HP 492A Service manual

HP Archive
This vintage Hewlett Packard document was
preserved and distributed by
www.hparchive.com
Please visit us on the web !
Scanned by on-line curator: Tony Gerbic
** For FREE Distribution Only ***

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
Lists of Illustrations and Tables
TABLE
OF CONTENTS
Section
Page Section
Page
IGENERAL 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
VMAINTENANCE.
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 492A or 494A
5-5
3-14.
Constant Gain or Constant 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
PRINCIPLES OF OPERATION.
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 View of Models 492A and 494A ., 5-2
5-2. Bottom ViewLooking Towards the Front
Panel of Models 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 and494A 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
from a TWT Amplifier
3-3
3-4.
Block Diagram of an Automatic Gain
Control to Maintain Constant Ampli-
fication from aTWT Amplifier
3-3
3-5. Typical Plot of Output Voltage vs Grid
Voltage of aModel 492A
3-4
3-6. Typical Plot of Output Voltage vs Grid
Voltage of aModel 494A.
3-5
3-7. RF Phase Shift Produced by Helix Mod
3-7
3-8. Offset Frequency Produced by Sawtooth
Modulation of the Helix . 3-7
3-9. Block Diagram of a Linear (Homodyne)
)
Detection System. 3-8
3-10. Block Diagram of a Circuit to Produce
an FM Signal with a TWT Amplifier 3-8
4-1.
Block Diagram, Models 492A and 494A
4-0
4-2.
TWT and How it Works .
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
7 gc to 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
Approximately 15 ns
Helix Modulating Voltage: Approximately 40 volts peak-to-
peak. Provides 3600 phase shift.
Input impedance lOOK
Approximately 50 volts peak-to-
peak. Provides 3600 phase 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%, 50 to 60 cps,
approximately 200 watts
63 Ib net, 84 Ib shipping
U5 volts ±10%, 50 to 60 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:
Approximately 15 ns
Approximately 50 volts peak positive pulse will produce a40 db change
in rf power output. Sensitivity approximately 1 db/volt
1-0 00144-2

Model 492A/494A
SECTION I
GENERAL
INFORMATION
Section I
Paragraphs 1-1 to 1-4
1-1. DESCRIPTION.
1-2. The t$ Models 492A and 494A Traveling Wave
Amplifiers are broadband, linear amplifiers provid-
ing adjustable amplification up to at least 30 db, be-
tween 4and 12.4 gc, and have a maximum power out-
put of at least 20 milliwatts to an external load of 50
ohms. The frequency range of the t$ Model 492A is
4 to 8gc; the frequency range of the Model 494A is 7
to 12.4 gc. These traveling wave tube (twt) ampli-
fiers are designed to be used also as buffer amplifiers
or modulators for any signal within their frequency
range. As buffers, their input impedances remain
constant with any reasonable load change at the output
terminal; the attenuation between input andoutput sig-
nals is at least 60 db, minus the gain of the amplifier.
As modulators, they can be used to amplitude, 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 adjustable by the front panelGRID BIAS con-
trol. Hum and spurious modulation generated within
the amplifiers are at least 45 db below the output sig-
nal level and the noise figure is less than 30 db.
1-3. The t$ Models 492A and 494A Traveling Wave
Amplifiers, amplify any type of rf signal: cw, swept,
sine-modulated, pulsed, multiple signals on different
frequencies, etc. Atwt amplifier used as a modulator,
in conjunction with a signal generator, can be used to
amplitude modulate an rf carrier to approximately 30%
with less than 2.5% harmonic distortion and up to 50%
with less than 5% distortion. Amplitude modulation
sensitivity is approximately 1 db/volt. Pulse modula-
tion is excellent; the rise time is less than 15 ns.
Phase modulation up to 3600with less than 1db
amplitude modulation is also possible. Wide-band
frequency modulation is simulated by a step-wise phase
modulation described in section Ill.
1-4. The front panel meter is provided for checking
and adjusting electrode currents in the traveling-wave
tube. The meter helps to obtain desired operating
characteristics during normal operation of the 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 back to normal due to tube ageing.
)
00144-2
Figure 1-1. Model 492A Traveling Wave Amplifier
1-1

Section I
Paragraphs 1-5 to 1-9
1-5. The ~ Model 492A and 494A are similar in that
one model may be changed to the other, by replacing
the twt, as covered in paragraph 5-17.
