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Grid Modulation; Helix Modulation - HP 492A Operating And Service Manual

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Section IV
Paragraphs 4-14 to 4-21
(3) Electron bunching caused by the electric field of
the cw signal (see detail).
(4) Amplification of the signal on the helix begins
as the field formed by the electron bunches interacts
with the electric field of the cw signal. The newly
formed electron bunch adds a small amount of voltage
to the cw signal on the helix. The slightly amplified
cw signal then produces a denser electron bunch which
in turn adds a still greater voltage to the cw signal,
and so on.
(5) Amplification increases as the greater velocity
of the electron beam pulls the electron bunches more
nearly in phase with the electric field of the cw signal.
The additive effect of the two fields exactly in phase
produces the greatest resultant amplification.
(6) Attenuators placed near the beginning of the helix
reduce all the waves traveling along the helix near to
This attenuator prevents regeneration and
possible oscillation due to undesired backward waves,
such as reflected waves from mismatched loads.
(7) The electron bunches travel through the attenu-
ator unaffected.
(8) The electron bunches emerging from attenuator
induce a new cw signal on the helix. The new cw sig-
nal is the same frequency as the original cw signal
(9) The field of the newly induced cw signal inter-
acts with the bunched electrons to begin the amplifi-
cation process over again.
(10) For a short distance the velocity of the electron
bunches is reduced slightly due to the large amount of
energy absorbed by the formation of the new cw signal
on the helix.
(11) Amplification increases as the greater velocity
of the electron beam pulls the electron bunches more
nearly in phase with the electric field of the cw signal.
(12) At the point of the desired amplification the
amplified cw signal is coupled out of the helix. Note
that the "amplified" cw signal is a new signal whose
energy is wholly supplied by the bunched electron
4-14. The traveling wave tube is completely enclosed
in the capsule shown in figure 4-4. The capsule sup-
ports and shields the tube and rigidly mounts the cap-
sule attenuator and the input-output couplers, cables
and connectors.
The capsule attenuator prevents
energy from being propagated down the capsule in a
coaxial mode using the helix as a center conductor
and the shield as the outer conductor.
The front-
panel INPUT connector connects through a coaxial
cable to the helically-wound directional coupler atthe
gun end of the helix; the OUTPUT connector connects
through a similar cable to an identical coupler at the
collector end of the helix. Impedance matching over
the extremely wide frequency range of the twt, is
obtained by cavity-coupling.
For an explanation of
Model 492A/494A
cavity-coupling refer to "The Use of Quasi-Static
Mode Approximations in the Design of Slow Wave
Structure Impedance Matches" by Wayne E. Raub,
dated August 1961.
Manual number 27-3. Reprints
available from Microwave Electronics Laboratory,
4061 Transport Street, Palo Alto, California.
4-16. The signal being amplified in the traveling wave
tube is amplitude-modulated by applying the modula-
ting signal between the cathode and first grid. Making
the potential on the grid more positive increases the
current passing through the center of the helix without
changing the velocity and results in greater density of
the electron bunches which in turn contribute more
energy to the rf wave being amplified on the helix,
and correspondingly increases the level of the output
Conversely, making the grid more negative
decreases the
4-18. The Helix Modulator, V5A, is a cathode follower
connected between the regulated voltage and the helix.
The front panel HELIX control varies the bias on V5A,
and hence the voltage applied to the helix. When the
twt amplifier is helix-modulated, the modulating sig-
nal is connected to the grid of the helix modulator
from the front panel BNC connector labeled HELIX
4-19. The
signal on the helix is phase-modulated
by superimposing the modulating signal on the normal
dc helix voltage. Changing the helix voltage changes
the velocity of the electrons in the beam through the
helix without changing beam density. A negative volt-
age slows the beam down and retards the phase of the
output signal; a positive voltage speeds up the beam and
advances the phase. Since the final signal taken from
the helix is the result of electron bunching in the beam,
altering the velocity of beam alters the relative position
of the bunches and results in a phase shift between the
input and output signals.
4-20. Since the amount of energy transferred from
the electron beam to the wave on the helix is in part
a function of the phase difference between the fields
of the helix and the electron bunches, altering the
electron velocity has some effect upon the energy
given to the signal on the helix resulting in some inci-
dental amplitude modulation. Special tubes are avail-
able which sacrifice gain to minimize this incidental
amplitude modulation, see paragraph 5-14.
4-21. A certain amount of stray and wiring capacity
exists between the helix of the twt and chassis which
must be charged and discharged as the helix is modu-
lated. When the helix modulating signal goes positive
this capacity can be charged very rapidly through the
low impedance of the helix modulator tube, V5A, and
the power supply. However, if the modulating signal
were a fast negative-going signal, V5A could be cut
off and the stray capacity would discharge through the
insulation (leakage) resistance which exists between
the helix and the chassis.
This is a relatively long



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