Method and apparatus for processing signals in an array antenna system

An antenna system (10), includes a plurality of antenna elements (21–25). Respective analog signals corresponding to respective antenna elements are subjected to respective phase shifts, are digitized using one-bit samples, are then subjected to an equal and opposite phase shift, and are subsequently combined so that the fundamental components add coherently and the harmonic components add noncoherently. A digital-to-analog converter is supplied with successive samples, and produces in response to each sample a pulse having a duration which is less than the time period between successive samples.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to array antenna systems and, more particularly, to a method and apparatus for processing signals in an array antenna system.

BACKGROUND OF THE INVENTION

A standard phased array antenna system includes an antenna section with a plurality of antenna elements that are arranged in a two-dimensional array of rows and columns. A central waveform generator and a transmitter produce an analog signal which is to be transmitted, and this analog signal is then supplied to each of the antenna elements through respective devices that impart to the signal a respective phase shift and/or time delay. In a large array which handles wideband signals, a simple phase shift is typically not sufficient, and the capability to effect time delays must be provided.

This involves for each antenna element a relatively large and heavy analog steering section, which is expensive and consumes a significant amount of power. In fact, the steering section commonly includes long-time delay units, short-time delay units, phase shifters, and switching arrangements for routing signals among the various time delay units and phase shifters. The steering sections for different antenna elements of the same system usually need to be matched and/or calibrated, which is cumbersome and adds to the expense. These existing steering sections are also subject to dispersion that results in transmission losses and smaller available signal bandwidths. Although the discussion here is presented in the context of waveform transmission, similar considerations apply with respect to waveform reception.

Attempts have been made to develop suitable alternative approaches. One pre-existing approach involved the provision of several waveform generators and transmitters which each served a respective portion of the array, such that problems due to dispersion and transmission loss could be reduced. However, problems involving size, weight, power, cost, matching and calibration of the steering sections were still present.

Another known approach is disclosed in U.S. Ser. No. 09/478,035 filed Jan. 5, 2000, which is assigned to the same Assignee as the present application. This application disclosed an approach for processing received signals using primarily digital circuitry rather than the traditional analog circuitry. While this approach was suitable for its intended purposes, it was not satisfactory in all respects. As one aspect of this, it focused on reception and was thus advantageous for a passive system which involved only reception of signals. But in the case of an active system which needed to transmit signals, it was still necessary to provide the traditional analog steering sections with time-delay units and phase shifters, with the associated disadvantages.

SUMMARY OF THE INVENTION

From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for processing signals primarily digitally within an array antenna system. According to a first form of the present invention, a method and apparatus are provided to address this need, and involve: producing first and second digital signals, the first digital signal representing a predetermined waveform with a first phase shift imparted thereto in relation to a reference, the second digital signal representing substantially the predetermined waveform with a second phase shift different from the first phase shift imparted thereto in relation to the reference; converting the first and second digital signals respectively into first and second analog signals; imparting to the first analog signal a phase shift which is substantially equal and opposite to the first phase shift in order to obtain a first adjusted signal, and imparting to the second analog signal a phase shift which is substantially equal and opposite to the second phase shift in order to obtain a second adjusted signal; and combining the first and second adjusted signals.

According to a different form of the present invention, a method and apparatus involve: generating a digital signal having a plurality of successive states; and converting the digital signal into an analog signal, including generating for each state of the digital signal a respective corresponding analog pulse which has a duration less than the duration of the corresponding state, and outputting a predetermined voltage between successive pulses.

According to yet another form of the present invention, a method and apparatus involve: producing first and second analog signals, the first analog signal representing a predetermined waveform and the second analog signal representing substantially the predetermined waveform; imparting to the first analog signal a first phase shift to obtain a first shifted signal, and imparting to the second analog signal a second phase shift different from the first phase shift to obtain a second shifted signal; converting the first and second shifted signals respectively into first and second digital signals; imparting to the first digital signal a phase shift which is substantially equal and opposite to the first phase shift in order to obtain a first adjusted signal, and imparting to the second digital signal a phase shift which is substantially equal and opposite to the second phase shift in order to obtain a second adjusted signal; and combining the first and second adjusted signals.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a block diagram of an apparatus which is a phased array antenna system10that embodies the present invention. The antenna system10includes an antenna12, and a central control circuit14.

The antenna12includes a plurality of physically separate antenna elements, five of which are shown diagrammatically at21–25. In the disclosed embodiment, the antenna elements are arranged in a two-dimensional array of rows and columns, and the illustrated antenna elements21–25represent a subset of the antenna elements from one row of the array.

