Abstract:
A method and system for signal processing, especially useful as a signal repeater, i.e. for simulating the characteristic echo signature of a preselected target. The system has a digital radio frequency memory (DRFM) and associated circuitry, including digital tapped delay lines, and a modulator in each delay line to impose both amplitude and frequency modulation in each line.  
     Use of digital semiconductor technology increases the bandwidth and sensitivity of such a repeater over prior analog based systems, reduces the noise of the repeated signal, reduces size and cost of such a system, and permits real time alteration of operating parameters, permitting rapid and adaptive shifting among different kinds of targets to be simulated. Preferably the associated circuitry is unitary with, and part of, the DRFM.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Systems which coherently sum a series of tapped delay lines are relevant to a wide range of signal processing applications, transverse filtering being a prominent example. Another example is that of decoy repeaters. An object will modify any signal reflected from it according to the object&#39;s shape, and the object&#39;s velocity relative to the signal. This permits a hostile interrogator to identify the nature of such objects, which, if the objects are military platforms such as warships or aircraft, is not desirable. One solution has been to artificially synthesize fake characteristic echo signatures in response to receipt of an interrogating signal. Thus, for example, a series of decoy buoys deployed at sea could simulate the presence of a naval flotilla, and thereby potentially disrupt enemy plans.  
           [0002]    [0002]FIG. 1 illustrates broadly how this is done for a ship  5  and an aircraft  3 , in the line of sight of an interrogating signal  2 . For illustrative purposes, signal  2  can be a radar pulse, but could as well be any linear signal pulse, of which sonar or acoustic signals are other examples. As signal  2  hits aircraft  3  and ship  5 , it bounces off of their major reflective surfaces, which, for ship  5  are the hull  4 , superstructure  6 , and smokestack  8 , and for the aircraft are the nose  9  and the wings  7 . The echo from craft  3 ,  5  will be the superposition of the echoes from surfaces  4 ,  6 ,  8 , and  9 ,  7 , and because these surfaces are at different places along the line of sight of signal  2 , the superimposed reflections will be out of phase with one another by the differing times of flight of signal  2  to each reflecting surface. This tends to lengthen the echo by an amount equal to the round trip time of flight of signal  2  between the nearest and farthest major reflector, in the case of ship  5  hull  4  and smoke stack  8 , and to make the echo of varying magnitude as dictated by the varying radar cross sections of the reflecting surfaces. Furthermore, movement of aircraft  3  or ship  5  relative to signal  2  will Doppler shift the returned echos. Thus any platform which reflects signal will in effect frequency modulate signal  2 , such that the returned echoes permit an interrogator to infer the nature and motion of the platform. The most common way to detect a Doppler shift is, responsive to a series of interrogation pulses, compare echoes from consecutive pulses. Thus, an imaging interrogator, such as a search radar, SAR, ISAR, etc., would infer Doppler by comparing consecutive echoes, and do so on a range bin by range bin basis. The interrogator can infer strength of reflection, e.g. radar cross-section, in a range bin from only one echo returned from the range bin by checking echo strength, although as a matter of prudence, it would likely check several such echoes to ensure that echo strength doesn&#39;t vary greatly.  
           [0003]    Any credible repeater decoy must simulate the temporal lengthening and amplitude modulation caused by plural, recessed, reflective surfaces, and a simulate a realistic Doppler shift for each surface.  
           [0004]    Conventionally this is done by analog systems which receive an interrogating signal and pass it through a length of cable having serial taps along its length, one tap per range bin (also called range cell, or downrange range cell). Each tap modulates the signal in amplitude and/or frequency to simulate reflection from the reflective surfaces within that range bin. Total path length of signals traversing the respective taps are selected to correspond to the differing times of flight of the interrogating signal to the respective range bins. Finally, the signals from the taps are summed, and the signal thus synthesized is retransmitted. In this manner, the system returns what appears to be an echo from an object located within the selected range bins, and having a signature indicative of the object to be simulated, e.g. a ship or aircraft in motion.  
