Middleton W.M. (ed.) Reference Data for Engineers: Radio, Electronics, Computer and Communications

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1-1

NG112-The frequencies 25.04, 25.08, 150.980,

154.585, 158.445, 159.480, 454.000, and 459.000 MHz may

be authorized to stations in the petroleum radio service for

use

primarily in oil spill containment and cleanup operations and

secondarily in regular land mobile communication.

NGl14-In the Gulf of Mexico offshore from the Louisi-

ana-Texas coast, the frequency band 476494 MHz (TV

Channels

15,

16, and 17) is allocated to the Domestic Public

and Private Land Mobile Radio Services in accordance with

the regulations set forth in parts 22 and 90 respectively.

KG115-In the 174

216 MHz band wireless micro-

phones may be authorized

operate on a secondary, nonin-

terfering basis, subject to terms and conditions set forth in Part

74 of these Rules and Regulations.

NGll7-The frequencies 156.050 and 156.175

MHz

may

be assigned

stations in the maritime mobile service for com-

mercial and port operations in the New Orleans Vessel Traffic

Service (VTS) area and the frequency 156.250 MHz may be

assigned to stations in the maritime mobile service for port

operating in the New Orleans and Houston VTS areas.

NGll8-Television translator relay

stations may

authorized to

use

frequencies in the 2025-2130 MHz band on

a secondary basis to stations operating in accordance with the

Table of Frequency Allocations.

NG120-Frequencies in the band 928-960 MHz may be

assigned for multiple address systems and mobile operations

on a primary basis as specified in 47 C.F.R Part 101.

NG124-Within designated segments of the bands that

comprise 30.8547.41 MHz, 150.8-159.465 MHz, and

453.0125467.9875 MHz, police licensees are authorized to

operate low-power radio transmitters

a secondary, nonin-

terference basis in accordance with the provisions

C.F.R.

2.803 and 90.20(e)(5).

NG127-In Hawaii, the frequency band 488494 MHz is

allocated exclusively to the fixed service for

use

by common

carrier control and repeater stations for point-to-point inter-

island communications only.

NG128-In

the hand 535-1705 !&z, AM broadcast lic-

ensees or permittees may

use

their

canier on a secondary

basis

transmit signals intended for both broadcast and non-

broadcast purposes. In the band 88-108 MHz, FM broadcast

licensees or permittees are permitted to

use

subcarriers on a

secondary basis to transmit signals intended for both broad-

cast and uonbroadcast purposes. In the bands 54-72, 76-88,

174-216,470-608, and 614-806 MHz, TV broadcast licens-

ees

or permittees are permitted to

use

subcarriers on a second-

ary basis for both broadcast and uonbroadcast purposes.

NG129-In Alaska, the bands 76-88 MHz and 88-100

MHz are also allocated

the Fixed service on a secondary

basis. Broadcast stations operating in these bands shall not

cause interference to non-Government fixed operations autho-

rized prior to January

1982.

NG133-Stations authorized in the band 73-74.6 MHz as

of December 1, 1961 may continue to operate until December

31, 1985. Such stations shall not be required to afford protec-

tion to radio astronomy observations within the United States

and possessions; however, such stations must afford protec-

tion to the observatories of other countries.

NG134-In the band 10.45-10.5 GHz non-Government

stations in the radiolocation service shall not cause harmful

interference to the amateur and amateur-satellite services.

NG135-In the 420430 MHz band the amateur service is

not allocated north of line A (def. Sec. 2.1).

NG139-Pending the adoption

further specific rules

Concerning usage of the band 12.2-12.7 GHz by the fixed and

broadcasting-satellite services, systems in these services may

be authorized subject to the condition that adjustments in cer-

tain systems design

technical parameters may become nec-

essary during the system lifetime. The necessity for such

adjustments, and their extent, will be dependent upon the

Final Acts of the 1983 Regional Administrative Radio Con-

ference and subsequent Commission decisions.

NG14O-Pending the adoption of further specific rules

concerning usage of the band 17.3-17.8 GHz by the fixed-sat-

ellite service for the purpose of providing feeder links

the

broadcasting-satellite service, systems may be authorized for

this purpose subject

the condition that adjustments in cer-

tain systems design or technical parameters may become nec-

essary during the system lifetime.

The

necessity for such

adjustments, and

their

extent, will be dependent upon the

Final Acts of

the

1983 Regional Administrative Radio Con-

ference and subsequent Commission decisions.

