Patent GB-1361480-A (GB)
Title: Method and circuits for attenuating a wideband background noise level and interference signals superposed on it
IPC: G01S7/35
Filed: 1971-12-02
Abstract: 1361480 Automatic gain control; sonar systems; pulse radar; interference suppression FRIED KRUPP GmbH 2 Dec 1971 [3 Dec 1970] 56034/71 Headings H4R H4D and H4L In an arrangement for reducing the effect of random noise and spurious signals of limited bandwidth and relatively long duration on wanted signals of shorter duration the signals are passed through a gain controlled amplifier having a control circuit time constant which is large in comparison with the duration of wanted signals but small relative the duration of spurious signals so as to derive a signal having the wanted signals superimposed on a constant background noise level. As described the signals are from a doppler echo ranging system having wanted short duration signals, whose frequency depends on relative velocity of the object and observer, superimposed on spurious signals, generated by reverberation or reflections from waves, together with wide band background noise. In a first embodiment, Fig. 2, the received signals 1 are applied to two parallel channels 7.1 and 7.2. In channel 7.1 the signals pass through a band pass filter 7.1.1. which passes only the spurious signals and wanted signals from objects stationary relative the observer. The filter output passes via a gain controlled amplifier 7.1.2 to one input of an adder 7.3. The gain of amplifier 7.1.2 is controlled by the output of a second narrower band, band pass filter centred on the same centre frequency as filter 7.1.1 so that the spurious signals are brought to a constant level, with the narrow band wanted signals superimposed at a higher level, since the a.g.c. is too slow acting to respond to them. In channel 7.2 the signals pass through a band elimination filter 7.2.1, tuned to the same band as the band pass filter 7.1.1 in the channel 7.1, and via a gain controlled amplifier 7.2.2 to the other input of the adder 7.3 with the control of the gain effected via a narrow band pass filter 7.2.3 tuned to a frequency outside the stop band of filter 7.2.1 so as to vary the gain to keep the wide band noise in the signal at a predetermined low level. The signal derived from the adder comprises therefore wanted signals from stationary objects, from channel 7.1, and wanted signals from moving objects, from channel 7.2, superimposed on a constant overall background noise and spurious signal level. In an alternative arrangement, Fig. 3, the incoming  signal passes through a band stop filter 8.1 whose damping, or attenuation, can be varied by a control voltage 8.2. From the filter the signal passes through a gain controlled amplifier (10) whose gain is controlled by a long time constant control circuit to maintain the output signal  level nearly constant, with a further band pass filter and a comparison circuit 9 which selects the signal stopped by the filter 8.1 and produces a control signal, via a longtime constant circuit, which adjusts the damping of filter 8.1 so as to maintain the signal level at that frequency, mainly spurious signals, substantially constant. The result again is to produce a uniform background level of noise and spurious signals with the short duration wanted signals superimposed thereon. In an echo ranging system an interference pulse at the start of each ranging period is suppressed by a pulse from monostable 15.