DesignandTestingofaDiode-BasedElectricFieldProbePrototype

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Design and testing of a diode-based electric field probe prototype
Conference Paper · January 2011
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Damir Senic
Antonio Šarolić
National Institute of Standards and Technology, Boulder, CO, USA
University of Split
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Design and Testing of a Diode-Based Electric Field Probe Prototype
Zlatko Živković, Damir Senić, Antonio Šarolić, Ante Vučić
FESB – Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture
Ruđera Boškovića 32, 21000 Split, Croatia
E-mail: [email protected], [email protected], [email protected]
Abstract: The paper presents the design and the response
measurements of the simple, one-axis diode-based electric field
probe, consisting of short dipole, diode detector, low-pass filter,
transmission line and monitoring instrument. Frequency
response measurement results were compared to the simulation
results, where simulation was achieved by a method of
combining the numerical FDTD simulation with dipole/diode
circuit simulation. The measured results showed good
agreement with simulation results in the operational range.
Additionally, the measurements of the probe response to
amplitude modulated and pulse modulated signals were
performed. As expected, the response to the modulated signals
showed considerable deviation from the sinusoidal CW
response. Measurement error for such signals was calculated
and presented.
1. INTRODUCTION
The electric field probes, based on the diode-loaded short
dipoles, are widely used in various aspects of EMC
measurements, including the radiation hazard survey [1]-[4].
These
probes
represent
wideband,
non-selective
instrumentation and, thus, are mainly used for quick,
wideband measurements in free-space or even in human
tissues [3], [4]. The modern diode-based probes should
exhibit very wide operational frequency range (from few
hundreds kHz to few GHz), flat response (up to ±3 dB) in the
defined operational range, wide amplitude dynamic range and
isotropic response (for three-axes probes). Furthermore, the
probe has to be electrically small since it must not perturb the
field distribution.
The fabrication of simple, one-axis electric field probe is
presented in this work. The frequency response and probe
linearity for sinusoidal continuous wave (CW) signal were
measured in Gigahertz Transverse Electromagnetic (GTEM)
cell. To make sure that the measured CW values were
correct, the probe was also modeled by combining numerical
SEMCAD X simulation software [6] and NI Multisim
dipole/diode system equivalent circuit. As it has been showed
that the measurement error of the diode based electric field
probes significantly depends on strength and modulation of
the applied signal [7]-[10], the additional error measurements
for large modulated signals were performed. Several
This research was supported by the Ministry of Science, Education and
Sports of the Republic of Croatia (Project No. 023-0000000-3273 and
No. 036-0361630-1631).
examples of amplitude modulated (AM) signals with
different modulation frequencies and pulse modulated (ASK
and GSM) signals with different durations and duty cycles
were applied, since it has been noticed that these signals
cause significant measurement errors [7], [8].
2. ELECTRIC FIELD PROBE DESIGN
The diode based electric field probe generally consists of
five main components [2]: short dipole printed on dielectric
substrate, nonlinear detector (zero-bias Schottky diode)
connected between dipole arms, low-pass filter, resistive
transmission line and monitoring instrumentation (Fig. 1).
The incident RF electric field induces the RF oscillating
voltage Uoc on the short dipole. The DC component of the
diode detector voltage (Ud) is then proportional to the voltage
Uoc induced between the dipole arms. The resistive
transmission line (which also acts as a low-pass filter)
transmits the DC signal component from the diode to the
monitoring instrument. Its high resistance per unit length
ensures reduced incident field reception and scattering by the
line. The additional low-pass filter prevents RF voltages to
reach the diode detector from the wrong side.
Figure 1 – Electric field probe – schematic view
The fabricated electric field probe is presented in Fig. 2.
The dipole with half-length l = 2 cm was printed on
electrically thin ( h = 1.6 mm ) FR4 dielectric substrate with
relative permittivity ε r = 4.6 . The zero-bias Schottky diode
(BAT62-03W) was soldered between the dipole arms. The
low-pass RC filter consisted of two resistors (1 MΩ each) in
combination with 10 pF capacitor which ensured the cut-off
frequency of f c = 8 kHz . Each lead of the parallel resistive
transmission line was realized as a large resistance of 20 MΩ,
which yielded the overall transmission line resistance of
40 MΩ.
where Prec is the received power, Aef is effective area, S is
incident wave power density, λ is desired wavelength, E is
the electric field strength set to 20 V/m, and D is directivity
obtained from SEMCAD X simulations for different
frequency values. Calculated Uoc, along with RA and CA
obtained from SEMCAD X, were used in NI Multisim
simulation for each different frequency. The output DC
voltage was captured at each frequency and later compared to
the measured one.
