Impedance match technique

If the curve, presenting the impedance ratios, is plotted for a series of plates made of different materials inp - W coordinates, then the intersection of this curve with the p= p DW line represents the CJ point (Figure 4.40). At that point, the impedance of the metal plate is equal to the impedance of the detonation products. This means that there is neither rarefaction nor slowing down of the detonation products, i.e., the detonation and the shock wave parameters at the explosive/plate interface are equal to the shock wave parameters. The technique described is known as the impedance match technique. 

The properties of reflected waves in the deton products were detd by a similar impedance match at the explosive-backing plate interface using the measured transmitted shock velocity in the backing plate whose Hugoniot curve was known. This technique was described by Al tschuler et al (Addnl Ref Fj) for detg the shock properties of inert solids... 

Both thin- and thick-film technology allow planar microstrip or coplanar transmission lines. However, requirements on line and coupling-gap accuracy do not always permit traditional screen printing. Etching techniques or photosensitive paste systems offer a potential solution. LTCC provides more design freedom. Impedance-matched line transitions (e.g., from microstrip to stripline or to an embedded waveguide) are possible. 

Thus, maximum power will be available to external load (R) when the external impedance is equal to the internal impedance (which is referred to as impedance matching). Impedance matching is the technique used to select a speaker to obtain the greatest output from a given HiFi system. 

To illustrate the technique, let s consider the case in which the system is stimulated by an applied voltage at a discrete frequency and the response current is measured at the same frequency. At zero frequency, or d.c., impedance is equivalent to resistance as defined by Ohm s law R = V/I. When the impressed voltage is oscillated at a particular frequency, the system responds by passing an oscillating current. If the amplitude of the input voltage is sufficiently small (typically < 10 mV), the system is linear, and the frequency of the response wave (current) matches the frequency of the perturbation (voltage). However, the response current wave may differ from the perturbation in amplitude and phase (Fig. 1). The ratio of the amplitudes of the perturbation to the response waveforms and the phase shift between the signals define the impedance function. 

For a low-frequency, Mach-Zehnder modulator, the impedance is dominated by the electrode capacitance and resistance as shown in Fig. 9.57. A Smith chart plot for a MZM is shown in Fig. 9.58. Unlike the laser diode, a modulator on lithium niobate has no junctions consequently the impedance is independent of modulator bias. As was the case with the diode laser, it is often important to match the modulator impedance to the system impedance, typically 50 or 75 real. However, unlike the diode laser, modulators fabricated in electro-optic materials that are also piezoelectric can have perturbations in their impedance due to coupling of some of the modulation signal into compressional waves. The coupling can be significant at frequencies where the modulator crystal is resonant. The dashed curve in Fig. 9.58 shows an acoustic resonance at 150 MHz. A number of techniques (Betts, Ray and Johnson, 1990) have been developed to suppress these acoustic modes to insignificant levels, as is evidenced by the solid curve in Fig. 9.56. 

Poly(vinylidene fluoride) has become an attractive material for use in uhrasook biomedical transducers because trf its large bandwidth, low acoustic impedance (about 4.5 MRayl, which is fairly close to human tissue long stability with time, conformability to shape, and low cost. This makes the material intrinsically broadband and permits construction of transducers with short impulse responses without the need for matching layers. The low electromechanical coupling factor is partially oftet by the use of time-delay spectrometry (16j. This technique allows a sig -to-noisc ratio (SNR) of at least 60-7S dB to be maintained in the frequency range from 1 to 40 MHz. 

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