Delay lines, acoustic

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

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. 

A delay line can also be formed from a slice of a special glass designed so that the velocity of sound is as nearly as possible independent of temperature. (The term isopaustic has been coined to describe such a material.) PZT ceramic transducers are soldered on two 45° metallized edges of the slice. The input transducer converts the electrical signal to a transverse ( shear ) acoustic... 

Figure 1.2 Schematic sketches of the four types of acoustic sensors discussed in detail in this book (a) Resonant quartz crystal like that used in electronic communications systems (after Lu [6]) (b) Suiface-acoustic-wave delay line with selective absorptive coating (after Wohltjen and Dessy [3]) (c) Acoustic-plate-mode delay line made from quartz crystal (after Ricco and Martin [7]) (d) Thin-membrane flexural-plate-wave delay line made by microfabrication techniques from a silicon wafer.

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... 

In the context of resonant acoustic devices, Q =fiJBW, where fn is the resonant frequency and BW is the bandwidth it can equivalently be defuied as loU Pj, where o> is the angular frequency. Up is the peak total energy present in the device, and is the power dissipated by the device. For resonant systems, BW is the range of frequencies over which the reflected power is within 3 dB (a factor of two) of its minimum value, attained at fit, for non-resonant systems (e.g., delay lines), BW is the range of iiequencies over which the transmitted power is within a factor of two of its maximum value. 

To complicate matters, appropriately designed two-port devices can readily be operated as resonators (though one-port devices are not practical for use as delay lines). One need only consider the second IDT, located a few acoustic wavelengths fiom the launching transducer, as shown in Figure 6.2, to understand how one- and two-port SAW resonators differ in their fundamental design. A discussion of the function of the second IDT is contained in Section 6.3, Acoustic-Wave Measurement Technology. 

For SAW resonators (either one- or two-port), design criteria are very similar to those discussed above for delay lines, with an important caveat the number of fingers utilized for the IDT must be significantly fewer than for delay lines in order to couple to the acoustic cavity (the ridge array) at the proper signal level, and simultaneously achieve the desired impedance. The result is that while... 

The velocity of the electrical signal in the cable is nearly 10 times faster than the acoustic signal in the delay line, hence the wavelength is nearly 10 times as long in the electrical cables as it is in the acoustic delay line. 

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. 

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