Glossary

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

3GPP

The 3rd Generation Partnership Project unites telecommunications standards bodies. Maintains specification 3GPP TS 25.171 Requirements for support of assisted GPS using mobile telephony

A

Acceleration , 2g tip-over

The ability of the oscillator to maintain its frequency as it changes orientation with respect to gravity

Accuracy

The degree of conformity of a measured or calculated value to its definition or with respect to a standard reference (see uncertainty).

Adjustment range

See mechanical frequency adjustment or voltage frequency adjustment

Ageing

The systematic change in frequency over time because of internal changes in the oscillator. For example, a 100 kHz quartz oscillator may age until its frequency becomes 100.01 kHz (see drift). Ageing is the frequency change with time when factors external to the oscillator such as environment and power supply are kept constant. See also "frequency Ageing"

Allan Deviation

A measure of the short term frequency stability of the oscillator made in the time domain. It is calculated and presented as the value of frequency variance over a specified time period (square of the standard deviation). It is a useful measurement for industries where accurate time intervals are required (e.g. global positioning, measurement of relativistic effects, etc.) see also Allen Variance.

Allan Variance

The standard method of characterizing the frequency stability of oscillators in the time domain, both short and long term, established by Dr. Allan

AM

Amplitude Modulated

Ambiguity

The properties of something that allows it to have more than one possible meaning. In the case of a clock, it displays 3 hours and 5 minutes, it could be morning or night. The clock is then said to be ambiguous to the hour.

ANSI

American National Standards Institute

ASCII

American Standard Code for Information Interchange

ATM

Asynchronous Transfer Mode (e.g. ATM switching)

Atomic Time Scale (TA)

A time scale based on atomic or molecular resonance phenomena. Elapsed time is measured by counting cycles of a frequency locked to an atomic or molecular transition. Earlier time scales were based on the rotational rate of the earth.

AU

administrative unit; a managed entity within the SDH structure

AV

Analog Video

B

B-ICI

Broadband Inter-Carrier Interface

B-ISDN

Broadband Integrated Services Digital Network

BER

Bit Error Ratio

BootP

Bootstrap Protocol

BPV

Bi-Polar Violation BVA BVA crystal resonator

C

Calibration

The process of identifying and measuring time or frequency errors, offsets, or deviations of a clock/oscillator relative to an established standard, such as UTC(NIST)

Calibration tolerance

The allowable offset in ppm from nominal frequency usually referenced to +25ºC

CAS

Channel Associated Signaling

CC

Composite Clock

CCITT

International Telegraph and Telephone Consultative Committee (now ITU)

CCS

Common Channel Signaling

CDMA

Code Division Multiple Access. A coding scheme, used as a modulation technique, in which multiple channels are independently coded for transmission over a single wideband channel. Note 1 : In some communication systems, CDMA is used as an access method that permits carriers from different stations to use the same transmission equipment by using a wider bandwidth than the individual carriers. On reception, each carrier can be distinguished from the others by means of a specific modulation code, thereby allowing for the reception of signals that were originally overlapping in frequency and time. Thus, several transmissions can occur simultaneously within the same bandwidth, with the nutual interference reduced by the degree of orthogonality of the unique codes used in each transmission. Note 2 : CDMA permits a more uniform distribution of energy in the emitted bandwidth

Center frequency

A term describing the frequency at the center of the tuning range. This may differ from the nominal frequency in many designs

Clipped sine wave

An output voltage format achieved by driving the output circuit into clipping to achieve a rectangular wave form. Some applications use this format to reduce EMI in the product. However, this technique becomes less reliable as frequency increases and the rise/fall times may be too great at any frequency to make a useful clock oscillator for modern technology

Clock

A device for maintaining and displaying time. CMOS see HCMOS

COFA

COFA Change Of Frame Alignment

Control voltage

A voltage applied to control the frequency of VCXOs

Control voltage range

The range of voltage over which the tuning of the oscillator is valid

Coordinated Universal Time (UTC)

A coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM), which forms the basis of a coordinated dissemination of standard frequencies and time signals. A UTC clock has the same rate as a Temps Atomique International (TAI) clock or international atomic time clock but differs by an integral number of seconds called leap seconds. The UTC scale is adjusted by the insertion or deletion of leap seconds to ensure approximate agreement with UT1

