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VHF AND UHF POWER GENERATORS

FOR

RF INSTRUMENTATION

BY

A. H. MORGAN AND P. A. HUDSON

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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, devices,

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,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's unique competence is required. The scope of activities is suggested by the listing of

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

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers: the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series,

Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in \ BS Circular 160. Publications of the National Bureau of Standards ($1.25) and its Supplement ($1.50). available from the Superintendent

of Documents, Government Printing Office, Washington 25, 13. C.
NATIONAL BUREAU OF STANDARDS

technical ^Tiote

77

September I960

VHF AND UHF POWER GENERATORS

FOR RF INSTRUMENTATION

by

A. H. Morgan and P. A. Hudson

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

1 TABLE OF CONTENTS

VHF AND UHF POWER GENERATORS

FOR RF INSTRUMENTATION

by

A. H. Morgan and P. A. Hudson

Page

ABSTRACT Hi

1. INTRODUCTION 1

1. 1 Purpose 1

1. 2 Design Specifications 1

2. EXCITER DESIGN 2

2. 1 General Description 3

2. 2 Oscillator Section 3

2. 3 Buffer Stage 3

2.4 Intermediate Amplifiers 4

2. 5 Exciter Output Stage 4

3. 100 WATT POWER AMPLIFIER DESIGN 5

3. 1 100 Mc and 200 Mc Circuits 5

3. 2 300 Mc and 400 Mc Circuits 5

4. AUTOMATIC LEVEL CONTROL CIRCUIT 6

5. PERFORMANCE CHARACTERISTICS 7

5. 1 Frequency Stability 7

5. 2 Output Level Stability 8

5. 3 Spurious Outputs 8

5. 4 Residual Modulation 8

5. 5 Shielding 8

6. ACKNOWLEDGMENT 9

11

Table of Contents (Continued)

7. FIGURES 10

7,

8,

Generator Circuit Diagram (400 Mc)

Bottom View of Chassis

100 Mc and 200 Mc Power Amplifier Circuit

Plate Cavity for 4X250B Tube 300 Mc and 400 Mc

Automatic Level Control (AJLC) Circuit

Block Diagram, Typical Connection Procedure When

Using Automatic Level Control

Generator Tube Layout

Front View of Generator With Shield in Place

Ill

ABSTRACT

The work described in this report was undertaken to provide

certain projects in the Radio Standards Division with a series of fixed

frequency rf generators having good stability with respect to both

frequency and power output level and continuously adjustable in power

output from 1 to 100 watts. Prior to the completion of the generators

the work of some of the standards projects was seriously hampered

because the rf generators available were lacking in either sufficient

power output, frequency stability, or power output stability. For

example, bolometer bridges used for rf voltage and power measurements are sensitive to power input variations as small as a few parts

4 3

in 10 . Power stability of at least one part in 10 is considered an

absolute necessity to obtain measurement accuracies of from 0. 5% to

1%.

1 VHF AND UHF POWER GENERATORS

FOR RF INSTRUMENTATION

A. H. Morgan and P. A. Hudson

1. INTRODUCTION

1. 1 Purpose

The work described in this report was undertaken to provide

certain projects in the Radio Standards Division with a series of

fixed frequency rf generators having good stability with respect to

both frequency and power output level and continuously adjusted in

power output from 1 to 100 watts. Prior to the completion of the

generators the work of some of the standards projects was seriously

hampered because the rf generators available were lacking in either

sufficient power output, frequency stability, or power output stability.

For example, bolometer bridges used for rf voltage and power measurements are sensitive to power input level variations as small as a

few parts in 10 . Power stability of at least one part in 10 is considered an absolute necessity to obtain measurement accuracies of

from 0. 5% to 1%. Good power stability is likewise desirable in the

use of rf calorimetric wattmeters andin the substitution method of

attenuation measurement.

Frequency stability is required in many measurements because

tuned elements, such as quarter-wave stubs and narrow band detectors, may be part of the measuring system.

A survey of commercially available generators was made but

nothing was found at that time (1955) which would satisfy all the requirements. Hence it was decided to design and construct a set of

generators in the laboratory.

