High-frequency One-port Colpitts SAW Oscillator for Chemical Sensing

Transcription

1 High-frequency One-port Colpitt SAW Ocillator for Chemical Sening S. Thoma, Z. Rácz, M. Cole and J.W. Gardner Microenor and Bioelectronic Laboratory Univerity of Warwick Coventry, UK Abtract Thi paper report upon the deign and development of a low cot, high enitivity, high frequency urface acoutic wave reonator (SAWR) baed ytem for ga ening application. The 262 MHz one-port SAWR operate in a grounded-bae Colpitt ocillator arrangement that wa developed baed on an equivalent circuit model. Electrical characteritic of the fabricated SAWR how good agreement with it equivalent device model at the reonant frequencie, and it wa found to have good tability and enitivity with a Q-factor in air of about 2,870 at it fundamental reonant frequency. The enor ytem i deigned to operate in a dual configuration in which one reonator i coated with a gaenitive polymer (polyethylene) coating, whilt the econd one i ued a a reference channel; thereby eliminating common mode interference on the baeline ignal. Ma enitivity wa found to be ca. 1 Hz/ng, which correpond to ub-ppm enitivity to ga/odour concentration. Keyword-acoutic wave; one-port; Colpitt ocillator; BVD model; SAW reonator;polymer coating I. INTRODUCTION Both bulk acoutic and urface acoutic wave (SAW) baed enor ytem have been reported in chemical ening application over the pat few decade [1 4]. Due to their high enitivity and imple drive/readout circuitry, more recent focu ha been on urface acoutic wave baed device where a SAW device form the frequency elective component within an ocillator circuit. Polymer-coated SAW baed chemical enor impart high enitivity and electivity to pecific volatile compound. The aborption of the ligand molecule onto the polymer change the phyicochemical and electrical behavior of the SAW device reulting in a change in it ocillation frequency. Common method to implement SAW ocillator circuit are typically baed on the feedback loop method or the negative reitance method [5], [6]. The frequency tability and vapor enitivity of the SAW enor ytem directly depend on the type of the employed ocillator circuit. Nimal et al. [7] have recently reported that one-port Colpitt ocillator are more enitive, but le table, than two-port Pierce ocillator. The enitivity can alo be improved by tuning the phae point et within the SAWR in the pa band thereby reducing the noie performance of the ocillator circuit [8]. In thi tudy, we preent one-port polymer-coated Rayleigh wave baed SAW reonator, fabricated on an STcut quartz wafer, for application in low-cot chemical enor. An invetigation into different equivalent circuit model i alo preented, which lead to the concluion that the mot uitable ocillator circuit for one-port SAWR enor i a Colpitt ocillator configuration. II. ONE PORT SAW RESONATOR SAW reonator are commonly available a one-port and two-port device employing either delay line or reonator configuration. Becaue of the potential for high Q-value, low noie level and higher tability, we have elected a oneport reonator tructure. Thee reonator are deigned to operate at a baeline frequency of 262 MHz in a dual configuration to obtain differential meaurement (Fig. 1). The deign and modeling of urface acoutic device are normally carried out uing the well-etablihed Coupling of Mode (COM) theory [9], [10]. Although a COM model allow for an accurate decription of the SAW reonator by imulating the admittance behaviour, the formula are omewhat cumberome and are not very informative - a far a circuit analyi and imulation i concerned. In addition, the accuracy of thi model i limited to a narrow frequency band around the reonance region. Hence, the COM theory mut aume near-reonance frequencie in order to derive a implified electrical model of the SAW reonator [11]. The Butterworth Van Dyke (BVD) model, a a imple electrical equivalent circuit model, i more uitable for circuit deigner. Morgan [12] etablihed that the electrical acoutic impedance behavior of a SAW device, obtained uing a lumped-element equivalent circuit model, i in good agreement with conventional COM analyi. Thi equivalent circuit model conveniently relate the acoutic perturbation due to urface ma loading in a SAW device to it electrical behavior. A BVD model [11] wa developed for the 262 MHz oneport SAWR, hown in Fig. 2., allowing quick imulation and deign of the aociated ocillator circuitry. The motional and tatic arm parameter were extracted uing the tranmiion parameter of the SAWR. A hown in Fig. 2, the electrical component R, L and C are the motional inductance, capacitance and reitance repectively, which form the motional arm producing the reonant frequency while the capacitor C o form the tatic arm providing the anti-reonant frequency. The motional arm ignifie the electro-acoutic propertie [13] of the piezoelectric material and it model the vibration of the crytal. R repreent the 13

