Abstract:
In a jamming cancellation system wherein a balanced modulator receives as two inputs thereto: the desired plus jamming signals, and the AM difference frequency produced by the beating of the desired and jamming signals, jamming suppression, in an environment containing two jamming signals, is achieved by peak detecting the output from the AM difference frequency producing means and applying same directly to the receiver video input or upconverting same to the receiver r.f. frequency and applying same to the system receiver.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a jamming cancellation system and more particularly to a multiple source jamming cancellation system. 
     An important consideration in the design of electronic systems such as radar, particularly those for military applications, is the provision of means for recovering the desired signal from a total signal which include noise jamming signals fo higher intensity than that of the desired signal itself. Most noise jammers consist fo an oscillator which is frequency modulated by a noise waveform at a high rate. The rates are high enough to shock excite the victor radar i.f. amplifier so that its output is indistinguishable from true random noise. 
     Various techniques to cancel jamming signals and thus recover the desired signal have been proposed. One such technique is described in a patent application for a “Jamming Signal Cancellation System”, Ser. No. 509,158, filed Sep. 24, 1974 and assigned to the assignee of the present invention. 
     In that application improved signal detection is achieved by detecting the AM difference frequency produced by the beating of the desired signal with the jamming signal and applying same to one input of a balanced modulator while simultaneously applying the received signal (containing both the desired signal and the jamming signal) to the other input of the balanced modulator whereby the output from the balanced modulator contains the desired signal but not the jamming signal. Actually, the video signal applied to the balanced modulator causes double-sideband suppressed-carrier modulation of the jamming signal. As a result the balanced modulator output contains two frequencies: one is the desired signal; the other is an image on the other side of the jamming frequency and frequency modulated with twice the deviation of the jammer. The image is outside the passband of the receiver and thus, only the desired signal is processed. 
     This single stage device only removed the strongest jammer signal. If a weaker jammer signal is also present it is preserved in its original ratio to the desired signal. Therefore, the apparent improvement factor can never exceed the ratio of the tow jammer powers and generally is a few db less. 
     A solution to this problem of multiple jammers is disclosed in U.S. patent application User. No. 589,490, filed Jun. 16, 1975 for “Multiple Source Jamming Signal Cancellation Systems” and assigned to the assignee of the present application. In that application suppression of jamming from multiple sources is achieved by detecting the AM difference frequency produced by the beating of a desired signal and the smaller of the multiple jamming signals with the stronger jamming signals and applying same to one input of a single-sideband modulator while simultaneously applying the received signal (containing both the desired signal and the jamming signals) to the other input to the single-sideband modulator whereby the larger jamming signal is suppressed. The sum and difference outputs of the single-sideband modulator containing the upper and lower sidebands, respectively, of the desired signal plus the smaller jamming signal are each processed by a jamming cancellation circuit to suppress the smaller jamming signal and their outputs combined to receive the desired signal. While this system performs adequately it does require that the multiple jamming signals be of different power levels and degradation of the system occurs as the relative power levels of the multiple jamming signals approach unity. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide improved means for reducing the effects of noise jamming. 
     It is another object of this invention to provide improved signal detection performance in a jamming environment. 
     It is a further object of this invention it provide means for reducing the effects of multiple jammers on signal detection. 
     It is yet another object of this invention to provide means for reducing the effect of multiple jammers on signal detection even in the presence of multiple jammers having substantially equal power. 
     Briefly, in one embodiment the signal received at an antenna, which consists of a desired signal and one or two jamming signals is split into two channels. One channel includes an AM Detector, a high-pass filter and a video amplifier. The AM detector detects the difference frequencies produced by the beating of the desired signal and the one or two jamming signals. The other channel may include a delay line. 
     The outputs from the delay line and video amplifier are coupled to the two inputs to a balanced modulator. The output form the video amplifier is also coupled to a channel consisting of a peak detector, amplifier and low-pass filter. The output of the low-pass filter is applied to a mixer having an r.f. oscillator coupled to the other input thereto. 
