Category Name > General
Glossary of Terms

Atomic Clock
An atomic clock is an extremely accurate timekeeping device regulated by the natural regular oscillations of an atom or molecule. An atomic clock powered by a hydrogen atom (maser) is accurate to 1 part in 2 quadrillion. A cesium atom powered clock has an error of about one second in one million years. GPS satellites carry multiple atomic clocks, regulated by these precise atomic vibrations, to ensure accuracy. The GPS Master Control Station uses cesium clocks and a hydrogen master clock.

Atomic Oscillator

An oscillator that uses the quantified energy levels in atoms or molecules as the source of its resonance.

A circuit board (usually a printed circuit board) that connects several connectors in parallel, forming a computer bus. It is used as a backbone to connect several printed circuit board cards together to make up a complete computer system. Backplanes are normally used in preference to cables because of their greater reliability. A backplane is generally differentiated from a motherboard by the lack of on-board processing power where the CPU is on a plug-in card.


Bandwidth is the range of frequencies that a communication cable or channel can carry. It generally refers to the amount of data that can be carried by a specific cable or bus for computer.


In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. A bus logically connects several peripherals over the same set of wires. (Also, part of a chip or circuit board designed to send and receive data. The bus on a computer’s motherboard contains slots for expansion cards.)


Bus Level Products
Plug-in timing boards for various computing platforms


Bus Level Timing
Board level products may be used to time synchronize one or more computers precisely and/or provide very precise time to the host or a application program.


Bus Timing Instrumentation
Bus Level Timing boards provide time stamping and synchronization requirements. Synchronizes one or more computers to an external time standard to provide very accurate time stamps to application programs.  Bus level timing also can be used to store the time occurrence of external events.



A comparison between a device under test and an established standard. The official time standard is known as UTC. This same standard is used to calibrate Frequency in hertz which is the inverse of seconds. When the calibration is finished it should be possible to state the estimated time offset and/or frequency offset of the device under test with respect to the standard, as well as the measurement uncertainty.


A hardware expansion unit that is installed inside the case of a computer, into an expansion slot on the motherboard.

Cesium Clock
A clock containing a cesium standard as a frequency-determining element.


Cesium Frequency Standard
Cesium frequency standards work by placing Cesium atoms from a reservoir that’s heated to boil off some atoms from a small piece of cesium liquid metal. These atoms are processed in such a way that they become an atomic frequency standard.


Cesium Oscillator
Often used as a primary frequency standard since the SI second is defined from the resonance frequency of the cesium atom (133Cs), which is 9,192,631,770 Hz. A properly working cesium oscillator should be close to its nominal frequency without adjustment, and there should be no change in frequency due to aging. Environmental conditions (motion, vibration, magnetic fields, and so on) do cause small frequency shifts.


A channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite.


Coaxial Cable
A type of cable that is commonly used due to its insensitivity to noise interference. Coaxial cable is often used in Ethernet networks; both thick (10Base5) and thin (10Base2). Ethernet cables are coaxial.


Code Phase GPS
GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.


Compact PCI
PCI bus technology implemented on a Eurocard. Eurocards provide more rugged packaging and a more secure plug and socket for embedded systems than the standard PCI card used in desktop computers. Compact PCI may support hot swap and provides higher performance (32-bit, 33MHz).


Crystal Oscillator
An oscillator that produces electrical oscillations at a frequency determined by the physical characteristics of a piezoelectric quartz crystal.


Compact Peripheral Component Interconnect – a bus standard


Atomic representation for cesium


Daisy Chain
A method of connecting devices on a bus. On a daisy chained bus, devices not requesting a signal respond to it by passing it on. The daisy chain scheme permits assignment of device priorities based on the electrical position of the device on the bus.


Decibel (dB)
A measure of the ratio between two quantities, and can be used to express a wide variety of measurements in acoustics and electronics. The decibel is a “dimensionless unit” like percent. Widely used as a measure of the loudness of sound.


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


DoD or DOD
Abbreviation for The United States Department of Defense, sometimes called the Defense Department. A civilian Cabinet organization of the United States government. The Department of Defense controls the U.S. military and is headquartered at The Pentagon. It is headed by the Secretary of Defense.


Electrically Erasable Programmable Read Only Memory is a type of ROM that can be erased electronically and reprogrammed in-circuit (or with a device programmer). . From the programmer’s perspective, EEPROM is very similar to flash memory. The biggest difference is that the bytes (words) of an EEPROM can be erased.


