JUL 2 6 1961

^jiouUer laboratories

PB 161608

92o. I07

A FIXED FREQUENCY, 9.1 Gc,

FIELD INTENSITY RECORDING RECEIVER

WITH EXTREMELY NARROW BANDWIDTH

BY R.W. HUBBARD AND J.V. CATEORA

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
TdS

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for

making measurements consistent with these standards: the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devi<'es. and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

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divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

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NATIONAL BUREAU OF STANDARDS

technical ^flote

107

June 1961

A FIXED FREQUENCY, 9-1 Gc, FIELD INTENSITY

RECORDING RECEIVER WITH EXTREMELY NARROW BANDWIDTH

by

R. W. Hubbard and J. V. Cateora

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $,75

Figure No.

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LIST OF FIGURES

X-Band Field Intensity Recording Receiver

X-Band Recording Receiver - Panel View

X-Band Recording Receiver - Rear View

SHF Head

SHF Input Section

SHF Input Section

Calibrated Variable Attenuator Characteristics

X-Band Recording Receiver

Calibration Unit

Calibration Unit (Schematic)

First Local Oscillator (Schematic)

First Local Oscillator

100 Mc Multiplier

Block Diagram - Reference Signal System

Local Oscillator AFC Loop

100 Mc Multiplier (Schematic)

100 Mc Multiplier Power Supply

10 Mc Amplifier - Discriminator

Block Diagram - Phase/Frequency Discriminator

Control Circuit

10 Mc Amplifier - Discriminator

Discriminator Frequency Characteristics

10 Mc Phase Monitor

10 Mc Phase Monitor (Schematic)

Klystron Power Supply

Klystron Power Supply (Schematic)

IF Annplifier

9.9 Mc Cascode Amplifier

LIST OF FIGURES (CONT'D)

Figure No.

26 9.9 Mc Cascode Amplifier (Schematic)

27 9.9 Mc Amplifier and Cathode Follower

28 Gain Stable Mixer - Couple

29 100 kc Gain Stable Amplifier Couple (Schematic)

30 100 kc Gain Stable Amplifier

31 Cascade Amplifier Gain Versus Plate Supply Voltage

32 Cascade Amplifier Gain Versus Filament SupplyVoltage

33 Frequency Response IF Feedback-Couple Stage

34 Frequency Response IF Cascade Amplifier

35 Attenuation Characteristic - Crystal Filter

36 Final IF Amplifier and Detector

37 Gain Stability - Complete IF Amplifier

38 Frequency Response - Complete IF Amplifier

39 Wiring Diagram - IF Amplifier Chassis

. 40 RF Cabling Diagram IF Amplifier Unit

41 IF Amplifier Power Supply

42 Voltage Regulator IF Amplifier Power Supply

43 IF Amplifier Power Supply (Schematic)

44 Servo Amplifier

45 Servo Amplifier (Schematic)

46 Servo Amplifier Power Supply

47 Frequency Response Low Pass Servo-Driven Attenuator Loop

48 High-Pass Recording Amplifier

49 High Pass Recording Amplifier (Schematic)

50 High Pass Recording Unit

^

Figure No.

LIST OF FIGURES (CONT'D)

I 51 Second Local Oscillator

52 Test Transnnitter Antenna Assembly

5 3 Test Transmitter Rack

54 100 Watt CW Klystron Oscillator

TABLE OF CONTENTS

Page

Abstract 1 Summary of Characteristics 2

Introduction 3

System Description 4

General 4

Complete Receiver 9

SHF Input Section 9

Calibration Unit 10

First Local Oscillator and

Freq. /phase Control System 11

IF Amplifier 13

Servo Amplifier and Power Supply l6

High-Pass Recording Amplifier 17

Second Local Oscillator 19

Conclusions 19

Acknowledgements 20

References 21

A FIXED FREQUENCY, 9. 1 Gc, FIELD INTENSITY

RECORDING RECEIVER WITH EXTREMELY NARROW BANDWIDTH

by

R. W. Hubbard and J. V. Cateora

ABSTRACT

A specialized field intensity recording receiver designed

to operate in the X-band (9- 1 Gc) portion of the radio spectrum

is described in this paper, and its operational characteristics

presented. The receiver was developed by the Radio Propagation Engineering Division, Central Radio Propagation Laboratory, National Bureau of Standards. Its purpose is to provide an advanced system to augment the facilities of the CRPL

in a program of basic tropospheric radio propagation research.

