NBSIR 80-2072

X-Cal — A Calibration System for

Electrical Measurement Devices

Used With Diagnostic X-Ray Units

Ronald H. McKnight and Robert E. Hebner, Jr.

Electrosystems Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

4 :-QC100

.056

80-2072

1980

Prepared tor; Calibration Coordination Group

Department of Defense

DoD/CCG Project 78-130
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NBSIR 80-2072

• * t X-CAL — A CALIBRATION SYSTEM FOR

ELECTRICAL MEASUREMENT DEVICES

USED WITH DIAGNOSTIC X-RAY UNITS

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Ronald H. McKnight and Robert E. Hebner, Jr.

Electrosystems Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

June 1980

Prepared for: Calibration Coordination Group

Department of Defense

DoD/CCG Project 78-130

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Table of Contents

Abstract i

Introduction 1

Specifications 2

Theory 5

System Description 21

Installation 27

Operating Instructions 31

Calibration Information 46

Maintenance and Calibration 50

Parts List 51

Circuit Diagrams 53

References 70

Acknowledgments 71
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Abstract

A high-voltage-divider calibration system (designated as X-CAL System)

was designed to calibrate the direct and alternating voltage measurement

capabilities of commercial high voltage dividers used to characterize

diagnostic x-ray units. The system includes capability for determining

the frequency response of these dividers and for the calibration of the

filament current and anode current measurement features of specific

commercial ly-available devices. The calibration system combines in a

single unit a range of capabilities which allows calibration of a device

under test to be accomplished with a minimum of set up time. This report

describes the rationale for measuring certain parameters of the high voltage

dividers. It provides the design details of the X-CAL system and certain

sections of this report serve as the operating manual for the system. The

report lists the specified accuracies and the ranges of applicable parameters

of the various subsystems, describes the five different calibrations which

can be performed using the X-CAL system, discusses techniques used in the

original calibration of the X-CAL system itself, and contains the basic

circuit diagrams for the system.
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1. Introduction

The X-CAL high voltage divider calibration system was designed to

calibrate the direct and alternating voltage measurement capabilities of

commercial high voltage dividers used to electrically characterize diagnostic

x-ray units. In addition there is capability for determining the frequency

response of these dividers. This set of measurements as applied to

dividers used to characterize diagnostic x-ray units was developed at NBS

and is a part of the calibration services of NBS. The X-CAL system was

developed for and with the support of the Calibration Coordination Group (CCG)

of the Department of Defense. It will be used to routinely calibrate

divider units used in military hospitals as part of a service-wide effort

to control radiation exposure. Because of CCG requirements, two test

functions have been included in X-CAL for the calibration of special measurement

features of one particular device, the Machlett Dynalyzer.^ These are

filament current and anode current measurements. Vlith minor X-CAL system

modification, these test functions can be used to calibrate any measurement

system offering these capabilities. The calibration system combines in a

single unit a range of capabilities which allows calibration of a device under

test (DUT) to be accomplished with a minimum of set up time.

This report is intended to give a user, or potential user, a description

of the motivation for, the operation of and the design details of the

X-CAL system. It is also intended and is written appropriately to serve as

an operating manual for the system. Section 2 lists the specified accuracies

and the ranges of applicable parameters of the various subsystems. The

motivation for the various electrical measurements is described in Section 3.

Section 4 describes the five different calibrations which can be performed
- 2

-

using the X-CAL system. System installation is described in Section 5.

Detailed operating instructions are given in Section 6. Section 7 discusses

techniques used in the original calibration of the X-CAL system itself.

Section 8 describes maintenance and recalibration procedures for the X-CAL

system. A suggested list of replacement parts is given in Section 9.

Section 10 contains the basic circuit diagrams for the system and Section 11

is a list of references.

2. Specifications

The X-CAL calibration system allows five specific calibrations

to be performed. These are listed below with statements of accuracies

and ranges of applicable parameters.

2.1 Direct Voltage Divider Ratio Measurement

Voltage Range - 0 to ±75 kV.

Accuracy of measurement - ± 1 %.

2.2 Alternating Voltage Divider Ratio Measurement

Voltage Range - determined by power supply provided by user.

Accuracy of measurement - ±1% using calibrated

standard high-voltage gas-filled capacitor.

2.3 Frequency Response of Divider Ratio

Frequency Range - fixed frequency oscillator (50,

100, 500, 1000, 5000, 10000 Hz) or variable frequency

function generator.

Accuracy of measurement - ± 1 % for ratios normalized

to dc ratio values.

2.4 Filament Current

Current Range - continuously variable from 0 to

20 amps for a 1 ohm load.
-3-

Accuracy of measurement - ±1%.

2.5 Anode Current

Current Range - selectable from 0.05 amp to 2 amp

(0.05, 0.1, 0.5, 1, 2) into a 200 ohm load.

Current Pulse Width - selectable from 0.01 s to 1 s

(0.01, 0.05, 0.1, 0.5, 1) with current pulse controller

limiting average power to 10 watts.

Accuracy of measurement - ±1%. This accuracy statement

is for the current pulses driving the 200 ohm load of a

Machlett Dynalyzer^ when the peak current value is converted

into the corresponding average current value. The table

of conversion factors is in Section 6.

The measurements described in 2.5 and 2.6 are designed specifically

for a Machlett Dynalyzer^ system and the specifications apply only to measurements on this system. The adaptation of the X-CAL system for calibration of

other units is detailed in Section 6.8.

2.6 Description of System Modules and Specific Control Functions

The following sections describe the system components which were

specially developed for the X-CAL system. Commercially available items

have been used when possible and the instruction manuals for these

items should be referred to for details of their operation.

2.7 System Control Module Switches

Power - Turns on control module and activates control

circuitry. Other calibration system modules have

independent power switches.

Test Function - Enables appropriate test circuitry and routes

test signals. Five separate test modes may be selected:

DCHV, ACHV, FREQ Res, ANODE I, and FIL I.
-4-

A/C - Selects for processing either the anode or cathode

output signal from the DUT.

Active/Passive - Differentiates between signals generated

by devices under test which contain active signal processing

components and passive devices. In the ACTIVE position,

the signal is routed to the appropriate signal processing

circuits. When the passive mode is selected, the input

signal is amplified by an amplifier, the gain of which can

be selected (by resistor replacement and trim pot adjustment)

to provide direct readout from the DUT.