The antenna12also includes a plurality of circuits, five of which are shown at31–35. Each circuit is associated with a respective antenna element. Thus, the number of circuits is equal to the number of antenna elements. Each of these circuits serves an interface between a respective antenna element and the central control circuit14. The circuits31–35are each physically located at the antenna, in relatively close physical proximity to a respective one of the antenna elements21–25.

The antenna system10can both transmit and receive electromagnetic signals. The broken line41represents the wavefront of an electromagnetic signal which is approaching the antenna system10in a direction42, where the direction42forms an angle with respect to the plane containing all of the antenna elements in the array. Consequently, the electromagnetic signal41will not reach all of the antenna elements simultaneously. For example, it will reach the antenna element25first, then the antenna element24, eventually the antenna element23, then the antenna element22, and then the antenna element21.

For purposes of discussing the disclosed embodiment, it is assumed that antenna12has a physically large array of the antenna elements21–25, and that the electromagentic signal41is a wideband signal. While the present invention can be utilized in systems that involve smaller arrays and/or narrowband signals, it is particularly advantageous in the context of a large array which transmits and receives wideband signals. In this regard, the antenna elements21–25should ideally sample the electromagnetic signal41at substantially the same point in time. but in the case of a large array and a wideband electromagnetic signal, the information in the signal may change between the point in time when the signal reaches the antenna element25and the subsequent point in time when the signal reaches the antenna element21. In this context, simply applying a respective phase shift to the signal from each antenna element21–25does not provide sufficient accuracy.

Therefore, a common pre-existing approach was to apply various time delays to the analog signals from the different antenna elements prior to sampling them, such that the electromagnetic signal41would effectively be sampled at the same point in time for each antenna element, instead of being sampled at different points in time in conjunction with the subsequent application of different phase shifts to the respective samples in order to obtain temporal alignment. However, this approach involved the use of programmable analog time delay units and phase shifters which are large and heavy, and which needed to be matched and calibrated. Further, analog circuitry was provided to receive the radio frequency (RF) signal and down convert it to an intermediate frequency (IF) signal before digitization, which required additional circuitry that added to the size and complexity of the analog circuit provided for each antenna element.

In contrast, in the disclosed embodiment, electromagnetic signals received at the antenna elements21–25are relatively promptly digitized while at RF frequencies, and subsequent processing, such as the implementation of time delays, is carried out digitally. Similarly, when the antenna system10is transmitting a signal, time delays and other processing of the signal components are carried out digitally, and then the component digital signals are each converted to an RF analog signal almost immediately before being transmitted. The following discussion explains in more detail how this is carried out. In this regard, the circuits31–35of the antenna12are identical in the disclosed embodiment, and therefore only the circuit31is described below in detail.

More specifically,FIG. 2is a block diagram showing the central control circuit14, the antenna element21, and the circuit31which interfaces the antenna element21to the central control circuit14. The circuit31has an analog section identified by broken line61, a digital high speed section identified by broken line62, a digital reduced speed section identified by broken line63, and a digital reference signal generator section identified by broken line64. The digital high speed section62operates at a clock speed of 10 GHz, and in the disclosed embodiment is implemented with high speed indium phosphide (InP) semiconductor technology of a known type. The digital reduced speed section63and the digital reference signal generator section64each operate at 1/32 the speed of the high speed section62, in particular at a clock speed 312.5 MHz. In the disclosed embodiment, the sections63and64are implemented with complementary metal oxide semiconductor (CMOS) integrated circuitry.

The analog section61includes a transmit/receive circuit71, a phase shifter72and a bandpass filter (BPF)73which are coupled in series between the antenna element21and the digital high speed section62. The transmit/receive circuit71operates in either a transmit mode or a receive mode, based on the state of a transmit/receive control line76from the central control circuit14. The phase shifter72can induce a phase shift into transmit or receive signals passing through it, the amount of the phase shift being programmable through control lines77from the central control circuit14. In the disclosed embodiment, signals are transmitted and received through the antenna element21at a frequency of 7.5 GHz, but it would alternatively be possible to use some other frequency. The BPF73has a 5 GHz pass band which is centered on a transmit/receive frequency of 7.5 GHz, or in other words has a pass band from about 5 GHz to about 10 GHz. However, it will be recognized that the characteristics of the pass band could be varied.

