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Quartz Crystal Microbalance (QCM) sensors are actively being implemented in various fields due to their compatibility with different operating conditions in gaseous/liquid mediums for a wide range of measurements. This trend has been matched by the parallel advancement in tailored electronic interfacing systems for QCM sensors. That is, selecting the appropriate electronic circuit is vital for accurate sensor measurements. Many techniques were developed over time to cover the expanding measurement requirements (e.g., accommodating highly-damping environments). This paper presents a comprehensive review of the various existing QCM electronic interfacing systems. Namely, impedance-based analysis, oscillators (conventional and lock-in based techniques), exponential decay methods and the emerging phase-mass based characterization. The aforementioned methods are discussed in detail and qualitatively compared in terms of their performance for various applications. In addition, some theoretical improvements and recommendations are introduced for adequate systems implementation. Finally, specific design considerations of high-temperature microbalance systems (e.g., GaPO4 crystals (GCM) and Langasite crystals (LCM)) are introduced, while assessing their overall system performance, stability and quality compared to conventional low-temperature applications.
Keywords:
quartz crystal microbalance, BVD model, impedance analyzers, QCM oscillators, Phase-Locked-Loop, QCM-D, Contactless QCM, Phase-Mass QCM, high-temperature microbalance
Piezoelectric crystals are widely considered as an essential component in contemporary electronic applications. Their operating principle relies on the piezoelectric effect that was first discovered by the Curie brothers in [1]. These crystals may be used as mass sensors, known as the Quartz Crystals Microbalance (QCM), conventionally through sandwiching a thin crystal between two conducting electrodes. That is, the crystal&#;s natural oscillation frequency is altered in response to small deposited mass adlayers. Sauerbrey introduced an expression relating the detectable shift in the crystal&#;s resonance frequency (Δfm) to the mass deposited (Δm) in gaseous/vacuum media in [2]. This resonance frequency shift is also dependent on the unloaded crystal&#;s resonant frequency (fo), its odd harmonic overtone (N = 1, 3, &#;), the crystal&#;s effective surface area (Ae), its density (ρq) and the shear modulus of the crystal&#;s material (μq).
ΔfMfo=&#;2NfoΔmAeμqρq
(1)
Equation (1) represents relative resonance frequency changes with respect to mass variations, depicting accurate responses for thin and rigid uniform films [3,4], by treating the deposited mass adlayer as an extension to the crystal&#;s thickness. Consequently, the thickness extension approximation loses its validity for frequency shifts exceeding 2% of unloaded crystal resonance [5,6]. Many applications are limited to lower frequency variations that are typically measured by part-per-million (ppm). Yet, an extended model based on the &#;Z-Match&#; method is presented in [7] to overcome this limitation. Unlike Equation (1), this model considers the acoustic properties (e.g., impedance) of the deposited rigid adlayer rather than treating it as a crystal material extension. Thus, extending the practical mass-frequency interpretation range to 15% of fo [7] and up to 40% for some applications [8,9].
Nonetheless, other factors (e.g., temperature) may also affect the QCM frequency stability. Conventionally, AT-cut quartz crystals are adopted for QCM sensors due to their near-zero temperature coefficient around room temperature. Typically, a 1 ppm/° change in oscillation frequency is observed for AT-cut quartz within the 10 to 50 °C range [10]. However, temperature-induced frequency variation (ΔfT) is observed when the sensor is required to operate under elevated temperature environments, where this shift should be compensated for accordingly [10,11].
In the early s, the use of QCM sensors was significantly expanded, especially in biosensing applications, by considering operation in liquid media [12,13]. The well-known work of Kanazawa and Gordon [14] led to the development of Equation (2) which governs resonance frequency-shift of QCM when operated in liquids.
ΔfLfo=&#;foρLηLπρqμq
(2)
where ρL and ηL are the damping medium&#;s density and viscosity, respectively. Thus, the overall resonance frequency shift in microbalance applications mainly incorporates a combined effect of mass, temperature and viscosity, as summarized in Equation (3).
Δf &#; ΔfM + ΔfT + ΔfL
(3)
The aforementioned effects are more representative for most microbalance applications, yet, other spurious factors (e.g., pressure and stress) may also cause additional minor frequency shifts that should be taken into account when present [15]. Generally, QCMs are used as chemical and biological sensors for mass, viscosity, temperature and humidity measurements, in addition to recently being used as gas sensors in some applications [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Quantitatively, 569 published works tackled molecular recognition studies based on QCM sensors from and [36], compared to 857 works focusing on QCM&#;s chemical and biochemical applications between &#; [37]. This huge number indicates the active nature of QCM applications and the necessity of identifying appropriate electronic interfacing circuit for the intended application.