Coupling

Description of the connection between the oscillator and the external circuit. The choices are AC or DC

D

Date

A unique instant defined in a specified time scale. NOTE: The date can be conventionally expressed in years, months, days, hours, minutes, seconds, and fractions. Also, Julian Date (JD) and Modified Julian Date (MJD) are useful dating measures (see Julian Date and Modified Julian Date)

dB (decibel)

A measure of relative amplitude of a sine wave

dBm

dBm Decibels referenced to 1 milliwatt, it must be specified with load impedance (usually 50 ohms)

DCC

data communications channel; the main management channel inside SDH

DCLS

Direct Current Level Shifted

DCS

digital cross-connect system; an electronic multiport switch for digital traffic

DDS

Direct Digital Synthesiser

Design Frequency

See nominal frequency

Deviation Linearity

See linearity

Deviation slope polarity

The slope of the change in frequency with respect to the control voltage. A positive polarity indicates the frequency increases when the control voltage becomes more positive

DGPS

Differential Global Positioning System

DHCP

Dynamic Host Configuration Protocol

Direct compensation

A technique of building crystal oscillators in which the circuit components affecting the crystal load reactance are selected to have temperature characteristics such, that the crystal is re-tuned over temperature to reduce the effects of temperature on the frequency of the oscillator

DLC

Digital Loop Carrier

DO - (Disciplined Oscillator)

An oscillator with a servo loop that has its phase and frequency locked to an external reference signal

DOCXO

Double Oven Controlled Crystal Oscillator. A crystal oscillator maintained, during operation, at a fixed temperature so that frequency changes due to ambient temperature changes do not affect its performance.

Drift

A measure, in hertz per second, of how quickly the skew of a clock is changing. See also "skew"

Drift (frequency)

The linear (first-order) component of a systematic change in frequency of an oscillator over time. Drift is due to aging plus changes in the environment and other factors external to the oscillator (see aging)

DTCXO

Digital Temperature Compensated Crystal Oscillator

DUT1

The approximate time difference between UT1 and UTC, expressed to the nearest 0.1 s. DUT1 = UT1 + or - UTC. DUT1 may be regarded as a correction to be added to UTC to obtain a better approximation to UT1. The values of DUT1 are given by the International Earth Rotation Service (IERS) in integral multiples of 0.1 s

Duty cycle

A term with identical meaning to symmetry. Use of the term "symmetry" is preferred to avoid confusion with other parameters. See symmetry

E

EGNOS

EGNOS European Geostationary Navigation Overlay Service

EIA-95

see TIA/EIA Standard

EMC

Electro-Magnetic Compatibility

EMI

Electro-Magnetic Interference

Ephemeris Time (ET)

An astronomical time scale based on the orbital motion of the earth around the sun (see Terrestrial Time)

Epoch

The beginning of an era (or event) or the reference date for a system of measurements

ESA

European Space Agency

ESD

Electro-Static Discharge

ETSI

European Telecom Standards Institute

F

Fanout

A specification of the number of TTL or HCMOS loads to which the oscillator will be subjected

FCC


Federal Communications Commission

Frequency


The rate at which a periodic phenomenon occurs over time

Frequency

The repetition rate of the signal output from the oscillator. It is measured in repetitions (cycles) per second or Hertz (Hz)

Frequency adjustment range

the range over which the oscillator's frequency may be adjusted above and below its nominal frequency. This may be achieved either by a mechanical trimmer or via a tuning voltage. Ranges are usually specified in ppm

Frequency ageing

This is the ability of the oscillator to maintain its design frequency over a specified time period. This is usually specified as a maximum variation of plus and minus parts per million over a specified time period or over a specified time period for a specified number of those time periods (e.g. 10 ppm/10 years or 1 ppm per year for 10 years)

Frequency drift


See drift and aging

Frequency load tolerance


A specification of the allowable change in frequency when the load impedance is changed from the minimum to the maximum specified values

Frequency offset

The frequency difference between the measured value and the defined value

Frequency shift

Change in frequency from a standard reference

Frequency stability

The ability of the oscillator to maintain its design frequency with changing environmental and electrical conditions. Parameters which may affect stability include temperature, time, supply voltage, load impedance, humidity, and vibration

