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TECHNICAL NOTE 539

ISSUED OCTOBER 1970

Nat. Bur. Stand. (U.S.), Tech. Note 539, 20 pages (Oct. 1970)

CODEN: NBTNA

NBS

Improvements in Oscilloscopic Measurements

in High-Speed Experiments

A. Cezairliyan, M. S. Morse, and H. A. Berman

Physical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

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Contents

Page

1. Introduction 1

2. Description of the System 2

2. 1. Differential Suppression

2. 2. Time Synchronization

3. Experimental Arrangement and Operation 3

4. Oscillogram Measurements and Calculations 5

5. Results and Discussion 7

6. Conclusions 9

7. References 9

it
Improvements In Oscilloscopic Measurements In

High-Speed Experiments*

A. Cezairliyan, M. S. Morse, and H. A. Berman

Two refinements in oscilloscopic recording have been

made which improve considerably the recording of isolated

events in heavy current discharge studies, where substantially rectangular pulses are employed. The accuracy of

the method employed has been verified experimentally to be

in the region of 0. 01 to 0. 1 percent. The first refinement

is a unit for the differential suppression of the incoming

signal by an adjustable amount, and the second refinement

is a system by which time markers are sent to several

oscilloscopes at adjustable time intervals simultaneously

with the actual incoming signal.

Keywords: High-speed measurements; high-speed recording;

oscilloscopes.

1. Introduction

Oscilloscopes in themselves are not accurate recording instruments. Under the most favorable conditions, that is with adjustments

for linearity and prior calibrations, the accuracy of the beam position

is about 1 percent. Additional difficulties result when more than one

oscilloscope is required for the simultaneous recording of several

quantities, for example in work involving single heavy discharges.

Although the same pulse may be used for triggering all oscilloscopes,

experience has shown that differences between oscilloscopes, even of

the same make and model, may cause differences in the time scale zero. Further errors may be caused by differences in sweep linearity.

In this study two refinements in oscilloscopic recording have been

made which have reduced the recording inaccuracy to 0. 1 percent or

better. The first is a unit for the differential suppression of the incoming

signal by an adjustable amount, and the second is a system by which

time markers are sent to every oscilloscope simultaneously with the

actual incoming signals. Extensive experimental work has been done to

confirm the accuracy of the method and equipment used.

*This work was supported in part by the Propulsion Division of

the U. S. Air Force Office of Scientific Research under contract

ISSA-69-0001.
The motive in undertaking this study was to improve the recording

accuracy and resolution in millisecond experiments in which thermophysical properties of matter are measured at high temperatures.

2. Description of the System

2. 1. Differential Suppression

The circuit diagram of a differential voltage suppression unit

designed for a maximum voltage of 9 volts is shown in figure 1. It will

be noted in figure 2 that the suppression unit is connected to the

observation point through a high-speed mercury relay. While physically

separated, the relay is an essential part of the suppression system,

since it enables the oscilloscope amplifier to remain under normal

operating conditions (where the oscilloscope trace is within the screen

area) until a time just prior to the pulse, or portion of the pulse, to be

observed.

The operation of the oscilloscope amplifiers under cut-off

conditions is thus avoided. All oscilloscope amplifiers with which the

authors are familiar, including those with built in suppression facilities,'

drift severely during cut-off conditions. In addition, amplifiers with

built in suppression facilities are not available with differential inputs,

thus rendering them essentially useless for work of this nature, where

the reference point is by necessity above ground by a variable amount.

The mercury relays used are capable of operating within a time

of approximately a millisecond, and are reproducible to about 0. 1 millisecond. As in most discharge work, the time of the pulse

occurrence is approximately known, so that the relays may operate in

advance of the pulse if desired.

It must be noted that a voltage suppression is effective only in

cases where the portion of the transient voltage to be examined does

not vary considerably with time; in other words, when the waveform is

of rectangular or flat trapezoidal shape. In these experiments it has

been possible to suppress 90 to 99 percent of the incoming voltage

signals. Since the suppression voltage is constant, it can be measured

very accurately under steady- state conditions by a potentiometer. The

oscilloscope inaccuracies are therefore applicable only to the unsuppressed portion of the signal.