Embedded System
A combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a dedicated function. In some cases, embedded systems are part of a larger system or product.


A data transport standard for Local Area Networks (LANs). All hosts are connected to a coaxial cable where they contend for network access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) paradigm.


Fiber Optic Distribution
A technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.


Frequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the length of the time interval. In SI units, the result is measured in hertz (Hz) after the German physicist, Heinrich Rudolf Hertz. 1 Hz means that an event repeats once per second. Other units that have been used to measure frequency include: cycles per second, revolutions per minute (rpm). Heart rate is measured in beats per minute.


Frequency and Time Standards
A radio communication service for scientific, technical and other purposes, providing the transmission of specified frequencies, time signals, or both, of stated high precision, intended for general reception.


Frequency Spectrum
The distribution of signal amplitudes as a function of frequency.


Frequency Standard
A stable oscillator used for frequency calibration or reference. (188) Note 1: A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points. Note 2: Frequency standards in a network or facility are sometimes administratively designated as “primary” or “secondary.”


GPS (Global Positioning System)
The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR GPS), is a satellite navigation system used for determining one’s precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit. It uses an intermediate circular orbit (ICO) satellite constellation of at least 24 satellites. The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The satellites orbit the earth at approximately 12,000 miles above the surface and make two complete orbits every 24 hours. The GPS satellites continuously transmit digital radio signals that contain data on the satellites location and the exact time to the earth-bound receivers. The satellites are equipped with atomic clocks that are precise to within a billionth of a second. By knowing how far away a satellite is, the receiver knows that it is located somewhere on the surface of an imaginary sphere centered at the satellite. By using three satellites, GPS can calculate the longitude and latitude of the receiver based on where the three spheres intersect. By using four satellites, GPS can also determine altitude.


Institute of Electrical and Electronic Engineers, an international organization that sets standards for electrical and computer engineering. It has the most members of any technical professional organization in the world, with more than 360,000 members in around 175 countries.

IEEE-1588 IEEE-1588 is the industry standard for synchronization delivery over packet networks required within the telecommunications industry. To read more about this standard, see this news release or look at our PTP (Precise Time Protocol) products.


Part of a computer, program, or peripheral that communicates with other components. Interface also refers to the user interface.


This is a signal on which the processor stops execution of the current command sequence and transfers control to the program-handler of the interrupt. The program-handler address is calculated by the interrupt vector table. An interrupt may be initialized either by user programs, while working with disks, screen, printer etc (program interrupts), or by hardware: keyboard, timer (hardware interrupts).


Interrupt Latency
The amount of time between the assertion of an interrupt signal and the start of the associated interrupt service routine.


Interrupt Request
On IBM, PC and compatible computers, hardware lines for devices send interrupts. Typically each device connected to the computer uses a separate IRQ. Expansion cards such as Ethernet cards or token ring adaptors also use IRQs. Setting the IRQ so that it does not conflict with other devices is an essential part of installing expansion cards and new devices.


I/O (Input/Output)
The interface between a processor and the world around it. The simplest examples are switches (inputs) and LEDs (outputs).


IRIG is a serial time format standard.


IRIG Time Codes

The IRIG time codes, originally developed by the Inter-Range Instrumentation Group (IRIG), now used in government, military and commercial fields.  There are many formats and several modulation schemes, but they are typically amplitude modulated on an audio sine wave carrier.  The most common version is probably IRIG-B.

Sends day of year, hour, minute, and second data on a 1 kHz carrier frequency, with an update rate of once per second (1PPS).

(Industry Standard Architecture) An expansion bus commonly used in earlier PCs that accepted plug-in boards for sound, video display and other peripheral connectivity. Originally called the “AT bus,” introduced with the IBM PC AT in 1984, the AT/ISA bus extended the PC bus from 8 to 16 bits.


LAN (Local Area Network)
A group of personal computers linked together in order to share programs, data, and peripherals.

LED (Light Emitting Diode)
Used for display in some electronic devices.


Low Phase Noise
The ratio of the power density of one phase modulation sideband to the total signal. It is usually specified as the single side band (SSB) power density in a 1Hz bandwidth at a specified offset frequency from the carrier. It is measured in dBc/Hz. The best short- and long-term stability can be obtained only if the frequency synthesis does not introduce noise that masks the atomic noise limit. In the best standards, the signal used for atomic interrogation must have exceptionally clean spectral purity.


