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492A

HP 492A, 494A Operating And Service Manual

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HEWLETT-PACKARD
COMPANY
4.2A/4.4A
TRAVELING-VVAVE AMPLIFIER
)
)
)
00144·2
OPERATING
AND
SERVICE
MANUAL
MODEL
492A
AND
MODEL
494A
SERIALS
PREFIXED:
010-
TRAVELING-WAVE
AMPLIFIERS
Copy,;gh'
HEWLETT.'ACICA.D
COM'
ANY
1962
lS01
'AGE
MILL
ROAD,
'ALO
ALTO,
CALlFO.NIA,
U.S.A.
Printed:
JAN
1962
Model
492Aj494A
Table
of Contents
ListsofIllustrations
and
Tables
TABLE
OF
CONTENTS
Section
Page
Section
Page
I
GENERAL INFORMATION
1-1
IV
PRINCIPLES OF OPERATIO
(cont'd)
1-1.
Description
1-1
4-10.
Traveling
Wave
Tube
4-1
1-6.
Instrument
Identification
1-2
4-15.
Grid
Modulation.
4-4
1-8.
Traveling
Wave
Tube
Warranty
1-2
4-17.
Helix Modulation
·
.
4-4
4-22.
Constant-Amplitude,
Linear
II
INSTALLATION
2-1
Sawtooth
Generator
4-5
2-1.
Mechanical
Inspection
2-1
2-3.
Power
Requirements.
2-1
V
MAINTENANCE.
5-1
2-5.
Power
Cable.
2-1
5-1.
Introduction.
·
5-1
2-8.
Installation
2-1
5-3.
Cleaning
the
Air
Filter·
5-1
2-12.
Repackaging
for
Shipment
2-1
5-5.
Test
Equipment .
5-1
5-7.
Repair.
5-3
III
OPERATING INSTRUCTIONS
3-1
5-8.
Cabinet
Removal
5-3
3-1.
Introduction
.
3-1
5-10.
Tube
Replacement
.
.
5-3
3-3.
Preliminary
Operating
Procedure
3-1
5-12.
Traveling
Wave
Tube
Capsule
3-6.
Helix
Control
3-1
Replacement
5-3
3-8.
Saturation
Power
Output
3-1
5-17.
Changing
the
Frequency
Range
3-12.
Bandwidth
Considerations
.
3-1
of
the
492Aor494A
5-5
3-14.
Constant
GainorConstant
Output
5-21.
Adjustments
5-5
Amplification
3-3
5-22.
Excessive
Helix
Current
5-5
3-18.
Buffer
Amplifications
3-3
5-24.
Chassis
Helix
Control
5-5
3-20.
Amplitude
Modulation
3-3
5-26.
Anode Voltage
Control
.
5-5
3-24.
Pulse
Modulation.
3-6
5-28.
Regulated
Power
Supply
5-5
3-27.
Limited
Phase
Modulation.
3-6
5-30.
Troubleshooting.
·
5-6
3-29.
Unlimited
Phase
Modulation and
5-32.
Performance
Checks
5-7
Frequency
Shifting .
3-6
5-33.
Gain
Check.
5-7
3-34.
Homodyne
Detection
3-7
5-36.
Output
Power
Check
5-7
3-37.
Frequency
Modulation
3-9
5-39.
Noise
Figure
Check
5-7
5-42.
Hum and
Spurious
Modulation Check·
5-8
)
IV
PRINCIPLESOFOPERATION.
4-1
4-1.
Introduction
.
4-1
VI
REPLACEABLE
PARTS
.
6-1
4-3.
Magnet
Power
Supply.
4-1
6-1.
Introduction
6-1
4-5.
Regulated
Power
Supply
4-1
6-4.
Ordering
Information
6-1
LIST
OF
ILLUSTRATIONS
AND
TABLES
Number
Illustration
Title
Page
4-3.
CutawayView
ofaTWT
Capsule
andMagnet
Showing
the
Important
Elements·
. . .
4-3
4-4.
Cutaway View of an
Encapsulated
TWT·
.
4-3
4-5.
Simplified
Circuit
ofa
Constant
Amplitude,
Variable
Slope Sawtooth
Generator··
4-5
5-1.
Top
ViewofModels 492A and 494A
.,
5-2
5-2.
Bottom ViewLooking
Towards
the
Front
PanelofModels
492A and 494A .
..
5-4
5-3.
Rear
View of Models 492A
and
494A"
5-4
5-4.
Test
Setup
for
Gain, Output
Power,
and
Noise
Figure
Performance
Checks
5-7
5-5.
Test
Setup
for
Hum and
Spurious
Modulation
Performance
Check . .