1-6. INSTRUMENT IDENTIFICATION.
1-7. Hewlett-Packard uses a two-section eight-digit
serial number (000-00000). If the first three digits
of the serial number on your instrument do not agree
WA •• ANTY CLAIM AND ADJUITMINT 'IOCIDUU
Model 492A/494A
with
those on the title page of this manual, change
sheets supplied with the manual will define differences
between your instrument and the Model 492A or 494A
described in this manual.
1-8. TRAVELING WAVE TUBE WARRANTY.
1-9. The Traveling Wave Tube Warranty is illus-
trated in figure 1-2. A sheet for your use is included
in the appendix of this manual.
MICROWAVE TUBE WARRANTY CLAIM
INFORMATION FORM
IMPORT ANT: Please ans .... er all questions fully -- InsuHlclent Information may delay processing
of your claim. -
for microwave tubes supplied by [he
HEWLETT· PACKARD COMPANY
for use In * Instruments
Microwave tubes supplied by the Hewlett-Packard Company. either 88 original or replacement.
for use in 8 instruments are actually warranted by the tube manufaclUrer and not by 9.
However, S will process warranty claims for you, and will promptly pass on all allowances
granted by the tube manufacturer.
In the event that your tube Is (ound to be repairable, 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 may be made on this sIngle form; merely fJII out the lnfonnallon on the reverse side
and rerurn this fonn. along .... lth thedefectlve rube. to your 8 engineering representallve. or tO~.
Please be sure each space on the form is filled In··lack of complete infonnation may delay
processing of yourcTiim.
Each tube manufacturer has his own warranty policy. Copies of Individual Conditions of War-
ranty are available from your * engineering representative or from 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
On P. O. number
The following instructions are included to aid you In preventing damage in transit. Package
your tube carefully •• no allowance can be made on broken tubes.
Tube is Original ( ) or Replacement (
1. Carefully wrap tube in 1/4 inch thick "klmpack". Cotton batting. or other sofl padding
material.
2. Wrap the above in heavy kraft paper.
3. Pack in a rigid container .... hlch Is at least 4 inches larger than the lube In each dimension.
4. Surround the tube with at least 2 inches of shock absorbing material. Be cenaln that the
packing Is tight all around the tube.
Date tube received _
Date first tested
Date placed in service _
Date of failure _
Hours use per day (average) _
Number of days In service _
Total hours filament operation
5. Tubes returned from outside the continental UnltedSrates should be packed In awooden box.
6. Mark container FRAGILE and ship prepaid via Air Freight or Railway Expreu. Do not
ship via Parcel POSt or AJr Parcel POSt since experience has shown that fragile Items are
more apt to be damaged when shipped by these means.
Tubes returned to the Hewlen·Packard Company should be addressed to:
SYMPTOMS: (Please describe conditions prior to and at time of failure. 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 failures at time of failure? 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 2-1 to 2-13
/
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 there is any
apparent damage, file a claim with the carrier and
refer to the warranty page in this manual.
2-3. POWER REQUIREMENTS.
2-4. The Model 492A and Model 494A require a power
source of 115 volts ±10%, single phase, 50 to 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 connector is the ground pin.
2-7. To preserve the protection fearure when opera-
ting the instrument from a two-contact outlet, use a
three-prong to two-prong adapter and connect the
green pigtail on the adapter to ground.
2-8. I NST ALLATION.
2-9. The only special precaution necessary for in-
stalling the twt amplifiers is that they should not be
operated close to very 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 than is practical to in-
clude in the design.
2-10. To operate, connect the instrument to a 115-
volt ac power source, check and adjust rube operating
00144-2
currents as described in preliminary operating pro-
cedure, section 1Il, and connect to the external equip-
ment with coaxial cables terminated in standard UG-
21D/U, type connectors.
2-11. In section V, beginning with paragraph 5-32,
is a list of performance checks for this instrument.
These procedures make agood test as part of incom-
ing quality-control inspection following initial rurn-on.
2-12. REPACKAGING FOR SHIPMENT.
2-13. The following list is a 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 paper or plastic
before placing it in the shipping container.
c. Use plenty of packing 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
bands to seal the container.
e. Mark the packing box with "Fragile", "Delicate
Instrument", etc.
Note
If the instrument is to be shipped to Hewlett-
Packard Company for service or repair, at-
tach to the instrument a tag 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 switch is used for selection of
circuit to be checked on the front-panel meter.