It is important to note that Equations (1) and (2) indicate an increase in the measurement sensitivity as a function of (fo). However, a practical maximum limit is set to fo as it is inversely proportional to crystal&#;s thickness and its quality factor [38,39]; where the latter should ideally be maximized. Accordingly, the development of High Fundamental Frequency (HFF) QCM resonators operating at 150 MHz was recently reported [40]. Electrodeless QCM biosensors operating at a fundamental resonance frequency of 170&#;180 MHz are also newly reported [41]. Specifically, recent works introduced wireless-electrodeless QCM operation to increase the resonator&#;s sensitivity and detection limits [42,43,44,45,46]. This is because parallel electrodes deteriorate the overall QCM sensitivity and range since they are also considered as deposited adlayers [42]. The effect of parallel electrodes geometry and distribution on the resonator&#;s sensitivity is recently discussed in [47], in addition to the effect of adlayers radial distribution. That is, QCM sensitivity is found to be the highest at the electrodes center and decreases exponentially as the radial distance increases [47,48]. A dynamic extended model is thus introduced in [47] to account for the aforementioned parameters, especially when geometrically symmetric, uniform adlayers are unexpected.
Other alternatives for applications requiring elevated sensitivities are the Film Bulk Acoustic Resonators (FBAR) sensors, with fundamental resonance frequencies within the GHz range [49]. Higher mass sensitivities are achieved by these sensors, at the expense of higher noise levels, establishing a practical tradeoff. A comprehensive comparison between these sensors is found elsewhere [41,50].
The discussion above has been focused on conventional QCM applications with temperature ranges within the quartz low-temperature coefficient regions; such applications make up for the majority of crystal microbalance applications. Yet, QCM systems have also been developed for use under extremely low temperatures (e.g., space exploration applications) [51,52]. On the other side of the spectrum, some applications require stable operation at higher temperatures. Quartz maximum operating temperature as a piezoelectric material is limited by its phase-transition, Curie temperature, of 573 °C [53]. In contrast, other types of novel, quartz-like, crystals are shown to possess similar piezoelectric characteristics to the conventional quartz for QCM applications at higher temperatures [53,54]. For instance, the Y-11.1° cut gallium orthophosphate (GaPO4) based crystals are able to maintain stable operation under temperatures as high as 900 °C with a linearly decreasing temperature coefficient, reaching 3 Hz °C&#;1 at 450 °C, based on the manufacturing specifications [53]. The phase-transition temperature of this material is reported around 920 °C, in addition to its higher mechanical quality factor (Q), indicating its superiority over quartz for such applications.
The utilization of GaPO4 based material in sensing applications, known as GCM, is reported in several works [53,54,55,56,57,58]. shows different types of QCM sensors with various sizes and Au/Cr-electrodes distribution, in addition to a Pt-electrodes coated R-30 GCM sensor from PiezoCryst for comparison. Finally, a comprehensive review of high-temperature crystal types and characteristics, including the emerging Langasite based LCM sensors, is found elsewhere [59].
Open in a separate windowSubsequently, the main focus of this paper is on analyzing the existing QCM electronic interfacing sensors based on their wider-utilization in literature. Some interfacing techniques are based on the described resonance, others are based on direct impedance or phase shift measurements that are representative of resonance shifts. Although QCM and GCM characterization is somewhat similar, a separate section of this paper is dedicated to GCM and high-temperature microbalance applications, with relevant design considerations and recommendations.
The structure of this paper is as follows: electrical modeling of crystal resonators is first introduced. After that, QCM electronic measurement systems and techniques are thoroughly analyzed in the following order: (1) Impedance-based measurements. (2) Electronic oscillator circuits, in addition to their enhanced lock-in oscillator systems. (3) Impulse Excitation/Decay technique (QCM-D). (4) QCM Phase-mass technique. High-temperature microbalance systems design considerations and recommendations are then discussed in terms of their electronic interfacing circuits and physical aspects. A qualitative comparison of the different electronic characterization systems is then presented, detailing the advantages/disadvantages of each category for different applications and operating mediums, followed by concluding remarks.
