Strobe Generator

This invention relates to radar circuitry and more particularly to novel circuitry for indicating the bearing or azimuth, relative to the radar set, of an enemy jamming source. Means are provided for generating a strobe pulse in response to received jamming signals, said pulse being applied to the radar PPI (plan position indicator) to produce a radial strobe line or fiducial mark thereon at a position which accurately corresponds to the direction of arrival of the jamming signals.

It is therefore an object of this invention to provide a strobe generator circuit of improved accuracy.

Other objects and advantages of this invention will become apparent from the following detailed description and drawings, in which:

FIG. 1 is a block diagram of the circuit, and

FIG. 2 illustrates the waveform in various parts of FIG. 1.

The radar has two antennas, one a rotating directional type and the other a fixed omnidirectional type, the latter arranged to cover the same elevation as the former. The gain of the omnidirectional antenna is made slightly higher than the side-lobe gain of the directional antenna, so that by substracting the signal in the omnidirectional channel from that in the directional channel spurious signals caused by pickup from the side-lobes of the directional antenna are canceled or blanked out. The subtraction is accomplished in differential amplifier 3 to which the detected video signals from both channels are fed. The waveform a, FIG. 2 shows the output of 3 as the rotating antenna sweeps past a source of CW jamming signals. The signal is applied through cathode follower 5 to diode clutter gate 7 which is open only during the last 25% of each sweep interval. The gating signal for 7 is obtained from the output of gate multivibrator 29 which is a monostable multivibrator which is triggered by the radar system pre-trigger pulses FIG. 2b, which are the same pulses used to trigger the radar set transmitter. The output of 29 is shown in FIG. 2c, the positive portions of which open gate 7 and apply the input thereof to storage device 9. The input to the storage device is a series of gradually increasing and then decreasing negative pulses, as illustrated in FIG. 2d. These pulses cause storage device 9, which may comprise a capacitor, to charge up in step fashion while the pulses are increasing in amplitude and then to discharge in like fashion as the pulses decrease in amplitude, as illustrated in FIG. 2e. It should be noted that gate 7 and storage device 9 together comprise a box-car detector. The charge on 9 is differentiated by 11 to determine whether the charge thereon has increased or decreased since the preceding pulse. The output of 11 will be a series of negative spikes if the charge on 9 is increasing and a series of positive spikes if the charge thereon is decreasing, as shown in FIG. 2f. The spikes are amplified by trigger amplifier 13 and the negative going spikes, indicative of increasing charge on 9, trigger first monostable multivibrator 15. Multivibrator 15 may be made responsive to negative spikes only by means of a unidirectional conducting device such as a diode, or by other well-known means. Multivibrator 15 has a pulse length of less than the radar imterpulse period or sweep interval and therefore will be triggered by each negative spike of sufficient amplitude applied thereto. The pulse output of the first multivibrator, FIG. 2g, is applied to a second monostable multivibrator 19, which has a pulse length or time constant longer than the radar interpulse period. The initial pulse from 15 sets 19 and succeeding pulses from 15 hold the second multivibrator in the set or unstable state. When the directional antenna is pointing at the jamming source the charge on storage device 9 stops increasing and no more negative spikes of sufficient ampltidue to trigger the first multivibrator are produced by differentiator 11, the first multivibrator stops producing pulses and the second multivibrator resets, FIG. 2h. The resetting of 19 is differentiated by 21, FIG. 2i, and is used to initiate the strobe pulse. It can be seen that the strobe pulse is initiated by the lack of increase in the output of the rotating antenna rather than a decrease therein. This yields increased azimuthal accuracy in the strobe pulse. It is difficult to detect a decrease in the antenna output near the center of the beam because the bluntness of the antenna pattern causes only slight differences in the received signal in this area. The spike output of 21 is applied to pulse stretcher 23, the output of which is shown in FIG. 2j. This pulse is inverted in 25 to form the positive going pulse of FIG. 2k. The output of 25 and the output of gate multivibrator 29, shown in FIG. 2m, which is a repeat of FIG. 2c, are applied to AND gate 27 to produce the strobe pulse of FIG. 2n, which accurately coincides with the azimuth of the jamming source and occupies the last 25% of the sweep interval in which it was initiated. It should be noted that the waveforms have been idealized to more clearly illustrate the principle of operation of the circuit. Actually the beamwidth of the directional antenna will normally contain about 35 sweep intervals.

While a preferred embodiment of the invention has been illustrated and described, it is obvious that many modifications thereof will be apparent to those skilled in the art, accordingly the scope of the invention should be limited only by the appended claims.