Abstract:
An integrated receiver includes an Instantaneous Frequency Measurement (IFM) device, an interferometer and switches. The IFM receives signals from a target and determines the frequency of the signals. The IFM includes the shared N-channel phase receiver. The interferometer also receives the signals from the target and determines the angle-of-arrival (AOA) of the signals. The interferometer includes the shared N-channel phase receiver and shares the shared N-channel phase receiver with the IFM. The switches selectively connect the shared N-channel phase receiver to the IFM when the IFM is determining the frequency of the signals, and selectively connect the shared N-channel phase receiver to the interferometer when the interferometer is determining the AOA of the signals. The shared N-channel phase receiver determines phase information indicative of the frequency of the signals and the AOA of the signals. A method for calculating the frequency and the AOA of the signals from the target includes the steps of receiving the signals, determining the frequency of the signals using the shared N-channel phase receiver, and determining the AOA of the signals using the shared N-channel phase receiver.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to Electronic Surveillance Measurement (ESM) receivers and more particularly relates to the use of phase receivers in determining frequency and angle-of-arrival (AOA) in ESM receivers. 
     2. Description of the Prior Art 
     Electronic Surveillance Measurement (ESM) receivers commonly require that both frequency and angle-of-arrival (AOA) calculations be performed on input signals from targets of interest. Usually, frequency is measured first, with the measurement spanning a wide instantaneous bandwidth to maximize probability of signal intercept. After a signal of interest is detected and its frequency is determined, the bandwidth of measurement is narrowed substantially for the determination of AOA. Centering a narrow measurement band around the signal frequency improves signal separation in a multi-signal environment, and reduces the influence of receiver noise on the accuracy of the AOA measurement. Prior art uses separate systems to measure each of these two parameters. The frequency of the input signals is often measured using an Instantaneous Frequency Measurement (IFM) device as illustrated in FIG.  1 A. The AOA is typically measured using an interferometer as illustrated in FIG.  1 B. Both the IFM and the interferometer use the difference in phase between the received input signals to calculate the frequency and the AOA of the input signals. 
     The IFM illustrated in FIG. 1A includes a receptor element  10 , a delay line  12  and an N-channel phase receiver  14 . The input signals  16  are received from the target by the receptor element  10 , such as an antenna. The received input signals  16  are then applied to the delay line  12 , which provides two or more output signals delayed in time and thus relative phases, to the N-channel phase receiver  14 . The frequency of the received input signals  16  are determined by the difference in phase between inputs to the N-channel phase receiver  14  by means well known in the art. Phase receivers are alternatively referred to as phase discriminators, phase correlators or quadrature mixers. Further detail regarding phase receivers is presented in the product specification catalog entitled  Anaren RF  &amp;  Microwave Components,  February 1997, distributed by Anaren Microwave, Inc., 6635 Kirkville Road, East Syracuse, N.Y. 13057, which is hereby incorporated by reference in its entirety. 
     The interferometer illustrated in FIG. 1B includes the receptor elements  10  (such as antennas), and the N-channel phase receiver  14 . The receptor elements  10  are offset by a predetermined distance d. The interferometer uses the difference in phase between the input signals received by the offset receptor elements  10  to determine the AOA. The sine of the AOA θ of the input signals  16  is proportional to the phase difference between the input signals received by the offset receptor elements  10  in accordance with equation (1) as follows: 
     
       
         Phase difference=2π sinθ  d/λ   (1) 
       
     
     Thus, both the interferometer and the IFM utilize N-channel phase receivers as a means for calculating the AOA and the frequency of the input signal, respectively. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an integrated interferometer and Instantaneous Frequency Measurement (IFM) receiver, which calculates the frequency and the angle-of-arrival (AOA) of input signals using a shared N-channel phase receiver. 
     It is a further object of the present invention to provide an integrated interferometer and IFM receiver, which is less costly to produce and maintain than interferometers and IFM devices manufactured as separate and distinct units. 
     It is still a further object of the present invention to provide an integrated interferometer and IFM receiver, which occupies less space than interferometers and IFM devices manufactured as separate and distinct units. 
     It is still a further object of the present invention to provide an integrated interferometer and IFM receiver, which is less complex to maintain than interferometers and IFM devices manufactured as separate and distinct units. 
     It is still a further object of the present invention to provide an integrated interferometer and IFM receiver, which is more easily calibrated than interferometers and IFM devices manufactured as separate and distinct units. 
