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Model 492A/494A
SECTION IV
PRINCIPLES OF OPERATION
Section IV
Paragraphs 4-1 to 4-13
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4-1. INTRODUCTION.
4-2. The twt amplifier contains very little signal cir-
cuitry external to the twt itself. The electrical cir-
cuits in the instrument provide the operating voltages
and the means for modulating the twt, as shown in the
block diagram, figure 4-1.
4-3. MAGNET POWER SUPPLY.
4-4. The 492A and 494A utilize a 400-gauss electro-
magnet (surrounding the traveling wave tube capsule)
to hold the emitted electrons in a very narrow beam.
The power supply for the magnet consists of a full-
wave selenium bridge rectifier connected directly
across the US-volt line and a capacitive-input filter,
and supplies approximately 0.7 ampere at an output
voltage of 120 volts dc with less than 1 volt rms ripple
when connected to the magnet. The magnet is covered
by a shield as a protection against stray magnetic
fields.
4-5. REGULATED POWER SUPPLY.
4-6. The operating voltages applied to the twt are
obtained from a voltage doubler followed by a voltage
regulator, VI, V2, and V4. The regulation is accom-
plished by varying the plate resistance of VI in ac-
cordance with the output voltage in the following man-
ner, see figure 4-1.
V4 is a constant voltage tube
which holds the voltage at the grid of V2 constant with
respect to the cathode of VI.
The cathode of V2 is
connected to a voltage divider between the cathode of
VI and the minus side of the supply.
If
the cathode
voltage of VI increases, the grid voltage of V2 will
also increase the same amount. However, the cath-
ode voltage of V2 will increase only by an amount
equal to the ratio of resistance below the potentiom-
eter arm to the total resistance times the total
voltage change.
Thus, a signal appears on the grid
of V2 which is proportional to the rise of voltage on
the cathode of VI. This signal on the grid of V2 is
amplified and inverted by V2 and applied to the grid
of V1, increasing the plate resistance of V1 and low-
ering the voltage at the cathode of VI which results in
a substantially constant output voltage.
If
the voltage
at the cathode of VI tends to decrease, the plate re-
sistance of VI decreases, holding the voltage at the
cathode of V1 substantially constant.
4-7. The chassis Helix control (R25) adjusts the level
of the regulated dc output to compensate for the varia-
tions in twt characteristics. The switch, 52, and the
associated voltage dividers permit the use of the same
power supply with two twt types and allows the opera-
tor to use the same instrument as a 492A (4-8 gc) or
as a 494A (7-12.4 gc) by changing the twt. Changing
the tube type is no more difficult than replacing the
twt with another tube of the same type (see para. 5-17).
00144-2
4-8. The bias voltage for the twt is supplied by V3
and controlled by the GRID BIAS control on the front
panel.
The grid of the twt is grounded through a
3900-ohm resistor, and the voltage on the cathode is
varied.
The circuit is arranged so that all the twt
electrode voltages except the control grid vary as the
cathode voltage is varied and therefore remain con-
stant with respect to the twt cathode.
If
the grid were
not grounded, but connected to a source of variable
negative voltage a blocking capacitor would be required
which would impair the response of the twt to low-
frequency modulating signals. A pin jack on the front
panel allows the bias voltage to be measured; an ex-
ternal voltmeter haVing 20,000 ohm/volt sensitivity or
higher should be used.
4-9. The anode voltage is controlled by a potentiom-
eter on the chassis of the instrument and is adjusted
to obtain normal cathode current for the twt amplifier;
see paragraph 5-26.
The front panel HELIX control
adjusts the helix voltage of the twt to obtain either
maximum gain and power at a particular frequency or
optimum broadband response.
4-10. TRAVELING WAVE TUBE.
4-11. The basic traveling wave tube consists of an
electron gun which projects a focused electron beam
through a helically-wound coil to a collector electrode,
shown in figure 4-3. The focused electrons are held
in a pin-like beam through the center of the helix by
a powerful magnet around the full length of the capsule.
4-12. A cw signal coupled into the input end of the
helix travels around the turns of the helix and thus has
its linear velocity reduced by the amount equal to the
ratio of the length of wire in the helix to the axial
length of the helix. The electron beam velocity, de-
termined by the potential difference between thecath-
ode and the helix is adjusted so that the electron beam
travels a little faster than the cw signal. The electric
field of the cw signal on the helix interacts with the
electric field created by the electron beam and in-
creases the amplitude of the signal on the helix, thus
producing the desired amplification.
4-13. Figure 4-2 is a diagram showing the principal
elements of a typical traveling wave tube in the upper
portion and the important steps in the amplification
process in the lower portion. The steps should be fol-
lowed by referring to the numbered captions below.
(1) An electron beam is directed through the center
of the helix.
(2) A cw signal is coupled into the helix. Arrows in
the detail show the direction and magnitude of force
exerted on the electron beam by the cw signal.
4-1
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