The receiver normally operates on a fixed frequency of

9100. 1 Mc but may be operated at ± 100 Mc intervals from

the nominal, in the band 8. 5 to 9. 6 Gc. It displays an extremely

narrow bandwidth of 45 cps, held by a stable and flat crystal

filter element. The latter may be switched out of the system,

providing a wider bandwidth of operation to 5 00 cps. The narrow bandwidth (approximately 5x10" percent at the X-band

frequency) makes possible a high signal-to-noise figure, a desirable characteristic for a receiver used in tropospheric

research. Maintaining this bandwidth of operation requires

an extremely high degree of stabilization of the klystron

oscillator frequencies. This has been accomplished in the

design, utilizing a unique frequency/phase type control loop

with an overall stability of approximately ± Z parts in 10°.

A true logarithmic response is obtained for a wide

dynamic range of the recorded input signal variations,

over the lower frequency portion of the recorded spectrum.

The higher portion of the spectrum may be recorded separately on a linear expanded scale record, with a dynamic

range up to 25 db. This system alleviates a common problem

in most field strength recording systems, i.e., where the

full dvnamic range is recorded on a single record. In these

systems the low frequency portion of the signal variations

•V-
-2-

have a wide dynamic range, and consequently they mask the

smaller amplitude variations of the higher frequency components. The actual division of the spectrum recording is accomplished by closing a servo-controlled loop around the

receiver from the output of the second detector to a variable

attenuator at the antenna input to the receiver. The loop

serves to maintain a constant output level over the response

time of the loop, and over a maximum dynamic range of

40 db (the range of "the attenuator). The higher frequencies

are then recorded at full gain of the receiver in a separate

channel, with a maximum dynamic range of 25 db. The time

response of the servo loop is variable in ten discrete steps.

The application of the receiver is primarily for remote

unattended operation. To this end an integral self-calibrating

system has been provided, which may be controlled remotely

at the transmitting terminal of any experimental propagation

path. The systenn makes use of a calibrated noise source as

a fixed in-line component, applying a noise input to the receiver of a known excess noise ratio when ignited. Also,

gain-stable amplifier design techniques and modular plug-in

type construction have been used whenever practical in the

development to reduce operational and maintenance problems.

SUMMARY OF CHARACTERISTICS

(1) Operating Frequency - 9100. 1 Mc (nornaal). May be operated at

100 Mc intervals thruout the range 8.5 to 9.6 Gc

(?.) Receiver Type - double conversion superheterodyne

(3) Bandwidth

(a) Narrow Band - 5 cps at -6db

45 cps at -3db

(b) Wide Band - 1000 cps at -6 db

500 cps at -3 db

(4) Noise Figure - 1 1 db

(5) Sensitivity - -146 dbm narrow band

-136 dbmi wide band
-3-

(6) Dynamic Range - 55 ^^ J

^^ ^^ ^'^'^ freq. channel

I 25 db high freq. channel

9

(7) Local Oscillator Stability - » 1 part in 1 per day

(8) Intermediate Frequencies - First: 9.9 Mc

Second: 1 00 kc

(9) Outputs

(a) Low Freq. Recording - servo driven recording potentiometer

(tape or graphic recorder)

(b) High Freq. Recording - low impedance d-c amplifier to magnetic tape recorder

(10) Recorded Spectrum

(a) Low Frequency - servo time response variable in 1 discrete

steps from 3 seconds to 5 minutes full scale

(40 db)

(b) High Frequency - from f of low frequency channel up to

50 cps

(11) Calibration

(a) Low Frequency - logarithmic calibration of the input attenuator

(b) High Frequency - linear voltage calibration of the high-pass

recording circuit

(c) The calibration is checked periodically with use of an automatic system which inserts a calibrated noise

source at a level of 14.6 db excess noise ratio.

INTRODUCTION

A great amount of effort has gone into the study of tropospheric

radio propagation in the UHF and VHF portion of the RF spectrum in

the last decade. The Central Radio Propagation Laboratory of the

National Bureau of Standards has been a recognized leader in this work.

Very little effort has been expended to date in the SHF region of the

spectrum, however, especially in beyond line-of-site conditions and

troposcatter circuits.
-4-

The development reported— in this paper represents the initial

phase of a plan by the Radio Propagation Engineering Division of CRPL

to extend their basic research program into the SHF region,

SYSTEM DESCRIPTION

A. General

A complete block diagram of the receiver is shown in Figure 1.