     In accordance with one form of the present invention, an integrated receiver including an IFM, an interferometer and switches for selectively connecting the shared N-channel phase receiver to the IFM when the IFM is determining the frequency of the signals, and selectively connecting the shared N-channel phase receiver to the interferometer when the interferometer is determining the AOA of the signals is provided. The IFM receives signals from a target and determines the frequency of the signals. The IFM includes the shared N-channel phase receiver. The interferometer also receives the signals from the target and determines the AOA of the signals. The interferometer includes the shared N-channel phase receiver, and shares the shared N-channel phase receiver with the IFM. The shared N-channel phase receiver determines phase information indicative of the frequency of the signals and the AOA of the signals. The integrated receiver may also include an amplitude measurement circuit responsive to the signals, which determines amplitude-based parameters of the signals. 
     In accordance with another form of the present invention, a method for calculating the frequency and the AOA of the signals from the target is provided, which includes the steps of receiving the signals, determining the frequency of the signals using a shared N-channel phase receiver, and determining the AOA of the signals using the shared N-channel phase receiver. The method may also include the step of determining amplitude-based parameters of the signals. 
     Previously, interferometers and IFM devices were operated as separate and distinct units having unique N-channel phase receivers. By implementing an integrated interferometer and IFM, the same N-channel phase receiver may be shared between the interferometer and the IFM and used for both AOA and frequency measurements, respectively. 
    
    
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a block diagram of an Instantaneous Frequency Measurement Device (IFM) of the prior art. 
     FIG. 1B is a block diagram of an interferometer of the prior art. 
     FIG. 2 is a block diagram of an integrated receiver for determining both the frequency and the angle-of-arrival (AOA) of input signals from a target using a shared N-channel phase receiver, formed in accordance with the present invention. 
     FIG. 3 is a block diagram of a receptor element, multiband converter and local oscillator suitable for use in the integrated receiver of the present invention. 
     FIG. 4A is a delay line of the prior art, which is configured in lengths forming a binary set of half wavelengths suitable for use in the integrated receiver of the present invention. 
     FIG. 4B is a delay line of the prior art, which is configured in lengths forming relatively prime ratios of half wavelengths suitable for use in the integrated receiver of the present invention. 
     FIG. 5 is a block diagram of a dual channel phase receiver of the prior art. 
     FIG. 6 is a block diagram of a second embodiment of a phase translation circuit illustrated in FIG.  2 . 
     FIGS. 7A and 7B are flowcharts of a method of the present invention for determining both the frequency and the AOA of the input signals using a shared N-channel phase receiver. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates a block diagram of an integrated receiver  20  for determining both the frequency and the angle-of-arrival (AOA) of input signals from a target using a shared N-channel phase receiver in accordance with the present invention. The integrated receiver  20  includes a plurality of receptor elements  10  (e.g., antennas), a reference receptor element  11  (e.g., an antenna), a multiband converter  22 , a tunable local oscillator  24 , narrow bandwidth filters  26 , a notch filter  28 , delay lines  30 , an N-channel phase discriminator, phase correlator or phase receiver  32 , a frequency read-only memory (ROM)  34 , an AOA ROM  35  and switches  36 - 44 . Each of the switches  36 - 44  is selected to make contact with corresponding terminals A during AOA measurements, or corresponding terminals B during frequency measurements. 
     During frequency measurements, the input signals  16  are received by the reference receptor element  11  and directed to the multiband converter  22 . FIG. 3 illustrates a block diagram of one embodiment of the multiband converter  22 , which is suitable for use in the present invention. The multiband converter  22  includes a coarse preselect filter  46  responsive to the input signals  16  received by the receptor elements  10  and  11 , an amplifier  48  coupled to the output of the coarse preselect filter  46 , a fine preselect filter  50  coupled to the output of the amplifier  48 , a mixer  52  which receives the output signal from the fine preselect filter  50  and selectable signals of various frequencies from the local oscillator  24  to down convert the received radio frequency (RF) signals to intermediate frequency (IF) signals, a wide bandwidth filter or IF bandpass filter  54  coupled to the IF output of the mixer  52 , and an IF amplifier  56  coupled to the output of the IF filter  54 . The IF bandpass filter  54  preferably has a wide bandwidth (for example, at least 500 MHZ). The local oscillator  24  includes a plurality of fixed or tunable local oscillators, which are switched through frequency bands of interest that the received RF signals are expected to occupy. The multiband converter  22  essentially functions to down convert the received RF signals to an intermediate frequency by means well known in the art. Further detail regarding the multiband converter and other superheterodyne concepts is provided in R. C. Dorf,  Electrical Engineering Handbook,  IEEE Press (1993), which is hereby incorporated by reference in its entirety. 
