Defence Glossary
Microwaves
Microwave | Power Supplies
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A
AM/PM Conversion
The change in phase angle between the input
and output RF signals as a function of drive level changes. Expressed in degrees
per decibel (dB), AM/PM conversion arises from a slowing of the electron beam as
electronic interaction increases when RF drive exceeds a level of about 20 dB
below saturation drive.
Amplitude Modulation (AM)
Modulation in which the
amplitude of a wave varies with time. It is usually defined as the ratio of the
modulation to the average amplitude in dB.
Bunching
This is any process that produces a velocity
modulated electron beam due to acceleration or deceleration of individual
electrons.
Collector
This is an electrode used
to absorb the energy in the electron beam after it has traversed through the
slow-wave circuit. Cooling is usually by means of pumped fluid, conduction to a
baseplate or forced air depending on the customer requirements.
Depressed Collector
When the Collector is held a potential
between the cathode voltage and the microwave tube body, usually earth
potential, then some of the kinetic energy of the beam can be returned to the
power supply. This provides a valuable increase in Electrical Efficiency of the
device. Several stages of depression with gradually increasing voltage can be
used to further enhance efficiency.
D
Duty Ratio or Duty
The product of the Pulse Duration and
the Pulse Repetition Frequency (PRF). This is also the ratio of the Output Power
(Average) to the Output Power (Peak).
Electrical Efficiency
The ratio of
the average RF output power of a tube to the total dc input power.
Electron Gun
This uses a Thermionic Cathode and associated
Electron Optics to provide a laminar stream of electrons. The electron beam can
be switched by means of a Control Grid. To reduce current to this grid a Shadow
Grid is usually used to limit electron emission from the cathode behind the
control grid.
Electron Optics
This is the arrangement of electrostatic
and magnetic fields used to produce and focus an electron beam from a Thermionic
Cathode.
Frequency Pulling
For an oscillator,
this is the change in the generated frequency caused by a change in the load
impedance.
Frequency Pushing
For an oscillator, this is the change in
frequency due to a change in the supply current (or voltage).
Gain
A general term used
to denote an increase in signal power in transmission from one point to another.
Usually expressed in dB, for a microwave tube the power gain is the ratio of the
output signal power to the drive signal power.
Gain Equaliser
A device which can be used to modify the
input signal to a Travelling Wave Tube to improve the Instantaneous Bandwidth by
preferentially attenuating the mid-band frequencies and hence boosting the band
edges.
Heater Surge Current
The maximum
instantaneous current flowing through the cathode heater when cold and thus when
the impedance is lowest.
Helix Slow-Wave Structure
This Slow-Wave Structure uses a
simple helix, usually made from Molybdenum or Tungsten tape. This type of
structure provides the maximum bandwidth, several octaves can be available in an
appropriately designed travelling wave tube.
Instantaneous Bandwidth
The
instantaneous electronic bandwidth of a microwave tube is the range of
frequencies for which the output power is greater than a specified
level.
Intermodulation
The modulation of different frequencies by
each other, resulting in waves having frequencies equal to the sums and
differences of integral multiples of the original frequencies.
Intermodulation Distortion
Distortion resulting from
Intermodulation. Odd order intermodulation products can an important fraction of
the output signals of an amplifier with the third-order product being the
highest level.
Ion pump
An electronic vacuum pump used to help maintain
or check the vacuum in a microwave tube.
Lineariser
A device which can be used to modify the input
signal to a Travelling Wave Tube to improve the Intermodulation Distortion
performance at close to saturation operation.
Microwaves
Electromagnetic waves whose frequencies lie
between the far infrared and some lower frequency limit. Although not clearly
bounded, the microwave region is commonly regarded as lying between 300 MHz and
300 GHz.
Noise Figure
The ratio in decibels of the total output
noise produced by an amplifier to the noise that would be present at the output
when the source temperature is 290 ºK.
Output Power (Average)
The power
delivered to the load averaged over time. Usually the Output Power (Peak)
multiplied by the Duty Ratio.