Basically, it is a conventional double-conversion superheterodyne

receiver with a first IF of 9-9 Mc and a second IF of 100 kc. The

receiver has been designed with an extremely narrow bandwidth,

which reqmres a very high degree of stabilization of the first local

oscillator klystron frequency. The latter is accomplished in a

1/ 3/

frequency/phase type control loop— ' — consisting of the 10 Mc reference oscillator, the xlO multiplier, and the frequency/phase discriminator unit indicated in Figure 1. The reference oscillator is a crystal controlled unit operating at 1 Mc with a stability of approxi-

-9

mately 1x10 . A multiplier is used to multiply the reference oscillator signal to a frequency of 100 Mc. This signal is then fed to a harmonic generating crystal at the top of the directional coupler in

the control loop. A very high order harmonic (x91) is extracted from

this crystal at 9100 Mc and propagated down the guide of the directional

coupler to the reference mixer crystal, where it is mixed with a small

portion of the local oscillator klystron power. The LO klystron is

mechanically tuned to a frequency of 9110 Mc, which yields a 10 Mc

beat frequency in the reference mixer crystal. This signal is then

amplified, and applied to a frequency discrinninator of the gated beam

tube type. The 10 Mc reference oscillator signal is also fed to the

quadrature grid of the gated beam disciminator as a phase reference

and a phase lock between this oscillator signal and the 10 Mc loop
-5-

signal is effected. The discriminator output is applied as an error

signal to the repeller of the klystron, and in this nnanner the local

oscillator frequency is held to approximately the stability of the

9 reference oscillator, i.e., a few parts in 10 per day.

The first IF amplifier consists of two amplifier stages each employing feedback for gain stability and low output impedance. The

first stage is a conventional cascode circuit with a cathode follower

output, and the second a pentode amplifier stage with a cathode

follower output. Both stages are tuned at 9.9Mc. The first stages

have been designed with low impedance outputs to facilitate installation of the receiver in locations which require the antenna to be

several hundred feet from the receiver. In this type of installation

the X-band portion of the receiver may be placed physically near

the antenna, and the output of either the first or second stage of the

first IF amplifier carried by coaxial cable to the rest of the receiver

in a remotely located operations building.

The second mixer and second IF amplifier consist of feedback4/ couple stages designed for the best gain stability possible—, and

narrow bandwidth operation. The 100 kc amplifier stages utilize

high-Q cup-core assennblies, with inverse current feedback from the

second stage of each couple to the input stage. A crystal filter unit

is employed as the coupling network in the IF output stage. This

filter may be inserted or removed with a front panel selector switch,

thus providing a variable bandwidth feature. The crystal filter used

has a 6 db bandwidth characteristic of 50 cps at 100 kc , and excellent

flat response over a wide portion of this bandwidth. This filter

establishes the narrow bandwidth of the receiver. The IF output stage

^^
-6-

provides a low impedance cathode follower output which can be used if

desired to drive directly a 1 ma or 5 ma low impedance recorder thru

a crystal diode detector. A low impedance 100 kc output is also available from this stage.

The IF output and detector stage normally feed a servo-control

amplifier which drives mechanically a precision variable calibrated

attenuator at the input to the receiver. This system closes a complete loop around the receiver, and is a technique used to provide a

split in the recorded frequency spectrum. The closed-loop servo

system maintains a constant signal-to-noise ratio throughout the

receiver over the low pass -band of the loop. The time response of

this loop is variable in fixed steps from 3 sec. to as high as 50 min.

full scale (40 db), the full range of the input variable attenuator. The

loop thus serves to take out of the input signal all of the slow variations

including the diurnal and other long term changes, which generally

represent the largest portion of the dynamic range of the received

signal. Because of this action only the input attenuator itself is re- quired to have a wide dynamic range, and the dynamic range and linearity of the receiver are of secondary importance.

The relatively fast fluctuations present in the received signal

represent a small portion of the overall dynamic range. This portion

of the spectrum is amplified at full gain by the receiver, and can be

displayed separately on an expanded scale type of record. The low

portion of the spectrum may be directly recorded fronn an electrical

analog of the servo shaft position, as indicated in Figure 1.

The variable attenuator employed in the loop has been modified

to provide a true logarithmic characteristic with shaft rotation. A

small precision cut cam is used in the drive mechanism, replacing

the original linear lead-screw,. This modification was necessary in
-7-

order to provide a constant rate -of- change of attenuation for anyoperating position of the attenuator within its calibrated range. It also provides a logarithmic calibration of the receiver response,

which is a desirable feature.

