Double-Superheterodyne Topology for Magnetic Communication

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Giant Magnetoimpedance Receiver With a

Double-Superheterodyne Topology for Magnetic Communication

KIBEOM KIM ¹, SEUNGHUN RYU ¹, JANG-YEOL KIM ², IN-KUI CHO ², HYUN-JOON LEE ², JAEWOO LEE ², AND SEUNGYOUNG AHN ¹, (Senior Member, IEEE)

1The Cho Chun Shik Graduate School of Green Transportation, Korea Advanced Institute of Science and Technology, Daejeon 34051, South Korea 2Radio and Satellite Research Division, Electronics and Telecommunications Research Institute, Daejeon 34129, South Korea

Corresponding author: Seungyoung Ahn (sahn@kaist.ac.kr)

This work was supported by the Institute of Information & Communications Technology Planning & Evaluation (IITP) grant

funded by the Korea Government (MSIT) (Magnetic Field Communication Technology Based on 10pT Class Magnetic Field for Middle and Long Range) under Grant 2019-0-00007.

ABSTRACT Radio reception relies on the medium which determines the propagation characteristics of the electromagnetic fields carrying the information. The permittivity varies greatly depending on the medium, but it remains nearly constant, except when magnetic materials are used. For this reason, magnetic fields, typically affected by permeability, can be utilized in microwave challenging environments. In this paper, a new approach based on the giant magnetoimpedance (GMI) effect is presented. The proposed GMI-based receiver has an effective double-superheterodyne topology, where ‘‘effective’’ means that the receiver actually has a single mixer but appears to have added a virtual mixer due to the GMI effect.

The magnetic field-to-voltage conversion ratio (MVCR), the spurious free dynamic range (SFDR) and the receiver sensitivity are characterized, and from these results the optimal operating conditions of the fabricated receiver are obtained. Additionally, wireless digital communication using on-off keying (OOK) is demonstrated and transmitted and received waveforms are compared, with the final demodulation result of the receiver showing that the transmitted digital data are precisely extracted.

INDEX TERMS Giant magneto impedance (GMI), magnetic communication, double superheterodyne receiver.

I. INTRODUCTION

In harsh wireless communication environments such as underground or underwater environments, the conventional technique of using microwaves faces many challenges, including path loss, multi-path fading and signal propagation delays due to the presence of soil, rock and water, which are composed of various components [1]–[3].

With a non-conventional medium, communication based on magnetic induction (MI) is a good alternative when attempting to address the challenges of microwave-based communication. Recently, this approach has been applied in various fields, from disaster detection to implantable medical devices, as well as military applications [4]–[6].

Although MI-based communication has low susceptibility

The associate editor coordinating the review of this manuscript and approving it for publication was Giambattista Gruosso .

in the non-conventional medium, it remains associated with the inherent constraint of a very limited communication range [7]. In the near-field range, the magnetic field is atten- uated at a rate of 1/d³, where d is the distance to the source.

Hence, as the receiver moves away from the transmitter, the radio signals containing the messages on the channel are transmitted at a high attenuation ratio of 60dB per decade.

A possible solution to this constraint is to take advantage of the giant magnetoimpedance (GMI) effect, characterized by the high magnetic field-to-voltage conversion ratio (MVCR).

This effect stems from the phenomenon of high impedance variation in a ferromagnetic wire, driven by the carrier, when subjected to a change of the external magnetic field par- allel to the direction in which the current inside the wire flows [8], [9]. Another important characteristic of the GMI effect for magnetic communication is that it is equivalent to its own a superheterodyne radio topology, which performs

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typical amplitude modulation (AM) with respect to the external magnetic signal. In an off-diagonal GMI-based magnetometer, the output voltage spectrum shows that the external magnetic signal is up-converted when modulated with the carrier driving the magnetometer [10]. Hence, the magnetometer can be considered as identical to the combination of an antenna and an up-conversion mixer.

The GMI effect can be characterized by its ultra-high sensitivity and its potential for wireless magnetic communication. This paper presents a system analysis and discusses the principle of the operation of a GMI-based receiver with the double conversion superheterodyne topology. To implement the receiver, an additional mixer is connected to the back end of an off-diagonal GMI-based magnetometer and is used to downconvert the signal up-converted by this magnetometer to a baseband signal (See Fig. 1). In a harsh wireless communication environment, the frequency for magnetic communication is typically as high as several hundred kHz [11];

accordingly, the receiver is greatly affected by flicker (1/f ) noise, inversely proportional to the frequency, which deterio- rates its sensitivity. The GMI-based receiver can achieve high sensitivity [12]. The external magnetic signal is up-converted due to the GMI effect and then amplified while remaining less sensitive to 1/f noise resulting from this process.

