National Bureau of Standards

Library, N.W. Bldg n

may 6 1964 Reference book not to be

^ecknic&l v£ote 235

A PORTABLE RUBIDIUM-VAPOR

FREQUENCY STANDARD

ROBERT J. CARPENTER

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS

^ecknical ^lote 235

ISSUED APRIL 6, 1964

A PORTABLE RUBIDIUM-VAPOR

FREQUENCY STANDARD

Robert J. Carpenter

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 25 cents

Page

Contents

Characteristics

1. Introduction -,

2. General Description of the System /

2.1 Optical-Microwave Section /

2.2 Oscillator-Servo System 5

2.3 Frequency Translator g 2.4 Power Supply o

3. Placing the Standard in Operation 9

4. Details of Circuits -,-,

4.1 Oscillator-Reference -q

4.2 Preamplifier 12

4.3 Tuned Amplifier l2

4.4 Phase Detector -jo

4.5 Frequency Multiplier -w 4.6 Power Supply -jj

4.7 Lamp Microwave Assembly -17 4.S Other ^

111
Characteristics of Rubidium-Vapor Frequency Standard

Output Frequencies

Approximate Frequency Displacement

from Nominal

Frequency Variation

Operating Ambient Temperature

Power Requirement

5,000,00.0.000 and 100,000.000 Hz

q Standard #1 — + 3.5 parts in

q

10

Standard #2 1 part in VY

Standard #3 —

10

Less than 1 in 10 for periods

over 1 sec.

15 to 35° C

105V to 130V 50-60 Hz

Operating Time on Internal Battery Approximately 5 to 15 hours

depending on ambient temperature,

Battery Recharging Time

Weight

Approximately 24 hours.

Electronics 39 lbs. (18 kg)

Power Supply 57 lbs. (26 kg)

IV
A Portable Rubidium-Vapor Frequency Standard

Robert J. Carpenter

Three portable optically-pumped rubidium-vapor

frequency standards have been constructed. Output

frequencies of 5.,MHz and 100 kHz with a stability of

better than 1:10 have been realized. The weight

without power supply is IS kg and the power required,

exclusive of temperature control, is about 11 watts.

The power supply, which also contains batteries for

up to 15 hours operation, weighs 26 kg. Sufficient

detail is presented to allow the construction of

similar units.

1. Introduction

Frequency stable energy level transitions have been observed in a number of materials. The potential frequency stability available from

the use of such transitions is such as to encourage substantial developmental effort toward frequency standards based on energy level transitions. The first type to be announced was an ammonia-vapor maser at

about 24 GHz.

More recently cesium and thallium have been used in a Ramsey atomic

beam apparatus. The national standard of frequency is established by

such a cesium beam device at the NBS Boulder Laboratories. Very

recently work has been directed toward the development of an atomic

hydrogen maser at 1420 MHz.

* This work was partially supported by the National Aeronautics and

Space Administration.

1

2

Reports on other work in the field of frequency standards may be

found in the Proceedings of the Signal Corps Frequency Control

Symposia for the years 1957 to date. See also IRE Trans, on Inst .

Vol. 1-11, No. 2 and 4, Dec. 62.

Mockler, Beehler and Snider, "Atomic Beam Frequency Standards", IRE

Trans, on Inst . . Vol. 1-9, No. 2, pp 120-131, Sept. I960.
A suitable transition exists in optically-pumped cesium and rubidium vapor. A frequency standard based on this effect has certain advantages over the beam-type since no material is consumed in operation

(resulting in longer life) and the frequency stability is not related

to the mechanical length of the reaction area. Unfortunately the frequency of a practical device is influenced by the type and pressure of

buffer gas, so that the optically-pumped standards are not suitable for use as primary standards.

In 1959 the National Bureau of Standards undertook the construction

of three rubidium-vapor frequency standards for use in a proposed earth

satellite experiment to investigate the gravitational "red shift". The

nature of the experiment established certain special requirements on the

standards to be built. Prime emphasis was placed on low power consumption and good long term stability, with less emphasis on small size and

short term stability. The original intention was that temperature con- trol of the environment to 45 °C would be furnished by the user and

would not be charged as a power requirement of the frequency standard.

After a considerable effort had been expended on this project, a laboratory experiment by the Mossbauer effect established the "red shift" to

approximately the same accuracy as was to be obtained by the frequencystandard-satellite experiment. Plans for the satellite experiment were

deferred, but work was continued to complete the standards in a portable

form so that they could be used to compare time in various locations,

calibrate radio -path delays, etc.

