HP 492A Operating And Service Manual
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Model 492A/494A
SECTION
IV
PRINCIPLES
OF
OPERATION
Section
IV
Paragraphs
4-1
to
4-13
)
)
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