2. 2. Time Synchronization

The most effective way of insuring a one-to-one correspondence

between the simultaneous points on the traces of every oscillogram is to

provide a means of making the same time signals on every oscillogram

during the transient experiment. A modified time -mark generator was
used to achieve this. Its output was fed into the oscilloscope vertical

amplifiers. This scheme provided small, sharp dots or vertical spikes

at regular time intervals on the oscilloscope signal traces. The frequency of the time markers could be selected according to the duration

of the experiment.

Two major modifications in the time -mark generator were made.

An amplifier and a pulse -length control unit were installed to adjust the

amplitude and the duration of the markers, and a gate unit to start the

markers at a given time. Events on different oscillograms were thus

provided with a common time base and uncertainties arising from the

triggering of the beams of different oscilloscopes and from nonlinearities of the time scales could thereby be reduced to a negligible level.

3. Experimental Arrangement and Operation

To assess the operational characteristics of the system described,

the value of a test resistor was determined by measuring the current

flowing through it and the voltage drop across its terminals under

transient conditions and comparing this measurement with its resistance

under steady- state conditions as determined by the potentiometric

method.

The experimental arrangement is shown, schematically in figure 2.

The main circuit was designed for high pulse currents (up to 5000 A).

The value of the pulse current was obtained by measuring the potential

across a reference resistor (0. 001 Q). This reference resistor consisted of a manganin strip bent into a single hairpin. Calculations indicated

that skin effects and inductance were negligible at the frequencies

involved in this work. The test resistor was an Tnconel tube (230 mm

long, 25 mm O. D. , 3.1 mm thick).

Because of the very low resistance, the high thermal capacity of

the electrical system, and the relatively stable power source (a 28 V

battery bank of 1200 ampere-hours capacity), current flowing through

the circuit did not change appreciably during the short pulse. This

implies that the current pulse was of rectangular form. In order to

simulate actual heating conditions in a test specimen in which the

current decreases with time as the resistance increases with temperature, a trapezoidal current pulse was obtained by inserting a tungsten

strip (100 mm long, 6. 2 mm wide, 0. 5 mm thick) in series with the test

resistance. The slope of the top of the trapezoidal pulse is then

determined mainly by the characteristics of the tungsten strip.

Experiments were conducted both with and without the tungsten

strip in the circuit. Figure 3 shows oscillogram reproductions of two

typical cases of rectangular and trapezoidal pulses. For each case, the

complete pulse is shown as well as the partial recording after suppression of the major portion of the pulse. The partial recording is on a

3
more sensitive scale. The actual oscillogram of a trapezoidal pulse

with and without suppression is shown in figure 4.

Because of the short duration of the current pulses, all operations

were synchronized with the aid of time-delay units. As may be seen in

figure 2, after manual triggering, the main switch was closed and the

current pulse was sent through the system. At this point, the voltage

developed across the test resistor triggered a voltage detector, which

sent a signal to the time -delay unit to trigger the oscilloscopes and

operate the mercury switches and the time-mark generator. The time

sequence of major events in an experiment is shown in figure 5.

The mercury switches prevented the error caused by overload conditions

(which can amount to several millivolts) by keeping the beam on the

screen at all times and preserving the position of the base line. Before

the pulse current was triggered and after it was terminated, the

mercury switches were open, isolating the circuit from the suppression

voltage. During the pulse, they were closed so that the oscilloscopes

sensed only the difference between the source voltage and the suppression voltage. In addition, the mercury switches provided superposition

of the base line of the actual pulse with that obtained under zero signal

conditions, as may be seen in figure 6. The two base lines obtained at

slightly different times were thereby compared and a correction could

be made in the event of a difference between them.

Two oscilloscopes instead of a dual-beam oscilloscope were

intentionally used in the tests. The time scales of the oscilloscopes

were set on arbitrary "uncalibrated" positions so that the experimenter

could rely completely on the time markers in correlating the corresponding current and voltage signals. Since the same time markers

appeared on both signal traces, non-linearities in the time scales did

not cause any errors; in fact, it was not even necessary to calibrate the

oscilloscope time scales.

The oscilloscopes used in the experiment were electrically

isolated from each other by means of isolation transformers at the

power -line connections. Isolation transformers were also used ahead

of all other inputs, such as trigger signals and time -marker signals.