5-8
5-6.
Models 492A and 494A
Schematic
Diagram'
5-9
Number
Illustration
Title
Page
1-1.
Model 492A
Traveling
Wave
Amplifier
1-1
1-2.
Traveling
Wave
Tube
Warranty
1-2
3-1.
Operating
Controls
.
3-0
3-2.
Typical
Gain
and
Power
Output
Characteristics
.
3-2
3-3.
Block
Diagram
of a
Circuit
used
to
Maintain
Constant-Level
Output
Power
fromaTWT
Amplifier
3-3
3-4.
Block
Diagramofan
Automatic
Gain
ControltoMaintain
Constant
Ampli-
fication
from
a TWT
Amplifier
3-3
3-5.
Typical
PlotofOutput VoltagevsGrid
Voltageofa Model 492A
3-4
3-6.
Typical
PlotofOutput VoltagevsGrid
Voltageofa Model 494A.
3-5
3-7.
RF
Phase
Shift
Produced
by Helix Mod
3-7
3-8.
Offset
Frequency
Produced
by Sawtooth
Modulationofthe
Helix .
3-7
3-9.
Block
Diagram
of a
Linear
(Homodyne)
)
Detection
System.
3-8
3-10.
Block
Diagram
of a
CircuittoProduce
an FM Signal with a
TWT
Amplifier
3-8
4-1.
Block
Diagram,
Models 492A and 494A
4-0
4-2.
TWT
and
HowitWorks
.
4-2
00144-2
1-1. 3-1.
5-1. 5-2.
5-3. 6-1. 6-2.
Table
Title
Specifications.
. . . . .
Maximum
Operating
Currents
for
Models
492A and 494A . . .
Recommended
Test
Equipment
Tube
Replacement
List
Troubleshooting
Chart
. . •
Reference
Designation Index
Replaceable
Parts·
. . . .
1-0 3-1
5-1 5-3
5-6 6-2 6-5
iii
Section I Table
1-1
Model 492A/494A
Table
1-1,
Specifications
~ Model 492A
<Fj!
Model 494A
Frequency
Range:
4
gc
to 8
gc
7gcto
12,4
gc
Maximum
Output
Power:
20 mw
minimum
into
50 ohm
load
20 mw
minimum
into
50 ohm load
Modulated
Pulse
Delay:
Approximately
20 ns
Approximately15ns
Helix Modulating Voltage:
Approximately40volts
peak-to-
peak.
Provides
3600phase
shift.
Input
impedance
lOOK
Approximately50volts
peak-to-
peak.
Provides
3600phase
shift.
Input
impedance
lOOK
Hum and
Spurious
Modulation:
At
least
45 db below
signal
level
At
least
45 db below
signal
level
Weight:
Power
Supply:
66
lb
net,
85 Ib shipping
US
volts
±10%, 50to60
cps,
approximately
200
watts
63 Ib net, 84 Ib
shipping
U5
volts
±10%, 50to60
cps,
approximately
225
watts
Accessories
Furnished:
AC-16Q
cable
assembly
AC-l6Q
cable
assembly
For
Both Models
Small
Signal Gain:
30 db
minimum
Meter
Monitors:
Cathode
current,
anode
current,
helix
current,
collector
current.
Input Impedance: 50
ohms,
swr
less
than 2
Output
Internal
Impedance: 50
ohms,
swr
less
than 3
Dimensions:
Cabinet
Mount:
7-3/8
in.wide,
U-l/2
in.high, 20
in.deep.
TypeN Less
than 30 db
t
:0:
IU]
REAR
,~,,".
:0:
'.1&
1.----19---.,
492AR/494AR
Rack
Mount:
Noise
Figure:
Connectors, RF
Input and Output:
Pulse
Rise
and Decay
Time:
Amplitude Modulating Voltage:
Approximately15ns Approximately50volts
peak
positive
pulse
will
produce
a 40 db
change
inrfpower
output.
Sensitivity
approximately1db/volt
1-0
00144-2
Model 492A/494A
SECTION
I
GENERAL
INFORMATION
Section I
Paragraphs
1-1to1-4
1-1.
DESCRIPTION.
1-2.
Thet$Models 492A and 494A
Traveling
Wave
Amplifiers
are
broadband,
linear
amplifiers
provid-
ing
adjustable
amplification
up toatleast
30 db,
be-
tween 4 and 12.4 gc, and
haveamaximum
power
out-
put ofatleast20milliwattstoan
external
load
of 50
ohms.
The
frequency
rangeofthet$Model 492A
is
4
to
8 gc; the
frequency
rangeofthe
Model 494A
is
7
to
12.4 gc.