Frequency stability

Statistical estimate of the frequency fluctuations of a signal over a given time interval.
Long term stability usually involves measurement averages beyond 100s.
Short term stability usually involves measurement averages from a few tenths of a second to 100s.
Generally, there is a distinction between systematic effects such as frequency drift and stochastic frequency fluctuations. Special variances have been developed for the characterization of these fluctuations. Systematic instabilities may be caused by radiation, pressure, temperature, and humidity. Random or stochastic instabilities are typically characterized in the time domain or frequency domain. They are typically dependent on the measurement system bandwidth or on the sample time or integration time

Frequency standard

An oscillator such as a rubidium, cesium, or hydrogen maser whose output is used as a frequency

Frequency voltage tolerance

A specification of the allowable change in frequency due to changes in supply voltage from the lower to the upper limit of specified supply voltage range

FSMS

Frequency Stability Measurement System

FTP

 File Transfer Protocol

G

GALILEO

Galileo will be Europe’s own global satellite navigation system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other global satellite navigation systems. This means that a user will be able to take a position with the same receiver from any of the satellites in any system

Global Positioning System (GPS)

A highly accurate, global satellite navigation system based on a constellation of 24 satellites orbiting the earth at a very high altitude. In addition to navigation, the system also provides very precise time

GLONASS

GLONASS (GLObal NAvigation Satellite System) is a satellite based radionavigation system which enables unlimited number of users to make all-weather 3D positioning, velocity measuring and timing anywhere in the world or near-Earth space

GMT Greenwich Mean Time

A 24 Hour system based on mean solar time plus 12 hours at Greenwich, England. Greenwich Mean Time can be considered approximately equivalent to Coordinated Universal Time (UTC), which is broadcast from all standard time and frequency radio stations. However, GMT is now obsolete and has been replaced by UTC

GPS

Global Positioning System

GPS Pcode

This is called the precise code or "protected code" and is a series of pseudorandom, binary byphase modulations on the carrier and has a chip rate of 10.23 MHz. The P code repeats about every 267 days. Each 1 week segment of the code is unique to a particular GPS satellite and is reset each week

GPS signals

GPS signals {broadcast signals of GPS and their functions are as follows}:
• L1 - 1575.42 MHz
- C/A and P codes and navigation data
• L2 - 1227.6 MHz
• L3 - 1381.05 MHz

GPS C/A code The standard GPS code known as the coarse/acquisition code or "civilian code." The code is a series of 1023 pseudorandom binary byphase modulations on the carrier and has a chip rate (bit transition time) of 1.023 MHz (often called "Standard Positioning Service

GSM

Global System for Mobile

H

Harmonic distortion

The levels of harmonic distortion are measured with respect to the fundamental signal and are specified in dB

Harmonics

A frequency is harmonic if it is an integer multiple of the fundamental frequency. The fundamental is the first harmonic (although it's generally referred to as the fundamental). The second harmonic is two times the frequency of the fundamental, the third harmonics is three times the fundamental, and so on. So with a fundamental of 100 Hz, the second harmonic is 200 Hz, the third is 300 Hz, the fourth is 400 Hz, etc

HCMOS

An output signal format defined as a rectangular wave capable of driving a multiple of CMOS loads. It has a positive signal peak greater than 90% of the supply voltage and a negative signal peak less than 10% of the supply voltage. The number of loads must be specified. Note that the "H" in HCMOS stands for high speed and does not affect the output voltage specification

HOLD OVER

To maintain time alignement if disconnected from the external source

HPFRS

High Performance Frequency Rubidium Standard

HSO

High Stability Oscillator

HTML

HyperText Markup Language

HTTP

HyperText Transfer Protocol

Hysteresis


When applied to a crystal oscillator, this refers to the difference in frequency at a given temperature or control voltage depending on how the set of conditions was approached. For example, as temperature increases to 25° C the frequency will be different than as the temperature decreases to 25°C. Hysteresis is expressed as ± 1/2 the frequency difference divided by the nominal: if the difference is 0.5 Hz and the frequency at 25°C is 10.000000 MHz then the hysteresis is ±2.5 x 10-8

