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N/ST Technical Note 1392

NIST Technical Note 1392

Time-Domain Calibrations of D-Dot Sensors

Robert T. Johnk

Arthur R. Ondrejka

Electromagnetic Fields Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

February 1998

MTES

U.S. DEPARTMENT OF COMMERCE, William M Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G Kammer, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol., Tech. Note 1392, 36 pages (February 1998)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1998

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

Page

1. INTRODUCTION 1

2. D-DOT SENSOR DESCRIPTIONS 2

3. SENSOR CALIBRATION PROCEDURES IN THE TEM CELL 2

4. SENSOR CALIBRATIONS ON THE CONE AND GROUND PLANE 4

5. NOISE REDUCTION 6

6. TRANSFER FUNCTION RESULTS 7

7. UNCERTAINTIES IN THE MEASUREMENT SYSTEMS 9

8. CONCLUSIONS 12

9. REFERENCES 12

ui
LIST OF TABLES

Table 1 . Type B expanded uncertainties (from Ref. [9]) for sensor calibrations in the NIST

1.2 m TEM cell field generation system with an assumed

confidence level of 95% 9

Table 2. Type B expanded uncertainties (from Ref [9]) for sensor calibrations in the NIST

1.2 m cone and ground plane cell field generation system with an assumed

confidence level of 95% 10

Table 3. Type A relative uncertainties computed from the Student's t-distribution [10,1 1] for a 95% level of confidence 11

Table 4. NASA D-dot sensor average receive transfer fianction results, along with associated

combined standard uncertainties (95% level of confidence). Note: bp denotes the support

structureof sensor #34 12

LIST OF FIGURES

Figure 1. Basic features of the D-Dot sensor used in the NASA evaluation effort 14

Figure 2. Support structures for sensors #33, #34, #35, and #37. Sensor #36 had no

supporting structure other than the small base plate shown in Figure 1 15

Figure 3. TEM cell sensor calibration setup 16

Figure 4. Low-frequency pulse generator output used for the TEM cell sensor calibrations. 16

Figure 5 . Reference waveform acquisition setup for both TEM cell and cone and ground

plane sensor calibrations 17

Figure 6. Sensor #36 output waveform obtained with the TEM cell standard field generation

system using the generator waveform of Figure 4 17

Figure 7. Cone and ground plane sensor calibration setup 18

Figure 8. High-frequency pulse generator output used for the cone and ground plane sensor

calibrations 19

Figure 9. Sensor #36 output waveform obtained with the cone and ground plane standard

field generation system using the generator waveform of Figure 8 19

IV
Figure 10. Pre-trigger common-mode signal removal using SMA coaxial barrel without a center conductor 20

Figure 1 1 . Raw sensor #37 output obtained with the cone and ground plane system without

averaging or common-mode signal removal 21

Figure 12. Raw sensor #37 output obtained with the cone and ground plane system with 1024

averages. Note the significant reduction in high-fi^equency random variations 21

Figure 13. Pre-trigger common mode signal detected by sensor #37. 1024 averages 22

Figure 14. Sensor #37 output after 1024 averages and pre-trigger common-mode signal

removal. The sensor response occurs approximately in the range of 3 ns to 9 ns, and the

environmental reflections begin at approximately at 11 ns 22

Figure 15. Sensor #36 receiving transfer function results obtained with the TEM cell standard field generation system 23

Figure 16. Sensor #36 receiving transfer fijnction results obtained with the cone and ground

plane standard field generation system 23

Figure 17. Intercomparison of sensor #36 receiving transfer fianction results obtained with the

TEM cell and cone and ground plane standard-field generation systems 24

Figure 1 8. Sensor #37 receiving transfer fianction results obtained with the TEM cell standard field generation system 24

Figure 19. Sensor #37 receiving transfer fianction results obtained v^th the cone and ground

plane standard field generation system 25

Figure 20. Intercomparison of sensor #37 receiving transfer fianction results obtained with

the TEM cell and cone and ground plane standard-field generation systems 25

Figure 21 . Sensor #33 receiving transfer fianction resuUs obtained with the TEM cell standard

field generation system. These data were obtained with the cylindrical post behind the sensor

(with respect to the incident wavefi-ont) 26

Figure 22. Sensor #33 receiving transfer fijnction results obtained with the cone and ground

plane standard field generation system. These data were obtained with the cylindrical post

behind the sensor (with respect to the incident wavefi-ont) 26

Figure 23. Sensor #33 receiving transfer fianction results obtained with the cone and ground

plane standard field generation system. These data were obtained with the cylindrical post in

fi-ont of the sensor (with respect to the incident wavefi^ont) 27
Figure 24. Intercomparison of sensor #33 receive transfer function results obtained with the

TEM cell and cone and ground plane standard-field generation systems. These data were

obtained with the cylindrical post behind the sensor (with respect to the

incident wavefi"ont) 27

Figure 25. Sensor #34 receiving transfer function results obtained with the TEM cell standard

field generation system. These data were obtained with the narrow baseplate dimension

parallel to the incident wavefront 28

Figure 26. Sensor #34 receiving transfer function results obtained with the cone and ground

plane standard field generation system. These data were obtained with the narrow baseplate

dimension parallel to the incident wavefront 28

Figure 27. Sensor #34 receiving transfer function results obtained with the cone and ground

plane standard field generation system. These data were obtained with the wide baseplate

dimension parallel to the incident wavefront 29

Figure 28. Intercomparison of sensor #34 receiving transfer function results obtained with

the TEM cell and cone and ground plane standard-field generation systems. These data were

obtained with the narrow baseplate dimension parallel to the incident wavefront 29

Figure 29. Ungated sensor #35 time-domain response obtained on the cone and ground plane

standard-field generation system. Note that the sensor response is extended by resonances in

the support structure 30

Figure 30. Sensor #35 receiving transfer function results obtained with the cone and ground

plane standard field generation system 30

VI
TIME-DOMAIN CALIBRATIONS OF D-DOT SENSORS

Robert T. Johnk and Arthur R. Ondrejka

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, Colorado 80303

This technical note covers in detail the procedures that are necessary to measure

the receiving transfer function of broadband electric-field sensors using directpulse, time-domain methods. The calibration techniques presented here are applied to a cone and ground plane system in the 50 MHz to 14 GHz fi-equency

range, as well as to a TEM cell in the 10 MHz to 100 MHz range. Measurement

results using both types of facilities are presented for selected D-Dot sensors. In

addition to a comparison of results fi^om the two measurement facilities in the

overlapping fi^equency range, a thorough combined uncertainty analysis is presented.