These
traveling
wave tube (twt)
ampli-
fiers
are
designed
to be
used
alsoasbuffer
amplifiers
or
modulators
for
any
signal
within
their
frequency
range.
As
buffers,
their
input
impedances
remain
constant
with any
reasonable
load
changeatthe output
terminal;
the
attenuation between input andoutput
sig-
nalsisat
least
60 db,
minus
the
gain of
the
amplifier.
As
modulators,
they
can
be
usedtoamplitude,
fre-
quency,
pulse,
or
phase
modulate
the
signal
being
amplified
with no
interaction
on
the
signal
source.
The
gain and
power
output of
the
amplifiers
are
con-
tinuously
adjustablebythe
front
panelGRID BIAS
con-
trol.
Hum and
spurious
modulation
generated
within
the
amplifiers
areatleast
45 db below
the
output
sig-
nal
level
and
the
noise
figure
is
less
than 30
db.
1-3.
Thet$Models 492A and 494A
Traveling
Wave
Amplifiers,
amplify
any type ofrfsignal:
cw, swept,
sine-modulated,
pulsed,
multiple
signalsondifferent
frequencies,
etc.
A twt
amplifier
usedasa
modulator,
in conjunction with a
signal
generator,
canbeused
to
amplitude
modulate
anrfcarriertoapproximately
30%
with
less
than 2.5%
harmonic
distortion
and upto50%
with
less
than
5%
distortion.
Amplitude modulation
sensitivityisapproximately1db/volt.
Pulse
modula-
tion
is
excellent;
the
rise
timeisless
than15ns.
Phase
modulation up
to
3600with
less
than 1 db
amplitude
modulation
is
also
possible.
Wide-band
frequency
modulationissimulated
by a
step-wise
phase
modulation
describedinsection
Ill.
1-4.
The
front
panel
meterisprovided
for
checking
and
adjusting
electrode
currentsinthe
traveling-wave
tube.
The
meter
helpstoobtain
desired
operating
characteristics
during
normal
operationofthe
ampli-
fier
and
also
assists
with
preventive
maintenance
and
troubleshooting.
An
anode
voltage
adjustment
on the
instrument
chassis
prOVided
to
adjust
the
cathode
current
of the twt backtonormal
duetotube ageing.
)
00144-2
Figure
1-1.
Model 492A
Traveling
Wave
Amplifier
1-1
Section
I
Paragraphs
1-5to1-9
1-5.
The~Model 492A and 494A
are
similarinthat
one
model
maybechangedtothe
other,byreplacing
the
twt,ascoveredinparagraph
5-17.
1-6.
INSTRUMENT
IDENTIFICATION.
1-7.
Hewlett-Packard
usesatwo-section
eight-digit
serial
number
(000-00000).
If
the
first
three
digits
of
the
serial
numberonyour
instrument
do not
agree
WA••ANTY
CLAIM
AND
ADJUITMINT
'IOCIDUU
Model 492A/494A
with
those
on
the
title
pageofthis
manual,
change
sheets
supplied
with
the
manual
will
define
differences
between
your
instrument
and
the
Model 492Aor494A
describedinthis
manual.
1-8.
TRAVELING
WAVE
TUBE
WARRANTY.
1-9.
The
Traveling
Wave
Tube
Warranty
is
illus-
tratedinfigure
1-2.Asheet
for
your
useisincluded
in
the
appendixofthis
manual.
MICROWAVE
TUBE
WARRANTY CLAIM
INFORMATION FORM
IMPORT
ANT:
Please
ans
....erall
questions
fully--InsuHlclent Information may
delay
processing
of
your
claim.
-
for
microwave
tubes suppliedby[he
HEWLETT·
PACKARD
COMPANY
for
use
In*Instruments
Microwave
tubes
suppliedbythe
Hewlett-Packard
Company.
either88originalorreplacement.
for
use
in8instruments
are
actually
warrantedbythe
tube
manufaclUrer
and
not
by
9.
However, S
will
process
warranty
claims
for
you, and will
promptly
passonall
allowances
grantedbythe
tube
manufacturer.
In
the
event
that
your
tube Is (ound
toberepairable,
the tube
manufacturer
reserves
the
right
to
repair
and
return
the tube in lieu of
Issuing
pro--Tata
credil.
For
your
convenience,
warrantyclalms(orall
microwave
tubes
supplied
by the
Hewlett-Packard
Company
maybemade on
this
sIngle
form;
merely
fJII
out the
lnfonnallon
on the
reverse
side
and
rerurn
this
fonn.
along
....
lth
thedefectlve
rube.toyour8engineering
representallve.
or
tO~.
