See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/289612089 Design and testing of a diode-based electric field probe prototype Conference Paper · January 2011 CITATIONS READS 0 1,327 4 authors, including: Damir Senic Antonio Šarolić National Institute of Standards and Technology, Boulder, CO, USA University of Split 34 PUBLICATIONS 217 CITATIONS 52 PUBLICATIONS 216 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: COST EMF-MED View project All content following this page was uploaded by Damir Senic on 04 January 2017. The user has requested enhancement of the downloaded file. SEE PROFILE 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. REFERENCES [1] Motohisa Kanda: “Standard Probes for Electromagnetic Field Measurements”, IEEE Transactions on Antennas and Propagation, October 1993, Vol. 41, p.p. 1349-1364. [2] Howard I. Bassen, Glen S. Smith: “Electric Field Probes – A Rewiev”, IEEE Transactions on Antennas and Propagation, September 1983, Vol. 31, p.p. 710-718. [3] Ronold W. P. King, Glenn S. Smith: “Antennas in Matter: Fundamentals, Theory, and Applications”, The MIT Press, 1981. [4] Thomas Schmid, Oliver Egger and Niels Kuster: “Automated E-Field Scanning System for Dosimetric Assessments”, IEEE Transactions on Microwave Theory and Techniques, 1996, Vol. 44, p.p. 105-113. [5] Motohisa Kanda, Lanny D. Driver, “An Isotropic Electric-Field Probe with Tapered Resistive Dipoles for Broad-Band Use, 100 kHz to 18 GHz”, IEEE Transactions on Microwave Theory and Techniques, 1987, Vol. 35, p.p. 124130. [6] SEMCAD X by SPEAG, www.speag.com [7] Antonio Šarolić, Borivoj Modlic, “Measurement of Electric Field Probe Response to Modulated Signals Using Waveguide Setup”, IEEE Antennas and Wireless Propagation Letters, October 2010, Vol. 9, pp. 1041-1044. [8] J. Randa and M. Kanda, “Multiple-source, Multiplefrequency Error of an Electric Field Meter,” IEEE Transactions on Antennas and Propagation, Jan. 1985, vol. AP-33, pp. 2–9. [9] Antonio Šarolić, Borivoj Modlic and Dragan Poljak, “Electric Field Probe Measurement Error Due to Sine Signal Calibration”, 2005 International Conference on Software, Telecommunications and Computer Networks - SoftCOM 2005 (CD ROM), September 2005, Split, Croatia, pp. 1-5. [10] Antonio Sarolic, Vesna Roje, and Borivoj Modlic, “Measurement of Electric Field Probe Error for Pulsed Signals,” in Proc. IEEE Int. Symp. Electromagn. Compat., Portland, OR, 2006, vol. 2, pp. 244–248. View publication stats