Key words: averaging; antenna; cone; common-mode; d-dot; electric-field; ground

plane; oscilloscope; pulse generator; TEM cell; uncertainty.

1. INTRODUCTION

The National Aeronautics and Space Administration (NASA) recently conducted field tests on

large commercial aircraft. The aircraft was flown close to various high-power electromagnetic

radiators (air surveillance radars, broadcast transmitters, etc.) in order to measure the resuhing

fields at key locations of the airfi-ame. The goal of this effort is to assess the impact of

electromagnetic radiation on aircraft avionics systems and potential electromagnetic interference

aircraft safety hazards.

In order to perform this study, NASA placed electromagnetic sensors at strategic internal and

external locations on the aircraft, such as the passenger compartment, cargo bay, and key avionics

bays. NASA chose linear electric-field sensors known as D-Dot sensors which, at sufficiently low

fi"equencies, produce output voltages that are proportional to the time derivative of an impinging

electric field [1]. At higher frequencies, for which the sensor critical dimensions become

comparable to the wavelength of the incident field, this derivative property is lost as the sensor

enters the resonance region and operates in a manner similar to a monopole antenna. The sensor

outputs were fed into calibrated spectrum analyzers that recorded the results at selected

frequencies in the 25 MHz to 6 GHz band. In order to determine the electric fields that were

present, the sensors must be calibrated with incident fields that have known spatial and temporal

properties, from which the mathematical relationships between the incident field and the output
voltage can be extracted. A sensor calibration over a wide frequency range is a requirement in

order to obtain meaningful electric field measurement results. The National Institute of Standards and Technology (NIST) in Boulder, Colorado has several

facilities that enable the accurate and efficient calibration of electromagnetic sensors over such a wide frequency range. Two facilities that were used to generate a standard reference fields for the

calibration of the NASA sensors were the TEM cell and the cone and ground plane system. The

TEM cell is a closed transmission line system that generates reference fields from DC to a frequency at which the first higher-order modes occur, depending on the dimensions of the cell. For the 1 .2 m TEM cell that was used in this effort, this frequency range is to 100 MHz. The

cone and ground plane system is used to extract the sensor receive transfer fianction in the range

of 50 MHz to 14 GHz. The cone and ground plane is an open 200 Q (nominal) transmission line

system that generates an incident field with a simple mathematical representation that depends

solely on geometry, making it a highly accurate primary standard. The only drawback of this

system is the time window limitation of 1 1 ns dictated by the length of the cone and area of the

ground plane. However, since the response times of a wide variety of electromagnetic sensors are

less than 1 1 ns, this facility is highly usefiil for calibrations.

2. D-DOT SENSOR DESCRIPTIONS

Five diflferent sensor configurations were calibrated in this effort. A sketch of the sensor that was

tested is provided in Fig. 1 . As the sketch shows, the sensor has a simple geometrical and

mechanical structure consisting of mounting plate, a coaxial connector, and a solid metal central

element. Structural support is provided by a plastic cylindrical shell. The sensor is designed to be

used on a large, highly conducting ground plane, with the connecting cable and associated

instrumentation located on the opposite side of the ground plane from the sensor element.

The sensors were deployed in five different configurations ranging from an actual ground plane

mount to four different combinations of sensor and metal plate support structures. Sketches of

the four sensor / metal plate structure configurations are depicted in Fig. 2. The first sensor

(Serial #36) had no associated structure, and it was deployed by bolting it directly to the ground

plane, in accordance with the manufacturer's specifications. The second sensor (Serial #37) had

an attached sheet-metal base plate. The third sensor (Serial #33) had a slightly more complex

structure associated with it, consisting of a metal base plate with a cylindrical post (used for

mounting purposes). The fourth sensor configuration (Serial #34) was mounted an angled piece

of sheet metal, with two of the sides left open to the environment. This sensor was elevated

approximately 0.1m above the ground plane. The fifth and final sensor (Sensor #35) was

mounted centrally on the top of a 0.77 m high metal box. NASA requested that four of the sensors be calibrated on these structures to see how performance would be affected.

3. SENSOR CALIBRATION PROCEDURES IN THE TEM CELL

The TEM cell sensor calibration setup is depicted in Fig. 3. The TEM cell is a 50 Q, matched

stripline transmission structure that generates electric and magnetic fields that can readily be

predicted from a known applied voltage [2]. The sensor under test is placed on the floor of the
cell along the longitudinal axis at the midpoint of the cell. This location is one of good field

uniformity, and it is convenient for connecting cables without inducing severe common-mode

signals. The cell is terminated with a precision broadband 50 Q. load, and is driven with a lowfrequency, low-voltage pulse generator with a spectrum to 200 MHz. The pulse generator

produces a unipolar waveform approximately 15 ns in duration at a repetition rate of 50 kHz. The

generator output waveform across a matched 50 Q load is depicted in Fig. 4.

In order to extract the required sensor parameters, two waveforms are needed: a reference signal

from the generator itself and one from the output of the D-Dot sensor. The method by which the

generator output reference waveform is obtained is shown in Fig. 5, where the output of the

pulse generator is fed into the input of 20 GHz digitizing sampling oscilloscope, through a

precision attenuator. The attenuator must be included to prevent severe nonlinear distortion or

physical damage to the sampling head. The precision attenuator used in these measurements has a very flat insertion loss characteristic (less than 0.2 dB variation) and a very linear insertion phase

over the frequency range of interest (10 MHz to 6 GHz), with resulting minimal dispersion. The

second required waveform is the sensor output which is obtained by a direct connection to the

sampling oscilloscope. In each case, the oscilloscope captures and digitizes both waveforms for

subsequent signal processing on a computer.

