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AM to AM conversion

Amplitude modulation-to-amplitude modulation conversion is used to describe a type of distortion in microwave amplifiers. It is the change in the output radio frequency (RF) voltage produced by variations in input signal level, usually expressed in dB/dB. A linear amplifier has an AM/AM conversion of 1 dB per dB.

AM to PM conversion

AM/PM is the amount of phase change in the output signal when the input amplitude is changed and is typically expressed in degrees per dB. A perfectly linear amplifier has no change in output phase when input drive is changed. As a tube is saturated the electrical length tends to become shorter.

Amplification factor (Mu)

Amplification factor in a triode is defined by the change in anode voltage divided by the change in grid voltage at a constant anode current and is assigned the Greek letter Mu (µ). Comparing constant current curves for tubes with different Mu factors, one can see that the slope of anode current varies directly with Mu. Likewise, tetrodes have a screen Mu factor.


An amplifier is a device that increases the power level of an input signal.

Amplitude pushing factor

The amount of power change experienced as the voltage on a particular tube element is varied, usually expressed in decibels (dB) per volt.


In an electron gun, the electrons are accelerated from a cathode to or through an anode that is at a positive potential with respect to the cathode. In klystrons and traveling-wave tubes (TWTs), the cathode is negative with respect to ground and the anode can be above ground, at ground, or between ground and cathode. In triodes and tetrodes, the anode is positive with respect to the cathode but either element may be grounded depending on the circuit design.

Antenna control unit

A device containing a processor calculating the target’s location and a servo system that controls the positioning drive motors of a communications antenna to optimize pointing toward the desired target. Several different types of antenna control units have been developed to support antennas of any size tracking satellites in any orbit, resulting in various levels of complexity. Large antennas operating at high microwave frequencies require very precise antenna pointing and tracking, making antenna control units operating under those conditions more complex.

Antenna feed

A device in a parabolic reflector antenna that focuses the desired RF energy from a waveguide or coaxial input onto the primary or secondary reflector. An antenna feed typically includes a feed horn, an Ortho-Mode Transducer and a diplexer/Transmit Reject Filter to separate uplink and downlink satellite signals. Antenna feeds cover specific frequency bands and signal polarizations and can support one or multiple RF paths simultaneously. They are optimized for the optics of the specific reflectors in which they are installed.

Antenna gain

The measurement of the ability of an antenna to focus RF energy in a desired direction. Antenna gain is usually specified in terms of dBi, or dB to isotropic, in which isotropic is equivalent to RF energy radiating equally in all directions as if on the surface of a sphere.

Average power

The average power is the peak instantaneous power averaged over one period. For a rectangular pulse the average power is equal to the peak power times the duty cycle, where the duty cycle is the ratio of the “on” time to the period. More complex definitions exist if the circuit has reactance.

Az/El mount

One of the least complex and most common forms of an antenna mount, in which an elevation axis is mounted above an azimuth axis and two separate adjustments are required to move from one satellite to another. Az/El mounts can be fixed, to point at only one satellite, or motorized to permit moving from one satellite to another.


The angle of horizontal rotation that an antenna must be rotated through to point at a desired target. Azimuth angles can be calculated based upon the latitude and longitude of an earth station and the location of a desired target. For convenience, Azimuth angles are usually defined relative to true North.



Backheating occurs in a power grid tube when electrons are repelled back onto the cathode surface, thereby raising its operating temperature. This occurs because of transit time effects at VHF, UHF and microwave frequencies. Carefully reducing the heater voltage will compensate for backheating and prevent a compromise in cathode life that may occur if the heater is operated otherwise.


Typically refers to the measure of an amplifier’s reduced output power from saturation to obtain more linear operation. With analog modulation formats, backoff often gave a reasonable estimate of linear power capabilities. With modern digital modulation formats, backoff is less significant than other measures.


Backstreaming refers to backstreaming electrons, which do not stay in the depressed collector, but travel backwards (toward the cathode) down the tube. These electrons can cause distortion of the RF signal and spurious signals.

Backward wave oscillations (BWO)

BWO is instability in a Helix TWT where infinite (or very high) gain exists in the opposite direction to the electron beam. Sometimes BWOs can occur when tubes are run at operating conditions beyond their respective specifications.

Beam efficiency

The RF output power divided by the beam power (cathode current times cathode voltage).

Beam power

The amount of power in the electron beam equal to the cathode voltage times the beam current.


The angle or conical shape an antenna projects, usually referred to in terms of the 3dB beamwidth, or the angle at which RF signal strength is reduced 3dB from signal peak. In parabolic antennas, beamwidth is a function of antenna diameter to wavelength, and it is an important consideration in determining how far satellite targets must be separated to ensure acceptable levels of RF signal interference.

Bit error rate (BER)

The fraction of a sequence of message bits in a signal that are in error over the measurement period.

BITE (Built in Test Equipment)

Any type of internal built-in capability which enabled the device to check on and report its status via an output signal of some type.

Block down converter (BDC)

A solid-state component in a satellite communications (satcom) system that converts the downlink RF signals to a lower IF frequency, usually in L-Band, for modem signal processing. The converter is called a "block down" because it translates a large number of frequencies from the downlink signal band to one or more IF frequency bands.

Block up converter (BUC)

A solid-state component in a satellite communications (satcom) system that converts the input signals, usually in L-Band, to higher frequencies for transmission to the satellite. The converter is called a "block" converter because it translates a large range of frequencies to the transmit band; "non-block" converters only translate one transponder of bandwidth at a time. BUCs come in several bands (C-Band, Ku-Band, X-Band, and Ka-Band are BUCs commonly produced by CPI). In some cases, one BUC can cover the entire transmit band, but in other cases such as Ku- and Ka-bands, the satellite bandwidth is so wide that several BUCs are required to cover the entire satellite’s frequency band.
Note: A BUC can also refer to a complete amplifier that includes not only the converter, but a built-in power amplifier unit that provides the power for transmission to the satellite. This alternate definition can cause confusion. See SSPB.

Body/helix protection

Helix protection is a circuit in a tube power supply that shuts the tube off when body or helix current exceeds a threshold that has been determined to damage the tube.