The digital high speed section62includes an electronic switch81which is controlled by the transmit/receive signal76from the central control circuit14. In the transmit mode, the switch81couples the BPF73to a digital-to-analog converter (DAC)82. In the receive mode, the switch81couples the BPF73to an analog-to-digital converter (ADC)83. The DAC82has an input which is coupled to an output of a 32-bit bi-directional shift register86, and the ADC83has an output which is coupled to an input of the shift register86.

The shift register86is supplied with a free-running 10 GHz clock, and the direction in which data shifts through the register86is controlled by the state of the transmit/receive line76from the central control circuit14. The shift register86is associated with a buffer register87, which is also responsive to the transmit/receive control line76. Words of 32-bit data can be transferred from the buffer register87to the shift register86in the transmit mode, and from the shift register86to the buffer register87in the receive mode.

A 5-bit counter provides a delay control function which is controlled by a delay control circuit92through control lines93, the delay control circuit92being part of the digital reduced speed section63. The counter91is clocked with a 10 GHz clock signal, and provides a divide-by-32 function. In particular, on every 32 d clock pulse, the counter91activates a load control line94. In the receive mode, the load control line94causes a 32-bit word received in the shift register86to be loaded into the buffer register87. In the transmit mode, the load control line84causes the shift register86to be loaded with data from the buffer register87each time the shift register has finished transmitting 32 bits of data. The initial value loaded into the counter92by the delay control circuit92determines when the load signal94is generated in relation to other activity within the circuit31, thus permitting a programmable time delay to be introduced into data being transmitted or received by the circuit31.

The central control circuit14outputs a 10 GHz clock signal96. The digital high speed section62includes a phase shifter97, which effects a phase shift of the clock signal96by an amount controlled at98by the delay control circuit92. The amount of this phase shift can be different in the various circuits31–35. The output of the phase shifter97is a phase-adjusted 10 GHz clock signal101, which is used to operate the various components of the digital high speed section62. A divide-by-32 circuit102converts the 10 GHz clock signal101into a 312.5 MHz clock signal103, which is supplied to the components of the sections63and64. The phase shifter97can adjust the phases of the 10 GHz clock signal101and the 312.5 MHz clock signal103by an amount which is less than one period of the 10 GHz clock signal96. This permits fine tuning of the timing of the operation of the circuit31relative to other circuits in the antenna, such as those shown at32–35inFIG. 1.

The digital reference signal generator section64includes an IREF signal generator111and a QREF signal generator112. The IREF generator111is used for both transmit and receive, and the QREF generator is used only for receive. Before each receive operation, the central control circuit14loads each of the generators111and112with a respective reference signal, which is 1.7 megabits in length. Before each transmit operation, the generator111is loaded with such a reference signal. Each reference signal may be viewed as a serial stream of binary bits which is 1.7 megabits long, and which is a digitized version of a reference analog waveform. The QREF signal and the IREF signal used for the receive mode represent the same analog waveform, but with a phase difference of 90°.

In theory, each of the generators111and112would output the 1.7 megabits of its respective reference signal one bit at a time, at a rate of 10 GHz. However, as mentioned above, the generators111and112operate at a clock speed of 312.5 MHz, which is 1/32 of 10 GHz. Consequently, in order to output bits at an effective rate of 10 GHz, each of the reference generators111and112outputs its serial bit stream in successive 32-bit segments at a rate of 312.5 MHz. The generators111and112can be implemented as random access memories which each contain a plurality of 32-bit storage locations, where the 32-bit words in the memory locations are successively accessed and output at a rate of 312.5 MHz.

The outputs of the IREF generator111are coupled to inputs of the buffer register87, and also to inputs of thirty-two separate exclusive OR gates, which are represented collectively inFIG. 2by a single gate symbol116. The outputs of the QREF generator112are coupled to inputs of thirty-two exclusive OR gates, which are represented collectively inFIG. 2by a single gate symbol117.

During transmit mode, the 1.7 megabit reference signal in the generator111is supplied in 32-bit segments to the buffer register87, and then to the shift register86, where the bits are sent serially to the DAC82at a rate of 10 GHz. The analog output from the DAC82is supplied through the switch81, BPF73, phase shifter72and transmit/receive circuit71to the antenna element21.

During receive mode, an electromagnetic signal received at the antenna element21is converted into an analog signal by the transmit/receive circuit71, and then passes through the phase shifter72, BPF73and switch81to the ADC83, where it is digitized into a 10 GHz bit stream which is supplied to the shift register86. The bits shifted into the shift register86at 10 GHz are supplied in 32-bit segments through the buffer register87to the inputs of all of the sixty-four exclusive OR gates116–117.