A tj^ical output waveform (sensor #36) obtained in the TEM cell depicted in Fig. 6. The initial portion of the waveform is due to the differentiating property of the sensor on the incident

unipolar E-field pulse; the slight distortion that is visible on the initial doublet, along with the

subsequent oscillations, are due to the occurrence of higher-order modes in the TEM cell. Previous studies [2], as well as the data obtained in this effort, indicate that the first non-TEM

mode occurs around 130 MHz. This limits the upper usable frequency limit (for purpose of

standard-field calibrations) of the cell to approximately 100 MHz.

The receiving transfer function [3,4] of the sensor is a fi-equency-domain quantity (with an assumed 50 Q load) that requires a Fourier transformation process for both the reference and

sensor time domain waveforms. The complete process of measuring the receive transfer function

involves the following steps:

1

.

Using the ungated sensor output time domain waveform, compute the frequency domain

spectrum amplitude [5] from 10 MHz to 100 MHz using a Fourier transformation. This can be effected using an FFT, chirp z-transform, or a series approximation to the Fourier integral.

We used the series approximation because of simplicity and the ability to specify arbitrary

frequency steps [6]. For the TEM cell, time gating is not required to eliminate the effects of

higher-order modes since the filtering action of the Fourier transform algorithm provides the

required suppression of higher-order mode effects and provides the required separation of

sensor effects.

2. Compute the reference waveform amplitude spectrum using the ungated reference waveform

using the same procedure as step 1 . The reference waveform is obtained by feeding the pulse

generator output through a precision 30 dB coaxial attenuator using the setup depicted in Fig.

5.

3. Take the ratio of the amplitude spectrum of step 1 to that of step 2. This division of spectral

amplitudes in the fi^equency domain is referred to as deconvolution [7].
4. The required receive transfer function magnitude is finally obtained by multiplying the ratio

obtained in step 3 by the TEM cell field geometry factors given in [2]. These geometry

factors are a fiinction of the cell dimensions as well as the location of the sensor under test.

This process is summarized in the following equations:

^ \vsensor (nP \J /

K

geom

,_ ^•«(/)| J

(1)

geom

^incident \J )

In eq (1), f is the frequency of operation, TFR(f) is the receive transfer fiinction; Vin(f), Vsensor (f) are the fi^equency-domain reference and sensor voltages; Eincident (f) is the standard electric field

illuminating the sensor. The applied voltage and the incident field are interrelated by the

fi^equency-independent Kgeom term that is a fiinction of only the TEM cell geometry (provided that

higher-order modes have been properly filtered out in the Fourier transformation process). This

constant is governed by the quasi-static electric field configuration within the cell. This constant can be readily determined fi^om tables and is provided in [2]. A method of moments code has

also been developed at NIST that solves an electrostatic integral equation and generates an

electric-field flux map of the of the cell [8]. Equation (1) assumes that this system, consisting of

the TEM cell, pulse generator, oscilloscope, cables, and load, has 50 Q impedances throughout.

Any variations fi"om this value, due to mismatches in the scope, load, cables and cell, along v^th

deviations fi'om 50 Q of cell and cable characteristic impedances generate systematic errors in the

predictions of eq (1). These mismatch errors are accounted for by increasing the estimated Type

B uncertainties [6,9]. This issue is dealt with in a later section of this report.

4. SENSOR CALIBRATIONS ON THE CONE AND GROUND PLANE

For the fi"equency range of 50 MHz to 14 GHz, the cone and ground plane system, depicted in Fig.

7, is used. This system uses a 200 Q conical transmission line to generate an axially symmetric

TEM spherical wave. The electric field generated by this system has the simple time-domain

analytic form [3];

^ rsin((9)ln[cot(<9o/2)]

where r, 9 denote the location of the sensor under test, 0o is the solid angle of the cone (4° for the

NIST system), and Vbase is the total time domain voltage at the base of the cone given by

gen

with

^'tee(0 = K„„(0(i+r), (3)

7 - 7

P_ cone gen

7 7

cone gen
subject to the time constraint:

0</<ll ns.

The elements in the numerator and denominator of the right side of eq (4) are the characteristic

impedances of the pulse generator and cone, which, for the NIST system, are nominally 50 Q and

200 Q, respectively. Equations (2) and (3) are valid with the assumption of a purely forwardtraveling wave that originates from the base of the cone. As soon as reflections occur, the

equations are no longer valid, and the model becomes far more complicated. The dimensions of

the cone and ground plane on the NIST system constrain the maximum usable time window to a maximum of 1 1 ns [3], which is quite satisfactory for the D-Dot sensors that were tested, as well

as for a wide variety of other types of broadband sensors and antennas.

Sample reference and sensor waveforms for sensor #36 are shown in Figs. 8 and 9. The pulse

produced by this generator is considerably faster than the one used for the TEM cell calibrations,

and it has a usable spectrum (when received by these D-Dot sensors) up to 10 GHz. The sensor waveform has two principal contributions, the sensor response itself (3 ns < t < 6 ns) and the

major reflections from the end of the cone, edges of the ground plane, and the walls, floor, and

ceiling of the room (t > 13 ns). The sensor response portion of the waveform of Fig. 9 possesses

a more complicated form than that obtained in the TEM cell, because this much faster pulse (with

its much higher frequency content) excites a higher number of "antenna-like" modes on the

sensor. The bipolar nature of the sensor response pulse is still indicative of low-frequency

differentiating properties of the sensor. The procedures used to calibrate the D-Dot sensor on the cone and ground plane are similar to

those used in the TEM cell. As is shown in Fig. 7, the sensor is mounted on the ground plane at a

suitable distance from the base of the cone that is within its projected volume. For the NASA

calibrations the distance is set at r = 2.3m and 9 = 90° to obtain the maximum usable time

window. The base of the cone is driven by an impulse generator that produces a 10 V, 65 ps

waveform, at a repetition rate of 50 kHz. The generator triggers a 20 GHz sampling oscilloscope

that records the output of the sensor under test. The calibration procedure involves the following steps:

1. Capture and time gate the sensor waveform. The objective of the time gating procedure is to

delete undesired portions of the received waveforms by simply "zeroing" them out. In the case of the sensor waveform, the gating process completely eliminates the spurious reflections

from the end of the cone, the edges of the ground plane, and the walls of the room.

