frequency (and usually their peak power) is very high.
A magnetron
oscillator is actually a tube within a magnet.
Even though magnetrons
differ physically from other types of oscillators, they have the same basic
requirements of a tuned circuit, tuned circuit excitation, and regeneration.
7.
All oscillators must have a tuned circuit.
All oscillators have a tuned or resonant circuit that determines the
oscillating frequency.
In most applications at frequencies below 500
megahertz, the tuned circuit is a coil and capacitor combination.
At
frequencies between 500 megahertz and 1000 megahertz, the tuned circuit is
usually a section of transmission line. At frequencies above 1000 megahertz
the tuned circuit used most often is a resonant cavity.
Most magnetrons
oscillate at frequencies above 1000 megahertz, and therefore they use
resonant cavities as tuned circuits.
8.
All oscillator tuned circuits must be excited.
You know that an oscillator tuned circuit must be excited. That is,
it must have some energy applied to it to start oscillations. For example,
the coil and capacitor tuned circuit used with an ordinary vacuum tube
oscillator is excited by a surge of current.
A tuned circuit made of a
section of transmission line is usually excited the same way. The resonant
cavities of a magnetron oscillator, however, are excited by the fields built
up by current flowing in their walls. You have already seen in a previous
lesson how a resonant cavity is shock-excited by an electromagnetic field.
Well, magnetron cavities are excited in a similar manner.
9.
All oscillators need regeneration.
a. One more requirement of an oscillator is regeneration. This means
simply that we must reinforce the energy generated, or it will die out.
Ordinary vacuum tube oscillators use positive feedback in which some of the
output energy is fed back in phase with the input energy. The energy fed
back adds to the input and keeps the oscillator working.
This happens
because the energy feedback is greater than the energy being lost.
b. A magnetron also needs regeneration to continue oscillating after it
has been excited. Energy generated in the resonant cavities is reinforced
by energy given off by electrons emitted from the magnetron cathode.
The
electrons add their energy in phase with the oscillating energy.
Because
the energy added is greater than the energy lost, oscillations don't die
out.
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