Breakdown power (minimum firing power)

In a receiver protector, breakdown power is the maximum full pulse leakage power which will occur just prior to the onset of hard limiting.

Brillouin Field

The amount of magnetic field that is required to maintain a (non-thermal) electron beam at a constant diameter when there is no flux threading the cathode. Any flux threading the cathode requires a higher magnetic field to maintain the beam at a constant diameter.

Broadcast satellite service (BSS)

The International Telecommunications Union definition of RF spectrum assignments for services intended to broadcast signals from satellites directly to subscribers. In the industry, this service is more commonly called DBS, or direct broadcast service.



A measure of a satellite carrier signal (“C”) to the noise density (“k” which is Boltzmann’s Constant and “T” which is noise temperature) within the occupied carrier bandwidth, which is usually specified in terms dB-Hz. A higher C/kT indicates a better, or stronger, satellite signal-to-noise ratio and vice versa.


A measure of a satellite carrier signal (“C”) to the total noise energy (“N”) within the occupied carrier bandwidth, which is usually specified in terms dB. A higher C/N indicates a better, or stronger, satellite signal-to-noise ratio and vice versa.


An electrode that emits electrons and may be negatively charged in beam tubes. Thermionic emitters are used in klystrons and TWTs, where electron emission results from heating the cathode to temperatures ranging from 800 degrees to 1100 degrees Celsius. Oxide cathodes are generally used for pulse applications. B type cathodes are uncoated tungsten cathodes with various types of impregnants. M type cathodes have a coating containing Osmium on the surface that reduces the workfunction, and thus the operating temperature, resulting in a longer wear-out life. Most power grid tubes employ either oxide cathodes of various types or thoriated tungsten (directly heated) filaments as thermionic emitters.

Cathode loading

The electron current density along the surface of the cathode. Spherically convergent cathodes generally have low current density at the center and higher current density at the edge. The minimum operating temperature is determined by the highest current density on the cathode surface.

Circular polarization

RF signals can be separated in space by radiating in different polarization senses. In circular polarization, RF energy is transmitted in a rotating corkscrew-like pattern. This pattern can rotate in either direction, termed right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). The OMT in an antenna feed determines the isolation of each of the polarization senses and permits one antenna to support both polarizations simultaneously.

Code-division multiple access (CDMA)

An access technique in which RF signals are separated by utilizing spread-spectrum and orthogonal codes. Multiple access techniques utilized with RF communications signals can separate signals by frequency with frequency-division multiple access (FDMA), by time with time-division multiple access (TDMA), or by codes with CDMA. CDMA is more common with terrestrial cellular systems than it is with satellite communications systems.

Cold match

The input or output voltage standing wave ratio (VSWR) of a TWT measured when the tube has no operating electron beam.


After the electron beam has interacted with the microwave circuit, such as a klystron, it proceeds into a collector, where the electrons are stopped in metallic electrodes and their remaining kinetic energy is converted to heat.

Conduction angle

The conduction angle is that portion of 360 degrees over which cathode current is allowed to flow to the anode. The operating class determines what the conduction angle will be: a general definition of class A is constant conduction (360 degrees), class B up to approximately 180 degrees, class C less than 180 degrees (typically 90 to 120 degrees).

Confined flow

An electron beam focusing approach where magnetic flux threads the cathode. The amount of magnetic field required is higher than the Brillouin Field.

Constant-current curves

The dynamic electrical characteristics of a power grid tube may be plotted using control grid voltage versus anode current, together with incidental control grid current and (in the case of a tetrode) screen grid current, all plotted on an x-y graph. Constant-current curves are used for computing theoretical tube performance and they also indicate other useful parameters including cut-off voltage and saturation potential.

Continuous wave (CW)

A vacuum electron device (VED) runs CW when the RF signal is on continuously. This is as opposed to “pulsing” the RF signal.

Control grid

An electrode in front of the cathode that allows the electron beam to flow through it and turns the beam off when a voltage more negative than the cathode is applied.

Conversion efficiency

See efficiency.

Coupled-cavity TWT

A vacuum electron device (VED) where the circuit is made up of a series of cavities electrically coupled by holes or slots between the cavities. These tubes have very high peak and average powers and medium bandwidths (up to 20 percent).

Crossed-field device (CFA)

An electron tube where the accelerating electric field perpendicular to a static magnetic field causes the electrons to follow a circular slow wave circuit.

Current density

The current per unit area usually referring to emission under continuous wave (CW) state. See cathode loading.

Current division

Electrons flowing between the cathode and anode will be attracted to and intercepted by positive charged conductors in the path such as the control grid and screen grid, thereby reducing the current that arrives at the anode. Current division is the ratio of current going to the grid(s) divided by that leaving the cathode.

Cutoff voltage

The value of grid voltage that just reduces anode current to a value slightly above zero, such as one milliamp in most cases. This is an important parameter useful for testing the internal geometry in power grid tubes.


Dark current

Cathode current that is present in the cut-off condition. This unwanted current can contribute to interpulse noise in pulsed devices.


See decibel.


A unit of power expressed in the number of decibels higher or lower than the carrier power. Commonly used to specify harmonic, spurious and other unwanted signals.


A measure of antenna gain relative to radiation in all directions equally isotropically as if onto the surface of a sphere. Antenna gain is typically specified in terms of dBi at a specific operating frequency.


A unit of power expressed on a decibel scale relative to a milliwatt.


A unit of power expressed on a decibel scale relative to a watt.


The Bel is a logarithmic scale for expressing gain, power and loss, based on logarithms to the base 10 (common logarithms). The decibel is defined as one tenth of a Bel and is abbreviated as dB.

Depressed collector

A depressed collector is a collector where the collection stages are depressed below the body potential to decelerate the electron beam before impact. This increases the efficiency of the amplifier and reduces the thermal power created by the impinging electron beam.

Diode limiter

A form of receiver protector which employs microwave diodes as the active limiting elements.


A component or subsystem that separates RF energy flow in a communications system by filtering the frequency passed by each transmission path. In satellite communications antennas, for example, uplink RF signals are separated from downlink RF signals by passing through different RF filter elements of a diplexer.