3. Pin jack is for measuring grid bias (measures
Ek; grid grounded).
4. GRID BIAS control is used to adjust fordesired
tube current. Normal cathode current is indi-
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 control is used to adjust for best broad-
band response, position 5, or for maximum
gain at anyone frequency.
6. INPUT rf connector is where the rf signal to be
amplified is coupled into the instrument.
7. OUTPUT rf connector is where the amplified rf
signal is coupled from the instrument.
8. GRID MOD. connector is the input for ampli-
tude modulation signals.
9. HELIX MOD. connector is the 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 3-1 to 3-13
3-1. INTRODUCTION.
3-2. This section contains operating instructions for
theModels 492A and494A 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 shown in figure 3-1.
This figure also shows the uses for the controls, indi-
cator, and connectors. Whenever the twt is turned
on, use the front panel meter to measure 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 electrode is shown in table 3-1.
ormal cathode current for your instrument is indi-
cated on the plate attached to the meter face. Cur-
rents are usually a little high when the instrument is
first turned on, but decrease to normal during warm-
up. Allow IS minutes warmup before making final
reading. The GRID BIAS control may be used to re-
duce the tube currents during warmup.
Table 3-1. Maximum Operating Currents for
Models 492A and 494A.
Adjusting the HELIX voltage control for maximum
gain at the upper frequency limit usually produces the
flattest frequency response over the band. Maximiz-
ing the gain at frequencies below the upper frequency
limit usually results in additional gain and power out-
put over that obtained when adjusted for flattest broad-
band operation. The final setting of the HELIX con-
trol is independent of the type of signal amplified.
3-8. SATURATION POWER OUTPUT.
3-9. Saturation power output is the 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 saturation is approached
and the gain begins to decrease. Eventually the output
reaches a peak which is the saturation power output
and any
further increase in the input causes the
output to decrease. If the collector current is reduced,
the power output at which 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 bunches 2) the energy transfer from the electron
beam to the helix causes the beam velocity to decrease
and gradually lose synchronism with the wave on the
helix. These effects become more pronounced as
saturation is approached, and cause the gain of the
twt to decrease.
3-6. HELIX CONTROL.
3-7. The HELIX voltage control on the front panel
maximizes the gain and power output of the twt at a
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 control to slightly above the safe
maximum current (see table 3-1), readjust the anode
voltage control, paragraph 5-26. The anode voltage
control is set to limit the twt to its normal cathode
current when the bias voltage on the twt is zero. If
the cathode current is limited to its normal value,
but the current to another electrode is excessive, the
amplifier requires service or adjustment, see para-
graphs 5-21 or 5-30.
)
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 content in the
output, increase the input signal level until satura-
tion is reached, then reduce the input until the out-
put decreases approximately 6db (an input reduction
of 10 to 15 db). If second harmonic content is un-
important, operation at saturation is usually very
satisfactory.
3-11. One advantage of operating the twt near satura-
tion is the constant output characteristic exhibited by
atwt
at saturation. At saturation the gain of atwt
varies inversely with the input level and input varia-
tions of 10 to 15 db cause the output to vary only
4
to 6db. If a more nearly constant output is desired,
see paragraph 3-14.
3-12. BANDWIDTH CONSIDERATIONS.
3-13. The graphs in figures 3-2A and Bshow typical
gain vs frequency and saturation power output curves
for the 492A and the 494A amplifiers using two differ-
ent conditions of helix voltage. One set of curves
shows the gain and saturation power output when the
helix voltage is optimized at each frequency. The other
set of curves shows the gain and saturation power out-
put when the HELIX control is set to the broadband
00144-2 3-1

Section III
Figure 3-2
Model 492A/494A
GAIN: GAIN AT SATURATION
POWER OUTPUT.
VH OPTIMIZED AT EACH
FREQUENCY.
~ SMALL SIGNAL GAIN: VH OPTIMIZED AT EACH
FREQUENCY.
SIGNAL GAIN; VH OPTIMIZED FOR
BROADBAND
OPERATION.
SATURATION POWER OUTPUT: VH OPTIMIZED AT EACH
FREQUENCY
492A
-rr
a. 15 +-----+-----+-----+--------""'1-----+-----+-------1
50
30
A
VH: HELIX VOLTAGE
Eo=0 VOLTS
456 7 8
G-L-248
FREQUENCY KMC
49'4 A
SMALL SIGNAL GAIN: VH OPTIMIZED AT EACH
FREQUENCY.
