Journal of The Electrochemical Society

OPEN ACCESS Review—Electronic Circuit Systems for Piezoelectric Resonance Sensors

To cite this article: Jong-Yoon Park and Jin-Woo Choi 2020 J. Electrochem. Soc. 167 037560

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Review—Electronic Circuit Systems for Piezoelectric Resonance Sensors Jong-Yoon Park1 and Jin-Woo Choi1,2,*,z

1School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, United States of America 2Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70803, United States of America

Piezoelectric mass sensors have been widely studied for a variety of applications as a biological or chemical sensing transducer. With an increasing range of application areas and performance requirements for fast measurement time, higher resolution and accuracy, and compact system size, different measurement electronic systems have also been investigated to fulfill the performance requirements. Selecting a proper type of measurement electronics is critical to develop an optimized sensing system for practical applications. In this review, we cover different types of measurement electronics configurations including impedance-based measurement, oscillator-based measurement, and ring-down technique. Also, we provide an overview of the recent advances of each measurement electronics configuration for piezoelectric resonator sensors. Finally, the pros and cons of each measurement electronic configuration are compared and discussed. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY- NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/1945-7111/ab6cf7]

Manuscript submitted November 1, 2019; revised manuscript received January 13, 2020. Published February 3, 2020. This paper is part of the JES Focus Issue on Sensor Reviews.

Piezoelectric mass sensing techniques have been widely studied odd harmonics (N = 1, 3, 5,…). The Sauerbrey’s equation is and developed as a mass sensor for various applications, such as applicable to cases of rigid, uniform, and thin-film deposits.13,14 volatile chemical detections,1,2 DNA sequence mismatch,3 protein Early applications of QCM were limited to measuring gas-phase binding,4,5 and so on. Piezoelectric mass sensors have been showing analytes such as volatile organic compounds,15,16 environmental their strong potential especially for biomolecular recognition be- pollutants,17 and gas-phase chromatography detectors.18 cause they provide advantages like real-time monitoring, label-free Applications of QCM have further been expanded, particularly to detection, quantitative/qualitative information, and their affordable biomolecular detections, by introducing QCM in liquid media in the cost compared to the labeling techniques with fluorescent markers, 1980s.19–21 In those papers, they demonstrated the actuation of a radioactive species, and so on.6 quartz crystal resonator (QCR) in high damping media and the Recent advances show that piezoelectric mass sensors have been relationship between frequency shift and mass change in such studied beyond their mass sensing ability by monitoring power conditions. The relationship in liquid media, also known as the dissipation with different harmonics. For instance, monitoring of Kanazawa and Gordon’s equation, is expressed in the equation power dissipation, which is represented as a quality factor or Q below. factor in resonators, provides additional information about binding fi ⎛ ⎞3 molecules and allows the capability of distinguishing speci c 3 rh 2 7,8 2 ⎜ L L ⎟ molecules having the same molecular mass. D=-F f0 ⎜ ⎟ []2 Technological innovations and breakthroughs commonly follow ⎝ prqq m ⎠ novel findings in natural phenomena. The history of piezoelectric ρ η devices started after the Curie brothers discovered the piezoelectric where L and L are the density and viscosity of a liquid medium, effect (or reverse piezoelectric effect) on quartz in 1880.9 Other respectively. studies found that a certain cutting orientation of a quartz crystal QCM has become one of the most common piezoelectric mass plate could provide better frequency stability from temperature sensing tools. A typical QCM is fabricated with two gold electrodes variations.10,11 In modern electronics systems, a quartz crystal has separately deposited on both surfaces of a quartz disc. The resonance become an essential component providing stable reference clock frequency of commercially available QCM discs is usually ranging signals. In 1959, a quartz crystal was first used as a mass sensor from 5 MHz to 20 MHz. The thickness of QCM is a few hundred known as quartz crystal microbalance (QCM), when Sauerbrey micrometers and the diameter varies from a few millimeters to uncovered a linear relationship between the resonance frequency centimeters. While QCM devices usually have a high Q factor and shift of a quartz crystal and the mass of molecules attached on the show good stability at room temperature, sensitivity is limited by the 12 applicable thickness of a quartz slice determining a resonance crystal surface in vacuum or a gas medium. This relationship, also 22,23 known as the Sauerbrey’s equation, is described below. frequency (f0). In recent years, a film-bulk acoustic resonator (FBAR) has 2Nf 2 attracted increasing attention due to its higher sensitivity, lower D=-Fm0 ´D []1 cost, smaller size, and design flexibility. FBAR is a type of Ae rmqq piezoelectric resonators and utilizes a piezoelectric thin film instead where ΔF and Δm are the resonance frequency shift of the quartz of a bulk crystal, which allows miniaturization of a resonator-based sensor to have an array of multiple thin film resonators. For this, crystal and the change of the mass. f0 is the resonance frequency of FBAR has been applied to lab-on-a-chip systems24–27 and integrated the unloaded crystal. Ae is the effective surface area, ρq is the circuits.28 Additional information can be found in papers discussing density, and μq is the shear modulus of the crystal. N is the crystal’s various types of FBAR sensors.29,30 Piezoelectric resonance sensors require a measurement electronic *Electrochemical Society Member. system that can generate oscillation and measure the resonance zE-mail: [email protected] frequency shift. Journal of The Electrochemical Society, 2020 167 037560

