannot yet

be envisioned as being available in the near future. The

following illustrates this important constraint: Saturated

absorption stabilized lasers are omitted because their use

as frequency standards or clocks is not possible at the present time because of the unavailability of a practical frequency synthesis chain in the infrared* which would allow

the generation of precise standard frequencies and time signals from these standards which otherwise have documented

competitive stability, and an interesting accuracy potential

[10-13]

AVAILABLE STANDARDS

Figure 1 is adapted from Ref. 1 and 2. It includes crystal

and superconducting cavity oscillators and various types of

laboratory and commercial atomic frequency standards.

Figure 1 shows that for short sampling times quartz crystal

oscillators and superconducting cavity oscillators or rubidium masers are the oscillators of choice. For medium- term

stability, the hydrogen maser and superconducting cavity

oscillator are superior to any other standard which is

available today. For very long-term stability or clock performance, cesium standards are presently the devices of choice.

Rubidium standards are not superior in any region of averaging times, however, as shown in Table 1, they excel in the

combination of good performance, cost and size.

It should be noted that in Fig. 1 the best available stabilities are listed for each class of standards regardless of

other characterization of the devices. In contrast, Table 1

(and the following tables) combine stability data with

operational data and other device characteristics. For each

listed device in Table 1 the data may be viewed as being

compatible, i.e., realizable in the very same device.

Frequently, one finds in publications or other reference

material that best performances are combined to create a super-device which is not actually available.

* The present realization is still too complex and lacks

precision [9]; however, this important problem is being

studied at various laboratories, and significant technical breakthroughs may be expected in the future.
Tables 1 and Fig. 1 illustrate that the choice of atomic

frequency standards should be a matter of very careful consideration and weighing of the trade-offs and actual requirements. For any system application of precision oscillators, it is important to first determine the actually

needed stability performance of the devices; secondly, to

consider the environmental conditions under which the standard has to perform; and thirdly, the size, weight, cost and

turn-on characteristics of the standard. Occasionally a

system designer will find that a standard with all the

characteristics needed does not exist yet on the market. In

this case, the designer has two alternatives: either to

adjust his system parameters to accommodate one of the

available standards or to choose a combination of these

standards to fulfill his need. The latter is an important

aspect; for example, we assume that a system requires very

good long-term stability and clock performance but at the

same time high spectral purity, i.e., very good short-term

stability. In addition, no cost, weight or size constraints

are imposed. An optimum combination for this case could be

a crystal oscillator paired with a cesium beam frequency

standard. The systems concept as a solution to a design

problem is a very powerful tool, and it can be realized

technically at no sacrifice to the performance of the individual components of the system. The only actual restrictions may be physical size and cost. It should be noted

here that many time scale generating systems are based on

clock ensembles which feature not only several clocks of the

same type but a combination of clocks of different design.

For example, at the National Bureau of Standards we routinely use a combination of crystal oscillators and cesium

standards when testing precision oscillators.

POTENTIALLY AVAILABLE STANDARDS

As was outlined in the Introduction, we list here only those

devices and concepts which appear to be easily realizable

.within today's technology. A great many of highly promising

and interesting concepts have been omitted at this time

because they are too far removed from practical realizations

or even practicality.

First, we assess existing standards (Table 1) and their

future development capability (Table 2). In particular, we

note that we expect stability improvements of about 1 order

of magnitude in all four devices: crystal oscillators,

hydrogen masers, cesium beam tubes, and rubidium standards.
In the case of crystals, this is due to better understanding

and control of the noise behavior [14,15]. In the case of

hydrogen, we expect an even better control of the cavity

pulling effects which transduce temperature, pressure and

vibrational effects into frequency fluctuations. A better

understanding and control of the aging of rubidium cells due

to improved control of the lamp intensity, as well as the

gas composition in the cell appears possible. In cesium, an

understanding of the flicker noise performance is expected

as well as improved signal levels. Flicker noise effects

may be due to cavity temperature gradients, microwave interrogation power fluctuations, magnetic field variations etc.,

all of which can be controlled to higher precision [1,2,16].