The operation of these frequency standards, in common with that of

most "atomic" frequency standards, is based on the correction of the

frequency of oscillation of a quartz- crystal oscillator to a submultiple

of an atomic energy level transition frequency. In the present case the

transition is in optically- pumped rubidium vapor and occurs at about

6, 834., 683, 4-05 Hz. The atomic transition in question can be detected by

a process in which the normal population of the various energy levels

is modified by the application of pumping radiation at an appropriate

optical frequency. If, at the same time, radio-frequency energy at the

frequency corresponding to the desired transition is applied, the amount

of pumping radiation whioh is scattered will be increased. The scattering of the pumping radiation can either be observed directly, or as

a reduction in the intensity of the radiation passing directly through

the vapor cell as illustrated in Figure 1. A servo system must be provided to control the frequency of the radio -frequency energy to exactly

that of the rubidium-vapor transition.

2. General Description of the System

The frequency standards may be thought of as consisting of four

major sections (Figure 2):
1. The optical-microwave section in which the transition is

detected;

2. The oscillator-servo system in which the microwave frequency is generated and locked to the transition;

3. The frequency translator in which the 4,996,113. 6-Hz

submultiple of the transition frequency is translated

to 5,000,000 Hz;

4. The power supply.

2.1 Optical-Microwave Section, Figure 3

Light of the pumping wavelength is generated by an electrodeless

discharge lamp excited by a vacuum-tube oscillator. The lamp contains

natural rubidium and krypton at a pressure of a few torr. The spectral

characteristics of the light from the discharge lamp are modified by a filter cell placed in the optical path. The filter cell contains rubidium-85 and 30 to 50 torr of argon or neon. The entire lamp and filter

cell assembly was built for the National Bureau of Standards by Varian

Associates. The spectral characteristic of the pumping light is further modified by' an interference filter which, passes the D-2 rubidium

line at 7800 A but not the D-l line at 7947 A. The interference filter

is used because the shift in frequency with light intensity for these

standards was found to be less for the D-2 than for the D-l line.

The rubidium vapor in which the transition is to be observed is

located in a TE 1 1 cylindrical microwave cavity which is resonant at

6.83 GHz. Due to the mode of resonance in the cavity, large openings

are possible, in the end faces and are used to admit the pumping light

and pass the transmitted light' on to the photocell. The rubidium vapor

at a pressure of about 10 torr and a buffer gas mixture at a total

pressure of about 39.6 torr are contained in a quartz cell. The mixture

is approximately 49.1$ nitrogen and 50.9$ methane, which gives maximum

frequency at about 45 °C. The frequency falls off quadratically above

and below this point with a 2 x 10" deviation at ± 2°G from the peak

frequency temperature. After pa'ssing through the gas cell, light is

focused on a silicon photo-voltaic cell by a Fresnel lens.

2.2 Oscillator-Servo System

The microwave frequency used to probe for the transition in the

rubidium vapor is generated by a quartz-crystal-controlled oscillator

operating at 4^996,113.6 Hz and followed by a frequency-multiplier chain.

In order to sweep across the microwave frequency which results in the

piayiTrnTt» light attenuation in the rubidium vapor, the radio frequency is

sinusoidally phase modulated at about 25 Hz to a peak deviation of about

100 Hz (at 6.8 GHz). Since the speed of response of the frequency
correction loop is rather limited, the short term stability of the output frequency of the frequency standard is determined by the crystal

oscillator. Since the crystal oscillator is called upon to provide ex- cellent short term stability, while its long term drift may be reduced

by the correction based on the rubidium vapor transition, the crystal

drive level of the oscillator is somewhat higher than is ordinarily en- countered in high stability oscillators. The oscillators used in these

standards were obtained from Sulzer Laboratories and the Bell Telephone

Laboratories. They are fitted with voltage-controlled capacitors to

permit variation of the oscillation frequency on the command of the frequency-correction servo system.