From the electrical point of view, piezoelectric thickness shear mode (TSM) crystals are modeled as acoustic transmission lines [60]. Yet, simplified versions of this model have been developed over time. Namely, the Lumped Elements Model (LEM), which models the loading effect as a complex impedance ZL series to the motional Butterworth-Van-Dyke (BVD) branch [60]. Consequently, unloaded QCM sensors are electrically modeled, near resonance, using the generic BVD circuit model, consisting of a series RLC motional arm, with (Cm) representing the energy stored per oscillation, (Rm) representing the energy loss, damping, per oscillation and (Lm) as the inertial element related to vibrating mass of the crystal unit [61]. Another capacitor (Co) is added across the motional arm to account for the electrodes capacitance with the piezoelectric crystal acting as the dielectric material, where its capacitance value is dependent on the crystal geometry and is typically in the pF range. The simple BVD circuit model is shown in and related to the sensor composition.
Open in a separate windowAs seen from the circuit in , two fundamentals resonance frequencies may be achieved at the resonator&#;s minimum and maximum impedance points. Namely, series (fs) and parallel resonance (fp). Series resonance is mainly related to the crystal itself and more representative of its behavior. Yet, the series minimum impedance frequency (fs) is different from the true motional series resonant frequency (MSRF), which is only dependent on LmCm resonance and is considered to be the frequency of interest for most QCM applications, representing actual piezoelectric crystal resonance. On the other hand, the parallel resonance frequency is mainly attributed to the parallel capacitor resonance with the motional branch. Both frequencies are obtained as in Equations (4) and (5), serving as accurate approximations for high quality resonators as depicted in .
fs&#;12πLmCm
(4)
fP&#;12πLmCmCo*Cm+Co*
(5)
where Co*=Co+Cext assumes a connected resonator to external apparatus and is replaced by Co in case of bare-electrodes, as discussed above. Equation (4) resembles MSRF, where fs = MSRF only if Rm = 0 or if the sensor is electrodeless. The simple BVD components values may be theoretically calculated as in Equation (6) for a given resonance frequency, or one of its harmonic overtones [62].
Co=&#;pAh
(6a)
Cm=8Ko2Co(Nπ)2
(6b)
Lm=1ωMSRF2Cm
(6c)
Rm=ηpcpC1(ωωMSRF)2
(6d)
where ω and ωMSRF are the operating and MSRF frequencies, respectively. Equation (6) may be inaccurate under certain conditions [38]; thus, practical characterization techniques may be alternatively used (e.g., based on impedance analysis) as will be discussed further down. also demonstrates the different operating regions of a high-quality QCM sensor, near resonance, based on a practical case of 5-MHz QCM crystals [9]. The BVD parameters are provided as: Lm = 30 mH, Cm = 33 fF, Rm = 10 &#; and Co*=20 pF. Sharp phase-transition between inductive and capacitive regions is observed at resonance from &#;90° to 90° and back. The aforementioned approximations are accurately valid for QCM applications in vacuum/gas phase, where the medium&#;s damping is mostly negligible [63] and the deposited uniform films are modeled through BVD as a small inductance, series to the motional arm, causing the resonance curves in to slightly shift to the lower-frequency side.
Open in a separate windowOn the other hand, liquid-phase applications are associated with significant damping of the QCM resonator due to viscous loading, in addition to further shifts in resonance frequency. Such changes are modeled by adding series Rliq and Lliq components to the motional branch of the equivalent circuit. Single-sided liquid immersion is used for most in-liquid applications. Yet, double-side immersion is still adopted by some works [39,64]. For the latter case, the liquid conductivity, modeled by Gliq and permittivity, modeled by Cliq are taken into account when modeling the resonator [39,65] and are omitted otherwise. Intuitively, using the sensor in highly-conductive liquids under complete-immersion conditions would short the sensor output. illustrates the detailed, loaded BVD model, near resonance, for a QCM resonator.
Open in a separate windowFor the standard 5 MHz crystal characterized in , a single-side immersion in pure water corresponds to increasing the overall motional resistance RsT=Rm+Rliq to 400 &#; (liquid-induced resistance Rliq = 390 &#;), in addition to a liquid-induced inductance Lliq = 8.48 µH [9], with LsT=Lm+LLD+Lliq. The other parameters ideally maintain their constant values. Significant increase in the total motional arm resistance, compared to the minimal inductance change, negatively affects the QCM quality factor (Q), which represents the ratio of the stored to lost energy for each crystal oscillation [66]. The basic formula for determining Q is expressed as a ratio of the resonator&#;s equivalent inductive reactance at resonance to its equivalent resistance, as in Equation (7).