Signal - INT/EXT - Determines the source of the low level

signal used in the frequency response measurements. In

the INT position, switch selectable discrete frequencies

are available. For more extensive measurements, the EXT

position allows use of a separate function generator.

Ratio Trans - HI/LO Selects either the high

frequency or low frequency sections of the ratio

transformer for use in frequency response measurements.

Frequency - Sets the frequency of the internal

oscillator to one of six values.

Reset - Resets the peak and hold detector used

in the ACHV measurement circuit.

2.8 Filament Current-Viewing-Resistor Test Module

Pwr - turns on ac supply but does not control the

ac panel meter.

AC I - Current control

AC Panel Meter - Indicates current in external circuit

directly. Unless the FIL I test mode is selected, the

meter display is blanked.
-5-

2.9 Anode-Current-Viewing-Resistor Test Unit

Pwr - Energizes main dc supply.

Peak Current - Selects nominal value of peak current

produced in a 200 ohm external load.

Pulse Duration - Selects pulse length. Peak current and

pulse duration circuitry are configured to limit average

power dissipation in a 200 ohm load to 10 W.

Peak Current Meter - Indicates peak current produced

by unit.

3. Theory

It has been estimated that exposure to medical and dental x-rays is

2

the source of 90 percent of the population dose from man-made sources.

The x-ray dose produced by an x-ray unit depends on x-ray tube construction,

the waveform of the potential difference across the tube, the current

through the tube and the duration of the voltage pulse. In practice the

operator has control of the last three parameters, and of these, the

peak potential difference across the tube is most important. There are

two different techniques used to determine the peak voltage level in

3-12

diagnostic x-ray units. One is to infer the peak voltage from a

measurement of appropriate parameters of the emitted beam of x-rays. The

other is to perform an electrical measurement of the voltage using

conventional high voltage measurement techniques. The relative advantages

of various measurement methods have been discussed and the consistency of

these measurements (under a limited range of voltage and time parameters

of pulses), when used to measure the same voltage waveform, has been

5 11 determined by other workers. ’
- 6

-

Unnecessary radiation exposure can result when the operation of I

diagnostic x-ray equipment is not well characterized. Direct measurements I

of x-ray energy spectrum and dose during each exposure are presently I

impractical. However, one means of controlling radiation exposure is to insure I

the accuracy of the measurement of electrical quantities associated with diagnostic!

x-ray units. I

The X-CAL unit provides electrical calibration of devices which perform

electrical measurements on diagnostic x-ray units. Because high voltage

measurements are made in a large number of applications outside the

field of diagnostic radiology, the application of standard calibration

techniques to x-ray unit high voltage dividers insures the consistency

of the calibration of the measurement apparatus to within the specified

uncertainty of the measurements among users of high voltage apparatus.

A basic theoretical model , which has been used to describe pulse | dividers, represents the divider as a matrix relating the input current

j and voltage to the output current and voltage: !

''in W It

1 > A -|2

• Vg (U)

lin W A21 (^) A22 11 0

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From Eq. 1, two parameters of the divider are defined. One is the voltage

transfer function, H (co), which is the ratio of the output voltage to the |

input voltage when the output current is zero, i.e., ^

I H (uj) = V. (oj)/V. (oj) = 1/A.n (o)). (2) t

The other is the divider ratio, N, which is defined as 1

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-7-

N = ( 0 ) = lim A^i (03).

OJr>0

( 3 )

To apply these equations to the dividers under test, it is useful

to consider the circuit in Fig. 3.1. The model is not adequate to predict

the exact shape of the frequency response of all the dividers tested, but

from this model, many of the significant properties of the device can be

inferred. For example, V^/V is given by

v„/v

1

(c + Ch) + j 0j(-^

H H

(^ + ^) + (C +

(4)

From 3, the divider ratio is

N = |-= lim A,, (uj) = (R + R|,)/R . (5)

0 03-K)

g This equation is the basis for the practice of calibrating this type of

pulse divider by determining the resistance ratio.

The expression derived from the equivalent circuit indicates that

A-|-| (oj) is a complex number. In general, all of the matrix elements A., (co)

are complex. Measurement systems are designed so that the ratio of the

input to the output voltage can be represented as a single real number.
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FIGURE 3.

R

O

V

1 Approximate lumped parameter equivalent circuit for a high voltage divider.
-9-

The divider ratio, as defined here, has that property and is the quantity

which has traditionally been used in the description of dividers used

to measure the voltage in diagnostic x-ray units. For the divider

ratio to be a valid quantity, however, it is necessary that (o)) |

not differ significantly from N over the frequency range of interest.

It can be seen from Eq. 4 that if

RC - C|^,

i.e., if the divider is compensated, then A^-j (oo) is frequency independent

and the divider ratio will be real. For a variety of reasons, e.g., inadequacy

of the model by not accounting for stray capacitances, construction errors, etc.,

the compensation in actual dividers is not exact. All dividers, therefore, have

a quadrature component large enough to be measured.

Using the X-CAL system, the divider ratio [Eq. (3)] is measured by

applying high voltages between 25 kV and 75 kV. This corresponds

to x-ray tube voltages of 50 kV to 150 kV. The test is carried out with

direct voltage so that it is also necessary to demonstrate that the divider

has the appropriate frequency response.
- 10 -

The frequency dependence of the ratio can be determined using the

circuit shown in Fig. 3.2. The inductive voltage divider shown is a seven

decade device, the division ratio of which is characterized by an

uncertainty of order one part per million of the full scale reading. It

4

should be emphasized that to measure a voltage ratio of 10 :l--a common

ratio for dividers used with x-ray units--with an uncertainty of a

few percent of the value of the ratio, requires that the inductive

divider be accurate to a few parts per million of full scale.

Oscillator voltages are usually less than 10 V rms, so the output from

_3

the resistor divider is of order 10 V. The voltmeter is a two-phase

or vector instrument used in the differential input mode. Typically, the input

Q

impedance of each channel in these voltmeters is 10 $7 shunted by

no more than 20 pF. To the measurement accuracy required, this input

impedance is sufficiently large so that the measuring device causes an

insignificant perturbation on the remainder of the circuit.