In the receive mode, respective bits of one reference signal from the IREF generator111are supplied to the inputs of the respective gates116, and respective bits of the other reference signal from the QREF generator112are supplied to the inputs of the respective gates117. Each of the exclusive OR gates116–117serves effectively as a 1-bit digital multiplier or mixer, such that the gates116–117collectively mix the received signal with the two reference signals from the generators111and112. In the disclosed embodiment, the reference signals from the generators111and112represent a waveform with a lower frequency than the frequency of the received signal, and thus the gates116and117effectively implement a down conversion of the frequency of the received signal.

The outputs of the thirty-two gates116are all summed in an adder121, in order to produce a single 5-bit number which is supplied to inputs of a 5-bit wide shift register123. Similarly, the outputs of the thirty-two gates117are all summed in an adder122, in order to produce a 5-bit number which is supplied to inputs of a different 5-bit wide shift register124. Thus, each 32-bit segment received through the shift register86and the buffer register87produces one 5-bit number in the shift register123, and one 5-bit number in the shift register124. The shift registers123and124each hold a plurality of these numbers.

A selector128can select any one of the 5-bit numbers in the shift register123, and supply it to the central control circuit14as an “I” signal. Similarly, a selector129can select any one of the 5-bit numbers in the shift register124, and supply it to the central control circuit14as a “Q” value. The particular location along each shift register from which the 5-bit numbers are extracted by the selectors is determined by control lines131from the delay control circuit92. It will be noted that the shift registers123–124and the selectors128–129effectively implement a programmable delay in the outputs of the adders121and122.

A distinctive aspect of the operation of the DAC82will now be described. In this regard,FIG. 3is a timing diagram showing at A an analog waveform201which is part of a reference waveform that might be represented digitally by the IREF information in the generator111. InFIG. 3, B represents a digitized version of the waveform201, in the form of a plurality of successive samples. If a straight line segment is drawn between the ends of each adjacent pair of the samples at B inFIG. 3, these line segments will together define a waveform which approximates the waveform201. If the samples shown at B were successively supplied to the input of a standard DAC, the standard DAC would essentially produce a series of pulses as shown at C, where each pulse has a magnitude equal to the magnitude of the corresponding sample, and has a duration equal to the time interval between successive samples.

FIG. 4is a diagrammatic view of several frequency characteristics.FIG. 4shows at A the frequency spectrum determined by Fourier transform for the digital waveform which is shown at B inFIG. 3. The presence of energy at negative frequencies is a reflection of the fact that this frequency spectrum is determined mathematically through Fourier analysis, rather by empirical measurement.FIG. 4shows at B a representation of the pass band of the BPF73(FIG. 2), which corresponds to the desired transmit spectrum. With reference to A inFIG. 4, reference numeral206denotes the portion of the energy which is within the desired transmit spectrum.

If the output from a standard DAC, as shown at C inFIG. 3, is subjected to the same Fourier transform, the result will be the frequency spectrum shown at B inFIG. 4. It will be noted that the magnitude of the energy distribution has a roll-off207which is relatively pronounced within the desired transmit spectrum, such that the portion of the energy208within the desired transmit spectrum has a distorted spectrum with an asymmetric reduction in magnitude. If the spectrum shown at B inFIG. 4was subjected to the band pass filtering characteristic shown at D for the BPF73, the result would be the spectrum208shown at E inFIG. 4.

In order to change the roll-off characteristic and thus avoid unwanted distortion within the desired transmit spectrum, the DAC82of the disclosed embodiment operates differently from a standard DAC. In particular, in response to each of the samples shown at D inFIG. 3, the DAC82would produce the series of pulses shown at D inFIG. 3, where each pulse has the same magnitude as the associated sample, but has a duration which is only half the time interval between successive samples. During each time interval between two successive pulses, the DAC output returns to a predetermined voltage which, in the disclosed embodiment, is zero volts. When the waveform shown at D inFIG. 3is subjected to the same Fourier transform discussed above, the result is the frequency spectrum shown at C inFIG. 4. It will be noted that the roll-off characteristic indicated by the broken line211maintains a suitable and uniform magnitude throughout the desired transmit spectrum, such that the portion212of the energy which is within the desired transmit spectrum has little or no distortion, and conforms relatively closely to the energy characteristic206in the spectrum A ofFIG. 4. If the signal shown at D inFIG. 3was subjected to band pass filtering by the filtering characteristic shown at D inFIG. 4, the result would be the spectrum shown at F inFIG. 4.