Master Clock
A generator which generates an accurate frequency signal for the control of other generators; Also known as a PRC


The MIL-STD-188 standards were created to address telecommunication design parameters based on proven technologies. To ensure interoperability, DISA made these standards mandatory for use in all new DoD systems and equipment, or major upgrades.  MIL-STD-188-164A states:

“Interoperability of SHF Satellite Communications Earth Terminals”, establishes interoperability and performance requirements for SATCOM earth terminals (ETs) operating with satellite transponders in the C-band, X-band, Ku-band, and commercial and military Ka-bands. The MIL-STD’s content and configuration is controlled by the Defense Information Systems Agency (DISA). The MIL-STD defines the requirements for ET certification, which is administered by the Department of Defense (DOD) for all ETs developed for use by U.S. forces over X-band and military Ka-band.”

To find out more about this click here for the link to the Military Satellite Transmission


The main circuit board, containing the CPU. All other functions and peripherals are connected in some way to the motherboard.


Multi-Channel Receiver
A GPS receiver that can simultaneously track more than one satellite signal


One-billionth of a second


Network Synchronization
A generic concept that depicts the way of distributing a common time and/or frequency to all elements in a network.


National Institute of Standards and Technology, the National Measurement Institute in the United States. The institute’s mission is to develop and promote measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. NIST is directly involved in standards development and testing done by the private sector and government agencies. U.S. technological innovation and progress depend on NIST’s unique skills and capabilities, especially in four key areas: biotechnology, nanotechnology, information technology and advanced manufacturing.


NTP – Network Time Protocol
Network Time Protocol provides a reliable way of transmitting and receiving time over TCP/IP networks and is useful for synchronizing the internal clock of computers to a common time source to millisecond accuracy.  Running as a continuous background client program on a computer, NTP send a periodic time request to servers, obtaining timing information from a Time Server and using them to adjust the client’s work.



Oven-controlled X-tal (crystal) Oscillator is used for holdover of time and frequency.  The Quartz Oscillator is housed in an “oven”, an insulated package with a heating element and temperature sensor, to improve accuracy as one of the largest contributors to frequency drift of quartz oscillators is due to temperature changes.


Operating System
In computing, the system software responsible for the direct control and management of hardware and basic system operations. Additionally, it provides a foundation upon which to run application software such as word processing programs and web browsers.An operating system typically consists of a set of system calls and a periodic clock tick ISR. The operating system is responsible for deciding which task should be using the processor at any given time and for controlling access to shared resources. There are a number of different operating systems, which have different design philosophies, provide different facilities, and have different looks and feels. Among popular operating systems are MacOS, MS-DOS, the various incarnations of Windows, Unix in its various flavors, OS/2, etc. See also real-time operating system.


An electronic circuit that produces an output signal of a specific frequency. An oscillator generally consists of an amplifier having part of its output returned to the input by means of a feedback loop; the necessary and sufficient condition for oscillation is that the signal, in passing from input to output and back to input via the feedback loop, arrive at the input with no change in amplitude or phase. If this condition is met for only a single frequency, the output is a pure sine wave; if it is met for more than one frequency, the output is a complex wave. Some oscillators are designed to operate under certain conditions so that the output is a square wave, a triangular wave, or a pulse.


Oscillator Crystal
A timing device that consists of a crystal and an oscillator circuit, providing an output waveform at a specified reference frequency.


Peripheral Component Interconnect, a local bus that supports high-speed connection with peripherals. It plugs into a PCI slot on the motherboard. The Peripheral Component Interconnect standard (in practice almost always shortened to PCI) specifies a computer bus for attaching peripheral devices to a computer motherboard. These devices can take the form of integrated circuits fitted on the motherboard itself (called planar devices in the PCI specification), or expansion cards that fit in sockets. The PCI bus occurs commonly in PCs, where it has displaced ISA and VESA Local Bus as the standard expansion bus, but it also appears in many other computer types. The bus will eventually be succeeded by PCI Express and other technologies, which have already started to appear in new computers.The PCI specification covers the physical size of the bus (including wire spacing), electrical characteristics, bus timing and protocols.


The PCI Local Bus is a high performance bus for interconnecting chips, expansion boards, and processor/memory subsystems. A high-speed bus for 486, Pentium and compatible systems. PCI stands for Peripheral Component Interconnect. Peripheral Component Interconnect is common to newer Macintosh and Pentium machines. This bus features faster bus throughput, as well as hardware compatibility across computer platforms, so a card could be used on both a PCI Mac and a PCI Pentium machine, though different software drivers would be needed. pci bus specification PCI is a synchronous bus architecture with all data transfers being performed relative to a system clock (CLK). The initial PCI specification permitted a maximum clock rate of 33 MHz allowing one bus transfer to be performed every 30 nanoseconds. Later, Revision 2.1 of the PCI specification extended the bus definition to support operation at 66 MHz, but the vast majority of today’s personal computers continue to implement a PCI bus that runs at a maximum speed of 33 MHz. The current PCI specification is 2.3. The specification can be purchased from the PCI Special Interest Group (PCISIG).