NG141-The frequencies 42.40 MHz and 44.10 MHz are

authorized on a primary basis in the State of Alaska for meteor

burst communications by fixed stations in the Rural Radio

Service operating under the provisions of Part 22 of this chap-

ter.

The

frequencies 44.20 MHz and 45.90 MHz are autho-

rized on a primary basis in Alaska for meteor burst

communications by fixed private radio stations operating

under the provisions of Part 90

the chapter. The private

radio station frequencies may be used by Common Carrier sta-

tions on

secondary, noninterference basis and the Common

Carrier frequencies may

used by private radio stations for

meteor burst communications on a secondary, noninterference

basis. Users shall cooperate to

the

extent practical

minimize

potential interference. Stations utilizing meteor burst commu-

nications shall not cause harmful interference to stations of

other radio services operating in accordance with the Table of

Frequency Allocations.

NG142-TV broadcast stations authorized

operate in

the bands 54-72, 76-88, 174-216, 470-512, and 512-806

MHz may

use

a portion of the television vertical blanking

interval for the transmission of telecommunications signals.

on the conditions that harmful interference will not be caused

to the reception of primary services and that such telecommu-

nications services must accept any interference caused

pri-

mary services operating in these bands.

NG143-In the band 11.7-12.2

GHz

protection from

harmful interference shall be afforded to transmissions from

space stations not in conformance with International Footnote

839 only if the operations of such space stations impose no

unacceptable constraints on operations or orbit locations of

space stations in conformance with 839.

NGl4LStations authorized as of September 9, 1983 to

use

frequencies in the band 17.7-19.7 GHz may, upon proper

application, continue

authorized for such operation.

NG145-In the band 11.7-12.2 GHz, transponders on

space stations in the fixed-satellite service may be used addi-

SPECTRUM MANAGEMENT

1-1

tionally for transmissions in the broadcasting-satellite service,

provided that such transmissions do not have a maximum eirp

greater than 53 dBW per television channel and do not cause

greater interference or require more protection from interfer-

ence than

the

coordinated fixed-satellite service frequency

assignments. With respect

the space services, this band

shall be used principally

for

the fixed-satellite service.

NGl474tations in

the

broadcast auxiliary service and

private radio services licensed as of July 25,1985, or on a

sub-

sequent date following as a result of submitting an application

for license

or before July 25,1985, may continue

operate

on a primary basis with the mobile-satellite service and the

radio-determination satellite service.

NG148-The frequencies 154.585 MHz, 159.480 MHz,

160.725 MHz, 160.785

MHz,

454.000 MHz, and 459.000

MHz may

authorized

maritime mobile stations for off-

shore radiolocation and associated telecommand operations.

NG149-The frequency bands 54-72 MHz, 76-88

MHz,

174-216 MHz, 470-512

MHz,

512-608 MHz, and 614746

MHz

are

also allocated to the fixed service

permit subscrip-

tion television operations in accordance with

Part

73 of the

rules.

NG151-In the frequency bands 824-849 MHz and 869-

894 MHz, cellular land mobile licensees are permitted to offer

auxiliary services on a secondary basis subject to the provi-

sions of Part 22.

NG152-The hand 219-220 MHz

also allocated to the

amateur service on a secondary basis for stations participat-

ing, as forwarding stations, in point-to-point fixed digital mes-

sage forwarding systems. including intercity packet backbone

networks.

KG153-The 2145-2150 MHz and 2160-2165

MHz

bands are reserved for future emerging technologies on a co-

primary basis with

the

fixed and mobile services. Allocations

to specific services will he made in future proceedings.

NG154-The 157.1875-157.45 MHz and 161.775-

162.0125 MHz bands are

also

allocated

the land mobile ser-

vice for assignment to stations as described

47 CFR Part 90.

NG155-The bands 159.500-159.675 MHz and

161.375-161.550

MHz

are allocated to the maritime service

as described in Part 80 of this chapter. Additionally, the

fre-

quencies 159.550, 159.575, and 159.600 MHz are available

for low-power intership communications.

NG158-The frequency bands 764-776 MHz and 794-

806

MHz

are

available for assignment exclusively to the pub-

lic safety services, to be defined in Docket

No.

96-86.

NGP59-Full-power analog television stations licensed

pursuant

applications filed

before

January 2,2001, and new

digital television (DTV) broadcasting operations in the 746-

806 MHz band, will be entitled to protection from harmful

interference until the end of the DTV transition period. After

the end of the DTV transition period, the Commission may

assign licenses in the 746-806 MHz band without regard to

existing television and DTV operations.