1. 2. Design Specifications

A set of design Specifications was arrived at after consultation

with the project leaders in need of better rf generators. The adopted

specifications were as follows;

Frequencies: 100, Z00, 300, 400, Mc

Frequency Stability: Within 0. 0005% after 15 minute

warmup

Power Output:

Power Output Stability:

Residual AM:

Harmonic Content:

Shielding:

Output Impedance:

Duty:

Size:

Continuously adjustable from 1 watt

to 100 watts into 50 ohm load

± 0. 1% per hour

Less than 0. 05% of output voltage

60 db below fundamental with

single filter

At least 80 db when generator is

enclosed in a shielded rack

50 ± 5 ohms

Continuous operation at full power

output

Suitable for standard rack mounting

Power output stability and frequency stability were considered

of major importance and design features were chosen which enhanced

these qualities.

2. EXCITER DESIGN

2. 1 General Description

The arrangement of the exciter part of each generator, with the

exception of the 100 Mc unit, consisted of an oscillator -double r, a

buffer stage, an intermediate amplifier and a combined tripler amplifier for the output. In the 100 Mc unit the output was from a straight

power amplifier. High frequency crystal units were employed to

minimize the number of multiplication stages and to reduce the number

of sidebands in the generator output. For example, in the 400 Mc

generator the crystal frequency was 66. 666 Mc. Such crystals were

readily available from commercial sources with the desired tolerance.

The choice of tubes in the output stage was determined by

available power from the exciter. Each stage of the exciter will be

described in the following paragraphs. A circuit diagram for a typical

exciter (400 Mc) is shown in Fig. 1.

3

2. 2 Oscillator Section

The oscillator is a modified Butler type and was chosen because

of its simplicity, good frequency stability, and reliability. In addition,

the circuit is not critical with respect to design, adjustment, or interchange of tubes (1).

A circuit diagram of the two-stage oscillator section is shown

in Fig. 1 a. No claim is made to originality of the design used because numerous modifications of the basic Butler circuit have been

made by others including, in all probability, the one described here.

The first stage of the oscillator includes a plate tank circuit

tuned to the crystal frequency fc while the plate tank of the second

stage is tuned to Z f c> Thus, frequency doubling is accomplished.

The plate tank in the first stage, which is a modification of the basic

Butler circuit, serves to increase the output of the oscillator. The

100 Mc generators employ 50 Mc crystals and, with the doubling, no

further frequency multiplication was necessary. In the 200. 300 and

400 Mc generators the crystal frequency is 1/6 of the exciter output

frequency with doubling and tripling required. This arrangement was

used because the exciter output tube, type 5894, has a high input

capacity and it is difficult to effectively employ input frequencies above

160 Mc.

2. 3 Buffer Stage

The purpose of the buffer stage as shown in Fig. 1 b is, of

course, to isolate the oscillator from the amplifier stages and prevent

frequency variations due to varying amplifier loading conditions. The

design and construction of the stage is straightforward, as shown in

Fig. 1 b. The oscillator output is coupled to the 6AH6 input by a

10fX(xf capacitor. With inductive coupling on the input to the 6AH6

higher output may be obtained, but under those conditions there was

a strong tendency to oscillate. The plate tank circuit of the buffer is

tuned to the oscillator output frequency with inductive coupling to the

following amplifier stage.

(1) "Handbook of Piezolectric Crystals for Radio Equipment Designers",

John P. Buchanan, Philco Corporation, Wright Air Development

Center, Technical Report 56-156, October 1956. For sale by

Office of Technical Services, U. S. Department of Commerce,

Washington 25. D. C.

4

2.4 Intermediate Amplifiers

The driving power required for the 5894 exciter output stage is

approximately 3 watts when used as a straight push-pull amplifier at

100 Mc. When used as a tripler-amplifier, to frequencies of 200, 300

and 400 Mc, the driving power must be at least 6 watts. Thus, considerable power amplification (approximately 18 db) must be provided

between the buffer and input to the type 5894 tubes. This gain is obtained with two stages utilizing the type 6360 twin- tetrode tube operating push-pull, class C. As shown in the schematic of Figure 1 c, butterfly capacitors are used on both input and output to balance the

tank circuits to ground.