The measurement techniques for piezoelectric mass sensors are stand for the motional effect of the resonator. Co and Cp represent usually categorized into impedance-based measurement, oscillator- the capacitance of a piezoelectric material and the parasitic based measurement, and ring-down analysis techniques. The im- capacitance when the resonator is connected to an external electrical pedance-based measurement system basically measures the elec- system. In the modified model in a liquid medium, LL and RL are trical impedance (or admittance) spectrum from the sensor by added to address the influence of the viscosity and the density of applying a sweeping frequency signal to the sensor. The resonance the liquid. LL causes a decrease in the resonance frequency of the frequency and the dissipation factor can be determined by analyzing piezoelectric resonator while RL decreases the Q factor of the the spectrum. On the other hand, in the oscillator-based measure- piezoelectric resonator. ment system, the sensor is actively interrogated in an oscillator In the BVD model shown in Fig. 1a, the impedance plot of the circuit. The oscillation frequency is usually measured by a frequency sensor has minimum and maximum impedance frequency points. counter and the power dissipation of the sensor can be monitored by The minimum impedance frequency or series resonance frequency gain control circuits adjusting the amplitude of the oscillating signal. (fs) represents the motional behavior of the sensor. The maximum In the ring-down analysis technique, the sensor is passively inter- impedance point is formed by the influence of the parallel * rogated by an excitation signal having a frequency near the capacitance (CO ) on the motional behavior of the sensor. The resonance frequency of the sensor. The excitation signal is applied maximum impedance frequency is commonly called a parallel to the sensor for a very short time and the decaying signal of the resonance frequency (fp). The estimated fs is very close to the actual sensor is monitored to determine the resonance frequency and motional series resonance frequency (MSRF) under a high Q factor 32 dissipation factor of the sensor. condition. fs and fp can be calculated by (3a) and (3b) below. The measurement techniques have been improved to fulfill increasing application areas and measurement requirements for 1 fs » []3a several decades. Since they have different operating principles, 2p LCmm each measurement system has different considerations to design the 1 electronics system and to improve its performance. Recent electronic f » []3b configurations have been focused on reducing the system size and p CCm o* cost while maintaining the system performance in acceptable ranges. 2p Lm * This review mainly focuses on introducing reported measurement CCm + o electronic systems for piezoelectric resonance sensors. It is our The other parameters can be also calculated by the following intention to overview the fundamental operating principles of those 32 measurement systems and discuss recent advances along with equations : performance comparisons among different resonance measurement 2 8KC0 0 electronic systems. Cm = []4 ()Np 2 Impedance-Based Measurement 1 L = []5 An electrical model of piezoelectric resonators.—Piezoelectric m 2 w MSRF Cm resonators are generally fabricated with a piezoelectric material ⎛ ⎞ which is sandwiched by two electrodes. Motional vibrations of the hq w piezoelectric resonator induce the electrical potential changes onto Rm = ⎜ ⎟ [6] cC¯ ⎝ w ⎠ the electrodes. The mechanical vibration of the resonator can be 66 m MSRF modeled by electrical lumped-elements like a resistor (R), a where K0 is the lossless effective electromechanical coupling factor, capacitor (C), and an inductor (L). A simplest electrical model ηq is the quartz viscosity, c66̅ is the piezoelectrically stiffened elastic which is known as the Butterworth-Van Dyke (BVD) model is constant, and ω and ωMSRF are the operating frequency and the illustrated in Fig. 1a.31 Also, the modified BVD model for the MSRF frequency of the quartz crystal. resonator in a liquid medium is shown in Fig. 1b.32 In both figures, The power dissipation factor (D) can be obtained with the Cm (related to the mechanical elasticity), Lm (the inertial component electrical model and it is simply the reciprocal of the Q factor (Q) of vibrating piezoelectric material), and Rm (mechanical energy loss) which is expressed in 7.

1 Edissipated jLw D ==,Q =0 m []7 Q 2pEstored Rm

In liquid media, RL and LL are added to the motional series branch. The Q factor is obtained by considering the RL and LL as expressed in 8. Because of a large increase in RL compared to LL increases, the quality factor of the sensor becomes significantly degraded. jLw ()+ L Q8= 0 mL [] ()RRmL+

Conventional impedance measurement system.—Impedance measurement systems are usually based on the measurement of the impedance of the sensor by applying frequency sweeping signals near the resonance frequencies. The frequency sweeping process is repeated at certain time intervals to keep tracking the behavior of the sensor for real-time monitoring. A conventional impedance analysis Figure 1. Lumped-element models of a piezoelectric resonator: (a) simpli- method has some advantages in the isolation of the sensors and no fied lumped-element model (BVD model) and (b) the modified BVD model external circuitry, which can electrically affect the behavior of the for liquid loading. They consist of the motional components (Cm, Lm, Rm, LL, sensor. These merits allowed that the impedance measurement * 33 RL) and a static component (CO = CO + Cp) in a parallel connection. method can be a more accurate tool than the other methods. Journal of The Electrochemical Society, 2020 167 037560

Figure 2. BVD model including branches for harmonic frequencies. 14,38,43 N N N Figure 3. Voltage division network adopted in. Ct is added to divide Additional motional branches (Cm , Lm , Rm ) are added in the simplified the frequency sweeping input (ui), and the output signal (uo) is connected to BVD model to express the motional behaviors of the sensor in the harmonic the signal processing unit. frequencies.

However, in multi-sensor applications, the system is connected to Frequency sweeping near the resonance frequency of the sensor multiple sensors with a multiplexing interface and the multiplexing 34 is also applied to the frequency division approach. While the interface causes the perturbation of the sensor response. Improving conventional impedance measurement system obtains information frequency resolution usually degrades the minimum measurement regarding the conductance and susceptance of the sensor, this time of the system. Moreover, because the conductance plot is technique measures the magnitude of the output signal in the voltage flattened in the high damping medium like water, a non-linear fitting fi division network. The transfer function spectrum of the voltage (Lorentzian curve tting) is necessary for accurate results. Other division network can be obtained through an experiment. The disadvantages include an expensive cost of the measurement system 35,36 transfer function is utilized to extract the BVD parameters based and bulky system size limiting the portability of the system. on 9. The conventional impedance-based measurement system is also Kankare et al. presented a double-sideband suppressed-carrier utilized to investigate the behavior of the QCM at the harmonic 38 = … (DSB-SC) modulation method with the voltage division network. frequencies (N 1, 3, 5, 7, ). The harmonic behaviors can be In the method, since the impedance analysis is performed with high expressed in the BVD model by adding motional R, L, C frequency signals which are modulated by a low-frequency signal, components corresponding to each harmonic frequency as shown in the conventional low-frequency lock-in amplifier can be used for Fig. 2. One of the recent harmonic analysis studies utilized the data analysis. This provides an advantage of the improved noise broadband impedance analyzer having a frequency sweeping range performance for high damping applications. As shown in Fig. 4,a from 20 Hz to 120 MHz.36 The given frequency range allows the DSB-SC signal (u1) is applied to the voltage division network as an measurement of the harmonic behaviors up to 11th overtone with a excitation signal. The excitation signal (u ) and an output signal of sensor having a 10 MHz fundamental resonance frequency. 1 the voltage division network (u2) are multiplied as a demodulation Many studies have been performed to overcome the inherent process. Since the demodulated signal has multiple frequency disadvantages of the conventional impedance-based measurement components, the signal needs to be filtered by the band-pass filter system.14,35,37–44 They mainly focused on solving the issues like an fi (BPF). The modulation and demodulation processes can be under- ef cient extraction of the BVD model parameters, fast data acquisi- stood by the numerical expressions of the important signals such as tion, lower cost, and miniaturization of the system. u1, u2, and uo described as — Improved impedance-based measurement systems. There was U0 uUsintcost10==-wwlh·(sin w+- tsin w t )[]10a another study on optimizing the electronic circuitry to improve data 2 acquisition time.40 The authors proved that the developed system could operate in the data acquisition time of 1–5 ms per each U0 u2 =+((fgww++ )sin ( ) ( w + )cos ( w + )) frequency point. That is, if 1,000 frequency points need to be swept, 2 it will take 1 s to obtain a full range of data. Even if 1 s acquisition U0 time is enough for the normal applications, it is not enough for the ++((fgww-- )sin ( ) ( w - )cos ( w - )) [10b ] 2 high-speed QCM applications.45,46 In addition, for a higher fre- quency resolution, the sweeping frequency points need to be 1 2 ukhUfol=-+(((www+- ) fcost ( )) 2 increased, which would subsequently cause an increase in the 8 0 acquisition time. +-((ggsintwww ) ( ))2 ) []10c Some interesting approaches have been studied by implementing +-l 14,38,43 a voltage division or impedance divider network. They where ωl is the modulation signal operating at a low frequency and adopted a voltage division network by placing a capacitor between ωh is the carrier signal operating at a high frequency. ω+ and ω- are a frequency sweeping unit and a resonance sensor. The voltage defined as ωh+ωl and ωh-ωl, respectively. f(ω) and g(ω) are the real division technique is illustrated in Fig. 3 and the transfer function of part and the imaginary part of the voltage division network the voltage division network is expressed in (9). impedance. k and h in 10c are the gain factors of the multiplier 9