Table 3 lists five new concepts of devices. The cesium gas

cell device is very much like the rubidium gas cell device

except that cesium is used which necessitates a different

lamp filter arrangement [17,18]. There is the potential

that some aging effects may be better controllable with a

cesium device because its different filter permits better

control and higher symmetry of the optical spectrum.

However, aside from this, the cesium gas cell device is

expected to have characteristics similar to the projected

performance of rubidium gas cell devices.

The dual-crystal concept is depicted in Fig. 2. The device

consists of a crystal oscillator which is locked to a crystal resonator with a reasonably long time constant. The

lock between the crystal oscillator and the passive crystal

resonator can be envisioned as being rather simple using the

dispersion lock technique studied in its basic feasibility

with the hydrogen maser [19,20]. The advantage of a combination of a passive crystal with an active crystal oscillator lies in the realization of exceedingly high short-term

stability in the oscillator, while the crystal resonator can

be specifically designed for excellent long-term stability.

In crystal oscillators short-term and long-term stability

have been opposing goals, because high short-term stability

typically requires rather high driving levels whereas

excellent long-term stability requires low drive levels at

the crystal resonator. A combination using two crystals

could optimize on both in the same package.
The passive hydrogen device has been studied in detail and

has demonstrated feasibility [19,20]. Its advantages rely

to a high degree on the significant reduction of cavity

pulling. As was mentioned already, cavity pulling serves as

the transducer for temperature fluctuations, pressure fluctuations, mechanical stress fluctuations, etc., into frequency fluctuations. The passive device allows cavity Q's

of 100 times or more below that of an oscillator and thus

leads to a corresponding reduction in the cavity pulling

effect. An increased environmental insensitivity coupled

with a simplified design and excellent long-term stability

without very high demands on the temperature stability can

be realized. Figure 3 shows a block diagram of such a device. The hydrogen resonance is interrogated by a signal

derived from a crystal oscillator. The signal is used to

lock the crystal oscillator to the hydrogen resonance. In

Fig. 3, dispersion locking is depicted which could simplify

the overall system. A low cavity Q can be realized by using

a lossy cavity but it appears advantageous to realize the

low Q by using a very high cavity Q with a well defined

mode, and lowering the Q electronically with negative feedback. This concept is shown in Fig. 3. In order to discriminate against long-term phase shifts in the electronics,

an amplitude modulation of the hydrogen signal may be added.

As shown in Fig. 3, this could be a hydrogen beam modulation.

Figure 4 shows the concept of a small and inexpensive atomic

frequency standard. Traditionally, atomic frequency standards have been devised, designed, and built in order to

achieve performances impossible to reach with crystal oscillators. In other words, the atomic resonance was used in

the past to achieve excellence in performance. Thus, the

selection of the atomic resonance as well as the whole

design concept was directed towards achieving the utmost in

stability and accuracy. A different design philosophy,

however, appears possible. The weaknesses of a crystal

oscillator are certainly not its size, weight, or power.

They are the fact that crystals do not have a precise frequency without calibration, and that the crystal shows

environmental sensitivity, in particular, with regard to

temperature and acceleration (constant load, vibration,

shock, etc.). If the atomic resonance is viewed only as a

means to reduce or eliminate these negative performance

characteristics of a crystal oscillator we are not necessarily constrained to resonances which lead to utmost stability and accuracy performance but others may be considered

that lead to simpler designs. We therefore propose that a simple atomic standard could be built based on the well known
inversion transition in ammonia*. Ammonia will not permit

the design of a standard exceeding signicantly a 10 ° performance level in stability, accuracy, and environmental

insensitivity, however, up to the 10~ level a rather

simple design concept should be realizable. Such an ammonia

standard is depicted in Fig. 4 and its projected performance

is depicted in Table 3. Again one could use the simple

dispersion lock concept to control the frequency of the

oscillator. The oscillator has not necessarily to be a

crystal oscillator. If the standard is to operate under

severe acceleration and vibration, the sensitivity of a

crystal against these influences may cause loss of lock to

the atomic resonance. Therefore, it may be advantageous to

use other oscillator concepts such as a conventional LC or a

Gunn effect oscillator. The device will have a performance

which is in certain ways inferior to that of laboratory type

crystal oscillators but it is projected that a combination

of low cost, size and environmental insensitivity can be

obtained which is not presently available with any other

design solution.

