Abstract:
A compressive receiver including a dispersive delay line ( 10 ) and a frequency translator ( 16, 18 ) is preceded by signal compressors ( 22   a-d ) that record the incoming signals at one speed and play them back at a higher speed. This increases the frequency spread and provides greater frequency resolution at the output of the receiver.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to compressive receivers. It is particularly concerned with increasing the frequency resolution of such receivers. 
     A compressive receiver can be thought of as a spectrum analyzer or Fourier-transform device. The input to the compressive receiver is a time-dependent signal, and the output of the receiver is a waveform whose value at a given time indicates the presence of spectral components in the incoming signal at a frequency corresponding to the given time. That is, an input sine wave of a given frequency will result in an output pulse at a particular point in time as the output of the compressive receiver, while a sine wave of a different frequency will cause a pulse in the output at a different point in time. 
     The typical compressive receiver consists of a dispersive delay line fed by a linearly swept frequency translator, such as a mixer and a linearly swept local oscillator. The dispersive delay line is chosen to have a substantially linear relationship of delay to frequency; that is, the difference between the delays experienced by two simultaneous frequency components having a given frequency difference is proportional to the frequency difference. 
     The local oscillator is swept in frequency in a sawtooth manner at such a rate that mixer outputs caused by all signals of a given frequency occurring at the input port of the mixer during a single sweep arrive at the output port of the delay line at the same time. The time of arrival of a pulse at the delay-line output port is thus an indication of the frequency of the signal that gave rise to it. 
     A two-dimensional compressive receiver employs the same principle, but it uses a two-dimensional delay line having a set of input ports distributed along one edge and a set of output ports distributed along the opposite edge. In a typical application, the input ports are driven by elements of an antenna array. Each antenna element provides essentially a time-shifted version of the signals from the other antenna elements. Accordingly, interference patterns are set up within the delay line. The geometry of the two-dimensional delay line is such that, if there is a linear relationship between input-port position and the time delay of a signal arriving at the input port, the signals within the delay line caused by a particular frequency component will all constructively interfere at a particular point along the opposite edge of the delay line, the position of this point depending on the time difference between the various input signals. For instance, if signals of a given frequency all arrive at the same time, constructive interference of all those signals might occur at the center output port, while the point of constructive interference might be at one of the left output ports if the signals at the left input ports are delayed more than those at the right input ports. If the input ports are driven by elements of a linear antenna array, the position of the output port having the greatest signal is thus an indication of the direction of the source of the signal. 
     Regardless of whether the delay device is of one dimension or more dimensions, the frequency resolution of the device is equal to the reciprocal of the difference, T, between the delays of the lowest and highest frequencies that it can process in a single sweep of the local oscillator. That is, a single-frequency signal at the mixer input port will cause a delay-line output pulse whose duration is long enough to prevent it from being distinguished from a pulse caused by a single-frequency signal of a different frequency unless the difference between their frequencies is greater than 1T. In the past, the only way to increase frequency resolution was to increase T which means to increase the delay-line length. 
     It is an object of the invention to increase the frequency discrimination in a compressive receiver without increasing delay-line length. 
     SUMMARY OF THE INVENTION 
     The foregoing and related objects are achieved in a compressive-receiver system in which an input signal of a given input bandwidth is time-compressed before it is applied to the dispersive delay line. That is, the input signal is recorded for a given time interval and then played back at a faster speed so that the playback lasts for a shorter time interval. This applies the information to the delay line in a wider range of frequencies. Since the spread in frequency of the input to the delay line is increased, the spread in delay time for the input bandwidth is also increased. This increases the frequency resolution without lengthening the delay line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features and advantages of the present invention are described in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a compressive receiver employing the teachings of the present invention; 
     FIG. 2 is a timing diagram depicting signals at various points in the circuit of FIG. 1; and 
     FIG. 3 is a block diagram of an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described in connection with a two-dimensional compressive receiver, although it will become apparent as the description proceeds that the teachings of the present invention can also be applied to one-dimensional receivers. 
     The two-dimensional compressive receiver includes a two-dimensional delay line  10  having a series of input ports  12   a - 12   d  arrayed along one edge, while a number of output ports  14   a - 14   d  are provided along the opposite edge. (The equality in number of the input and output ports in the illustrated embodiment is coincidental.) 
     A frequency translator consisting of a mixer  16  and a linearly swept local oscillator  18  feed input port  12   a , while similar translators, not shown in the drawings, feed each of the input ports  12   b-d.    
