Surface delay line

Fig. 21. A surface acoustic wave dual-delay line oscillator. The sensitise layer is placed in the propagation path of one of the two SAW devices. The differenee in Ireqnency (At) between the two channels provides a dtrecl result of the mass loading and electric field effects associated w ith the sensor layer...

The simplest SAW sensor is a two-terminal transmission (delay) line in which the acoustic (mechanical) wave is piezoelectrically launched in one oscillator, called the transmitter. It travels along the surface of the substrate and is then transformed back into an electrical signal by the reverse piezoelectric effect at the receiving oscillator (Fig. 4.18). 

Filters and delay lines of the form shown in Fig. 6.34(b) are made which exploit the surface acoustic wave (see Section 6.5.2 above). The SAW is propagated in the direction normal to the overlap of the interdigitated electrodes, the wavelength launched being related to the electrode spacing and width. For a combined space and half-width of 15 /nn (2sulfacc = 30 /nn) the structure will propagate a centre frequency of 100 MHz. That is it operates as a filter. 

The discovery by R. M. White of the University of California at Berkeley that surface acoustic waves could be excited and detected by lithographically patterned interdigital electrodes on the surface of piezoelectric crystals [42] has led to widespread use of SAW devices in a number of signal-processing applications. These include frequency filters, resonators, delay lines, convolvers, and correlators [43,44]. 

An example of a two-port device is the surface acoustic-wave (SAW) delay line shown in Figure 6.3. Acoustic plate mode (APM) devices utilize a two-port configuration that is conceptually identical to that of the SAW for the flexural plate wave (FPW), there is typically a third connection to its ground plane (see Section 6.2.3). In principle, the ground plane connection is unnecessary, but in practice more stable operation results when this connection is made. Notice that there... 

ZnO is a wide band gap semiconductor, which is used for various applications. Based on textured ZnO films one can build highly effective piezo field emitters. On the other hand ZnO is a very effective electron-excited phosphor. ZnO films easily withstand electron fluence more than 1 W/cm. ZnO films doped with Al, Ga, or In have a low resistivity of about 10 " Qcm and a high transparency of about 90%. This is sufficient for applications as a front contact in solar cells, liquid crystal displays etc. Dielectric ZnO films have a high electromechanical coupling factor that allow using ZnO in various surface acoustic wave (SAW) devices such as delay lines, delay-line filters, resonators, transducers and SAW convolvers. 

A new Pt(II) polyyne polymer, P15, prepared from the reaction of cfs-[Pt(PPh3)2Cl2] with l,4-diethynyl-2,5-dihexadecyloxybenzene using the extended one pot polymerization route, was tested for its sensing properties and showed fast and reproducible response to relative humidity variations and methanol vapor in surface acoustic-wave (SAW) sensors.46 A SAW sensor was fabricated from polymer P15 as a sensitive membrane, and the polymer was deposited as thin film on the surface of SAW delay lines implemented on three different piezoelectric substrates. High sensitivity and reproducibility were recorded for such devices. The acoustic characterization of the polymer film was also studied with the aid of theoretical results obtained by the perturbation theory. 

Figure 2. Surface acoustic wave gas sensor consisting of dual delay line oscillator[4].

There are few works involved in the detection of aliphatic hydrocarbons, due to their very week interactions between the analytes and Pcs. Urbanczyk and coworkers employed SAW technique to detect trichloroethylene [15]. The acoustic waveguide was fabricated on the y-cut of the LiNb03 piezoelectric substrate. The changes in the physical properties of the CuPc layer placed on a piezoelectric crystal surface can be recorded as a change in differential frequency in a dual delay-line oscillator system, under the exposure of the vapors of the VOCs. The sensitivity is normally quoted as differential response, that is, Afp/ppm of gas, and the greatest sensitivity (approximately 0.1 Hz/ppm) was obtained for trichloroethylene. 

An electronic tongue based on dnal shear horizontal surface acoustic wave (SH-SAW) devices was developed to discriminate between the basic tastes of sour, salt, bitter, and sweet [57]. Sixty MHz SH-SAW delay line sensors were fabricated and placed below a miniature PTFE housing containing the test liquid. All the tastes were correctly classified without the need for a selective biological or chemical coating. 

Fig. 3.9 (Left) Surface acoustic wave (SAW) resonator and (right) SAW dual delay line device (on penny). Quartz-based SAW sensors, coated with chemically selective films, can detect chemical vapors. Sensor arrays, with diverse coatings, can detect multiple chemical vapors, image courtesy of pacific northwest national laboratory...

The direct sound and the first order reflections (bouncing off of only one surface) are shown in Figure 12.10, but there would be many second, third, and higher order reflections as well. These individual paths can be modeled as simple delay lines, with attenuation due to the spherical sound waves... 

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