VH OPTIMIZED FOR
BROADBAND OPERATION.
SATURATION POWER OUTPUT: VH OPTIMIZED AT EACH
FREQUENCY,
GAIN: GAIN AT SATURATION
POWER OUTPUT •
VH
OPTIMIZED AT
EACH FREQUENCY.
rr
15 -1-----+-----+-----+-----+----"""""" ...... -..---+-------1
25
45
.c
."
I 35
z
«
(9
B
VH: HELIX VOLTAGE
Eg:0 VOLTS
78910
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 3-14 to 3-22
is developed which is then applied to the 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 power is directly
proportional to bandwidth, it may be desirable to
limit the bandwidth and therefore noise by maximizing
the gain at a particular frequency with the HELIX
control and by installing suitable filters at the 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
(SET TO OBTAIN 0 VOLTS TO TWT
GRID MOD. CONNECTOR, AT DESIRED
AMPLIfiCATION)
3-19. The 492A and 494A serve as very effective
buffers to isolate a microwave signal source from a
load. Mismatches, changes in external circuitry, or
the introduction of modulation do not affect the con-
stant 50-ohm input impedance of the twt and thus will
not affect a signal source connected to the input. The
attenuation between the output and input terminals is
60 db due to attenuators placed along the helix. How-
ever, when the output signal is reflected from a mis-
matched load back to the input, the effective signal
isolation is the 60 db minus the gain of the amplifier.
For example: with an amplifier gain of 25 db, an open
or short circuit on the twt output can result in a maxi-
mum reflected signal 35 db below the input level
(approximately 1/56 of the input signal), which corre-
sponds to a swr of less than 1.04.
Note
The GRID MOD. connector is direct-coupled
to the grid of the twt amplifier. If a dc poten-
tial accompanies a modulating voltage applied
to this connector, the grid-bias voltage will
be altered. The GRID BIAS control may be
used to compensate for the change in grid bias
voltage due to a dc component at the input.
Figure 3-4. Block Diagram of an Automatic
Gain Control to Maintain Constant Amplifica-
tion from a TWT Amplifier
3-18. BUFFER AMPLIFICATIONS.
3-20. AMPLITUDE MODULATION.
3-21. To amplitude modulate an rf signal applied to
the twt amplifier with a minimum of envelope distor-
tion in the output signal, carefully establish the opti-
mum rf drive, grid bias, and modulating signal
amplitude for agiven setup.
3-22. For minimum distortion, the twt grid voltage
must not leave the linear region of the grid voltage vs
rf output characteristic (see figures 3-5 and 3-6).
Also, the rf drive must be adjusted so that the modu-
lation peaks are at least 2db 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 can be used to
provide nearly constant output power, installing suit-
able feedback circuitry provides a constant output for
input signal variations as great as 20 db.
Figure 3-3. Block Diagram of a Circuit
used to Maintain Constant-Level Output
Power from a TWT Amplifier
3-16. An arrangement for obtaining a constant-level
output signal from the twt, in spite of variations in
input signal level or variations in amplifier gain, is
illustrated in figure 3-3. In this circuit a portion of
the rf signal is coupled from the traveling-wave tube
output, through a directional coupler to a detector
such as a crystal rectifier. The rectified voltage is
then amplified in a dc coupled amplifier and applied
to the GRID MOD. connector on the twt. Any tendency
for the output level from the twt to increase is immed-
iately detected, amplified, and fed back to reduce the
gain of the traveling wave tube amplifier in proportion.
Conversely, any reduction in output level increases
the gain of the amplifier to hold the output level con-
stant. The flatness of the rf output power level will
be affected by the frequency response of the detector,
directional coupler and the amplifier gain. The band-
width of the amplifier must be great enough to pass
any rate of change at which the output level may vary.
3-17. A variation of the basic automatic power level
control circuit can be used to obtain constant ampli-
fication with atwt 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: The rf input and output
signals are sampled, rectified, and the resulting dc
voltages amplified and compared. The gain or output
from each half of the circuit can be adjusted to estab-
lish the desired ratio of input to output level. If the
rf input to output ratio changes, a difference 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
: 6000 Me
: OPTIMIZED AT Eg =0
FOR EACH CURVE.