In recording the high-pass portion of the data spectrum, it is

necessary to compensate this output of the receiver for the dynamic

changes introduced by the response of the input attenuator to the

low-pass spectrum. Compensation is accomplished automatically

in the receiver by using a logarithmic potentiometer, driven by the

servo motor shaft, as a variable attenuator in the high-pass recording

amplifier. The compensation is provided with a slope equal but

negative to that of the input attenuator characteristic, over the entire

calibrated range of 40 db. The dynamic range of the receiver proper

is approximately 25 db which may be utilized completely for the

high-pass portion of the data spectrum, and the full 40 db of the

input attenuator loop utilized for the low portion. In this manner, the

overall dynamic range of the receiver system is 65 db.

It can be seen that once the receiver sensitivity has been determined, the calibration of the system is completely dependent upon the

calibration of the input attenuator, which is a fixed and stable characteristic. To monitor the performance of the electronic sections, it is

then only necessary to make a periodic check of the gain of the receiver, and its noise level. For this purpose, an X-band calibrated

noise source has been provided as an in-line component in the input

section of the receiver. This tube is a small gaseous noise generator

of the argon type with a very low VSWR in the extinguished state. It provides a broad-band noise spectrum at a fixed and constant excess

noise ratio of 14.6 db when fired. The noise source thus provides a calibrated check point for the entire receiver, the output being recorded on the high-pass channel record. This system lends itself
-8-

readily to automatic calibration procedures as well as unattended

operation. The noise source is controlled by the limit s^witch on the

servo shaft in Figure 1. Thus, when received signal is lost, the

limit switch is actuated by the input attenuator seeking its lower

limit, turning on the noise source. The calibration cycle can therefore be controlled remotely at the transmitter ternninal by merely

turning off the transmitter for a short period of time. A time delay

is provided in the calibration system to enable the receiver to partially distinguish between a complete signal fade and a transmitter

outage. During this delay, the receiver noise level is recorded on

the output record.

Since the primary application of this receiver is for remote unattended operation, several important design features have been incorporated. First, all of the electronic circuitry has been designed

in a modular plug-in form for rapid replacement, thus eliminating

most field nnaintenance. Solid state circuitry has been employed in

all power sources, and each section of the receiver provided with its

individual source. As mentioned earlier, gain-stable techniques have

been used wherever possible in the electronics to provide reliable and

stable operation over long periods of time. These features may be

seen more in detail in the following sections of this paper.

Another important feature of the receiver should also be described, which is a modified mode of operation. In locations where

the received signal is fairly strong and not subject to rapid fading,

the local oscillator klystron nnay be phase-locked directly to the received signal, providing even a higher degree of stability. In this

mode, the AFC loop may be fed directly from the signal mixer crystal,

with the klystron retuned to 9110. 1 Mc . Thus the first IF becon^es a

10. Mc beat, and it is necessary to retune slightly the first IF stages.
-9-

The freq. /phase loop operates as described above, and a second local

oscillator of 10. 1 Mc is substituted for the 10 Mc reference oscillator

in this capacity. The second local oscillator need only have a stability

6

on the order of one part in 10 or better. An oscillator having a sta7

bility of approximately 1 part in 10 has been provided for this application.

B. Complete Receiver

Figures 2 and 3 are photographs of the completed receiver, showing the front panel view and the rear view with the rack door open.

C. SHF Input Section

This unit contains all of the WR-90 waveguide components including the input variable attenuator, the gaseous noise source, a 10 db

directional coupler for inserting the local oscillator power, a variable attenuator for adjusting the local oscillator power level, and a tunable crystal mount for housing the signal mixer crystal. This unit

also contains the servo motor, induction generator, limit switches,

the low-pass recording potentiometer, and the logarithmic compensation potentiometer. A schematic diagram of the unit is shown in

Figure 4.

A variable speed mechanical drive assenably is used in this section

to provide the following gear reduction ratios between the servo motor

and the calibrated attenuator shaft: 1 : 1, 2 : 1, 5 : 1, 10 : 1, 20 : 1,

50 : 1, 100 : 1, 200 : 1, 500 : 1, and 1000 : 1. The gear reduction

ratio is used to establish the time response of the low-pass servo

loop, and is selected by a turret control knob on the variable speed

assembly box. The recording and compensation potentiometers,

limit switch assembly, and the input attenuator are all geared to the

output of this unit by the ratios indicated in Figure 4.