2. Fourier transform the sensor waveform obtained in step 1, along with the reference waveform

in a manner identical to that of the TEM cell.

3. Perform the required deconvolution procedure by computing the ratio of the sensor amplitude

spectrum to the ratio of that of the reference sensor.

4. Multiply the result of step 3 by the analytical ratio of the frequency-domain input drive voltage

to incident electric field to obtain the desired sensor receive transfer function magnitude.

This process is summarized as follows:
rF^(/)U0.625 ^cgp

Vsensor if)\ y J f v.M) gen

(4)

with

K^^^335rsm9.

In eq (4), Kcgp is the ratio of the voltage at the base of the cone to the generated electric field for

the NIST system. This ratio depends solely on the location (r,6) of the sensor under test. Since

the voltage at the base of the cone and the resulting electric field have the same functional

dependence in the time-domain, the Fourier-transformed ratio Kcgp of these quantities is independent of fi'equency for the fiandamental, axially symmetric mode. Once again, only

fi'equency-domain magnitudes are used in this case since phase information was not a

requirement.

5. NOISE REDUCTION

Two significant interfering signal components were encountered during the sensor calibration

process: random noise and common-mode noise. The random noise component was significant,

due primarily to the ultrawide (20 GHz) bandwidth of the sampling oscilloscope. This component

of noise is amenable to reduction by averaging over an ensemble of received waveforms [3]. The

sampling oscilloscope that was used for this effort has a waveform built in averaging feature, and

all of the captured waveforms were averaged 1024 times. The net effect of this process is a 30

dB reduction of random noise. The other interfering component, which is particularly severe in

the cone and ground plane measurements, is due to common-mode signals that are generated by

the test setup. Recent investigations at NIST have traced the source of this common-mode

component to radiation from the rear panel of the pulse generator. This radiation generates a pretrigger common mode signal that is synchronized with the trigger signal, and contaminates the

entire received waveform with a deterministic error component. This effect is particularly severe with the cone and ground plane calibrations, due to the open system architecture. Although a common-mode signal was present in the TEM cell calibrations, the observed level was far smaller

than that of the cone and ground plane. Observations of this signal component indicate that it is completely deterministic, so waveform averaging has no effect in reducing it. The impact of the

common mode signal is twofold: it makes gating the received time signals ambiguous since there

are no obvious points at which to set the gate, and it causes systematic errors in the frequencydomain receive transfer function measurements.

In order to reduce greatly the common-mode component, the following three-step subtraction

procedure was found to be very effective:

1

.

The averaged sensor waveform with the common-mode signal component is acquired and

stored.

2. A coaxial barrel is inserted in series with the generator feed line at the coaxial connector at the

base of the cone. The sensor waveform is then averaged, captured, and stored. This barrel.
depicted in Fig. 10, has its inner conductor removed, providing isolation from the pulse

generator and still permitting the common-mode currents to flow on the outer conductor of

the generator, trigger, and sensor coaxial cables. The net result of this measurement is a

deterministic common-mode signal.

3 . Subtract the waveform obtained in step 2 from that of step 1 . The result is a signal with at least a 40 dB reduction in common mode.

The effects of signal averaging and common-mode subtraction are illustrated in Figs. 1 1 through

14. Figure 1 1 depicts a single time-domain waveform captured by the sampling oscilloscope

using sensor #37 on the NIST cone and ground plane system. The waveform is heavily

contaminated with high-frequency random noise. A deterministic common mode signal is also

readily visible. The result of applying 1024 waveform averages is shown in Fig. 12, where the

random noise component is significantly reduced, but the common mode component remains.

The common-mode waveform is shown in Fig. 13. This component is clearly visible on portions

of the waveform of Fig. 12, where the sensor response and reflections are not present. The

subtraction of the common-mode signal from that of Fig. 12 results in the clean and easily gated

result in Fig. 14. Once the previously described three-step process is completed, the resulting

time-domain waveform is ready to time-gate and transform into the frequency domain.

A virtually identical noise-reduction process was carried out for the low-frequency TEM cell calibrations, except that the SMA barrel was placed at the input cormector of the cell, instead of

at the base of the cone.

6. TRANSFER FUNCTION RESULTS

Sensor #36 was the only sensor that did not have an associated metallic structural component and,

therefore, provided a good indication of true sensor performance. During the measurements, it was deployed as a true ground plane sensor. It was securely mounted to the ground plane with

machine screws, and an excellent conductive contact was maintained. The measured results for

this sensor are depicted Figs. 15 to 17. The plot of Fig. 15 depicts the average sensor receive

transfer function results (for three measurements) obtained in the TEM cell for the frequency

range of 10 MHz through 100 MHz. The differentiating property of this sensor is clearly evident

in the logarithmic frequency plot, where the sensor response is nearly a perfect straight line with a

slope very close to 20 dB/decade. The receive transfer function results are extended with

measurements on the cone and ground plane system to cover the frequency range of 1 00 MHz to

6 GHz. The average of eight measurements is shown in Fig. 16, where the differentiating

property rapidly breaks down above 1 00 MHz, and the sensor begins to operate as an antenna.