Finally, in Fig. 5, we depict the superconducting cavity

oscillator. This oscillator concept has been recently developed and studied, and it has demonstrated stability performance which exceeds that of any other known oscillator

[23,24], In fact, stabilities in the 10~ 1 6 region have been

realized at averaging times of hundreds of seconds [25]

The superconducting cavity oscillator appears adaptable to

commercial design and would be the best oscillator for medium-term stabilities (averaging times of 10 to 1000 s) . It

could therefore be of interest to users such as those engaged in very long baseline interferometry . It appears,

however, unlikely that the superconducting cavity oscillator

can become a very small and rugged device and it is equally

unlikely that its environmental sensitivity can be reduced

significantly from those values projected in Table 3.

* The ammonia molecule has served in the first atomic

clock" device [21] as well as in the ammonia maser

[22] which opened up the modern field of quantum

electronics. Ammonia was discarded for clock applications because 25 years ago it was technologically

cumbersome to reach K-band and because ammonia is

inferior with regard to the realization of superior

accuracy and stabilities.
So far, we have only discussed stabilities for averaging

times of 1 s or longer. Stabilities in the millisecond

region correspond to very high spectral purity. This spectral purity is especially needed in the generation of frequencies in the infrared and visible radiation region from

microwave sources. The two oscillators which play a crucial

role in this regard are superconducting cavity oscillators

and crystal oscillators. Studies of both of these devices

have shown that significant improvement in the millisecond

stability region should be possible which, in turn, should

allow multiplication of these signals into the infrared

region without the need for intermediate oscillators. Such

oscillators (lasers) presently serve as spectral filters in

the infrared synthesis work and speed of light measurements

[9].

It has been projected that linewidths of less than about 100

Hz in the near infrared region should be possible without

intermediate oscillators using either improved crystal

oscillators or superconducting cavity oscillators, or using

today's crystal oscillator paired with today's superconducting cavity as a filter at X-band [26] . Such linewidths

would be totally adequate to do high precision metrology,

since 100 Hz in the near infrared region represents parts

in 10 1

3

. Thus the realization of a unified standard for length of

time [27,28] and a control of frequencies in the infrared

and visible region is crucially tied to the availability of

new oscillators.

Acknowledgments

The author is indebted to the many individuals, laboratories

and companies who provided information which made possible

the compiling of data of Tables 1 and 2 and Fig. 1. D. W.

Allan and F. L. Walls contributed important information in

many discussions. S. R. Stein provided data on the superconducting cavity oscillator; Fig. 5 is adapted from his

thesis [25]

.
Tables 1-3

The listed data are average values for the respective types

of standards taken from publications, reports and manufacturers specifications. There may be significant deviations towards better or worse data for certain models under

certain operating conditions. The listed data for one type

have generally been realized in at least one existing device. Therefore, some stability data are not as good as

those of Fig. 1 which refer to the best achieved values

(regardless of operational, physical, or environmental

characteristics) . The following is an explanation of

the terms used in the second half of the Tables (the terms

in the first part of the Tables are self-explanatory)

Warmup is the time required to reach a frequency within

-9

10 of the "final" frequency (i.e., after several days).

In atomic standards it is practically equivalent to the

time required for reaching a locked condition. Retrace

is the ability of the device to reproduce - after a complete turn-off (long enough to return to shelf storage

temperature) - the frequency before the power interruption.

