Radar Basics

Backward-Wave Oscillator

The backward-wave oscillator (BWO), also called Carcinotron (from karkinox, the Greek word καρκίνος for the backward-swimming crayfish), a brand name of the company Thomson-CSF, now part of Thales, is an electron tube for oscillation generation which has the advantage of electronic tunability over a wide frequency range. Backward-wave tubes are examples of velocity-modulated tubes.

There are two types of backward-wave tubes. They are distinguished as type “M” or “M-BWO” and type “O” or “O-BWO”. Type “M” tubes are cross-wave tubes similar to a magnetron. The “O” from the type designation “O-type” comes from the French word for wave l’onde and designates that the magnetic field is in the same direction as the electron beam and the slow-wave structure.

Invention of backward-wave oscillator was simultaneously and independently presented by Rudolf Kompfner (an Austrian scientist living in England) from Bell Labs, and by Bernard Epsztein from Thomson-CSF. Epsztein demonstrated the operation of an M-type backward-wave oscillator on March 31, 1951. A patent application was filed (French Patent No. 1 035 379;[1] British Patent No. 699 893; U.S. Patent No. 2 880 355). Kompfner invented the O-type backward-wave oscillator in 1951. This patent was filed on May 15, 1952, under US 2 985 790, and granted on May 23, 1961.[2]

RF-output

Dummy-load

Figure 2: Schematic circuit of an “O” type backward-wave oscillator with serpentine waveguide as slow-wave structure

RF-output

Dummy-load

Figure 2: Schematic circuit of an “O” type backward-wave oscillator with serpentine waveguide as slow-wave structure

“O”-Type Backward-Wave Oscillator

The backward-wave oscillator of type “O” has a similar construction as a traveling wave tube. The electron beam is focused by a strong magnetic field but this magnetic field is not involved in the interactions. The slow-wave structure is usually a folded waveguide whose dimensions determine the bandwidth of the tube. A bifilar helix may also be used for lower output powers. As in the traveling-wave tube, the mode of operation is based on the synchronous interaction between the high-frequency wave in the slow-wave structure and the electron beam. While in a traveling wave tube the direction of the wave in the slow-wave structure travels with the electron beam, in the backward-wave oscillator the direction is reversed. When the wave and electron beam travel in opposite directions, the electron beam acts as a feedback element by feeding back the induced velocity changes from the output to the input.

Oscillations are generated if there is a phase dispersion between the velocity of the electrons and the phase velocity of the wave on the slow-wave structure. The velocity of propagation in the slow-wave structure is constant. The velocity of the electrons in the electron beam can be changed by the collector potential (practically: change of the anode voltage!). Thus the frequency of the generated oscillations can be changed in quite a wide range.

anode with slow-wave structure

RF-output

cold kathode

kathode

absorber

collector

control-
electrode

Figure 2: Schematic circuit of an “M” type backward-wave oscillator

anode with slow-wave structure

RF-output

cold kathode

kathode

absorber

collector

control-
electrode

Figure 2: Schematic circuit of an “M” type backward-wave oscillator

“M”-Type Backward-Wave Oscillator

The backward-wave oscillator type “M” uses an electric field E between the cathodes and the anode and a magnetic field B perpendicular to it, similar to the magnetron, for the circular deflection of an electron beam. The electrons move perpendicular to field E and field B with a velocity V_e. A strong interaction occurs at a phase dispersion, that is, when the phase velocity V_φ of the high-frequency wave in the slow-wave structure is approximately equal to the electron velocity V_e. The electrons acquire an additional velocity component, which leads to a bunching of electrons. Electrons that are in a decelerating electric field of the wave in the slow-wave structure lose the energy they received from the static electric field E and give this energy to the high-frequency backward-wave. The backward-wave has the velocity V_rh and forms a harmonic resonance in space with the electron bunches, i.e. the electric field strength of the backward-wave obtains a maximum exactly at the points where the electron bunches also have a maximum.

The cold cathode is more negative than the heated cathode to avoid the electrons being captured by the cold cathode, which have absorbed energy during the interaction with the field of the wave in the slow-wave structure. This action also causes the radius of the electron paths to change shortly after leaving the heated cathode. The collector C has anode voltage potential and closes the circuit of the backward-wave oscillator.

“M” type backward-wave oscillators can also operate in a slow-wave structure resonant region. This gives them better efficiency, but still allows them to retain the characteristic of electrically tunable frequency within a certain bandwidth. Usually it is sufficient to include only one resonator as a reflector.

Electrical characteristics

The power levels achievable with this type of tube range from 50 to 1 000 mW. The frequencies that can be generated extend into the terahertz range and are limited only by the slow-wave structure. The tunable bandwidth can typically be more than 10% of the center frequency. Compared to other oscillator tubes, it has a rather low efficiency of only about 20 to 30% (O-type) and up to 40% (M-type), which decreases even more with increasing frequency.

backward-wave oscillators were used in the early 1960s to the late 1970s in jamming equipment against radar. After that, they were generally replaced there by semiconductor circuits. From 1980 onwards, laboratory applications up to the upper terahertz range became interesting.