In the disclosed embodiment, and as discussed above, the DAC82operates at a frequency of 10 GHz. To facilitate digital to analog conversion at this frequency, the digital samples supplied to the input of the DAC82are each a single binary bit. Each sample can thus only have one of two different states. In this regard, and as discussed above, the IREF information output by the generator111is a serial stream of binary bits having a length of 1.7 megabits, and is a digital representation of an analog waveform, where each digital sample is a single bit. Thus, for example, if the IREF information was a representation of the waveform201shown at A inFIG. 3, the digital samples would be as shown at E inFIG. 3, rather than at B.

The samples at E each have a uniform magnitude, although some are positive and some are negative. Those with a positive magnitude would each be represented by a binary “1”, and those with a negative magnitude would each be represented by a binary “0”. In a sense, this is simply the sign bit of each sample shown at B inFIG. 3, since each sample at B with a positive magnitude has a sign bit of “1”, and each sample at B with a negative magnitude has a sign bit of “0”. When the samples shown at E inFIG. 3are applied in sequence to the input of the DAC82, the resulting output would be as shown at F inFIG. 3. The waveform at F inFIG. 3enjoys the same favorable roll-off characteristic which is shown at211inFIG. 4, rather than the distorted roll-off characteristic shown at207inFIG. 4.

As discussed above, the reference waveform represented in digital form by the IREF information in generator111is configured in the disclosed embodiment as a series of successive samples which are each one binary bit. That is, each sample is either a binary “1” or a binary “0”. Thus, for example, the reference waveform201ofFIG. 3will be represented by samples such as those shown diagrammatically at E inFIG. 3. If a waveform is reconstructed directly from the samples shown E inFIG. 3, it will have generally the shape of the waveform241shown at G inFIG. 3.

The waveform241is, of course, an approximation of the waveform201, and is not identical to the waveform201. In essence, the waveform241represents the waveform201with the addition of various harmonics. In other words, when a waveform such as that shown at201is digitized using 1-bit samples, the resulting digital signal includes unwanted harmonics. The disclosed embodiment includes provisions which substantially reduce these unwanted harmonics. This is explained in more detail with reference toFIG. 5.

More specifically,FIG. 5is a block diagram showing two of the antenna elements21and22fromFIG. 1, and the associated circuits31and32. The circuit31has already been described in detail with reference toFIG. 2, and selected components of this circuit are depicted inFIG. 5, including the transmit/receive circuit71, phase shifter72, BPF73, DAC82, shift register86, and IREF generator111. As mentioned above, the circuit32is identical to the circuit31, andFIG. 5thus shows components251–253and256–258of the circuit32which are respectively equivalent to the components71–73,82,86and111of the circuit31.

For purposes of the following discussion ofFIG. 5, assume that the antenna system is to transmit electromagnetic signals corresponding to a reference waveform, which is the waveform201ofFIG. 3. Ideally, this waveform would be digitized into 1-bit samples as shown at E inFIG. 3, in order to obtain the IREF information, and the same IREF information would be stored in each of the IREF generators111and258. The timing of the shift registers86and257would be controlled to introduce appropriate respective time delays into each signal so that, when each signal is transmitted from a respective antenna element21or22, the resulting wavefront will be directed or steered in the desired direction relative to the antenna.

However, as discussed above, the representation of the reference waveform with 1-bit samples introduces unwanted harmonics into the digitized version of the waveform. Therefore, instead of the ideal approach just described, the disclosed embodiment takes a different approach which reduces the unwanted harmonics. To achieve this, the waveforms stored in the generators111and258are not identical. Instead, the reference waveform201is given an arbitrary phase shift φ1 relative to a reference, and is then digitized into the 1-bit samples which are stored in the IREF generator111. Separately, the same reference waveform201is given a different phase shift φ2 with respect to the same reference, and is then digitized into 1-bit samples which are stored in the IREF generator258. The shift registers86and257then impart appropriate time delays to the respective signals in order to properly steer the waveform which is to be transmitted. The digital signals from the shift registers are then converted into respective analog signals by the DAC83and DAC256, and then the resulting analog signals are subjected to bandpass filtering at73and253.