2 ⎛ 1 ⎞ RL2 +-⎜w ⎟ mm⎝ ⎠ u wCm o = []9 ui ⎛ ⎛ ⎞⎞2 ⎛ ⎞2 RCmo 11wLCmo Co ⎜Rm ++-+--⎜ ⎟⎟ ⎜wLm ⎟ ⎝ ⎝ Ct ⎠⎠ ⎝ wCm Ct wwCCtm C t⎠ Journal of The Electrochemical Society, 2020 167 037560

44 dimensions. They added an inductor in series with the Co as shown in Fig. 5a. The resonant frequency determined by La and Co is more than double of the QCM resonant frequency value and can be found by the impedance measurement. The Ra is used to protect the short circuit at the resonant frequency. Thus, the Co value is estimated based on the La value at the resonant frequency (fr), which is expressed in 11. 1 C = o 2 []11 Lf()2p r

The Rm can be obtained with the same test setup but without La. They applied the frequency sweeping signal and found the resonant frequency of the QCM. At the resonant frequency, the impedance of Cm and Lm are canceled and the network has only Ra and Rm. Thus, the Rm can be calculated based on the known value of Ra, the input signal (ui), and the output signal (uo). It is expressed in 12.

Ruao´ Rm = []12 uuio- Figure 4. The double-sideband suppressed-carrier modulation method in which the authors recommended choosing Ra and La components involving the voltage divider.38 The system consists of the double-sideband with high precision and accuracy for the more accurate estimation of 44 suppressed-carrier (DSB-SC) signal generator, the voltage divider network, Co and Rm. the multiplier for demodulation, band-pass filter, and the signal processing There were other reports on a compact electronics system having unit. the capability to replace a bulky and expensive conventional impedance analyzer.35,37 The fundamental principle to obtain the and the band-pass filter. After mixing u1 with u2 and applying the BVD parameters was similar to the conventional method involving signal filtering with a band-pass filter, the output signal (uo) only has the passive interrogation of the sensor with a frequency sweeping a frequency component related to a low frequency (ωl). Also, the signal and an analysis of the impedance information including the output signal involves the information about the impedance of the curve fitting process. Even if the final signal analysis was still sensor, f(ω) and g(ω). Thus, the output signal is utilized to obtain processed on a computer, the electronics components including the the parameters in the BVD model by a non-linear fitting technique. frequency sweeping signal generator, the sensor interrogation stage, The main advantage of the configuration is to enhance noise and signal conditioning stages, were compacted in a single printed performance when it is comparing to the conventional impedance circuit board. analyzer. Specifically, because the phase and magnitude information The system block diagram of a compact impedance analyzer is corresponding to the impedance of the sensor is analyzed with the illustrated in Fig. 6.35 The system consists of the frequency sweeping output signal operating at a lower frequency, it is possible to utilize signal, QCM voltage measurement circuit, buffers, demodulators for electronic components operating at lower frequencies, which nor- the current and voltage signals, analog-to-digital converters for the mally show a better signal-to-noise ratio. Any additional noise can digitization of the signals, and a computer. A frequency synthesizer be canceled because the analyzed signal is a differential form of two drives a current into the sensor through a resistor (RS). The current is coherent signals. detected by the differential amplifier while the voltage signal from Some researchers suggested a more convenient method to extract the sensor is buffered. Next, the voltage and current signals are the BVD parameters. The extraction of the motional parameters (Cm, demodulated as in-phase and quadrature terms by using a 90° phase Lm, Rm) is started from a calculation of the Co value which depends shifter (PS). The signals go through a low-pass filter and finally on the dimensions of the sensor. In the method, they developed a digitized by analog-to-digital converters (ADC) to communicate with way to estimate the Co value without the effort to obtain the sensor a computer. The circuit system generates four signals (II, IQ, VI, VQ)

Figure 5. Illustrations of the system to obtain (a) Co and (b) Rm. The inductor (La) is serially connected to the resonator for the Co estimation while it is not used 44 in the system for the Rm extraction. Journal of The Electrochemical Society, 2020 167 037560

