Patent GB-1395933-A (GB)
Title: Method and apparatus for attenuating side maxima in ultrasonic echo sounding processes
IPC: G01S15/00
Filed: 1973-02-08
Abstract: 1395933 Echo sounding F HOFFMANN-LA ROCHE & CO AG 8 Feb 1973 [11 Feb 1972] 6360/73 Heading H4D A method and apparatus for echo-sounding uses a converging ultra-sonic pulse having an annular cross-section which is transmitted with constant phase over the cross-section, a second converging annular pulse is transmitted using a varying phase over the cross-section and echoes from both pulses are processed to reduce the response from reflectors displaced laterally from the focal line. A ring of transducers 1-12, Fig. 1, is arranged such that they are inclined towards a fixed point on the annular axis. The transducers are piezoelectric elements mounted on an epoxy resin/tunsgten damping block and are insulated from the holder 21 and each other by cork 22. Epoxy resin 23 provides mechanical protection and impedance matching to water. In the transmitting side (Fig. 2, not shown) a pulse generator is fast connected to all the transducer elements simultaneously. For the next pulse, the generator output is fed to a delay line which provides five phase shifts in 60 degree steps. The delay line taps are connected to diagonally opposite pairs of transducer elements and the generator output is itself connected direct to a reference pair (1, 7). In the receiver, Fig. 3, when the first pulse is transmitted, the transducer elements are connected, via switch contacts I to adder 53 whose output is detected and fed to delay 54 of length equal to the transmitter pulse period. For the second pulse, the transducer outputs pass via phase shift elements 57-61 to adder 62. The magnitudes of the phase shifts 57-61 are equal to the phase differences in the transmitted pulse, but the corresponding values are inverted. The adder output is detected and passed to subtractor 55 along with the delay 54 output. The Specification shows that the response to the first pulse is from all reflectors in the focal field, but to the second pulse is only from reflectors displaced from the focal axis. Thus the subtractor output which is displayed on a CRT has considerably reduced side-lobe response. To further reduce the sidelobe response, subsequent pulses may be transmitted with different phase distributions and the results summed in a weighted adder. Other methods of providing the transmitter and receiver phase shifts are disclosed including using three separate annular transducers-one planar for the first pulse and two helical one raised, one indented for transmission and reception of the second pulse respectively; providing acrylic resin rings in front of planar annular transducers, the thickness of the rings varying to provide the desired phase-shifts; dividing the transducers into sectors and displacing them step-wise in a helical manner; providing permanent phase shift elements in the connecting leads to segmented transducers. If necessary the simple detectors shown in Fig. 3, may be replaced by phase correcting rectifiers (Figs. 6, 7, not shown). In both these the two differences between cos wt multiplied by one pulse and sin wt multiplied by the other pulse are found and the root of the sum of the squares of the differences is used to drive the CRT display.