Low-power television and television translators in the

746-806 MHz hand must cease operations in the band at the

end of the DTV transition period.

lot

Allotment

575

and 27 585

kHz

for Short-Dis-

tance Low-Power Service

These allotments

are

provide for intermittent mis-

cellaneous

Government short-distance low-power

radiocommunications, radio signaling,

and

the control

of remote objects or devices

means

of radio (where

the radiated power exceeds

the

limit established under

Part

7.9).

The designated frequencies

are

allotted for use by

Government agencies and may be authorized for

use by agencies as required upon application.

All

sta-

tions operating

these frequencies shall

meet

the

conditions and standards established for

this

service.

The

designated frequencies

are

available

shared

basis only and

will

not be authorized for exclusive

use

any

one

agency.

protection from interference

can

assured to

any

station operating in

this

service.

Services involving safety of life

and

property should

not employ these frequencies in view of

their

unpro-

tected status.

All

transmissions

are

restricted to

official

Government business that requires the

use

of radio.

Stations

this service shall utilize FCC type-

accepted or type-approved Citizens Radio

Band

equip-

ment or the equivalent. The maximum transmitter out-

put power shall be

5 W.

Stations shall

identified in accordance with the

regulations of each agency.

The only class of station authorized is Mobile

(including portable-type operation).

Frequencies

575

and

585

kHz

with 6KA2A,

6KA2D, and

6KA3E

emission are designated for

the

Government short-distance low-power radio ser-

vice.

All

applications for the use

these frequencies

must bear the note S159, which reads,

“US

Govern-

ment

short-distance low-power service.”

Allotments in the Band 1710-1 850

MHz

for Fixed

Security Surveillance Systems

The frequencies

720,

740,

760,

780, and

800

MHz are allotted for use in fixed security surveil-

lance systems, on a secondary basis

other stations

operating in accordance with

the

Government Table of

Frequency allocations.

Allotment

163.100, 418.050, and 418.575

MHz

for Wide Area Use

(Assignments made under these procedures shall

longer be authorized after December

31,2004.)

The frequencies 163.100,418.050, and

418.575

MHz are allotted for use by

all

Government

agencies and

are

to satisfy intermittent wide

area

requirements of

transient nature. Coordination

accordance with Section 8.3.18 is not required.

All

operations shall be authorized

accordance

with Chapter 9 of

the

Manual. The frequencies

are

available on a shared nonpriority basis only

1-1

REFERENCE

DATA

FOR ENGINEERS

and will not be authorized nor are they intended

for the exclusive use of any one agency. No pro-

tection from interference will be provided to any

station operating on these frequencies from other

stations operating on the same frequency. The use

of equipment with coded squelch is strongly

encouraged to reduce nuisance interference from

other users.

The intent of these allotments is for use by Gov-

ernment stations in the Land

Maritime Mobile

Services, which are unlikely to cause harmful

interference to other stations operating in these

Services (Table of Services, Station Classes, and

Stations, Section

6.1.4),

and the following restric-

tions apply:

The-minimum

El2P

(Effective Radiated

Power) necessary to support the intended use

shall be employed.

The maximum base

or mobile station trans-

mitter output power shall not exceed

The gain of the base station antenna shall not

exceed

dBi.

The height of the base station antenna shall

not exceed

m above the height of the struc-

ture supporting the antenna.

Station classes

are

limited

FB, FC, ML,

MLP, MS, MSP. However, these station

classes may be suffixed with the letter

“R’

applicable and restricted by 3.(e)( 1) below.

Mobile and/or transportable repeater

transmitting stations

(as

defined in Sec-

tion 6.1.2, Paragraph

and Section

9.8.2,

Paragraph

1%)

may be authorized only

on 163.1 or

418.05

MHz but are

restricted to a maximum placement

period

days and

maximum trans-

mitter output power

(For trans-

portable repeater stations, Note

S362

applies.) The repeater receive frequencies

are 168.35 or

408.4

MHz.

Stations in the Fixed Service

(FX

station

class) may be authorized but are restricted to

transportable stations (Note

S362

applies).

All equipment shall conform

Part

5.6

the Manual.

All applications utilizing these allotted frequen-

cies must be affixed with Record Note

S352

“This

assignment is for intermittent wide area reqnire-

ments of transient, itinerant nature pursuant to

Section

4.2.3

the Manual.”

Allotment

168.350, 408.400,

and

418.075

MHz

for Common Use Frequencies

(Assignments made under these procedures shall no

longer be authorized after December

31,2004.)