The output of the crystal oscillator is fed to both the frequency

multiplier and the frequency translator. In the frequency- multiplier

section the first tuned circuit is shunted by a voltage variable capacitor used to apply phase modulation to the signal. Incidental phase and

amplitude modulation of the radio frequency due to short term instability

of the crystal oscillator and imperfections in the multiplier chain produce non-harmonically related microwave frequencies in addition to the

desired harmonic. Since such modulations distribute the available microwave power among a number of frequencies variously removed from the desired harmonic, they tend to reduce the amplitude of the observed transition, broaden it, and create spurious locking points where there is

sufficient mistuning for a distant modulation sideband to lie at the

transition frequency.

In the development of this standard, the frequency-multiplier stages

originally employed varactors in three cascaded stages (X3, X2, X2) to

multiply 5 MHz to 60 MHz (nominal). While the varactor multipliers could

be made to operate and did not seriously widen the spectrum of the microwave energy, adjustment was very difficult. One had to be very careful

to avoid a situation in which output would not resume if the input was momentarily interrupted. These difficulties, plus a problem in reproducing results from one unit to the next, led to a search for a more

stable solution even if potentially less efficient. Though conventional

diode multipliers are less efficient than the varactor type and might

therefore be expected to be more noisy, they were investigated. The

diode multiplier stages used two diodes connected so as to discriminate

against odd order harmonics in the doubler stages and against even order

harmonics in the tripler stage. Each multiplier was followed by a grounded-base transistor amplifier with a gain of greater than one for

the complete stage. The performance of the chain of these multipliers

(5 to 60 MHz) proved to be fully satisfactory. The only serious problem

For further information contact Sulzer Laboratories, Inc., 621 Lofstrand

Lane, Rockville, Maryland.

Mff. L. Smith. "Miniature Transistorized Crystal-Controlled Precision

Oscillators*, IKE Trans, on Inst .. Vol. 1-9, No. 2,pp 141-148, Sept. 60.

4
encountered was in the amplitude control feedback system. Initially, reverse bias was used to reduce the gain of the input amplifier, but

this resulted in the accentuation of the amplitude modulation unavoidably present on the 4.996 MHz input signal from the crystal oscillator.

Under conditions of large input signal and high reverse bias the non- linearity of the gain-controlled stage generated extremely strong spurious sidebands. A change to forward bias gain control resulted in

satisfactory performance. Amplification to a power level of about 30 mW

follows the multiplication to 60 MHz. Multiplication from 60 MHz to

6,8 GHz takes place in a single varactor diode mounted on the gas-cell

cavity located in the microwave-optical section of the standard. Even

though the efficiency of multiplication by 114 in the varactor diode is

extremely low, enough microwave energy is obtained to adequately observe

the atomic transition.

Since the microwave energy applied to the rubidium vapor is slightly phase (frequency) modulated at a low rate, the intensity of the light

after passing through the vapor also varies slightly in a related manner. If only the fundamental-modulation-frequency component of the variation

in light intensity is observed, it will have essentially zero amplitude

when the microwave frequency exactly corresponds to the atomic -transition frequency. A frequency error of one sense will cause light variations in phase with the applied modulation while an error in the other

sense will cause light variations of the opposite phase. It is very

important that the phase (frequency) modulation be sinusoidal. Any

second-harmonic content in the modulation will result in spurious generation of substantial fundamental-modulation-frequency energy even when

the microwave signal is centered on the peak of the atomic transition.

This spurious signal will cause the servo system to balance at the point

at which the error signal in one sense just equals the spurious signal

in the opposite sense. The frequency-correction system which controls

the crystal oscillator contains a synchronous-detection system to re- cover the sense of the error from the phase of the error signal. The

correction signal, a direct voltage, acts to change the frequency of the

crystal oscillator. The correction system is of the type in which an

error must exist for a correction signal to be generated. For this reason variations in the crystal-oscillator frequency are reduced, not

eliminated. The amplification of the correction system is high enough

so that this type of system is acceptable.

2.3 Frequency Translator

The frequency-translator portion of the frequency standard, Figure

4, is a product of Sulzer Laboratories . In the translator a signal at

3886.4 Hz is derived from the 4 > 996, 113. 6-Hz output of the quartz-crystal

oscillator. These two frequencies are added to give the desired

5,000,000-Hz output. An additional output of 100,000 Hz is available

from a separate 5 -MHz to 100 -kHz divider.
2.4 Power Supply

The power supply for the rubidium-vapor frequency standard is a portable unit with batteries to run the standard for approximately 5 to

15 hours depending on the ambient temperature. The supply incorporates

provisions for charging the batteries and operating the unit from the

117V a-c line. When operating from alternating current, the supply

"floats" the batteries to maintain them near full charge. The circuit

is designed so that the changeover to battery operation is automatic

should the power line fail. Therefore the unit serves as an emergency

power supply as well as a unit for portable operation of the standard.