•H
-10-

Figures 5 and 6 are photographs of the SHF section in which all

of the components mentioned above are visible. The micrometer dial

of the input attenuator and the control for the local oscillator power

level attenuator have been provided on the front panel as seen in

Figure 5. Also visible in this figure is the gaseous noise tube at the

upper left, with its power cable attached. Figure 6 is a rear view

of the Tonit, with a clear picture of the cam drive mechanisnn on the

input attenuator. The variable speed drive unit is partially visible

at the lower center of the photo.

Figure 7 is a plot of the input attenuator characteristics.

D. Calibration Unit

The calibration unit contains the gaseous noise source power

supply, and the timing and control circuitry used in the automatic

calibration cycle. In manual position, the timing and control unit

is bypassed, and the noise source may be turned on manually for any

period. In the automatic setting of the control switch (Figure 8), the

unit is controlled by the limit switch in the SHF unit. When the limit

switch is closed due to loss of received signal, a time delay relay is

energized. After approximately 30 seconds the timing and control

unit is energized and first turns on the filament power to the noise

source. During a short period, the receiver is thus reading out its

own noise level. The noise source plate supply is then turned on,

and the new noise level recorded for a short period. The timer then

extinguishes the noise source, and the receiver again records its own

noise level until the transmitter signal is restored. The length of

each portion of this cycle may be varied by cam adjustnaents in the

timing section.
-11-

Figure 8 is a photograph and Figure 9 is a schematic diagram of

the calibration unit. The meter in Figure 8 monitors the noise source

tube current, and the controls are for coarse and fine adjustment of

this current.

E. First Local Oscillator and Freq. /Phase Control System

The first local oscillator of the receiver is a type VA 203 B reflex

klystron oscillator operating in a closed loop frequency control system.

The schematic diagram of this unit is shown in Figure 10. Figure 11

is a photograph of the unit with the top shield plate removed. The

klystron is mounted within the shielded enclosure, and a small centrifugal blower provides a cooling air stream directly on the tube. All of

the essential controls of this unit are available on the front panel, including the mechanical tuning control, the attenuator control for adjusting the power level to the mixer crystal, and the direct reading reactive type frequency meter. A crystal and 0-1 ma panel meter are

provided as a power monitor for the klystron, as well as an indicator

in conjunction with the frequency meter. The crystal mount at the

lower left of Figure 11 houses the harmonic generating crystal in the

system. The mixer crystal is mounted at the opposite end of the same

directional coupler. Both are type 1N23B crystals.

5/ The harmonic generating crystal— is driven from the output of the

100 Mc multiplier shown in Figure 12. This photograph shows the

multiplier strip removed from the power supply panel in the foreground. The multiplier is a balanced circtiit which multiplies the

reference oscillator frequency of 10 Mc to a push-push output of 100 Mc.

The 91st harmonic of this reference signal is generated in the harmonic crystal, supplying the final reference of 9100 Mc to the mixer

crystal. This system is shown in block diagram form in Figure 13.

The schematic diagram of the 100 Mc multiplier and its power supply

are presented in Figures 14 and 15.
-12-

The 10 Mc signal developed in the nnixer crystal is fed into the

Amplifier-Discriminator unit shown in Figure l6. The sub-chassis

assembly of the amiplifier and discriminator has been removed from

the supporting panel in this photo. A block diagram of the unit is

shown in Figure 17 and a complete schematic diagram in Figure 18.

The unit is composed of three stages of amplification followed by a limiter stage which drives the 6BN6 discriminator. The latter stage

is of the gated beam type in which the phase reference signal is

transformer coupled into the quadrature grid. The averaged d-c

output of the discriminator is applied as an error signal to the repeller of the local oscillator klystron completing the control loop. The

characteristic of the discriminator is presented in Figure 19. The

front panel meter of Figure l6 is provided to nneasure the mixer

crystal current. A vernier control of the repeller voltage is also

available, providing a small range of electronic tuning of the oscillator to initially establish a phase lock.