The frequencies

168.350,408.400,

and 418.075

MHz are allotted for use by all US Government

agencies and are to provide for radiocommunica-

tions that do not justify the assigning of a radio

frequency exclusively to that use, i.e., the fre-

quency can be shared with other users. Coordina-

tion

in accordance with Section

8.3.18

is not

required.

All operations shall be authorized in accordance

with Chapter

of the Manual. The frequencies

are available on

shared nonpriority basis only

and will not be authorized nor are they intended

for the exclusive use of any one agency.

pro-

tection from interference will be provided to any

station operating on these frequencies from other

stations operating on the same frequency. The use

equipment with coded squelch is strongly

encouraged to reduce nuisance interference from

other users.

The intent of these allotments is for use by Gov-

ernment stations in the Land or Maritime Mobile

Services, which are unlikely to cause harmful

interference

to other stations operating

these

Services (Table of Services, Station Classes, and

Stations, Section

6.1.4),

and the following restric-

tions apply:

(a)

The minimum ERP necessary

support the

intended use shall be employed.

(b) The maximum base or mobile station trans-

mitter output power shall not exceed 30

the base station antenna shall not

exceed 6 dBi.

TIME SCALES

The world operates on a single international time

system, Coordinated Universal Time (UTC), an aver-

age of the best clocks in the world. The time scales

the Department of Commerce National Institute

Standards and Technology (NIST) and the

Naval

Observatory (USNO) are synchronized to the interna-

tional time scale maintained by the Bureau Interna-

tional des Poids et Mesures (BIPM)

Sitvres, France.

NIST and USNO

also

maintain atomic clocks that con-

tribute to the UTC average.*

There is an international agreement that UTC scales

within a given country are to be kept (if possible) to

within one microsecond of this international scale.

the United States, there are two principal timing cen-

ters: UTC(N1ST) operated by NIST in Boulder, CO.

and UTC(USN0) operated by the USNO Time Ser-

vices Division in Washington, DC. UTC(NIST) serves

the civilian sector, and the UTC(USN0) has direct

responsibilities for Department of Defense activities.

The official United States time is maintained and dis-

tributed by both NIST and USNO.

Since both UTC(NIST) and UTC(USN0) are syn-

chronized to UTC, they are

also

synchronized

each

Bodson,

D.,

Adair, R.

T.,

and Meister,

“Time and

Frequency Information in Telecommunications Systems

Standardized by Federal Standard

1002A.”

Proceedings

the

IEEE,

Vol.

79,

No.

July

1991, pp.

1077-1078.

SPECTRUM MANAGEMENT

1-1

other, typically within 50 ns. Greater accuracies with

respect to these sources can be achieved by obtaining

the necessary corrections from the respective agency.

new United States standard for time reference,

Federal Standard 1002A, was developed in 1991 by

the Federal Government in cooperation with the tele-

communications industry.

International Definition

the

Second

The second is one of seven base units in the Intema-

tional System of Units (SI). The base units are used to

derive other units

physical quantities. Use of the SI

means that physical quantities such as the second and

hertz are defined and measured in the same way

throughout the world.

There have been several definitions of the

second.

Until 1956, the definition was based on the

mean solar

day,

or one revolution of the earth on its axis. The

mean solar

second

was defined as 1/86 400 of the

mean solar day and provided the basis for several

astronomical time scales known as Universal Time

(UT):

UTO-The original mean solar time scale, based on

the rotation of the earth on its axis. UT0 was first

kept with pendulum clocks. When quartz clocks

became available, astronomers noticed errors in

UT0 caused by polar motion and developed the

UT1 time scale.

scale, UT1 improves upon UT0 by correcting for

longitudinal shifts of the observing station caused

by polar motion. Because the earth's rotational

rate is not uniform the uncertainty

UT1 is

about 2 to 3 ms per day.

smoothed version of UT1 that corrects for devia-

tions in the period of the earth's rotation caused

by angular momenta of the earth's core, mantle,

oceans, and atmosphere.

The

ephemeris second

served as the SI second from

1956 to 1967. The ephemeris second was a fraction

the tropical year, or the interval between the annual

vernal equinoxes, which occur on or about March 21.

The tropical year was defined as 31 556 925.9747

ephemeris seconds. Determining the precise instant of

the equinox is difficult, and this limited the uncertainty

Ephemeris Time (ET) to

k50

ms over a 9-year inter-

val. ET was used mainly by astronomers and was

replaced by

Terrestrial Time

(TT) in 1984, equal

International Atomic Time (TAI)

32.184

The

uncertainty

TT is k10

~s.