A two-phase voltmeter is useful because the quantity measured,

l/A-|i (co), is complex, i.e., Eq. 4 can be expressed in the form

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''in 'll

= a + j6 . ( 6 )

If the inductive divider is adjusted so that the in-phase component

of the differential voltage is zero, the value of a can then be read directly

from the inductive divider. The magnitude of the quadrature component
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FIGURE

3.2

Test

circuit

for

determining

frequency

response

of

divider.
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- 13-

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FIGURE

3.4

Time

response

characteristics

of

a

divider

with

poor

hiph

frequency

response.
-14-

is read from the voltmeter.

As shown above, the voltage ratio obtained for dividers intended to

be used with diagnostic x-ray units often depends on the freouency of

the applied voltage. By using the measured frequency response, approximate

measurement errors can be calculated for any assumed input waveform. It is

anticipated, however, that few users would want to calculate, and mathematically

correct for, measurement errors because dividers with suff iciently-smal

1

frequency dependence are available. It is, therefore, relatively convenient

to obtain a divider which is designed for the measurement application.

The measured frequency dependence data can serve as a guide to judge the

adequacy of a divider for a given measurement. This can be understood

by considering some specific examples. The first example is shown in

Fig. 3.3. In this case the voltage ratio is frequency independent and

the divider output voltage is a replica of the input. In Fig.

however, the ratio of the input voltage to the output is smaller at high

frequencies than at direct voltage. The magnitude and duration of the

overshoot depend on the details of the frequency response. This type of

waveform can cause particular difficulty when measured with an electronic peakdetect-and-hol d circuit. The overshoot at the beginning of the pulse--

which in this case is due to the divider and not present in the high voltage

pulse—can lead to a significant error in the determination of the peak voltage.
-15-

To avoid this difficulty, some dividers are designed so that the ratio of

the input voltage to the output voltage is larger at high frequencies than

at low frequencies. With this design the pulse waveforms shown in Fig. 3.5

are obtained. From the model in Fig. 1 it can be seen that this situation

results if the capacitance in the low voltage arm is increased. This also

leads to a non-zero quadrature component of the voltage transfer function.

In both Figs. 3.4 and 3.5 it can be seen that the divider output

eventually attains its expected value, i.e., it eventually has a magnitude

of V /N. It is, therefore, possible to obtain the true value of the

P kV using this divider if care is taken to make measurements at the

P appropriate time.

By contrast, if the entire voltage pulse is short compared to the

divider response time, the output would be similar to the case shown

in Fig. 3.6. Although it is expected that this situation is rare, dividers

with response times longer than a few milliseconds have been observed.

This fact, coupled with the short exposure times available on multiphase

x-ray units, could lead to significant measurement errors.

One final consequence of a frequency-dependent voltage ratio is

summarized in Figs. 3.7 and 3.8. These examples illustrate the results if

the voltage ratio at low frequencies, e.g., 50-500 Hz, is different from

the divider ratio, which is the value of the voltage ratio under direct
- 16- A

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FIGURE

3.6

Time

response

characteristics

of

a

divider

with

a

response

time

long

relative

to

input

pulse

duration.
-18-

voltage. The resulting error depends on whether the high voltage pulse

applied to the x-ray is produced by a unit powered by three phase or single

phase voltage. In the case of a three phase pulse waveform, the error

could be insignificant. To illustrate this, consider a three phase waveform which

can be modeled as a smooth, approximately rectangular, pulse with an

added, small sinusoidal ripple (Fig. 3.7). It is assumed that the pulse

duration is sufficiently long that only steady-state conditions need

be considered. Under these assumptions, the peak value of the rectangular

pulse component would be correctly reproduced. At the divider output,

however, the ripple would be a different fraction of the peak value than

it would be at the divider input. However, because the ripple is usually a small

fraction of the high voltage pulse, the error in the ripple measurement

may contribute only a very small error to the measurement of the pulse.

The situation is, however, substantially different for a single phase

unit. If it is assumed that the waveform is a full wave rectified sine

wave (Fig. 3.8), elementary Fourier analysis indicates that the low

frequency components contribute significantly to the voltage waveform.

Therefore, if it is assumed that the divider ratio, which is measured

under direct voltage, is valid in this application, a substantial error

could result.

The current through the tube is pulsed and roughly follows

the voltage waveform. A traditional measurement technique is to apply

the pulsed current to a resistor and measure the voltage drop across
+

a

FIGURE

3.7

Representation

of

a

multiphase

waveform

as

an

approximately

rectangular

pulse

with

small

super

ini[)osed

ripple.
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- 21 -

the resistor. That technique was chosen in this application because

the impedance of the current-viewing resistor is independent of frequency

to the accuracy, and over the frequency ranae, required. The voltage

across this resistor is monitored using a peak-detect-and-hold circuit

so the measuring circuit displays the magnitude of the peak current.

The circuit is designed to calibrate measurement systems having an input

impedance of approximately 200 Q and which do not require that the

current-carrying line be grounded to the device under test. A possible

modification of the X-CAL which would permit calibration of pulse

transformers is suggested in Section 6.

The filament current is a steady-state, 60-Hz current with a magnitude

up to 20 A. The measurement approach is to use a very accurate current

transformer having a wel 1 -characterized resistive burden to provide a

voltage signal which is proportional to the filament current. This

voltage is then measured using a digital voltmeter. The system is

designed to display the rms value of the filament current.

4. System Description

4.1 The X-CAL calibration system allows five specific calibrations

to be performed. These are;
- 22 -

4.1.1 Direct Voltage Divider Ratio Calibration

4.1.2 Alternating Voltage Divider Ratio Calibration

4.1.3 Frequency Dependence of the Divider Ratio

4.1.4 Filament Current

4.1.5 Anode Current.

Each of these functions is discussed in more detail in the following sections.

4.2 Direct Voltage Divider Ratio Calibration

The electrical system used in this calibration is illustrated

schematically in Fig. 4.1. A direct voltage, variable from 0 to ±75 kV, is

14

applied to a Park divider' and to the device under test. The low side

of the Park divider is active as indicated in Fig. 4.1. The active low

side configuration offers a significant advantage over passive low sides

in that deleterious effects such as loading due to the indicating instrument

are virtually eliminated. In this configuration, the voltages indicated

by the calibration system and the device under test may be compared

directly if the device under test has a self-contained readout capability.

