4-4
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.
4-15. GRID MODULATION.
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
signal.
Conversely, making the grid more negative
decreases the
rf
output.
4-17. HELIX MODULATION.
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
MOD.
4-19. The
rf
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
01144-2