BWO vs. BWA: Understanding the Differences in RF Microwave

This page explores the distinctions between Backward Wave Oscillators (BWO) and Backward Wave Amplifiers (BWA) in the realm of RF and microwave technology. We’ll cover their applications, principles of operation, and key features.

Backward Wave Oscillator (BWO)

Figure 1: Cross-section of a Backward Wave Oscillator.

As shown in Figure 1, a BWO typically has a larger diameter compared to a Traveling Wave Tube (TWT). It features a helix at the center of the tube. The output signal is derived near the cathode terminal, and the electron beam is terminated at the collector. A magnetic field, provided by a magnet, confines the beam within the helix structure.

The BWO operates based on the velocity modulation of electrons. Specifically, it functions with an RF wave traveling backwards, in the opposite direction to the electron beam within the helix structure. The frequency of oscillation is directly dependent on the voltage applied between the cathode and the helix. The beam’s velocity is carefully set to be slightly greater than the phase velocity by adjusting the beam voltage.

The velocity of the electron in a BWO can be expressed as:

Vo = (2 * e * Va / m)^0.5

Where:

• e = electron charge
• m = mass of electron
• Va = anode voltage

The phase constant of the electron beam is:

Βe = w / Ve

The condition for oscillation is defined as:

D * N = (2 * k + 1) / 4

Where:

• D = D factor
• k = integer
• N = Βe *L / (2*π)

Key Features of BWO

• Non-resonant oscillator: Operates without a resonant cavity.
• Broadband oscillator: Capable of generating a wide range of frequencies.
• Voltage-tunable oscillator: Frequency can be adjusted by changing the applied voltage.
• Wide Frequency Range: Can achieve frequency ranges greater than 5:1.

Applications of BWO

• Continuous wave generator
• Generation of wide range of frequencies

Backward Wave Amplifier (BWA)

Figure 2: Cross-section of a Backward Wave Amplifier.

Figure 2 illustrates the cross-section of a Backward Wave Amplifier. It consists of two terminals and includes components such as an electron gun, cathode, focusing electrodes, a collector, magnetic RF input, and RF output. Notice that the input port is positioned near the collector side, while the output is located near the cathode side.

The operation of a BWA is similar to that of a BWO, with a key difference: the helix wave in a BWA is dependent upon the input RF wave. When an RF input signal is applied, the wave travels in the reverse direction relative to the electron beam. Velocity modulation is produced by the interaction between the electron beam and the helix wave. The phase difference between the input wave and the bunched electrons causes the electrons to slow down, transferring energy to the wave. This added energy amplifies the input RF signal.

The BWA’s frequency can be tuned by adjusting the helix voltage.

Vdc = (2 *e * Va / m)^0.5

Phase constant:

Βe = w / Vdc

Where, Cyclotron angular frequency, Wc is expressed as follows:

Wc= (e/m)*Βm

Here, Βm is crossed magnetic flux density

The BWA gain is expressed as follows :

G = [(Ia * Zo) / (4 * Va)] ^1/3

Where, Ia = Anode Current and Zo = Characteristic impedance

BWO vs. BWA: Key Similarities and Differences