Patent GB-1353801-A (GB)
Title: Frequency dispersive delay devices
IPC: H03H9/44
Filed: 1971-08-05
Abstract: 1353801 Dispersive delay lines; pulse radar TEXAS INSTRUMENTS Inc 5 Aug 1971 [26 Aug 1970] 36869/71 Headings H3U and H4D A pulse, comprising a waveform which sweeps linearly or otherwise from a minimum to a maximum frequency or vice-versa as it is produced, Fig. 2 (not shown), and which may also be shaped in amplitude, or divided up into further sub-pulses during the frequency sweep, is applied to a frequency dispersive acoustic surface wave delay device which delays the pulse by at least its duration and compresses its time duration, Fig. 3 (not shown). As shown in Fig. 1 a single crystalline prize substrate 14, of lithium niobate, quartz, zinc oxide or cadmium sulphide supports broadband input transducers 15, 19, 23 and output transducers 17, 21, 25, which may be uni- or bi-directional, belonging to respective sections 10, 11, 12, series connected. Frequency dispersive arrays 16, 20, 24 have interdigitated electrodes so spaced that, at the instant when the initial low frequency portion of the pulse provided by source 46 reaches electrodes 49, 51 of array 24, the last, high frequency pulse portion has just reached electrodes 36, 37 of array 24, and the electrode spacing throughout the arrays 16, 20, 24 is matched to the instantaneous wavelength of the surface waves passing, being one half the corresponding instantaneous wavelength, so that a high amplitude spike (32), Fig. 3 (not shown), appears at the delay device output A, A<SP>1</SP> connected to the three arrays 16, 20, 24 in parallel. Alternatively, a pulse of decreasing frequency sweep may be applied to transducer 25. Fig. 4 (not shown) displays the frequency delay characteristic, the discontinuities occurring at frequencies matched at the ends of the respective arrays. The pulse may comprise three subpulses separated by the transit time of regions B, to avoid consequent loss of signal. In Fig. 7, the substrate 14 supports frequency dispersive arrays 87, 88, 89; 90, 91, 92 each of which has electrode spacings corresponding to the entire frequency range of the input pulse. The initial lowest frequency pulse portion, emitted from the left hand end of transducer 87 reaches the right hand end of transducer 93 where the electrode spacing is adapted to detect it, at the same time as the highest frequency arrives at the left hand end of the transducer, having traversed the distance from the adjacent end of transducer 87. Subsequent transducers provide two further delays of identical characteristics, as displayed in Fig. 6 (not shown). In Fig. 5 (not shown), the left hand transducer of each section corresponding to those of Fig. 7 is a broadband interdigital transducer (60, 62, 64) at the expense of higher input impedance. To avoid excessive ripples as the number of cascaded frequency dispersive arrays is increased, Fig. 8 (not shown), such arrays may have electrodes whose width and overlap are varied (119-122), Fig. 9 (not shown), in accordance with Fourier Transform design techniques, which are applicable to all embodiments. All devices are reversible. The electrodes are Al or Au formed by metallization and etching using a photoresist. The device is used in a radar installation.