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\begin{document}

\title{A Compact High-sensitivity Tangential E-Probe With Resonator and Coupled
Balun}



\author[1]{Shang Zhang}%
\author[1]{Zhen-Guo Liu}%
\author[1]{Zhipeng Chen}%
\author[1]{Mengzi Li}%
\author[1]{Weibing Lu}%
\affil[1]{Southeast University}%


\vspace{-1em}



  
  \date{June 11, 2024}


\begingroup
\let\center\flushleft
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\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
A compact high-sensitivity probe for sensing up tangential E-field is
proposed and measured in this paper. The probe is consisted of an
electric dipole with patches used to induce the tangential E-field, a
resonator made up of open circuit stubs (capacitor) and short circuit
stubs (inductor), for achieving a specific resonance, and a compact
coupled balun for transforming the differential- mode voltage into the
single output voltage and impedance matching. The probe is fabricated on
five-layer dielectric lamina. Compared with the referenced broadband
probe, the measured \textbar{}S12\textbar{}of the probe is enhanced by
18.81 dB. The proposed probe in this paper is validated with simulation
and measurement.%
\end{abstract}%



\sloppy


A Compact High-sensitivity Tangential E-Probe With Resonator and Coupled
Balun

Shang Zhang\textsuperscript{1,2}, Zhenguo Liu\textsuperscript{1, 2, 3},
Zhipeng Chen\textsuperscript{1, 2}, Mengzi Li\textsuperscript{1, 2},
Weibing Lu\textsuperscript{1, 2, 3}

\textsuperscript{1} \emph{State Key Laboratory of Millimeter Waves,
Southeast University, Nanjing China.}

\textsuperscript{2} \emph{School of Information Science and Engineering,
Southeast University, Nanjing, China.}

\textsuperscript{3} \emph{Purple Mountain Lab., Nanjing, China.}

Email: liuzhenguo@seu.edu.cn

A compact high-sensitivity probe for sensing up tangential E-field is
proposed and measured in this paper. The probe is consisted of an
electric dipole with patches used to induce the tangential E-field, a
resonator made up of open circuit stubs (capacitor) and short circuit
stubs (inductor), for achieving a specific resonance, and a compact
coupled balun for transforming the differential- mode voltage into the
single output voltage and impedance matching. The probe is fabricated on
five-layer dielectric lamina. Compared with the referenced broadband
probe, the measured \({|S}_{12}|\) of the probe is enhanced by 18.81
dB. The proposed probe in this paper is validated with simulation and
measurement.

\emph{Introduction:} With the increase of operating frequency modern
radio frequency (RF) integrated circuits have become more and more
miniaturized, which results printed circuit board (PCB) has become more
dense. Thus, unexpected electromagnetic radiation coupling between some
sensitive components will affect the normal operation of RF integrated
circuits, resulting in very serious electromagnetic compatibility (EMC)
issues {[}1{]}. To improve the performance of RF integrated circuits, a
proper electromagnetic analysis model is needed to determine the
location of interference source in the RF integrated circuits, which is
the key to solve EMC issues.

In recent year, the near-field scanning technology for detection
location of interference sources has been developed rapidly {[}2-3{]},
which play significant role in EMC problems. The probe used to sample
E-field component is the key part of the near-field scanning
measurements. Much effort has been devoted to studying the normal
E-field probe {[}4-6{]}, such as how to design the miniaturized probe
and improve the working frequency band of probe for wideband
measurements, but less attention has been paid to the probe detected the
tangential component of E-field. Yan proposed a broadband tangential
E-probe loaded with electric dipole and integrated balun for the
near-field measurement covering the GPS band {[}7{]}. However, for
integrated circuits working in narrowband, the high-sensitivity probe
detected tangential E-field can convey more power to the instruments,
therefore, a weaker electric field can be detected. A resonant
tangential E-probe {[}10{]} has high sensitivity for sensing up the weak
electric field, compared with it, the structure of the resonator in the
probe proposed in this work is simpler and the coupled balun without via
fence has more concise structure. Moreover, the performance of this
probe is better.

In this paper, a compact high-sensitivity probe for capturing tangential
E-field with resonator and coupled balun is suitable for locating the
noisy sources or the electromagnetic interference sources of circuits
working in narrow band. The proposed probe has been fabricated and
measured.