Hz

Hertz

I

IEEE

Institute of Electrical & Electronic Engineers

Indirect compensation

A technique for temperature compensation in which a temperature sensitive circuit external to the oscillator is used to modify the crystal load reactance with temperature so that oscillator tuning is modified to reduce or eliminate the effects of temperature on the frequency of the oscillator

Initial frequency accuracy

A specification of the allowable range of frequencies within which the oscillator will operate at the time of shipment from the manufacturer and at a temperature of 25°C unless otherwise specified. This applies only to oscillators that are of fixed frequency (no ability to tune)

Input impedance

The load represented by input connections on the oscillator used for voltage control or tri-state control

International Atomic Time (TAI)

An atomic time scale based on data from a worldwide set of atomic clocks. It is the internationally agreed upon time reference conforming to the definition of the second, the fundamental unit of atomic time in the International System of Units (SI). It is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium – 133 atom. The TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in France. Although TAI was officially introduced in January 1972, it has been available since July 1955. Its epoch was set so that TAI was in approximate agreement with UT1 on 1 January 1958 (see second).

IP Internet protocol;

a component of protocol for many computer links including the Internet

IRIG

InterRange Instrumentation Group

ISA

Industry Standard Architecture; desktop PC adapter board specification

ISDN

Integrated Services Digital Network

ITU-T

International Telecom Union –Telecom Standardization Sector

ITU–TS

International Telecommunications Union–Transmission Standards

J

Jitter


A measure of the short term frequency stability of the oscillator. It applies only to rectangular wave forms. It is measured as the uncertainty in the location of one edge of the signal with respect to other edges. It is usually specified in units of time (nano-seconds or pico-seconds), but may also be specified in degrees. This measurement has particular application to the digital communications industry.

Julian Date (JD)

The Julian Day number followed by the fraction of the day elapsed since the preceding noon (1200 UT). {Example: The date 1900 January (1) 0.5 day UT corresponds to JD = 2 415 020}.

Julian Day

Obtained by counting days from the starting point of noon on 1 January 4713 B.C. (Julian Day zero). One way of telling what day it is with the least possible ambiguity

Julian Day - Modified Julian Day (MJD)

Equal to the Julian day. Shifted so its origin occurs at midnight on 17 November 1858. The MJD differs from the Julian date by exactly 2 400 000.5 days.

Julian Day Number (JDN)

The number of a specific day from a continuous day count having an initial origin of 1200 UT on 1 January 4713 BC, the start of Julian day zero. {Example: The day extending from 1900 January (1) 0.5 day UT to 1900 January 1.5 days UT has the number 2 415 020}.

K

kbit

kilobit

kHz

 kiloHertz

L

LAN

Local Area Network

LATA

Local Access and Transport Area

LCD

Liquid Crystal Display

Leap second

An intentional time step of one second used to adjust UTC to ensure approximate agreement with UT1. An inserted second is called a positive leap second, and an omitted second is called a negative leap second. A positive leap second is presently needed slightly more often than once per year.

LEC


Local Exchange Carrier

LED

Light Emitting Diode

Linearity

A term that applies only to VCXO’s and TCVCXO’s. It describes the maximum frequency error in following a straight line curve fitted to the actual frequencies measured at different control voltages within the control voltage range. This error is measured in percent according to the maximum deviation from the curve as a percentage of the total tuning range.

Linearity error

See linearity.

Load

See Load impedance.

Load impedance


The characteristics of the circuit connected to the output. Normally this specification will be used as part of the performance specification.

Load stability


The ability of the oscillator to maintain its design frequency over a range in load impedance. This is usually specified as a maximum variation of plus and minus parts per million over a specified percentage change in load impedance.

Long term frequency stability

An oscillator will change frequency over time due to internal and environmental changes This is often referred to as frequency drift. Causative factors include ageing of components, temperature and humidity, electrical environment, and shock or vibration. Long term frequency stability is a design criterion or set of criteria that specify the ability of the oscillator to maintain its design frequency over time. Long term stability measured in terms of seconds to years. Long term stability usually involves measurement averages beyond 100s. see also ageing

LORAN-C

Long Range Navigation, version C LOS Loss Of Signal

LPFRS

Low Profile Frequency Rubidium Standard

LTCRO


Low Temperature Coefficient Rubidium Oscillator

M

MAN

metropolitan-area network; an area network for public broadband traffic

Mbit

Megabit

Mbps

Mega bits-per-second

MCFRS

Magnetron Cavity Frequency Rubidium Standard

Mean Solar

Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun

Mechanical frequency adjustment

The range in parts per million, both above and below the center frequency to which the oscillator can be tuned by an internal or external trimmer

MHz

MegaHertz

Modified Julian Day (MJD)

Modified Julian Day (MJD): Equal to the Julian day. Shifted so its origin occurs at midnight on 17 November 1858. The MJD differs from the Julian date by exactly 2 400 000.5 days^.