The era of atomic time keeping formally began in

1967, when the

second was redefined based on the

resonance frequency of the cesium atom:

The duration of 9,192,631,770 periods

the

radiation corresponding to the transition between

UT1-The most widely used astronomical time

UT2-Mostly of historical interest, UT2 is a

two hyperfine levels of the ground state of the

cesium-133 atom.

Owing to the atomic second, time interval and fre-

quency can now be measured with less uncertainty and

more resolution than any other physical quantity.

Today, the best time and frequency standards can real-

ize the SI second with uncertainties of

Physical realizations of the other base SI units have

much larger uncertainties (Table 4).

International Atomic Time (TAI) is an atomic time

scale that attempts

realize the SI second as closely

as possible. TAI is maintained by the Bureau Interna-

tional des Poids et Mesures in Skvres, France. The

BIPM averages data collected from more than 200

atomic time and frequency standards located at more

than 40 laboratories, including the National Institute of

Standards and Technology.

Coordinated Universal Time (UTC) runs at the same

rate as TAI. However, it differs from TAI by an integral

number of seconds. This difference increases when

leap seconds

occur. When necessary, leap seconds are

added to UTC on either June

or December 31. The

purpose of adding leap seconds is to keep atomic time

(UTC) within f0.9

of astronomical time (UT1).

Some time codes contain a UT1 correction that can be

applied

UTC to obtain UT1.

Leap seconds have been added to UTC at a rate of

slightly less than once per year, beginning in 1972.

UT1

currently losing about 700 to 800 ms per year

with respect to UTC. This means that atomic seconds

are shorter than astronomical seconds and that UTC

runs

faster than UT1. There are two reasons for this.

The first reason involves the definition of the atomic

second, which made it slightly shorter than the astro-

nomical second to begin with. The second reason is

that the earth's rotational rate is gradually slowing

down and the astronomical second is gradually getting

longer. When a positive leap second is added to UTC,

the sequence of events is:

23 h 59 m 59

23 h59 m 60

OhOmOs

TABLE 4. UNCERTAINTIES

PHYSICAL

mALIZATI0NS

THE

BASE

UNITS

Base Unit Physical Quantity Uncertainty

candela luminous intensity

mole amount

substance

10"

ampere electric current

o-'

kilogram

mass

10-8

meter length

10-11

second time interval

10-l~

kelvin thermodynamic temperature

1-1

REFERENCE

DATA

FOR ENGINEERS

The insertion of the leap second creates a minute that

long. This “stops” UTC for

that UT1 can

catch up.

Time

Zones

The world is divided into

time zones, each

15”

longitude, or

hour angle, apart. The meridian of

Greenwich, England, is the center of the zero zone,

which extends

7.5”

east and west. Proceeding east-

ward from Greenwich, the zones are numbered

with the prefix “plus”

indicate the hour angle to be

added to Universal Time to obtain local “standard

time”. Proceeding westward. the zones are numbered

with the prefix ‘‘minus’’ to indicate the hour angle

to be subtracted from Universal Time to obtain local

“standard time.” For example, Washington, DC, at lon-

gitude

77”

West, is in time zone

-5.

The actual boundaries of time zones are defined by

law or custom in the various countries and generally

do not coincide with the theoretical zone, even in some

places at sea. In many areas, local legal “standard

time” differs by 60 or

minutes from theoretical

standard time. Fig.

in this chapter is

map showing

the standard time zones of the world; Chart

in Chap-

ter

relates the times in several major cities of the

world.

TIME AND FREQUENCY

STANDARDS

The stability of time and frequency standards

closely related to their quality factor, or

The Q of an

oscillator is its resonance frequency divided by its res-

onance width. The resonance frequency is the natural

frequency

the oscillator. The resonance width is the

range of possible values where the oscillator will run.

A high-Q resonator will not oscillate at all unless it is

near its resonance frequency. Obviously

high reso-

nance frequency and a narrow resonance width are

both advantages when seeking

high

Generally

speaking, the higher the

the more stable the oscilla-

tor, since

high Q means that an oscillator will stay

close to its natural resonance frequency.

Quartz oscillators achieve the highest Q of any

mechanical-type device. Oscillators based on the

atomic resonance of rubidium, hydrogen, and cesium

achieve much higher

factors. Each oscillator type is

briefly described below and Table

provides

sum-

mary.