If not, the calibration unit contains a low side system which may be

operated in either a passive or active mode to provide a digital display

of the output voltage from the device under test.

4. 3 Alternating Voltage Divider Ratio Calibration

The measurement technique employed in this calibration is shown in

Fig. 4.2. The ac power supply (provided by user) applies an alternating
Park

Divider

-23-

A

I-3

O

FIGURE

4.1

Test

circuit

for

determining

the

dc

ratio

of

a

divider

under

test

(OUT).
- 24 -

I' 3

Bii
-25-

voltage to a standard capacitor (provided as part of the X-CAL system) which

again incorporates an active low side. In this measurement, the low side

voltage is introduced into a peak and hold circuit, which acquires and

holds the maximum voltage impressed upon it during the measurement

period. This voltage is read and displayed by a digital voltmeter.

4.4 Frequency Response of the Divider Ratio

A measurement of the frequency response of the divider ratio over the

frequency range 50-10,000 Hz can provide a direct indication of the physical

condition of a divider and can be used to predict the response of the

divider to voltage pulses. The electrical schematic of the technioue used in

this measurement is shown in Fig. 4.3. A source of alternating voltage provides

a signal to the device under test and to a ratio transformer. A well-defined

fraction of the input voltage may be selected by the ratio transformer. A

phase sensitive detector, operated as a vector voltmeter, measures the

difference between input voltages "A" and "B". If the ratio transformer

is adjusted to provide a null indication on the vector voltmeter, the reading

of the ratio transformer is equal to the in-phase component of the divider

ratio. There will also be a quadrature voltage, since the divider has both

resistive and reactive components, and this quadrature voltage is read directly

from the vector voltmeter. These readings may be combined with the input

voltage to provide a normalized divider ratio as described in Section 6.

4. 5 Filament Current Calibration

The Machlett Dynalyzer^ provides the user with the capability

of measuring the filament current of the x-ray tube in the x-ray unit
- 26 -

Ratio

Transformer
-27-

under test. The X-CAL system may be used to calibrate the filament current

measuring system as is indicated in Fig. 4.4. An alternating current, variable

between 0 and 20 amperes, passes through a current viewing resistor

(nominally 1S7). The current value measured by the commercial system can

be compared to that determined by the precision instrument current

transformer which is a part of the X-CAL unit.

4.6 Anode Current Measurement Calibration

The X-CAL system provides a current pulse (variable up to 2 amperes)

which may be used to calibrate a Machlett Dynalyzer^ unit as illustrated

in Fig. 4.5. The pulse generator, v;hen triggered, drives current through the

nominal 200-ohm load of the device under test. The current waveform, which is

not intended to duplicate an actual x-ray machine current pulse, is approximately

rectangular. There is some droop to the pulse with the maximum current

occurring immediately after triggering. The maximum value of the current

is determined by amplifying the voltage drop across a current viewing resistor

(CVR) and applying this voltage to a peak and hold circuit which captures

the maximum voltage level (corresponding to maximum current). The commercial

unit measures the average value of the current pulse so a conversion factor

relating the peak current value and the average current has been determined

for the pulser in the X-CAL unit driving the nominal 200 ohm load of Machlett

Dynalyzer^ to allow direct comparison of the two readings.

5. Installation

The X-CAL system consists of two enclosed racks of electric instruments,

connecting cables and a standard capacitor. A necessary component of

the system is a Park resistor, which is to be provided by the user.
- 4. 0 -
FIGURE

^.5

Test

circuit

for

assessing

the

accuracy

of

the

anode

current

measuring

capability

of

the

divider

system

under

test.
-30-

Upon receipt of the system, uncrate all cartons and inspect the

contents for damage. The high-voltage transformer assembly should he

removed from its case. This transformer assembly is to be mounted in

the high voltage cabinet. This installation should be such that the

outlet for the high voltage cable is toward the rear of the cabinet. Be

careful not to damage the x-ray connectors located on the front panels

of the high voltage cabinet when installing the transformer assembly.

After the transformer assembly has been placed in the cabinet, it should

be filled with oil according to the manufacturer's specifications.

CAUTION : Avoid introducing any foreign material

into the transformer assembly during filling.

When the assembly has been filled with oil, the

high voltage cable may be inserted and then

connected to the corona shield attached to the

x-ray connector used for the high voltage dc tests.

The main electronics rack should be inspected to make sure that all

cables are in place and all plugs securely inserted in their corresponding

sockets.

Coaxial cables should be connected between the appropriate input

and output terminals of the ratio transformer and the corresponding

bulkhead feedthrough mounted on the bulkhead panel.

The high voltage cabinet should be located in a secure area in

accordance with the users' safety regulations. Interconnecting cables

allow approximately 7 m between the main cabinet and the high voltage cabinet.
-31-

Output cables (anode and cathode) should be connected to the

corresponding input and output connectors. The HV-interlock cable and

the control cable between the high voltage control unit and the transformer

assembly should also be connected.

A good grounding system is necessary to insure proper system

operation as well as for safety reasons. A ground strap should be run to

the grounding terminal on the transformer assembly, the high voltage

cabinet and between the two cabinets. In addition, the ground terminals

on the high voltage control unit and the current pulser should be connected

to the rack ground which is then attached to a reliable ground in the

users laboratory.

The best technique for ascertaining that the system is functioning ,

properly is to calibrate an x-ray divider unit. Detailed operating

instructions are given in Section 6.

Conditioning procedures for the dc high voltage supply are given by

the manufacturer and should be followed whenever the high voltage tank

is drained and refilled with oil.

6. Operating Instructions

6.1 The following sections describe in detail how the measurements

leading to the calibration of an x-ray divider are to be done. In each

case, the user should become more adept at making measurements as he

gains experience. Local operating procedures may be generated, providing

the integrity of the measurements are not compromised. Local safety

rules regarding high voltages should be followed. The high voltage supply
-32-

provided with the system generates lethal voltage and current levels and

should be treated accordingly. Cables connecting x-ray dividers to the

high voltage cabinet may retain considerable charge after high voltage

tests are completed and the judicious use of a grounding stick is highly

recommended.

i

6.2 Dividers with Display Units

The instructions in the following sections assume the divider under

test is one which has a display unit as an integral part of the system.

For dividers not meeting this requirement, refer to Section 6.8.