2 acoutic attenuation in the reonator and capacitance C o the capacitance of the piezoelectric crytal. where ω =2πf = 1, LC (2) 1 (3) f= 2π LC Here, f i the erie reonance frequency of the SAWR, modeled by the motional arm. The unloaded quality factor of a SAWR i given by Q = (4) R u ωl Due to the high Q-factor of a SAWR, R can be neglected. Thu (1) become, Fig.1. Optical micrograph of the 262 MHz one-port dual SAW reonator enor, fabricated in aluminium on a ST-cut quartz wafer ubtrate. The top reonator i coated with a chemically-enitive non-conductive polymer (polyethylene) and the bottom reonator i uncoated thu acting a a reference channel. The deigned 1-port SAWR comprie finger pair with 3 µm finger width forming the inter-digitatedtranducer (IDT), and 500 reflector on each ide to create a tanding wave pattern with an overall die ize of 7.4 mm 2.4 mm. The dual reonator configuration [14] with a reference channel eliminate common mode interference on the baeline ignal, uch a change in ambient temperature or preure. The SAWR were fabricated on an ST-cut quartz ubtrate with aluminum IDT uing UV lithography (PacTech, Germany) ω Z()= 2 C 2 Co + 1+ ω Co Thi how that the reonator exhibit a parallel reonance at: a T (5) 1 (6) f= 2π LC where CCo (7) C T = C+C o Fig.2. Illutration of the BVD equivalent circuit lumped element model of a one-port SAW reonator. In addition to the fundamental mode of operation, the SAWR exhibit everal overtone frequencie, which can be modelled by adding additional erie-reonant branche to the BVD model. For operation around a certain reonant frequency, the crytal can be modelled by the circuit with a ingle motional arm. The impedance of thi modelled circuit i given by [15]: R (1) 2 2 +ω + L Z()= 2 R C 2 Co ω L Co Fig.3. Real (Solid line) and imaginary part (dotted line) of the impedance preented by 262 MHz one-port SAWR. The erie, parallel and center frequencie are marked on the diagram. The real and imaginary part of the one-port SAWR impedance exhibiting a minimum reitance at reonance and a maximum reitance at anti-reonance frequencie, obtained by an RF network analyzer (E5071B, Agilent Technologie), i hown in Fig. 3. The erie reonance frequency, f, i MHz, the parallel reonance, f a i MHz and the center frequency, f c i MHz. Thi alo demontrate that the SAWR center frequency lie 14

3 a expected between the erie and parallel reonant frequencie. The phae curve in Fig. 3 alo how that the Barkhauen criterion of 0 phae condition for ocillation i atified at the center frequency of the SAWR. III. COLPITTS OSCILLATOR DESIGN Several imilar tranitor-baed circuit configuration are available for the realiation of SAW ocillator, uch a Pierce, Colpitt, and Clapp, with the main difference lying in the tranitor grounding option. The performance of the three configuration varie with the difference in the poition of the biaing reitor and capacitance. The mot deirable option i the Pierce configuration due to it implicity, robutne and ability to work at higher frequencie (> 500 MHz) becaue it i arguably the leat affected by tray capacitance [15]. However, the Pierce ocillator can only work with a two-port SAW reonator within a feedback loop to attain the required 180 phae hift. The Colpitt ocillator, however, allow the SAWR to operate in a 1-port configuration [7], and therefore wa elected for thi work. The chematic of the Colpitt SAW ocillator circuit with a grounded bae configuration, where the SAWR input i connected to the tranitor bae and the output port i connected to the ground, i hown in Fig. 4. reduce paraitic capacitance allowing radio frequency (RF) ocillator operation. To obtain the tuned ocillation frequency cloe to the SAWR Q-factor, tight tolerance component were elected for the capacitor and the inductor value. In thi configuration, the reonator how an inductive behavior between the erie (f ) and parallel reonance (f p ). The tranitor along with the feedback capacitor C 1 and C 2 provide the negative reitance to compenate for reitive loe in the reonator. The major limitation of uch an ocillator circuit i that the paraitic capacitance begin to affect the effective operation of the circuit at frequencie above 500 MHz. IV. CHEMICAL DETECTION SYSTEM SETUP A robut, high-enitivity chemical detection ytem baed on polymer-coated one port SAW enor ha been deigned and implemented. The SAW ocillator ha been realized by interfacing the dual SAW reonator to Colpitt ocillator circuitry. A two layered Printed Circuit Board (PCB) ha been deigned uing Altium Deigner oftware. Figure 5 how the photograph of the dual Colpitt SAW ocillator baed chemical enor ytem. The PCB enure minimal cro-talk aociated with high frequency ignal. The phae hift linked with the RF ignal path to the reonator have alo been taken into account during the PCB deign. Fig. 4. Simplified chematic of the Colpitt ocillator circuitry ued to drive the 1-port SAW reonator enor. The Colpitt ocillator offer good tability at higher frequencie, lower harmonic, lower component count and hence lower cot than other type including feedback-baed ocillator. The tranition frequency of the tranitor, f T, limit it frequency of operation, when the capacitor needed for obtaining the ocillation frequency are comparable to the tranitor terminal capacitance. Thi may be avoided by uing a high f T value (a few gigahertz) BJT in the ocillator circuit or by uing the crytal in a erie reonance configuration [15]. The ue of RF tranitor (BFR92P, Infineon) rather than an operational amplifier alo Fig.5. Photograph of the SAW reonator with aociated Colpitt ocillator circuit on the backide of a cutom PCB. The experimental arrangement demontrating chemical detection uing SAWR ocillator conit of a cm 3 ga/odor chamber (photograph of the etup i hown in Fig. 6) to which a nemesys multi-channel yringe pump (Cetoni GmbH, Germany) i attached. The microliter preciion yringe deliver the chemical into the chamber via capillary line, where it get vaporied. The SAW enor, arranged in dual configuration, where one i coated with the ening polymer polyethylene and the other 15

