First the transducer uses C-mode
imaging to scan the entire thickness of the sample in multiple narrow (vertically) gates.
Moreover, black and red arrows indicate the [pi]- and c-mode components, respectively.
This discrepancy in the voltage fraction resulted in the development of the c-mode waves preceding the original [pi]-mode pulse.
Because the c-mode pulse is significantly attenuated, pulses in a TWFET are free from modal distortions.
5(a) and (b) show the measured waveforms of the c-mode inputs monitored at n =10 and n = 40 for [V.sub.G] = -2.0 V (< [V.sub.TO]) and [V.sub.G] = -1.17 V (> [V.sub.TO]), respectively.
For lower frequencies, the c-mode pulse is significantly influenced by dispersive distortions.
We found that the [pi]-mode gains amplitudes, while the c-mode loses them.
For definiteness, we call the fast and slow modes as the [pi]- and c-modes, respectively; therefore, the dispersion having the upper (lower) sign is for the c- ([pi]-) mode, which is denoted as [k.sub.c]([pi])([omega]).
Although there exists two c- and two [pi]-mode characteristic impedances for the considered coupled structures, one can choose their dimensions, [W.sub.1], [W.sub.2], S, and S' (for a shielded structure), to produce equal c-mode impedances and equal [pi]-mode impedances.
Figure 4 shows a comparison between this analysis and the computed even- and odd-mode characteristic impedances and effective dielectric constants for an offset structure with equal strip widths, as a function of S/W ((0.5h+d)/W = 1 and 0.5h/W = 0.2).(11) As a result, the [Z.sub.oe] c-mode impedances are the same for both lines.
The [pi]-mode effective dielectric constant for strip 1 or 2 decreases significantly, whereas that of the c-mode is virtually unchanged with an increase in the strips' distance.
As the spacing between the side walls is increased, the c-mode characteristic impedances increase, while those for the [pi]-mode remain virtually the same.