Pleasebesure
each
space
on the formisfilled
In··lackofcomplete
infonnation
may
delay
processingofyourcTiim. Each
tube
manufacturer
has
his
own
warranty
policy.
Copies
of Individual Conditions of
War-
ranty
are
available
from
your*engineering
representativeorfrom Ihe He
....
len·Packard
Company.
SHIPPING
INSTIUCTIONS
FROM:
(rube
Owner)
Company
Address
Tube
type
Tube
serial
No.
_
Tube
mfr.
Use
in~Model _
Instrument
serial
no. _
Date
FOR FURTHER INFORMATION CONTACT:
Name
_
Title
_
Company _
Address
_
Tube
purchased
from
OnP.O.
number
The
following
instructions
are
includedtoaid youInpreventing
damageintransit.
Package
your
tube
carefully••no
allowance
canbemade on
broken
tubes.
TubeisOriginal ( )orReplacement
(
1.
Carefully
wrap
tube in
1/4
inch thick
"klmpack".
Cotton batting.orother
sofl
padding
material.
2. Wrap the
above
in heavy
kraft
paper.
3.
Pack
in a
rigid
container
....
hlch Isatleast4inches
larger
than the lube In
each
dimension.
4.
Surround
the
tube
withatleast2inches
of shock
absorbing
material.Becenaln
that the
packing Is tight
all
around
the tube.
Date
tube
received
_
Date
first
tested
Date
placedinservice
_
Dateoffailure
_
Hours
use
per
day
(average)
_
NumberofdaysInservice
_
Total
hours
filament
operation
5.
Tubes
returned
from
outside
the continental
UnltedSrates
shouldbepackedIna wooden box.
6.
Mark
container
FRAGILE and
ship
prepaid
via
Air
FreightorRailway
Expreu.Donot
ship
via
Parcel
POStorAJr
Parcel
POSt
since
experience
has
shown that
fragile
Items
are
more
apttobe
damaged
when
shippedbythese
means.
Tubes
returnedtothe
Hewlen·Packard
Company should be
addressed
to:
SYMPTOMS:
(Please
describe
conditions
priortoandattimeoffailure.
along with
description
of
lube's
defect,
If known) _
CUSTOMr.
snvlcr
H.~".,.d.rd
COfftpel'ly
]95
,.~
Mil
R_d
1'.
Alto. C.lifomi.,
U.S.A.
01
II.W....
,..I.r.,.J
H_lett·,.d.,dS.A. kvedIiVifillli",dNo.l
G.I'l......S
...
itter!Mtd
Were
there
other
circuit
component
failuresattimeoffailure?
Which ones?
Signature
_
Title
9/12/61
1-2
Figure
1-2.
Traveling
Wave
Tube
Warranty
00144-2
)
Model 492A/494A
SECTION
II
INSTALLATION
Section
II
Paragraphs
2-1to2-13
/
2-1.
MECHANICAL
INSPECTION.
2-2.
Unpack
the
instrument
upon
receipt
and
inspect
it
for
signs
of
phy
ical
damage
such
as
scratched
panel
surfaces,
broken
knobs,
etc.
If
thereisany
apparent
damage,
fileaclaim
with
the
carrier
and
refertothe
warranty
pageinthis
manual.
2-3.
POWER
REQUIREMENTS.
2-4.
The
Model 492A and Model 494A
requireapower
source
of
115
volts
± 10%,
single
phase,50to 60
cps,
which
can
deliver
approximately
225
watts.
2-5.
POWER
CABLE.
2-6.
For
the
protection
of
operating
personnel,
the
ational
Electrical
Manufacturers'
Association
(NEMA)
recommends
that
the
instrument
panel
and
cabinet
be
grounded.
This
instrument
is
equipped
with a
three-prong
conductor
power
cable
which, when
plugged
into
an
appropriate
receptacle,
grounds
the
instrument.
The
offset
pin on
the
power
cable
three-
prong
connectoristhe
ground
pin.
2-7.
To
preserve
the
protection
fearure
when
opera-
ting
the
instrument
fromatwo-contact
outlet,
use
a
three-prong
to
two-prong
adapter
and
connect
the
green
pigtailonthe
adaptertoground.
2-8.INST
ALLATION.
2-9.
The
only
special
precaution
necessary
for
in-
stalling
the
twt
amplifiers
is
that
they
should
not be
operated
closetovery
large
magnetic
fields,
such
as
60-cycle
fields,
unless
externally
shielded.
While
the
twt
amplifiers
are
shielded
within
the
cabinet,
complete
protection
against
large
low-frequency
fields
would
require
more
shielding
thanispracticaltoin-
cludeinthe
design.