Q=jωoLsTRsT
(7)
Practical consequences of Q degradation on the QCM frequency response are illustrated in a. That is, the minimum motional impedance (Zs) point, corresponding to MSRF and maximum conductance and represented by Equation (4), occurs after the overall resonator&#;s minimum impedance point (ZQCM), represented by fs. Such behavior is mainly influenced by the parallel RsTCo combination near resonance, which is no longer dominated by RsT for damped mediums and considerably affects the points of absolute minimum/maximum impedance and their phase. As a result, series and parallel resonance points occurred at &#;26.95°, compared to the case of unloaded resonator (i.e., Rm = 10 &#;), where both resonance frequencies occurred around the zero-phase crossing point. In addition, the inductive phase-transition hardly reaches 30° for the given liquid-loading scenario due to the narrow inductive region that, combined with the large damping resistance, prevents the inductive reactance to build-up significantly and dominate the sensor&#;s overall impedance before resonating with Co*.
Open in a separate windowSubsequently, the total parallel static capacitance Co* negative effects on the measured resonance frequency in viscous mediums require thorough evaluation. The reported Zmin value in a was 377 Ω, compared to 400 Ω for the actual RsT, indicating a false interpretation of the sensor damping and the corresponding liquid viscosity if the absolute minimum impedance point is taken as references for MSRF and RsT estimation. Also, a difference of ~254 Hz between the MSRF and the actual detected minimum impedance frequency fs is measured for the given scenario. These variations are for the case of operating in pure water and are amplified significantly for more viscous mediums [9]. Thus, the maximum conductance (G) point should be taken as a global reference for accurate MSRF estimation instead as it is independent of Co*. RsT is then simply calculated as the reciprocal of Gmax.
b qualitatively describes the practical implications of the parallel capacitance effect through admittance locus, adopted from [63] with some improvements, where the outer circle represents an unloaded QCM scenario (i.e., low RsT) and the inner circle represents a damped resonator scenario (only Rliq is considered for simplicity in this scenario, since the goal is to study the combined Co*, Rliq effect, rather than Lliq mass loading). The orange horizontal dashed line represents the maximum conductance trajectory, independent of Co* (i.e., the MSRF line). On the other hand, the slightly inclined red dashed line represents the measured fs at the minimum impedance point. This inclination represents the electrical susceptance (B: reciprocal of reactance) behavior of the sensor, where the angular frequency ωs varies based on the minimum impedance point as in b. A recent paper discussed the influence of motional resistance and parallel capacitance variations on the resonator&#;s series resonance point for liquid-phase applications [67]. Detailed resonance-band characterization may also be reviewed from the following recommended references [39,63,65].
Finally, it should be noted that additional modifications can be introduced to the BVD model based on several additional factors, such as coating the QCM with polymer films, or adding extra components to indicate the viscoelastic behavior of some loaded film. Such adjustments are important for the BVD model accuracy when it is used as part of the sensor characterization, by making it more representative of the actual application [68].
AT-cut quartz are widely employed for manufacturing QCM sensors due to their extremely good mechanical and temperature stability within 10&#;50 °C range as discussed earlier, given that the majority of QCM applications operate within this range (i.e., around the zero-temperature coefficient spectrum). However, other applications require operating the microbalance sensor at elevated temperatures, such as assessing the thermal stability of viscous fluids [10], in addition to high temperature controlled thin-film Atomic Layer Deposition (ALD) [11,56].
Consequently, the utilization of AT-cut quartz crystals at high temperatures causes significant frequency drifts influenced by the increased operating temperature [10]. Several compensation techniques were introduced over time to separate the temperature-induced frequency variations from those caused by actual sensor loading. For instance, the use of dual-resonator setup is experimentally verified in [11,15] through exposing two identical resonators to the same temperature conditions, while only exposing one of them to the actual testing environment and physically isolating the other, reference resonator, to limit its detected frequency shifts to temperature variations and prevent film growth on its surface. Thus, establishing a temperature-frequency baseline. The subtraction of both resonators frequency output signals results in a compensated signal that, ideally, represents the surface interactions of the non-isolated resonator. The presented methodology proved an experimentally adequate operation up to 565 °C in [15] and 500 °C in [11].
In contrast, the use of mathematical models to compensate the temperature-frequency drifts is discussed in [11] and compared to the dual-resonator technique. That is, the baseline set earlier through a reference oscillator is set mathematically instead through a polynomial equation, typically of 3rd order, of the form indicated in Equation (30).