FIGURE 1. Block diagram of the GMI-based receiver. Two signals produced by the Rx DDS connected to the GMI-based magnetometer and the mixer can ensure that the second IF is phase synchronized.

Here, an experimental demonstration vehicle corresponding to the proposed GMI-based receiver was implemented.

The receiver system configuration and its experimental setup are presented in Section II. A Helmholtz coil was used to measure the characteristics of the magnetic responses to the receiver with regard to a certain magnetic field strength and to demonstrate wireless digital data communication. Section III describes the characteristics of the MVCR and receiver sensitivity for the detection of external magnetic signals and the spurious free dynamic range (SFDR) for on-off keying (OOK) modulation. Waveform

FIGURE 2. Experimental environment configuration to measure the characteristics of the GMI-based receiver and demonstrate wireless magnetic communication.

FIGURE 3. Schematic view of the GMI-based magnetometer. The GMI magnetometer was fabricated using the design method described in [7].

Port 1 is connected to DDS driving the GMI wire and Port 2, which is the output of the pick-up coil, is connected to the voltage buffer (see Fig. 1).

results representing a series of digital signals indicate the possibility of magnetic communication using the GMI effect.

The concluding remarks are given in the final section of this paper.

II. SYSTEM CONFIGURATION

A block diagram of the GMI-based receiver is illustrated in Fig. 1. In the transmitter system, a message signal in the OOK format produced by a direct digital synthesizer (DDS) is fed into a power amplifier and emitted from a Helmholtz coil. As shown in Fig. 2, a uniform magnetic field with the message is directed toward an off-diagonal GMI-based magnetometer located centrally between two identical circu- lar magnetic coils forming a Helmholtz pair. The magnetic flux density at the midpoint can be defined by the equation B =(4/5)³^/2µ0NI/R, whereµ0is the permeability constant, I is the current applied to these coils, and N and R are the turns and radius of the coils, respectively [13].

As mentioned in the introduction, the magnetometer corresponding to the first stage of the receiver receives the external magnetic signal and then up-converts it to the first inter- mediate frequency (IF). The sensing components, presented in Fig. 3, consist of a CoFeSiB-based soft-amorphous wire (which is referred as the GMI wire in this paper) in the cavity

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TABLE 1. Design parameters of GMI-based magnetometer.

of an alumina insulation tube and an insulated pick-up coil wound around the GMI wire, both of which are mounted on a printed circuit board (PCB). The design parameters of the magnetometer are listed in Table 1. The GMI wire is driven by a carrier at several MHz. The sinusoidal voltage produced by the DDS of the receiver is converted to a current by an injection resistor and by the inherent resistance of the wire.

Here, the MVCR is one of the most important characteristics determining the receiver performance. The optimal values of the frequency and current of the carrier to maximize the MVCR depend on the physical parameters and material prop- erties of the GMI wire. When the frequencies of the external magnetic signal and carrier driving the wire are f_extand f_GMI, respectively, the output of the pick-up coil is the first IF with f_ext± f_GMIcontents caused by the off-diagonal (mutual) impedance between the GMI wire and the pick-up coil [14].

The sensing element is followed by a voltage buffer, which is used to prevent noise from being generated by the intrinsic impedance of pick-up coil and to minimize the power loss of the first IF. The bandpass filter (BPF) is used to eliminate unwanted intermodulation frequencies before the first IF is amplified. When compared to a single conversion heterodyne receiver, the high-frequency first IF stage in this receiver enables the received signal with a very low frequency range (∼kHz) to be less affected by the 1/f noise. The next stage is similar to typically non-coherent demodulation in a commer- cial AM.

Once the signal has passed through the first stage, it is then passed through the mixer to produce a baseband by modu- lating the up-converted first IF with a signal of a frequency identical to that of the carrier driving the GMI wire. The direct current (DC) block used here is such that a zero-Hz component generated by the mixer is removed. Here, the form of the output signal is identical to that of the signal transmitted from the Helmholtz coil. The envelop detector extracts the signal’s outline and the low pass filter (LPF) then removes the high-frequency contents that remain in the signal after the detector. Finally, the amplified signal is converted to a digital signal by passing the analog-to-digital converter (ADC). The digital signal is separated into either binary 1 or 0 by the compare function at the data acquisition soft- ware, and the message is completely detected at the receiver system.

III. SFDR, MVCR AND RECEIVER SENSITIVITY

For magnetic communication, the MVCR, SFDR and receiver sensitivity are useful metrics for evaluating the performance of a receiver system. In the demonstration setup,

FIGURE 4. SFDR from the power spectrum measurement using an external magnetic field strength of 116.2nT in the linear region (see Fig. 5) at f_ext=60 kHz.