     Referring again to FIG. 2, the down converted IF signals from the multiband converter  22  are directed to the notch filter  28  via switch  42 . The notch filter  28  is typically tunable and essentially functions to filter out continuous wave (CW) interference in the down converted IF signals. An example of a commercially available, tunable notch filter includes a ferrite device, and is based on yttrium—iron—garnet (YIG). Such a bandpass or band reject device is available from Micro Lambda Corporation, 48041 Fremont Blvd., Fremont, Calif. 94538. The output of the notch filter  28  is fed via switch  44  to a multiple tap delay line  30 , which terminates in distinct inputs I 1 -I 4  of the N-channel phase receiver  32 . 
     The delay line  30  can be configured in lengths forming a binary set of half wavelengths, or in lengths forming relatively prime ratios of half wavelengths, lengths being determined at the highest intermediate frequency. FIG. 4A illustrates the delay line  30  configured in the binary set of half wavelengths including taps for 1, 2, 4 and 8 half wavelengths. Such a configuration is often used in Instantaneous Frequency Measurement (IFM) devices. FIG. 4B illustrates a configuration of the delay line  30  in lengths forming relatively prime ratios of half wavelengths including taps for 3, 7 and 11 half wavelengths. This configuration of lengths is copied from the spacings often used between receptor elements in interferometers, which is not practiced in the prior art of IFM devices. The configuration in FIG. 4B requires fewer delay line lengths for a given accuracy of frequency measurement than that in the configuration illustrated in FIG.  4 A. The lengths of the delay line  30  illustrated in FIGS. 4A and 4B are not drawn to any particular scale, and are merely intended as illustrative examples without limiting the scope of the present invention in any form. 
     FIG. 5 illustrates a block diagram of one embodiment of a phase receiver  32  well known in the prior art, but suitable for use in the present invention. The phase receiver  32  includes a phase correlator  46 , IQ amplifiers  48 , video bandpass filters  50 , video amplifiers  52 , analog-to-digital converters  54  and a memory or programmable-read-only memory (PROM)  56 . Although only two channels of the phase receiver  32  are illustrated in FIG. 5, the concepts discussed below with respect to the dual channel phase receiver  32  illustrated in FIG. 5 may be extended to N-channel phase receivers by means well known in the art. The phase correlator  46  measures the phase difference between inputs I 1  and I 2  and outputs differential quadrature video signals I+, I−, Q+ and Q−. The differential quadrature video signals vary sinusoidally as the phase varies between the signals input at I 1  and I 2 . The differential quadrature video signals are applied to differential IQ amplifiers  48  or alternative, substantially equivalent summing devices. Each output of the IQ amplifiers  48  is then filtered to the appropriate video bandwidth by the video bandpass filters  50 . 
     The voltages of the differential quadrature video signals I+, I−, Q+ and Q− typically require scaling to ensure that the maximum expected voltage corresponds to the maximum voltage required by the analog-to-digital converters  54 . Gain adjustment, offset adjustment, and temperature compensation circuits are commonly required to maintain the differential quadrature video signals within a predetermined range of acceptable voltages, thereby enabling the analog-to-digital converters  54  to reduce errors due to dynamic range variations. The outputs of the video amplifiers  52  are fed into ladder type analog-to-digital converters  54 , which typically digitize these signals into eight bits of information. Such analog-to-digital converters  54  require a stable and precise voltage reference circuit and a clock. The digitized information is then applied to the PROM  56 , which performs the Arc Tan function, and outputs digitized phase information  60  representing the phase difference between the I 1  and I 2  inputs. 
     Referring again to FIG. 2, a phase translation circuit  58  converts the digitized phase information  60  embodied in the output signal from the N-channel phase receiver  32  to frequency information  62  embodied in an output signal from the translation circuit  58 . The digitized phase information  60  is used to address the frequency read-only memory (ROM)  34 . The data at the address in the frequency ROM  34  pointed to by the digitized phase information  60  represents the frequency of the input signals  16  corresponding to the digitized phase difference  60 . This data is outputted as a signal from the phase translation circuit  58 , and corresponds to frequency information  62 . 
     During AOA measurements, the input signals  16  are received by the receptor elements  10  and reference receptor element  11  and directed to separate channels of the multiband converter  22 . Since each of the switches  36 A 4  is selected to make contact with terminals A during AOA measurements, the outputs of the multiband converter  22  are applied to the narrow bandwidth filters  26 . The narrow bandwidth filters  26  are typically bandpass filters having a preferred narrow bandwidth of approximately 10 MHZ. The output signals from the narrow bandwidth filters  26  are provided through switches  36 - 40  and  44  to the N-channel phase receiver  32  at inputs I 1 -I 4 . The N-channel phase receiver determines the phase difference associated with inputs I 2 -I 4  relative to a reference input such as input I 1 . 
