Patent GB-704217-A (GB)
Title: Improvements in or relating to delay devices for signals
Inventor: BEVERIDGE HAROLD NORMAN
IPC: H03H9/36
Filed: 1952-01-31
Abstract: 704,217. Acoustic delay lines. RAYTHEON MANUFACTURING CO. Jan. 31, 1952, No. 2664/52. Class 40 (8) A delay line for acoustical wave energy comprises a body of acoustical wave-transmissive medium within which a directed beam is transmitted between two transducers and suffers one or more reflections within the medium, the reflecting devices being limited in area to the desired cross-sectional area of the beam in order to prevent divergence. The medium may be a solid such as quartz or a liquid such as mercury. In the device of Fig. 2 the delay medium is mercury contained within a shallow stainlesssteel tank 6. The propagating and receiving transducers 4, 5 consist of quartz crystals which are mounted with one face in contact with the mercury at a small angle to the walls of the tank. The walls of the tank are ground smooth except at the positions 9 where circular, sandblasted (or etched) areas of the same dimensions as the faces of crystals 4, 5 are provided for the reflection of the beam. These sand-blasted areas are almost totally reflecting, whereas the smooth surface of the rest of the tank absorbs the beam to the extent of 10 db. Almost all the energy in the beam strikes the sand-blasted " target areas", and the small amount of energy in the side lobes of the beam, which would normally cause spurious, mis-directed reflections, is almost totally absorbed by the smooth wall of the tank. Each target area 9 may be surrounded by an obliquely cut groove 20, Fig. 7, the divergent part of the beam being substantially absorbed by the bottom of the trough, which is ground smooth. The small amount of energy reflected from the inclined bottom of the groove is directed to the bottom of the tank. Fig. 5 shows diagramatically a further form of the device in which a reflecting surface or target area 11a is arranged in an oblique position between two walls of the tank. Three or more delay lines may be arranged in the same tank, by using a corresponding number of pairs of propagating and receiving transducers arranged on opposite walls of the tank together with the required number of reflectors. Each transducer may be in the form of a quartz crystal disc 50, Fig. 11, removably held between a metal electrode 51 and a resilient washer 49. In order to adjust the length of the line the crystal disc may be moved in the direction of the beam by means of three screws 43 which bear at circumferentially spaced points on a metal annulus 41 whose movement is transmitted to the electrodes 51 through an insulating washer 45'. The angular position of the crystal is also adjustable by means of the screws 43. The position of the reflectors 62 may be adjusted by means of screws 66. A removable stud 71 limits the movement of the reflectors and prevents them from rotating. A solid delay line of fused quartz may be constructed using the principle of the invention, the quartz being coated with lead over those areas where absorption is desired and quartz-air interfaces being left in areas where the beam is to be reflected.