The combined results in the overlapping frequency range of the two calibration systems are shown

in Fig. 17 where the results agree well within the estimated measurement (see the last section of

this report) uncertainties of the two measurement systems. A systematic offset is apparent

between the two sets of measured results. The reason for this offset is not known at this time, but one strong possibility might be attributed to local field distortion effects inside the TEM cell,

caused by the presence of the sensor. One aspect of the data that suggests this is the frequencydependent variation in slope that are present on the TEM cell results, a feature not present on the cone and ground plane results. Measurements in TEM cells of various sizes and numerical
modeling of the cell/sensor combination will be required to resolve this issue in a quantitative

fashion.

The measurement results for sensor #37 are summarized in Figs. 18 through 20, where

characteristics very similar to those of #36 have been obtained. The results are averaged over a sequence of repeated measurements: three for the TEM cell results; 8 for the cone and ground

plane. The low-frequency TEM cell results, depicted in Fig. 1 8, are very close to those obtained

for sensor #36, with a slope that is also very close to 20 dB/decade. The cone and ground plane

data of Fig. 19 track those of sensor #36 up to a frequency 3 GHz, beyond which there are noticeable differences. These differences are probably due to the discontinuity at the junction of

the metal sheet and the ground plane. The overlap data, depicted in Fig. 20, exhibit similar

characteristics to those previously obtained for sensor #36; and a similar offset is observed

between the two measurement systems.

Although the low-frequency data of Fig. 21, obtained for sensor #33, closely track those

previously obtained, dramatic differences occur with the higher frequency results of Figs. 22 and

23. A study of these two figures shows the major effects due to the presence of the metal post

attached to the base plate. As expected, the loading effect of the post is most dramatic when the

post is located between the sensor and the incident wave front. This is due to the shadowing

effect of the post which becomes more pronounced at the higher frequencies. Although the

effects of locating the post behind the probe are not as pronounced, significant effects are still noted. The lower frequency ripple in the data (in the 2 to 4 GHz range) of Fig. 23 strongly

suggests an interaction of the direct wave, generated by the cone, and reflections from the post.

Improved agreement is obtained between the two systems in the overlapping frequency range of

Fig. 24. The improvement might very well be due to serendipitous low-frequency tuning effects

caused by the post.

Both the high and low-frequency results for sensor #34 in Figs. 25 through 28 show pronounced

shifts from those of sensor #36. This is primarily due to the loading effects of the elevated

(approximately 0.1m) base plate on which the sensor is attached. This effect manifests itself in the

upward shift (approx. 2 dB) of the low-frequency receive transfer fimction characteristics of Fig.

25 and in the pronounced differences between the 100 MHz to 6 GHz results of Figs. 26 and 27

for two different base plate orientations. One dramatic effect is present in Fig. 26, where a deep

resonance is visible at 3.6 GHz. As can be seen by comparing Figs. 26 and 27, this resonance is strongly a fimction of the base plate orientation. Since the base plate is basically a rectangular

waveguide transmission line section with two open ends, it forms a resonant structure that can

easily be coupled to by external fields. The 3.6 GHz null is probably due to a waveguide

resonance in the base plate structure. The overlap region between the TEM cell and cone and

ground plane systems is shown in Fig. 28, where the agreement is excellent. It appears that the

base plate provides a tuning effect in the TEM cell similar to the post of sensor #33, resulting in

better agreement than occurred for sensor #36.

The final set of results are depicted for sensor #35 in Figs. 29 and 30. The ungated time-domain

waveform for this sensor is shown in Fig. 29, where it is apparent that the sensor response does

not decay out before the environmental reflections occur. The longer response is due to structural resonance effects of the large aluminum box on which the sensor is mounted. Because of the sensor response does not ring down before the first major environmental reflections, this sensor cannot be calibrated with a reliable statement of uncertainty. However, receive transfer fiinction
results are shown in Fig. 30 in which the maximum time gate has been employed (up to the point

at which environmental reflections occur in the time-domain waveform). The low-frequency

behavior of the sensor is totally "washed out" by the presence of the box. In fact, there is no

longer a low-frequency differentiation property for this sensor.

7. UNCERTAINTIES IN THE MEASUREMENT SYSTEMS

There are two main contributing factors to the measurement uncertainty:

1

.

Uncertainties introduced by the field generation system and the associated instrumentation and

cables.

2. Uncertainties introduced by the placement of the sensor in the generated standard field. The measurement uncertainties introduced by both the TEM cell and cone and ground plane

standard-field generation systems have been carefiiUy analyzed by Ondrejka and Johnk in a NIST

technical note that is currently in preparation [9]. These standard-field generation systems are analyzed in this technical note using a Type B uncertainty analysis that uses analytical formulas in

conjunction with assumed uniform a priori probability distributions [10]. The results of this

analysis are summarized in Tables 1 and 2 below.

Table 1. Type B expanded uncertainties (from Ref. [9]) for sensor calibrations in the NIST

l.lm TEM cell field generation system with an assumed confidence level of 95%.

Source of error Nominal value of system Relative measurement

parameter uncertainty

Ceptum distance, D 0.55 m ± 0.2%

Generated field unifoimity ± 20% + 2.3%

Sensor aperture integration ± 10% ±4.0%

effects

Measured voltage ratio (variable) ± 0.2%

Reference attenuator 0.001 (30 dB) ± 0.6%

Cable mismatches 1.0 ±0.04 ± 0.07%

Sensor impedance effects iri = ±1 ;r = ±0.02m ± 5.7%

Total relative uncertainty in

(variable) ± 7.7%

ItFrI
Table 2. Type B expanded uncertainties (from Ref. [9]) for sensor calibrations in the NIST

1.2 m cone and ground plane cell field generation system with an assumed confidence level

of 95%.