ΔFT = a3T3 + a2T2 + a1T + a0
(30)
The constants a0 &#; a3 are temperature coefficients that are dependent on the crystal cut and properties [10], T is the operating temperature and ΔFT is the modeled temperature-induced frequency variation. Comparatively good results to those obtained by the dual-resonators system are produced through the model based technique in [11]. Clearly, utilizing the latter technique is advantageous in terms of minimizing the experimental system size and cost by using one resonator only, in addition to overcoming the complexities associated to the reference oscillator isolation. Model-based compensating techniques are also employed for liquid-phase in [123] for determining the state-of-charge of lead acid batteries, while neutralizing electrolyte temperature variations during charging/discharging process.
The aforementioned compensation techniques are meant for adjusting frequency measurements, without addressing the implicit variations in terms of the resonator&#;s quality factor and BVD parameters, proportional to temperature, from an electrical point of view. Practically, Q is found to deteriorate with increasing temperature, such variations are negligible under normal operating conditions within the range of 30&#;160 °C as reported in [124] and supported by the results presented in [10,56]. Yet, extending the operating temperature further beyond 300 °C and near the reported limit point of 565 °C significantly deteriorates Q and increases Rm (i.e., imposing similar experimental effect to that experienced through highly-viscous loading) and necessitating similar experimental considerations for the electronics interfacing circuits design under such harsh operating conditions, such as adequate Co compensation for oscillator circuits.
Eventually, quartz crystal use as piezoelectric resonators is only limited to their phase-transition, Curie-temperature of 573 °C, taking into account that operating QCM resonators near that limit is not recommended given their exponentially increasing temperature coefficient [53]. That is, other piezoelectric crystals with similar sensing properties and higher phase-transition temperatures are more adequate for such applications, maintaining higher mechanical stability and enhanced performance.
Several piezoelectric crystals exhibiting high phase-transition temperatures are used for microbalance applications, mainly, the GaPO4 (GCM) and more recently, the langasite (LCM) crystals [59,125,126]. GCM sensors exhibit irreversible phase-transition around 920 °C, compared to ~ °C for LCM resonators [59]. Yet, it is reported that GCM crystals operating above 750 °C exhibit significant Q reductions [127], compared to ~ °C for LCM sensors [59], supporting the previous recommendation of not operating crystal close to their piezoelectric limits. Extensive practical comparison is performed between GCM and conventional QCM sensors in [53,54,56], demonstrating the former superiority for high-temperature applications. Namely, the GCM motional resistance increments with increasing temperature is significantly less than that of QCM sensors, compared to sharper relative degradations at higher temperatures above 400 °C in terms of GCM quality factor as reported in [56], while maintaining relatively high absolute Q values.
In contrast, the Pt-electrodes used for commercial GCM sensors inherently require an adhesion layer that firmly attaches it to the crystal. Several materials have been investigated in literature for this purpose, including tantalum (Ta), zirconium (Zr) and titanium (Ti) [128]. Commercial GCM sensors typically utilize a Ti layer, however; titanium diffusion through platinum at temperatures ~600 °C for applications involving oxide-rich environments can form oxide precipitates that affect the adhesion and thus the stability of the electrodes and the overall frequency response as a result. Thus, it is advised to verify the electrodes/adhesion compatibility with the operating environment to avoid any hard-to-detect results misinterpretations that may arise from such problems.
Commercial GCM sensors may be customized to the application requirements by setting its zero-coefficient temperature (To) around the expected operated range, up to 650°, as per the main GCM sensor supplier, PiezoCryst [129]. That is, adjusting Equation (30) as follows.
ΔFT&#; = ΔFT(To)[a3(T &#; To)3 + a2(T&#;To)2 + a1(T &#; To) + a0]
(31)
Yet, associated quality factor degradations around higher operating temperatures, away from the given To limit for wide-temperature range applications, should be closely monitored when designing the electronic interface due to the aforementioned reasons, mainly, ensuring simultaneous mathematical and practical compensation of any temperature-induced effect.
Consequently, high-temperature microbalance applications, especially with in-situ measurements, require customized sensor holders that are capable of withstanding the harsh operating environment while adequately connecting the sensor to the rest of the readout circuit that is usually kept around room-stable temperature. Non-reactive Gold or Platinum wires are typically used to extend electrical connections as in [57,59,130]. An interesting option for applications requiring customized holder designs, is based on the &#;pyrophyllite&#; material. This mineral, in its natural state, is easily machined to the required design shape through conventional drilling tools or Computer Numerical Control (CNC) machines, before being fired to elevated temperatures (800 &#; °C). The fired holder may then continuously operate at high temperatures within the given range. Pyrophyllite based holders have been successfully implemented in several high-temperature GCM applications [8,130].