Dispenser cathode

A cathode made of porous tungsten containing an active material that is evaporated or dispensed into the vacuum. The material coats the tungsten surface, reducing the surface workfunction and allowing electrons to be emitted at a relatively low (1000 degrees Celsius) temperature.


Dispersion refers to the amount the phase velocity of an RF wave changes as the frequency is changed.


Refers to the amount of RF power fed into a vacuum electron device (VED).

Dual-mode TWT

Commonly refers to a TWT that runs in two modes, a pulsed mode and a lower power CW mode.


A component or subsystem which directs the flow of power in a radar system or communications antenna, thus enabling a single antenna to be used for both transmit and receive. The duplexer also keeps the receiver isolated from the transmitter. There are different duplexer topologies. Some, such as a circulator, are singular components. Others, such as a branched or balanced hybrid duplexer are sub-assemblies. In radar systems, receiver protectors are used in concert with the duplexer to provide additional receiver protection where needed. In some cases, the receiver protector provides both the duplexing and receiver protection functions.

Duty cycle

The percentage of time during which the power is present in one period. Duty cycle is calculated as the pulsewidth times the PRF or pulsewidth divided by the pulse period. For communications systems, duty cycle is the ratio of active transmission to the total time measured. See PRF.


A satellite signal transmission modulation format defined by the European Technical Standards Institute using phase modulation of various orders and a collection of various forward error correction code rates. DVB-S2 is the second generation of these standard modulation formats and the most common format of satellite signal transmission modulation. It enables interoperability of various different transmission and reception equipment by defining how individual data bits are to be interpreted.



In an amplifying device, the total efficiency is the peak RF output power divided by the total input power to the TWT or amplifier. Conversion efficiency is the peak RF power divided by the beam power. Circuit efficiency is the peak RF output power divided by the peak RF output power plus the RF loss. The collector efficiency is the power recovered by the power supply divided by the spent beam power.
The RF output power for the efficiency values above can be measured at band center, at band edge, at saturation or at rated power. Comparing device efficiencies can be misleading unless the definitions are consistent.
Anode efficiency in power grid tubes is the net conversion of dc anode input power to useful output power. Any anode input power that is not delivered to the load is given off as heat.
In an antenna, efficiency is a measure of how well the RF energy input to the antenna is actually radiated. Antenna gain usually includes both the directive gain as well as the antenna efficiency and results in a stated antenna gain in terms of dBi.

EIRP (effective isotropic radiated power)

A measure of the uplink RF power relative to an isotropic radiator. It is calculated as the sum of the antenna gain, in dBi units, added to the RF signal power, normally in dBW units, input to the antenna. EIRP is then typically specified in dBW units and is given at a specific frequency. EIRP is typically one of the most important specifications defining Earth terminal performance.

Electromagnetic spectrum

The electromagnetic spectrum entails the entire range of electromagnetic radiation extending in frequency up to approximately 1023 Hertz, or in corresponding wavelengths, down to 10-13 centimeters. In order of increasing frequency, the main bands are: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays, gamma-rays and cosmic rays. Between microwaves and infrared radiation are millimeter waves and the Terahertz regime.

Electron device

A device that uses electrons to create or amplify an electromagnetic signal.

Electron gun

An emitter used to supply a stream of electrons, usually in a well-defined beam within a vacuum electron device (VED). A commercial product called an "electron gun" consists of a cathode, heater and grid assembly designed to be readily available for use in linear accelerators or other types of vacuum chambers where a cylindrical electron beam is required.

Electron intercept

See current division.

Electronic counter-measures (ECM)

Equipment that renders unfriendly radars (and other detection methods) ineffective.

Electronic power conditioner (EPC)

An EPC is another name for a power supply/modulator that provides power to a tube and other electronic housekeeping functions.


The angle of vertical tilt that an antenna must be rotated through to point at a desired target. Elevation angles can be calculated based upon the latitude and longitude of an Earth station and the location of a desired target. For convenience, elevation angles are usually defined relative to the horizon with zenith represented by 90 degrees elevation.

Emitter (electron)

The emitter or source of electrons is the cathode in electron devices.


An active or passive device that has a loss versus frequency profile or group delay versus frequency profile opposite to the gain or group delay performance of an RF or IF cable, amplifier or complete signal chain, so a constant input drive can be used. Equalizers are generally used with broader band devices, such as long cable runs or helix TWTs. Prior to the use of L-Band IF systems, equalizers were more commonly used to correct for group delay distortion within the bandwidth of individual satellite transponders.

Equatorial orbit

A satellite orbit that is generally aligned with the Earth’s equator. Low Earth orbit (LEO), medium Earth orbit (MEO) and geostationary orbit (GEO) satellites can all utilize equatorial orbits, but all GEO satellites are, by definition, only in equatorial orbits. Equatorial orbits can be inclined relative to the equator but, if they are, they only cross directly over earth stations at latitudes lower than their degrees of inclination.


A standard for computer networking that allows hard-wired high-speed data communications between devices in a local area network. The Ethernet standard, which is the prevailing standard for the Internet, has been evolving since it was standardized in 1983. Ethernet standards refer to the physical and the data link levels only; protocols and transport of data are covered in other standards (i.e., TCP/IP).

EVM (error vector magnitude)

A measure of the maximum error vector resulting from a digital RF signal passing through a transmitter, receiver or amplifier. An error vector is a vector in the I-Q plane between the optimal constellation location and that measured in the receiver. EVM depends heavily upon the exact modulation being used but gives a definitive measure of the overall quality of the signal transmission path.


Federal Communications Commission (FCC)

Regulates all civil use of the electromagnetic spectrum in the United States. FCC regulations cover technical and administrative controls of RF communications in United States territory such that use of the electromagnetic spectrum for satellite and other communications are interoperable and limit interference.

Ferrite limiter

A form of receiver or amplifier protector in which the active element is magnetically biased ferrite material.

Field effect transistor (FET)

A semiconductor device, usually fabricated from GaAs or GaN material when used at microwave frequencies, that can act as a current switch or amplifier device. In solid-state power amplifiers (SSPAs) used for satellite communications uplink applications, several FET devices are combined, both in series and parallel, to achieve sufficient output power capability.

