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Velocity-modulated Tubes
The microwave tube uses transit time in the conversion of dc power to radio-frequency power. The interchange of power is accomplished by using the principle of electron velocity modulation and low-loss resonant cavities in the microwave tube.
Velocity modulation is then defined as that variation in the velocity of a beam of electrons caused by the alternate speeding up and slowing down of the electrons in the beam. This variation is usually caused by a voltage signal applied between the grids through which the beam must pass. The direction of the electron beam and the static electrical field goes to each other parallelly (linearly) into linear beam tubes. Against this the fields influencing the electron beam stand vertically by the electron beam at the cross field tubes.
The following table compares with characteristic quantities of the velocity-modulated tubes used in radar technology. Although the planar tube isn't a velocity-modulated tube, it was included into this table for comparison purposes.
| Klystron | Traveling Wave Tube | Magnetron | Carcinotron | planar tube | |
| frequency | up to 35 GHz | up to 95 GHz | up to 95 GHz | up to 5 GHz | up to 1.5 GHz |
| bandwidth | 2 - 4 % | 10 - 20 % | any megahertzes | 2 GHz | 30 - 50% |
| power output | up to 50 MW | up to 1 MW | up to 10 MW | 1 W | up to 1 MW |
| amplification | up to 60 dB | up to 50 dB | - | - | up to 20 dB |
| function as | small-band power amplifier | wide-band, lownoise voltage amplifier | high power oscillator at one frequency | frequency-controlled oscillator (VFO) | amplifier, oscillator |
Table 1: Comparing of velocity-modulated tubes