Temperature and acceleration sensitivities are self-explanatory, however, it must be noted that crystals are

inherently sensitive to these effects due to the fact that

the crystal resonator itself changes frequency under these

environmental loads, whereas atoms are inherently unaffected

However, the proper design of temperature control can

reduce these effects significantly. Acceleration refers

quantitatively to constant g-loads; however, the values

indicate qualitatively the related sensitivities to vibration and shock. Barometric effects are transduced

into frequency variations via design features, and thus

are reducible by improved design. Magnetic effects are

acting directly on the atoms changing their frequency,

but - as in the case of temperature for crystals - this

effect can be reduced almost arbitrarily by shielding

(though affecting cost and size) for all atomic standards.
REFERENCES

[1] Hellwig, H. , "Atomic frequency standards: a survey,"

Proc. 28th Annual Symp. on Freq. Contr. , pp. 315-339,

May 1974.

[2] Hellwig, H. , "Atomic frequency standards: a survey,"

Proc. IEEE, vol. 63, pp. 212-229, Feb. 1975.

[3] McCoubrey, A. 0. , "A survey of atomic frequency standards," Proc. IEEE, vol. 54, pp. 116-135, Feb. 1966.

[4] Kartaschoff, P., and Barnes, J. A. , "Standard time and

frequency generation," Proc. IEEE, vol. 60,

pp. 493-501, May 1972.

[5] Audoin, C, "Revue des oscillateurs atomiques et

moleculaires, " L'Onde Electrique, vol. 53, pp. 39-45,

Feb. 1973.

[6] Hellwig, H. , "Areas of promise for the development of

future primary frequency standards," Metrologia,

vol. 6, pp. 118-126, Oct. 1970.

[7] Beehler, R. E. , "A historical review of atomic frequency standards," Proc. IEEE, vol. 55, pp. 792-805,

June 1967.

[8] Ramsey, N. F., "History of atomic and molecular frequency control of frequency and time," Proc. 2 5th

Annual Symp. on Freq. Contr., pp. 46-51, Apr. 1971.

[9] Evenson, K. M. , Wells, J. S., Petersen, F. R.

,

Danielson, B. L. , Day, G. W. , Barger, R. L. , and

Hall, J. L. , "Speed of light from direct frequency

and wavelength measurements of the methane-stabilized

laser," Phys. Rev. Lett., vol. 29, No. 19, pp. 1346-1349,

Nov. 1972.

[10] Hall, J. L. , and Borde, C, "Measurement of methane

hyperfine structure using laser saturated absorption,"

Phys. Rev. Lett., vol. 30, pp. 1101-1104, May 1973.

[11] Barger, R. L. and Hall, J. L. , "Wavelength of the 3.39pm

laser-saturated absorption line of methane," Appl. Phys.

Lett., vol. 22, pp. 196-199, Feb. 1973.
[12] Hellwig, H., Bell, H. E. > Kartaschoff, P., and

Bergquist, J. C, "Frequency stability of methanestabilized He-Ne lasers," J. Appl. Phys., vol. 43,

pp. 450-452, Feb. 1972.

[13] Brillet, A., Cerez, P., and Clergeot, H., "Frequency

stabilization of He-Ne lasers by saturated absorption,"

IEEE Trans. Quant. Electron, vol. QE-10, pp. 526-527,

June 1974.

[14] Wainwright, A. E. , Walls, F. L. , and McCaa, W. D. , "Direct

measurements of the inherent frequency stability of

quartz crystal resonators," Proc. 28th Annual Symp.

on Freq. Contr. , pp. 177-180, May 1974.

[15] Walls, F. L. , and Wainwright, A. E. , "Measurement of the

short-term stability of quartz crystal resonators and

the implications for crystal oscillator design and

applications," IEEE Trans, on Instrum. & Meas.,

vol. IM-24, March 1975 (to be published).

[16] Howe, D. A., Bell, H. E., Hellwig, H. , and DeMarchi, A.,

"Preliminary research and development of the cesium

tube accuracy evaluation system," Proc. 28th Annual

Symp. on Freq. Contr., pp. 362-372, May 1974.

[17] Beverini, N. , and Strumia, F., "High efficiency hyperfine

filter for the cesium resonance lines," Optics Commun.