It was found that neglect of such precautions introduced stray currents

and erroneous signals. Each oscilloscope was completely floating

electrically and its relative ground (reference) point was at the negative

potential of either the reference or the test resistor.

A permanent record of the signal traces was made by photographing the oscilloscope screen. The traces on the oscillograms thus

obtained were measured with the use of a micrometer microscope, and

the measurements corrected as described in Section 4.

The foregoing paragraphs have presented the experimental

arrangement for measuring the value of a test resistor under pulse

conditions, and comparing it with its value under steady-state conditions.

However, to measure certain thermodynamic quantities under transient

4
conditions, it is necessary to know the power imparted to the specimen

as a function of time. Although both resistance and power are obtained

through the measurement of the same two variables, the errors associated with them may be different from each other inasmuch as one is

represented by a quotient, the other by a product. If the systematic

errors in voltage and current are in the same direction, the resulting

relative error in resistance will be the difference between these relative errors, whereas the corresponding power error will be their sum;

conversely, if these errors are of opposite sign, the resistance error

will be their sum and the power error their difference.

An additional experiment was performed to assess the errors

involved with the high-speed measurement of one quantity, such as

current or voltage. In this experiment the same basic circuit as that

given in figure 2 was used. The original 0, 001 0, reference resistor was

replaced by one of 0. 1 Q. A direct current of approximately 12 A was

allowed to flow under steady- state conditions through the main circuit.

The potential across the reference resistor was measured with a potentiometer, and, at the same time, recorded with the improved

oscilloscopic technique following the procedure developed for the pulse

experiments. The comparison of the two results obtained by the two

different methods gives the accuracy of the high-speed recording of one

variable under quasi-pulse conditions. This procedure was used separately to check each of the two oscilloscopes. The only difference

between this quasi-pulse method and that of a pulse experiment is the

absence of possible fields and skin effects. It is shown in Section 5 that

errors of this origin are negligible.

Thus, the results of two separate experiments, (1) pulse experiments yielding a comparison of steady-state resistance with that under

pulse conditions, and (2) quasi-pulse experiments yielding a comparison

of one quantity under steady -state and quasi-pulse conditions, determine

the accuracy of measurement of the quantities in transient experiments

by the improved oscilloscopic technique.

4. Oscillogram Measurements and Calculations

The signal traces on the oscillogram were converted into the

corresponding voltages by performing a calibration experiment in which

a known constant voltage (measured potentiometrically) was applied to

the oscilloscope and the ratio of this voltage to the deflection in the

oscilloscope was determined. Because of oscilloscope imperfections,

this ratio, or "calibration factor, " is a function of three semiindependent parameters: The horizontal position of the trace on the

oscillogram, its vertical level, and the magnitude of the deflection.

Ideally, for every point measured, the known applied voltage

should be chosen to produce a deflection equal to that accompanying the
unknown pulse signal and at the same horizontal and vertical positions.

Rapidly changing pulses would then require one calibration for each

point of measurement, or a study of the variation of deflection with the

three parameters would be needed for each oscilloscope used. However,

it is possible to avoid an excessive number of calibrations by making

two calibration measurements for each pair of pulses, one equal in

deflection to that of the extreme left end of the pulse range to be

measured (usually a positive deflection at the start), and a corresponding one for the extreme right end (usually a negative deflection at the

finish in a trapezoidal pulse). The calibration factor k for any intermediate point x is then interpolated as follows:

kx =k

1 + (kn -k

1 )(tx

- tl)/(tn

- tl) (1)

where k and k are, respectively, the initial and final calibration

factors and t , t , t are the corresponding times.

This technique assumes that the calibration factor has a linear

variation with each of the two position parameters. It automatically

corrects for the effect of these two parameters regardless of drift in

the calibration from day to day or with a change in oscilloscope. The

correction for pulse deflection magnitude was found to be negligible in

comparison with those for position. The errors due to interpolation

and neglect of the deflection magnitude parameter were minimized by

the fact that pulse experiments were designed to intersect the baseline

approximately at the midpoint of the trace. Thus, the calibration factor

was accurately measured at the extreme positions where the contribution of oscilloscope error to the total signal is greatest, and interpolated at intermediate points where the contribution of the more

accurate potentiometric measurement was more important.