     The outputs from the mixer and balance modulator are applied to a switching circuit for switching one of the mixer and balanced modulator outputs to a receiver. If one jammer is present the balanced modulator output is applied to the receiver while if two jammers are present the output of the mixer is applied to the receiver. 
     The switching circuit is controlled by a mode control circuit which measures whether the output of the peak detector exceeds a threshold indicating the presence of two jammers and, thus, switches to the receiver the output of the mixer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a multiple source jamming cancellation system; 
     FIGS  2 A- 2 D are a series of waveforms illustrating operation of the multiple source jamming cancellation system; 
     FIG. 3 is a block diagram of an alternate embodiment of a multiple source jamming cancellation system; and 
     FIGS. 4A-4D are a series of schematic diagrams of representative peak detector circuits employed in the systems of FIGS. 1 and 3. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1 of the drawings, there is illustrated thereby above the dashed line  10  a jamming signal cancellation system  11  as set forth is said U.S. patent application Ser. No. 509,158. In that system an input signal  12  containing both a desired signal and a jamming signal of higher intensity is split into two paths by a power divider  13 . One path drives a balanced modulator  14 , the other an AM detector  16 . The desired signal and jamming signal beat together producing AM at the difference frequency. This AM is detected by AM detector  16  and applied to the other input of balanced modulator  14  via a high-pass filter  18  and video amplifier  20 . This video signal applied to balanced modulator  14  causes double-sideband suppressed-carrier modulation of the jamming signal. As a result the balanced modulator output  19  contains two frequencies: one is the desired signal; the other is an image on the other side of the jamming frequency and frequency modulated with twice the deviation of the jammer. 
     High-pass filter  18  blocks d.c. such that the jamming signal cannot push itself through. The r.f. input to the balanced modulator  14  is the strong signal which switches the diodes; the video input is the weak signal which controls the output amplitude (and polarity). As set forth in the aforementioned patent application (and as shown in FIG. 3 by circuit  25 ) AGC may also be used to restrict the dynamic range of signal presented to the balanced modulator in order to permit a better balance to be maintained. Also, an auto-AGC circuit (circuit  27  of FIG. 3) can be provided at the output of the balanced modulator  14  to cancel the gain variations introduced by the AGC in order to restore the antenna modulation of a target. A delay line  21  is also provided to compensate for the delay in the video amplifier. 
     If it is expected that the hammers to be handled will have AM thereon then high-pass filter  18  should cut off at roughly the AM frequencies. This may cause some loss of the desired signal since whenever the beat frequency is below the filter cutoff the signal will not get through. However, for typical practical parameters this loss will be small. 
     In practice an automatic mode control should be provided to monitor the level of jamming (d.c. at detector  16 ) such that below a predetermined threshold level a d.c. bias be applied to the balanced modulator, thus, in effect, by passing the jamming cancellation system. This would be used to prevent loss of the desired signal in the absence of jamming. 
     The jamming cancellation system  11  of FIG. 1 described above removes the strongest jamming signal only. If a weaker jamming signal is present, it is preserved in its original ratio to the desired signal. Therefore, the apparent improvement factor can never exceed the ratio of the two jammer powers and if two equal or nearly equal jammer powers are used there is no improvement provided by the system. When two jammers are present their beat frequency will appear in the video of system  11  of FIG. 1 described above as shown by waveform (A) of FIG.  2 . For tutorial purposes it is assumed that neither jammer has AM so that the amplitude of the beat is constant. When a signal pulse is added it will appear as shown in waveform (B) of FIG.  2  . 
     Under these conditions an additional mode  23  is employed to recover the desired signal. This is shown below the dashed line  10  of FIG.  1 . The output of video amplifier  20  of the basic jamming cancellation system is applied to a peak detector  22 . The output of peak detector  22  will appear as shown in waveform (C) of FIG.  2 . Note that most of the energy at the beat frequency has disappeared but the rise in peak voltage due to the signal is preserved. This signal is amplified by an amplifier  24  and further filtered by a low-pass filter  26  matched to the radar pulse to yield an output as shown by waveform (D) of FIG.  2 . The output from low-pass filter  26  is applied directly to the radar display via a switch  31 . The output from balanced modulator  14  is disabled for this mode by a switch  33 . 
