Patent GB-1290935-A (GB)
IPC: G01S3/84
Filed: 1970-11-13
Abstract: 1290935 Spectrum analysis FRIED KRUPP GmbH 13 Nov 1970 [14 Nov 1969] 54185/70 Heading G1U [Also in Division H4] Apparatus for indicating on a CRT received acoustic or electromagnetic waves comprising a scanner operating at a scanning frequency to provide a function A = g(x) where A is the signal strength at the receiver and x is angle of incidence or frequency; a deflection device providing circular deflection of the CRT beam at the scanning frequency and radial deflection at a submultiple thereof and a brightening circuit wherein a polar co-ordinate display is produced with the signal strength represented by radial distances from a zero circle. In a ship borne system for monitoring sound waves generated by other vessels the output of a circular group of hydrophones 7 are combined by a scanner 9, as described in Specification 1220629, to produce a beam effectively rotating at a frequency fa. A sine wave at this frequency is divided down to pulses k at a frequency fb which trigger a sawtooth generator 13 whose output m is multiplied by the signal fa. The composite signal is applied to the deflection plates of a CRT, directly and through a 90 degree phase delay producing a spiral raster rotating at fa and deflecting radially at fb. Alternatively, the sawtooth generator 13 is followed by a sample and hold circuit which provides an intermittently increasing voltage for multiplier 14 to produce a concentric circle raster. The scanner 9 output is passed to a Schmitt trigger 19 which has a threshold level b, decreasing with time, triggered by the sync pulses k and produced by a generator 18 e.g. a discharging capacitor. As soon as the received signal level exceeds the instantaneous threshold level the Schmitt trigger output which is connected to video amp 20, intensity modulates the CRT beam. The received signal amplitude will now always exceed the threshold b at the angle of incidence until b is reset. Thus a display as shown in region 6.2 of Fig. 10, is produced where the screen is illuminated outside the curved representing signal strength plotted against angle of incidence. In order to display the direction of the signal maxima only, the scanner 9 output is diverted through a maximum filter 36 which reduces the signal to a narrow pulse and the display only shows radial spokes, Fig.3 (not shown). If switch 38 is operated a second threshold b<SP>1</SP> equal to b plus a constant U is applied to a second Schmitt trigger 19a, also receiving the scanner 9 output. This Schmitt trigger output is inverted and applied to AND gate 30a along with the Schmitt trigger 19 output. Thus the trace is intensity modulated when the signal strength is between b and b<SP>1</SP> producing a continuous line display, Fig. 2 (not shown). An alarm 23 is provided which sounds if the received signal exceeds a given level n. The threshold is fed to a comparator 26 which produces a pulse, when b falls below n, which triggers a monostable 21 whose output is connected to a AND gate 22 which produces a pulse output at the arrival of a sync pulse z derived from the scanning signal fa. The gate 22 output triggers a monostable 29 of duration Ta = 1/fa which provides an input to the video amplifier 20 which thus produces a ring indicating the threshold level n on the CRT. This intensifying pulse also enables AND gate 27 whose second input is the scanner output from gate 30a, thus if a signal is received while the ring is being displayed and passes Schmitt trigger 19 the alarm 23 sounds. The reference ring and alarm enabling signal may be inhibited for a fraction of Ta by a pulse from monostable 34 which is triggered by a sync pulse z delayed by delay line 32. In a second embodiment, Fig.8, two displays are shown on the same screen. Initially pulse k acts bi-stable 42 to apply the decaying threshold from generator 18a to the Schmitt trigger 19 through switch contact 37 which also connects the Schmitt trigger to the scanner 9 through the maximum filter 36. When the raster sawtooth m reaches a pre-set value, trigger 39 operates switch 37 and monostable 41 provides a blanking pulse to video amplifier 20 and also resets bi-stable 42 to provide the decaying threshold from generator 18b. The trigger 19 now receives its signal input from receiver 9 and the display is thus comprised of an inner portion showing the signal maxima and an outer portion showing a normal display, Fig.7 (not shown). Each portion has separate variable alarm thresholds as described above. In a further embodiment the central portion 6.1 is used to display a frequency spectrum of signals received from any particular direction selected by sector switches e.g. 6.4 situated around the perimeter of the CRT screen. In this case the system is initially switched to a scanner which scans a bank of band pass filters connected through the sector switches to the elements of the hydrophone array. In a different embodiment the scanner is only sensitive to pulse type signals to detect sonar apparatus used by the other vessels. It is stated that the system may be used with electromagnetic waves.