           [0005]    Unfortunately, analog systems have drawbacks which limit their usefulness as decoys. They are inherently noisy, and can hold an incoming signal only a short time for processing before the signal deteriorates below noise. This limits system bandwidth, and permits effective simulation of only small objects. Further, analog systems are costly and very bulky, the latter being a particular concern for military platforms, where space is extremely limited. Finally, analog systems cannot readily change operating parameters such as relative delays among taps, or the amount of modulation in the various taps. This means that analog repeaters cannot switch among different simulated objects on the fly, but rather must typically be fabricated for one specific target.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, an object of the invention is to increase the bandwidth of tapped delay line processors of the kind above described.  
           [0007]    Another object is to permit such delay line processors to hold received signals as long as necessary for a given application.  
           [0008]    Another object is to increase the size of objects which such delay lines can simulate when used as repeater decoys.  
           [0009]    Another object is to reduce the size and cost of such delay line processors.  
           [0010]    In accordance with these and other objects made apparent hereinafter, the invention concerns a signal synthesizer which has a digital radio frequency memory (DRFM), and an associated digital processing circuit having a plurality of tapped delay lines, a summer disposed to sum the output of the delay lines, and signal modulator in each of the delay lines. A DRFM is a semiconductor device which can rapidly and permanently record digital information, most notably digitized samples of an incoming signal, and read it back equally rapidly when needed, as well as carry other circuitry. Because of this, the processing circuitry associated with the DRFM can be digital also, with its attendant values of speed, and hence greater bandwidth, reliability, small size, and modest cost. Because the DRFM can hold data indefinitely, the duration of the synthesized signal is not limited, as with analog systems, thus permitting (in the example of FIG. 1) simulation of larger objects by adding more taps to accommodate more range bins. Because the associated circuitry can be digital, and most especially because the circuitry can be dedicated to its processing task, rather than requiring extensive programming to perform its tasks, the speed of the synthesizer can be especially great. In a preferred embodiment, the associated circuitry is made part of the DRFM on the same monolithic chip in order to increase synthesizer speed even more. This is in contrast to a computer, or programmable processor, which, in conjunction with a fast and permanent memory like a DRFM, could in principle do the necessary processing. But the time needed to execute the large number of programming instructions necessary to process data makes this far less desirable than the invention, and, for the specific problem of decoy repeaters, largely ineffective.  
           [0011]    Please note that, although the name DRFM suggests radio frequency signals, and thus suggests use in RF, radar, and other microwave applications, use of the term DRFM is not intended to limit the invention to these applications. Rather, the term DRFM has come to be associated with a specific class of device in the semiconductor art, and is here used only to denominate such devices, not suggest limitations on their uses.  
           [0012]    These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic drawing illustrating a problem to which the invention is particularly useful.  
         [0014]    [0014]FIG. 2 is a circuit schematic diagram of an embodiment according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]    With reference to the drawing figures, wherein like numbers indicate like parts throughout the several views, FIG. 2 shows an embodiment according to the invention. Member  13  is disposed to receive a signal, which, consistent with the non-limiting example of a radar system given above, is illustrated as an antenna, and which could be a radar pulse from a (possibly hostile) search radar. Analog to digital converter  11  digitizes the analog signal from member  13 , producing a stream of digital samples which are stored in digital radio frequency memory (DRFM)  10 , preferably in the form of phase samples. By phase sample, it is meant simply a digital representation of phase only. Phase data such as this does permit ultimate reconstruction of the original signal. See, T. T. Vu et al.,  A GaAs Phase Digitizing and Summing System for Microwave Signal Storage , IEEE J OURNAL OF  S OLID  S TATE  C IRCUITS , vol. 24, p. 104 (no. 1, February, 1989). Associated with DRFM  10  is digital circuitry  15  for processing the samples. The digitized samples are read in due course from DRFM  10  via delay line  12 , having plural taps  18 . Placement of delays  14 ,  14 ′ as shown in FIG. 2 is per conventional usage, although they could as well be elsewhere, e.g. immediately before summer  22 . The circuit of FIG. 2 shows two taps,  18  and  18 ′, but this is illustrative and in principle one would have the number of taps which the particular application dictates: in the case of a repeater decoy, the number of major reflective surfaces of the simulated target.  