Figure 6. The block diagram of the system suggested in.35 The circuit components were designed on a single PCB as a handheld format. providing information on the real and imaginary parts of the current controlled by the programmable logic device (PLD) and the and voltage. The impedance of the sensor can be obtained by microcontroller. The PLC is programmed to control a programmable analyzing the signals as expressed in (13). frequency synthesizer chip (DDS) generating a frequency sweeping signal. The digital-to-analog converter is utilized to adjust the ∣∣V q Z1= []3amplitude of the DDS output signal ranging from 0.05 Vrms to 8.33 ∣∣I f Vrms in 4096 steps. An 80 MHz clock is used as a primary clock for where ∣V∣∠θ and ∣I∣∠ϕ represent the voltage drop across the sensor the PLD and the DDS. The output signal of the DDS goes through a fi fi and current flowing through the sensor, respectively. In the paper, low-pass lter, and the amplitude of the signal is ampli ed by an fi the minimum achievable frequency resolution was not reported, but operational ampli er. The frequency sweeping signal is applied it only mentioned that the frequency resolution was better than 1 Hz through the AC-coupling network (AC cpl.). The impedance without the measurement time. Additionally, although some portions responses of the sensor are transferred to the PLD by passing of the impedance analyzer were built on a single printed circuit through the RMS-to-DC converter (TRMS) and the analog-to-digital board (PCB), it still needed aids of a computer for the control of the converter (ADC). A USB applicable microcontroller collects the fi frequency synthesizer and signal processing to obtain the important data from the PLD and nally send the data to a personal computer parameters corresponding to the behavior of the sensor. (PC). The main tasks for the sensor parameters extraction including fi Another system was proposed as a miniaturized system showing the curve tting are performed on a computer. With the primary rapid impedance scanning and high-performance curve fitting.37 Its clock of 80 MHz, they reported that a full spectrum with a bandwidth of 20 kHz can be theoretically obtained within 200 ms block diagram is illustrated in Fig. 7. All circuits components are 37 also designed on a single PCB and the functional blocks are at a resolution of 0.2 Hz. Oscillator-Based Measurement In the oscillator-based measurement system, the piezoelectric sensor is actively interrogated by an oscillator circuit generating an analog output signal whose frequency changes are utilized to interpret the mass loading effect, whereas the sensor is passively interrogated by an external frequency sweeping signal in the impedance-based measurement systems. In general, oscillator-based measurement systems would be the best choice for the monitoring of the sensor behaviors for gas phase applications due to the high Q factor under gas phase media. For the cases, the typical oscillators such as Pierce, Colpitts, Miller, and so on, can be applicable.33 However, In the high damping media like water, the oscillation frequency is affected by the mass loading effect as well as dissipation that is represented by the motional resistance, 33,47–49 Rm. This implies that the dissipation must be considered to accurately measure the resonant frequency in a high damping media. The dissipation monitoring has been achieved by Automatic Gain fi Figure 7. The block diagram of the system suggested in Ref. 37. All the Control (AGC) ampli ers which automatically adjust the gain of an circuits were built on a single PCB and connected to a computer for the fast amplifier to maintain the amplitude of the oscillating signal at a 50,51 fitting of the collected data. certain level. It has been mentioned that the required amplifier Journal of The Electrochemical Society, 2020 167 037560

Figure 8. Typical oscillator circuits: (a) with an inverting amplifier (pierce); (b) with a non-inverting amplifier, and (c) with a Colpitts oscillator.34 gain in the AGC technique is proportional to the dissipation various oscillators can be found in the published papers; emitter- 46 34,49,53 54,55 represented as Rm. However, it was claimed that the ideal coupled oscillators, lever oscillators, active bridge operation of the AGC system is limited by the parallel capacitance oscillator,56,57 and balanced bridge oscillators.58,59 * 52 (CO ). Even if the oscillator-based measurement system has advantages in the cost, the size of the system, and a relatively Automatic Gain control and parallel capacitance compensa- easy design of the frequency tracking system, it should be noted that tion.—The basic operating principle of the AGC amplifiers is that the oscillators should be carefully designed and optimized for an the AGC amplifier tries to keep maintaining ∣Aβ∣=1 by adjusting the * ideal operation of AGC amplifier and a perfect compensation of CO open-loop gain (A) which compensates the decreases in the feedback when the system is designed for the high damping applications.52 gain (β) due to dissipation. The gain of the AGC amplifier is utilized In the chapter, the concept of the typical oscillators is briefly to obtain the damping characteristic of the sensor. introduced, and the advanced oscillator-based systems which are The AGC system is usually designed with several essential capable of dissipation monitoring, parallel capacitance compensa- components such as precision rectifier, PI-controller, and gain- tion, and so on, are explained. The common frequency measurement controllable amplifier.34,49 The block diagram explaining the basic methods, which are usually composed of digital systems, are also concept of the AGC oscillator is shown in Fig. 9. The precision discussed. rectifier generates a DC voltage corresponding to the amplitude of the oscillation signal of the oscillator. The DC voltage is applied to Principles of general oscillators.—The oscillator is basically the PI-controller which improves the measurement precision by designed as a positive feedback network that enables the power limiting the permanent control error. The dissipation of the quartz recovery of the oscillating signal from the energy loss by the energy crystal can be monitored by the AGC output signal. The voltage-to- dissipation component (Rm). “Barkhausen criterion” is commonly frequency converter (VFC) is used to provide conveniences of AGC implemented to determine the stability of the electronic circuits by output monitoring in signal processing. assuming a linear behavior of the circuit and no consideration of the The AGC system was implemented to a practical QCM pre-oscillation. The Barkhausen criterion is: oscillator.50 As mentioned above, the AGC circuit monitors the amplitude of the oscillating signal of the sensor and maintains the ∣∣AAnbbp 1,= 2( n = 0, 1, 2, ¼) []14 amplitude at a constant level for the variable loading conditions. As where A is an open-loop gain provided by an amplifying network shown in Fig. 10, the presented AGC circuit consists of a voltage and β is a feedback gain that is mainly determined by the sensor. buffer, a peak detector, and an integrator. The peak detector Figure 8 shows the typical oscillator principles. They have generates a sine wave that is clamped to the ground. The signal basically categorized into two oscillator types; an inverting amplifier can be regarded as a pulse signal operating at the oscillator and a non-inverting amplifier. The inverting amplifier in Fig. 8a, a frequency. In the integrator, the adjustable voltage (Vaj)isa well-known as a pierce oscillator, provides a 180° phase shift. Thus, reference voltage and the output voltage of the integrator is inversely the additional 180° phase shift should be accomplished by the proportional to the reference voltage. The output voltage of the fi feedback network consisting of R1, C1, C2, and the quartz crystal integrator adjusts the gain of the variable gain ampli ers. Finally, (a piezoelectric sensor) to satisfy the phase condition in the they proved that the AGC voltage is directly proportional to the Barkhausen criterion. In the non-inverting amplifier shown in motional resistance (Rm) of the sensor, showing that the AGC Fig. 8b, the sensor acts as a series resonator and satisfies the phase voltage can be utilized for monitoring the resonator losses.50 condition at the series resonance frequency. A Colpitts oscillator As an example of the practical application of the AGC system which is shown in Fig. 8c is also a type of the non-inverting into the oscillator, the lever oscillator is shown in Fig. 11.54,55 The amplifier, but the capacitor (C1), the resistor (R1), and the sensor are lever oscillator was designed for the series resonant mode and one connected in parallel. Thus, the sensor behaves as an inductor of side of the quartz crystal is grounded because this configuration high quality like a parallel resonance oscillator.34 allows the minimized parasitic capacitance effect. In order to In the series resonance mode, the oscillation frequency is at * MSRF, or near MSRF depending on the damping condition and C0 compensation, whereas the resonance frequency is somewhere between fs and fp in the parallel resonance mode. The desirable operating mode can be varied depending on the applications. In the parallel mode oscillators, the operating frequency can be tuned by changing the component values which are connected to the quartz crystal. On the other hand, the series mode is advantageous in * obtaining accurate fs value with the compensation of C0 when it is important to measure the absolute frequency. Most of the oscillators developed for QCM applications under high damping media have adopted the series mode oscillator due to its capability for actuating * the sensor at MSRF with the AGC system and C0 compensation under the high damping media. The specific design techniques of the Figure 9. Block diagram of the basic AGC oscillator configuration. Journal of The Electrochemical Society, 2020 167 037560