The deflections themselves in the pulse and calibration oscillograms were measured by reading the difference between the center of

the pulse trace and the center of a baseline which represented the position of the beam -when only the suppression voltage was present. The

pulse trace (figure 6) shows initial and final traces at the baseline level

and an intermediate trace at the pulse level. Because the absent portion

of the base level may not be linear, a separate baseline trace, superimposed on the pulse trace 2, was taken before the pulse, and a baseline level was thus obtained for each pulse position measured. A

correction had to be made, however, for an imperfection which

occurred in superposition of the base level of the pulse trace and the

level of the baseline. This correction was obtained by algebraically

adding the pulse-baseline difference at the ends to that measured at the

intermediate points. The overlap was different at the two ends of the

pulse and therefore had to be interpolated between them as for the

calibration factor. The total pulse deflection, A , at a given time, tx, was obtained as follows:

6
A = Bx" A

x

+ 2[D

l - Ci + (°n - Cn

+ Cr

D

i) 0* - V

/(tn " V] (2)

where

Bx

- A^. is the uncorrected pulse deflection at point x, A

representing the pulse trace center and B the

corresponding baseline center,

C is the level of the baseline just after it diverges from the

overlap,

D is the level of the overlapping pulse and baselines just

before divergence,

Subscript n refers to the corresponding points after the end of

the pulse, Cnbefore convergence, D after convergence,

t , tn

, tx

are the corresponding times.

These points are shown on figure 6.

5. Results and Discussion

The method was assessed by comparing the resistance of an

Inconel tube measured as described in Section 3 under steady-state and

under pulse conditions at 25 "C, The following measurements were

made:

1. A steady- state potentiometric measurement of the resistance

of the Inconel tube before the series of pulse measurements

and, three months later, after the series. The mean of six

measurements (at 25 °C) in each case was 3. 07755 mQbefore

and 3. 07778 mQ after. The average of the two means,

3. 07767 mQwas used as a standard for the pulse measurements.

2. A group of 14 rectangular 100 ms pulses of about 600 A,

in which an average of 12. 6 percent of the pulse amplitude was

recorded on the oscilloscope, the remainder measured before

and after the pulse by potentiometer. Five measurements on

the current and voltage oscilloscope traces were made at

equally spaced time intervals for a total of 70 measurements.

3. A group of three rectangular pulses similar to the last, but

with an average of 0. 5 percent of the pulse amplitude

recorded.

4. A group of eight trapezoidal 90 ms pulses of about 500 A, in

which an average of 6 percent of the pulse was recorded at the

start, 1. 5 percent at the midpoint, and -6 percent at the end,

that is, the suppression voltage measured potentiometrically

was greater than the amplitude of the last half of the pulse

resulting in a negative deflection on the oscilloscope. These

pulses are representative of the pulse shape to be expected in a high-speed thermodynamic experiment in which the specimen

7
temperature is changing rapidly over a range of several hundred or

thousand degrees. For each pulse, a total of five measurements at

equally spaced time intervals were made.

5. A group of 11 trapezoidal 50 ms pulses of about 1300 A, in

which an average of 4 percent of the pulse was recorded at the

start, 0. 2 percent at the midpoint, and -4 percent at the end.

Table 1 lists the difference between steady- state and pulse

resistance values and the standard deviation of each group of measurements described in items 2 to 5. Figure 7 shows the dependence of the

above on the fraction recorded on the oscilloscope for both rectangular

and trapezoidal pulses. In general, if the fraction recorded was kept as

low as 6 percent of the total, an accuracy of 0. 1 percent or better and a

reproducibility of i 0. 07 percent or better were possible, and for

fractions of 1 percent or less, errors of the order of 0. 01 percent in

resistance could be achieved.

The average systematic error of all the determinations reported

in this paper was positive. This positive systematic error decreased

almost to zero as the fraction recorded approached zero, whereas the

resistance corresponding to the negative limit of the standard deviation

remained almost constant. There was apparently a bias of unknown

origin in the oscilloscope recordings.

The possibility of decreasing the number of calibration measurements is indicated by the fact that calibration factors at a given

oscilloscope position were reproducible from one experiment to another

on a given day to about 1 percent. The results reported in these

experiments were obtained with one calibration between each two pulses.