         [0016]    The digital samples from DRFM  10  are sequentially read into taps  18 ,  18 ′ by any conventional clocking scheme (not shown), the signals in the respective taps being delayed with respect to one another by preselected amounts dictated by delays  14 ,  14 ′. For simplicity, the following discussion references tap leg  18  only, at the expense of tap leg  18 ′; however, the function of each leg is identical. Signals in leg  18  pass through accumulator  26  having an associated look up table  28  of values of a sine and cosine for a 2π cycle, for use in constructing I &amp; Q components, and through a selectable gain  30 , which multiplies the signal in leg  18  by a preselected amount. Together, members  26 ,  28 ,  30  constitute a signal modulator for signals traversing leg  18  in the following manner.  
         [0017]    Accumulator  26  frequency modulates the signal traversing tap leg  18  by phase rotation, which is a technique that is per se known, and is referred to as serrodyning, or serrodyne modulation. The phase φ of any signal subjected to a linear frequency modulation such as Doppler shift is given by φ=(ω+ω d )t, where ω is signal angular frequency, ω d  is the change in frequency due to the modulation, and t is time. Thus at each point in time the difference in phase between the modulated and unmodulated signal is ω d t. For a digitally sampled signal, the phase of the nth sample φ n =n(ω+ω d )Δt, where n is an integer counter and Δt is the period at which the signal is sampled. Here, the phase difference due to Doppler frequency is nω d Δt. Thus one can shift the frequency of a digitally sampled signal by an amount ω d  by rotating each nth phase sample by nω d Δt, which is to say incrementing the phase nωΔt of each nth sample by nω d Δt. Importantly, however, because Doppler is typically inferred by sampling echoes at the pulse repetition rate, and inspecting these samples for Doppler induced phase differences between the echoes, one can simulate a Doppler shift of ω d  simply by repeating the pulses from an interrogator, with each pulse phase shifted with respect to the next by an amount ω d Δt, where Δt is here the pulse repetition interval.  
         [0018]    In operation, member  32  sets nominal values of ω d  and ω d′  per instructions from DRFM  10 . An interrogator sends a burst of N pulses having a pulse repetition period of Δt (the same Δt as above, for the reasons discussed below). Member  13  acquires each of the N pulses, converter  11  converts them to phase samples, and stores them in DRFM  10 , which ultimately pipes them to data line  12 . Tap leg  18  receives the phase samples from the first pulse, and accumulator  26  rotates the phase of each sample by an amount ω d Δt. The resultant phase samples are scaled by gain  30 , and in the absence of output from leg  18 ′, returns them to DRFM  10 , and thereafter to converter  11 ′ which reconstructs from the samples an analog pulse, which member  13  retransmits. The waveform of the retransmitted pulse is identical to that of the received pulse, save that it is phase rotated by ω d Δt. After processing this pulse, DRFM  10  changes the phase of member  32  to 2ω d Δt, rotates each phase sample of the second pulse by 2ω d Δt, and, again assuming no output from circuit  18 ′, retransmits the reconstructed pulse. This continues through the N pulses of the burst, with the phase samples of each pulse rotated by an amount nω d Δt, where n is pulse number, i.e. n=1, 2, . . . , N, and Δt is the pulse repetition interval. In the absence of output from leg  18 ′, the result is a stream of analog pulses from member  13 , each of which is unchanged in shape and duration, but are different in phase from one pulse to the next by ω d Δt. An interrogator detecting these echoes would interpret the constant pulse to pulse phase shift of ω d Δt as a Doppler shift from a single reflector. Tap leg  18 ′ does the same thing, by use of a different ω d , and by the phase sample being delayed with respect to those passing through leg  18 ′ by the difference in delays  14 ,  14 ′. Summer  22  then combines output of legs  18 ,  18 ′. The signal which summer  22  returns to DRFM  10  is the superposition of the signals exiting legs  18 ,  18 ′. This means that for each nth pulse of the N pulses, summer  22 &#39;s output will be the superposition of two copies of the nth pulse, delayed with respect to one another by delay  14 ′, scaled differently by gains  30 ,  30 ′, and one phase rotated by nω d Δt, the other by nω′ d (Δt). An interrogator which receives the corresponding N analog pulses will interpret this as having come from two reflectors located in range bins separated by delay  14 ′ with reflective cross sections respectively proportional to gains  30 ,  30 ′. Because the pulse to pulse phase difference between these pulses is ω d Δt for the range bin corresponding to delay  14 , and ω d ′(Δt) for the bin corresponding to  14 ′, the interrogator will conclude that the reflectors in these two range bins produced Doppler shifts of ω d  and ω d ′, respectively.  
