Patent GB-552072-A (GB)
Title: Means for measuring time intervals by impulses
IPC: G01S1/02
Filed: 1941-12-16
Abstract: 552,072. Measuring distances by reflected electromagnetic waves ; directive wireless signalling ; oscillographs. STANDARD TELEPHONES & CABLES, Ltd. (International Standard Electric Corporation). Dec. 16, 1941, No. 16264. [Class 37] [Also in Group XL] Apparatus for measuring short time intervals, as for example between a transmitted impulse and the reflected impulse in obstacle-detecting systems, comprises a cathode-ray oscillograph wherein the reflected impulses and corresponding reference impulses of the same frequency are brought into coincidence bv a time-delay network, the adjustment of which measures the time interval. The scanning of the oscillograph is effected at a frequency corresponding with that of the transmitted impulses but arranged to begin before the impulses are transmitted, so that the impulses appearing on the screen are not interfered with owing to return of the spot. A transmitter 1 radiates impulses to an obstacle by means of antenna 2T, the reflected impulses being received at antenna 2R and sent to receiver 10 and thence to a field oscillograph 9 and vernier oscillograph 14 for fine adjustment. 'An impulse generating system 4 ... 6 of, say 200,000 p.s. impulses is connected to a frequency divider 7 where the impulses are stepped down, say, 5000 p.s. and adjusted so that each is, say, 0.5 microseconds duration, one being shown at A, Fig. 3. These impulses are fed to a delay network 12 introducing a 5-microseconds delay and thence applied through modulator 3 to the transmitter 1, the corresponding delayed impulse B being shown in Fig. 3. The impulse C received at 10 is shown in Fig. 3 as occurring 12.5 microseconds later. The impulses A are also applied to the time-base 11 of field oscillograph 9 which is designed to sweep 5,000 times per second, each sweep lasting 200 microseconds. Impulses A are applied also to a device 16 which inverts their polarity and the output of 12, 16, namely B plus inverted A, is mixed at 17, delayed at 13 by an adjustable amount which makes the coarse measurement of time, and separated by amplitude selection at 18. The delayed impulses B are separated in the form D and fed to the oscillograph 9 as reference impulses whereas the inverted A impulses in the form G are fed to the time base 15 of the vernier 14. The output of 6 is fed to a delay network 19 the output E of which, in the form of impulses of 5 microseconds interval, supplies the reference impulses to 14. The network 13 has a range of zero to 200 microseconds in 2 microseconds steps, whereas 19 has one of zero to 5 in steps of 0.1, and the two are coupled together and preferably arranged so that every time 19 is adjusted through 2 microseconds, 13 is stepped up 2 microseconds, 19 being arranged also to return to zero and increase again every time it reaches its maximum. In use, 13 is adjusted for approximate coincidence in 9 and then 19 is adjusted for the vernier reading, the two readings being then added. To facilitate observation, the test and reference impulses may be of opposite polarity on the screens, and 13, 19 may be calibrated directly in distance. In Fig. 2, the same reference pulses are used in 9, 14, and two further oscillographs 23, 24 are provided for bearing and altitude indication. The output of 13, 19 in this form, namely, B, E, and inverted A impulses, is mixed at 20 as shown in curve F of Fig. 3, where b denote delayed B impulses, a inverted A impulses, and e the impulses E, and it will be seen that e is added to b and depressed by a. The amplitude selector 21 separates the impulses exceeding a level m and also those which have crossed the level n twice, the former being fed as reference impulses through device 22 to all the oscillographs, whereas the latter are fed through 15 to 14, 23, 24. With this arrangement it is necessary for the A, B impulses to be as wide as the steps of network 13 to ensure that the impulses e will be raised or depressed for any setting of 19 between successive steps of 13. The receiver 28 supplies the impulses C to 9, 14, 23 and for distance measurement may be connected to one aerial of the directional pair 25, 26, the commutator 27 being then operated for bearing measurement and the aerials adjusted for coincidence of image. The altitude-measuring system operates similarly, but by suitable timedelay in operation the output of 31 could be fed to 28 and 23 and separate measurements of altitude and bearing made in the latter. The vernier time-base 15 may comprise, Fig. 4 (not shown), a condenser in the plate-circuit of a valve which condenser is allowed to charge to provide the sweep and is then discharged by a further valve controlled by the reference impulses. The first-valve circuit includes also a spot-extinguishing resistance. The details of elements 13, 19 ... . 21 of Fig. 2 are described with reference to Fig. 5 (not shown). Automatic gain-control means may be provided to compensate varying amplitude of reflected impulses, Fig. 7 (not shown), and also means for sensitizing the receiver valves only for the pericd of the impulses, Fig. 6 (not shown). The latter. arrangement may be modified to perform both functions.