A crossed-field amplifier (CFA) is a specialized

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...

, first introduced in the mid-1950s and frequently used as a

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...

in very-high-power

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...

s. Raytheon engineer

William C. Brown William C. Brown (May 22, 1916 – February 3, 1999) was an American electrical engineer who helped to invent the crossed-field amplifier in the 1950s and also pioneered microwave power transmission in the 1960s. Brown received his BSEE fro ...

's work to adapt magnetron principles to create a new broadband amplifier is generally recognized as the first CFA, which he called an Amplitron. Other names that are sometimes used by CFA manufacturers include Platinotron or Stabilotron. A CFA has lower

gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...

and bandwidth than other microwave amplifier tubes (such as klystrons or traveling-wave tubes); but it is more efficient and capable of much higher output power. Peak output powers of many megawatts and average power levels of tens of

kilowatts The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

can be achieved, with efficiency ratings over 70 percent. Their current use is in satellite ground stations and deep space communications networks.

Operation

A CFA's electric and magnetic fields are perpendicular to each other ("crossed fields"). This is the same type of field interaction used in a magnetron; as a result, the two devices share many characteristics (such as high peak power and efficiency), and they have similar physical appearances. However, a magnetron is an oscillator, and a CFA is an amplifier (although a CFA can be driven to oscillate by application of improper low voltages as can any amplifier); a CFA's RF circuit (or slow-wave structure) is similar to that in a coupled-cavity TWT. The CFA has the useful property that when power is shut off, the input simply passes to the output with little loss. This avoids the need for RF bypass switching in the event of failure. Two CFAs can be connected sequentially with only one powered; if it fails, power can be removed from the primary tube and applied to the secondary as a backup. This approach with built-in redundancy was used on the

S-band The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the conventional ...

downlink transmitter on the

Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed ...

, where high efficiency and reliability were needed. A large negative voltage is placed on the green electrode in the center, and a large magnetic field is directed perpendicular to the page. This forms a thin spinning disk of electrons with a flow pattern like spinning water as it drains from a sink or toilet. A slow-wave structure is located above and below the spinning disk of electrons. Electrons flow much slower than the speed of light, and the slow wave structure reduces the velocity of the input RF enough to match the electron velocity. The RF input is introduced into the slow wave structure. The alternating microwave field causes the electrons to speed up and slow down alternately. These disturbances grow larger as electrons spiral around the device, and electrons slow down as the RF energy grows. This produces amplification. There is a small amount of RF feedback from output to input. This creates a slight random phase jitter when the device is pulsed.

References

{{Electronic components Valve amplifiers Microwave technology American inventions