     The phase translation circuit  58  converts the digitized phase information  60  from the N-channel phase receiver  32  to AOA information  64 . The digitized phase information  60  is used to address the AOA read-only memory (ROM)  35 . The data at the address in the AOA ROM  35  pointed to by the digitized phase information  60  represents the AOA of the input signals  16  corresponding to the digitized phase information  60 . This data is outputted as a signal from the phase translation circuit  58  and corresponds to AOA information  64 . 
     The input I 1  is optionally tapped just prior to the N-channel phase receiver  32  and applied to an amplitude measurement circuit  72  including a detector/log video amplifier  73  and an analog-to-digital converter  75 . The amplitude measurement circuit  72  derives amplitude-based parameters from the input signals received from the reference receptor element  11  such as pulse width, pulse repetition rate or frequency and amplitude. The amplitude information is then optionally digitized and output as pulse descriptor information  77 . 
     Alternatively, a second embodiment of the phase translation circuit  58  illustrated in FIG. 2 includes a microprocessor  66 , input buffers  68 , program ROM  70 , random-access memory (RAM)  72  and output buffers  74  linked by an address/data/control bus as illustrated in FIG.  6 . The digitized phase information  60  is applied to the input buffers  68  by the N-channel phase receiver  32 . The microprocessor  66  reads the digitized phase information  60  from the input buffers  68  and converts it to either frequency information  62  or AOA information  64  using software residing in the program ROM  70  and variables residing in the RAM  72 . The microprocessor  66  then writes the frequency or AOA information to the output buffers  74 , which output the frequency information  62  and the AOA information  64 . 
     FIGS. 7A and 7B illustrate a flowchart of a method for determining both the frequency and the AOA of the input signals using the shared N-channel phase receiver. The input signals are received in step  78  and down converted from RF frequencies to IF frequencies in step  80 . The down converted input signals are then filtered with a filter having a wide bandwidth (for example, at least 500 MHZ) in step  82 . 
     The frequency of the input signals is then determined in step  84 , which may include filtering the input signals to remove CW interference in step  86 . In order to determine the frequency, the input signals are delayed using delay lines configured in relatively prime ratios of half wavelengths in step  90  or using delay lines configured in a binary set of half wavelengths in step  92 . The phase difference between the input signals or the phase information is then determined by the shared N-channel phase receiver in step  94  and converted to frequency information indicative of the frequency of the input signals in step  96 . 
     Following the determination of the frequency of the input signals in step  84  and prior to the determination of the AOA in step  100 , the shared N-channel phase receiver is switched from the IFM, which is used to determine the frequency, to the interferometer, which is used to determine the AOA in step  98 . During the determination of the AOA of the input signals, the local oscillator may be tuned to a frequency band of interest in step  102  and the signals may be filtered using a narrow bandwidth of approximately 10 MHZ in step  104 . The phase difference between the input signals or phase information is then determined in step  106  and converted to AOA information indicative of the AOA of the input signals in step  108 . Following the determination of the AOA of the input signals in step  100 , the amplitude-based parameters of the input signals may be determined in step  110 . Following step  100  or step  108 , the method returns to step  78  in order to receive additional input signals from the target. 
     The following modifications to the embodiments of the present invention described above are considered well within the scope of the present invention: 
     1. altering the quantity and type of receptor elements and/or reference receptor elements (greater resolution and accuracy is achieved in measuring the phase difference and ultimately the AOA and the frequency of the input signals as the number of receptor elements is increased); 
     2. altering the characteristics of the wide and narrow bandwidth filters and notch filters including such parameters as bandwidth, cutoff frequency, and passband; 
     3. altering the configuration of the delay lines to something other than a binary set or relatively prime ratios such as a fixed or exponential progression of lengths; 
     4. altering the implementation of the multiband converter to any of numerous alternative designs well known in the art; and 
     5. altering the implementation of the N-channel phase receiver to any of numerous alternative designs well known in the art. 
     Thus, the integrated interferometer and IFM receiver of the present invention calculates the frequency and the AOA of the input signals using a shared N-channel phase receiver. By virtue of the fact that only one N-channel phase receiver is required, the integrated receiver is less costly and less complex to produce and maintain; occupies less space; and is more easily calibrated than interferometers and IFM devices manufactured as separate and distinct units. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.