Figure of merit (G/T)

A measure of an Earth station downlink gain (G) (in dB units) divided by the Earth station system noise temperature (T) (in degrees Kelvin), typically specified as dB/K and defined at a specific frequency. G/T is one of the most common, and most important, specifications for an Earth station’s performance and permits a determination to be made of the received signal-to-noise ratio from a given satellite downlink signal.

Flat leakage power

The steady state component of a receiver protector’s leakage power that occurs after the onset of hard limiting.

FM to AM conversion

When an FM signal is applied to a narrow-bandwidth amplifier stage, an AM component will result in the output. In FM broadcast this phenomena is called “synchronous noise” or “incidental AM.” This is generally due to the input circuit having a high “Q”.

Focus electrode

An electrode in an electron gun that focuses the electron beam through the anode structure. Some focus electrodes are designed to be at a separate potential for modulation purposes.

Folded-waveguide circuit

A folded-waveguide circuit is a type of slow-wave microwave circuit where the beam interacts with the waveguide at discreet phase lengths. A more sophisticated type of folder waveguide circuit is the couple-cavity TWT.

Forward error correction (FEC)

A technique in digital RF signal modulation in which additional coding bits are transmitted with user data bits to enhance the ability of a demodulator to decode the transmission, resulting in an improved bit error rate (BER). Different FEC techniques are available and can be used with higher or lower rates of coding bits to data bits. The use of more coding bits relative to data bits on a signal increases the transmitted symbol rate but reduces the required receiver signal-to-noise ratio needed to achieve a specific BER.

Free space loss

Refers to the attenuation of electromagnetic signals due to the distance between the point of transmission and reception. Since electromagnetic energy radiates as if on to the surface of a sphere, isotropically, free space loss is a function of the square of the wavelength over the square of the distance, or in dB units: LdB = 21.98 + 20 log10 (distance/wavelength).


The number of cycles or completed alternations per unit time of a wave or oscillation. Frequencies are generally measured in hertz, including kilohertz (thousand), megahertz (million), gigahertz (billion) and terahertz (trillion). Hertz are frequently abbreviated as “Hz” for hertz, “kHz” for kilohertz, “MHz” for megahertz, “GHz” for gigahertz and “THz” for terahertz.

Frequency reuse

A technique that maximizes the capacity of a communications satellite by separating RF energy beams either spatially, by polarization or both, such that the same frequency bands can be used multiple times by a specific satellite. Effective frequency reuse drives improved Earth station antenna performance in terms of polarization isolation.

Frequency shift keying (FSK)

A modulation method that transmits digital information by modulating the frequency between two frequencies, one frequency for a “1” and one frequency for a “0.”

Frequency-division multiple access (FDMA)

An access technique in which RF signals are separated by utilizing separate RF frequencies for each signal. Multiple access techniques utilized with RF communications signals can separate signals by frequency with frequency-division multiple access (FDMA), by time with time-division multiple access (TMDA), or by codes with code-division multiple access (CDMA). FDMA access techniques are very commonly used in satellite communications systems.



See figure of merit. G is the antenna gain and T is the noise temperature of the system.


Gain commonly refers to the ratio of output power to the input (drive) power of an amplifier or antenna. Power gain is usually expressed in decibels. See dB, dBm, dBw, dBc and dBi.

Gallium arsenide (GaAs)

A synthetic compound of the elements gallium and arsenic. It is used as a semiconducting material in high-frequency applications such as cellphones, DVD players and fiber optics. It is also used in devices, such as FET and MMIC amplifiers, that are used in solid-state power amplifiers (SSPAs).

Gallium nitride (GaN)

A synthetic compound of the elements gallium and nitrogen. It is used as a semiconducting material in high-output LEDs, optoelectronics, high-voltage switches and diodes, and high-power and high-frequency devices. It is also used in devices, such as FET and MMIC amplifiers, that are used in solid-state power amplifiers (SSPAs).

Geostationary orbit (GEO)

A circular satellite orbit directly over the equator at an altitude of 35,785 km (22,236 miles) such that the satellite appears to hover in a fixed location above one spot on the Earth’s equator. The accuracy of geostationary satellite station-keeping can vary. At higher frequencies, with the use of larger antenna sizes, more sophisticated antenna pointing and tracking systems are required.


A material placed inside a vacuum that absorbs residual gasses. Some getters require activation using an applied voltage and others do not.


A grid is mounted between the cathode and anode to control the flow of electrons. The control grid can modulate the electron beam. Focus grids, shadow grids and Unigrids are all structures at cathode potential that shadow the electrons from the control grid.

Group delay

A measure of the time delay of the signal envelope propagating through the device or signal chain. It is defined as the derivative of phase delay versus frequency. Flat group delay is required for avoiding waveform distortion.

Group velocity

The group velocity is the velocity at which the signal energy is propagated along the RF circuit.


A device for producing microwave energy that utilizes a strong axial magnetic field in a cavity resonator to produce azimuthal bunching of an electron beam. A gyrotron can produce megawatts of output power up to frequencies of over 500 GHz and can be designed as an oscillator (no input drive required) or an amplifier.


Harmonic drive

The useful bandwidth of a TWT may be improved by injecting a harmonic signal in addition to the fundamental. The amplitude and phase relative of the harmonic signal to the fundamental must be determined for best performance.


Used to warm the cathode to its operating temperature. Heaters are usually a coil of tungsten alloy wire thermally connected to the cathode. Sometimes a dielectric potting is put around the heater for mechanical stability.

Heater hum

An AC voltage applied to a heater can induce an ac signal at the same frequency on the RF signal. The heater’s induced magnetic field modulates the electrons as they leave the cathode, causing this effect.

Helix TWT

A traveling-wave tube that uses a helix as a slow wave circuit. Helix TWTs are commonly used in satellite communications traveling-wave tube amplifiers (TWTAs) and are useful for electronic warfare applications due to their multi-octave bandwidths.

Highly elliptical orbit (HEO)

Satellites in this type of orbit, such as the Russian Molniya satellite system, are typically inclined polar orbits that have higher velocity when near the Earth, in perigee, and appear to hang with lower velocity when farthest from Earth, in apogee. HEO satellites can thus provide better coverage over the Earth’s polar regions than GEO satellites, but have the disadvantage of requiring Earth station pointing and tracking systems.

High-power amplifier (HPA)

High-power amplifiers are typically used to amplify an RF signal to a sufficient level for transmission. An HPA is generally a VED and power supply - or a semiconductor amplifier device, FET or MMIC, and power supply - but can also contain RF conditioning equipment on the input and output side of the TWT device, cooling and housekeeping functions. Solid-state power amplifiers (SSPAs) are versions of high-power amplifiers based upon solid-state amplifier devices. TWTAs are versions of HPAs based upon traveling wave tube amplifier devices. HPAs are available for several different RF frequency bands and can have output power capabilities from a few watts to multiple kilowatts.

Hot match

The VSWR of a VED when the beam is operating.



In-flight entertainment and connectivity, also sometimes called in-flight entertainment and communication

Inductive Output Tube (IOT)

A special version of the tetrode, employing four electrodes: a cathode, a control grid, an anode (instead of a screen grid) and a collector (instead of the traditional anode). The IOT is unique in that it uses a reentrant output cavity like that of a klystron, which is inserted between the anode and collector. This configuration replaces the traditional lumped-element or coaxial output circuits that connect between the screen grid and anode in a traditional tetrode amplifier. In the IOT, both the anode and collector can remain at dc ground and are devoid of RF voltage; thus, very high power can be produced at frequencies up to a few GHz with no concern of screen dissipation, which can be a significant problem in conventional tetrodes. At the same time, the IOT achieves higher operating efficiencies and has considerably better power gain than tetrodes. The Klystrode® IOT is a registered trademark for the IOT developed by CPI's Eimac operations.

Insertion loss

Insertion loss is the reduction in signal strength when power passes through a structure or a device. It is measured in dB and calculated as -10 times the log (output power divided by input power).
In a VED or solid-state amplifier, high insertion loss is commonly required from the output back to the input so the input components do not fail when the output load reflections are high.
In a receiver protector or switch, insertion loss is small signal loss and normally specified for input power levels below 0.1 mW (– 10 dBm).
In a radome, insertion loss is the RF signal attenuation incurred by the RF signal as it passes through the radome material. In radomes, the lowest possible insertion loss is desired and is commonly on the order of less than 2 dB.

Intercepting grid

An intercepting grid is a type of control grid that intercepts electrons. There is no shadow grid for this type of structure.

Intermodulation distortion

Distortion due to the sum and difference frequencies from a multi-carrier input signal. The frequencies are the result of the non-linear behavior of an amplifier. The third order intermodulation products are generally used to measure the relative linearity of a device.

Ion pump

An Ion pump uses electrons produced by field emission (cold cathode) to ionize residual gas molecules which are then attracted to a getter. An Ion pump with voltage applied will help maintain vacuum quality of an electron device while it is in storage and can be used as a relative indicator of the gas level within the vacuum envelope while a vacuum electron device (VED) is operating.



Klystrode® is a registered trademark synonymous with the CPI Eimac-brand IOT. See inductive output tube.


A klystron is a microwave tube which uses the interaction between an electron beam and the RF energy in microwave cavities to provide signal amplification. Klystron interaction takes place at discrete locations (the cavities) along the electron beam.



A passive receiver protector. See receiver protector.

Linear polarization

RF signals can be separated in space by radiating in different polarization senses. In linear polarization, RF energy is transmitted as a plane. This plane wave can be in any orientation but is usually referred to as vertical or horizontal, skewed from the Earth terminal horizon. The OMT in an antenna feed determines the isolation of each of the polarization senses and permits one antenna to support both polarizations simultaneously.

Linear power

An RF power level at which an amplifier does not generate excessive intermodulation or signal degradation. No amplifier is perfect, so consideration of linear power is important in determining an amplifier output power capability given the operating conditions. There are many definitions for linear power as the acceptable level of intermodulation products and intersymbol interference, and other factors are different for different forms of modulation. Consult your CPI representative for assistance in determining acceptable linear power conditions for the specific application.


In an amplifier, a linearizer is a device that improves the AM to AM and AM to PM conversion of an amplifier by compensating for the deviations from linear phase and gain of the amplifier. The use of a linearizer reduces the intermodulation distortion, spectral regrowth and noise power ratio (NPR).
In an attenuator, a linearizer is an electronic driver which transforms the natural non-linear attenuation versus current characteristic of the attenuator to a linear attenuation versus voltage function.

Link analysis

To determine the transmitted power and signal modulation format required to support a specific communications link, satellite system engineers perform a link analysis, which considers several parameters of the satellite, the Earth station, the transmission path attenuation and RF uplink and downlink signal levels to determine the potential capacity offered for a desired satellite communications link. A satellite link analysis is typically required, for example, to determine the required output power capability of a satellite Earth station high-power amplifier to ensure suitable BER performance and availability.

Low noise amplifier (LNA)

A solid-state device used on the downlink (receive) side of a satellite communications (satcom) system. It is designed to amplify the very low level signals received by the satellite antenna while adding the minimum amount of noise. The signal-to-noise ratio of the received signals is largely controlled by the noise added by this amplifier.

Low noise block down-converter (LNB)

A block down-converter that is combined with an LNA in a single package. The block down-converter converts the signals received from the satellite to a lower frequency, usually L-Band. Like a BUC, the LNB has a large bandwidth and, in some bands, can cover the bandwidth of the entire satellite.



A crossed-field microwave oscillator tube containing concentric cylinders; the inner cylinder is the cathode and the outer cylinder is an anode that contains embedded resonant cavities. A strong axial magnetic field causes a cloud of electrons to orbit between the cathode and the anode. The RF voltages across gaps in the resonators modulate the velocities of the electrons. This causes the orbiting electrons to form into “spokes” that rotate around the tube axis. As the spokes of electrons rotate past the resonator gaps, they induce currents that excite the cavities. The RF voltages build up to large levels. High power output is obtained at moderately high efficiency.


Match refers to the VSWR or reflection characteristics looking into a port of a microwave device. A good match is where the VSWR is very nearly equal to one. See VSWR.


Microwaves refer to electromagnetic energy of extremely high frequency, typically from 1 GHz to 100 GHz.

Microwave power module (MPM)

An MPM is a small transmitter that includes a low noise, solid-state amplifier (SSA) gain section, a high efficiency, high power helix TWT output section and a very compact high voltage power supply all packaged together. These devices are ideal for high efficiency, light weight applications.

Monolithic microwave integrated circuit (MMIC)

A device that uses photolithographic methods (as in other integrated circuits) to provide broad functionality in a very small package. For instance, MMICs are used to provide very high performance transceivers in smart phones.


See dual-mode TWT.


A type of receiver protector which uses the phenomenon of multipaction in a contructive way to limit high levels of RF power. Multipactors are characterized by a moderate peak, high duty cycle power handling capability and extremely fast recovery time.

Multi-stage collector

An electron collector with more than a single stage being “depressed” below ground potential. Also termed “Multi-Stage Depressed Collector” or “MSDC.”


National Telecommunications and Information Administration (NTIA)

Regulates all federal and government use of the electromagnetic spectrum in the United States. NTIA regulations cover technical and administrative controls of RF communications by federal entities, e.g. the military and other government agencies, in United States territory such that use of the electromagnetic spectrum for satellite and other communications are interoperable and limit interference.

Noise figure

The ratio of the signal-to-noise on the output of a device or system to the signal-to-noise on the input. It is usually expressed in dB. A lower noise figure means that the specified system can receive signals of weaker signal amplitude.

Noise power density (NPD)

The NPD is the noise power generated by an amplifier within a given bandwidth (usually 1 Hertz) when measured at the output port when the input port is terminated.

Noise power ratio (NPR)

NPR is equivalent to the intermodulation distortion created by a large number of simultaneous carriers. One way to measure NPR is to measure random noise through a notch filter. The ratio of noise power in the passband to the noise power in the notch is the NPR. It is usually expressed in dB.

Noise temperature

The effective temperature that would generate the quiescent noise level present on the output of an antenna, amplifier or device. It is usually expressed in degrees Kelvin, or “K.” A lower noise temperature means that the specified system can receive signals of weaker signal amplitude.

Non-intercepting grid

A non-intercepting grid is a control grid that does not intercept electrons due to another grid that shadows the former.

Normal operating power

The amount of power that a receiver protector will normally see when the system is in normal operation. Recovery time is always specified only up to the normal operating power.


Octave (bandwidth)

One octave is when the highest frequency is equal to twice the lowest frequency.

Orthomode transducer (OMT)

A waveguide component that is commonly referred to as a polarization duplexer. Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths, which can be linearly or circularly polarized. Orthogonal polarization is when two signals are able to travel in the same place and time because they travel at right angles to one another. Typically one of the paths forms the uplink, which is transmitted over the same waveguide as the received signal path, or downlink path. The LNA or LNB is usually connected directly to the receive port of the OMT to reduce the amount of noise added by waveguide losses.


An oscillator is a circuit that produces an alternating output signal of a certain frequency determined by the characteristics of the circuit components. No input power is needed; as in the case of an amplifier.


Vacuum electron devices (VEDs) operate at very high vacuum. The materials of which these devices are made contain trace amounts of gas that, in this vacuum, come out of the material. Special processing is generally performed to minimize the deleterious effects of outgassing.


To overdrive an amplifier is to apply an input drive signal level greater than that required to saturate the device. In this condition, distortion due to non-linear effects becomes large.

Overload power (fault power)

The absolute maximum amount of power that a receiver protector will be expected to handle in the event of a system fault. This is usually the full transmitter power.


Peak envelope power (PEP)

Peak envelope power is that instantaneous value of Root Mean Square (RMS) power occurring at the crest of the waveform envelope. PEP is used for linearity measurements with a two-tone signal (usually sinusoidal) applied to an amplifier.

Peak power

The maximum power of a sinusoidal signal; the maximum power present during the time of an RF pulse or under any modulation condition.


An electron device having five elements: a cathode, control grid, screen grid, a suppressor grid and an anode. The suppressor grid controls secondary electrons coming off the anode and thereby reduces screen current under some operating conditions and improves linearity.

Periodic permanent magnet (PPM) focusing

PPM is a type of focusing of an electron beam in a VED where many permanent magnets of opposite polarity are placed side by side along the length of the tube. The advantages of this approach compared to others are size and weight.


In triodes, tetrodes and microwave vacuum electron devices (VEDs), the perveance is a factor determined by internal geometry between tube elements. Perveance is the beam current divided by beam voltage raised to the 1.5 power. It is only a function of geometry. So if the voltage is changed on a VED, the current will change such that the perveance is constant. There are two perveances when discussing a mod anode electron gun, the gun perveance, which uses the cathode-to-anode voltage, and the beam perveance, which uses the cathode-to-circuit voltage. Microperveance is often used (perveance times 1,000,000) because the normal perveances are approximately 0.000001.

Phase pushing factor

The phase pushing is the amount by which a change in the voltage on an electrode changes the phase length in the TWT. The phase pushing factor is usually given in degrees per volt.

Phase shift keying (PSK)

A modulation method that transmits digital information by modulating the phase of a CW signal. Some modulation methods transmit one bit per phase change (BPSK – Binary Phase Shift Keying) and others transmit two bits per phase change (QPSK – Quadrature Phase Shift Keying), higher numbers of bits are transmitted by each phase change as the number of phase states increases (8PSK, 16PSK, etc.). Where larger numbers of bits are required, both the phase and amplitude of the modulated signal changes (as in 16QAM, 64QAM, etc.). Where each symbol falls in the combination of phase and amplitude of the PSK signal is considered the symbol constellation. The number of bits transmitted in each phase change is commonly referred to as Bits/Symbol.

Phase tracking

In a VED: phase tracking is an attribute of a VED design that allows unit-to-unit variations in phase versus frequency to be highly repeatable. This allows efficient power combining, thus this characteristic is commonly required when VED are power combined through a waveguide or spatially.
In any other component: phase tracking (phase matching) is the small signal insertion phase length of the device as measured in relation to a standard. The standard could be unit or length of transmission line. Phase tracking can be specified in terms of all units to a universal standard or in terms of matched sets of units.

Phase velocity

The velocity at which a constant RF phase point advances along the RF circuit. TWTs are designed to synchronize this velocity with the electron beam.

Planar triode

A triode with the cathode, grid and anode all in parallel planes.


A commonly used acronym for the anode. Some text refers to “plate” instead of anode. See anode.


The common name for an orthomode transducer (OMT). It can also refer to a device that goes between an OMT and the antenna’s feed horn and converts the linear polarization of an OMT to circular polarization.

Power added efficiency (PAE)

A definition of efficiency that is commonly used for low gain devices (where, because of their low gain, the input RF power is significant compared to the output RF power). PAE = (RF output power – RF input power) / DC power input.

Power combining

A method of obtaining more RF power than a single amplifier, TWT or amplifier device can produce alone by combining power through a microstrip, stripline or waveguide or spatially.

Power module/booster

A device inside of a solid-state power amplifier (SSPA) (or SSPB) that provides the final power amplification of the amplifier.

Pre-TR tube

A form of receiver protector in which the high RF power initiates a gas plasma discharge. This type can be designed to handle very large amounts of power.

Pulse modulator tube

A pulse modulator tube is used to switch current into a load which may be another VED or a transformer, etc. Pulse modulator tubes may be either tetrodes where constant current characteristics are desirable or triodes where fast rise time is a requirement. Special techniques are employed in manufacturing and special high voltage processing is used to produce VEDs that have performance intended for use as pulse modulators; these products often make poor linear amplifiers because of compromises made for improving performance as a switch.

Pulse period

The time between one pulse and the subsequent pulse in a pulse train.

Pulse repetition frequency (PRF)

The inverse of the pulse period.

Pyrolytic graphite (PG)

Pyrolytic graphite is used to make grids in power grid tubes where the material’s low thermal coefficient of expansion, excellent thermal conductivity and high operating temperature are required. PG is chemically deposited on a mandrel, machined to shape then cut with a laser or other means. PG grids feature precise mechanical tolerances and excellent reproducibility.


Radial beam tube

An electron tube with the cathode, grid(s) and anode all concentric about one axis. In these tubes, electrons are formed into radial beams by electrostatic focusing.

Radio frequency (RF)

A radio frequency (RF) signal is an electromagnetic wave with a frequency from about 15 kilohertz to above 100 GHz.


A dome or other structure designed to protect an antenna from the environmental elements and sometimes physical damage. Radomes are fabricated with materials transparent to the radio frequency used and are available to support antennas of virtually any size mounted on the ground, shipboard, submarines, land vehicles or aircraft. Their performance is typically specified in terms of antenna size, operating frequency and acceptable levels of attenuation.

Receiver protector

A microwave component that will limit a high-power RF signal to a level low enough to be successfully handled by the components that follow it. In terms of functionality, there are three broad categories of receiver protector:

  • Active receiver protectors require an external control and/ or bias supplies in order to operate.
  • Passive receiver protectors (also called limiters) require no external control. They react automatically to the presence of a high power signal.
  • Quasi-active receiver protectors have a combination of active and passive functionality.

Recovery time

The time it takes for a passive receiver protector to transition back from its protection state to its insertion loss state. Recovery time is measured from the point at which the trailing edge of the high-power input pulse reaches zero to some point in time at which the receiver protector recovers to within a pre-determined level of its quiescent insertion loss. Unless otherwise specified, this level is assumed to be 3 dB as an industry standard.


A redundant subsystem, such as an amplifier, converter or other device, is a design approach commonly found in communication systems in which more than one component is operating simultaneously so that a backup component subsystem can be used without interruption if there is a fault. Redundancy can be provided with switching systems, such that subsystem elements are switched in and out of the active signal path and remain at the same level with slight interruption. Redundancy can also be provided with passive signal combining and dividing, such that power levels change if one subsystem fails.

Reflection coefficient

The reflection coefficient is the voltage in the reflected wave divided by the voltage in the incident wave. See also match and VSWR.

Return loss

The return loss is equal to -20 times the log of the reflection coefficient. See also match, reflection coefficient and VSWR.

Ring-bar TWT (Ring Loop TWT)

A TWT with an RF circuit that is composed of coaxial rings tied together with bars. A Ring Loop TWT uses loops to tie the rings together. These devices are capable of higher power levels than conventional helix TWTs, but have significantly less bandwidth.


Saturated power output

The saturated output power is the maximum output power obtained as the input drive is increased from the small signal region. It is also where an increase in input power does not increase the output power. At this point there are as many electrons in the accelerating phase of the RF signal as there are in the decelerating phase.

Saturation region

The saturation region is that area in the constant-current curves where the anode curves begin to slope upwards at low anode voltage. This illustrates the fact that the anode has decreased ability to attract electrons and most of the cathode current begins to flow to the screen grid or, in the case of a triode, to the control grid.

Screen grid

The screen grid is used in tetrodes and pentodes as an accelerating grid that also provides an electrostatic shield between the anode and control grid. The overall gain of a tetrode is determined to a high degree by the value of screen voltage applied to it.

Secondary emission

Secondary emission results when electrons flowing from the cathode to the anode strike a conducting surface such as a grid or the anode and produce additional low energy electrons. These unwanted electrons can be attracted to nearby elements and cause instability or distortion. Carefully controlling the materials and processes used in manufacturing power grid tubes reduces secondary emission to very low values.

Shadow grid

A shadow grid is between the cathode and control grid and is used to prevent electrons from impacting the control grid. It is tied to cathode potential.

Single-stage collector

A single-stage collector has only one stage for collecting electrons which may be grounded or operated at a voltage between ground and cathode potential.

Slow-wave circuit

An RF structure that has an effective axial phase velocity slower than the speed of light.

Solid-state power amplifier (SSPA)

An amplifier that uses solid-state components, typically FETs or MMICs. It includes a power supply and other supporting components so that it can provide sufficient power for the transmission of a signal. An SSPA does not include a converter and does not provide any frequency translation. SSPAs can be found inside block up converters (BUCs).