Quartz Oscillators

Quartz crystal oscillators are by far the most com-

mon time and frequency standard. An estimated two

billion

lo9)

quartz oscillators are manufactured

annually. Most are small devices built for wrist-

watches, clocks, and electronic circuits. However, they

are also found inside test and measurement equipment,

such

counters, signal generators, and oscilloscopes,

and interestingly enough, inside every atomic oscilla-

tor.

A pal Q for

quartz oscillator ranges from

lo4

The maximum Q for

high-stability quartz

oscillator can be estimated

millionf, wheref

is the resonance frequency in MHz.

Environmental changes such

temperature, humid-

ity, pressure. and vibration can change the resonance

frequency of

quartz crystal, and there are several

designs that reduce the environmental problems. The

oven-controlled crystal oscillator

(OCXO)

encloses

the crystal in

temperature-controlled chamber called

an oven. An alternate solution to the temperature prob-

lem is the

temperature-compensated crystal oscillator

(TCXO). In

TCXO, the signal from a temperature

sensor generates

correction voltage that is applied

voltage-variable reactance, or varactor. The varactor

then produces a frequency change equal and opposite

the frequency change produced by temperature.

TCXOs are used when high stability over

wide tem-

perature range is not required.

Quartz oscillators have excellent short-term stabil-

ity. An OCXO might be stable

(qr,

lo-’’.

The limitations in short-term stability are mainly

due to noise from electronic components in the oscilla-

tor circuits. Long term stability is limited by

aging,

change in frequency with time caused by internal

changes in the oscillator. Aging

usually

nearly lin-

ear change in the resonance frequency that can be

either positive or negative, and occasionally,

reversal

in aging direction occurs. Aging has many possible

causes including

buildup

foreign material on the

crystal, changes in the oscillator circuitry, or changes

in the quartz material or crystal smcture. A high-qual-

ity OCXO might age at

rate of

per year,

while

TCXO might

age

100 times faster.

Because of aging and environmental factors such

temperature and vibration, it is hard to keep even the

best quartz oscillators within

lo-’’

of their nominal

frequency without constant adjustment. For this rea-

son, atomic oscillators are used for applications that

require higher long-term accuracy and stability.

Rubidium Oscillators

Rubidium oscillators are the lowest priced members

of the atomic oscillator family. They operate

6 834

682

608 Hz, the resonance frequency of the rubidium

atom (87Rb), and use the rubidium frequency to control

the frequency of

quartz oscillator. A microwave sig-

nal derived from the crystal oscillator is applied to the

S7Rb vapor within

cell, forcing the atoms into

par-

ticular energy state.

optical beam

then pumped

into the cell and is absorbed by the atoms

it forces

them into

separate energy state.

photocell detector

measures how much of the beam is absorbed and tunes

quartz oscillator to a frequency that maximizes the

amount of light absorption. The quartz oscillator is

then locked to the resonance frequency of rubidium,

SPECTRUM MANAGEMENT

1-1

Fig.

Standard

time

zones

the

world

and their

relationship

UTC.

1-146

REFERENCE

DATA

FOR ENGINEERS

TABLE

SUMMARY

OSCILLATOR

TYPES

Oscillator Quartz Quartz Commercial

TYPe (TCXO)

(OCXO)

Rubidium Cesium

Beam

Hydrogen Maser

io4

io6

3.2

lo6

MHz)

lo7

108

io9

Resonance Various

various

6.834 682 608

GHz

9.192 631 770

GHz

1.420 405 752

GHz

frequency

Leading cause None

None

Rubidium lamp Cesium beam tube Hydrogen depletion

failure

(15

years

plus)

years)

years

plus)

Stability,

10-~

10-l~

lo-"

10-1'

10-12

Noise floor,

10-12

10-l~

10-15

Frequency

offset

10-8

10-10

10-12

fly

(7). 7

ffy

17)

after warm-up to

10-10

10-12

10-l~

x 10-12

(7=

io3

io5

10-12

(T=

io5

io7

(7=

lo2

(T=

io2

(T=

io3

io5

Aging/year

10-7

10-10

None

10-l~

Warm-up

period

min

to 1

lo-'

min

lo-''

min

x lo-''

lo-"

and standard frequencies are derived and provided as

outputs.

Rubidium oscillators continue to get smaller and

less expensive, and they offer perhaps the best price/

performance ratio of any oscillator. Their long-term

stability is much better than that of a

quartz

oscillator

and they are also smaller, more reliable, and less

expensive than cesium oscillators.

The

rubidium oscillator

about

lo7.