2-10.
To
operate,
connect
the
instrument
to a
115-
voltacpower
source,
check
and
adjust
rube
operating
00144-2
currents
as
described
in
preliminary
operating
pro-
cedure,
section
1Il, and
connecttothe
external
equip-
ment
with
coaxial
cables
terminatedinstandard
UG-
21D/U,
type
connectors.
2-11.
In
section
V, beginning with
paragraph
5-32,
isalistofperformance
checks
for
this
instrument.
These
procedures
make
a good
testaspartofincom-
ing
quality-control
inspection
following
initial
rurn-on.
2-12.
REPACKAGING
FOR
SHIPMENT.
2-13.
The
following
listisa
general
guide
for
re-
packaging
an
instrument
for
shipment.
If you
have
any
questions,
contact
your
authorized
Hewlett-
Packard
sales
representative.
a.
If
possible,
use
the
original
container
designed
for
the
instrument,
b.
Wrap
the
instrument
in
heavy
paperorplastic
before
placingitin
the
shipping
container.
c.
Use
plentyofpacking
material
around
all
sides
of
the
instrument
and
protect
the
panel
with
card-
board
strips.
d.
Use
heavy
cardboard
carton
or
wooden box to
house
the
instrument
and
use
heavy
tape
or
metal
bandstoseal
the
container.
e.
Mark
the
packing
box with
"Fragile",
"Delicate
Instrument",
etc.
Note
If
the
instrumentistobeshippedtoHewlett-
Packard
Company
for
service
or
repair,
at-
tachtothe
instrumentatag
identifying
the
owner
and
indicating
the
service
or
repair
to
be
accomplished.
In
any
correspondence
be
sure
to
identify
the
instrument
by
model
number,
serial
prefix,
and
serial
number.
2-1
Section
III
Figure
1-3
POWER
1.
Turns
on
all
circuits
of
the
amplifier
(allow
IS
minutes
warmup).
2.
Meter
selector
switchisused
for
selection
of
circuittobe
checkedonthe
front-panel
meter.
3.
Pin
jackisfor
measuring
grid
bias
(measures
E
k
;
grid
grounded).
4. GRID BIAS
controlisusedtoadjust
fordesired
tube
current.
Normal
cathode
currentisindi-
cated
on
the
meter
plate.
Do not
exceed
maxi-
mum
operating
currents,
refer
to
table
3-1.
Model 492A/494A
~-~--;3
MAG
C1RJD
Il100
8
7
L
S. HELIX
controlisusedtoadjust
for
best
broad-
band
response,
position
5,
or
for
maximum
gainatanyone
frequency.
6.
INPUTrfconnectoriswhere
therfsignaltobe
amplifiediscoupled
into
the
instrument.
7.
OUTPUTrfconnectoriswhere
the
amplified
rf
signaliscoupled
from
the
instrument.
8. GRID MOD.
connector
is
the
input
for
ampli-
tude
modulation
signals.
9. HELIX MOD.
connectoristhe
input
for
phase
and
frequency
modulation
signals.
3-0
Figure
3-1.
Operating
Controls
00144-2
)
Model 492A/494A
SECTION
III
OPERATING
INSTRUCTIONS
Section
III
Paragraphs
3-1to3-13
3-1.
INTRODUCTION.
3-2.
This
section
contains
operating
instructions
for
theModels
492A and 494A
Traveling
WaveTube
Ampli-
fiers.
Figure
3-1
gives
basic
operating
instructions.
The
remainder
of
this
section
supplements
these
in-
structions.
3-3.
PRELIMINARY
OPERATING
PROCEDURE.
3-4.
The
front
panel
controls,
indicator,
and
con-
nectors
for
the
492A and 494A
are
showninfigure
3-1.
This
figure
also
shows
the
uses
for
the
controls,
indi-
cator,
and
connectors.
Whenever
the
twtisturned
on,
use
the
front
panel
metertomeasure
the
current
to
each
electrode
in
the
traveling-wave
tube with
the
GRID BIAS
control
set
for
zero
bias.
The
safe
maxi-
mum
current
for
each
electrodeisshown in
table
3-1.
ormal
cathode
current
for
your
instrumentisindi-
cated
on
the
plate
attached
to
the
meter
face.
Cur-
rents
are
usuallyalittle
high when
the
instrument
is
first
turned
on, but
decreasetonormal
during
warm-
up. Allow IS
minutes
warmup
before
making
final
reading.
The
GRID BIAS
control
maybeusedtore-
duce
the
tube
currents
during
warmup.