I

6.3 High Voltage (DC) Divider Ratio

The test configuration is shown in Fig. 6.1. The high voltage

cable is connected to the device under test (either anode or cathode)

and then passed through the divider unit to a Park divider. It should be

noted that x-ray divider units are designed to be inserted between the x-ray

machine power supply (transformer) and the x-ray tube itself. The high voltage

divider assembly (shown as a resistor divider in Figs. 4. 1-4.3) is connected

internally to the high voltage cable. All high voltaae connections are made

by using standard x-ray high voltage cables and connectors which results in

a configuration with no exposed high voltage. In Fig. 6.1 and following

figures in this section, connections involving high voltage cables are shown

symbolically as originating from the top of the divider unit under test. The

measurement mode should be set to DCHV and the output from the Park resistor connect6d

to PARK on the input bulkhead panel. With the system turned on and allowed to

stabilize for 15-20 minutes, a reading of zero (±1-2 mV) should be observed on the digital readout on x-ray". The high voltage polarity appropriate for the
X-CAL

System

-33-

FIGURE

6.1

X-CAL

system

configuration

for

dc

ratio

measurements.
II

-34-

test should be selected. Measurements over the desired range may now be made.

Reconnection of the high voltage leads and a change in high voltage supply

polarity is all that is required to go from one divider section to another.

CAUTION: OBSERVE HIGH VOLTAGE SAFETY RULES!

Refer to the manufacturers instruction manual

for proper operating procedures for the high

voltage supply.

A direct comparison of the voltage recorded by the device under test

and the input voltage as given by the Park divider may be made from the

readings of the individual displays of the X-CAL and device under test.

The X-CAL system has been calibrated to be direct reading with a Park

divider. If V^.^ is the voltage indicated by the standard Park divider

and V the x-ray divider indication, the divider ratio is given by

A

Ratio (7)

6.4 High Voltage (AC) Divider Ratio

The test configuration for this measurement is shown in Fig. 6.2.

A source of alternating voltage (supplied by the user) is connected to

the divider in a pass-through configuration and to the standard capacitor.

The low side of the standard capacitor goes to the CAP input of the X-CAL

system. The measurement function on the X-CAL system is ACHV. Pressing

the reset button on the main chassis reduces the X-CAL readout to zero

and enables the peak and hold circuit. When the ac supply is activated,

either in a continuous or pulsed mode, the peak and hold circuit will
X-CAL

System -

h-OC o

<0 -“g C 2

CO gCQ

C/5

O

FIGURE

6.2

X-CAL

system

conf

ifiuration

for

ac

ratio

measurements.
-36-

acquire the peak voltage which will then be displayed on the DPM.

Because of droop in the peak and hold circuit, it will be necessary to

read the meter immediately after energizing the ac supply if a pulsed

mode of operation is used. The system has been calibrated to read

directly with the standard capacitor supplied. As with the dc high

voltage measurement, proper safety procedures should be followed when

using the ac supply. Make sure the case of the standard capacitor is

connected to system ground. The divider ratio is calculated from the

reading of the standard V.^ and the reading from the x-ray divider as

follows

Ratio ( 8 )

6.5 Frequency Dependence of the Divider Ratio

The purpose for making a measurement of the frequency response of

the divider has been discussed in Section 3. These measurements are the

most involved of the various calibration procedures but become straightforward

with increasing user experience. The experimental arrangement is shown

in Fig. 6.3. The switches on the control unit should be set as follows:

test function - FREQ RES

signal source - INT

anode/cathode - Divider section under test

- HI or LO (both positions will be

used)

ratio tran
-37-

A coaxial cable may be connected between the front panel "ref"

I

I coaxial connector on the vector voltmeter and the input of the frequency

I counter if the exact frequencies of the internal oscillator are to be

I recorded. The function generator provides a continuously variable

j siqnal and if this siqnal source is to be used, the sianal source selector

i on the control unit should be set to "EXT" and a cable connected from

I the signal generator to the "EXT" input on the feedthru panel. The

I output level of the signal generator should be adjusted to approximately 5 V rms.

I

i A digital voltmeter with ac response flat to 10 kHz should be connected

I to the chosen input of the ratio transformer. The 0.35 f section is the "HI"

frequency section and 2.5 f the "LO" frequency section.

The operation of the vector voltmeter is summarized in this section.

I This device measures the voltage difference between two input signals

"A" and "B". If the divider and ratio transformers were purely resistive,

then all voltages and currents would be in phase and a simple voltmeter

would suffice. Indeed, there would be no frequency dependence of the

ratio. This, however, is not the case and because of the stray capacitance

1

1 in the divider, there will be some relative phase shift between the

voltages in the circuits.

The vector voltmeter is initially adjusted so that one of the input

signals, say "A", is in phase with a reference signal derived from the

signal source. When operated in the difference mode, the difference of

I the two inputs "A" - "B" is displayed. Since "A" and "B" are in general

I not in phase, their difference is determined in terms of two components.

One is in phase with "A", and the other in quadrature.

I
DVM

AC

Volts

OUT
-39-

After the initial adjustment of phase, the ratio transformer is

varied until the in phase component of "A" - "B" is zero. At this

point, the measurement of the ratio for a given frequency is complete.

The quantity a defined in Section 3 is the reading of the ratio transformer.

The quantity 6 also defined in Section 3 is just the ratio of the quadrature

It should be noted that there are corrections to be made to the a and 3 values

for 5- and 10-kHz signals. These corrections are for the ratio transformer

and are of the order of 3% for the 10-kHz ratios. The additive corrections

da and d3 which are taken from the data sheet supplied for the ratio transform

should be applied to the uncorrected values a and 3 obtained according to the

procedure described above. The divider ratio may then be simply calculated as

The initial adjustment of the vector voltmeter is done in the

following way. The initial control positions on the vector voltmeter

voltage to the input voltage determined by the DVM, V^.^;

(9)

(10)

are:
-40-

SIGNAL

Input Mode - Flat

- Float

- A

Sensitivity - 100 mV

High Pass - Switches (

Low Pass

Reference

Switches <

Phase - 0 deg.

Mode

In Phase

f

ext

Zero Offset - 0.0

Time Constant - 0.1 Hz

Output Expand - xl

Pref il ter

Quadrature

0.15 s

Zero Offset - 0

Time Constant - 0.1 Hz

Output Expand - xl

Pref il ter - 0.1 s

Outputs - in - phas

Quad
-41-

The ratio transformer should be set to 0.0001 (for a 10^ to 1 divider).