DB BELOW SATURATION
OUTPUT AT Eg=O
OUTPUT
--------- 0 DB
-'--'-' 2DB
-_ .. __ __ .. - 4 DB
-,,-,,-,,-6 DB
----IODB
----12 DB
OUTPUT
FREQUENCY
HELIX VOLTAGE
dJp-- MODEL 492 A
TYPICAL MODULATION CHARACTERISTICS
\ • I I ,
\\ '\\
'! I
:.,
\ \'
: ~ \
'\ T\ TT
\\\\
\\ \'
:: I'
\ !
\\ ~
: .,
\;\,
\\\\ ~
. i ,
\:,
I \ 1\ ,
: ! . ,
"
.~k. ,
r···, ,
I ,
,,
." I ,
r, I
1700
1600
I
1500
1400
0
et
1300
0
..J
~
1200
%:
0
0
11.00
\t)
1000
C/)
~
..J
900
0
>
..J
800
..J
~
700
z
600
~
::)
500
0.
~
::)
400
0
300
200
100
0
+10 +5 o
-5
-10 -15 -20 -25 -30 -35 -40 -45 -50 -55
G RID B I A S VOL T S G- M-36
Figure 3-5. Typical Plot of Output Voltage vs Grid Voltage of aModel 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 Eg=0
\\\\\\\\\
,\,,
.\
\.
\.
,. .....
,. ,
,. ,
\\\
::::".
\'\
"\
\\
\\
800 -fI<--------t--\-----J'l---t---1
~\.\ ',\ \
I \ \ \ \
6 00 +--4-~.____--+-.:,.-----4---;.-+-----l..-____,r----__r___---......,.....---_+---__l
'.'. \\
\>\\ \\
400+-------''rl-----T---'';---+-ir----1-----+-----+-----+----~
\\.\
..• ',\
\\ ..... 1\\
'. '. ~~
\. '\ \ .. \
200+------+------>,r-"""'d-....:...;-"""\r----11-----+-----+-----+----~
~~" .•.. ,~~
'~ "~
~ ............. ~,.
~:.-.:.:..~
0~---_+_---_+_----1f_--==:..::.!!!~---_+_---_+---___l
1400+------+--~
1200
0
<
0
.J
:E
:I:
01000
0
It)
z
V>
~
.J
o
>
.J
.J
)+10 o
-10
GRID
-20 -30
BIAS VOLTS
-40
-50
G-M-37
-60
Figure 3-6. Typical Plot of Output Voltage VB Grid Voltage of aModel 494A
00144-2
3-5

Section III
Paragraphs 3-23 to 3-30
output. The linear modulation region extends from
approximately +4 volts (where beam de-focusing
occurs) to approximately -15 volts (where the rf out-
put becomes an exponential function of the grid volt-
age). This linear operating region permits up to 30%
modulation with less than 2.5% envelope distortion
and up to 50% modulation with less than 5% distortion.
Envelope distortion increases rapidly above 50% modu-
lation. In pulse work the twt may be biased near or
below cut-off and the rf drive adjusted for saturation
output at the peak amplitude of the modulating pulse.
However, the grid voltage at the peak of the modu-
lating signal must not cause de-focusing (approxi-
mately +4 to + 8 volts) nor excessive average electrode
current, see table 3-1. The transient response of the
492A or 494A to a step function applied to the grid is
approximately 15 nsec.
3-23. Amplitude modulation is accompanied by some
incidental phase modul'btion of the rf signal, amount-
ing to approximately 90 phase shift of the rf carrier
for a10 db change in the modulated rf output level.
In practice this phase modulation is unimportant when
using the conventional square-law crystal detectors,
but is important in detection systems where the out-
put is afunction of the rf carrier phase.
3-24. PULSE MODULATION.
3-25. There is considerably latitude in the adjustment
of modulation characteristics when pulse modulating
an rf signal using the 492A or the 494A; see figures
3-5 and 3-6. The cw input level, the modulation-
pulse amplitude, and the grid bias determine the
characteristics of the rf output pulse as follows:
a. The cw input signal primarily determines the
maximum possible level of the rf output pulse and
whether or not the twt can be operated into saturation.
b. The peak-to-peak amplitude of the modulating
pulse primarily determines the on-off ratio of the rf
output pulse.
c. The grid bias level primarily determines the rf
output levels attained during the pulse-on and pulse-
off times and also, in conjunction with the modulating
pulse, determines the rf input level necessary to
saturate the twt. The GRID BIAS control always
should be set so that the twt grid will not draw cur-
rent (approximately 4 volts positive) during the pulse-
on period.