FIGURE 5. MVCR from the first IF voltage at f_ext=60kHz. The MVCR is obtained by the least square method of linear functions of the first IF measured in accordance with the magnetic field increasing by two times.

a Helmholtz coil with N = 90 turns and R = 150mm generates an external magnetic field with f_ext ranging from 1kHz to 100 kHz. Here, f_GMI and voltage driving the GMI wire are 5MHz and 1.63V, respectively.

SFDR is a measure by which the largest noise or harmonics, called spurious signals, interfere with or distort the fundamental signal. In a non-coherent demodulation system using an envelope detector, even-order harmonics are the major cause of envelope distortions of an oscillation signal, resulting in an increased bit error rate (BER) of the data when converted from analog to digital. The strength of the AM sig- nal is determined by the power spectral density (PSD) P_buff(f) of the voltage output of the voltage buffer, corresponding to the measurement point M₁ in Fig. 1. The dBc level of the SFDR is given by 10log(P_buff(f )/P₀) with an arbitrary but largest spurious P₀. Fig. 4 shows the method used to quantify the SFDR with the data measured from f_ext = 60kHz. The up-converted signal (AM) of the GMI-based magnetometer has fundamental frequencies (f_GMI + f_ext = 5.06MHz and f_GMI − f_ext = 4.94 MHz) and second harmonics (fGMI + 2f_ext=5.12MHz and fGMI−2f_ext=4.88MHz). The analysis reveals a single overtone at 5.12MHz which is 37.7dB weaker than the first IF at 5.06MHz.

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FIGURE 6. OOK waveform generated by the Helmholtz coil and demodulation results of the demonstration receiver. The top plot presents the current waveform applied to the Helmholtz coil corresponding to a transmitter antenna, while in the receiver the voltage waveform to which the received magnetic signal is converted and its envelope are displayed in the bottom plot. The transmitted and demodulated characters and corresponding digital data are shown in the middle plot.

A larger MVCR for wireless magnetic communication means that weaker magnetic signals can be detected. This is similar to the gain of an RF amplifier. MVCR is the slope of the output voltage v_buff(B) of the voltage buffer termi- nated with 50 measured against the strength of the external magnetic field B, expressed as dvbuff(B)/dB. Fig. 5 reveals the MVCR of demonstration receiver at the fundamental frequency of 5.06MHz. Here, vbuff(B), by which the power obtained from the spectrum analyzer is converted, shows a linear increase from 215.7µV (−60.3dBm) to 53.1mV (−12.5dBm) as the strength of the external magnetic field increases from 1.07nT to 273.5nT; additionally, its MVCR is 194kV/T.

The receiver sensitivity for magnetic communication determines the weakest detectable magnetic signals, which can be obtained by dividing noise-PSD (N-PSD) P_N into the MVCR. Using the MVCR calculated above and with P_N=10.1µV/√

Hz (−86.9dBm/Hz), the receiver sensitivity is 52.1pT/√

Hz.

The SFDR, MVCR, N-PSD and receiver sensitivity values for the demonstration receiver are summarized in Table 2.

The SFDR increases as the frequency increases, while at fext = 60kHz it has the maximum MVCR and minimum receiver sensitivity. When comparing the characteristics at fext = 60kHz and 100kHz, the SFDR difference is 2.5dB, but at 60kHz the MVCR is approximately 28 times better, detecting fine magnetic field strengths as low as 1/28 times the original level. These results show that the optimal communication frequency of the receiver is 60KHz.

TABLE 2.SFDR, MVCR, N-PSD and receiver sensitivity for DUT.

IV. OOK WIRELESS COMMUNICATION RESULT

To verify the full functionality of the proposed GMI-based receiver in wireless magnetic communications, a demonstration of OOK digital data transmission is conducted.

Fig. 6 shows the waveforms for each measurement point indicated in Fig. 1, the characters, and the corresponding digital data. The DDS of the transmitter produces the message signal ‘‘Hello World,’’ which is converted to binary ASCII code. Here, the presence of a carrier frequency of 60kHz over 100ms represents the binary of 1, while its absence over the same duration represents the binary of 0.

At M₂, the RMS current value of the signal fed into the Helmholtz coil is I = 0.305mA with respect to binary 1; the Helmholtz coil generates an external magnetic flux density of B =116.2nT at the location of the GMI-based magnetometer.

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The waveform measured at M₃ is in good agreement with the transmitted signal, and the envelope waveform at M₄ shows that the message data can be detected accurately by the receiver.