Modulation

A term referring to the ability of a VCXO to be voltage controlled using a time-varying signal.

Modulation bandwidth

See modulation frequency response.

Modulation frequency response

The range of frequencies over which the modulation signal produces frequency variations of the same magnitude of Hz/volt. It can be measured by observing the sidebands produced by the modulation to be of fixed amplitude within 3 dB

Modulation range

The point at which the frequency deviation of the oscillators tuning decreases to 70.7% of its d.c. value while being modulated.

ms

millisecond

MTBF

Mean Time Between Failure

MTIE

Maximum Time Interval Error

MTTR

Mean Time To Repair

N

NMEA

National Marina Electronics Association specifies the format of GPS receiver communications that includes position, velocity and time.

 

Nominal frequency

The frequency for which the oscillator is designed and to which it is tuned.

Nominal value

The defined value of a quantity, or an ideal value whose uncertainty is 0. Frequency offset can be estimated by using the measured frequency (f1) minus the nominal frequency (f2) over the nominal frequency.

Non-harmonic distortion (Spurious responses)

Unwanted non-harmonic or spurious responses are found in the output signal of an oscillator. Their content is specified in dBc.

ns

nanosecond

NT1

Network Terminating equipment - type 1 (ISDN)

NTP

Network Time Protocol

O

OCXO

Oven Controlled Crystal Oscillator. A crystal oscillator maintained, during operation, at a fixed temperature so that frequency changes due to ambient temperature changes do not affect its performance.

On Time Marker (OTM)

The part of a time code that is synchronized to the UTC second.

Operable temperature

The temperature range over which the oscillator may be operated without damage. The oscillator is not required to perform within specification over this range. To avoid confusion this parameter is normally not specified and will never be specified when its values are the same as operating temperature.

Operating current

See supply current

Operating temperature

The temperature range over which the oscillator will operate within its specifications.

Operating voltage

See supply voltage.

Output impedance

A specification that allows calculation of the change in output amplitude with changing load impedance.

Output power

The output power calculated from the rms output voltage and the load impedance or measured directly. It is usually specified in dBm into a 50 ohm load. See Dealing with dBm for further information.

Output voltage

The specified output voltage magnitude and waveform. It may be specified in several different ways: TTL output, HCMOS output, 1 volt +/- 25% Sine wave output, or as output power.

Overall frequency accuracy

The maximum permissible frequency deviation of the oscillator due to all combinations of specified operating and environmental parameters within a specified period of time. This applies to fixed frequency oscillators.

Overall frequency stability

The maximum permissible frequency deviation of the oscillator due to all combinations of specified operating and environmental parameters within a specified period of time and at a fixed tuned frequency. This applies to oscillators which may be tuned.

Overshoot

A characteristic of circuits employing high performance rectangular wave techniques. Overshoot is the tendency of the rising and falling edges of the signal to have a damped oscillation superimposed upon them causing the voltage to momentarily rise above the normal peak signal voltage by as much as several volts.

P

Package

A definition of the style of enclosure in which the oscillator will be packaged. This may be done by drawing or may be selected from a set of standard packages. The package includes all dimensions and connection definitions.

Parts per million

A measure of frequency compared to the center frequency of the oscillator. It is defined as one Hertz per Megahertz of the oscillator frequency.A measure of frequency compared to the center frequency of the oscillator. It is defined as one Hertz per Megahertz of the oscillator frequency.

PBX

Private Branch Exchange

PCM

Pulse Code Modulation

PCS

Personal Communications Services

PDH

Plesiochronous Digital Hierarchy

PDOP

Position Dilution of Precision

Period

The time required for one repetition (cycle) of the output waveform measured in seconds or fractions of seconds. It is mathematically identical to the inverse of frequency (1/frequency).