The quasi-pulse experiments described in Section 3 (with steadystate current of approximately 12 A and effective measurement duration

of 5 ms) produced the following results: for 20 measurements at each

suppression level recorded, the voltage oscilloscope showed a mean

systematic error of -0. 027 percent at 90 percent suppression and

-0, 003 percent at 98. 9 percent suppression; the current oscilloscope

+ 0. 024 percent at 92 percent suppression and -0. 004 percent at 99. 2

percent suppression. At the higher fractions recorded, the systematic

errors for voltage and current were each about one -third the systematic

error of a resistance measurement and were opposite in sign. This

limited group of data would favor the idea that the power measurements

would be no less accurate than the resistance measurements.

There are several sources of error that contributed to the

measured difference between steady-state and pulse resistance values.

Steady-state resistance measurements were one or two orders of magnitude more accurate than pulse measurements. Therefore, most of

the difference can be attributed to errors in pulse measurements.

A list of major sources of error and their estimated magnitudes in

oscilloscopic recording of a quantity in pulse experiments is given in

8
Table 2. It may be noted that the table includes also errors due to

phenomena affecting the components of the overall system in addition to

the suppression unit-oscilloscope combination.

6. Conclusions

The results of the present investigation have shown that it is

possible to measure quantities in high-speed experiments with an

inaccuracy of 0. 1 to 0. 01 percent using an improved oscilloscopic

technique. This method is applicable primarily to experiments in which

the pulse shape is rectangular or flat trapezoidal.

The method was checked by conducting pulse experiments of

millisecond resolution, in which the resistance of an Inconel tube

measured under pulse conditions was compared with its direct-current

value. An increase of one or two orders of magnitude in the measurement accuracy was achieved by (1) suppressing the major portion of the

signals by known constant voltages, and (2) providing time synchronization between traces on different oscilloscope screens. It was observed

that the increase in accuracy was approximately proportional to the

suppression level.

Although high-speed digital data acquisition systems show great

promise especially in millisecond resolution experiments [l, 2] their

use, at present, is limited because of electronic complexities and

relatively high prices. Thus, the improved oscilloscopic recording

system may have immediate applications in various high-speed

(millisecond resolution) measurements of thermodynamic, transport,

and other related properties at high temperatures. The basic concept

of this method, after some modifications, may also be applied to other

experiments of even higher time resolution (microsecond), such as

exploding conductor, gaseous discharge, shock, etc.

The authors wish to extend their appreciation to Dr. C. W. Beckett for

his encouragement of research in high-speed measurement techniques.

7. References

Ll] Cezairliyan, A. , M. S. Morse, H. A. Berman, and C. W. Beckett,

J. Res. Nat. Bur. Stand. (U.S.) , 74A (Phys. and Chem. )

, 65

(1970).

L.2] Kollie, T. G. , M. Barrisoni, D. L. McElroy, and C. R. Brooks,

High Temp. - High Pres. , 1, 167(1969).
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Table 2. List of Maj

Recording

or Sources of Error in Oscilloscopic

of a Quantity Using Suppression

Source of Error

Error (percent)

On Overall

Signal

On Recorded

Portion

On Suppressed

Portion

Skin effect <0. 01

Self -inductance * 0. 0002-0. 02

Mutual - inductanc e <0. 01

Temperature variations <0. 01

Frequency response** <0. 01

Time synchronization 0. 05

Oscillogram reading 0. 1

Calibration 1 Suppression voltage 0. 001

Potentiometer, etc. *** 0. 005

*The exact value depends upon the nature of the pulse and the

geometry of the resistance across which potential measurements are

made.

**The frequency response of the suppression units was checked for

frequencies up to 10 kHz.

***The potentiometer error does not enter into a comparison of pulse

and steady- state resistances, voltages, or currents, but does affect

power measurements in a thermodynamic experiment.

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suppression.

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4 8 12 FRACTION RECORDED, PERCENT

16

Figure 7. Variation of resistance difference and standard deviation of

an individual determination of resistance as a function of

percentage of signal recorded on oscilloscope.
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TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's research, developmental, cooperative and

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professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

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literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards

adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

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