     Switch  33  is opened and switch  31  closed in the event of detection of two jammers. This is accomplished by a mode control circuit  35 . If only one jammer is present the output of video amplifier  20  is a d.c. voltage and, thus, the output of peak detector  22  will be zero. If two jammers are preset, the beat between the two is detected by peak detector  22 . This d.c. voltage is applied to mode control circuit  35  which detects a threshold being exceeded and operates switched  31  and  33 . 
     In this mode the system is a noncoherent system such that the basic radar sensitivity will be somewhat reduced. However, this will only be noticeable at low jamming levels since at moderate and higher jamming levels the sensitivity will be limited by the jamming, not be receiver noise. Additionally, in this noncoherent mode MTI will not function. Therefore, there will be no MTI in any sector which is being doubly jammed. Unlike the system is not limited in performance by the power ration of the two jammers and will even work for two equal power jammers. This system is also considerably simpler and less expensive than the aforementioned system. 
     If one or both jammers have AM it will appear on the envelope of the beat and peak detector  22  will follow it. The effective percentage AM will be reduced considerably, however, by the low-pass filter  26  because the filter will typically have a bandwidth on the order of 50 KHz whereas the AM noise typically is concentrated in the band form 1-5 MHz. Therefore, the energy in the low-pass filter band will be primarily that due to beats between the two AM envelopes. 
     An alternate embodiment of the invention is shown in FIG. 3 of the drawings. Instead of applying the video output directly to the display as described with respect to the embodiment of FIG. 1, the video is translated back to the radar frequency and put through the radar receiver via an output  29 . This has some advantages: it requires fewer connections to the radar; it permits putting the signal through the anti-AGC circuit  27  and thereby restoring the proper dynamic range and azimuth resolution; and it permits use of the existing radar signal processing and ECCM features. 
     As shown in FIG. 3, the translation is accomplished by feeding the video from low-ass filter  26  to a balanced modulator (mixer)  28  where it modulates an r.f. carrier obtained from an oscillator  30 . Oscillator  30  is tuned to the radar center frequency. 
     A mode control  35  operates a diode switch  32  to determine which signal is applied to the receiver via line  29 . 
     A typical peak detector  22  is shown in circuit (A) if FIG. 4 of the drawings comprising a diode  32 , shunt capacitor  34  and shunt resistor  36 . However, in order to obtain a sufficiently high ratio of discharge to charge time constant several stages are required as shown in circuit (B) of FIG.  4 . In this circuit the output of the simplified peak detector of circuit (A) is amplified by an amplifier  38  and includes one or more additional stages here shown as a single stage comprising a diode  40  and shunt capacitor and resistor  42  and  44 , respectively. Enhancement of the difference between the signal pulse and noise fluctuation is achieved by providing long time constant a.c. coupling between the stages. This is illustrated by the circuit (C) of FIG. 4 by capacitor  46  and resistor  48 . Further improvement in any of the peak detector circuits described is achieved by the use of compound detector loads as shown in circuit (D) of FIG.  4 . In this circuit a high impedance high time constant load is placed in series with the normal load to absorb slow variations and levels, leaving only the fast variations (signal pulses) to be passed on to the next stage. In circuit (D) of FIG. 4 resistor  50  should be made of higher value than resistor  52  and the time constant of the combination of resistor  50  and capacitor  54  should be very much larger than that of the combination of resistor  52  and capacitor  56 . These peak detectors of FIG. 4 are only illustrative of typical peak detector and are not intended to be exhaustive thereof. 
     While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that the specification is presented by way of example only and not as a limitation of the scope of my invention as set forth in the accompanying claims.