Phase

A measure of a fraction of the period of a repetitive phenomenon, measured with respect to some distinguishable feature of the phenomenon itself. In the standard frequency and time signal service, phase time differences such as time differences between two identified phases of the same phenomenon or of two different phenomena are mainly considered.

Phase deviation

The difference of the phase from a reference.

Phase jump

A sudden phase change in a signal.

Phase locked loop

A circuit in which a crystal oscillator is used as a time base for a voltage controlled oscillator maintaining frequency and phase lock between the two oscillators. This is a useful technique for achieving a frequency programmable crystal oscillator and output frequencies that are out of the range of a crystal manufactured to achieve certain performance specifications. However, short term frequency stability is one or two orders of magnitude worse than for a true crystal oscillator.

Phase noise

In an oscillator, rapid, short-term, random fluctuations in the phase of a wave, caused by time-domain instabilities. Note: Phase noise, in decibels relative to carrier power (dBc) on a 1-Hz bandwidth, is given by = 10log[0.5(S (f ))] where S (f ) is the spectral density of phase fluctuations.

Phase shift

An intentional change in phase from a reference.

Phase signature

A deliberate phase offset for the purpose of signal identification. For example, NIST's radio station WWVB broadcast is deliberately phase shifted at 10 minutes after the hour, so a person knows that WWVB is being tracked and not some other signal.

PLL

See phase locked loop.

ppm

See parts per million.

PPS

Pulse Per Second.

Precision

The degree of mutual agreement among a series of individual measurements. Precision is often, but not necessarily, expressed by the standard deviation of the measurements.

Primary Server

Another name for a stratum 1 server. See also "stratum".

PRN

Pseudo/Random Noise Code.

PRR

Primary Reference Receiver.

PRS

Primary Reference Source.

PSTN

Public Switched Telephone Network.

R

RAFS

Rubidium Atomic Frequency Standard.

RAM

Random Access Memory.

Rectangular wave

An output waveform which is characterised by having short rise and fall times and stable at one of two flat or fixed amplitudes for most of each cycle. It may be specified as TTL or HCMOS output.

Reproducibility

With respect to a set of independent devices of the same design, it is the ability of these devices to produce the same value. With respect to a single device, it is the ability to produce the same value and to put it into operation repeatedly without adjustments.

Resolution

The degree to which a measurement can be determined is called the resolution of the measurement. The smallest significant difference that can be measured with a given instrument. For example, a measurement made with a time interval counter might have a resolution of 10 ns.

Retrace

Retrace limits the accuracies achievable with oscillator in applications where the oscillator is on-off cycled.Typical Rubidium retrace specifications: < 5 x 10-11 within 1 h after 24 h off. This one of the main advantages of a Rubidium oscillator.

Ringing

See overshoot.

Rise/fall times

A measure of the sharpness of the edges of a rectangular wave form. It is usually specified in units of time (nano-seconds) from 10% to 90% of the rated output voltage.

RMO

Rubidium Master Oscillator.

S

S-PHM

Space Passive Hydrogen Maser.

SA

Selective Availability (GPS accuracy limits for civilian users)

SDH

Synchronous Digital Hierarchy

SDN

Integrated Services Digital Network

SDV

Switch Digital Video (=BMFB or BMFS)

Second

The basic unit of time or time interval in the International System of Units (SI) which is equal to 9 192 631 770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133.

Secondary Server

Another name for a stratum 2 server.

Shock

A specification referring to the ability of the oscillator to withstand, without performance degradation, a non-repetitive acceleration. A specification is given for three mutually perpendicular axes.

Short term frequency stability

An oscillator will change frequency over time due to internal or external causes in a temporary manner. That is, there are no permanent frequency changes due to this type of instability. Short term stability can usually be measured in terms of the period of the signal frequency. Causative factors include thermal noise in active components, shock and vibration, construction parameters, and external electrical fields.

Short term stability

Short term stability usually involves measurement averages from a few tenths of a second to 100s. Also called Sigma Tau and Allan Variance or Allan Deviation. See also Allan Variance and Allan Deviation

Sidereal time

The measure of time defined by the apparent diurnal motion of the vernal equinox; hence, a measure of the rotation of the Earth with respect to the reference frame that is related to the stars rather than the sun. Two types of sidereal time are used in astronomy: mean sidereal time and apparent sidereal time. One sidereal day is equal to about 23 hours, 56 minutes, and 4.090 seconds of mean solar time. Also, 366.2422 mean sidereal days equal 365.2422 mean solar days.