4 enor form the reference channel are attached to the far end of the chamber. A commercial FQ4 interface intrument (JLM Innovation, Tubingen, Germany) wa connected to the ocillator output for frequency meaurement. The ocillation frequencie of the individual enor were monitored to obtain the SAWR differential ignal. ocillator provide a highly-enitive ytem for chemical detection. The repone ha a low level of noie a hown in Fig. 8. On the introduction of 10 µl of the inect pheromone Z9-14:OAc into the odor chamber, a differential frequency hift of about 6 khz wa meaured at the SAW output, which how that the average enor repone to the pheromone compound i about 0.6 Hz/nl of liquid, i.e., ubppm level of phermone in air. The repone time of the ytem i relatively low (~100 ) and it i aociated with the evaporation and diffuion of the volatile compound inide the chamber. However, the actual repone time of the SAWR itelf i below one econd. Fig.6. Experimental arrangement for chemical detection enor ytem coniting of an odour chamber, a venting pump, yringe pump, and SAW enor with the Colpitt circuitry. V. EXPERIMENTAL RESULTS Figure 7 how the ocillator reonant frequency output obtained by an RF ocillocope (LeCroy LT342 Waverunner). The meaured frequency value i in good agreement with the theoretically modeled value. The load enitivity i ignificantly le for thi ocillator circuit. The output of the SAWR ocillator i practically noie and ditortion free. Fig.7. Photograph of the baeline frequency (261.9 MHz with amplitude of 4.2 V) of a Colpitt SAW ocillator enor ytem hown at the channel 2 of an RF Ocillocope. The typical frequency hift of a dual SAWR enor after the detection of a volatile chemical compound (here an inect ex pheromone) how that the one-port SAW Fig.8. Differential frequency repone of polymer-coated SAWR enor to pheromone Z9-14:OAc demontrating the high enitivity of the polymer-coated SAWR enor. VI. CONCLUSION A high frequency one-port Colpitt SAWR ocillator ha been deigned and fabricated for application in a low-cot, low-power ga enor. An equivalent model ha been developed, which formed the bai of an ocillator circuit deign for a highly enitive chemical enor. The SAWR exhibit a high quality factor of 2,870 and ha an etimated 0.5 Hz/ng ma enitivity after coating with a thin ga enitive non-conducting polymer film. Further tudie are being carried out on the detection of pecific blend (i.e. mixture) of chemical compound. In addition, technological development of thi work include the creation of a mart low-cot, low-power chemical enor on a chip - by the integration of the SAWR enor with full cutom CMOS ocillator circuitry thu reulting in an application-pecific integrated circuit (ASIC) BioMEMS chip. ACKNOWLEDGMENT The author wih to thank firt Mr. Frank Courtney (Warwick Univerity, UK) for hi aitance in all mechanical and technical matter and econdly Mr. Ian Griffith (Warwick Univerity, UK) for hi help in the manufacturing of the ocillator PCB. 16

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