PCI Card
A card designed per the PCI specification. PCI (Peripheral Component Interconnect) is an interconnection system between a microprocessor and attached devices in which expansion slots are spaced closely for high-speed operation.


PCI Express
A high-speed peripheral interconnect from Intel introduced in 2002. Note that although sometimes abbreviated “PCX,” PCI Express is not the same as “PCI-X”. As a result of the confusion, “PCI-E” is the accepted abbreviation. PCI Express was designed to match the higher speeds of today’s CPUs, accommodating Gigabit and 10 Gigabit Ethernet and even support chip-to-chip transfers. PCI Express provides a high-speed, switched architecture. Each PCI Express link is a serial communications channel made up of two differential wire pairs that provide 2.5 Gbits/sec in each direction. Up to 32 of these “lanes” may be combined in x2, x4, x8, x16 and x32 configurations, creating a parallel interface of independently controlled serial links. The bandwidth of the switch backplane determines the total capacity of a PCI Express implementation.


Phase-Locked Loop (PLL)
An electronic circuit that controls an oscillator so that it maintains a constant phase angle (i.e., lock) on the frequency of an input, or reference, signal. A PLL ensures that a communication signal is locked on a specific frequency and can also be used to generate, modulate and demodulate a signal and divide a frequency.


PCI Mezzanine Card.


Precise Positioning Service (PPS), is the most accurate positioning, velocity, and timing information continuously available, worldwide, from the basic GPS. This service is limited to authorized U.S. and allied Federal Governments; authorized foreign and military users; and eligible civil users.


Precision Frequency
Frequency assigned for usual use on a particular circuit. The first-choice frequency that is assigned to a fixed or mobile station for radiotelephone communications.


Precise Time
A time mark that is accurately known with respect to an accepted reference time standard.


Primary Reference Source
Equipment that provides the highest quality sync for telecom networking as required by international standards. Typically derive source timing from GPS or CDMA signals or from stand-alone sources such as cesium.


Precision Time Protocol (PTP)
The protocol defined by the IEEE 1588 standard.

IEEE 1588
IEEE 1588 is a standard published by the IEEE Instrumentation and Measurement Society (Std IEEE 1588-2002). Per the standard it is a protocol to synchronize independent clocks running on separate nodes of a distributed measurement and control system where a high degree of accuracy and precision is specified. The protocol is independent of the networking technology, and the system topology is self-configuring.


P(Y) Code
See P-code

The Precise code. A very long sequence of pseudo random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz which repeats about every 267 days. Each one week segment of this code is unique to one GPS satellite and is reset each week.

Quartz Crystal Oscillators
A timing device that consists of a crystal and an oscillator circuit, providing an output waveform at a specified reference frequency


Real-Time Clock
The CMOS circuitry that keeps track of the year, day and time of day for the system. Abbreviated RTC


Real-Time System
Any computer system, embedded or otherwise, that has timeliness requirements.


RF Distribution
Delivering a centralized reference signal to a number of different equipment stations without degrading the signal or creating crosstalk.



The Restriction of Hazardous Waste Directive is a European Union standard that restricts the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment including lead.


In telecommunications,TIA 232 (formerly EIA-232 ,formerly EIA RS-232, the ‘RS’ meaning Recommended Standard) is a standard for serial binary data interconnection between a DTE (Data terminal equipment) and a DCE (Data communication equipment). It is commonly used in computer serial ports. This standard was originally used for specifying the connection between an electromechanical teletypewriter and a modem. When electronic terminals began to be used, they were often designed to be interchangeable with teletypes, and so supported RS-232. It is used for all sorts of remote communications, especially through modems, including computer to computer.


EIA-422 (formerly RS-422) is a serial data communication protocol that specifies 4 wire, full-duplex, differential line, multi-drop communications. It provides for balanced data transmission with unidirectional/non-reversible, terminated or non-terminated transmission lines. In contrast to RS485 (which is multi-point instead of multi-drop) EIA-422 does not allow multiple drivers but only multiple receivers.


EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and 7 volts will allow correct operation of the receiver. EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol.