The

shifts in the resonance frequency are mainly caused

by collisions

the rubidium atoms with other gas

molecules. These shifts limit the long-term stability.

Stability

(CT~T,

is typically

lo-",

and

about

at one day. The frequency offset of a

rubidium oscillator ranges from

x lo-''

after a warn-up period of a few minutes, and

they

meet the accuracy requirements

most applications

without adjustment.

Cesium

Oscillators

Cesium oscillators

are

primary frequency standards

since the

second is defined using the resonance fre-

quency of the cesium atom

(133Cs),

which

192

631

770

Hz. A properly working cesium oscillator should

be close

its nominal frequency without adjustment,

and there should be no change in frequency due to

aging.

Commercially available oscillators use

cesium beam

technology. Inside a cesium oscillator,

133Cs

atoms are

heated to a gas in an oven. Atoms from the gas leave the

oven

a high-velocity beam that travels through a vac-

uum tube toward a pair of magnets. The magnets serve

a gate that allows only atoms of a particular mag-

netic energy state to pass into a microwave cavity,

where they are exposed to a microwave frequency

derived from a quartz oscillator.

the microwave fre-

Shielded

Cavity

pency Quartz

Servo

thesizer

Oscillator Feedback

831

682

608

Hz-y

-Hr\rips

MHz

Fig.

Rubidium oscillator.

1-1

quency matches the resonance frequency of cesium, the

cesium atoms will change their magnetic energy state.

The atomic beam then passes through another mag-

netic gate near the end of the tube. Those atoms that

changed their energy state while passing through the

microwave cavity are allowed to proceed to a detector

at the end of the tube. Atoms that did not change state

are deflected away from the detector. The detector pro-

duces

feedback signal that continually tunes the

quartz oscillator in a way that maximizes

the

number

of state changes

that the greatest number of atoms

reaches the detector. Standard output frequencies are

derived from the locked quartz oscillator (Fig.

5).

The

of a commercial cesium standard is a few parts

lo8.

The beam tube

typically

0.5

length, and

the atoms travel

velocities of

100 ms inside the

tube. This

limits

the observation time to a few millisec-

onds, and the resonance width to a few hundred hertz.

Stability

(CT~T,

is typically

and

reaches a noise floor near

about one day,

extending

out

to weeks or months. The frequency off-

set is typically near

lo-''

after a warm-up period of

minutes.

The current state-of-the-art

cesium technology is

the

cesium fountain

oscillator, named after its foun-

tain-like movement of cesium atoms. A cesium foun-

tain named NIST-F1 serves

the primary standard of

time and frequency for the United States.

A cesium fountain works by releasing

gas

cesium atoms into a vacuum chamber.

Six infrared

laser beams are directed at right angles

each other at

the center of the chamber. The lasers gently push the

cesium atoms together into a ball.

the process of

creating this ball, the lasers slow down the movement

of the atoms and cool them to temperatures

few thou-

sandths of

degree above absolute zero. This reduces

their thermal velocity to a few centimeters per second.

Two vertical lasers gently toss the ball upward and

then all of the lasers are turned

off.

This little push is

just enough

loft the ball about

meter high through

microwave-filled cavity. Under the influence of grav-

ity, the ball then falls back down through the micro-

wave cavity. The round trip up and down through the

microwave cavity lasts for about

and is limited only

by the force of gravity pulling the atoms to the ground.

During the trip, the atomic states of the atoms might or

might not be altered

they interact with the micro-

wave signal. When their trip is finished, another laser

is pointed at the atoms. Those atoms whose states were

altered by the microwave signal emit photons

state

known

jquorescence)

that

are

counted by

detector.

This process is repeated many times while the micro-

wave signal in the cavity

tuned to different frequen-

cies. Eventually, a microwave frequency is found that

alters the states of most of the cesium atoms and maxi-

mizes their fluorescence. This frequency is the cesium

resonance (Fig.

6).

The

of a cesium fountain is about lo'', or about

100

times higher than

traditional cesium beam.

Although the resonance frequency is the same, the res-

onance width is much narrower

Hz), owing to the

longer observation times made possible by the combi-

nation of laser cooling and the fountain design. The

combined frequenc uncertainty of NIST-F1 is esti-

mated at

lo-'

Hydrogen

Masers

The

hydrogen maser

is the most elaborate and

expensive commercially available frequency standard.

The word

maser

acronym that stands for micro-

wave amplification by stimulated emission of radia-

tion. Masers operate at the resonance frequency of the

hydrogen atom, which is

420

405

752

Hz.