Nominal value of system Source of error Relative Measurement

parameter Uncertainty

(variable) Distance from cone base to sensor ±0.1%

90° Sensor position angle

(see eq 2)

± 0.05%

40 Cone angle 0o ± 0.4%

(variable) Measured voltage ratio ± 0.2%

0.001 (30 dB) Reference attenuator ± 0.6%

1.0 ±0.04 Cable mismatches ±0.1%

|r| = ±1 ; r = + 0.02 m Sensor impedance effects ± 9.9%

(variable) Total relative uncertainty in

±10.1%

ItFrI

While the uncertainties listed in the tables provide a complete description of the components of

errors in the two systems in terms of intrinsic field generation capabilities they do not completely

account for effects introduced by the sensor under test. One significant source of sensor effects

that are not accounted for in the Type B analysis is due to the sensor support structures (see Fig.

2). Since the support structure is exposed to the illuminating field, it becomes part of the sensor

itself and affects the receive transfer fianction. These effects are caused by structural resonances,

contact with the ground plane (or TEM cell floor), and the resulting modifications in the sensor

receiving aperture plane. These effects could be accounted for using a Type B analysis, at least to

some extent, using sophisticated numerical modeling of the measurement system and the sensor under test, but such an endeavor would be a formidable task indeed. Clearly, a significant effort is needed to account for the systematic effects introduced by the sensor support structures. However, such an undertaking is beyond the scope of this effort. The random effects of the sensor mounts that are not covered in the Type B uncertainty analysis,

however, can be accounted for by a Type A statistical analysis, based on repeated sensor measurements. In this effort, each sensor measurement was performed either three or eight times. In addition, at the conclusion of each measurement trial, the sensor was completely dismounted

from the measurement system, and the interconnecting sensor and generator cables were

10
completely disconnected in order to eliminate potential statistical bias. In order to assess system

stability and reproducibility, the system was shut down for a minimum of 24 hours after several

trials were completed. The sequence of measurements was then completed at later date after a

suitable warm-up time for the equipment. The results of the Type A analysis are summarized in

Tables.

Table 3. Type A relative uncertainties computed from the student's t-distribution [10,11]

for a 95% level of confidence.

Measurement Freq. Sensor 36 Sensor 37 Sensor 33 Sensor 34 Sensor 34R

system MHz type A

relative

type A

relative

type A

relative

type A

relative

type A

relative

uncertainty

(n= # of

uncertainty

(n= # of

uncertainty

(n= # of

uncertainty

(n= # of

uncertainty

(n= # of

meas.) meas.) meas.) meas.) meas.)

TEM Cell 25 ±4.0% ± 4.6% ± 4.0% ±1.6%

(n = 3) (n = 3) (n = 3) (n = 3)

Cone and 173 ±1.7% ±1.4% ±1.8% ± 0.9% ±1.4%

Ground

Plane

(n = 8) (n = 8) (n = 8) (n = 8) (n = 3)

Cone and 430 ±1.9% ±2.3% ±4.5% ±1.9% ±3.9%

Ground

Plane

(n = 8) (n = 8) (n = 8) (n = 8) (n = 3)

Cone and 2700 ± 2.4% ± 2.4% ± 7.3% ±1.6% ±0.5%

Ground

Plane

(n = 8) (n = 8) (n = 8) (n = 8) (n = 3)

Cone and 5400 ±7.9% ± 15.2% ± 4.0% ± 7.8% ±3.7%

Ground

Plane

(n = 8) (n = 8) (n = 8) (n = 8) (n = 8)

Because of the small number of trials involved, the correct distribution for the evaluation of the

uncertainties is the student's t-distribution given in refs. [10,1 1,12]. This distribution is used for

statistical evaluations when the number of repeated experiments is less than 30 [1 1,12]. The

student's t-distribution rigorously accounts for the higher uncertainties that resuU from

conducting fewer measurement trials. As an inspection of Table 3 reveals, it is interesting to note

that in some instances, the measurement uncertainties increase dramatically with frequency. This

occurs, for example, for sensor #37 where the combined measurement uncertainty increases

fivefold from 2700 to 5400 MHz. The data obtained indicated that this increase in uncertainty

was primarily due to the contact variations between the sensor support structure and the ground

plane.

11
The overall measurement uncertainties can now be found by combining the Type A and Type B

uncertainties in quadrature [12], by taking the square root of the sum of squares of the uncertainty

components. The combined measurement uncertainties, along with the nominal receive transfer

function are tabulated in Table 4 below.

Table 4. NASA D-dot sensor average receive transfer function results, along with

associated combined standard uncertainties (95% level of confidence). Note: bp denotes the

support structure of sensor #34.

Frequency Sensor #36 Sensor #37 Sensor #33 Sensor #34 Sensor #34R

MHz JTFrI dB ITFrI dB ITFrI dB ITFrI dB ITFrI dB

post behind narrow bp bp rotated sensor dim II to

incident

wavefront

90°

25 -43.5 ±0.7 -43.0 ±0.8 -42.8 ±0.7 -50.0 ±0.7

173 -25.9 ±0.9 -25.6 ±0.9 -25.4 ±0.9 -25.4 ±0.9 -21.9 ±0.9

430 -24.5 ±0.9 -24.0 ± 0.9 -23.5 ±0.9 -21.2 ±0.9 -26.4 ± 0.9

2700 -40.5 ±0.9 -40.5 ±0.9 -41.3 ±1.0 -42.3 ± 0.9 -40.9 ±0.9

5400 -46.5 ±1.0 -46.1 ±0.9 -50.9 ±0.9 -50.6 ±1.0 -45.8 ±1.1

8. CONCLUSIONS

The results of the measurements demonstrate the power and simplicity of direct-pulse timedomain methods in performing broadband sensor calibrations. The sensor data obtained using the

NIST 1.2 m TEM cell and the cone and ground plane standard-field generation system covered a frequency band spanning nearly three decades. Accurate broadband response data were obtained on the five sensors for use in NASA electromagnetic compatibility studies, and they enabled

NASA to obtain vital electromagnetic field intensity data for a large commercial aircraft flown

close to high-power electromagnetic radiators.