Soft silvery metallic element of the alkali metal group; burns in air and reacts violently in water; occurs in carnallite and lepidolite and pollucite.


Rubidium Frequency Standard
A frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency. Note: A rubidium standard consists of a gas cell, which has an inherent long-term instability. This instability relegates the rubidium standard to its status as a secondary standard.


Rubidium Oscillator

The lowest priced members of the Atomic Oscillator family, rubidium oscillators operate at 6,834,682,608 Hz, the resonant frequency of the rubidium atom (87Rb), and use this rubidium frequency to control the frequency of a quartz oscillator so it is much more accurate.

Selective Availability Anti-Spoofing Module-  This module was chosen by the US Joint Chiefs as the security architecture of GPS devices.  It is required for any US military GPS use. To find out more see Brandywine’s PTS-SAASM


Serial Port
A computer input/output port that transmits data sequentially, one bit at a time. The serial ports on the back of the computer are the DB-9 and DB-25 male connections. (DB connectors are the ‘D-shaped’ ports). Serial ports are used for modems, mice, certain printers and communication devices.


Serial Time Code
Uniquely coded digital serial stream of data (usually in a pulse code modulation form) representing time of year, time of day or countdown time.


Simple Network Management Protocol

A TCP/IP-based protocol network management protocol that is used to monitor networks and devices.  Also can be used for exchanging network management information and outlining the structure for communications among network devices.


Simple Network Time Protocol

A subset of the Network Time Protocol, SNTP simplifies the access strategy for servers and clients using NTP, SNTP, can operate in unicast (point to point), multicast (point to multi-point) or anycast (multi-point to multi-point) modes. SNTP can be used when the ultimate performance of the full NTP implementation is not necessary.


Source Code
A program in the form of an ASCII text file. Programs as they are written in a programming language, such as C and Pascal. In order to run the program, source code must be translated into machine code by either an interpreter or a compiler.


Stratum Level
Standards set by Network Time Protocol RFC 1305. The highest level are Stratum 0 devices such as GPS, which get their time from a primary time source such as a national atomic clock. Stratum 1 servers, such as TymServe, source their time from a Stratum 0 device. Stratum 2 and beyond obtain their time from Stratum 1 servers. The further away a network is from a primary source, the greater the chance of signal degradations due to variations in communications lines and other factors.



The process of setting two or more clocks to the same time


Transmission Control Protocol over Internet ProtocolProvides dependable communication and multiplexing. It is connection-oriented, meaning it requires a connection be established data transfer. It sits on top of the Internet Protocol (IP), which provides packet routing. This is connectionless, meaning each data packet has its source and destination data embedded, so it can bounce around a network and still get to its destination.


Time Code
A code (usually digital) that contains enough information to synchronize a clock to the correct time-of-day. NIST time codes can be obtained from the WWV, WWVH, WWVB, GOES, ACTS, and Internet Time Services.


Time Stamp
A record mathematically linking a document to a time and date.


Time Standards
A time standard is a specification of either the rate at which time passes, or points in time, or both. For example, the standard for civil time specifies both standard time intervals and time-of-day.


Universal Serial Bus, a port that allows the hot-plugging of multiple external plug-and-play devices. A Universal Serial Bus is an external bus with a maximum transfer speed of 12 megabits per second. It’s hot-swappable, which means that a device can be connected or disconnected while the computer is running. The Operating System can recognizes and uses the device as soon as its plugged in. Up to 127 devices can be daisy-chained together.



UTC (known in English-language speaking countries as Coordinated Universal Time) is the international standard for time.  UTC time is based on atomic clocks and is linked to the solar-day by an automatic adjustment of leap second whenever the two are out of sync by one second.


Video over IP. Similar to Voice over IP but typically at higher and constant bit rates (CBR).


Versa Module Europe; VMEbus specification governed by the VSO.


(Versa Module Europa) is a fast, flexible, open-ended bus system that uses the Eurocard standard. VME bus supports a variety of computing tasks in industrial applications. It is defined by the IEEE 1014-1987 standard. VMEbus supports many specialized applications and industries, including: industrial controls, military, aerospace, telecommunications, medical, etc.


Voice over IP. A set of facilities for managing the delivery of voice information over a packet-switched network.


VXI (VME eXtensions for Instrumentation)
A peripheral bus based on VME specialized for data acquisition and realtime control systems. Introduced in 1987, VXI uses all Eurocard form factors and adds trigger lines, a local bus and other functions suited for measurement applications.

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