Figure 10. Block diagram of the oscillator circuit providing automatic gain Figure 12. The manual compensation method of the parallel capacitance control proposed by.50 The output of the integrator (AGC voltage) in the (C *) used in the QCM200 product. feedback loop adjusts the amplitude of the resonating signal. O maintain the oscillation meaning the open-loop gain (A) ⩾ 1, the R includes the AGC amplifier and the resonator represented by the f * needs to be larger than the R . If the oscillator operates at the zero BVD model, and RL. Without CO , the phase shift is 0° at the MSRF. c * ’ phases, the oscillation frequency is the MSRF of the sensor and the However, CO cannot be ignored because it contains a sensor s fl * impedance of the sensor can be expressed by the motional resistance physical properties. The current owing through CO affects the (Rm). In the condition, the open-loop gain of the lever oscillator can phase condition. The basic idea is to generate a current that has an 54 * be approximated by (15). opposite polarity to the current which is injected via CO . In order to realize the idea, they used a transformer. The transformer provides R the two voltage signals having different polarities. An inverted Open-» loop gain() A C RRCm voltage generates a current via the adjustable capacitor (CV) which 2h + eventually cancels the C * current. The perfect C * compensation RRRCf++ m O O can be achieved when C * = C . Also, the motional capacitance 26 mV O V  1, h = []15 (Cm) and inductance (Lm) can be ignored at the MSRF. At the ⎛ I ⎞ fi ⎜⎟ MSRF, only Rm, RL, and the AGC ampli er remain in the loop, thus, ⎝ ⎠ 2 the Rm can be obtained with the known the AGC gain (A) and RL as shown in 16. where I is the current flowing through the transistor Q3 which is designed as the voltage-controlled current source. In the denomi- RRAmL=-()11[]6 nator, the motional resistance (Rm) is multiplied by the term RC/(RC + Rf+Rm) which is reversely proportional to Rm. Thus, since the Unfortunately, no oscillators abovementioned can actuate the open-loop gain becomes less sensitive to the Rm variations, the sensor at the perfect MSRF. The fundamental reason for the oscillator can operate a wider range of Rm. It was mentioned that limitation can be found in its active interrogation of the sensor. the AGC loop is required to stabilize the amplitude of the oscillation That is, in order to maintain the oscillation, the oscillator consisting signal by controlling the current of Q3. They successfully proved the of the resonator and necessary components should satisfy the operation of the lever oscillator in the wide dynamic range of the Barkhausen criterion. Especially, achieving the ideal phase condition resistance of the sensor for high damping applications.34,54 is not easy due to the errors in the compensation of the parallel * Stanford Research Systems (SRS) developed a commercial QCM capacitance (CO ) and non-ideal phase-frequency characteristic of characterization system called QCM200. They also adopted the the active components in the oscillator. AGC system for the oscillator.51 The QCM system is employed for However, most applications show almost no changes in the both gas and liquid phase applications and its applications can be motional resistance (R ) and the parallel capacitance (C *) during – m O commonly found in many pieces of literature in various fields.60 62 experiments. Also, a frequency shift is a more important parameter In the system, we mainly focus on the parallel capacitance rather than an absolute frequency in most applications.33 Since cancellation technique. the frequency shift is not affected by the static phase condition, the * Figure 12 shows the illustration of the concept for the CO oscillator-based sensing systems are still attractive systems for the * compensation used in the QCM200 product. The oscillator circuit applications where the Rm and CO are maintained at constant values during experiments. The oscillator-based system will not be a good * choice for the applications where the Rm and CO keep changing during experiments. For those applications, the impedance-based measurement system can be a better choice for more accurate measurements.

Lock-in techniques.—Interesting approaches, called lock-in techniques, have been suggested to overcome the limitation of the oscillator-based system while maintaining its advantages like simple post-processing, a low cost, and portability. They are also kinds of oscillator-based system, but they implemented the passive interroga- tion of the sensor into the oscillator circuit system. In the lock-in techniques, the sensor is not a part of an oscillator. Instead, the external oscillator, which is usually composed of a voltage-con- trolled oscillator (VCO) or phase-locked loop (PLL), is locked at the MSRF of the sensor based on the informative signal from the passively interrogated sensor. They can be categorized into two Figure 11. A lever oscillator configuration showing a practical application different methods; (1) zero-phase lock-in technique and (2) max- of the AGC technique to the oscillator.54,55 imum conductance lock-in technique. In the first technique, the Journal of The Electrochemical Society, 2020 167 037560

