Backward wave oscillator
Encyclopedia
A backward wave oscillator (BWO), also called carcinotron (a trade name for tubes manufactured by CSF
, now Thales
) or backward wave tube, is a vacuum tube
that is used to generate microwaves up to the terahertz
range. It belongs to the traveling-wave tube family. It is an oscillator with a wide electronic tuning range.
An electron gun
generates an electron beam that is interacting with a slow-wave structure. It sustains the oscillation
s by propagating a traveling wave backwards against the beam. The generated electromagnetic wave power has its group velocity
directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun.
It has two main subtypes, the M-type, the most powerful, (M-BWO) and the O-type (O-BWO). The O-type delivers typically power
in the range of 1 mW
at 1000 GHz to 50 mW at 200 GHz
. Carcinotrons are used as powerful and stable microwave sources. Due to the good quality wavefront
they produce (see below), they find use as illuminators in terahertz
imaging.
The backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein, (French patent 1,035,379; British patent 699,893; US patent 2,880,355) and O-type by Rudolf Kompfner
. The M-type BWO is a voltage-controlled non-resonant extrapolation of magnetron interaction, both types are tunable over a wide range of frequencies by varying the accelerating voltage
. They can be swept through the band fast enough to be appearing to radiate over all the band at once, which makes them suitable for effective radar jamming, quickly tuning into the radar frequency. Carcinotrons allowed airborne radar jammers to be highly effective. However, frequency-agile radar
s can hop frequencies fast enough to force the jammer to use barrage jamming
, diluting its output power over a wide band and significantly impairing its efficiency.
Carcinotrons are used in research, civilian and military applications. For example, the Kopac passive sensor
and Ramona passive sensor
employed carcinotrons in their receiver systems.
The needed slow-wave structures must support a Radio Frequency
(RF) electric field with a longitudinal component; the structures are periodic in the direction of the beam and behave like microwave filters with passbands and stopbands. Due to the periodicity of the geometry, the fields are identical from cell to cell except for a constant phase shift Φ.
This phase shift, a purely real number in a passband of a lossless structure, varies with frequency.
According to Floquet's theorem (see Floquet theory), the RF electric field E(z,t) can be described at an angular frequency
ω, by a sum of an infinity of "spatial or space harmonics" En
E(z,t)=
where the wave number
or propagation constant kn of each harmonic is expressed as:
kn=(Φ+2nπ)/p (-π<Φ<+п)
z being the direction of propagation, p the pitch of the circuit and n an integer.
Two examples of slow-wave circuit characteristics are shown, in the ω-k or Brillouin
diagram:
A periodic structure can support both forward and backward space harmonics, which are not modes of the field, and cannot exist independently, even if a beam can be coupled to only one of them.
As the magnitude of the space harmonics decreases rapidly when the value of n is large, the interaction can be significant only with the fundamental or the first space harmonic.
The M-type carcinotron, or M-type backward wave oscillator, uses crossed static electric field E and magnetic field
B, similar to the magnetron, for focussing an electron sheet beam drifting perpendicularly to E and B, along a slow-wave circuit, with a velocity E/B. Strong interaction occurs when the phase velocity
of one space harmonic of the wave is equal to the electron velocity. Both Ez and Ey components of the RF field are involved in the interaction (Ey parallel to the static E field). Electrons which are in a decelerating Ez electric field of the slow-wave, lose the potential energy
they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic.
f/f=1/2[1/(1+|vΦ/vg|)](
V0/V0)
The oscillation frequency is also sensitive to the beam current (called "frequency pushing"). The current fluctuations at low frequencies are mainly due to the anode voltage supply, and the sensitivity to the anode voltage is given by
f/f=3/4[ωq/ω/(1+|vΦ/vg|)](
Va/Va)
This sensitivity as compared to the cathode voltage sensitivity, is reduced by the ratio ωq/ω, where ωq is the angular plasma frequency; this ratio is of the order of a few times 10-2.
Thomson-CSF
Thomson-CSF was a major electronics and defence contractor. In December 2000 it was renamed Thales Group.-History:In 1879 Elihu Thomson and Edwin Houston formed the Thomson-Houston Electric Company in the United States....
, now Thales
Thales Group
The Thales Group is a French electronics company delivering information systems and services for the aerospace, defense, transportation and security markets...
) or backward wave tube, is a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
that is used to generate microwaves up to the terahertz
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
range. It belongs to the traveling-wave tube family. It is an oscillator with a wide electronic tuning range.
An electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
generates an electron beam that is interacting with a slow-wave structure. It sustains the oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s by propagating a traveling wave backwards against the beam. The generated electromagnetic wave power has its group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun.
It has two main subtypes, the M-type, the most powerful, (M-BWO) and the O-type (O-BWO). The O-type delivers typically power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
in the range of 1 mW
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
at 1000 GHz to 50 mW at 200 GHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. Carcinotrons are used as powerful and stable microwave sources. Due to the good quality wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
they produce (see below), they find use as illuminators in terahertz
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
imaging.
The backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein, (French patent 1,035,379; British patent 699,893; US patent 2,880,355) and O-type by Rudolf Kompfner
Rudolf Kompfner
Rudolf Kompfner was an Austrian-born engineer and physicist, best known as the inventor of the traveling-wave tube .Kompfner was born in Vienna to Jewish parents...