     A chirp signal produced by the linearly swept local oscillator  18  is mixed in the mixer  16  with a signal received from a signal path that begins with a signal line  20   a . Line  20   a  carries a signal that typically is the filtered and frequency-translated output of an antenna element. Accordingly, the signal on line  20   a  falls within a given frequency band, say, between f 1  and f 2 . A signal compressor  22   a  receives this band-limited signal. The signal compressor  22   a  is a device for recording the signal present on input line  20   a  and storing it until a time arrives for its stored signal to be played back. The stored signal is then played back at a higher rate. In the illustrated embodiment, the playback rate is four times the rate at which the signals were recorded. The result is that the information signal generated by the signal compressor  22   a  does not fall between the frequencies f 1  and f 2  but rather between 4f 1  and 4f 2 . That is, the frequencies are higher, and, more importantly, the range of frequencies is wider. 
     The signal compressor  22   a  can be realized in many forms. For example, it can include a sampling circuit followed by an analog-to-digital converter that feeds the digitized data into a memory. The information is read out of the memory at a rate faster than that at which it was read in, converted back to analog form, and filtered by a filter  26  to restrict it to frequencies between 4f 1  and 4f 2 , 
     The filtered, time-compressed output of filter  26  is applied by the filter  26  to the mixer  16 , where it is mixed with the chirp signal generated by the local oscillator  18 . The output of the mixer  16  includes components whose frequencies are the differences between the frequencies of the time-compressed signal and those of the chirp signal. Since the input port  12   a  of the two-dimensional delay line  12  includes a transducer that is primarily sensitive to frequencies within this difference range, there is no need to filter out, e.g., sum-frequency components. In some circumstances, though, it may be necessary to insert a filter in the circuit to remove the unwanted components. 
     Accordingly, the mixer  16  and local oscillator  18  act as a frequency translator. Signals applied to the mixer  16  are translated downward in frequency. Since the frequency of the local oscillator sweeps repetitively in a sawtooth manner, the downward frequency translation of a given signal component increases throughout a given sweep of the local oscillator. The speed of propagation of a signal within the two-dimensional delay line  10  increases with decreasing frequency, so the frequency-translated version of a signal appearing at the input to mixer  16  is translated so that the portions of the signal arriving later are caused to propagate more rapidly through the two-dimensional delay line. 
     The rate at which the local oscillator  18  changes frequency is such that delay-line  10  signals caused by later-occurring portions of a given frequency component in the mixer input reach the output edge of the two-dimensional delay line at the same time as those caused by earlier portions of the same frequency components. Thus, the time of occurrence of a delay-line output depends on the frequency of the mixer input component that gave rise to it, not on the time during the sweep when the component occurred. The output signal plotted as a function of time is thus a plot of the frequency content of the mixer input. 
     The description so far has concentrated on the signal applied to input port  12   a . However, a similar signal path leading from another antenna element is used to feed port  12   b , and signals from further antenna elements feed ports  12   c  and  12   d . The sources of the signals at these ports are typically separate elements of the same antenna array, so their signals are delayed versions of each other. The two-dimensional delay line  10  is arranged for constructive interference at points on its output edge determined by the spatial frequency of the ensemble of signals at its input ports. If there is a linear relationship between the phases of the signals caused by a given source and the positions of the input ports  12   a-d  at which they occur, there will be constructive interference at the output edge of the delay line  10  at a point determined by the proportionality constant of that linear relationship. The antenna elements are usually arranged in a line, although such an arrangement is not necessary for the purposes of this invention. If they are in the typical linear arrangement, and if the positions of the associated input ports  12   a - 12   d  correspond to this arrangement, then the linear delay relationship will obtain, and the position at which the signals constructively interfere will be an indication of the direction of the signal source. This is because the relative delays between the signals at the various input ports are determined by the direction from which the signals are received. The point of constructive interference is in turn determined by the relative delays and the frequency of the interfering components—i.e., it is determined by the spatial frequency at the delay-line input ports. 
     The compressive receiver as described so far provides a two-dimensional Fourier transform; there is a transformation from the time and position domains to the time- and spatial-frequency domains. According to the present invention, the time-frequency resolution of using the signal compressor  22   a , which increased the difference between the arrival times of signals caused by inputs of different frequencies on line  20   a.    
     In order to achieve this increase in resolution, it was necessary to restrict the input bandwidth, f 2 -f 1 , to one-fourth the delay-line bandwidth. To cover the same bandwidth as could have been covered without the signal compressor, four separate bands are processed sequentially. In addition to receiving signals from output line  24   a  of time compressor  22   a , the filter  26  receives inputs at staggered intervals from other lines  24   b-d  from further time compressors. All of the time compressors that feed filter  26  receive signals from the same antenna element, but each of the lines  24   a-d  contains information from a different portion of the frequency spectrum of the antenna-element signal. This may be accomplished, for example, by inserting in each of the input lines  20   a - 20   d  a frequency translator that translates a different frequency band to the range from f 1  to f 2 . The frequency translator would be followed by a bandpass filter, each bandpass filter having the same bandpass, from f 1  to f 2 . Thus, the signals received by time compressors  22   a-d  are all in the same frequency range but originate in different portions of the frequency spectrum of the original antenna signal. 