3-26. To pulse modulate the rf signal being amplified
in the 492A or 494A, refer to figures 3-5 or 3-6 and
proceed as follows:
a. Determine if the twt is to be driven into satura-
tion and if the rf output must be at a specific level.
b. Set the GRID BIAS control for zero bias.
c. Connect the rf input signal to the twt and adjust
its level to produce the desired rf pulse output level.
d. Determine the on-off voltage ratio required in
the rf output pulse.
3-6
Model 492A/494A
e. Using the graph in figure 3-5 determine the
magnitude of modulation pulse required to produce the
desired on-off ratio.
f. Set the GRID BIAS control to obtain the voltage
determined in step e., Le., the peak voltage of the
modulating pulse. The bias VOltage may be measured
at the pin jack on the front panel.
g. Connect the modulating pulse to the GRID MOD.
connector and adjust its amplitude to the voltage de-
termined in step eto produce the desired on-off ratio
in the rf output pulse. Since the grid of the twt is con-
nected directly to the GRID MOD. jack, a dc com-
ponent in the modulating signal will affect the grid
bias. Also, if capacitive coupling is used the modu-
lating signal will drive the grid of the twt above and
below
the dc level established by the grid bias, an
amount determined by the duty cycle of the modulating
signal. The GRID BIAS control must be adjusted to
compensate for both of these effects.
h. To increase the on-off ratio of the rf output
pulse, increase the amplitude of the modulation pulse,
at the same time adjust the grid bias so that the grid
will not be driven beyond 4 volts positive, see the Note
paragraph 3-21.
Note
Large input modulating pulses, above 15
volts, tend to shock-excite the helix, produc-
ing ringing on the top of the rf output pulse
and aslow rise time. If the traveling-wave
tube is operated near saturation this effect is
minimized and better pulse characteristics
are obtained.
3-27. LIMITED PHASE MODULATION.
3-28. The signal being amplified in the 492A or 494A
can be phase-modulated by applying voltage to the
HELIX MOD. connector. This voltage varies the
electron-beam velocity by changing the potential be-
tween the cathode and the helix--a positive voltage
change accelerates the electron bunches and advances
the phase of the rf output signal; a negative change
slows them and retards the phase of the output signal.
The resultant phase deviation in the output signal is
directly proportional to the applied voltage. The de-
gree of phase deviation produced is limited by the
range of helix voltages that produces amplification,
and by the amount of incidental amplitude modulation
permissible in the rf output. Phase deviation of 3600
is possible with the output amplitude held to variations
of approximately 1-1/2 db and is obtained with a helix
voltage variation of less than 50 volts. The actual
voltage required for a phase shift of 3600 varies with
the operating frequency and from tube-to-tube.
3-29. UNLIMITED PHASE MODULATION AND
FREQUENCY SHIFTING.
3-30. Although the limited phase deviation described
in paragraph 3-27 is useful in some applications, un-
limited phase deviation has a much wider range of
usage. It is particularly useful because the frequency
00144-2

Model 492A/494A Section III
Paragraphs 3-31 to 3-36
G-l-7
TIM E
DESIRED OUTPUT
(Ftl
FR EQU ENCY +- __ -'---_----' __ ---A. __ --L.._---l
-- --- ---1--- INPUT FREQUENCY (Fl
UNDESIRED OUTPUT ( F )
FREQUENCY 2
HIGH
Figure 3-8. Offset Frequency Produced by
Sawtooth Modulation of the Helix
LOW
SAWTOOTH MODULATION
APPLIED
TO HELIX
3-31. In practical applications, a constant amplitude
linear-slope sawtooth generator is used to produce
the sawtooth waveforms. If the amplitude 8f the saw-
tooth voltage is adjusted to produce a360 shift, one
cycle of the cw signal will be added or subtracted
during each sawtooth, and the frequency shift pro-
duced in the output will be equal to the sawtooth repe-
tition rate. Sawtooth voltages having negative slopes
(see figure 3-7) produce a decrease in the output fre-
quency (delay in phase). Conversely, sawtooth volt-
ages of the opposite slope cause an increase in output
frequency.
of the input signal can be shifted by a predetermined
amount. Unlimited phase deviation is effectively sim-
ulated by continuously repeating exact 3600 phase
deviations. This is accomplished by modulating the
traveling wave tube helix wiSh asawtooth waveform,
each sawtooth producing 360 phase shift, as shown
in figure 3-7. An rf output frequency that is shifted
in relation to the input frequency is thus produced, as
shown in figure 3-8.