The unit supplies + 150V at 25 mA, regulated by a duty-cycle variation system, for the tubes in the lamp of the standard, as well as 6.3V

a-c for the filaments. The transistor modules and the heater in the

standard are supplied regulated direct current at +13V and -13V.

3. PLACING THE STANDARD IN OPERATION

The standard should be placed into operation in the following

manner. Power is applied by connecting the two units (power supply and

electronic) with the connecting cable provided and supplying 117V a-c

to the power supply unit. After the thermometer on the end of the electronic unit has stabilized at about 40°C to 45 °C for approximately a half hour, an effort should be made to lock the quartz oscillator to the

atomic transition. With the modulation and RF level controls set to

produce full RF level and half-full modulation, the 4,996,113.6-Hz frequency being fed from the multiplier to the translator sections should

be measured with a digital frequency meter. Adjustment of the Volt . Adj.,

control on the Phase Detector section of the electronics should allow

this frequency to be adjusted through the desired value. If not, insert

a variable-voltage source in series with the correction voltage to the

quartz oscillator. Now observe the Lissajous pattern made by the reference and error signals when the radio frequency is varied slightly

by means of the above mentioned Volt . Adj.. Control, or external voltage

source. When the frequency is adjusted closely enough to the center

of the atomic transition there will be a 1:1 pattern. The modulation

should be maximized by tuning the filter on the 25 -Hz OscillatorReference unit and the error signal should be brought into or 180°

phase relation to it by varying the tuning of the filter on the Tuned

Amplifier unit. At this point the RF level and modulation may be reduced slightly by means of the controls on the Multiplier until a mistuning of the 4.966-MHz oscillator by a 90° rotation of the Fine Volt . Ad

j

. control on the Phase Detector module results in a maximum error

signal. Do not reduce the RF level too far; low supply voltages that

may be present when operating from the batteries may result in insufficient RF energy. At this point the correction system should be

operating as evidenced by the reduction of error signal to nearly zero shortly after each intentional small mistuning. The Volt . Adj.. should

be slowly changed, all the while keeping the system locked, to cause a
zero dc error signal from the phase detector. The 4, 996, 113. 6 -Hz

oscillator is now locked to the atomic transition.

4. Details of Circuits

To the extent possible, the same circuit designs have been used in more than one place in the servo system. For example, essentially the same circuit is used as the modulation-frequency oscillator and in two

filters. The same squaring-circuit design is used in both the error signal and the reference signal channels. Silicon transistors have been

used extensively in the audio-frequency portion of the servo system.

4.1 Oscillator-Reference, Figure 5

A frequency of about 25 Hz is generated by the RC tuned oscillator

using Q101 and Q102. The tuned network consists of the two 4.7-fiF capacitors and the 22k and 100-ohm resistors in the emitter circuitry. The

waveform distortion of this oscillator is more than can be tolerated, so

a regenerative RC tuned filter follows the oscillator. Except for the

fact that less feedback is used, that there is no clipping, and that the

frequency is variable, this circuit is identical to the oscillator. The

filter supplies a modulation signal to the phase modulator in the Frequency Multiplier section of the system. In addition a reference

signal is supplied to a coaxial connector on the exterior of the case to permit the observation of Lissajous patterns by comparison with the

filtered error signal (from the error channel Tuned Amplifier). Since

the Phase Detector section of the standard operates on rectangular waves,

the sinusoidal signal from the filter is "squared" in the two 1N137

diodes between the last two transistors Q105 and Q106 of this unit.

4.2 Preamplifier, Figure 6

The error signal from the photocell in the Microwave-Optical sec- tion is at a very low level. This signal is amplified in two stages in

the Preamplifier. The first stage Q401 uses a low- noise silicon transistor, type 2N2175. In order to reduce the noise output of the photocell, a DC bias current is applied to the photocell. The 100k potentiometer is adjusted until the voltage at the terminals of the photocell

(measured with a "VTVM at the cell voltage test point) is within 5 millivolts of ground.

