- 09 ELECTRONICS Mehran UET

```Learning Lesson
“There are only two forces in the world, the
sword and the spirit. In the long run the
sword will always be conquered by the spirit.”
-Napoleon Bonaparte
Velocity Modulation
 Velocity modulation is 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.
 The electron beam passes through a pair of closely spaced grids,
called BUNCHER GRIDS.
Buncher grids
Polarity of AC voltage
Electron beam via Buncher Grids
Buncher and Catcher Cavities
Drift Space
Drift Space
The energy gained by the accelerated electrons is balanced by the energy lost
by the decelerated electrons.
 A new and useful beam distribution will be formed if the velocity modulated
electrons are allowed to drift into an area that has no electrostatic field.
Two cavity klystron amplifier
 A klystron is a microwave vacuum tube using cavity resonators to
produce velocity modulation of the electron beam and to produce
amplification.
 Input cavity (buncher cavity) RF energy is coupled in, and the
electron beam is velocity modulated .
 Output cavity (catcher cavity) the RF energy is coupled through
the electron beam by placing the second cavity into the proper
position at an optimum distance.
 The RF interacting with the electron beam causes a kinetic energy
loss from the beam that result in gain.
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+
(Accelerator grid)
(AC voltage)
Two cavity klystron
Two-cavity klystron oscillator
The two-cavity amplifier klystron is readily turned into
an oscillator klystron by providing a feedback loop
between the input and output cavities.
Multicavity Klystron
 In all modern klystrons, the
number of cavities exceeds
two.
 A larger number of cavities
may be used to increase the
gain of the klystron, or to
increase the bandwidth.
Three-cavity klystron
Reflex Klystron Oscillator
 A reflex klystron consists of an electron gun, a cavity with a pair of
grids and a repeller plate.
 In this klystron, a single pair of grids does the functions of both the
buncher and the catcher grids.
 The feedback required to maintain oscillations within the cavity is
obtained by reversing the beam and sending it back through the cavity.
 Due to this, they deliver energy to the cavity, the result is the
oscillation at the cavity producing RF frequency.
+
+
-
Grid voltage variation - Electron bunching
The variation in grid voltage causes the electrons to enter the space between
the grid and the repeller at various velocities.
Traveling Wave Tube (TWT)
 Traveling-wave tube (TWT) is a high-gain, low-noise, wide-bandwidth
microwave amplifier.
 The unique feature of the TWT is a helix or coil that surrounds the
length of the tube and the electron beam passes through the centre or
axis of the helix.
 Surrounding the tube are either permanent magnets or electromagnets
that keep the electrons tightly focused into a narrow beam.
 Attenuators placed along the RF circuit prevent the reflected wave
from traveling back to the cathode.
VELOCITY MODULATION
The electromagnetic field produced by the helix causes the electrons to
be speeded up and slowed down, this produces velocity modulation of
the beam which produces density modulation.
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The Backward Wave Oscillator (BWO)
 The electron beam (from an electron gun) passes through a wire
helix and generates an electric field that induces voltage with the
helix wire. The resonating electric fields (in and out) produce
microwaves in the direction opposite to the electron beam.
 The frequency of the radiation is varied by controlling the beam
velocity and the helix potential.
Applications of Microwave tubes
 A klystron can be used either as a generator or as an amplifier of
power, at microwave frequencies.
 Klystron as power output tubes
1.
2.

Satellite communication ground station
The reflex klystrons are used in
1.
2.
 BACKWARD WAVE OSCILLATOR (BWO)
1.
Shorter & Thicker TWT
2.
Microwave CW oscillator
3.
1-1000 GHz
Applications : Travelling Wave Tubes(TWT)
Low noise RF amplifier in broad band
2. Due to long tube life (50,000 hours against ¼th
for other types), TWT is power output tube in
communication satellite.
3. Continuous wave high power TWT’s are used in
troposcatter links (due to larger power and
larger bandwidths).
4. Used in Air borne and ship borne pulsed high