Two-port device

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... 

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. 

Two-port devices include SAW, FPW, and APM delay lines and the two-port-resonator variations of these devices. For all the delay lines, the transmitted signal S]2 (between ports) is most important Just as in one-port measurements, a network analyzer is the instrument of choice to measure frequency response the setup is depicted in Figure 6.9(a) (page 362). Frequency response can also be measured using the synthesized oscillatorA VM combination shown in Figure 6.9(b). As for the one-port measurements, this setup is most convenient when computer controlled. 

Figure 6.9 (a) Use of a network analyzer to measure two-port-device frequency response. (b) Setup for measuring two-port frequency response using the synthesized os-cillatorA VM combination. 

An important general distinction can be drawn between one- and two-port devices that applies to all the measurement schemes discussed throughout this chapter perturbations of one-port devices result in significant changes in device impedance, without major changes in phase for two-port devices, perturbations... 

For two-port devices (e.g., SAW delay lines), separation of the sensor from the electronics can be accomplished by simply connecting a transmission line —... 

FIGURE 7.8 The BJT as a two-port device (a) block representation, (b) common emitter, (c) common base, (d) common collertor. 

There are scores of microwave devices that need to be measured. The most common measurements tirat are done are the frequency measurements, but we are also interested in phase measurements depending on the device under test (DUT). We can use various equipments like spectrum analyzer, vector network analyzer, etc. [33]. Most of the devices that we measure will be two-port devices that is, the input is applied at one port and the output is taken at the other port. So we will basically be measuring devices, a group of devices that form a part of tiie system, and microwave circuits. 

A laboratory where the measurement takes place must be free from odor and is typically air-conditioned with air filtration. The odor sample is placed in an olfactometer that basically is a device for dilution of the sample. Typically, the meter has two outlet ports diluted odorous air flows from one, and clean odor-free air flows from the other. In dynamic olfactometry, panel members assess the two ports of the olfactometer. The assessors indicate from which of the ports the diluted sample is flowing. The measurement starts with a dilution that is large enough to make the odor concentration beyond the panelists threshold. This concentration is normally increased by a factor of two in each successive presentation. Only when the correct port is chosen and the assessor is certain that the choice is correct and not just a guess, is the response considered a true value. 

An example of a one-port device is the bulk resonator shown in Figure 6.1, which has a single, planar electrode on each side of a slab of piezoelectric material (these two electrodes together comprise a single port). Most often, the material takes the form of a disk and the electrodes are circular, covering less than the entire surface of the disk. Connection to an external circuit is typically made via a coaxial cable, with one of the two electrodes connected to the shield and the other to the center conductor. This device is known as a resonator because an external circuit (see Section 6.3.3.2) excites the piezoelectric substrate in such a way that a standing wave is set up in the crystal, which thus resonates. 

For comparison, the frequency response of a two-port SAW resonator is shown in Figure 6.11 (page 364). Note that it resembles the response of the delay line, with the addition of a sharp spike, where the insertion loss is considerably lower, at the center of the pass band. The similarity of the delay line and resonator frequency responses is a consequence of both devices using the same transducer pattern, while the spiked region of much lower insertion loss is a result of the ridge-reflector array utilized to set up a standing wave. Unlike the highest point of the delay-line spectrum, there is no 6-dB theoretical insertion loss limit for the peak of the resonator spectrum — loss can approach 0 dB. 

External cabling uses thick, shielded cables run from adapter to device to device in a fashion known as daisy-chaining (see Figure 4.15). Each device has two ports on it (most of the time). When hooking up external SCSI devices, you run a cable from the adapter to the first device. Then you run a cable from the first device to the second device, from the second to the third, and so on. 

The initially amplified signal is then usually detected in the receiver using a three-port device acting as a double-balanced mixer. Three port devices, which can be used as gates, mixers or phase-sensitive detectors, consist of a series of diodes and transformer coils. The phase-sensitive detector has two input signals, the NMR signal (wq) and the frequency of the synthesiser (Wref)- The output is proportional to... 

Strain gages are passive two-port electric devices, in which a force-induced... 

A double balanced mixer is a three port device. When two frequencies are presented at two of the ports, the sum and difference frequencies appear at the third port. A modern DBM is a broadband device, for example 2-200 MHz, and comes in extremely compact packages with or without connectors. DBM s are very useful in a variety of applications. 

The power divider is a broadband multi-port device which permits the rf inserted in one port to be equally divided between the outputs. The outputs have the same phase and the same amplitude and are highly isolated from each other. Conversely, if two rf signals are put into the output ports of the power divider, they will be added vectorially. Hence the... 

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