Figure 2 – Fabricated electric field probe
3. SIMULATION AND MEASUREMENTS OF
ELECTRIC FIELD PROBE CHARACTERISTICS
For the simulation purposes the NI Multisim model
combined with SEMCAD X simulation results was used.
The simple developed SEMCAD X model is presented in
Fig. 3. The simulations were used to obtain directivity and
the input impedance of printed dipole antenna. Since the
transmission line, low-pass filter and the diode should not
considerably change the directivity and input impedance of
the dipole, and for the sake of simplicity, they were omitted
from the simulation. The dimensions and electrical
parameters of simulated dipole antenna on dielectric substrate
were identical to the real, fabricated model.
Figure 3 – Simulation model of the printed dipole on
substrate
The dipole/diode equivalent circuit was created using NI
Multisim. All relevant parameters of the designed probe were
included into equivalent circuit model, presented with Fig. 4,
where Uoc presents the rms open-circuit voltage across dipole
input terminals, RA is dipole resistance, and CA is dipole
capacitance. The low-pass filter and the transmission line
were omitted from equivalent circuit since they should not
affect the output steady-state DC voltage. Dipole impedance
was obtained from SEMCAD X simulation results and Uoc
voltage was then calculated from well-known equations:
Prec = Aef ⋅ S ,
(1)
U oc2
λ2
E2
,
=
⋅D⋅
4 RA 4 π
120π
(2)
λ ⋅ E D ⋅ RA
⋅
,
π
120
(3)
U oc =
Figure 4 – Simplified NI Multisim model of electric field
probe
All measurements were performed in TESEQ 750 GTEM
cell having the maximum septum high of 75 cm. The
schematic layout of measurement setup is shown in Fig. 5.
The input CW signal was generated by Rohde & Schwarz
SM300 signal generator and amplified by RF power
amplifiers AR 150W1000 (80 – 1000 MHz) and Ophir 5140
(0.7 – 3 GHz). The produced electric field probe (DUT) was
placed inside GTEM cell on styrofoam base, inside the
defined test volume, next to the commercial HI-4455
isotropic probe, which was used to control the electric field in
the vicinity of DUT. To avoid the polarization loss, the probe
was oriented parallel to the incident electric field. The tested
probe was connected to the HP 3490A digital multimeter
which was used as DC voltmeter. Due to fact that DC voltage
is measured at the end of the highly resistive transmission
line (40 MΩ), the measuring instrument input resistance has
to be high enough (>GΩ) to ensure accurate results.
Several measurements were performed using described
setup. For the probe response analysis in the frequency range
from 100 MHz to 2.4 GHz, the value of the incident electric
field was kept constant at 20 V/m. The measured DC voltage
at probe’s terminals showed flat response within ±3 dB up to
1 GHz, with the value of measured voltage of approximately
0.35 V (presented with solid blue line in Fig. 6). This is in a
good agreement with simulation results in the proposed
operational range (red dashed line in Fig. 6).
The linearity of the electric field probe was measured at
100 MHz for incident electric field strength from 5 V/m to
185 V/m, as shown in Fig. 7. Generally, the diode-based
electric field probes operate in two different regimes,
depending on the incident signal strength. If α·Ud >> 1 [3],
(where α is a diode specific parameter and equals to 38.7 for
this diode, and Ud presents the DC component of the diode
detector voltage) the diode operates in large signal regime. In
the large signal regime the rectified DC voltage is linearly
proportional to the amplitude of the incident field, while for
the small signals it is proportional to the square of the
incident field amplitude. For the intermediate signals, the DC
voltage exhibits neither purely linear nor purely quadratic
proportionality. Due to the applied incident field strengths
and the resulting induced voltages, the electric field probe
operated mainly in the large signal regime. Measured
linearity response, shown in Fig. 7, was quite linear for
incident electric field from 5 V/m to 185 V/m.
4. MEASUREMENTS OF ELECTRIC FIELD PROBE
RESPONSE TO MODULATED SIGNALS
In the small signal regime, the probe will respond to the
true RMS field strength, while in the large signal regime it
will respond to the peak field strength, even if the measured
signal is of the complex waveform [7]-[10]. Hence, the
significant measurement error could be expected for the large
modulated signals, especially for amplitude modulated (AM)
or time-division multiple access (TDMA) signals, such as
pulsed radar signals or GSM signals.
As observed in previous section, fabricated probe operates
in large signal regime for the applied incident field strength
dynamic range. Thereby, significant error in the probe
detection is expected when the probe is exposed to modulated
signals. To verify the former, the probe has been exposed to
various modulated signals; AM, ASK and GSM TDMA, and
measurement error was monitored.