The discussed characterization techniques may be compared as follows, where summarizes the practical advantages and limitations of each discussed electronic interface. Overall, conventional impedance analyzers are superior in terms of their accuracy since the sensor response is interrogated in isolation of external components that may negatively affect its output, in addition to its ability to characterize the sensor at different odd harmonic overtones. Yet, such superiority is conditional in literature to other characterization alternatives such as compact impedance-based systems and oscillator circuits. Based on the operating environment and the sensor damping, parallel capacitance compensation may be pivotal for obtaining accurate results for QCM oscillator circuits, where some oscillator-based PLL systems are designed to automatically compensate for Co* and track the motional resonance frequency. Exponential decay techniques are also considered powerful and adequate in many applications due to their simultaneous f and D measuring capability at multiple overtones and have been recently expanded to include wireless sensor interrogation, which is significant for some in-situ sensing applications. On the other hand, phase-mass characterization is particularly advantageous for high-sensitivity applications when the use of high-frequency oscillating circuits is inadequate due to their amplified phase-noise and reduced SNR. This relatively new characterization technique also supports working with multisensory arrays for extended analysis spectrum.
Highest accuracy.
Supports complete sensor characterization and harmonic overtones analysis.
Isolated QCM measurements.
Adequate for all mediums, provided that appropriate resolution is supported.
Highest setup cost.
Large physical size.
Some models require time consuming processing/data fitting for accurate parameters extraction.
Significantly cheaper than conventional analyzers with more compact sizing.
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Provides essential characterization parameters for different applications (e.g., BVD parameters from [ 4 80 ]).
Less accurate compared to conventional analyzers.
Sensor is not measured in isolation (part of a circuit).
Requires time consuming processing algorithms.
Some circuits provide limited characterization [ 38 ].
Maintains most conventional impedance analyzers advantages.
Rapid and high-resolution measurements capability [ 72 ].
Low cost and high integration capability (handheld size).
Less accurate compared to conventional impedance analyzers.
Highly integrated low cost circuitry.
Good accuracy.
Direct frequency measurement.
Dissipation monitoring capability for some designs.
Sensor response is influenced by circuit components.
Inadequate for overtones study.
Requires high components stability.
fosc &#; MSRF without Co* compensation in-liquid.
Maintains QCM oscillators advantages.
Locks in to MSRF.
Supports simultaneous interrogation by different overtones for extended results.
Provides dissipation monitoring capability through signals indicating Rm variations.
Requires Co* compensation for MSRF tracking.
Circuit components influence may affect the tracking accuracy.
Requires high-resolution VCO for flat conductance peaks.
Circuit components influence may affect the tracking accuracy.
Accurately measures series and parallel resonance.
Simultaneous f and D tracking capability and overtones excitation.
Fairly expensive due to its high-quality components.
Mainly suitable for lab-based applications (i.e., low portability).
Maintains most conventional QCM-D advantages.
Extends the applications spectrum to closed volumes (i.e., no QCM wiring).
Electrodeless setup increases the measuring sensitivity.
Contactless setup in [ 114 ] is limited for lightly-loaded applications.
The wireless interrogation distance is still fairly low for various applications.
Achieves higher sensitivity and resolution compared to conventional circuitry.
Supports high-frequency crystals characterization with minimized signals noise and interference.
Highly integrated setup, supporting sensing arrays.
Current setup provides partial sensor characterization (e.g., limited dissipation monitoring).
Limiting the applications spectrum.
The special design considerations/compensations discussed for high-temperature applications should also be taken into account when selecting the appropriate characterization technique. For applications involving customized sensor connections, the wiring material/lengths should be kept to a minimum to minimize Cext and maintain it within the compensation range of the selected setup. Finally, microbalance sensors must be handled with extreme care due to their inherently delicate structure to avoid breakage, especially when using high-temperature microbalance sensors as they are much more expensive than conventional QCMs.