         [0019]    Leg  18  can, and preferably does, convert, via member  28 , the phase rotated signal into separate inphase and quadrature (I &amp; Q) components so as to be compatible with conventional microwave systems. This conversion of course can occur at any point between accumulator  26  and summer  22 . This is conventionally done by constructing the I &amp; Q components of the signal by using values of Cos (φ n ) and Sin (φ n ). Look up table  28  stores values of the cosine and sine functions for a complete cycle of 2π radians, and thus the magnitude of Cos(φ n ) and Sin(φ n ) can be had for any given phase φ n , and these are used to generate the I and Q outputs of leg  18 . Although leg  18  could readily calculate Cos (φ n ) and Sin (φ n ), doing so is less computationally efficient than use of look up table  28 , and thus would reduce overall system speed.  
         [0020]    Gain  30  scales the signal appropriately, e.g. for the examples of FIG. 1, scales the signal in proportion to the radar cross section of the reflective surfaces of aircraft  3  or ship  5  within the range bin to which leg  18  corresponds. Preferably, gains  30 ,  30 ′ scale the signals in legs  18 ,  18 ′ after the signals are converted to I and Q form. This greatly reduces the number of entries needed in lookup tables  28 ,  28 ′.  
         [0021]    Thus for the examples of FIG. 1, the summed signal, returned by data line  24  to DRFM  10 , has the shape and duration of a signal reflected from an object having major reflecting surfaces of radar cross sections proportional in size to gains  30 ,  30 ′, and having relative speeds corresponding to Doppler shifts ω d  and ω d ′. Of course, because the preferred output of each tap leg is in the form of inphase and quadrature components, summer  22  must sum the I&#39;s and Q&#39;s separately, as is well known to workers in this art. DRFM  10  would preferably forward the synthesized signal to digital to analog converter  11 ′ to convert the synthesized signal to analog form, and to 13 for transmission of the signal. For convenience, and to further increase system speed, converter  11 ,  11 ′ may be integral with DRFM  10 .  
         [0022]    Decoder  38  and latch  34  of circuit  15  update values of frequency shift and gain for the tap legs. Latch  34  receives from DRFM  10  via  36  new gain or ω d  values for tap legs  18 ,  18 ′. Decoder  38 , responsive to DRFM  10  via  36 , receives information about where and when to send the data in latch  34  (e.g. send a new value of ω d ′ to member  32 ′ at a given clock pulse) Controller  17  preferably directs the operation of members  34 ,  36 ,  38 ,  40  via DRFM  10 , in order to forward system parameters from DRFM  10  through data lines  36 ,  40  to the tap legs, as above described. Controller  17  is preferably a process computer interfaced with DRFM  10 , to permit an operator to change these parameters on the fly, in real time. Alternatively, controller  17  can do this automatically. This is particularly important if ω d  in any tap leg varies with time. In the example of FIG. 1, aircraft  3  flies directly at signal  2  at a constant speed, and would Doppler shift signal  2  by a constant, positive, amount. Ship  5 , on the other hand, could be rocking back and forth in the water along the line of sight of signal  2 , and thus the Doppler shift corresponding to this motion would oscillate in time.  
         [0023]    The invention has been described in what is considered to be the most practical and preferred embodiments. It is recognized, however, that obvious modifications to these embodiments may occur to those with skill in this art. Accordingly, the scope of the invention is to be discerned from reference to the appended claims, wherein: