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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.

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.



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.


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.

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 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

Brillouin Field is 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.



A cathode is an electrode which 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

Cathode loading is 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.

Cold match

The input or output Voltage Standing Wave Ratio (VSWR) of a TWT measured when the tube has no operating electron beam. See VSWR.


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.

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 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

Cutoff voltage is 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.


CW is an abbreviation meaning “continuous wave.” A vacuum electron device runs CW when the RF signal is on continuously. This is as opposed to “pulsing” the RF signal.


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 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.

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.

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, 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. 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. See PRF.



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.

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 electron gun is 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.

Emitter (electron)

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


An equalizer is a passive device that has a loss versus frequency profile opposite to the gain of tube, so a constant input drive can be used. Equalizers are generally used with broader band devices, such as helix TWTs.


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).


Ferrite limiter

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

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.


Frequency is 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 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.”



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

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 amplifiers, that are used in 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 amplifiers, that are used in SSPAs.


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

Group delay is a measure of the time delay of the signal envelope propagating through the device. 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 Gyrotron is 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. They can be designed as oscillators (no input drive required) or as amplifiers.


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.


A heater is 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 helix TWT is a traveling-wave tube that uses a helix as a slow wave circuit. Helix TWTs are useful for electronic warfare applications due to their multi-octave bandwidths.

High-power amplifier (HPA)

An HPA is generally a VED and power supply, but can also contain RF conditioning equipment on the input and output side of the TWT, cooling and housekeeping functions.

Hot match

The VSWR of a VED when the beam is operating. See VSWR and VED.



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

Inductive Output Tube (IOT)

The IOT (Inductive Output Tube) is 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. See tetrode or Klystrode®.

Insertion loss

Insertion loss is the reduction in signal strength when power passes through a device. It is measured in dB and calculated as -10 times the log (output power divided by input power).

In a VED: 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).

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 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.


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.

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.”


Noise figure

The noise figure is the ratio of the signal-to-noise on the output of a device to the signal-to-noise on the input. It is usually expressed in dB.

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.

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. 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 devices 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.


A pentode is 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 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.). 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) (see 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 or TWT can produce alone by combining power through a waveguide system or spatially.

Power module/booster

A device inside of an 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.

Receiver protector

A receiver protector is a microwave component which 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 amplifier is a design approach where more than one amplifier is operating simultaneously, so in case of a fault, the backup amplifier can be used without interruption. These are commonly found in communication systems.

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. It includes a power supply and other supporting components so that it can provide sufficient power for the transmission of a signal to a satellite. 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. See BUC.

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.



Telemetry is the science of measuring quantities, transmitting the results to a distant station and interpreting, indicating and/or recording the quantities measured.

Temperature limited current

A level of current emitted from a cathode where an increase in cathode voltage doesn’t 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 tetrode is 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 tetrode 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.

TR tube

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


Tracking is 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.


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 klystron, 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.


A transmitter is a VED, low- and-high voltage 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 triode is 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 VE is 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 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 workfunction is 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.