BWO vs BWA-Difference between BWO and BWA in RF Microwave
This page compares BWO(Backward Wave Oscillator) vs BWA(Backward Wave Amplifier) and mention difference between BWO and BWA including applications, principle of operation and features.
BWO-Backward Wave Oscillator
The figure-1 depicts cross section of Backward Wave Oscillator.
BWO has larger diameter compare to TWT.
As shown in the figure, it consists of helix at the centre of tube.
The output is derived near the cathode terminal.
The electron beam terminates at the collector.
The magnet provides magnetic field which confines the beam in helix structure.
Refer Difference between BWO and TWT➤.
It operates on principle of velocity modulation of electrons. It operates with RF wave travelling backwards in opposite direction to the electron beam in helix structure as shown. Frequency of oscillation depends on voltage between cathode and helix. Here velocity of beam is made slightly greater than phase velocity by adjustment of beam voltage.
The velocity of electron in BWO is expressed as follows:
Vo = (2*e*Va/m)0.5
Where,
e = electron charge, m = mass of electron, Va = anode voltage
Phase constant of electron beam is expressed as follows:
Βe = w/Ve
Condition of oscillation is expressed as follows:
D*N = (2*k + 1)/4
Where,
D = D factor
k = integer
N = Βe* L /2*π
Some of the silent features of Backward Wave Oscillator (BWO are as follows:
➨It is a non-resonant oscillator.
➨It is a broadband oscillator.
➨It is a voltage tunable oscillator.
➨Frequency range greater than 5 to 1 can be achieved.
Applications of BWO
Following are the applications of BWO:
• Used as continuous wave generator.
• Used to generate wide range of frequencies.
BWA-Backward Wave Amplifier
The figure-2 depicts cross section of Backward Wave Amplifier. As shown in the figure, it consists of two terminals. It has electron gun, cathode, focusing electrodes, collector, magnetic RF input and RF output. As shown input port is located near collector side and output is located near cathode side.
The operation of BWA is similar to BWO except that helix wave depends upon input RF wave in BWA(Backward Wave Amplifier). With the application of RF input, the wave travels in the reverse direction to the beam direction. In BWA, velocity modulation is produced by interaction between electron beam and helix wave. The phase difference between input wave and bunched electrons results into slow down of velocity of electrons. As a result energy is given to the wave. This additional energy of input RF wave creates amplification. BWA can be tuned to desired frequency by adjustment of 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
Following table summarizes similarities and difference between BWO and BWA.
| BWO(Backward Wave Oscillator) | BWA(Backward Wave Amplifier) |
|---|---|
| Operates on principle of velocity modulation of electrons. | Principle of operation is velocity modulation. |
| Velocity of electrons is controlled by cathode to helix voltage. | Frequency of operation depends on helix voltage. |
| velocity modulation is produced by interaction between electrons and helix. This interaction produces bunching of electrons. | velocity modulation is done by interaction between beam and helix. |
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