This work presented a comprehensive review covering microbalance sensing applications based on different types of crystals, mainly QCM and GCM. The specific scope of this work was focused on reviewing the existing QCM electronic interfacing circuits and sensor characterization techniques, while identifying the advantages and disadvantages of each in terms of complexity, cost, size and reliability under various operating conditions. The reviewed systems were based on impedance analyzers, electronic oscillators including PLL-based circuits, exponential decay (QCM-D) technique, as well as the emerging phase-mass characterization. Special design considerations for high-temperature applications are also discussed in this work with appropriate compensation techniques and alternative crystals that can operate normally well beyond the quartz Curie temperature limit. Finally, the various covered systems and operating conditions by this work provide the reader with an insightful overview of the existing systems to facilitate her choice for the intended application.
This publication was made possible by the National Priority Research Program (NPRP) award [NPRP9-313-2-135] from the Qatar National Research Fund (QNRF); a member of the Qatar Foundation. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of QNRF.
A.A. surveyed the literature, prepared the material (e.g., simulations and figures) and wrote the paper. M.B. and D.B. reviewed the content and contributed to enhancing its quality and presentation.
The authors declare no conflict of interest.
Quartz Crystals, Xtals Tutorial Includes:
Quartz crystals: xtals
What is quartz
How a crystal works
Crystal overtone operation
Quartz crystal frequency pulling
Quartz crystal cuts
Quartz ageing
Crystal resonator manufacture
How to specify a quartz crystal
VCXO
TCXO
OCXO
Crystal filter
Monolithic crystal filter
Ceramic resonator & filter
Ceramic filter specifications
Quartz crystal resonators are used to provide very high Q resonant elements within many electronic designs and particularly within many RF circuit designs within oscillators and filters.
Often in a circuit design, these electronic components may be referred to as "Xtals" and circuit design references to them may be given as xtal1, etc.
Wire leaded HC49 quartz crystal resonatorQuartz crystals can be cheap to produce even though they offer exceptional performance and can be used for everything from electronic designs for microprocessor clock oscillators to high performance filters, highly stable oven controlled oscillators, temperature compensated crystal oscillators and many more general and RF circuit designs.
As the name implies quartz crystal resonators are made from quartz which is is a naturally occurring form of silicon. However most of the quartz used for the electronics industry is manufactured synthetically.
Quartz crystal resonators are available in many sizes and formats to suit the requirements of most applications. From small surface mount devices right through to larger through hole mounted crystals as well as those for sockets, there are many sizes and formats for these electronic components.
Quartz crystal resonator technology relies on the remarkable properties of quartz for its operation. When placed into an electronic circuit a quartz crystal acts as a tuned circuit. However it has an exceptionally high Q.
Ordinary LC tuned circuits may exhibit values of a few hundred if carefully designed and constructed, but quartz crystals exhibit values of up to 100 000.
Apart from their Q, crystal technology also has a number of other advantages. They are very stable with respect to temperature and time. In fact most crystals will have these figures specified and they might typically be ±5 ppm (parts per million) per year for the ageing and ±30 ppm over a temperature range of 0 to 60 °C.
A crystal of naturally occurring quartzIn operation the quartz crystal uses the piezo electric effect to convert the electrical signals to mechanical vibrations. These cause the crystal to vibrate and the mechanical resonances of the crystal then act on the mechanical vibrations. The piezo-electric effect then links back to the electrical domain and the signals are converted back having been affected by the mechanical resonances.
The overall effect is that the quartz crystal links the very high Q mechanical resonances to the electrical domain, enabling very highly stable and high Q resonances to affect electrical signals.
Read more about . . . . what quartz is.The circuit symbol for a quartz crystal resonator used in electronic design schematics is straightforward. The quartz crystal symbol shows the two plates either side of the main quartz element. It has two lines, one top and the other at the bottom with a central rectangle.
In many ways, the circuit symbol is a good representation of the actual crystal itself, especially as early crystal resonators consisted of a quartz slab held between two conducting plated.
Circuit symbol for a quartz crystal resonator, xtalUnlike many other circuit symbols, there are very few variations of the quartz crystal circuit symbol, and accordingly it is widely recognised.
The operation of the quartz crystal is based around the fact that quartz exhibits the piezo-electric effect. This means that when a stress is set up a cross the crystal, an electromotive force or electric potential is seen. The reverse is also true, then when a potential is applied across the crystal, it deflects slightly.
Selection of quartz crystal resonators fro vintage to the latest SMD typesThis means that piezo electric effect enables the mechanical and electrical domains to be linked.
In terms of the operation of the quartz crystal as a high Q resonator, the quartz crystal may have an electrical signal such as a signal in a radio receiver, placed across it. This is converted into a mechanical vibration.