,

vol. IM-23, Dec. 1974, to be published.

[18] Leschiutta, S., and Strumia, F., "Optically pumped cesium

cell frequency standard," Conf. on Prec. Electromag. Meas

Conference Digest, London, July 1974.

[19] Hellwig, H., and Bell, H. , "Experimental results with

atomic hydrogen storage beam systems," Proc. 2 6th

Annual Symp. on Freq. Contr., pp. 242-247, June 1972.

[20] Hellwig, H. , and Bell, H. , "Some experimental results

with an atomic hydrogen storage beam frequency standard," Metrologia, vol. 8, pp. 96-98, Apr. 1972.

[21] Lyons, H., "The Atomic Clock," NBS Technical News

Bulletin, vol. 33, No. 2, pp. 17-24, Feb. 1949.

[22] Shimoda, K. , Wang, T. C, and Townes, C. H. , "Further

aspects of the theory of the maser," Phys. Rev.,

vol. 102, pp. 1308-1321, June 1956.

10
[23] Stein, S. R. , and Turneaure, J. P., "The development

of superconducting cavity stabilized oscillators,"

Proc. 27th Annual Symp. on Freq. Contr., pp. 414-420,

1973.

[24] Jimenez, J. J., and Septier, A., "S & X-band superconducting cavity stabilized oscillators," Proc.

27th Annual Symp. on Freq. Contr., pp. 406-413,

1973.

[25] Stein, S. R. , "The superconducting cavity stabilized oscillator and an experiment to detect time

variation of the fundamental constants," High

Energy Physics Lab. , Stanford University, HEPL

Report 741, Oct. 1974.

[26] Walls, F. L. , and DeMarchi , A., "RF spectrum of a

signal after frequency multiplication; measurements and comparison with a simplified calculation,"

to be published.

[27] Hellwig, H. , and Halford D. , "Accurate frequency

measurements: Survey, significance, and forecast,"

NBS Special Publication 343, pp. 17-26, Aug. 1971.

[28] IAU Resolution, Sydney, Australia, 197 3.

11
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H-BEAM

CONTROL

FIG. 3 Passive hydrogen concept. The crystal oscillator

is locked to the hydrogen resonance using the

phase sensitive dispersion lock technique. The

cavity-Q is lowered using negative electronic

feedback.

14
NH 3 FILLED

WAVEGUIDE -i

1

1

4> DETECTOR '^*

\ / V MULTIPLIER

FILTER - SERVO TUNING OSCILLATOR DIVIDER OUTPUT

FIG. 4 Utility standard concept. The oscillator is locked

to a resonance in ammonia (K-band) using the phase

sensitive dispersion lock technique. The oscillator

may not necessarily be a crystal oscillator but a Gunn effect oscillator or other conventional source.

VOLTAGE

CONTROLLED

MICROWAVE

OSCILLATOR

PHASE

MODULATOR

SUPERCONDUCTING

CAVITY

FREQUENCY

DISCRIMINATOR

FRE

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FIG. 5 Superconducting cavity oscillator. A Gunn effect

oscillator is servoed to a superconducting cavity using conventional lock loop techniques [23,24].

15
AVAILABLE TODAY

COST

(K$)

SIZE

(* ) WEIGHT

(lb)

POWER

(W)

STABILITY

Is FLOOR DRIFT

(per day)

X-tal 0. 7 - 3

1 1 3

11 -11 10 10" 10 10"

Rb(gas

cell)

3. 5 - 8

1 2 15 io- " io- " io12

Cs (tube) 15 20 40 30 io- " io"

13 IO""

H (maser) 100 100 90 20 io"

12 10"" IO""

WARN

TIME

for 10" 9

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RETRACE TEMP

(per °C)

ENVIRO]

ACCL.

(per g)

MMENT

BAROM.

(per mbar)

MAG.

FIELD

(per G)

X-tal lh io1P io11 10" 9 - -

Rb ( gas

cell)

10 min. io11 10" u 10" 1?