In order to prevent oscillation in the intermediate amplifiers a

grounded metal shield is placed across the bottom of each tube socket

so that the grid tank is shielded from the plate tank. Such an arrangement is shown in Figure 2. The 6360 tubes have tube shields of the

heat dissipating type and adequate ventilation also is provided.

2. 5 Exciter Output Stage

The exciter output stage is operated class C, with grid leak bias,

as shown in Fig. 1 d. The value of the grid leak resistor is fairly

critical as it determines the proper conduction angle. This is particularly true for the 300 Mc and 400 Mc generators. Operating voltages

are given in the standard tube manuals available from the tube manufacturer. The power output is 16 watts for the 400 Mc exciter. At

lower frequencies the output powers are as follows: 300 Mc - 22 watts,

200 Mc - 30 watts, 100 Mc - 40 watts. The exciter output is brought

out to a type N jack on the front panel and is connected to the 100 watt

power amplifier input jack by a short length of cable. It may be used

independently of the power amplifier when desired.

The power output is controlled either manually or by the automatic level control circuit (this is described in Paragraph 4. 0).

Selection of the method of control is made by means of the triple-pole,

double-throw switch which connects the automatic level control circuit

to the screen grid of either the 5894 exciter output or the 4X250 B

power amplifier. With the switch in the "PA" position the exciter

output is controlled by the rheostat (R ?n in Fig. 1).

It should be noted that lumped -constant tank circuits are used at

all frequencies and that all rf coils are wound with #10 A. W. G. wire

for good efficiency and stability,

3. 100 WATT POWER AMPLIFIER DESIGN

3. 1 100 Mc and ZOO Mc Circuits

As was mentioned earlier, the 4X250 B tube is used as the

power amplifier in all of the generators described here. This tube is

a small radial -beam tetrode rated at 250 watts plate dissipation at

frequencies up to 400 Mc. With 900 volts on the plate a power output

of 120 watts into a 50 ohm load is easily obtainable with class C operation. Link coupling is used on the input to the tube with grid-leak bias

of 90 volts as shown in Fig. 3. At 100 and 200 Mc approximately 8

watts of driving power are required.

The plate tank circuits at these frequencies are lumped -constant

type with coils made of relatively large diameter wire. The circuits

are series resonant to provide a relatively low impedance for the tube

plate in accordance with the tube manufacturers' recommendation. In

general, better coupling can be achieved on both the input and output if

the coils are mounted coaxially rather than side -by- side. A perforated

shield enclosure surrounding the tube, plate tank, and output link

improved the performance as well as reduced unwanted radiation.

The tube manufacturers' recommendations for forced-air cooling

of the tube were followed.

3. 2 300 Mc and 400 Mc Circuits

At frequencies of 300 Mc and above, the use of lumped-constant

tank circuits and output loops with the 4X250 B appeared to be marginal. That is, with 900 volts on the plate of the tube, an output of 100

watts could only be obtained with the tube operating at maximum

allowable plate dissipation. Since this type of operation shortens tube

life considerably it was decided to use a quarter -wave cavity for the

plate tank. Sectional views of the 400 Mc cavity are shown in Fig. 4.

The length of the cavity was made 0.6 X -j, at the operating frequency,

in order to better match the output impedance of the tube and to allow

for capacitive tuning. With two capacitors the 400 Mc cavity tunes

from 340 Mc to 430 Mc while the 300 Mc cavity tunes from 270 Mc to

330 Mc. Construction of the cavity is relatively simple and can be

done in any good machine shop. Contact to the plate of the 4X250 B is

made by a ring of beryllium -copper fingers soldered to the inner

cylinder of the cavity.

Using plate cavities for the 4X250 B, load power outputs of 120

watts were obtained with the tube operating at 70% of maximum plate

dissipation. In addition the cavity helps suppress the sub-harmonics

normally present in the output of the tripler -exciter.