\emph{Probe structure:} Fig. 1 shows the overall structure of the probe
with resonator and coupled balun based on 4-layer PCB. Fig. 2(a)
exhibits the layout of the probe with physical dimensions. The structure
from top to bottom is the top layer, the middle layer 1, the middle
layer 2 and the bottom layer. Both the top and bottom layers are copper
planes, which can provide the currents return paths and shield the
external field. The middle 1 layer is the metal grounding plane of
coplanar waveguide. In the middle 2 layer, the folded electric dipole
and the resonator composed of open- and short-circuit stubs as well as
the coupled balun are cascaded in turn. Fig. 2(b) shows the stack-up of
the resonator probe, which is made up of three layers of 0.254-mm thick
Rogers4350B and two layers of 0.2 mm thick Rogers4450F.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1} \emph{Overall top view of the E-probe}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Fig. 2(a)} \emph{Top view and layout of each layer with physical
dimensions (unit: mm)}

The E-probe is mainly divided into improved dipole, resonator and
coupled balun. The folded dipole arm connected with three patches
{[}10{]} through hole vias is used\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Fig. 2 (b)} \emph{Stack-up of the probe}

as the induced end in the front of the probe, mainly to increase the
contact area with the tangential electric field to enhance the
sensitivity of this probe. The end of the resonator is connected with
the dipole and the other end is connected to the coupled balun which
converts the differential mode voltage into the common mode voltage. The
other end of the coupled balun is connected with the strip line which is
connected to conductor-backed coplanar waveguide CB-CPW through a signal
via. The current generated at the probe tip through the electromagnetic
field to be measured, and then flows to the SMA connector through the
resonator, the coupled balun, the signal via and the CB-CPW.

\emph{A. Resonator:} Compared with the broadband probe, the electric
field probe with resonator has higher sensitivity working at narrow-band
frequency {[}8{]}. The capacitor and inductor are the parts of the
resonator, open-circuited and short-circuited transmission line can be
used for the resonator which has a high quality factor. The resonator
shown in Fig. 3 is loaded into the probe designed in the paper, the
function of open-circuited stub is equivalent to capacitor, and the
function of short-circuited stub is equivalent to inductor. The length
of open-circuited stub and short-circuited stub can be used to tune the
resonant frequency. Fig. 3 shows the specific dimensions of
open-circuited stub and short-circuited stub when the probe works at
1.320 GHz resonant frequency. The initial value is set to be less
than\(\lambda/8\) and the specific values can be obtained by
sweeping parameters to achieve specific resonance in HFSS.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Fig. 3} \emph{The resonator with physical dimensions (unit: mm)}

\emph{B. Coupled balun:} The function of the coupled balun is to achieve
the transition from balanced transmission line to unbalanced
transmission line. Since the electric dipole at the front of the probe
is the balanced transmission, but the output port of CB-CPW is
unbalanced port, thus, a balun is needed to achieve the transformation
from balanced to the unbalanced. Fig. 4 shows the coupled balun and its
specific dimensions adopted by the probe in this paper. The coupled
balun is used to transform the differential-mode voltage induced by the
electric dipole into the common-mode voltage and the transformation of
the input impedance at the resonator to the matched 50\selectlanguage{greek}Ω \selectlanguage{english}impedance is
obtained {[}9{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig. 4} \emph{The embedded balun with physical dimensions (unit:
mm)}

\emph{Experiments and analysis:} In order to demonstrate the performance
of the tangential E-probe with resonator and coupled balun proposed in
the paper, a conventional broadband tangential E-probe published in
{[}7{]} is used to compare. Fig. 5(a) is the physical picture of this
probe. The measurement setup is shown in Fig. 5(b), and the distance
between the tip of the probe and the microstrip line is 1 mm. In the
measurement, it should be noted that in order to obtain the maximum
electric field component, the probe was placed away from the center of
the standard microstrip line. After HFSS optimization simulation, it is
found that the peak value of the electric field of the microstrip line
is at 2 mm away from the center of the microstrip line. The comparison
between simulated results and measured ones is shown in Fig. 5(c). As
show in Fig. 5(c), measured results agree well with simulated ones
except for a little deviation in frequency between them.The simulated
result of the peak value captured by the probe is -24.39 dB at 1.320
GHz, while the measured one is -24.75 dB at 1.334 GHz. The possible
reasons is that there are machine tolerances in the manufacturing
process and at same time it is difficult to accurately control the gap
between the microstrip line and the tip of probe. Compared with the
traditional broadband probe, the value of
\textbar{}\(S_{12}\)\textbar{} is improved by improved by
18.81-dB at 1.334 GHz.

\emph{Conclusion:} To improve the measurement sensitivity in narrow
band, a compact high-sensitivity tangential E-probe with resonator and
coupled balun is proposed and tested in this paper. The resonator
composed of open-circuited and short-circuited transmission line can
enhance the sensitivity. The coupled balun is to complete the transition
from differential-mode voltage into common-mode voltage. The
\textbar{}\(S_{12}\)\textbar{} measured by this probe is
improved by 18.81 dB at 1.334 GHz. The compact high-sensitivity
tangential E-probe is more suitable for weak signal measurement.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Fig. 5(a)} \emph{Photograph of the probe}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Fig. 5(b)} \emph{The measurement setup of the probe}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Fig. 5(c)} \emph{\textbar{}S\textsubscript{12}\textbar{}
comparisons with the broadband tangential probe}

\emph{Acknowledgments:} This work was supported by the National Natural
Science Foundation of China under Grant 61671147.

References

\begin{enumerate}
\tightlist
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\end{enumerate}

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