At this point both the in phase and quadrature meters should deflect. The phase

control should be adjusted to make the Quadrature reading zero. The

"full scale sensitivity" and the "output expand" switches for the quadrature

component can be changed to give an extremely sensitive indication of

proper adjustment. For the 50-Hz measurement, some "beating" between

the 50-Hz signal and the ac line frequency (60 Hz) may be present as

evidenced by an oscillation of both the in-phase and the quadrature

meters. The user should adjust the instrument so that the indicator

excursions are centered on zero.

The A, A-B switch should now be placed in the A-B position.

Again, both in-phase and quadrature indicators should deflect. Now, the

ratio transformer, should be adjusted to make the in-phase reading zero.

This can be done very precisely by increasing meter sensitivity during

the nulling adjustments. When this process is complete, the ratio

transformer setting and the quadrature voltage should be recorded. Mow,

the frequency may be changed, the initial adjustments made and the

measurement process repeated.

A complete set of measurements should be done using both the 0.35 f

and 2.5 f sections of the ratio transformer. The mid-frequency measurements

for both sections should be the same. The 50-Hz value should be taken

from the 2.5 f data and all others from the 0.35 f data. A useful presentation

of the results of the frequency response measurement may be obtained by

normalizing the frequency dependent ratio to the dc ratio measured at low

voltages ('^^ 500 V)
-42-

6.6 Filament Current

The Machlett Dynalyzer^ indicates the filament current flowing to

the x-ray tube by measuring the voltage across a 1-ohm current viewing

resistor. When the X-CAL system is configured as indicated in Fig. 6.4,

this current reading may be compared to that obtained in the X-CAL system from

precision current transformer. The current may be adjusted up to 20

amperes and readings compared directly. In this measurement, the function

switch should be in the "fil I" position.

CAUTION : Do not enable the anode current pulser

when the system is configured for cathode testing.

Damage to the DUT could result if the pulser

used for anode current testing is discharged

through the filament current viewing resistor.

6.7 Anode Current

The anode current in an x-ray tube is normally pulsed and the X-CAL

system contains a dc pulser to provide a rectangular pulse which provides

peak currents from 0.05 amp to 2 amp. The function switch should be in

the "anode I" position and the system connected as shown in Fig. 6.5.

CAUTION : Do not enable the anode current

pulser when the system is configured

for cathode testing. Damage to the DUT

could result if the pulser used for

anode current testing is discharged through

the filament current viewing resistor.
X-CAL

System

/: ^ o

H-3

Q

FIGURE

6.4

X-CAL

system

configuration

for

filament

current

calibration

measurement.
-44-

The current pulser is manually triggered with the duty cycle being

controlled such that average power dissipation in a 200 ohm load does not

exceed 10 watts. The trigger switch on the pulser unit is lighted when the

system is ready for pulsing. When the system is pulsed, the indicator in the

X-CAL unit will show the peak current in the pulse. For the Dynalyzer^ system

(divider unit and digital display) which reads average current, a conversion

factor between peak current and average current into 200 ohms is given in Table 6.1

The delays involved for higher current pulses are long. For example,

with a 2 amp peak current pulse, the minimum time between pulses is 80 s

for a pulse width of 1 s. Pressing the trigger button when it is not

lighted cannot cause additional pulses. The OUT should be reset between pulses.

6.8 Dividers without Display Units

For dividers with no display units only certain of the procedures

described in the sections above are applicable. The following sections

indicate the proper procedure for these devices.

The X-CAL system may be used as a comparison system for other

dividers and current measurement systems.

6.8.1 Direct Voltage Divider Measurements

The calibration system and divider under test should be configured

as in Fig. 6.1. If the DUT has no internal readout, a suitable indicator

should be provided. In this case, the comparison is between the standard

and the DUT plus the chosen readout.
-45-

6.8.2 Alternating High Voltage Measurements

To compare the DUT under alternating high voltage with the calibration

system, the arrangement is as shown in Fig. 6.2. Here the output of the

DUT is compared directly with the standard reading provided by the

X-CAL system.

6.8.3 Frequency Response Measurements

The procedure to be followed in determining the frequency response

of the DUT is just that detailed in Section 6.10 - 6.15. What is measured

in this procedure is the frequency response of the divider in combination

with the cable nominally used to connect the divider to its associated

readout system. Because of this, the frequency response measurement

should be done by using this cable to connect the DUT to the X-CAL

system.

6.8.4 Filament Current Measurements

The variable supply used as a current source for the filament

current calibrator is designed to drive a 1-ohm load. Other loads may

be used with the proviso that the output current be limited to 20 amperes or less.

6.8.5 Anode Current Measurements

The dc pulser incorporated in the anode-current calibration unit is

to be used with a. 200-ohm load. If this unit is to be used to evaluate

the performance of current measuring devices having input impedances

less than 200 ohms, series resistance should be added to the circuit to

make the effective load at least 200 ohms. It should be remembered that
-46-

the calibration system indicates peak current in the external load. The

conversion between the peak current indicated by the calibration system

and the average current determined by the DUT listed in Table 6.1 are

for units with nominal 200 ohm loads only. Anode current

measuring systems which indicate peak current may be compared directly

with the calibration system. The anode current pulser unit cannot be

used with systems which require that one side of the current path be

connected to ground at the unit under test.

7. Calibration Information

Calibration procedures employed in establishing the stated accuracies

for the X-CAL system are summarized in the following sections.

The direct high voltage measurement uses as a standard a userprovided Park resistor. A calibration of the active low side was made by

using NBS standards and a MBS Park divider. Divider ratios may normally

be determined to ±0.1% levels of uncertainty, so the stated uncertainty of

the calibration system of 1.0% is conservative.