9. REFERENCES

[1] C. E. Baum, E.L. Breen, J. C. Giles, J. O'Neill, and G. D. Sower, "Sensors for

electromagnetic pulse measurements both inside and away from nuclear source regions," IEEE

Trans. Electromag. Compat., vol. 20, no. 1, Feb. 1978.

[2] M. Kanda and R. D. Orr, "Generation of Standard Electromagnetic Fields in a TEM Cell,"

Nat. Bur. Stand. (U.S.) Tech. Note 1319, Aug. 1988.

12
[3] R. A. Lawton and A. Ondrejka, "Antennas and the associated time-domain range for the

measurement of impulsive fields," Nat. Bur. Stand. (U.S.) Tech. Note 1008, Nov. 1978.

[4] A. Ondrejka, J. Ladbury, and H. Medley, 'TEM Horn Antenna Design Guide," Natl. Inst. Stand. Technol. Tech Note, to be published.

[5] J. R. Andrews, M. G. Arthur, "Spectrum Amplitude—^Definition, Generation and

Measurement," Nat. Bur. Stand. (U.S.) Tech. Note 699, Oct. 1977.

[6] A. V. Oppenhiem and R. W. Schafer, Digital Signal Processing, Prentice-Hall, Inc.,

Englewood Cliffs, N. J., 1975.

[7] N. S. Nahman and M. E. Guillaume, 'T)econvolution of time-domain waveforms in the

presence of noise," Nat. Bur. Stand. Tech Note (U.S.) 1047, Oct. 1981.

[8] R.T. Johnk, "Two-dimensional electrostatic TEM cell field mapping code," Unpublished

Fortran Code, (developed at NIST Oct. 1992).

[9] A. Ondrejka and R. T. Johnk, " Sensor and antenna calibration methods using NIST standard

field generation techniques and the three antenna method," Natl. Inst. Stand. Technol. Tech.

Note, in preparation.

[10] B.N. Taylor and C.E. Kuyatt, "Guidelines for evaluating and expressing the uncertainty of

NIST measurement resuhs, Natl. Inst. Stand. Technol. Tech. Note 1297, Sept. 1994.

[1 1] G. W. Snedecor and W. G. Cochran, Statistical Methods, Iowa State University Press,

Ames, Iowa, 1972.

[12] J. R. Taylor, An Introduction to Error Analysis, University Science Books, Sausalito, CA,

1982.

13
DIELECTRIC

SPACER

SOLID

METALLIC

ELEMENT

BASEPLATE

COAXIAL CONNECTOR

D-DOT SENSOR

(SIDE VIEW)

Figure 1. Basic features of the D-Dot sensor used in the NASA evaluation effort.

14
D-DOT

SENSORv

H

CYLINDRICAL

POST

Si

SENSOR #37 SENSOR #33

HI

SENSOR #34

SENSOR #35

SENSOR MOUNTS (SIDE VIEW)

Figure 2. Support structures for sensors #33, #34, #35, and #37. Sensor #36 had no

supporting structure other than the small base plate shown in Figure L

15
PULSE

OUTPUT

PULSE

GENERATOR

D-DOT SENSOR ^

TRIGGER

LINK

son

LOAD

• •

(r-^

SAMPLING

OSCILLOSCOPE COMPUTER

Figure 3. TEM cell sensor calibration setup.

10 15 20 25 30

Time (ns)

35 40 45 50

Figure 4. Low-frequency pulse generator output used for the TEM cell sensor calibrations.

16
PULSE

GENERATOR

PRECISION

30 DB

ATTENUATOR

SAMPLING

OSCILLOSCOPE

Figure 5. Reference waveform acquisition setup for both TEM cell and cone and ground

plane sensor calibrations.

0.1

0.06

0.06

i" 0.04-

ID % 0.02 + >

O JQ.QfZ O

M

V)

-0.04

-0.06

0.08

-0.1 10

-H h- 20 30

Time (ns)

40 50

Figure 6. Sensor #36 output waveform obtained with the TEM cell standard field

generation system using the generator waveform of Fig. 4.

17
CONE

GROUND PLANE

COMPUTER

GENERAL

FIELD

POINT at

(r,0)

PULSE

OUTPUT

PULSE

GENERATOR

TRIGGER

LINK

(r

SAMPLING

OSCILLOSCOPE

Figure 7. Cone and ground plane sensor calibration setup.

18
u.o - > 0.25 -

a> 2 0.2 o^ 0.15

-

o. o o-i - -

§ 0.05

o> n \ . \t 5 "

« ijf

-0.05 1 h- \ 10

Time (ns)

15 20

Figure 8. High-frequency pulse generator output used for the cone and ground plane sensor

calibrations.

0.02

Figure 9. Sensor #36 output waveform obtained with the cone and ground plane standard

field generation system using the generator waveform of Figure 8.

19
CONE and

GROUND

PLANE SYSTEM

PULSE

GENERATOR

SMA BARREL W/O

CENTER

CONDUCTOR

ATTACHED TO

CONE

BASE FEED POINT

GENERATOR CABLES

ATTACHED WITH

SMA CONNECTORS

^NuCVS/^l^^/\/V/4/\'^S^

^7V\/\/N/\/yVTs/S^C?

SMA BARREL

WITH

CENTER CONDUCTOR

REMOVED

Figure 10. Pre-trigger common-mode signal removal using SMA coaxial barrel without a

center conductor.

20
\J.\JH - _ 0.03 - >

T 0.02

o 0.01

: , , > 11 Jj A i 1 1 !L inkJl S

a. § -0.01

i.w -0.02 - c

0) W -0.03 _

-0.04 ^

\

' 10

Time (ns)

15 20

Figure 11. Raw sensor #37 output obtained with the cone and ground plane system without

averaging or common-mode signal removal.