Figure 17 indicates an automatic reset circuit for the phaselock system. This circuit ennploys a small thyratron, normally held

cut-off by a bias developed in the amplifier -linniter stage in the

presence of normal signal. When a phase-lock is lost, the thyratron

is allowed to fire and a resonant tank in its plate circuit applies a ringing voltage of approximately 5 cps to the klystron repeller. The

ringing voltage swings the repeller voltage of the klystron across the

critical value until phase-lock is reestablished. The circuit has

proven to be very fast and reliable in reset action. Also in Figure 17, a cathode ray tube m.onitor is noted. The monitor is provided so that

a continuous visual 1 : 1 Lissajous pattern is presented on the scope

to observe the phase lock. The Phase Monitor uses a 2" cathode
-13-

ray tube with single stage vertical and horizontal amplifier circuits.

A photograph of the unit is shown in Figure 20 and the schennatic

diagram in Figure 21.

Power for both the Amplifier Discriminator unit and the local

oscillator klystron is derived from the Klystron Power Supply of

Figure 22. A schematic diagram of the supply is presented in

Figure 23.

F. IF Amplifier

The IF Amplifier strip shown photographically in Figure 24 is

composed of five modular plug-in amplifier stages, a crystal filter

coupling network, and a precision step attenuator. The first two

plug-in stages comprise the first IF amplifier at a frequency of 9. 9 Mc,

The first stage is a conventional cascode input amplifier followed by

a low impedance cathode follower output stage. Negative feedback is

used on the input cascode stage for gain stability. The overall gain

of this stage is 32 db with a bandwidth of approximately 100 kc. A

photograph of this plug-in is shown in Figure 25. The construction is

typical for all of the plug-in sections of the IF amplifier as well as

other sections of the receiver. Miniature coaxial cable and fittings

have been used to interconnect the sections of the amplifier strip.

The input and output coaxial jacks are visable in Figure 25. A

schematic diagram of the cascode amplifier is given in Figure 26.

The grid of the input stage is returned to a front panel meter of the

strip to monitor the signal mixer crystal current. The second plugin consists of a tuned plate pentode amplifier stage and a cathode

follower with an output impedance of 5 ohms. The latter is to match

the input impedance of a strip attenuator provided between this stage

and the input to the second mixer. A schematic diagram of the amplifier and cathode follower is shown in Figure 27. The gain of this
-14-

stage is approximately 1 db with a 3 db bandwidth of 100 kc. The

input of the third plug-in assembly is a pentagrid converter stage

used as a second mixer in a feedback-couple circuit at 100 kc. This

circuit and the following gain-stable amplifier at 100 kc, which is

also a feedback-couple, compose the main second IF amplifier. Their

schematic diagrams are shown in Figures 28 and 29 respectively.

Typical construction of each couple may be seen from the photograph

of the gain- stable amplifier in Figure 3 0.

Both of these amplifier- couples have been designed with approximately 3 db of negative feedback to provide excellent gain- stable

characteristics which are shown in Figures 31 and 32. Figure 31 presents the gain stability of the two couples in cascade vs. the plate

supply voltage, and Figure 32 the same parameter plotted against

the filament supply voltage. These results are not only representative

of the stability expected with operating voltage variations, but also

serve to indicate somewhat the performance of the amplifiers over

long periods of time with gradual changes in g of the vacuum tubes.

It is for this reason that gain-stable features are important.

Care must be taken in tuning of these couple stages in order to

prevent regeneration. As seen in the schematic and in Figure 30, a small toggle switch has been provided to open the feedback path. The

tuning procedure is to peak each stage independently with the feedback

path open, then close the feedback switch and retune only the output

stage. The coils of the amplifiers are powdered-iron cup-core assemblies with high Q characteristics. The entire assembly is housed

in a silver plated cylinder, with a top mounting tuning slug.

Figure 33 is a plot of the frequency response for the 100 kc Gain

Stable Amplifier- Couple and Figure 34 is the response for both the

mixer-couple and the amplifier- couple in cascade. Figure 34 indicates
-15-

a slight degree of non-symmetry after following the tuning procedure

outlined above. If this degree is too severe, it may be corrected by

slightly adjusting the tuning of the input stages of each couple.

The final unit of the IF strip is a pentode amplifier and a cathode

follower stage, designed to match the input and output impedances of

a crystal filter coupling network. This filter establishes the narrow

bandw^idth of the receiver when inserted. It may be removed from

the circuit with a front panel switch seen in Figure 24. When removed

the operating bandwidth is established by the amplifier -couples with

the characteristic of Figure 34. Attenuation characteristics of the

crystal filter are shown in Figure 35. The response is seen to be

flat within l/Z db over a bandwidth of approximately 20 cps. The

insertion loss of the network is approximately 4 db.