(est. )

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12

Cs (tube) 30 min. io1? io- 13

13 10" IO"" IO"

12

H (maser) 1 min. io"

12 io13 io- 12

( est.)

IO" " io'

12

TABLE 1 Available devices.

16
POTENTIALLY AVAILABLE

COST

(K$)

SIZE

(£)

WEIGHT

(lb)

POWER

(W)

STABILITY

Is FLOOR DRIFT

(per day)

X-tal 0. 7 - 30. 5 0. 5 2 lo"

11 io- 12 io"

in

Rb (gas

cell)

2 1 2 10 10"" io13 io"

13

Cs (tube) 10 10 20 20 10"" 10" 14 10-is

H (maser) 80 100 90 20 10"" 10

-15

IO" 15

WARI

TIME

(for IO" 9

)

4-UP

RETRACE TEMP

(per °C)

ENVU

ACCL

(per g)

*ONMENT

BAROM

(per mbar)

Mag

Field

(per G)

X-tal

I hr. io" 10 io"

u 9 10" - - - -

Rb (gas

cell)

10 min 10" 12 IO"" io

-13 io"

13 IO'"

Cs (tube) 1 5 min 10"" 10

_13

io"

13 io"

14 10""

H (maser) 1 min 10"" 10" 13 IO"" io" 14 10""

TABLE 2 Potential of presently available

devices for the near future.

17
POTENTIALLY AVAILABLE

COST

(K$)

SIZE WEIGHT

(lb)

POWER

(W)

Is

STABILITY

FLOOR DRIFT

(per day)

Dual

X-tal

1 - 3 1 1 2 10" 13 10" 13 10 _n

Cs (gas

(cell) 4 - b 2 3 10 10" 1? 10" 13 10' 13

H

(passive)

50 60 70 15 io- ,? io- 1E 10" 16

Utility

atomic

standa rd

1 - 2 1 2 2 10"' 10" u

1

10" 12

1

SCC-Osc. ZO 100 100 100 10' 13 10" 16 10

-13

TABLE 3a Potential of new design concepts judged

realizable in the near future.

18
POTENTIALLY AVAILABLE

WARM-UP ENVIRONMENT

TIME

(for 10" 9

) RETRACE TEMP

(per °C)

ACCL.

(per g)

BAROM.

(per mbar)

1 MAG.

FIELD

(per G )

Dual

X-tal

10 min io'nio- 11 10" ?- -

Cs (gas

cell)

10 min io- 12 io- 12 -13 10 io

" 13 io"

12

H

(pas sive)

1 min io-13 .p- ,0- 10" 1E IO" 12

Utility

atomic

standard

Is io- 10 io12 IO"

14 io" 15 IO"

13

SCC-Osc 3h 10"'' (io"

;,;

) 10" do"

14

) - - - -

assuming that the device stays at superconducting temperatures, other-wise 10

-7

TABLE 3b Potential of new design concepts judged

realizable in the near future.

19
NLrS»ll4A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS Technical Note 662

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

A REVIEW OF PRECISION OSCILLATORS

3. Recipient's Accession No.

5. Publication Date February 1975

6. Performing Organization Code

7. AUTHOR(S)

H. Hellwig

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2770242

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as Item 9.

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Precision oscillators used in PTTI applications

include quartz crystal, rubidium gas cell, cesium

beam, and hydrogen maser oscillators. A general

characterization and comparison of these devices

is given including accuracy, stability, environmental sensitivity, size, weight, power consumption, availability and cost. Areas of special

concern in practical applications are identified

and a projection of future performance specifications is given. An attempt is made to predict

physical and performance characteristics of new

designs potentially available in the near future.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Accuracy; Atomic clocks; Clocks; Crystal oscillator;

Frequency standards; Stability; Survey of clocks

18. AVAILABILITY X ' Unlimited

_ For Official Distribution. Do Not Release to NTIS

J Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

J Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

24

22. Price

$ .70

ftGPO 1974 — 677-239/1246 REGION NO. 8 USCOMM-DC 29042-P7
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