Sine wave

An output waveform described by a sine curve. A voltage or decibel value and tolerance must be included to complete the specification.

Slew

To adjust gradually the time of a clock until it tells the correct time. Compare with "step"

SNR

Signal to Noise Ratio

Solar time

SOH section overhead; a group of management communications facilities in SDH
Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis. The UT is sometimes designated GMT, but this designation should be avoided.

SONET

synchronous optical network; the North American variant of SDH.

SPE

Synchronous Payload Envelope.

Spurious

These extra vibration modes produce additional, unwanted electrical responses which distort the filters response. These are commonly called 'spurious responses'. The most troublesome spurious responses fall on the high side of the passband, generally from 100 kHz to 1 MHz above center frequency.

Spurious responses

See Non-harmonic distortion.

Square wave

A rectangular wave output waveform having a 50% symmetry. If specified, the tolerance to the symmetry must be specified. TTL, LSTTL, CMOS and HCMOS outputs are described as square wave and comply to the wave form drawing at right.

SRO

Synchronized Rubidium Oscillator.

SSU

Synch Supply Unit.

Start-up time

The maximum time that the oscillator requires to reach stable operation within specifications after application of the supply voltage.

Step

To change the time of a clock to the correct time with no intermediate adjustments. Compare with "slew".

STM

synchronous transport module; the basic unit of transmission in SDH.

Storage temperature

The temperature range over which the oscillator may be stored, non-powered, without damage.

Stratum

The term "stratum" is referenced in both the network world and the telecommunications world. Dr. Davis Mills, University of Delaware, is the author of the RFC 1305 that is NTP (Network Time Protocol). He developed a hierarchical structure in which Stratum 0 is the USNO clock. Stratum 1 is a radio receiver that receives the time from Stratum 0. Stratum 2 is a client that receives the time over a network connection from a Stratum 1 clock. Stratum 3 is a client that gets the time from Stratum 2... and so on to a theoretical Stratum 15.
In the telecommunication world, stratum refers to the holdover performance of an oscillator in the event of loss of synchronization. Stratum 1, Stratum 2, Stratum 3, Stratum 4 are the most typical.

Stratum clocks Accuracy requirements placed on clocks in four strata. Accuracy of stratum clocks refers to clock performance when the clock receives no input reference.

STS-n

Synchronous Transport Signal level n.

Supply Current

The maximum operating current of the oscillator at the nominal supply voltage.

Supply voltage

The operating voltage (or range) of the oscillator.

Supply voltage stability

See frequency voltage tolerance.

SVN

Satellite Vehicle Number.

Symmetry

The ratio of the positive portion of the output signal to the entire period of the output signal. This applies only to rectangular wave outputs. The voltage at which to measure the symmetry is 1.4v for TTL and 50% of supply voltage for CMOS/HCMOS. Other rectangular output formats should include a voltage at which to measure symmetry.

Synchronization

The process of measuring the difference in time of two time scales such as the output signals generated by two clocks. In the context of timing, synchronization means to bring two clocks or data streams into phase so that their difference is 0 (see time scales in synchronism).

Synchronize clocks

To set two clocks to the same time and ensure that they are running at the same "speed". The speed at which a clock runs is determined by its frequency, that is, how often it "ticks" to the next fraction of a second. The design of a particular clock determines how small that fraction is.

Syntonization

Relative adjustment of two frequency sources with the purpose of cancelling their frequency difference but not necessarily their phase difference.

T

TAI

International Atomic Time.

TCG

Time Code Generator.

TCP/IP

Transmission Control Protocol/Internet Protocol.

TCVCXO

Temperature compensated voltage controlled crystal oscillator.

TCXO

Temperature Compensated Crystal Oscillator.

TDEV

Time DEViation.

TDOP

Time Dilution of Precision.

Temperature

As applied to crystal oscillators, temperature is specified in degrees Celsius. Normally the center of all temperature ranges is +25 degrees Celsius.

Temperature compensation

A technique used to stabilise the frequency of a crystal oscillator over a specified temperature range. The temperature range and the limits of frequency change are specified.