O·
HIGH
SAWTOOTH MODULATION
APPLIED
TO HELIX
LOW
PHASE DIFFERENCE
BETWEEN INPUT AND
OUTPUT WAVES
frequency shift with its relatively small power content
would be rejected by most narrow band circuits. In
systems where the undesired frequency shift falls
within the pass band of the equipment under test, a
negative pulse can be applied to the GRID MOD. con-
nector to cut off the twt beam current during the fly-
back time. This method reduces the undesired fre-
quency shift although it produces some small tran-
sients and leaves small time intervals during which
there is no signal output. Practical applications of
offset frequencies include the measurement of ex-
tremely high swr's accurate calibration of attenu-
ators over wide amplitude ranges (paragraph 3-34),
frequency shifting of microwave radio relay channels
for retransmission, production of mixer frequencies
for radar and other microwave receivers, etc.
G-l-I
TI ME ..
3-34. HOMODYNE DETECTION.
Figure 3-7. RF Phase Shift Produced by
Helix Modulation
3-32. With sawtooth modulation, the desired output
frequency shift (F in figure 3-8) occurs during the
sawtooth formatioJ time, and is proportional to the
rate of change of voltage. During the sawtooth fly-
back time the output phase is shifted in the opposite
direction producing an undesired frequency shift (F 2
in figure 3-8). If the flyback time is made extremely
short, this frequency is far removed from the desired
frequency since the degree of frequency shift is in-
versely proportional to the flyback time. In addition
to being far removed from the desired frequency the
power in the undesired frequency is very small since
it is proportional to the ratio of flyback time to saw-
tooth time.
3-33. In a typical case involving a desired 50-kc fre-
quency shift, a I-microsecond flyback time would
produce a I-megacycle frequency shift in the opposite
direction and would contain only 5% of the total power
in the output wave. In practice, this undesired
3-35. The ability of atwt to produce an offset fre-
quency that is stable with respect to the signal source
makes it an ideal instrument to use in ahomodyne
(linear) detection system. The difference frequency
will be dependent upon the stability of the sawtooth
generator used to modulate the twt helix, a problem
of no consequence at the low modulating frequencies
involved.
3-36. A typical linear detector system suitable for
calibrating microwave attenuators is illustrated in
figure 3-9. The signal generator supplies a signal
(f) both to a crystal mixer and to a traveling wave
tube amplifier. The traveling wave tube amplifier is
sawtooth modulated to produce an offset frequency
(f-f )which is applied to the attenuator to be cali-
brated. The output signal from the attenuator is then
combined with the original signal (f) in the mixer to
produce a beat frequency (f )whose amplitude is di-
rectly proportional to the ahtplitude of (f-f
j)
so long
as the amplitude of (f-f
j)
remains within the square-
law region of the cryStal. This beat frequency is
amplified by the tuned amplifier and the output is in-
dicated by an ac voltmeter. The lower sensitivity
01144-2 3-7

Section III
Figures 3-9 and 3-10
Model 492A/494A
SAWTOOTH
GENERATOR
f,
OFFSET
FREQUENCY
SINGLE SIDEBAND GENERATOR
-------------1
I
A TTENUATOR BE ING CALIBRATE"
/ I
I
I
I
I
I
I
f-f, I
I
I
I
I
I
_ _J L_
I-
I
TRAVELING WAVE TUBE TUBE
AMPLIFIER
SIGNAL
GENERATOR
COUPLER
/
f
VOLTMETER
TUNED AMPLIFIER
LO-l-'"
Figure 3-9. Block Diagram of a Linear (Homodyne) Detection System
INT. SWEEP
OSCILLOSCOPE
MI XER
30 DB PAO
'T' CONNECTOR
TWT AMPLIFIER
)20 OB (
DIRECTIONAL
COUPLER
SIGNAL GENERATOR
4 TO 8 KNC 0 USE -hp- MODEL 492 A
7 TO 12.4 MC. USE ·hp· MODEL 494 A
USE APPROPRIATE DIRECTIONAL COUPLERS.