* 52 Figure 13. A PLL-based zero-phase lock-in system including a manual compensation of the parallel capacitance (CO ) requiring a relatively easy calibration. zero-phase condition of the resonator can be achieved with the have the same phase, resulting in the lock-in condition. The lock-in * compensation of the parallel capacitance (CO ). Moreover, because condition can happen when the system works at the MSRF because the sensor is passively interrogated, the non-ideal phase-frequency Cm and Lm are canceled. Thus, in the lock-in condition, the VCO response of the active components does not affect to the sensor output signal (fexp) can be used for the MSRF tracking. Also, the behavior. In the second technique, the external oscillator is locked at amplitudes of UA and UB at the MSRF is utilized to monitor the the frequency showing the maximum conductance of the sensor. motional resistance (URm) which can be used for a dissipation Since the conductance of the sensor is not related to the parallel monitoring. * capacitance (CO ), the compensation of the parallel capacitance Another zero-phase lock-in system with a manual compensation * 63 * 66 (CO ) is not required. of the parallel capacitance (CO ) has been presented. The block diagram of the system is shown in Fig. 14. The principle of the Zero-phase lock-in techniques.—In the zero-phase lock-in tech- compensation method is similar to the method in QCM200, but it niques, the external oscillator is designed to be locked at the zero- adjusts the compensation current by an adjustable resistor (R2) 51 phase frequency of the motional branches including Rm, Cm, and Lm. instead of the adjustable capacitance used in. An advantage of the The main contributions of the related studies have focused on the system is that it can improve the quality factor by controlling the * accurate compensation of the parallel capacitance (CO ). Some gain of the current-to-voltage converter. After the parallel capaci- * researchers presented zero-phase lock-in systems with a manual tance (CO ) compensation, the gain of the current-to-voltage * 64,65 compensation of the parallel capacitance (CO ). However, the systems require complex calibration methods to make sure the lock- in condition at the MSRF with a precise compensation of parallel * capacitance (CO ). Some similar zero-phase lock-in systems have been introduced, but they do not need to do complex calibration procedures.52,66 The system that is shown in Fig. 13 implemented a phase-locked loop (PLL) technique to lock the operating frequency at the MSRF of the sensor.52 The operating principle of the system follows the PLL technique. When the adjustable capacitor (CV) is the same value * with the parallel capacitance (CO ), the entire current which is * injected via CV flows through the parallel capacitance (CO ). That is, * the parallel capacitance (CO ) does not have any influence on the circuit behaviors, meaning the compensation of the parallel capaci- * tance (CO ). With the compensation, only the motional series components (Rm, Cm, and Lm) affect the circuit behavior. In the PLL system, the output of the phase-frequency detector (PFD) generates signals corresponding to the phase difference between its inputs. The PFD keeps changing the control voltage of the voltage- Figure 14. Zero-phase lock-in system with a manual compensation of the φ ’ ’ * controlled oscillator (U ) until the two inputs of the PFD (A and B ) parallel capacitance (CO ) and quality factor improvability. Journal of The Electrochemical Society, 2020 167 037560

converter (I-V converter) is controlled by a resistor (Rf) providing Q signals shown in the previous techniques. However, they utilize a factor improvability. The effective quality factor is expressed in 17. low-frequency signal like 50 kHz for an auxiliary loop to compen- However, it is noticed that Rf should be set to a lower value than Rm sate for the parallel capacitance. The non-identical frequency because the larger value of Rf can cause instability due to negative characteristics in a pair of amplifiers that are designed with different damping and self-oscillation caused by electronic noises.66 gains can be avoided by using the low-frequency signal.67 In this approach, the differential voltage is used for the ACC, whereas the wMSRFL m phase difference is used in the previous systems.68 Qeff = []17 RRmf- Multi-harmonic oscillators were suggested by implementing the simultaneous excitation of the sensor at different frequencies.71–73 However, the manual compensation of the parallel capacitance The sensor is simultaneously excited at both the fundamental can be a problem in the cases where the parallel capacitance is frequency and the third harmonic. Two PLLs are used for locking variable due to the influence of the medium and temperature. Some at each frequency and tracking the frequency changes at both studies have been proposed the zero-phase lock-in systems with harmonic frequencies. The switchless harmonic oscillators showed – automatic capacitance compensation (ACC).67 70 They basically the capability of using the oscillator-based system in dual-harmonic utilize the two PLLs which operate at a high frequency and low applications. One of the systems showed the capability of the frequency, and the sensor is simultaneously excited at the two motional resistance monitoring at each harmonic.71 different frequencies. One PLL is used for the capacitance compen- Another type of PLL-based MSRF tracking system was intro- sation and another PLL is utilized for the MSRF tracking. duced in.74 The system consists of the digital chips, including a As an improved version of the previous system which is shown in digital signal processor (DSP), an analog-to-digital converter Fig. 13, an automatic capacitance compensation technique has been (ADC), and a 4-channel direct digital synthesizer (DDS). Four proposed.68 The system block diagram is illustrated in Fig. 15. The signals generated by numerically controlled oscillators in DDS are sensor is interrogated by the two different frequencies. The high- used for the sensor excitation, the compensation of the parallel frequency signal is fixed at around 4 times higher frequency than the capacitance, and ADC control. The main contribution of the system resonance frequency of the sensor. The phase difference between is that the system is digitally controlled instead of using the analog V1H and V2H controls the PLL which is responsible for the parallel circuit system presented in the literature above. capacitance compensation. The VCO generates a lower frequency signal operating near the MSRF of the sensor. The two different Maximum conductance lock-in techniques.—The compensation frequency signals are added for simultaneous interrogation of the of the parallel capacitance is very important in the zero-phase lock-in sensor at two frequencies, and they are separated by LPF and HPF technique for the accurate tracking of the MSRF. An alternative before phase-frequency detectors (PFD). The zero-phase tracking technique is to lock the external oscillator frequency at the maximum mechanism is very similar to the system which is previously conductance of the sensor. In most cases, the maximum conductance introduced.52 It is noticed that the performance of the PFD mainly is shown at the MSRF.75 The advantage of the technique is that the limits the degree of the capacitance compensation.67 compensation of the parallel capacitance is not necessary because A different concept for the ACC was adopted in.67,69,70 The the conductance of the sensor is independent of the parallel sensor is also interrogated by the sum of the two different frequency capacitance.

Figure 15. Block diagram of the automatic capacitance compensation system consisting of two PLLs operating at different frequencies.68 Journal of The Electrochemical Society, 2020 167 037560

Figure 16. The circuit block diagram showing the basic concept of the maximum conductance lock-in technique.

