低周波メカニカル遅延線路

抄録

A delay Line having a delay time of a few millisecond at low frequencies is useful in signal processing in sonar, voice and data communications. A compact mechanical delay line utilizing torsional waves propagating in a central wire can be constructed to meet these demands. Cross-bars are spot-welded to the central wire at regular intervals so that the propagating speed of the wave I greatly reduced (Fig. 1). The central wire acts as compliance and cross-bars act as inertia. Hence characteristics of the line can be represented by an equivalent low-pass filter circuit (Fig. 2). Design formulas are derived (Fig. 4). It is shown that the product of the delay time per unit section and the cut-off frequency is nearly constant (Fig. 3). The effect of anti-resonance of cross-bars on the delay characteristics is also discussed. Performance of the line at the base band and an intermediate frequency is presented. At the base band, a bimorph ceramic transducer is used at the input and an electrodynamic transducer is used at the output. Supression of spurious flexural waves is achieved by either push-pull excitation (Figs. 5, 7) or orthogonal arrangement of input and output transducers (Figs. 6, 8). A delay of 2 msec over 3kHz bandwidth is obtained in a line of 30 cm in length (Table 1, Figs. 9, 10). Supression of spurious reflection is found to be difficult. At an intermediate frequency of 36 kHz ceramic torsional transducers are used. A delay of 0. 8 msec over 4. 9 kHz bandwidth is obtained in a line of 30 cm in length (Fig. 14, 15, Table 2). The transducer consists of circular ceramic sectors bonded together to make a disk which is sandwiched between metal disks. Poling direction of sectors are so arranged that a circumferential poling can be approximated (Fig. 11). The effect of the number of sectors on the capacitance ratio is analyzed (Fig. 12). It is found that four sectors are good enough in practice.