Table
3-1.
Maximum
Operating
Currents
for
Models
492A and 494A.
Adjusting
the
HELIX
voltage
control
for
maximum
gainatthe
upper
frequency
limit
usually
produces
the
flattest
frequency
response
over
the
band.
Maximiz-
ing
the
gainatfrequencies
below
the
upper
frequency
limit
usually
resultsinadditional
gain
and
power
out-
put
over
that
obtained
when
adjusted
for
flattest
broad-
band
operation.
The
final
settingofthe
HELIX
con-
trol
is
independentofthe
typeofsignal
amplified.
3-8.
SATURATION
POWER
OUTPUT.
3-9.
Saturation
power
outputisthe
maximum
output
power
obtainable
with a
given
collector
current
and
optimized
helix
voltage.
As
the
input
signal
is
increased
from
the
noise
level,
the
output-vs-input
characteristic
is
linear
until
saturationisapproached
and
the
gain
beginstodecrease.
Eventually
the
output
reachesapeak
whichisthe
saturation
power
output
and any
further
increase
in
the
input
causes
the
outputtodecrease.Ifthe
collector
currentisreduced,
the
power
outputatwhich
saturation
occurs
will be
decreased.
The
primary
effects
which
lead
to
saturation
are:
1)
the
forces
between
the
electrons
in
the
beam
begin
to
limit
the
electron
density
in
the
bunches2)the
energy
transfer
from
the
electron
beamtothe
helix
causes
the
beam
velocitytodecrease
and
gradually
lose
synchronism
with
the
wave on
the
helix.
These
effects
become
more
pronounced
as
saturation
is
approached,
and
cause
the
gainofthe
twttodecrease.
3-6.
HELIX
CONTROL.
3-7.
The
HELIX
voltage
control
on
the
front
panel
maximizes
the
gain
and
power
outputofthe
twtata
selected
frequency
or
optimizes
the
gain
and
power
output
over
the
entire
band;
see
figures
3-2Aand
3-2B.
3-5.
If,
following
tube
replacement,
or
for
some
other
reason,
the
cathode
current
can
be
increased
with
the
GRID BIAS
controltoslightly
above
the
safe
maximum
current
(see
table
3-1),
readjust
the
anode
voltage
control,
paragraph
5-26.
The
anode
voltage
control
is
settolimit
the
twttoits
normal
cathode
current
when
the
bias
voltageonthe
twtiszero.
If
the
cathode
current
is
limitedtoits
normal
value,
but
the
currenttoanother
electrodeisexcessive,
the
amplifier
requires
service
or
adjustment,
see
para-
graphs
5-21or5-30.
)
CAUTION:
DO
NOT EXCEED:
Cathode
current
Helix
current
Collector
current.
Anode
current
3
ma
0.5
ma
3
ma
SO
/la
3-10.
Operating
the
492A
or
494A
near
saturation
power
output will
produce
second
harmonic
content
in
the
output.
To
obtain
maximum
power
output
from
the
twt without
excessive
second
harmonic
contentinthe
output,
increase
the
input
signal
level
until
satura-
tionisreached,
then
reduce
the
input until
the
out-
put
decreases
approximately
6 db (an
input
reduction
of
10
to
15 db).Ifsecond
harmonic
contentisun-
important,
operation
at
saturation
is
usually
very
satisfactory. 3-11.
One
advantageofoperating
the
twt
near
satura-
tionisthe
constant
output
characteristic
exhibited
by
a twt
at
saturation.
At
saturation
the
gain
of a twt
varies
inversely
with
the
input
level
and
input
varia-
tions
of
10 to 15 db
cause
the
outputtovary
only
4
to
6 db.Ifa
more
nearly
constant
outputisdesired,
see
paragraph
3-14.
3-12.
BANDWIDTH
CONSIDERATIONS.
3-13.
The
graphsinfigures
3-2A
and B show
typical
gainvsfrequency
and
saturation
power
output
curves
for
the
492A and
the
494A
amplifiers
using
two
differ-
ent
conditions
of
helix
voltage.
One
setofcurves
shows
the
gain
and
saturation
power
output when
the
helix
voltageisoptimizedateach
frequency.
The
other
setofcurves
shows
the
gain
and
saturation
power
out-
put when
the
HELIX
controlissettothe
broadband
00144-2
3-1
Section III Figure
3-2
Model 492A/494A
GAIN:
GAIN AT
SATURATION
POWER
OUTPUT.
VHOPTIMIZED
AT EACH
FREQUENCY.
~
SMALL
SIGNAL
GAIN: VHOPTIMIZED
AT
EACH
FREQUENCY.