Output Power (Peak)
The power at the maximum of a pulse of
power, excluding spikes.
Output Power (Saturation)
The maximum output power of an
amplifier. If the input power required to achieve this level of output power is
increased the this will produce a reduction off output power.
Periodic Permanent Magnet (PPM)
Focusing
An series of periodic permanent magnets which produces a sinusoidal
magnetic field whose diameter is independent of length and is used to focus the
electrons in a linear beam tube. This gives the lightest possible focussing
arrangement for Travelling Wave Tubes. A typical focussing stack uses Rare Earth
Samarium Cobalt axially charged ring magnet material. The ring magnets form
magnetic lenses which are periodic.
Perveance
Is a measure of the
relationship between the beam current and voltage of an electron beam. It is
defined as the ratio of the cathode current to the 3/2 power of the anode
voltage. Due to the size of this unit the normal unit used is the
microperv.
Phase Modulation (PM)
Modulation
in which the phase angle of a wave varies with time.
Phase Shift
The phase difference
between the output carrier wave and the input carrier wave, usually measured
between small signal operation (10 dB or more below saturation) and saturated
operation. Phase shift is a measure of tube linearity.
Pulse
Momentary flow of energy of
such short time duration that it may be considered to be an isolated
phenomenon.
Pulse Duration
The time interval
between the beginning and end of a Pulse at which the amplitude reaches a stated
fraction of the peak pulse amplitude.
Pulse Repetition Frequency
The
number of pulses per second.
Pushing Figure
In an amplifier this is the rate of
frequency for a variation in one of the control electrodes voltage of current.
It is usually expressed in dB/volt or dB/amp.
Ring-Loop and Ring-Bar Slow-wave Structures
These
Slow-Wave Structures use a series of rings and joining loops (or bars) to
provide the equivalent of a double contra-wound helix. This provides less
bandwidth than a simple helix structure but is capable of higher peak output
power.
Saturation
This is the phenomena in
an amplifier where for a further increase in input signal there is no increase
in output signal.
Slow wave Structure
A microwave circuit along which an
electromagnetic wave is propagating. The circuit is constructed so that the
velocity of the electromagnetic wave is near synchronous with the beam velocity.
The electron beam and the RF wave interact to transfer energy from the electron
beam to the RF wave to produce amplification.
Small Signal Gain
The power Gain in decibels provided by
an amplifier when the output power is well below Saturation. This is normally at
least 10 dB below saturation.
Solenoid
A cylindrical coil consisting of a multiple
winding. It is commonly used as an electromagnet to assist in the focussing of
an electron beam.
Space Charge
The electric charge carried by a cloud or
stream of electrons or ions in a vacuum or a region of low gas pressure, when
the net charge density is sufficient to produce local changes in the potential
distribution.
Standby Heater Voltage
To enable a tube to be more
rapidly, ready for use the heater voltage may be left applied, but at a lower
than usual value. This is the “standby heater voltage”.
Standing Wave Ratio (SWR)
Any transmission line, unless
terminated by its characteristic impedance, will exhibit a superposition of
standing and progressive waves. The standing wave ratio is defined as the ratio
of the maximum amplitude of voltage to the minimum amplitude of voltage measured
along the path of the waves.
Thermionic Cathode
This is a source
of electrons which are emitted from a hot surface. The work function of the
surface is lowered by the use of a coating or impregnant usually of barium.
Conventional Oxide cathodes have been replaced by Dispenser cathodes which
contain a mixture of barium-calcium aluminates within a porous structure.
Emission from this type of cathode can be enhanced by the addition of coatings
such as Osmium. These cathodes are known as B-type and M-type.
Travelling Wave Tube
A linear beam device which uses a
Slow-Wave Structure to provide Microwave or RF amplification.
Travelling Wave Tube Amplifier
The combination of a
travelling wave tube and a power supply which provides the necessary electrode
voltages and control and monitoring signals. Other components which might
include a Lineariser, Gain Equaliser, Output Monitor and Input Monitor could
form part of the amplifier.
VSWR
See Standing Wave Ratio.
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