The alternating high voltage measurements were calibrated by direct

comparison of the X-CAL system and NBS standards. Both steady state

and pulsed voltage levels were utilized.
X-CAL

System

FIGURE

6.5 X

CAL

system

configuration

for

anode

current

calibration

measurement.
-48-

Table 6.1

Peak Current Pulse Width I average

(amperes) seconds I peak

0.060 0.01 0.985

/ 0.05 0.985

0.1 0.980

0.5 0.965

1.0 0.960

0.116 0.01 0.975

0.05 0.975

0.1 0.975

0.5 0.970

1.0 0.969

0.602 0.01 0.980

0.05 0.960

0.1 0.940

0.5 0.915

1.0 0.910

1.20 0.01 0.960

0.05 0.942

0.1 0.910

0.5 0.880

1.0 0.876

1.83 0.01 0.967

0.05 0.895

0.1 0.866

0.5 0.875

1.0 0.819
-49-

Accuracies of measurements of the frequency dependence of the

divider ratio are determined primarily by the ratio transformer accuracy.

Vendors of these units maintain standards directly traceable to NBS

standards. In addition to the vendor's certificate of accuracy, specific

measurements of corrections to the divider ratio were made for 5-kHz

and 10- kHz signals. A summary of these corrections is provided in

Section 6. Since the measurement technique involves a nulling

procedure, the actual uncertainty in a measurement and the reproducibil ity

of such measurements depends largely on the laboratory techniques of the

user.

The accuracy of the filament current measurements was determined

using a precision current transformer constructed by NBS personnel

and precision current viewing resistors. Proper operation of the filament

current module was verified by direct comparison of module readings with

a direct reading 0.1?^ ac ammeter.

The anode current measurement requires a comparison between

the peak current as indicated by the calibration system and the average

current over the pulse duration when discharged into a known load, in this

case 200 ohm. The calibration procedure involved two separate measurements. The

voltage drop caused by a current flowing through a precision non-inductive resistor

was amplified and sampled by a peak and hold detector. This measurement

yields a value for the peak current in the current pulse. The accuracy of this

peak reading was determined by observing the voltage across the current viewing

resistor with a calibrated fast digitizing scope. By numerically calculating the
-50-

average current of the current pulse for different peak currents and

pulse widths a direct relation between peak current and average current

was determined. Table 6.1 summarizes the results of this calibration

procedure.

8. Maintenance and Calibration

There is no routine maintenance required for the X-CAL system.

Because of the developmental nature of the system, no explicit procedures

for troubleshooting system failures are available. Loss of a particular

test function would suggest investigation of the active circuitry involved

using conventional voltage checks and signal tracing.

Since the X-CAL system is designed to be used to calibrate x-ray

dividers in a comparator mode at the 1% accuracy level, a properly

equipped standards lab may routinely check the accuracy of certain units

of the system.

The DCHV test uses as a standard for comparison a Park resistor and an

active low side with a digital display. By using a second Park divider

and associated readout, the X-CAL system may be adjusted to be direct

reading. This adjustment is made by means of TRIM I, located on the

low side board in the system control module. This board contains three

separate active low sides and associated calibration adjustments. TRIM

I adjusts the Park divider low side, TRIM II the capacitive divider

low side and TRIM III the low side used with passive x-ray dividers.

Also on the low side board are pairs of trim pots used for zero offset

adjustments. The calibration trim pots are nearest the board and are

labeled I, II and III.
-51-

The ACHV calibration adjustment TRIM II should only be adjusted if a suitable ac divider system is available.

A third active low side is provided for use with passive dividers. The

gain of this system may be adjusted to make the output of a given divider

direct reading.

The filament current module uses a current transformer v/ith a precision

resistor burden and should maintain accuracy for an extended period barring

component failure.

The anode current unit was calibrated using a digitizing scope and would

require a similar unit for recal ibration. It is suggested that appropriate

NBS personnel be contacted for consultation in the event this unit is suspected

to be malfunctioning.

other X-CAL modules are commercially manufactured and the vendors Instruction manual

should be used in the event of failure of these units.

9. Parts List

No extensive parts lists have been developed for the X-CAL system

since no exotic components were used in the system design and most

replacement parts required are available from various vendors. It is

suggested that spares be acquired for the active components listed below

since lead times for procuring most semiconductors are long.
-52-

List of Selected Active Devices

Device No. Vendor^

OP-2 Precision Monolithics

OP-16 Precision Monolithics

4085 Burr-Brown

556 Signetics

TL-083 Texas Instruments

8038 Intersil

Mi sc. TTL Texas Instruments
- 53-

10. Circuit Diagrams

The following set of figures are circuit diagrams for system modules and

intermodule cabling. All component values are indicated on the diagrams.
- **' J*p-.

-tJ- W ’S'

->» ' ._ •^/ •-tK„fia»5'

' letyii* WtrtMH »Mipe» itun^J ;0J

bi« ^srubom *f0^ ‘aiwr-t^s^b itunfD >o pnfwtiftoY sHT

«?!i«^Sel05Sf'*Mc ha3M^t>flt abufftv .pnTFdM 6Tuboirifin|^

f .-flio 1 i tMcs

-'.-fPfc'SfMw Mdjmjtitbfcs / _

.' .. . gi "

JC

• f-

* r

'

iA ! ,..7»#

»y<-«

ft

r*^'-
S303

-55-

6 6 < -

6 m

FIGURE

10.2

Circuit

didyram

for

anode

pulser

module.
J2U3 - 56-

•“ <Q

Q<
-37-

1

2

3

4

P102 5

6

7

8

9

10

1

2

P201 3

4

5

i

!

i

I

j 1

‘ P401 3

1

2

3

P304 4

I 5

I 6

110 Lo

110 Hi : 110 Hi -—

0 Hi

GND

Blaink

-« +15

+ 5 ^ Relay

^

Cables Not Shown

1 . ± 400

2. Filament

3. Output to OUT

4. Interlock

5. AC PWR

6. input COAX

FIGURE 10.4 Intermodule cabling.
- 30 -

Q.

3

o

A

a

c
10K

-59- 3o

Q.<o c

Q.<O I

FIGURE'

10.6

Circuit

cliagraiii

for

active

low

side

(capacitor

divider).
91.-

•

vJ\

o

9I.+

-oulO TH.

CM

3. lO CM

00

a

o

a

o

CO 00 o o CM

T“ O o o lO CO • o d o

a o
FIGURE 10.3 Circuit diaaram for anode oulser controller.
-62-

5

-15

+15

GND

To J302

FIGURE 10.9 Circuit diagram for gain and calibration stages for

anode pulser peak-detect-and-hold circuitry.
r ^

-\J0

FIGURE

10.10

Circuit

diagram

for

dual

OP-16

to

16

pin

DIP

package

adapter.