     All of the time compressors  22   a - 22   d  may record signals concurrently, but they generate output signals at different times. This is illustrated in FIG. 2 which includes a plot of the frequency of the local oscillator  18  as a function of time. FIG. 2 also includes timing diagrams for reading in and reading out by the various signal compressors  22   a - 22   d , and it depicts the output of the output port  14   a, b, c , or  d  having the highest signal strength. 
     As the diagrams illustrate, the readout from signal compressor  22   a  is timed to occur during a sweep of the local oscillator. The sweep lasts for a period equal to 2T, where T is the difference in propagation time through the delay line  10  between the frequency-translated version of a signal at frequency 4f 1  and that of a signal at frequency 4f 2 . An arrow  30  in FIG. 2 associates the readout from time compressor  22   a  with the resultant output of the compressive receiver. The delay-line output caused by a readout lasting for a duration of 2T lasts for a duration of T. This is because, due to the sweep of the local oscillator, signals arriving later at the mixer  16  take less time to propagate through the delay line  10  than do the earlier-arriving signals. 
     After signal compressor  22   a  has completed its readout, it begins to read information in again, as FIG. 2 indicates. In most embodiments of the present invention, this type of timing will be the easiest to realize. However, those skilled in the art will recognize that it is clearly possible to build a signal compressor that reads in and out simultaneously. 
     After a recovery time of length R, the local oscillator begins its sweep again, and the readout of time compressor  22   b  begins at the same time. An arrow  32  depicts the causal relationship between the output signal of signal compressor  22   b  and a portion of the output of the delay line. Similar arrows  34  and  36  depict the causal relationship between the outputs of the other compressors and other portions of the delay-line output. It will be noted that the read-in time for each signal compressor is 8T, or four times the read-out time. Thus, the frequency spread, as was observed above, is four times as great in the signal-compressed outputs as it is in their inputs. 
     Since the inputs to the time compressors  22   a-d  originate from different parts of the frequency spectrum of the antenna-element signals, successive outputs of the delay line represent different frequency bands, a given frequency band being repeated on every fourth output segment. Thus, the effective bandwidth of the circuit is not reduced by using the signal compressors. 
     In an alternate arrangement of the present invention, the frequency translation is performed before the time compression. This alternate arrangement is illustrated in FIG. 3, in which four antenna elements  110   a-d  of a linear antenna array feed input circuits  112   a-d  that in turn feed input ports  114   a-d  of a two-dimensional delay line  116  similar to delay line  10  of FIG.  1 . Each input circuit is similar to input circuit  112   a , which includes four branch lines  118   a-d  that carry a common signal from the same antenna element  110   a.    
     Branch line  118   a  carries the antenna-element signal to a band-pass filter  119 , which restricts the signal from the antenna to a particular pass band. Similar filters are present in the other lines  118   b-d , but each of these filters passes a different band. The input circuits  112   b-d  have similar branches whose band-pass filters having the same pass bands as those in input circuit  112   a.    
     A frequency translator  120  is similar to the combination of the mixer  16  and local oscillator  18  of FIG.  1 . If necessary, the frequency translator  120  may also include appropriate filtering to remove sum frequencies or other unwanted components. Like the corresponding elements in FIG. 1, the frequency translator  120  sweeps in frequency at such a rate that all signals in the delay line  10  generated during one translator sweep from a given frequency in the antenna signal arrive at an output port of the delay line  116  at the same time. That is, the sweep rate of the frequency translator  120  is chosen so that the circuit will operate as a compressive receiver. However, the rate at which the translator sweeps is only one-quarter that at which the local oscillator  18  of FIG. 1 sweeps. This is because the output of the frequency translator  120  is compressed in time—and thus expanded in frequency—by a signal compressor  122 , so the frequency sweep is multiplied by four. 
     The frequency translators in the other lines  118   b-d  of input circuit  112   a  sweep at the same rate as frequency translator  120  does. However, the ranges through which they sweep differ in accordance with the band-pass filters in their respective branch lines so that their output signals will all fall within the same frequency range. These signals on the several lines are then fed to signal compressors similar to the signal compressor  122  in branch line  118   a . Like the signal compressors in FIG. 1, the signal compressors  122  read out their signals at staggered times at rates that are four times the rates at which the signals were originally recorded. Processing in the two-dimensional delay line  116  is then performed in the same manner as that in which processing is performed in the two-dimensional delay line  10  of FIG.  1 . 
     It is thus apparent that, by following the teachings of the present invention, it is possible to increase the frequency resolution of a compressive receiver significantly.