ATTENUATORS. MIXERS. ETC.
TO
HELIX
SLOPE - MODULATED
SAWTOOTH GENERATOR
.h
p'
202A
FUNCTION GENERATOR
M.O.P.A. MICROWAVE FREQUENCY MODULATION SYSTEM
LD-L-40B
Figure 3-10. Block Diagram of a Circuit tc? Produce an FM Signal with aTWT Amplifier
3-8
00144-2

)
Model 492A/494A
limit is determined by the crystal and IF amplifier
noise and is approximately -100 dbm.
ote
When a swr indicator (such as the <[j) Model
4158) calibrated for use with a square-
law detector is used in place of the tuned
amplifier and voltmeter, the db readings
must be doubled.
3-37. FREQUENCY MODULATION.
3-38. Narrow-band frequency modulation can be ob-
tained by applying the modulation signal directly to
the helix of the twt; however, to frequency modulate
with an appreciable frequency deviation it is first
necessary to produce an offset frequency as described
in paragraph 3-29. The deviation of the offset fre-
quency should be slightly greater than 1/2 the total
frequency deviation desired. The offset frequency is
then varied by varying the slope of the sawtooth.
3-39. A sawtooth waveform produced by the special
generator described in paragraph 4-20 can be slope-
00144-2
Section 1II
Paragraphs 3-37 to 3-40
modulated by any waveform before being applied to
the traveling wave tube helix. In this manner complex
signals can be used to frequency-modulate the signal
applied to the twt and the center of the output fre-
quency will be fixed by the sawtooth repetition rate
without slope modulation. In no case should the p~ase
shift due to a single sawtooth cycle exceed 360 so
that the amplification properties of the traveling wave
tube amplifier will not be adversely affected regard-
less of the magnitude of the apparent phase deviation
when the sawtooth wave is modulated.
3-40. Figure 3-10 is a block diagram of a system for
the generation of frequency-modulated offset signals.
In this arrangement, the slope-modulated sawtooth
voltage which is applied to the twt helix produces an
offset frequency, the instantaneous frequency of which
is proportional to the slope of the sawtooth. Varying
the slope of the sawtooth voltage varies the offset fre-
quency. Thus the output signal from the twt is an
fm signal. The detected fm signal (the difference
between the signal generator frequency and the fre-
quency-modulated offset frequency) is shown on the
oscilloscope.
3-9

"'"
,
o
'rjC/)
.....
CD
OQ
n
c:
~.
'i
0
CD
::s
"'"-
,<
......
3::
8.
CD
>-
"'"
'Cl
N
;>
-....
"'"
'Cl
"'"
;>
J4
rG'iiiDl
l!QQJ
J5
PIN
JACK
ON
FRONT
PANEL
FOR
MEASURING
BIAS
J2mD
J3m
(
~Jll
HELlXI
MOO.
Figure
4-1.
Block
Diagram
~
Models
492A and 494A
~
-
SRI
HALF
WAVE
RECTIFI
ER
+
SR2
~
VI
V5A
SR3
SERIES
HELIX
VOLTAGE
REGULATOR
MODULATOR
I
DOUBLER
6BQ6
V4
1126BQ7A
REFERENCE
ew
I
-
VOLTAGE
I
HELIX
~
0
OA2
STRAY
CAPACITY
'--c.--
T1
V2
V5B
..,- ,,'
-",
CONTROL
DISCHARGE
II
POWER
TUBE
TUBE
,,
Ii
~
TRANSFORMER
6AU6
1
12
6BQ7A
IW I
~
II
elY
-:
~
u
><@
HELIX
e>
elY
I
-
--
I""
ANODE
\~.J
~
,"
'--
""
~
<
V3
~o
I
GRID
BIAS
I
<:>
BIAS
eIY~
REGULATOR
OB2
80-
L-44B
JF1
I.SA
F2
1.6
A
~
SR4
~
TIT
MAGNET
SI
SUPPLY
L----J
L-
o
o
......
"'" "'"
,
N

)
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 1volt 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 a492A (4-8 gc) or
as a494A (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 ablocking capacitor would be required
which would impair the response of the twt to low-
frequency modulating signals. Apin 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
apowerful
magnet around thefull length of the capsule.
4-12. Acw 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) Acw 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