SIGNAL
GAIN;
VHOPTIMIZED
FOR
BROADBAND
OPERATION.
SATURATION
POWER
OUTPUT:
VHOPTIMIZED
AT
EACH
FREQUENCY
492A
-rr
a.
15
+-----+-----+-----+--------""'1-----+-----+-------1
50
30
A
V
H
:
HELIX
VOLTAGE
E
o
= 0
VOLTS
4
5
6 7
8
G-L-248
FREQUENCY
KMC
49'4
A
SMALL
SIGNAL
GAIN:
VHOPTIMIZED
AT
EACH
FREQUENCY.
VHOPTIMIZED
FOR
BROADBAND
OPERATION.
SATURATION
POWER
OUTPUT:
VHOPTIMIZED
AT
EACH
FREQUENCY,
GAIN:
GAIN
AT
SATURATION
POWER
OUTPUT
•
V
H
OPTIMIZED
AT
EACH
FREQUENCY.
rr
15
-1-----+-----+-----+-----+----""""""
......
-..---+-------1
25
45
.c
."
I
35
z
«
(9
B
V
H
:
HELIX
VOLTAGE
E
g
:
0
VOLTS
7 8
9 10
II
12
G-L-248
FREQUENCY
KMC
Figure
3-2.
Typical
Gain and
Power
Output
Characteristics
3-2
00144-2
Model 492A/494A
Section
III
Paragraphs
3-14to3-22
is
developed
whichisthen
appliedtothe
twt
grid
to
hold
its
amplification
constant.
)
pOSltlOn.
To
obtain
the
most
nearly
constant
amplifier
gain
over
the
full
frequency
range
set
the
HELIX
con-
trol
to 5,
the
setting
which
yields
the
optimum
broad-
band
helix
voltage.
Since
noise
powerisdirectly
proportional
to bandwidth,
it
may
be
desirable
to
limit
the
bandwidth and
therefore
noisebymaximizing
the
gain
ataparticular
frequency
with
the
HELIX
control
and by
installing
suitable
filtersatthe output.
3-14.
CONSTANT
GAIN
OR
CONSTANT OUTPUT
AMPLIFICATION.
SIGNAL
GENERATOR
TRAVEL
ING
-WAVE
TUBE
ANPLIflER
TO
GRID
MOD
INPUT
) (
DIRECTIONAL
COUPLER
Rf
OUTPUT
":>--------o------=<o-----+--
OUT
LO-l-66
00
L=J-=-~/~
__
,"""",,---_--,
BALANCE
CONTROL
(SETTOOBTAIN0VOLTSTOTWT
GRID
MOD.
CONNECTOR,ATDESIRED
AMPLIfiCATION)
3-19.
The
492A and 494A
serve
as
very
effective
buffers
to
isolateamicrowave
signal
source
from
a
load.
Mismatches,
changesinexternal
circuitry,
or
the
introductionofmodulation
do not
affect
the
con-
stant
50-ohm
input
impedanceofthe
twt and
thus
will
not
affectasignal
source
connectedtothe
input.
The
attenuation
between
the
output and
input
terminals
is
60 db
duetoattenuators
placed
along
the
helix.
How-
ever,
when
the
output
signalisreflected
fromamis-
matched
load
backtothe
input,
the
effective
signal
isolationisthe
60 db
minus
the
gainofthe
amplifier.
For
example:
withanamplifier
gain
of 25 db,anopen
or
short
circuitonthe
twt
output
can
result
in a
maxi-
mum
reflected
signal
35 db below
the
input
level
(approximately
1/56ofthe
input
signal),
which
corre-
spondstoa
swrofless
than 1.04.
Note
The
GRID MOD.
connectorisdirect-coupled
to
the
gridofthe
twt
amplifier.Ifadcpoten-
tial
accompaniesamodulating
voltage
applied
to
this
connector,
the
grid-bias
voltage
will
be
altered.
The
GRID BIAS
control
may
be
usedtocompensate
for
the
changeingrid
bias
voltage
due
to a
dc
componentatthe
input.
Figure
3-4.
Block
Diagram
of an
Automatic
Gain
ControltoMaintain
Constant
Amplifica-
tion
fromaTWT
Amplifier
3-18.
BUFFER
AMPLIFICATIONS.
3-20.
AMPLITUDE
MODULATION.
3-21.
To
amplitude
modulate
anrfsignal
applied
to
the
twt
amplifier
with a
minimumofenvelope
distor-
tion in
the
output
signal,
carefully
establish
the
opti-
mum
rf
drive,
grid
bias,
and
modulating
signal
amplitude
for
a given
setup.
3-22.