The cathode follower of the final unit provides a low impedance

output jack for the 2nd IF, as well as a driver for a crystal diode

detector circuit. The schematic diagram of this stage is shown in

Figure 36. This circuit is capable of driving directly either a 1 ma

or 5 ma strip chart recorder, or a variety of high impedance voltage

recorders when desired. In normal operation, the detector output is

fed to the Servo Control Amplifier, and the IF output is used to drive

the High-Pass Recording Amplifier discussed in a later section.

Measurements of the overall gain stability and response of the

complete IF Amplifier have been made, and the results presented in

Figures 37 and 38. A schematic wiring diagram of the assembly

chassis is shown in Figure 39 and the RF cabling diagram in Figure

40.

A separate power supply is provided for the IF strip which is shown

in Figure 41. A plug-in voltage regulator is used in this supply, and its

schematic diagram shown in Figure 42. The schematic of the supply is

Figure 43

.
-16-

G. Servo Amplifier and Power Supply

The Servo Amplifier and Power Supply, used to drive the input

variable attenuator of the receiver is shown in Figure 44. The servo

amplifier sub-assembly is shown removed from the power supply

panel in this figure. The schematic diagram of the amplifier plug-in

is shown in Figure 45. A miniature 60 cps synchronous -chopper input

network is used to compare differentially the output level of the detector and a stable reference voltage. The reference voltage is supplied by a mercury cell housed in a panel nnounted receptacle on the

power supply panel. The reference level is adjustable by a precision

potentiometer control, also mounted on the panel. Both of these components may be seen in Figure 44. Internal as well as induction or

rate generator feedback is applied to the amplifier for stability. The

output circuit is a pair of power pentodes in push-pull, transformer

coupled directly to the control phase of the servo motor. A shunting

diode is used to balance the input to the amplifier whenever received

signal is lost and the limit switch closes. When signal returns the

diode balance is upset, and the amplifier returns to normal operation.

A schematic diagram of the power supply section is shown in Figure

46. Regulated plate voltages are used on all of the preamplifier stages,

and an unregulated supply for the final power amplifier.

A typical frequency response for the low-pass servo driven attenuator loop is shown in Figure 47, with the selectable gear reduction

ratio as a parameter. This response was measured with a 10 db

peak-to-peak variation in the received signal at the indicated frequencies. Dotted portions of the curves represent extrapolated values

and not direct measurement. This set of curves may be used as a universal response for any peak-to-peak value of input signal variation by applying the following nnultiplier to the frequency scale:
-17-

Multiplier (M) = -rwhere A is the peak-to-peak signal variation in db. It should be noted

from Figure 47 that the response is almost inversely proportional to

the gear reduction ratio except at the lower ratios (higher speed) where

the torque load of the driven components is more pronounced.

H. High-Pass Recording Amplifier

The High-Pass Recording Amplifier is used to provide a low

impedance output, either balanced or unbalanced, for compensating

and recording the high frequency data spectrum, above the cut-off

of the servo loop. A photograph of the unit is shown in Figure 48

and the schematic diagrams in Figures 49 and 5 0. The amplifier is

a plug-in sub assembly like those shown in previous sections.

Compensation of the gain variations due to the dynamic action of

the servo loop attenuator is accomplished by passing the amplifier

output thru a logarithmic potentiometer, driven by the servo motor.

The potentiometer thus acts as a variable attenuator in the amplifier

output. The attenuation characteristic vs. shaft rotation of this

potentiometer is made equal to that of the X-band attenuator (Figure 7)

but with negative slope. The d-c level of the detector is balanced out

at the input to the amplifier by a positive potential developed in a voltage divider circuit. A front panel nnounted precision potentiometer

is provided for this balance control. A smaller potentiometer is also

provided to balance the circuit against the receiver noise level for

calibration purposes.

The amplifier consists of a pair of cathode follower input stages,

a phase inverter, and a push-pull cathode follower output stage. The

circuit is isolated above chassis ground, allowing either balanced or
-18-

unbalanced output selected by a front panel switch. A miniature balance control potentiometer is provided in the amplifier as a screw

driver adjustment. One position of a panel mounted operational

selector switch marked BAL opens the input to the amplifier for

adjusting and checking the d-c balance. A voltmeter in the output

circuit with a selected range of 10-0-10 volts or 1-0-1 volt is used

for nnaking all balance adjustments. The operational selector switch

is also used to insert or remove a filter capacitor at the input of the

amplifier providing either a narrow bandwidth (2 cps) or a full wide

bandwidth for the recorded spectrum up to 50 cps.