Vacuum

Cavity

uartz

Servo

192

631

770

Fig.

Cesium beam oscillator.

1-1

REFERENCE

DATA

FOR ENGINEERS

s-i

Microwave

Cavity

Pr-

I-

make it through the gate enter a storage bulb sur-

rounded by a tuned, resonant cavity. Once inside the

bulb, some atoms drop to a lower energy level, releas-

ing photons of microwave frequency. These photons

stimulate other atoms to drop their energy level, and

they in turn release additional photons. In this manner,

a self-sustaining microwave field builds up in the

bulb, The tuned cavity around the bulb helps to redi-

rect photons back into the system to keep the oscilla-

tion going. The result is a microwave signal that

locked to the resonance frequency of the hydrogen

atom and that is continually emitted as long as new

atoms are fed into the system. This signal keeps a

quartz crystal oscillator in step with the resonance fre-

quency of hydrogen (Fig.

7).

The resonance frequency of hydrogen is much lower

than that of cesium, but the resonance width of a

hydrogen maser is usually just a few hertz. Therefore,

the

is about

lo9,

at least one order of magnitude

better than a commercial cesium standard. As

result.

the short-term stability

better than a cesium standard

for periods

out

to a few days-typically

lo-"

(u~T,

and reaching a noise floor of

lo-''

after about

hour. However, when measured for more

than a few days or weeks, a hydrogen maser might fall

below a cesium oscillator's performance. The stability

decreases because

changes in the cavity's resonance

frequency over time.

Laser

TIME

AND

FREQUENCY

BROADCASTS

Laser

Fig.

Cesium fountain oscillator.

Short-Wave Services- WWV

and

WWVH

hydrogen maser works by sending hydrogen gas

through a magnetic gate that only allows atoms in cer-

tain energy states to pass through. The atoms that

The Department of Commerce National Institute of

Standards and Technology operates two high-frequency

(short-wave) radio stations,

WWV

at Fort Collins,

Phase-Locked

Shielded

Cavity

rT==l

Feedback

MHz

PPS

Fig.

Quartz

Oscillator

Hydrogen maser oscillator.

Hydrogen

Pump

SPECTRUM MANAGEMENT

1-149

Colorado, and WWVH at Kauai, Hawaii. Both stations

broadcast continuous time and frequency signals on

2.5,

10,

and

MHz.

WWV

also broadcasts on 20

MHz. All frequencies provide the same information.

Although radio reception conditions in the high-fre-

quency band vary greatly with factors such as location,

time of year, time of day, the particular frequency

being used, atmospheric and ionospheric propagation

conditions, and the type of receiving equipment used,

at least one frequency should be usable at all times. As

a general rule, frequencies above 10 MHz work best in

the daytime, and the lower frequencies work best at

night.

Services provided by

WWV

and WWVH include:

Time announcements

Standard time intervals

Standard frequencies

UT1

time corrections

BCD time code

Geophysical alerts

Marine storm warnings

Global Positioning System (GPS) status reports

Figs. 8 and 9 show the hourly broadcast schedules of

these services.

The station addresses and telephone numbers are:

NIST Radio Station

WWV

2000 East County Rd. 58

Fort Collins, CO 80524

(303) 497-3914

NIST Radio Station WWVH

P.O.

Box

417

Kekaha, Kauai, HI 96752

Accuracy and Stability-WWV and

WWVH

are

both referenced to the primary

NIST

frequency stan-

dard and atomic time scales located in Boulder, Colo-

rado. The transmitted frequency is accurate to within a

few parts

lOI3

and stable

per day. How-

ever, the

received

accuracy is far less due to various

propagation effects. The usable

received

accuracy is a

few parts in

lo9

for frequency and about

millisecond

for timing.

Facilities and Services-The facilities and services

of the NIST radio stations are summarized in Table 6.

The

WWV

antennas are half-wave vertical antennas

that radiate omnidirectional patterns. There are actually

five antennas at the station site, one for each frequency.

(808) 335-4361

STATION

440

1-HOUR MARK

GPS

REPORTS

OMEGA

REPORTS

GEOALERTS

Beginning

each hour is identified

0.8second-long,

1500-Hz

tone.

Beginning of each minute is identified

O.8-second-long.

1000Hz

tone.

The 29th

and

59th

second

pulses

each

STATION

Fig.

The

hourly

broadcast schedules

WWV.

minute

are

omitted

440

tone

omitted during

first

hour

of each

day.