For
minimum
distortion,
the
twt
grid
voltage
must
not
leave
the
linear
regionofthe
grid
voltage
vs
rf
output
characteristic
(see
figures
3-5
and
3-6).
Also,
therfdrive
mustbeadjustedsothat
the
modu-
lation
peaks
areatleast
2 db below
saturation
power
DIRECTIONAL
COUPLER
LO-L-'2
00
TRAVELING-WAVE
TUBE
AMPLIfiER
TO
AMPLITUDE
MODULATION
SIGNAL
INPUT
GENERATOR
!
3-15.
Although
the
traveling
wave tube
amplifier's
saturated
power
output
characteristic
canbeused
to
provide
nearly
constant
output
power,
installing
suit-
able
feedback
circuitry
providesaconstant
output
for
input
signal
variationsasgreatas20 db.
Figure
3-3.
Block
Diagram
of a
Circuit
usedtoMaintain
Constant-Level
Output
Power
fromaTWT
Amplifier
3-16.
An
arrangement
for
obtainingaconstant-level
output
signal
from
the twt, in
spiteofvariations
in
input
signal
level
or
variationsinamplifier
gain,
is
illustrated
in
figure
3-3.
In
this
circuitaportion
of
therfsignaliscoupled
from
the
traveling-wave
tube
output,
throughadirectional
coupler
to a
detector
such
asacrystal
rectifier.
The
rectified
voltage
is
then
amplified
in a
dc
coupled
amplifier
and
applied
to
the
GRID MOD.
connectoronthe
twt. Any
tendency
for
the
output
level
from
the
twttoincreaseisimmed-
iately
detected,
amplified,
and fed
backtoreduce
the
gainofthe
traveling
wave tube
amplifierinproportion.
Conversely,
any
reduction
in output
level
increases
the
gainofthe
amplifiertohold
the
output
level
con-
stant.
The
flatnessoftherfoutput
power
level
will
be
affectedbythe
frequency
responseofthe
detector,
directional
coupler
and
the
amplifier
gain.
The
band-
width of the
amplifier
must
be
great
enoughtopass
any
rateofchangeatwhich
the
output
level
may
vary.
3-17.Avariation
of
the
basic
automatic
power
level
control
circuit
canbeusedtoobtain
constant
ampli-
fication
with a twt
even
though
the
gain
changes
with
frequency,
power
line
voltage
and tube
characteris-
tics.
This
circuit
is
illustrated
in
figure
3-4.
The
circuit
operates
as
follows:
Therfinput and output
signals
are
sampled,
rectified,
and
the
resulting
dc
voltages
amplified
and
compared.
The
gainoroutput
from
each
halfofthe
circuit
canbeadjustedtoestab-
lish
the
desired
ratioofinputtooutput
level.Ifthe
rf
input
to output
ratio
changes,adifference
voltage
)
)
00144-2
3-3
Section
III
Figure
3-5
Model 492A/494A
\
\,
INPUT
-
13
DBM
-24DBM
-27.5DBM
-30.0DBM
-34.0DBM
-36.0DBM
:
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:
OPTIMIZED
AT E
g
=0
FOR
EACH
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DB
BELOW SATURATION
OUTPUT AT
Eg=O
OUTPUT
---------
0
DB
-'--'-'
2DB
-_..__
__..- 4
DB
-,,-,,-,,-6
DB
----IODB
----12
DB
OUTPUT
FREQUENCY
HELIX
VOLTAGE
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MODEL
492
A
TYPICAL
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CHARACTERISTICS
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-35
-40
-45
-50
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G
RID
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VOL
T S
G-
M-36
Figure
3-5.
Typical
Plot
of Output
VoltagevsGrid
Voltageofa Model 492A
3-4
00144-2
Model 492A/494A
Section III
Figure
3-6
)
1600+-----+-----1
-.::..bp--
MODEL
494A
TYPICAL
MODULATION
CHARACTERISTICS
INPUT
-6
OBM
-21
OBM
-250BM
-280BM
-31
OBM
OUTPUT
-------
0
DB
-'-'-'30B
............- _ 6
DB
-,,-,,-9
DB
----12
DB
FREQUENCY
:
9000
Me
HELIX
VOLTAGE:
OPTIMIZED
AT
Eg=O
FOR
EACH
CURVE.
OUTPUT:DB
BELOW SATURATION
OUTPUT
AT
E
g
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\
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+10
o
-10
GRID
-20
-30
BIAS
VOLTS
-40
-50
G-M-37
-60
Figure
3-6.
Typical
PlotofOutput
VoltageVBGrid
Voltage
of a Model 494A
00144-2
3-5
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