The output of this unit is used in nnaking the automatic noise

source calibration check for the entire receiver. For this purpose,

a relay circuit is provided to perform various switching functions

when the receiver is calibrated. The relay is energized when the

servo limit switch is closed. First, the relay switches the input

balance reference from the signal reference potentiometer to the

noise reference potentiometer. This balances the recording amplifier to the receiver noise level. Secondly, the compensating potentiometer is switched out of the circuit allowing the stage to operate with

maximum sensitivity. The relay also inserts the filter capacitor mentioned above, filtering out all of the high-frequency noise components.

When the X-band noise source is turned on during calibration, the

recorded output is then a measure of the d-c unbalance in the recording amplifier due to the increased noise output of the receiver. In this

fashion, the calibration check point is a function of the entire receiver

response, all the way from the X-band input to the recording terminals.

A direct reading of the receiver noise figure, as well as a connparative check on the calibrated output record is automatically provided

and recorded.
-19-

I. Second Local Oscillator

The Second Local Oscillator is a 10. 1 Mc crystal controlled

oscillator for use when the receiver is operated in the modified nnode

described previously in paragraph A of this section. A photograph of

this unit is shown in Figure 51. The oscillator is constructed in a plug-in assembly, and a crystal oven is used to house the crystal

unit. The voltage regulator in the power supply is the same unit as

shown in Figure 42. Further details on the oscillator are omitted,

since it is not considered an integral part of the receiver design.

6

Any oscillator with a stability of 1 part in 10 or better, with an output of 2 to 3 V rms is suitable.

CONCLUSIONS

The receiver described, with its very narrow bandwidth and

high sensitivity, lends itself to a great variety of tropospheric circuits

to be studied in the SHF band. Tests of the system have been performed over a short line-of-sight path between the Boulder Laboratories and a field site at Table Mesa, northeast of Boulder, Colorado, a distance of approximately 10 miles. A target transmitter using a VA 203 3 klystron oscillator was constructed for the tests. The test

transmitter equipment is shown in Figures 52 and 53.

Operational transmitting equipment is under development by the

Radio Propagation Engineering Division and will be described in a subsequent report. A CW klystron of the fixed tuned type, controlled

in a similar AFC loop as described for the receiver local oscillator, is

being employed. This tube has a nominal power rating of 100 watts,

with good efficiency and requiring light water cooling. A photograph

of the tube is presented in Figure 54. A high power transmitter using
-20-

a recently developed klystron capable of 2 kw CW power output is also

under development. These facilities will provide practically unlimited

horizons for propagation studies at the SHF frequencies.

It should also be mentioned here that the receiver described may

be very useful in the lower VHF and UHF propagation studies. The

practicality of designing input heads at these frequencies using parametric type up-converter amplifiers is being studied. In this way all

of the features of the X-band receiver could be utilized at the lower

frequencies

.

The receiver as described has been used as a prototype for construction of three additional receivers, and several experimental programs for their use are being planned.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the work of D. V. Glen, L. J.

Maloney, J. A. Sykes and Boyce Talbutt who assisted in the construction and testing of the receiver system, and the helpful suggestions

of M. J. Vetter and D. M. Waters. They also wish to acknowledge the

partial support of this project by the U.S. Army Signal Corps under

Contract MIPR R-56- 0027 -SC-91

.
.21-

REFERENCES

1. Hubbard, R.W., and J.V. Cateora, An X-band field intensity recording receiver with extremely narrow bandwidth. Presented

at the URSI-IRE Fall Joint Meeting, Oct. ZO, 1959, San Diego,

Calif.

2. Peter, M. , andM.W.P. Strandberg, Phase stabilization of microwave oscillators, Proc. IRE 43, 869-873 (July 1955).

3. Thompson, M.C., Jr., and M. J. Vetter, Single path phase measuring system for three-centimeter radio waves, RSI 29, No, 2,

(February 1958)

4. Montgomery, G. Franklin, Intermediate -frequency gain stabilization

with inverse feedback, Proc. IRE 38 , 662-667 (June 1950).

5. Thompson, M.C. Jr., and J. V. Cateora, High-order harmonics for

X-band oscillator stabilization, RSI 28 (August 1957).
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DEPARTMENT OF COMMERCE

Luiher H. Hodges, Secretary

lONAL BUREAU OF STANDARDS

A. V. Astin. Director THE NATIONAL BUREAU OF STANDARDS

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