Backward-wave oscillator

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Miniature O-type backward-wave oscillator tube produced by Varian in 1956. It could be voltage-tuned over an 8.2 - 12.4 GHz range and required a supply voltage of 600 V. Backward wave oscillator.jpg
Miniature O-type backward-wave oscillator tube produced by Varian in 1956. It could be voltage-tuned over an 8.2 - 12.4 GHz range and required a supply voltage of 600 V.
Backward wave oscillator at Stockholm University operating in the terahertz range Backward-wave Oszillator-Stockholm.jpg
Backward wave oscillator at Stockholm University operating in the terahertz range

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. Belonging to the traveling-wave tube family, it is an oscillator with a wide electronic tuning range.

Thomson-CSF electronics and defence contractor

Thomson-CSF was a major electronics and defence contractor. In December 2000 it was renamed Thales Group.

Thales Group is a French multinational company that designs and builds electrical systems and provides services for the aerospace, defence, transportation and security markets. Its headquarters are in La Défense, and its stock is listed on the Euronext Paris.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.


An electron gun generates an electron beam that interacts with a slow-wave structure. It sustains the oscillations 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.

Electron gun

An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode ray tubes (CRTs), used in nearly all television sets, computer displays and oscilloscopes that are not flat-panel displays. They are also used in field emission displays (FEDs), which are essentially flat-panel displays made out of rows of extremely small cathode ray tubes. They are also used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, and gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation, by emission mechanism, by focusing, or by the number of electrodes.

Oscillation repetitive variation of some measure about a central value

Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. The term vibration is precisely used to describe mechanical oscillation. Familiar examples of oscillation include a swinging pendulum and alternating current.

Group velocity physical quantity

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.

It has two main subtypes, the M-type (M-BWO), the most powerful, and the O-type (O-BWO). The output power of the O-type is typically 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.

In physics, power is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower. Being the rate of work, the equation for power can be written:

The watt is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second, and is used to quantify the rate of energy transfer. In dimensional analysis, power is described by .

Hertz SI unit for frequency

The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz (103 Hz, kHz), megahertz (106 Hz, MHz), gigahertz (109 Hz, GHz), terahertz (1012 Hz, THz), petahertz (1015 Hz, PHz), and exahertz (1018 Hz, EHz).

The backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein [1] 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 radars 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.

Rudolf Kompfner was an Austrian-born engineer and physicist, best known as the inventor of the traveling-wave tube (TWT).

Voltage difference in the electric potential between two points in space

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

Radar object detection system based on radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

Carcinotrons are used in research, civilian and military applications. For example, the Czechoslovak Kopac passive sensor and Ramona passive sensor air defense detection systems employed carcinotrons in their receiver systems.

Ramona passive sensor electronic support measures system

Ramona was the second generation Czechoslovak electronic support measures (ESM) system that uses measurements of time difference of arrival (TDOA) of pulses at three or four sites to accurately detect and track airborne emitters by multilateration.

Basic concept

Concept diagram. The signals travel from the input to the output as described in text within the image. Diagram of basic principle of backward-wave oscillator.svg
Concept diagram. The signals travel from the input to the output as described in text within the image.

All travelling-wave tubes operate in the same general fashion, and differ primarily in details of their construction. The concept is dependent on a steady stream of electrons from an electron gun that travel down the center of the tube (see adjacent concept diagram). Surrounding the electron beam is some sort of radio frequency source signal; in the case of the traditional klystron this is a resonant cavity fed with an external signal, whereas in more modern devices there are a series of these cavities or a helical metal wire fed with the same signal. [2]

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second to around three hundred billion times per second. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.


A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian, which is used as an amplifier for high radio frequencies, from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters, satellite communication, radar transmitters, and to generate the drive power for modern particle accelerators.

As the electrons travel down the tube, they interact with the RF signal (see concept diagram). The electrons are attracted to areas with maximum positive bias and repelled from negative areas. This causes the electrons to bunch up as they are repelled or attracted along the length of the tube, a process known as velocity modulation. This process makes the electron beam take on the same general structure as the original signal; the density of the electrons in the beam matches the relative amplitude of the RF signal in the induction system. The result is a signal in the electron beam that is an amplified version of the original RF signal. [2]

As the electrons are moving, they induce a magnetic field in any nearby conductor as illustrated in the concept diagram. This allows the now-amplified signal to be extracted. In systems like the magnetron or klystron, this is accomplished with another resonant cavity. In the helical designs, this process occurs along the entire length of the tube, reinforcing the original signal in the helical conductor. The "problem" with traditional designs is that they have relatively narrow bandwidths; designs based on resonators will work with signals within 10% or 20% of their design, as this is physically built into the resonator design, while the helix designs have a much wider bandwidth, perhaps 100% on either side of the design peak. [3]


The BWO is built in a fashion similar to the helical TWT. However, instead of the RF signal propagating in the same (or similar) direction as the electron beam, the original signal travels at right angles to the beam. This is normally accomplished by drilling a hole through a rectangular waveguide and shooting the beam through the hole. The waveguide then goes through two right angle turns, forming a C-shape and crossing the beam again. This basic pattern is repeated along the length of the tube so the waveguide passes across the beam several times, forming a series of S-shapes. [2]