The basic concept of the maximum conductance lock-in techni- ques starts with how to monitor the conductance of the sensor. In order to obtain the conductance information of the sensor in a certain range of frequency (near MSRF), the sensor is passively interrogated by a sweeping signal near the MSRF of the sensor like the impedance measurement system, which is illustrated in Fig. 16.76 fl With a sweeping signal, the current owing through the sensor is Figure 17. Automatic maximum conductance locking system including the converted to the voltage by the current-to-voltage converter (I-V sensor unit illustrated in Fig. 1663: (a) block diagram of the system including converter). The output of the I-V converter is proportional to the the frequency modulation and an additional feedback loop and (b) illustration admittance of the sensor. The output of the I-V converter is explaining the concept of the automatic maximum conductance locking. multiplied with the sweeping signal which is used to interrogate the sensor. Finally, the multiplied signal goes through a low-pass provide high precision, easy analysis, and simple design, which fi lter to obtain the signal VG which is proportional to the conductance makes the oscillator-based measurement systems suitable for low of the sensor and to the Q factor of the sensor. A peak hold circuit cost and portable measurement system.77,78 can be used to detect the peak value of the signal VG while the In the simplest frequency counter, the frequency counter counts sweeping frequency of the interrogating signal, resulting in the the number of the input pulses during a certain pre-defined time. The detection of the maximum conductance. Like the general impedance- measurement of the time gate can be done by a reference clock based measurement system, it was reported that the system also had having a known period with high stability. The concept of the an issue in detecting an accurate conductance peak when it is used simplest frequency counter is shown in Fig. 18a. The frequency of for low Q factor applications due to the flatness of the conductance the input signal is calculated as in (18). curve.76 Jakoby et al. presented a system that is designed for a continuous N f = []18 and automatic lock-in frequency at the maximum conductance of the t 63 sensor by applying a feedback loop, which is shown in Fig. 17. where f is the input signal frequency, N is the number of the input The sensor unit consists of the same functional blocks shown in pulses, and t is the time gate (measurement time). In the frequency Fig. 16. For the automatic lock-in function, they applied the counter, the frequency resolution (Δf) is related to the ±1 count error frequency modulation (FM) to the VCO output by adding a low- and the measurement time. Even if the frequency counter allows frequency signal (Vm) generated by the external oscillator. That is, very simple implementations in most cases, the drawback of this the control voltage of the VCO is the sum of the integrator output, method is that the measurement time should be increased to enhance determining a center frequency of the VCO, and the external the frequency resolution. For example, it has a frequency resolution oscillator output which determines the modulation frequency range of 1 Hz at 1 s gate time. If the gate time reduces to 0.1 s, the depending on its amplitude. The integrator (controller) adjusts the frequency resolution is degraded to 10 Hz. A 4-channel QCM center frequency of the VCO based on the signal VG and the 79 fi measurement system was presented in. In the system, the modulation signal Vm. Speci cally, when the VCO output frequency oscillating signals of the QCM oscillators are connected to each is smaller than the maximum conductance frequency, the FM microcontroller. The frequency measurement is performed by a 16- induced conductance variations (VG) are in phase with the signal bit timer and a counter unit imbedded in microcontrollers. They Vm. Reversely, when the VCO output frequency is larger than the employed the simplest counter and the system has 1 Hz frequency maximum conductance frequency, the FM induced conductance resolution at a measurement time of 1 s. They demonstrated the variations (VG) are out of phase (180°) with the signal Vm. Thus, by system by monitoring the signal with the LabVIEW system. the mechanism, the center frequency of the VCO is locked at the Reciprocal counters have been widely implemented in QCM maximum conductance frequency. The output signal of the inte- applications as an alternative to the simple counter.77,78,80 The grator is used to determine the maximum conductance frequency 63 reciprocal counter calculates the frequency of the input signal based under lock-in conditions. It is mentioned that the system is also on the measurement of the input signal period. The number of the limited in the low Q factor applications because the accuracy of the input pulses are counted for a certain measurement time that is maximum conductance tracking can be decreased in the high estimated by the high-frequency reference signal. The concept of the damping applications. In order to enhance the accuracy of the reciprocal counter is illustrated in Fig. 18b. In the reciprocal measurements, the low gain VCO, such as voltage-controlled crystal counters, the frequency resolution is expressed by 19.81 oscillator (VCXO), can be considered, but its operating frequency range is reduced. f D=f in [19] ft Frequency measurement system.—Electronic counters are prob- ref ably the most common and simplest method to measure the where Δf is the frequency resolution, fin is the frequency of the input frequency of any oscillating signals. The electronic counters usually signal, fref is the frequency of the reference signal, and t is the Journal of The Electrochemical Society, 2020 167 037560

Figure 19. The frequency measurement system using the analog-digital phase-locked loop as the frequency-to-voltage convertor for the fast QCM application.

Figure 18. Concept of (a) the simplest frequency counter and (b) the reciprocal counter. measurement time. According to the principle, if the input frequency of 10 MHz is measured, the frequency resolution of 0.1 Hz is achievable with 1 GHz reference signal at 100 ms measurement time. However, it should be noticed that various kinds of noise (e.g. phase jitter of the signals) can affect the frequency resolution and Figure 20. The PLL-based frequency shift measurement system for high- degrade the performance in practice, particularly, in liquid media 92 with a bio-functionalized sensor.82,83 throughput FBAR sensor applications. In the reciprocal counter, it may be difficult to achieve a faster measurement for the applications requiring measurements of quickly analyzer or optical system to measure the resonance frequency shift. changing frequency (e.g. AC Electrogravimetry), while maintaining The conventional measurement systems are the bulky size and high acceptable frequency resolutions. For instance, in order to achieve cost, which are not desirable for the high-throughput sensing both the measurement time below 1 ms and the frequency resolution applications with real-time monitoring. The key operating principle of 1 Hz with 10 MHz QCR, the frequency of the reference signal of the system is similar to the system proposed by Torres et al. in needs to be higher than 10 GHz. Fig. 19, which also utilized PLL as a linear FVC. However, they Torres et al. presented an interesting system for the fast used a mixer for a downconversion of the resonance signal of the frequency change measurement, particularly for AC- sensor because the resonance frequency of the FBAR sensor ranged Electrogravimetry applications, which is called the electrochemical from 1.1 GHz to 1.4 GHz which were too high to be used as a PLL – quartz crystal microbalance (EQCM).84 86 EQCM is an application input. Wang et al. showed the applicability of the proposed system of QCM technologies specifically implemented in electrochemistry. for FBAR sensors array as a hand-held system. EQCM provides quantitative (or qualitative) information regarding the mass transfer at the working electrode with the charge move- Ring-Down Technique (QCM-D) ments. Those applications can be found in recent studies utilizing – The ring-down analysis method, commonly called as quartz EQCM with electrochemical experiments.87 91 The system block crystal microbalance with dissipation monitoring (QCM-D), has diagram is presented in Fig. 19. They utilized a phase-locked loop become one of the dominant analysis methods for QCM applica- (PLL) as a frequency-to-voltage converter (FVC). The main tions. In 1995, Rodahl et al. firstly introduced the ring-down operating principle is as follows. The main PLL loop is locked at technique.93 In the following year, 1996, they showed some the output frequency of the oscillator which is designed for the applications of the technique.13,94,95 Later, the technique was electrochemical quartz crystal microbalance (EQCM) and generates commercialized by the Q-Sense instruments,96 and it has become a a signal controlling the voltage-controlled crystal oscillator (VCXO). well-known technology as QCM-D. The VCXO control signal is proportional to the frequency changes In this method, the piezoelectric sensor is shortly excited with a of the EQCM. Thus, it can be used to measure the frequency change pulse signal whose frequency is near the resonance frequency. The of the EQCM. Also, since the VCXO has a very narrow frequency exciting signal is turned off when t is equal to zero. The signals from range, they added a second loop consisting of a numerical control the sensor, which can be either the voltage or current depending on oscillator (NCO) tuned by a field-programmable gate array (FPGA). the test setup,95 exponentially decay. The exponentially decaying The second loop extends the frequency range of the operation by signal is expressed as in 20. means of “Coarse tuning.” They successfully proved that the system can measure the frequency changes as fast as 1 kHz measurement -t Ut()»+ Uet sin (2,t0pf ft )  [] 20 rate which is equal to the measurement time of 1 ms. 0 Wang et al. recently proposed a PLL-based sensing system for where U0 is the amplitude of the signal at t = 0, τ is the decay time FBAR sensors for high-throughput sensing applications92 as illu- constant, f is the output frequency, and ϕ is the phase. The strated in Fig. 20. The FBAR sensors usually require the impedance experimental setup can be different depending on the measurement Journal of The Electrochemical Society, 2020 167 037560 mode of the sensor; the motional series resonance frequency (MSRF or fs) and the parallel resonance frequency (fp). The experimental setups for each mode are shown in Fig. 21. In the series resonance frequency measurement setup, the sensor is shorted, thus the influence of the parallel capacitance is removed. After the excitation signal is disconnected from the sensor, the current from the sensor is measured by the I-V probe and the output voltage of I-V probe is digitized by the ADC probe for signal processing. In the parallel resonance frequency measurement setup, the sensor is directly connected to the ADC. The dissipation factor (D), the MSRF (fs), and the parallel resonance frequency (fp) are calculated based on the fitting parameters which are shown in (21).88

