Electronics and Communication Engineering - Microwave Communication - Discussion

27. 

Assertion (A): A backward wave oscillator has an internal positive feedback.

Reason (R): A positive feedback is necessary for sustained oscillations.

[A]. Both A and R are correct and R is correct explanation of A
[B]. Both A and R are correct but R is not correct explanation of A
[C]. A is correct but R is wrong
[D]. A is wrong but R is correct

Answer: Option B

Explanation:

It is somewhat similar to TWT and can deliver microwave power over a wide frequency band.

It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT.

The growing RF wave travels in opposite direction to the electron beam.

The frequency of wave can be changed by changing the voltage which controls the beam velocity.

Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current.

It features are:

1. Frequency range - 1 GHz to 1000 GHz.

2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed).

It is used as signal source in transmitters and instruments.


Abc said: (May 3, 2014)  
A traveling-wave tube (TWT) is a specialised vacuum tube that is used in electronics to amplify radio frequency (RF) signals to high power, usually as part of an electronic assembly known as a traveling-wave tube amplifier (TWTA).

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