V. CONCLUSION

Previous literatures introduced the off-diagonal GMI-based magnetometer for the purpose of measuring only the magnetic field strength existed in the medium [15]–[18]. These topologies merely measure peak value of the voltage induced by a GMI-based magnetometer. However, in order to use the GMI-based magnetometer for wireless communication, a topology which selects a certain frequency or frequency bands and demodulates radio signal into informative data is required. The paper proposes a GMI-based receiver with a double-superheterodyne topology for wireless magnetic communication. The GMI effect, which is the most important physical phenomenon for the implementation of the receiver, can reduce the 1/f noise and achieve a high MVCR. With the proposed GMI-based receiver, the SFDR, the MVCR and the receiver sensitivity, which are the pertinent characteristics for the sensing of a magnetic signal and to demonstrate wireless magnetic communication, are analyzed. These characteristics were extracted from different frequency ranges of the external magnetic signal, with the result indicating that the test vehicle has the most alert response at 60kHz. OOK was adopted in a demonstration. The comparison result of transmitting and receiving waveforms indicates that the proposed GMI-based receiver with the double-superheterodyne topology can be utilized to wireless magnetic communication.

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KIBEOM KIM received the B.S. degree from Koreatech, Cheonan, South Korea, in 2011, and the M.S. and Ph.D. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea, in 2014 and 2019, respectively. His current research interests include electromagnetic interference/electromagnetic compatibility for three-dimensional integrated circuits packages and magnetic sensors.

SEUNGHUN RYU received the B.S. degree from Koreatech, Cheonan, South Korea, in 2020. He is currently pursuing the M.S. degree with The Cho Chun Shik Graduate School for Green Transporta- tion, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea. His research interests include magnetic field communication and AC coupled interconnection design.

JANG-YEOL KIM received the B.S., M.S., and Ph.D. degrees in information and communication engineering from Chungbuk National University, Cheongju, South Korea, in 2010, 2012, and 2017, respectively. Since 2012, he has been with the Electronics Telecommunications Research Insti- tute, Daejeon, South Korea. His research interests include antenna design, thermal therapy algorithms, microwave sensing, and electromagnetic sensors.

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IN-KUI CHO received the B.S. and M.S. degrees from the Department of Electronic Engineering, Kyungpook National University, Daegu, South Korea, in 1997 and 1999, respectively, and the Ph.D. degree in electrical engineering from the Korea Advanced Institute of Science and Tech- nology, Daejeon, South Korea, in 2007. Since May 1999, he has been with the Electronics and Telecommunications Research Institute, Daejeon, where he has designed and developed optical back- plane, optical chip-to-chip interconnect system, and magnetic resonance wireless power transfer. His current research interest includes simulation and development of WPT components, such as planar magnetic resonators and magnetic resonators for three-dimensional WPT.

HYUN-JOON LEE received the B.S., M.S., and Ph.D. degrees from the Department of Physics, Pusan National University, Busan, South Korea, in 2008, 2011, and 2018, respectively. From 2018 to 2019, he was a Postdoctoral Associate with the Korea Research Institute of Standards and Science working on optical magnetometry.

Since 2019, he has been with the Electron- ics and Telecommunications Research Institute, Daejeon, South Korea. His research interests include ultra-low field magnetic resonance and the development of highly sensitive quantum sensors.

JAEWOO LEE received the B.S. degree in electrical and electronics engineering from Korea Uni- versity, Seoul, South Korea, in 2000, the M.S.

degree in information and communication engineering from the Gwangju Institute of Science and Technology (GIST), Gwangju, South Korea, in 2002, and the Ph.D. degree in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, South Korea, in 2017. After the Graduate School (GIST), in 2002, he joined the Micro-System Team, Electronics and Telecommu- nication Research Institute (ETRI), Daejeon. For a period of three years, he focused on developing bridge-type RF MEMS switches using an Au membrane on a GaAs substrate for RF application. Since 2006, he has been working on implementing MEMS acoustic sensors using the full-CMOS process (ETRI 0.8 um CMOS fab.) and specific MEMS processes. He has developed RF modules and sensor nodes for wireless communication applications.

SEUNGYOUNG AHN (Senior Member, IEEE) received the B.S., M.S., and Ph.D. degrees in electrical engineering from the Korea Advanced Insti- tute of Science and Technology, Daejeon, South Korea, in 1998, 2000, and 2005, respectively. From 2005 to 2009, he worked as a Senior Engineer with Samsung Electronics, Suwon, South Korea, where he was in charge of high-speed board design for laptop computer systems. He is currently an Asso- ciate Professor with The Cho Chun Shik Graduate School of Green Transportation, Korea Advanced Institute of Science and Technology. His research interests include wireless power transfer system design and electromagnetic compatibility design for electric vehicles and high-performance digital systems.