Figure 5 – Schematic layout of measurement setup
According to [8] and [10], measurement error (∆) is
expressed as logarithmic ratio of the field strength meter
reading Edisplay and true rms field strength:
∆ = 10 log10
2
Edisplay
E
2
rms
= 20 log10
Edisplay
Erms
(4)
To determine the probe measurement error, it is necessary to
compare the displayed field strength Edisplay with the true rms
value of field strength Erms as defined in (4).
According to [7], the measurement setup should ensure
controllable waveform, amplitude and frequency, and
measurement of true rms value of incident electric field.
Figure 6 – Frequency response
Controllable waveform is necessary to estimate the probe
response for various waveforms, as the measurement error is
expected to be dependable of waveform parameters. Beside
the waveform, the measurement error is also dependable on
the incident electric field amplitude. This is significantly
emphasized at the upper part of probe’s amplitude range
where the probe's response should be purely linear. Hence, it
is necessary to produce high fields with magnitude of
100 V/m or even higher.
Measurement of true rms value of incident field could be
achieved by measuring the true average power from
amplifier. This power is proportional to the true rms value of
incident electric field. For this purpose the Rohde-Schwarz
NRP Z21 true average power sensor was used.
The measurement error should not depend on carrier
frequency as long as the frequency is inside the probe’s
working range.
Figure 7 – Measured linearity response at 100 MHz
Used measurement setup for measuring probe response to
various modulated signals, was similar to setup used for
measuring probe’s characteristics, shown in Fig. 5. Only
difference is the directional coupler that is connected between
the amplifier and GTEM cell. Its coupled port was used for
power measurements. The reflected power from GTEM cell
was also measured with and without probe inserted, in order
to check for possible field perturbations caused by probe’s
presence. Modulated signals were monitored with spectrum
analyzer and oscilloscope to make sure that amplifier did not
cause the signal deformation. No deviations were observed
during this testing procedure.
error was in all cases considerably lower than for ASK
signals.
As expected, the error increased at higher incident field
amplitudes. However, it varied by only 1 dB from 20 to 100
V/m, which again confirmed that the probe was well into the
linear regime already at 20 V/m.
The initial measurements were performed using CW signal
adjusting its electric field magnitude to the predefined value.
For every level of the predefined incident electric field value,
the true average power was measured and DC voltage at
probes terminals was noted. This presented the reference
level, corresponding to the true rms field strength in (4).
After that, the different types of modulation were
generated with Rohde & Schwarz SM300 signal generator.
Employing modulation, instead of CW signal, generally
causes change in power reading depending on modulation
parameters. By setting the modulated signal power level
(measured by true average power sensor) to the average
power of the CW signal, the rms field strength of the
modulated signal should be the same as the rms field strength
of the CW signal. However, the measured DC voltage was
generally different and measurement error could be
calculated according to (4). For this calculation, the output
voltage was used instead of measured electric field in (4).
Figure 8 – Measured DC voltage at probe’s terminals as a
function of incident true rms field value
The measurements were performed for three different
modulation types: AM with 10 kHz, 40 kHz, and 80 kHz
modulating frequency and modulation index of 80%; ASK
with 1 kHz repetition frequency and duty cycles of
DC = 1/10 and 1/2, and finally GSM TDMA scheme. The
GSM TDMA scheme was achieved as ASK with 217 Hz
repetition frequency and duty cycle of DC = 1/8.
The carrier frequency was set to 100 MHz for all
modulations – the error should not depend on the carrier
frequency but only to the waveform and modulation
parameters.
Results of measured voltage for different modulations
compared with CW signal is shown in Fig. 8, and probe’s
measurement error for three types of modulation with
different parameters is shown in Fig. 9. Significant error in
probe’s display could be observed depending on the type of
the applied modulation.
Error was highest for the ASK signals with small duty
cycles (ASK with 1/10 duty cycle and GSM with 1/8 duty
cycle) and reached -9dB. The negative sign presents the
underestimation error. This is especially dangerous since it
can lead to undetected overexposure i.e. radiation hazard.
Considering the amplitude modulation, the error increased
as the modulation frequency decreased. However, the AM
Figure 9 – Electric field probe measurement error
5. CONCLUSION
The electric field probes, based on diode-loaded short
dipole, are often used in EMC measurements including
radiation hazard surveys. The simple one-axis design of such
electric field probe was presented in this paper. Its frequency
response and linearity response for sinusoidal CW signal
were measured and presented. The measurement results
showed good agreement with model developed by a
combined numerical FDTD simulation and dipole/diode
circuit simulation.
In addition, the probe's response to amplitude and pulse
modulated signals was measured in terms of measurement
error. The obtained results showed significant deviation
compared to CW probe response. This leads to considerable
measurement errors when measuring such modulated signals.
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