What is a Crossed Field Amplifier?

A Crossed Field Amplifier or CFA is a broadband microwave amplifier that utilizes the principles of magnetron and traveling wave tube (TWT) systems to amplify or increase the magnitude of the radio frequency (RF) input signal and deliver the amplified signal to the output port. It functions like a magnetron – a specialized high-power vacuum tube that generates microwave signals by controlling the flow of electrons using an applied magnetic field, and functions like a TWT at the same time – which amplifies the RF signal by absorbing power (or energy) from a beam of electrons as it propagates through the tube of the CFA. Utilizing these two techniques helps CFA to deliver high peak output power and efficiency compared to standard power amplifiers. This makes them suitable for satellite ground stations and deep space communication networks where a high output power is required to efficiently communicate with satellites over very long distances. They are also used in generating large amounts of power in very high power transmitters in radars and other wireless communication systems over a frequency range from 1 GHz to 10 GHz.

The word “crossed-field” indicates that the electric and magnetic fields generated inside the CFA are perpendicular to each other and that both the fields alternate with time as they travel through the system.

Operation of Cross-Field Amplifiers 

A typical crossed-field amplifier consists of a slow-wave structure or the delay line, a cathode (negative) at the center, an anode terminal (positive), round resonant cavities, input and output waveguides, and an electronic system as shown in the below figure.

An external electron gun can be used to inject electrons into the slow wave structure through which the RF signal will travel, get amplified, and reach the output. Other means of injecting electrons may also be used, such as the interaction of different kinds of electromagnetic fields that result in an electron cloud and so on. In this case, the cathode at the center of the CFA can be used to produce the electron beam. When the RF signal is applied at the input waveguide port, it naturally propagates at the speed of light. However, there must be a mechanism that reduces its speed down to the electron beam's speed to allow interaction between the electron beam and the RF signal for amplification. This is achieved by using a slow-wave structure. The structure is usually a helix shape or a bar-line, depending on the application requirement, and the resulting RF signal inside this structure is referred to as the slow wave. An additional attenuator block is also inserted to prevent spurious oscillations from interfering with the RF signal.

Since the resonant cavities are positively charged (anode) and cathode is negatively charged, the electric field from both accelerate and decelerate the electrons, depending on the direction of the electric field. This permits the speed of the electron beam to change (and reduce) at different locations in the structure. When the RF signal is injected into the input, the velocity of the RF signal is reduced, allowing it to travel at the same speed as that of the electron beam. Therefore, both the RF signal and electron beam are in-phase i.e. they have the same phase value at any given time. This allows them to interact with each other constructively, which adds more energy to the RF signal as it propagates. In other words, the electron beam transfers its energy to the RF signal as it propagates. This causes amplification of the RF signal. CFAs can deliver a peak output power in the order of megawatts, an average output power in the order of kilowatts, with an efficiency of up to 80%.

When the direction of the RF signal and electron beam are the same, this is called as forward wave crossed field amplifier (FW-CFA) and when the directions are opposite, the design is referred to as a backward wave crossed field amplifier (BW-CFA). Either or both of them can be used depending on what is required in any application setting. 

When both the input and output ports are properly matched with equal impedance values, maximum RF signal power can be delivered to the output. Otherwise, a finite amount of RF feedback power will be sent back to the input. This will introduce phase jitter, causing distortion and ripples in the RF output signal. The effects are significant when the CFA produces a pulsed waveform, which results in random phase jitters. 

Advantages of Crossed Field Amplifiers 

1. CFAs can provide a high output power and efficiency compared to standard TWT amplifier designs, making them suitable for high precision radar applications. 

2. It can handle a high amount of power of up to 4 MW (at 2 GHz) and 1 MW (at 10 GHz) when a pulsed-based CFA design is used. 

3. The use of a slow-wave structure similar to TWTs allows CFA amplifiers to adopt similar manufacturing designs used to develop TWT, thereby maintaining the overall cost while ensuring a relatively high output performance.

Disadvantages of Crossed Field Amplifiers 

1. CFA amplifiers provide a low gain (around 10-15 dB) and effective bandwidth compared to Klystron or TWTs 

2. The amount of isolation between the input and output depends on the type of isolator used. Sophisticated isolators may be used to improve isolation, but raise the cost of the amplifier.

3. There is a small amount of feedback from the output to the input which increases the noise power at the output. This can be alleviated by fine-tuning the slow-wave structure design.