CFA Theory of Operation

Beverly Microwave Division

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CPI Beverly Microwave Division, +1 (978) 922-6000 • FAX: +1 (978) 922-8914 • bmdmarketing@cpii.com

Rev. 10/17

The CFA is a vacuum electron device (VED) which

might best be described as part magnetron and part

TWT (Traveling Wave Tube). It is like a magnetron

because it utilizes the same type of electronic inter-

action that a magnetron does (crossed electric and

magnetic fields) and it derives many of the magne-

tron's characteristics as a result. It is like a TWT

because electronic interaction is with a traveling

wave and, of course, it is an amplifier (In a magne-

tron the electronic interaction is with a standing

wave.) Power can be generated with high efficiency

for the same reasons that a magnetron can do so.

Power is also generated at voltage levels similar to

those of a magnetron. In fact many CFA's look quite

like a magnetron with the same form factor but with

the addition of an input port.

The CFA derives its name from the fact that elec-

tronic interaction occurs in a region of crossed elec-

tric and magnetic fields, like the magnetron. This

form of interaction distinguishes the CFA from the

conventional TWT. Otherwise the CFA is made up

of the same basic elements as a TWT. It has a slow

wave circuit, an input-output system, and an elec-

tronic system.

The slow wave circuit, or delay line as it is some-

time called, is a periodic structure which has the

circuit characteristics of a band pass filter. It must

be capable of propagating RF energy over the

frequency range of interest. It must at the same

time provide fringing electric field lines with which

electrons may interact. These fields must have a

phase velocity approximately equal to the velocity

of the electron stream.

The input-output system provides the impedance

transformation between the RF transmission line

system external to the amplifier and the slow wave

circuit itself. These impedance transformations are

called circuit matches and the bandwidth over

which the VSWR of these matches can be kept low

often determines the useful bandwidth of the CFA

itself.

The electronic system generates electrons,

confines them to the interaction area, and collects

them when they are spent. Some cross-field ampli-

fiers have electron guns which inject a beam of

electrons into the interaction area. These amplifiers

have been called injected beam amplifiers. Some

amplifiers have a relatively large cathode which

extends the entire length of the slow wave circuit.

Electrons are generated along the entire length of

the cathode. CFA's with this type of electron source

are called distributed emission amplifiers.

Figure 1 provides a pictorial reference for the major

CFA elements. The cathode is also called the sole

and distributed emission amplifiers are sometimes

referred to as emitting sole amplifiers. We shall use

the words cathode and sole interchangeably. In

Figure 2, the magnetic field is shown normal to the

plane, and, during operation, the cathode will be

negative with respect to the slow wave circuit. The

magnetic field in this type of amplifier provides

focusing forces for the electron stream and estab-

lishes the crossed-field which give the amplifier its

desirable characteristics.

Pg. 1

The CFA - Part Magnetron, Part TWT

Figure 1

SLOW WAVE CIRCUIT

DRIFT

REGION

OUTPUT

MATCH

MAGNETIC

FIELD

INPUT

MATCH

CATHODE

(SOLE)

CFA Theory of Operation, pg. 2

Beverly Microwave Division

www.cpii.com/bmd

For information on CPI products visit our webpage at www.cpii.com/bmd, or contact:

CPI Beverly Microwave Division, +1 (978) 922-6000 • FAX: +1 (978) 922-8914 • bmdmarketing@cpii.com

Rev. 10/17

The distributed emission amplifier can be arranged

in a number of ways. It can be made in either a

linear or a circular format. Amplifiers made with the

circular format may collect electrons at one end of

the circuit, or the input and output sections may be

brought close enough together so that electrons

from the output are permitted to reenter the interac-

tion area at the input. Reentrancy is employed in

may amplifiers to enhance efficiency. When reen-

trancy is employed, however, it is possible that the

reentering electrons are modulated with information

which will subsequently be amplified. This is equiv-

alent to providing an RF feedback and this feed-

back must be considered in determining the behav-

ior of the amplifier. It is possible, however, to obtain

reentrancy after demodulating the electron stream

to eliminate the RF feedback

Historically the crossed-field amplifier principle was

reduced to practice causing a backward wave

circuit which was related to the magnetron circuit.

The CFA was subsequently developed with a

considerable support from the United States Gov-

ernment in that form to enable its application in

radar systems. However, as will be seen in the

following sections, the backward wave amplifier is a

voltage tuned amplifier and the modulator voltage

applied to it must be varied with frequency.

Pg. 2

Circuit Considerations

The forward wave amplifier on the other hand can

realize its bandwidth at constant voltage like the

TWT. For some applications the forward wave

amplifier is more desirable, primarily because of

modulator voltage considerations. At the present

time, forward wave circuits which have been recent-

ly developed also offer the potential for greater

bandwidth and are desirable for that reason.

Perhaps the most important element of a CFA is its

slow wave circuit. In the CFA the slow wave circuit

acts as the collector for the spent electron beam.

Since CFA's are mostly used for high power appli-

cations, as opposed to small signal use, the slow

wave circuit must be capable of dissipating the

collected beam and transferring that energy to a

heat sink.

• It must have the thermal capability to handle

the required power dissipation.

• It must be capable of interacting efficiently

with the electron stream. When it does it is

said to have a high or a good interaction (or

electronic) impedance.

• It must have dispersion characteristics suffi

cient to meet the bandwidth requirements of

the application in which it is to be used.

• It must have properties which will permit an

impedance transformation from the external

transmission line system to the slow wave

circuit. This transformation is called the

circuit match or simply the match, and it

must generally present to the external cir-

cuit a low VSWR over bandwidths which are

large compared with the intended use band

width of the amplifier.

• It must be practical enough in its mechanical

configuration to permit economic manufac-

ture. In this sense it must also be capable of

enduring system environmental conditions

without detriment to its life or performance.

OUTPUT

MATCH

Figure 2

SLOW WAVE CIRCUIT

MAGNETIC

FIELD

INPUT

MATCH

CATHODE (SOLE)