4.3 Tuned Amplifier, Figure 7

The signal from the Preamplifier is delivered to the Tuned Amplifier

section of the servo system. It is further amplified by the first stage

using Q201. The next two transistors Q202 and Q203 form an RC tuned re- generative filter similar to that used in the Oscillator-Reference Section. This filter is tuned to the fundamental- frequency component of

the error signal from the Optical-Microwave assembly by adjustment of

the 100-ohm variable resistor connected to the emitter of Q202. The
sine-wave output of this filter is supplied to a panel connector and is

used in a 1:1 Lissajous figure comparison with the Reference signal to

permit visual observation of the performance of the servo system, particularly when the system is being put into operation. The sine-wave is

also supplied to the next stage Q204, which is a simple amplifier. This

stage drives a double-diode clipper using type 1N137 silicon diodes.

The approximately rectangular wave signal from the clipper drives Q205,

the output amplifier from this unit. The output signal from this unit

forms one input to the Phase Detector unit.

4.4 Phase Detector, Figure 8

The Phase Detector unit of the servo system produces the correction

voltage to be applied to the 4,996,113.6-MHz oscillator on the basis of

the relative phases of the error signal (from the Tuned Amplifier) and

the reference signal (from the Oscillator-Reference). The error signal

is further clipped by the 1N137 silicon diodes at the Signal Input of

the Phase Detector unit. This signal is then passed through Q301 which

amplifies the signal. A phase-inverted replica of the output of Q301

is produced by Q302. Thus the outputs of Q301 and Q302 form a push-pull

error signal. The output of the Oscillator-Reference unit drives Q303,

an emitter follower.

The phase -detector proper is formed by Q304 and Q305. The collectors of both of these transistors are driven by the reference rectangular wave through Q303. This rectangular wave has limits of about ± 12 V.

The error rectangular wave (from Q301 and Q302) is fed in push-pull to

the bases of the phase -detector transistors. The phase-detector transistors conduct fully when the error-signal input is negative; thus if

the reference is negative at this time (in phase with the error signal)

the phase detector will produce a negative output. When the error and

reference signals are opposite in phase the phase-detector output will

be positive.

The output of Q304 and Q305, in the form of positive or negative

pulses is then passed through a long time constant RC filter. Due to

the demands of size and weight, a tantalum electrolytic capacitor had

to be used. Unfortunately a polarizing potential, provided by a 4-V

mercury battery, is needed for the capacitor. A further potential must

be subtracted from the voltage across the electrolytic capacitor before

it can be applied to the crystal oscillator as the frequency- correction

signal. This further potential, also from a 4-V mercury battery, is

made adjustable by a multiturn 1-megohm resistor with fine adjustment

by a 10k ohm resistor. In practice the 10k ohm resistor is set at midscale and is used primarily to produce small frequency offsets when adjusting for optimum system operation. The setting of the 1-megohm re- sistor is made so that the battery-induced potential maintains the os- cillator at the proper average frequency and the correction signal from

the phase detector is only needed for small errors. An external meter

indicates the output of the phase -detector transistors so that the

8
1-megohm resistor may be adjusted for an average zero output from the

phase detector,

4.5 Frequency Multiplier, Figure 9

The frequency- multiplier unit takes the 4 > 996, 113. 6-MHz output of

the crystal oscillator, phase-modulates it, and multiplies the frequency

to approximately 60 MHz. At the input of this unit the 4. 996-MHz signal

is amplified to Q502. This transistor amplifier is gain-controlled to

hold the,-60-MHz output signal at a constant amplitude. "Forward" AGO

is used. The tuned circuit in the collector of Q502 is shunted by a voltage-variable capacitor which produces phase modulation at the reference frequency due to a signal from the Oscillator-Reference unit. Frequency multiplication takes place in three stages of diode multipliers,

each followed by a grounded-base transistor amplifier. The first multiplier is a tripler. The output of this stage at 15 MHz is amplified by

Q504. The next multiplier is a doubler, followed by an amplifier Q505

at 30 MHz. The final multiplier, also a doubler drives the 60-MHz output amplifier Q506. The output amplifier is matched to the load exhibited by the varactor multiplier in the Optical-Microwave section by a very small capacitor in series with the output coaxial transmission line,

The value of this capacitor must be determined for each unit. The RF

potential on the output tank circuit is measured by a diode detector

with a back-bias to control the level at which detector output begins.

Output level is varied by adjusting the detector bias. The detector output is filtered to DC and applied to the two-stage amplifier Q501, Q503,

and controls the bias and therefore the gain of Q502, the 4.996-MHz input stage.