4. AUTOMATIC LEVEL CONTROL CIRCUIT

Measurements and tests on the first prototype rf generator

(100 Mc) indicated that the specification on output level stability could

not be met without extra circuitry for automatic control. This result

was at least partly anticipated in view of some prior tests made on

commercial rf power generators. It would thus appear that, in general,

equipment of this type does not have the inherent output level stability

necessary for instrumentation use.

The automatic level control circuit for the generator described

here was designed to control the dc voltage to the screen of the output

stage. The control grid of the tube could not be used for this purpose

due to the fact that the stage is operated class C and thus the grids are

driven to saturation. It was also desired that the circuit not include

semiconductor diode rectifiers and reference batteries because of the

inherent temperature and peculiar instability of these devices. In the

final circuit, shown in Figure 5, a temperature -limited diode is used

as the sensing and reference device. Since the diode is temperature

limited its plate current is directly proportional to the filament

emission current. This emission obeys Richardson's equation and is

a very rapidly varying function of filament temperature. That is,

doubling the temperature may increase the emission by a factor of

10 (2). The filament temperature is, of course, proportional to power

input. Even though the filament impedance is small while the plate

load impedance can be quite large (of the order of 10 ohm) it is possible

to achieve a voltage gain. In this circuit the gain is approximately 2000.

The diode filament is heated directly by a portion of the rf energy from

the generator.

Variations in output power cause variations in the plate voltage

of the diode which are 180° out of phase with those of the rf output.

This phase difference is maintained through the output of the cathode

follower V3. The stage V?

is a cathode coupled dc amplifier with a

(2) Karl R. Spangenberg, "Vacuum Tubes", McGraw-Hill Book Co. , 1948,

gain of 10. The VR tube V4 provides a constant voltage drop between

V3 and the screen of the generator output stage. More than one VR

tube may be used, when necessary, to provide the proper screen voltage for the generator output tube. In these generators the complete

circuit is mounted on the same chassis as the other components and

tubes of the generator. A type N jack is provided on the front panel to

feed power to the diode filament. The capacitor C2 may be either

fixed or variable and serves to tune out the inductance of the diode

filament. When the diode filament emits sufficient current to cause

the proper plate current to flow then the control circuit will oppose

any further variations in the filament temperature of the diode. Thus

there was no need for an external reference voltage in the circuit.

Adjustment of the generator, with the automatic level control,

is as follows: Connect the generator to a load through a tee. On one

leg of the tee, connect a variable capacitor in series with the center

conductor. The capacitor may be mounted inside a small metal box

with coaxial connectors on both ends. Small piston type capacitors

have been found quite satisfactory. The output end of the capacitor

box is then connected back to the input to the temperature -limited

diode filament.

The presence of the feed-back capacitor in parallel with the load

introduces some mismatch and it is necessary to tune the generator

plate -tank and output circuits for minimum dc plate current. A block

diagram of a typical setup is shown in Figure 6.

It should be noted that when using a capacitor for feed-back to

the diode the load sees a nearly constant -voltage -generator at the tee

where the capacitor is connected. Output impedance of the power

generator is approximately 0. 1 ohm. With inductive coupling the

generator becomes a nearly constant -current- source and, with a

directional coupler, a source of nearly constant incident -power.

Adjustrre nt of the output level with the latter two devices is, however,

not as convenient as with the variable capacitor.

5. PERFORMANCE CHARACTERISTICS

5. 1 Frequency Stability

The output frequency stability of the generator is essentially

that of the oscillator. The crystals used have a turnover point of 75°C

and are operated in relatively simple 75°C ovens. After warmup the

stability is better than 5 parts in 10° per hour and 10 parts in 10" per

day.

8

5. 2 Output Level Stability

Using the automatic level control with a typical generator, the

output voltage variations did not exceed 0. 5% per hour. For periods

of 10 minutes a maximum variation of the order of 0. 05 to 0. 1% may

be expected. These figures are from 10 to 20 times better than were

found for the same generators without the automatic level control.