Temperature stability

The ability of the oscillator to maintain its design frequency over a temperature range. This is usually specified as a maximum variation of plus and minus parts per million over the specified temperature range.

Terrestrial Time (TT)

The new 1991 International Astronomical Union replacement for what was once called Ephemeris Time. On 1 January 1997, TT = TAI + 32.184 seconds, and the length of the second is chosen so that it agrees with the International Second (SI) on the geoid. The TT scale differs from the old Ephemeris Time in its conceptual definition. Practically, however, it is realized by means of International Atomic Time (TAI).

Test load impedance

Usually identical to the load impedance, this is the characteristic of the external circuit connected to the oscillator during performance testing.

THD

See total harmonic distortion.

TIA/EIA

Standard Mobile Station-Base Station compatibility standard for wideband spread spectrum cellular systems.

TIE

Time Interval Error.

Time code

A system of symbols (digital or analog) used for identifying specific instants of time. An information format used to convey time information.

Time interval

The duration between two instants read on the same time scale.

Time scale

A system of unambiguous ordering of events. A time scale is meant to be stable and homogeneous.

Time stability

See frequency ageing.

Time standard

A continuously operated device used for the realization of a time scale in accordance with the definition of the second and with an appropriately chosen origin.

Time step

A discontinuity in a time scale at some instant. A step is positive (+) if the time scale reading is increased and negative (-) if the reading is decreased at that instant.

Tolerance

The allowable variation in specified parameters where the oscillator is considered to be within operating specifications.

Total harmonic distortion

The remaining signal appearing at the output of the oscillator after the signal at the operating frequency is subtracted. It is expressed in percent of the rms value signal at the operating frequency. This term only applies to sine wave output wave forms

Transfer function

See Deviation slope polarity.

Tri-state output

A definition of the ability to turn the output signal on and off. It must include a definition of the off state (logic high, logic low, high impedance, etc.)

Trimmer

A mechanical device used to tune the crystal oscillator. It may be a variable capacitor or some other suitable device.

TSG

Timing Signal Generator.

TTL

An output signal format defined as a rectangular wave capable of driving a multiple of TTL loads. It has a positive signal peak greater than +2.4 volts and a negative signal peak less than +0.5 volts. The number of loads must be specified.

TU-T

International Telecom Union -Telecom Standardization Sector.

TVAR

Time VARiance.

U

UI

Unit Interval.

Uncertainty

The limits of the confidence interval of a measured or calculated quantity. NOTE: The probability of the confidence limits should be specified, preferably as two standard deviations.

Universal Time (UT)

Family Universal Time (UT) is the general designation of time scales based on the rotation of the Earth. In applications in which a precision of a few tenths of a second cannot be tolerated, it is necessary to specify the form of UT such as UT1 which is directly related to polar motion and is proportional to the rotation of the Earth in space. The UT1 is further corrected empirically for annual and semi-annual variations in the rotation rate of the earth to obtain UT2.
Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis. The UT is sometimes designated GMT, but this designation should be avoided. Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun.

us

microsecond

USO

Ultra Stable Oscillator

UTC

Universal Time Coordinated

V

VCXO

Voltage Controlled Crystal Oscillator

VF

Voice Frequency

Vibration

A specification referring to the ability of the oscillator to withstand, without performance degradation, a repetitive acceleration. Usually a specification is given for one or more frequency ranges.

VLBI

Very Long Base Line Interferomertry

Voltage coefficient

The amount of frequency deviation caused by a change in the oscillators supply voltage.

Voltage frequency adjustment

The range in parts per million, both above and below the center frequency to which the oscillator can be tuned by the control voltage.

W

WAN

Wide Area Network

Warm-up

When power is applied to a frequency standard, it takes a finite amount of time before the equilibrium frequency stability is reached. The warmup time of an oscillator is a function of the thermal properties of the resonator, the oscillator circuit, the input power, and the oscillator's temperature prior to turn-on.

WCDMA

Wideband Code Division Multiple Access (WCDMA) is a wideband radio technique that
provides far higher data rates than other radio techniques available today.

X

XO

Crystal oscillator. This nomenclature is often used to refer to a crystal oscillator of fixed frequency that is not temperature compensated.