0.04

0.03 - >

a> 0.02 - O)

B

o 0.01

>

3 . a

3

O -0.01 - %-o

v>p

-0.02 - a> (0 -0.03 -

-0.04

10

Time (ns)

15 20

Figure 12. Raw sensor #37 output obtained with the cone and ground plane system with 1024

averages. Note the significant reduction in high-frequency random variations.

21
w.ut -

c« 0.03

>^mf

S, 0.02- -

iSo 0.01 - - >

fl> o

o

^r

S

i -0.01 - o1 -0.02 - - o« -0.03 - -

-0.04 1 ^ ^ 10

Time (ns)

15 20

Figure 13. Pre-trigger common mode signal detected by sensor #37. 1024 averages.

0.04

0)

o>

3

a

o

(0c

a> 0)

0.03 +

0.02

0.01

-0.01 -

-0.02 -

-0.03

-0.04 \ 10

Time (ns)

15 20

Figure 14. Sensor #37 output after 1024 averages and pre-trigger common-mode signal

removal. The sensor response occurs approximately in the range of 3 ns to 9 ns, and the

environmental reflections begin at approximately at 11 ns.

22
-^\J -

ffl S-25-_ c

o

"C -30 - . c ^^^^ 3 ^^^^^ t -35 ^y^^^

a> ^^^^^

«^ ^^^^^^

(A ^^^^^ S

-40 ^^^'^""^ ^ ^^^^^^ 1- ^^^^^

o) -45 ^^^^^

c ^^^^^ > ^^^^^

S

-50 -^,^^^

a> Q^

-55 1 1 1 1 1 1—1—1

10 100

Frequency (MHz)

Figure 15. Sensor #36 receiving transfer function results obtained with the TEM cell standard field generation system.

1000 2000 3000 4000

Frequency (MHz)

5000 6000

Figure 16. Sensor #36 receiving transfer function results obtained with the cone and

ground plane standard field generation system.

23
-A«» -

- - - Cone & Ground

S" Plane

S

c

-30 - ^^^^^^^^^^ ^ o ^——-r^^^-^^

c3 ^^^^^^ ^^m' """"

tl. £ -35

(0 ^^^*^-^"''*

c ^^n -" S ^^-^^B*"

K

^^^

o>c

B -40 . u

«

a:

-45 ^ ^ i ^ 50 60 70 80

Frequency (MHz)

90 100

Figure 17. Intercomparison of sensor #36 receiving transfer function results obtained with

the TEM cell and cone and ground plane standard-field generation systems.

ffi "2^ c

2u

c

3

u.

(A

O)c

>

'Su

0)

10 100

Frequency (MHz)

Figure 18. Sensor #37 receiving transfer function results obtained with the TEM cell standard field generation system.

24
1000 2000 3000 4000

Frequency (MHz)

5000 6000

Figure 19. Sensor #37 receiving transfer function results obtained with the cone and

ground plane standard field generation system.

-25

ffl

c-40 -

u

a> -45

50

Cone & Ground Plane TEM Cell

60 70 80

Frequency (MHz)

90 100

Figure 20. Intercomparison of sensor #37 receiving transfer function results obtained with

the TEM cell and cone and ground plane standard-field generation systems.

25
10 100

Frequency (MHz)

Figure 21. Sensor #33 receiving transfer function results obtained with the TEM cell standard field generation system. These data were obtained with the cylindrical post

behind the sensor (with respect to the incident wavefront).

-20 1

1000 2000 3000 4000

Frequency (MHz)

5000 6000

Figure 22. Sensor #33 receiving transfer function results obtained with the cone and

ground plane standard field generation system. These data were obtained with the

cylindrical post behind the sensor (with respect to the incident wavefront).

26
2000 3000 4000

Frequency (MHz)

6000

Figure 23. Sensor #33 receiving transfer function results obtained with the cone and

ground plane standard field generation system. These data were obtained with the

cylindrical post in front of the sensor (with respect to the incident wavefront).

S" -25

•o

50

Cone & Ground Plane —TEM Cell

60 70 80

Frequency (MHz)

90 100

Figure 24. Intercomparison of sensor #33 receive transfer function results obtained with the

TEM cell and cone and ground plane standard-field generation systems. These data were

obtained with the cylindrical post behind the sensor (with respect to the incident

wavefront).

27
100

Frequency (MHz)

Figure 25. Sensor #34 receiving transfer function results obtained with the TEM cell standard field generation system. These data were obtained with the narrow baseplate

dimension parallel to the incident wavefront.

1000 2000 3000 4000

Frequency (MHz)

5000 6000

Figure 26. Sensor #34 receiving transfer function results obtained with the cone and

ground plane standard field generation system. These data were obtained with the narrow

baseplate dimension parallel to the incident wavefront.

28
1000 2000 3000,.... , 4000

Frequency (MHz)

5000 6000

Figure 27. Sensor #34 receiving transfer function results obtained with the cone and

ground plane standard field generation system. These data were obtained with the wide

baseplate dimension parallel to the incident wavefront.

-25

OQo

.E -40 >

o

-45

Cone & Ground Plane •TEM Cell

50 60 —\

1

70 80

Frequency (MHz)

90 100

Figure 28. Intercomparison of sensor #34 receiving transfer function results obtained with

the TEM cell and cone and ground plane standard-field generation systems. These data

were obtained with the narrow baseplate dimension parallel to the incident wavefront.

29
0)

3

a

3

o

o

lAC

«> (0

0.03 -

0.02- • ^ A

0.01 - A A i -

-0.01 -

'' \ ^\ A-\/

V-- ^ V^ I

-0.02 - - .

-0.03 -

-0.04 \ \ 1 10

Time (ns)

15 20

Figure 29. Ungated sensor #35 time-domain response obtained on the cone and ground

plane standard-fleld generation system. Note that the sensor response is extended by

resonances in the support structure.

1000 2000 3000 4000

Frequency (MHz)

5000 6000

Figure 30. Sensor #35 receiving transfer function results obtained with the cone and

ground plane standard field generation system.

30

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