The mechanical properties of the quartz crystal act as a very high Q resonator. The effect of this is then converted back into the electrical domain. The overall result is that it appears to the electrical circuit that a very high Q electrical filter is present.
In any electronic circuit design it is useful to see the equivalent circuit of the crystal so that the electronic design can be completed correctly. The normal equivalent circuit for the quartz crystal resonator is given below.
Quartz crystal resonator equivalent circuitQuartz crystals are used in two main forms of application: as the resonant element in oscillators, and in filters. In both applications the very high Q of the quartz crystal resonator enables very high performance levels to be achieved, and this is why they are used in many general circuit designs for low cost clocks as well as more demanding RF circuit design applications.
Some of the uses of these electronic components along with their abbreviations are outlined in more detail below:
Generally the requirements for accuracy these oscillators are not excessively high and therefore costs can be kept to a minimum by using a quartz crystal. When used in these applications, quartz crystals are cheaper than many other solutions that would not perform as well. Obviously straightforward crystal oscillators are used in many other areas as well.
Voltage Controlled Crystal Oscillator - VCXO: For some applications a small degree of change of the oscillator frequency may be needed. A VCXO or Voltage Controlled Xtal Oscillator is relatively easy to construct.
The circuits are straightforward and generally involve using a variable voltage to drive a varactor diode in the crystal circuit. The change in reactance of the varactor changes the overall resonant frequency of the crystal and its associated circuitry.
However in view of the high Q of the crystal resonator, only relatively small changes in frequency are possible. These circuits can be built, or they are available as commercial modules.
Read more about . . . . VCXOs.Temperature compensated crystal oscillator - TCXO: One of the main causes of frequency change of a crystal oscillator is temperature change. Where more frequency stability is required than can be supplied by a standard oscillator, then a TCXO, Temperature Compensated Xtal Oscillator is an option. As the name implies, this form of oscillator applies temperature compensation to the oscillator. Although they do not have the same performance as an oven controlled crystal oscillator, they are nevertheless able to provide very high levels of stability and performance for many circuit designs.
Read more about . . . . TCXOs.Quartz crystal technology offers very many advantages, but against this there are also some other points to be placed into the equation when considering their use:
Advantages of quartz crystal resonators:
Disadvantages of quartz crystal resonators:
As with any technology, these electronic components have the positives and negatives. Understanding these issues and the benefits they bring can enable the best to be made of them in the electronic design stage.
SMD quartz crystal in an HC49 packageSince the first signs of the piezo electric effect and the action of quartz crystals, it has taken many years for their development to be taken to the stage where it is now.
Early investigations demonstrated the effect, and it was some years before radio technology was developed and the action of quartz crystal resonators or xtals could be demonstrated and then refined.
Quartz crystals have become an essential part of today's electronics providing a high performance resonator at low cost. These components have developed over many years with many people and organisations being involved in their development.
Read more about Quartz crystal history.
Quartz crystal resonators are manufactured in vast quantities. The manufacturing process starts with the raw silicon which is converted into synthetic quartz and then the individual quartz crystal resonators are manufactured from there. Once the basic quartz crystals have been manufactured they are trimmed and then encapsulated.
In some areas of the quartz crystal resonator manufacturing process, some elements bear some similarities to that of semiconductor manufacture, although the products being manufactured are very different.
Processes like etching, deposition and the like are all used in the quartz crystal manufacturing process.
Read more about . . . . quartz crystal resonator manufacture.When choosing a quartz crystal resonator for a general circuit design or an RF circuit design there are many parameters that need to be selected. Many of these are specific to the crystal operation and are not normally seen with other electronic components.
Typically manufacturers will require a number of parameters, often set out on a specific form before they are able to manufacture and supply the required crystal element.
The decisions about the various parameters to be selected may depend upon other electronic components in the circuit, or the overall electronic design.
Understanding the different parameters to be selected and the way in which they may affect the electronic design and selection of other electronic components ensures that the correct decisions are made.
Read more about . . . . how to specify a quartz crystal resonator.Quartz crystal resonators are widely used within the electronics industry. They can be used in quartz crystal oscillators and crystal filters where they provide exceptionally high levels of performance. In addition to this, low cost elements with lower tolerance specifications are widely used in crystal oscillators for microprocessor board clocks where they are used as cheap resonator elements. Whatever its use a quartz crystal resonator provides an exceptionally high level of performance for the cost of its production.
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