Solid-state power amplifier and block up converter (SSPB)

The term "solid-state power amplifier and block up converter (SSPB)" is used to remove the ambiguity of the term "BUC", as BUC can mean just the converter or the converter with an amplifier.

Space charge field

The total electrical field produced by electrons in a vacuum.

Space charge limited current

A level of current emitted from a cathode where an increase in temperature does not increase the cathode current and an increase in cathode voltage changes the cathode current by the perveance relationship. The other type of current is temperature limited emission.

Spectral regrowth

The ratio in decibels between the peak of a modulated carrier and the sidelobes caused by non-linear distortion in an amplifier.

Spike leakage energy

The energy contained within the spike leakage pulse. This is calculated as: spike energy = spike power x spike width

Spike leakage power

The transitional component of a receiver protector’s leakage power which occurs during the risetime of the input pulse, after the onset of hard limiting.

Spike width

The spike pulsewidth as measured 3 dB below the peak spike power.

Switch tube

A power grid tube used to switch current, either as a current sink or source. See pulse modulator.



The science of measuring quantities, transmitting the results to a distant station and interpreting, indicating and/or recording the quantities measured. In satellite communications, telemetry refers to the information communicated from a satellite itself to a ground station to report on operating conditions such as voltages, currents, temperatures and other parameters within the satellite.

Temperature limited current

A level of current emitted from a cathode where an increase in cathode voltage does not increase the cathode current and an increase temperature changes the cathode current by an exponential relationship. The other type of current is space charge limited emission.


A vacuum device having four elements: a cathode, control grid, screen grid and an anode.

Third order intercept point

The point on an output power versus input power plot where the extended slope of the small signal gain crosses the extended slope of the third-order intermodulation, plotted on the same scale. The intercept point is a purely mathematical concept, and does not correspond to a practically occurring physical power level. In many cases, it lies far beyond the damage threshold of the device. Typically the slope of the third-order intermodulation is three times the slope of the small signal gain.

Three-halves power law

When it is desired to calculate tetrodes performance at screen voltages other than that used for making a specific set of constant current curves, the three-halves power law may be used. The “Care and Feeding of Power Grid Tubes” text includes a table of commonly used voltage factors taken to the three-halves power. This law neglects cases where high secondary emission or an emission-limited cathode occurs.

Time-division multiple access (TDMA)

An access technique in which RF signals are separated by utilizing separate time allocations on one or more RF frequencies for each signal. Multiple access techniques utilized with RF communications signals can separate signals by frequency with frequency-division multiple access (FDMA), by time with time-division multiple access, or by codes with code-division multiple access (CDMA). Virtually all VSAT networks utilize a form of TDMA access techniques so it is very commonly used in satellite communications systems

TR tube

A form of receiver protector in which the high RF power initiates a gas plasma discharge.


A characteristic of a bit sync where the output bit rate varies with (follows) the input bit rate. Track range is a measurement of tracking and defines the range in percent of bit rate from a nominal value in which tracking will be maintained without bit slips. In satellite communications, tracking refers to the ability of a satellite communications antenna to follow the relative movement of the satellite as seen from the Earth station. Any errors in tracking result in less-than-optimal antenna gain to and from the satellite and are indicated as an additional transmission loss. Several different tracking modes have been developed, permitting trades in transmission loss and tracking system complexity and cost. See antenna control unit.


A system that contains all of the active components for transmission and reception in one package.

Transfer curve or transfer characteristics

There are two types of curves referred to as transfer curves. The first is cathode current versus grid voltage. The second is output power versus input power. In triodes and tetrodes, transfer characteristics are defined as anode current versus grid voltage.

Transit time

As it relates to power grid tubes (specifically, the input portion of triodes and tetrodes), transit time is the time it takes electrons to travel from the cathode to the control grid. This phenomenon is critical because at a given frequency, if the power grid tube is to perform well, the transit time must be short compared to the period of one cycle of the input signal waveform. Otherwise, the reversing polarity of the drive voltage will repel electrons from the grid back to the cathode, reducing the optimum bunching that would have occurred at lower frequencies. As a result, in triodes and tetrodes, as the operating frequency is increased, both power gain and efficiency will decrease until the performance would be considered unacceptable. In a klystrons, the transit time effect results in velocity modulation of the electrons leaving the cathode, which has the benefit of bunching occurring at this initial region. Like the triode and tetrode, an IOT uses RF voltage impressed between the grid and cathode to density modulate the emission, creating bunching. The maximum usable frequency in an IOT is determined to a first order by the transit time in the grid-cathode region.


In satellite communications, a transmitter is a high-power amplifier. In radar, a transmitter is a vacuum electron device (VED) or solid-state device, power supply and associated RF equipment such as waveguides, switches, solid state amplifiers, etc. in a common chassis.

Traveling-wave tube (TWT)

A microwave tube that uses a slow wave structure such as a coupled cavity circuit or a helix circuit to extract beam energy and create RF output power.

Traveling-wave tube amplifier (TWTA)

A combination of a traveling-wave tube and power supply, packaged together.


A vacuum device having three elements; a cathode, a control grid and an anode.


Vacuum electron device (VED)

A vacuum electron device or VED is an amplifier or oscillator where the electrons used to produce or amplify the microwave signal travel through a vacuum assembly. They are known for their extremely high output power capabilities and very high total efficiency.

Vacuum envelope (VE)

The core of a vacuum device that encloses the space that is evacuated. The VE contains the electron gun, RF circuit and collector. It generally does not include focusing magnets and packaging for heat dissipation.

Voltage standing wave ratio (VSWR)

A measure of the reflections produced by impedance mismatches in an RF circuit. The VSWR is the ratio of the voltage at a peak divided by the voltage at a minimum point on the standing wave pattern.



A common unit of power.

Work function

The amount of energy required to get an electron out of the cathode metal and into the vacuum. It is measured in electron volts (eV). Every material and combination of materials has a specific work function. The lower the workfunction value the less heat needed to obtain the required energy to emit electrons.



X-rays are a form of electromagnetic radiation, similar to light but of shorter wavelength and capable of penetrating solids and of ionizing gases.