5. 3 Spurious Outputs

The harmonic content of the output voltage varied with the frequency of the generator and with generators at the same frequency.

In all cases the harmonic voltages were at least 30 db down from the

fundamentals. Where necessary, commercially available filters have

been used to reduce spurious outputs to at least 60 db below the fundamental.

5.4 Residual Modulation

Residual amplitude modulation in all the generators was found to

be mostly at the 60-cycle and 120-cycle frequencies. At these frequencies the residual amplitude modulation was not greater than 0. 1%.

Some trouble was experienced with low frequency modulation due

to the vibration of the cooling fans when they were mounted directly on

the chassis. This was overcome by mounting the fans remotely and

connecting to the chassis with flexible tubing.

5. 5 Shielding

As was mentioned earlier each tube and tank circuit extending

above the chassis was shielded individually by a tube shield or a perforated copper enclosure. In addition the entire generator was enclosed in a copper box with screened openings for forced air cooling.

All wires entering the box were through low pass electrical filters.

Finallys the generators were mounted inside special racks which had

close-fitting doors and filters for the input power. In Figures 7 and

8 photographs are shown of the shielding, filter box and tube layout.

9

6. ACKNOWLEDGMENTS

The authors are indebted to Mr. C. M. Allred for several

suggestions including the use of the temperature -limited diode in the

automatic level control circuit and to the following members of the

staff for contributing to the development of the generators:

Mr. J. H. Shoaf, Mr. D. H- Russell, and Mr. L. E. Mann

10

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DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

ATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Joulder, Colo., is suggested in the following listing of the divisions and sections engaged in technical work. In

sneral, each section carries out specialized research, development, and engineering in the field indicated by its

;itle. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover.

Electrochemistry. Electrical Instruments. Magnetic Measurements.

Photographic Research. Length. Engineering

Molecular Kinetics. Free

WASHINGTON, D.C. ELECTRICITY. Resistance and Reactance.

Dielectrics. METROLOGY. Photometry and Colorimetry. Refractometry.

Metrology. Mass and Scale. Volumetry and Densimetry.

HEAT. Temperature Physics. Heat Measurements. Cryogenic Physics. Rheology.

Radicals Research. Equation of State. Statistical Physics. Molecular Spectroscopy.

RADIATION PHYSICS. X-Ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological

Equipment. Nucleonic Instrumentation. Neutron Physics.

CHEMISTRY. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances. MECHANICS. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Combustion Controls. ORGANIC AND FIBROUS MATERIALS. Rubber. Textiles. Paper. Leather. Testing and Specifications.

Polymer Structure. Plastics. Dental Research.

METALLURGY. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal Physics.

MINERAL PRODUCTS. Engineering Ceramics. Glass. Refractories. Enameled Metals. Constitution and

Microstructure.

BUILDING RESEARCH. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. APPLIED MATHEMATICS. Numerical Analysis. Computation. Statistical Engineering. Mathematical

Physics.

DATA PROCESSING SYSTEMS. Components and Techniques. Digital Circuitry. Digital Systems. Analog

Systems. Applications Engineering.

Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics.

Electron Devices. Electronic Instrumentation. Mechanical

Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas

ATOMIC PHYSICS.

Atomic Physics.

INSTRUMENTATION. Engineering Electronics Instruments. Basic Instrumentation.

Office of Weights and Measures. BOULDER, COLO.

CRYOGENIC ENGINEERING

Liquefaction.

IONOSPHERE RESEARCH AND PROPAGATION. Low Frequency and Very Low Frequency Research. Ion- osphere Research. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. RADIO PROPAGATION ENGINEERING. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

RADIO STANDARDS. High frequency Electrical Standards. Radio Broadcast Service. Radio and Microwave

Materials. Atomic Frequency and Time Standards. Electronic Calibration Center. Millimeter-Wave Research.

Microwave Circuit Standards.

RADIO SYSTEMS. High Frequency and Very High Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Space Telecommunications.

UPPER ATMOSPHERE AND SPACE PHYSICS. Upper Atmosphere and Plasma Physics. Ionosphere and

Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

uscoMM-wsa.
NBS