The original RF signal enters from what would be the far end of the TWT, where the energy would be extracted. The effect of the signal on the passing beam causes the same velocity modulation effect, but because of the direction of the RF signal and specifics of the waveguide, this modulation travels backward along the beam, instead of forward. This propagation, the slow-wave, reaches the next hole in the folded waveguide just as the same phase of the RF signal does. This causes amplification just like the traditional TWT. [2]

The difference in the two systems is that in the TWT the speed of propagation in the helix has to be similar to that of the electrons in the beam. This is not the case in the BWO. The waveguide places strict limits on the bandwidth of the signal and sets its propagation speed as a basic function of its construction, but the speed of the signal induced into the electron beam is relative to the speed of the electrons. That means the frequency of the output signal can be changed by changing the speed of the electrons, which is easily accomplished by changing the voltage of the electron gun. [2]


This image shows the effect of four carcinotron-carrying aircraft on a typical 1950s pulse radar. The aircraft are located at roughly the 4 and 5:30 locations. The display is filled with noise any time the antenna's main lobe or sidelobes pass the jammer, rendering the aircraft invisible. Carcinotron jamming a pulse radar unit.png
This image shows the effect of four carcinotron-carrying aircraft on a typical 1950s pulse radar. The aircraft are located at roughly the 4 and 5:30 locations. The display is filled with noise any time the antenna's main lobe or sidelobes pass the jammer, rendering the aircraft invisible.

The device was originally given the name "carcinotron" because it was like cancer to existing radar systems. By simply changing the supply voltage, the device could produce any required frequency across a band that was much larger than any existing microwave amplifier could match - the cavity magnetron and klystron worked at a single frequency defined by the physical dimensions of their resonators, and could be tuned only within a small range of that design. [2]

Previously, jamming a radar was a complex and time-consuming operation. Operators had to listen for potential frequencies being used, set up one of a bank of amplifiers on that frequency, and then begin broadcasting. When the radar station realized what was happening, they would change their frequencies and the process would begin again.

In contrast, the carcinotron could sweep through all the possible frequencies so rapidly that it appeared to be a constant signal on all of the frequencies at once. Typical designs could generate hundreds or low thousands of watts, so at any one frequency, there might be a few watts of power. However, at long range the amount of energy from the original radar that reaches the aircraft is only a few watts anyway, so the carcinotron can overpower them. [2]

The system was so powerful that it was found that a carcinotron operating on an aircraft would begin to be effective even before it rose above the radar horizon. As it swept through the frequencies it would broadcast on the radar's own pulse frequency at what were effectively random times, filling the display with random dots any time the antenna was pointed near it, perhaps 3 degrees on either side of the target. There were so many dots that the display simply filled with white noise in that area. As it approached the station, the signal would also begin to appear in the antennas sidelobes, creating further areas that were blanked out by noise. At close range, on the order of 100 miles (160 km), the entire radar display would be completely filled with noise, rendering it useless. [2]

The concept was so powerful as a jammer that there were serious concerns that ground-based radars were obsolete. Airborne radars had the advantage that they could approach the aircraft carrying the jammer, and, eventually, the huge output from their transmitter would "burn through" the jamming. However, interceptors of the era relied on ground direction to get into range. This represented an enormous threat to air defense operations. [4]

For ground radars, the threat was eventually solved in two ways. The first was that radars were upgraded to operate on many different frequencies and switch among them randomly from pulse to pulse, a concept now known as frequency agility. Some of these frequencies were never used in peacetime, and highly secret, with the hope that they would not be known to the jammer in wartime. The carcinotron could still sweep through the entire band, but then it would be broadcasting on the same frequency as the radar only at random times, reducing its effectiveness. The other solution was to add passive receivers that triangulated on the carcinotron broadcasts, allowing the ground stations to produce accurate tracking information on the location of the jammer and allowing them to be attacked. [4]

The slow-wave structure

(a) Forward fundamental space harmonic (n=0), (b) Backward fundamental Space-harmonics.svg
(a) Forward fundamental space harmonic (n=0), (b) Backward fundamental

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

where the wave number or propagation constant kn of each harmonic is expressed as

kn = (Φ + 2nπ) / p (-π < Φ < +π)

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.

M-type BWO

Schematic of an M-BWO M-bwo.svg
Schematic of an M-BWO

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.

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.

Frequency stability

The frequency–voltage sensitivity, is given by the relation

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.


Measurements on submillimeter-wave BWO's (de Graauw et al., 1978) have shown that a signal-to-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 1000–3000 K.


  1. FRpatent 1035379,Bernard Epsztein,"Backward flow travelling wave devices",published 1959-03-31
  2. 1 2 3 4 5 6 7 8 9 Microwave Principles. US Navy. September 1998. p. 103.
  3. Gilmour, A. S. (2011). Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers, and Gyrotrons. Artech House. pp. 317–18. ISBN   1608071855.
  4. 1 2 Morris, Alec (1996). "UK Control & Reporting System from the End of WWII to ROTOR and Beyond". In Hunter, Sandy. Defending Northern Skies. Royal Air Force Historical Society. pp. 105–106.

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