1 Edissipated 1 D == = []21a Q 2pptEfstored f f » []21b s 1 1 - ()2Q 2

Cm ffps=+1 []21c C0* Later, the QCM-D technique was applied for the study regarding the influences of the shear oscillation of the sensor to the adsorption kinetics or binding events by implementing the dual-harmonic measurement.97 The sensor operates at the two harmonic frequen- cies. The first harmonic is used for the excitation of the sensor at the resonance frequency with variable signal amplitude, while another harmonic is used for monitoring the frequency (f) and dissipation (D) as explained earlier. The impulse excitation and decay monitoring technique is recently combined with a wireless and contactless sensor technique which wirelessly transfers the excitation and the response signals of the sensor through a transformer.98,99 The contactless interrogation of the sensor provides a capability of in situ applications where the electrical connection to the sensor is not applicable like the quality monitoring of packaged foods.98 However, it should be noticed that the prediction of contactless sensor behavior was not proved under the high-damping condition. Thus, the theoretical prediction can cause significant variations in the MSRF monitoring under the high- damping media. Generally, the QCM-D technique has an advantage in the cost of the system compared to the network analyzer. However, since the size of the entire system is fairly large and the quality of the required Figure 21. Experimental setups of QCM-D technique (a) for the series resonant frequency measurement and (b) for the parallel resonant frequency components is high for the accurate measurement, the technique is measurement. more suitable for the experiments under the laboratory environment.33 systems considering the data acquisition time, data fitting time, and Discussion and a Comparison of the Measurement Techniques the acceptable level of noise in practice.100 Based on the literature reviewed above, the measurement systems On the other hand, the oscillator-based measurement system has for the piezoelectric sensor can be qualitatively summarized as advantages in simple measurement, relatively cheap cost, and high shown in Table I. The conventional impedance analysis method can integration capability. The lock-in techniques provide accurate provide the most accurate results of the series and parallel resonance tracking of the resonance frequency at the MSRF and dissipation frequencies, the motional resistance, and harmonic frequencies. monitoring (Rm) by utilizing an automatic capacitance compensation However, it requires a high cost, bulky size, long data processing technique and automatic gain control system. Interestingly, the study time for the data fitting to extract accurate BVD parameters. The using PLL as an FVC proved that it can be useful for rapid frequency advanced versions of the impedance-based measurement technique change measurements (e.g. EQCM), which is difficult to achieve in showed a capability to be designed as a miniaturized system by other types of measurement systems. However, it requires a utilizing discrete functional electronics, such as a sweeping signal sophisticated design for the oscillators to achieve accurate measure- * generator, signal filters, DACs/ADCs, a microcontroller unit, and so ments of MSRF with the parallel capacitance compensation (C0 ) on, which can be fabricated in a single PCB. Even if they improved and the dissipation monitoring. Even if the reported studies some drawbacks of the conventional impedance analyzer like size, introduced the dual-harmonics oscillators, the capability of multi- cost, and measurement time, the measurement accuracy and the harmonic analysis is still one of the challenges in the oscillator-based bandwidth are degraded. Most studies did not clearly mention the measurement system. measurement time and the frequency resolution of the developed QCM-D technique has proved its various applications by systems. Also, it was reported that it is difficult to achieve a simultaneous measurement of the frequency and dissipation factor measurement time under 500 ms in most impedance measurement at multi-harmonic frequencies. Recently, it was implemented to the Table I. Qualitative comparisons of the developed measurement systems.

Types of the system Advantages Disadvantages Society Electrochemical The of Journal

Impedance measurement • High accuracy. • Bulky size. Conventional system • Multi-harmonic analysis. • High cost. • Long measurement time. • Dissipation analysis. • Requiring data fitting. Advanced system • Compact circuitry system34,36 • Less accuracy than the conventional impedance measurement system. • Improved measurement speed.39 • Narrow bandwidth (20 kHz).36 • High portability. • Required C0* compensation in liquid media. Oscillator-based measurement • Low-cost system. • Required careful circuit design. Conventional oscillator • Relatively easy measurement of the • Not suitable for harmonic analysis. resonance frequency.80–82 • High-speed measurement achievable.77–79 • Dissipation monitoring by the

AGC.49,50,53 2020 , • Automatic lock-in at the MSRF with • Increase in design complexity. dissipation monitoring.70 167 Lock-in technique • The capability of dual-harmonic • High-performance VCO is necessary for accurate measurement (in the 63–65

monitoring. maximum conductance measurement system). 037560 • Still limited harmonics analysis. • Simultaneous measurement of f and D. • Fairly high cost due to the high-quality system components. Ring-down technique (QCM-D) • Harmonic analysis. • Limited in the experiments under the lab environment. • In-situ applications (wireless and contactless technique)91,93 Journal of The Electrochemical Society, 2020 167 037560

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