An isolating amplifier, Q507, provides a 4.996-MHz output signal to

drive the Frequency Translator section of the frequency standard. In

one of the units this stage is contained in a separate box.

4.6 Power Supply, Figure 10

The +13V and -13V are supplied by identical independent supplies.

The Stancor RT202 power transformer is used both in a full wave bridge

rectifier circuit to supply -34V, and in a full wave center-tapped circuit to provide -17V. The -34V" is used to supply base bias to the 2N174

(Q801) series -regulating transistor and with the 25- ohm, 25- watt resistor to provide a current source for charging the battery. The -17 volts

supplies the collector of the 2N174. With the charging switch in the

normal operating position, the output voltage is approximately equal to

the battery voltage which is kept from exceeding 14V by the 14V, 10 -watt zener diode. In the charging position "C", the zener diode sets the output voltage at about -14V and the battery voltage is allowed to reach a _ John C. Sinclair, "Forward or Reverse Transistor AGO?", Electronic

Design , Vol. 11, No. 22, pp. 64-69; Oct. 25, 1963
maxium of about 15.3V due to the forward drop in the 1N3208 silicon

diodes. The battery voltage will approach this value when fullycharged. The charging rate is 0.8A and the charging time is about 24

hours.

The + 150V and 6.3V are obtained from a 2kHz transistor oscillator

supply (Q803,Q804) operating from the +13V and -13V d-c supplies. The

output of the oscillator is controlled by a feedback circuit which

varies the low voltage direct current supplied to the oscillator. The

oscillator output is stepped down by the Merit A2814 transformer with

a 1000-ohm wire-wound variable resistor in the primary to control the

output level. The high voltage from the Triad TX89 transformer is rectified by the full wave bridge and filtered to provide + 150V d-c.

The + 150V feedback regulating circuit consists of a blocking oscillator (Q807) which produces positive pulses which turn on Q808. This

discharges the luf capacitor between the collector and ground. The

capacitor then begins to charge toward the supply voltage until it is

again discharged by the next pulse. The result is a sawtooth. The

1N100 diode is forward biased until the voltage becomes more positive

than that on the anode of the diode which is obtained from a voltage

divider across the + 150V. This produces a sawtooth which is clipped at

the top. The level of this clipping ultimately determines the voltage

applied to the oscillator transistors (Q803, Q8O4). The clipped saw- tooth is amplified and applied to a Schmitt trigger (Q811, Q812) which

produces pulses with a duty cycle that is dependent on the clipping level. These pulses are amplified by Q813 and Q814 and then used to

switch Q815. When Q815 is on, its collector is at +13V. When it is off,

its collector is at -13V and current is supplied through the 1N2071

diode. The collector output is then filtered and the resulting direct

current supplied to the oscillator transistors (Q803, Q8O4).

Transistors Q805 and Q806 control the heater in the standard. The

mercury thermostat in the standard opens for heat and closes for cooling.

4.7 Lamp - Microwave Assembly

The lamp, its exciting oscillator, a gas filter cell r and their

package were made for this standard by Varian Associates. The circuit

diagram of the exciting oscillator is reproduced as figure 11.

In order to verify that the lamp is operating, one may measure the

output of the Hoffman -type HOC photovoltaic cell. If the connection

between this cell and the Preamplifier is disconnected, the cell voltage

is a measure of the lamp output as attenuated by the filter cell and

microwave -cavity gas cell. When the system is at normal operating temperature the cell voltage is 50 to 100 mV as measured by a VTVM. A

larger voltage is present before the system reaches operating temperature,

The microwave cavity is magnetically shielded to reduce the effect

10
of external shields. An internal solenoid adds a fixed field of about

one microtesla to swamp out the small remaining external fields.

4. 8 Other

The circuit diagrams of the Sulzer 4.9961136 MHz oscillator and the

5 MHz to 100 kHz frequency divider are included as figures 12 and 13.

Acknowledgeme nt

This report reflects the work of a number of persons at the National Bureau of Standards. Major contributions were made by the following:

Earl C. Beaty

Peter L. Bender

George A. Dines

G. Franklin Montgomery

Sachio Saito

Robert 0. Stone

Kenneth W. Yee

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Figure 14 Interconnection Diagram

<r U. S. GOVERNMENT PRINTING OFFICE : 1964 O - 725-889


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