. The M-type BWO is a voltage-controlled non-resonant extrapolation of magnetron interaction, both types are tunable over a wide range of frequencies by varying the accelerating voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. They can be swept through the band fast enough to be appearing to radiate over all the band at once, which makes them suitable for effective radar jamming, quickly tuning into the radar frequency. Carcinotrons allowed airborne radar jammers to be highly effective. However, frequency-agile radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
s can hop frequencies fast enough to force the jammer to use barrage jamming
Barrage jamming
Barrage jamming: jamming accomplished by transmitting a band of frequencies that is large with respect to the bandwidth of a single emitter. Barrage jamming may be accomplished by presetting multiple jammers on adjacent frequencies, by using a single wideband transmitter, or by using a transmitter...
, diluting its output power over a wide band and significantly impairing its efficiency.
Carcinotrons are used in research, civilian and military applications. For example, the Kopac passive sensor
Kopác passive sensor
Kopáč was an early electronic warfare support measures system developed in Czechoslovakia in the early 1960s that used measurements of time difference of arrival of pulses at three sites to accurately detect and track airborne emitters...
and Ramona passive sensor
Ramona passive sensor
Ramona was the second generation Czechoslovakian electronic support measures system that uses measurements of time difference of arrival of pulses at three or four sites to accurately detect and track airborne emitters by multilateration.-History:...
employed carcinotrons in their receiver systems.
The Slow-wave structure
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) electric field with a longitudinal component; the structures are periodic in the direction of the beam and behave like microwave filters with passbands and stopbands. Due to the periodicity of the geometry, the fields are identical from cell to cell except for a constant phase shift Φ.
This phase shift, a purely real number in a passband of a lossless structure, varies with frequency.
According to Floquet's theorem (see Floquet theory), the RF electric field E(z,t) can be described at an angular frequency
Angular frequency
In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
ω, by a sum of an infinity of "spatial or space harmonics" En
E(z,t)=
where the wave number
Wavenumber
In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...
or propagation constant kn of each harmonic is expressed as:
kn=(Φ+2nπ)/p (-π<Φ<+п)
z being the direction of propagation, p the pitch of the circuit and n an integer.
Two examples of slow-wave circuit characteristics are shown, in the ω-k or Brillouin
Léon Brillouin
Léon Nicolas Brillouin was a French physicist. He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid state physics, and information theory.-Early life:...
diagram:
- on figure (a), the fundamental n=0 is a forward space harmonic (the phase velocityPhase velocityThe phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
vn=ω/kn has the same sign as the group velocityGroup velocityThe group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
vg=dω/dkn), synchronism condition for backward interaction is at point B, intersection of the line of slope ve - the beam velocity - with the first backward (n = -1) space harmonic,
- on figure (b) the fundamental (n=0) is backward
A periodic structure can support both forward and backward space harmonics, which are not modes of the field, and cannot exist independently, even if a beam can be coupled to only one of them.
As the magnitude of the space harmonics decreases rapidly when the value of n is large, the interaction can be significant only with the fundamental or the first space harmonic.
M-type BWO
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
B, similar to the magnetron, for focussing an electron sheet beam drifting perpendicularly to E and B, along a slow-wave circuit, with a velocity E/B. Strong interaction occurs when the phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
of one space harmonic of the wave is equal to the electron velocity. Both Ez and Ey components of the RF field are involved in the interaction (Ey parallel to the static E field). Electrons which are in a decelerating Ez electric field of the slow-wave, lose the potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic.
O-type BWO
The O-type carcinotron, or O-type backward wave oscillator, uses an electron beam longitudinally focused by a magnetic field, and a slow-wave circuit interacting with the beam. A collector collects the beam at the end of the tube.O-BWO Spectral Purity and Noise
The BWO is a voltage tunable oscillator, whose voltage tuning rate is directly related to the propagation characteristics of the circuit. The oscillation starts at a frequency where the wave propagating on the circuit is synchronous with the slow space charge wave of the beam. Inherently the BWO is more sensitive than other oscillators to external fluctuations. Nevertheless its ability to be phase- or frequency-locked has been demonstrated, leading to successful operation as a heterodyne local oscillator.1-Frequency Stability
The frequency-voltage sensitivity, is given by the relationf/f=1/2[1/(1+|vΦ/vg|)](V0/V0)The oscillation frequency is also sensitive to the beam current (called "frequency pushing"). The current fluctuations at low frequencies are mainly due to the anode voltage supply, and the sensitivity to the anode voltage is given by
f/f=3/4[ωq/ω/(1+|vΦ/vg|)](Va/Va)This sensitivity as compared to the cathode voltage sensitivity, is reduced by the ratio ωq/ω, where ωq is the angular plasma frequency; this ratio is of the order of a few times 10-2.
2-Noise
Measurements on submillimeter-wave BWO's (de Graauw et al., 1978) have shown that a signal-noise ratio of 120 dB per MHz could be expected in this wavelength range. In heterodyne detection using a BWO as a local oscillator, this figure corresponds to a noise temperature added by the oscillator of only 1000-3000K.External links
- Virtual Valve Museum Thomson CSF CV6124