-65-

I 1

I

I

I

I

! Inactive Components

I

1 555 1

1 3038

OP2 —

Pins

1 +15

2 -15

3 GND 4+5

5 OSC Sel

6

7 Output

8 Ref Output

9

11

12

13

14

15

FIGURE 10.12 Major parts layout for fixed frequency oscillator board.
Front

4085 4085

74121

PINS

1 +15

2 -15

3 GND

4 +5

5 P&H A In (From X-Ray Div. Low Side)

6 P&H A Out

7 P&H B In (From Cap Div. Low Side)

8 P&H B Out

9 Reset (No to GND)

10

Key

FIGURE 10.13 Major parts layout for peak-detect-and-hold board.
-67-

Trim III Trim II Trim I (X-Ray) (CAP) (Park)

OP16

Pair

TL083

OP16

Pair

3-4

Key

Pins

1 +15

2 -15

5 }Key

5 GND

6 Input From X-Ray Div.

7

8 Active Input

9 Active Output

10 Cap Div. Output

11 Cap Div. input

12 Park Div. Output

13 Park Div. input

14 X-Ray Div. Output

15

FIGURE 10.14 Major parts layout for active downside board.
-ob2N3439 o o 2N3904

INACTIVE

4 5

KEY

PINS

1 -15

2 +15

3 TO CALIBRATION OP AMP

4 TO S 304

5 TO S 302

6 TO S 303

7 ”

8 It 9 ti 10 ti 11 TO S 303

12 +5

13 FROM CVR OP AMP

14 GND

15 TO S 302

741 21

4085

FIGURE 10.15 Major parts layout for anode pulser controller board.
-G9-

0P2 0P2

PINS

1 -15

2 +15

3 GND

4 FROM CVR

5 TO S 302

6 TO PEAK AND HOLD

7 TO DPM

8 FROM PEAK AND HOLD

9

10

11

14

15

FIGURE 10.16 Major parts layout for anode pulser gain and calibration board
-70-

1 1 . References

1. Certain commercial equipment, instruments, or materials are identified

in this paper in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the

material or equipment identified is necessarily the best available

for the purpose.

2. Report of the Advisory Committee on the Biological Effects of Ionizing

Radiations, National Academy of Sciences - National Research Council,

The Effects on Populations of Exposure to Low Levels of Ionizing

Radiation (1972).

3. L. Stanton, D. A. Lightfoot, S. Mann, Radiology 87_, 87 (1966).

4. P. M. Joseph, Med. Phys. 201 (1975).

5. P. M. Joseph, Med. Phys. 2, 208 (1975).

6. J. C. Giarratano, R. G. Waggener, J. M. Hevezi, and R. J. Shalek,

submitted for publication in Med. Phys.

7. G. M. Ardran and H. E. Crooks, Br. J. Radiol. 193 (1968).

8. M. Davison and D. Reekie, Phys. Med. Biol. T3» 643 (1968).

9. E. G. Murphy, unpublished report to Bureau of Radiological Health,

FDA, Rockville, Maryland.
-71-

10. J. D. Gilbertson and A. G. Fingerhut, Radiology^, 1033 (1969).

11. E. R. Epp and H. Weiss, Phys. Med. Biol. H, 225 (1966).

12. A. F. Jacobson, J. R. Cameron, M. P. Siedband, and J. Wagner, Med.

Phys. 3, 19 (1976).

13. International Research Group Renardieres on Impulse Measuring

Systems, Electra 157 (1974).

14. In this report, the term "Park divider" is used to denote a divider

consisting of a high voltage resistor of the type developed by

Park (J. H. Park, J.

low side resistor.

Research NBS, 66C, 19 (1962)) and an appropriate

12. Acknowledgments

The authors would like to acknowledge the assistance of R. L. Kahler,

R. A. Bullock, A. Lomnicki and J. D. Neal in the design and construction

of the system and S. B. Kelley in the preparation of this report.
« 1

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2072

Aecassion Now >

4. TITLE AND SUBTITLE

X-CAL — A Calibration System for Electrical Measurement

Devices Used with Diagnostic X-Ray Units

5. Publication Date June 1980

$* PerformneOrgantzatiofl Code

7. AUTHOR(S)

R.H. McKnight and R.E. Hebner

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

llli Proiectyra^/Wiortt: Unit Not

11. Conlract/Grant No. DoD/CCG Proj. 78-130

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ("Sfreet. City, ZIP)

Combined Calibration Group

U.S. Army Metrology and Calibration Center, Bldg. 5435

Redstone Arsenal , AL 35809

13. Type of Report & Period Covered Final

14».Spons<KtagLAsency Code

15. SUPPLEMENTARY NOTES

: ; Document describes a computer program; SF*185, FIPS Software Summary, is attached.

18. ABSTRACT (A 200~word or leaa tactual aammary ot moat aignifieant information. If document includea a aignificant bibliography or literature auryey, mention it here.) The X-CAL high-voltage-divider calibration system was designed to calibrate the direct

and alternating voltage measurement capabilities of commercial high voltage dividers

used to characterize diagnostic x-ray units. In addition, there is capability for

determining the frequency response of these dividers and for the calibration of the

filament current and anode current measurement features of specific commercial lyavailable devices. The calibration system combines in a single unit a range of j capabilities which allows calibration of a device under test to be accomplished with

a minimum of set up time. This report gives a description of the motivation for,_

the operation of and the design details of the X-CAL system. It lists the specified

accuracies and the ranges of applicable parameters of the various subsystems, ,, describes the five different calibrations which can be performed using the X-CAL :i system, discusses techniques used in the original calibration of the X-CAL system

|| itself, and contains the basic circuit diagrams for the system. i

17. KEY WORDS ('six to twelve entriea; alphabetical order; capitalize only the firat letter of the firat key word unleaa a proper name;

aeparated by aemicolona)

Calibration; diagnostic x-rays; electrical measurements; health; radiation;

radiation safety; safety; x-rays.

18. AVAILABILITY Unlimited

For Official Distribution. Do Not Release to NTIS

Order From Sup. of Ooc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

rX~ Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

UNCLASSIFIED 75

20. SECURITY CLASS 22. Price

^ (THIS PAGE)

$7.00

UNCLASSIFIED

USCOMM-OC