In the present context, where the digital reference signal is effectively a square wave signal, odd harmonics are more dominant than even harmonics. Moreover, with respect to the phase shifts φ1 or φ2 which are added before digitization, the harmonics do not receive the same phase shift as the fundamental signal. More specifically, and with reference to block271inFIG. 5, at the output of BPF73the fundamental signal will have a phase shift of φ1, the third harmonic will have a phase shift of 3φ1, the fifth harmonic will have a phase shift of 5φ1, the seventh harmonic will have phase shift of 7φ1, and so forth. Similarly, and with reference to block272inFIG. 5, at the output of the BPF253the fundamental will have a phase shift of φ2, the third harmonic will have a phase shift of 3φ2, the fifth harmonic will have a phase shift of 5φ2, the seventh harmonic will have a phase shift of 7φ2, and so forth.

In the example under discussion, the phase shifter72in the circuit31is set to implement a phase shift of −φ1, which is opposite and equal to the phase shift01introduced before digitization of the IREF information for the generator111. The phase shifter252in the circuit32is set to implement a phase shift of −φ2, which is opposite and equal to the phase shift φ2 introduced before digitization of the IREF information for the generator258. Therefore, and with reference to block273inFIG. 5, at the output of the phase shifter72the fundamental will have a phase shift of zero, the third harmonic will have a phase shift of 2φ1, the fifth harmonic will have a phase shift of 4φ1, the seventh harmonic will have a phase shift of 6φ1, and so forth. Similarly, with reference to block274, at the output of the phase shifter252the fundamental will have a phase shift of zero, the third harmonic will have a phase shift of 2φ2, the fifth harmonic will have a phase shift of 4φ2, the seventh harmonic will have a phase shift of 6φ2, and so forth.

Consequently, when the signals output by the phase shifters72and252are thereafter respectively transmitted as electromagnetic signals through the antenna elements21and22, as indicated diagrammatically at277and278, the fundamentals in each of these electromagnetic signals will have a phase shift of zero and will therefore add coherently in free space, and the time delays introduced into these signals by the shift registers86and257will cause appropriate steering of the resulting wavefront for the fundamental.

In contrast, the third harmonic transmitted by antenna element21will have a phase shift of 2φ1 whereas the same harmonic transmitted by antenna element22will have a different phase shift of 2φ2, and the respective electromagnetic signals representing the third harmonics will therefore tend to add non-coherently in free space.FIG. 5shows only two antenna elements21and22with their associated circuits31and32, but it will be recognized that where a variety of phase shifts are used for all of the respective antenna elements in the array, the result will be non-coherent addition of the third harmonics in an effective and efficient manner that causes the wavefront for the fundamental to have little or no significant presence of the third harmonic. In a similar manner, the other harmonics are also out of phase and tend to add non-coherently in free space, and thus have little or no significant presence in the wavefront for the fundamental.

Although the reduction of harmonics through the addition and removal of phase shifts has been described above in the context of the transmit mode, it can also be used for the receive mode. In this regard, the phase shifters72and252can be used in the receive mode to introduce respective different phase shifts into respective signals received by the antenna elements21and22, after which these phase-shifted signals are digitized into 1-bit samples. As discussed above, this digitization technique introduces unwanted harmonics. Thereafter, these signals are converted into the I and Q signals which are delivered to the central control circuit14(FIGS. 1 and 2). The central control circuit14can apply respective phase shifts equal and opposite to those introduced by the phase shifters72and252, and can then combine the resulting signals, so that the components representing the fundamental add coherently and the components representing harmonics add non-coherently.

The present invention provides a number of technical advantages. One such technical advantage results from the use of low precision digital devices to generate a transmit waveform in a highly distributed manner. This digital generation of waveforms involves circuitry of reduced size, weight, power and cost in comparison to pre-existing circuits for analog waveform generation. Respective time delays for respective component signals are readily accomplished in the digital domain using shift registers and other logic. A related advantage is that, in the context of a radar system, the circuitry of the antenna is primarily digital circuitry rather than radio frequency circuitry, and interfaces between the antenna and a central control system are all digital.

Another advantage results from the generation of transmit waveforms using digital reference waveforms defined by multiple samples which are each a single binary bit. By introducing a respective phase shift before digitization of the reference waveform for each antenna element, and removing the same phase shift after digitization, harmonics resulting from use of one-bit samples are greatly reduced in the transmitted wavefront, because the fundamentals of the transmit waveform add coherently in space, while the harmonics add noncoherently.

Another advantage results from the use of digital-to-analog converters that produce for each sample a pulse having a duration less than the time interval between samples, the output of the converter returning to zero during the time interval between adjacent pulses. This technique permits transmission of a waveform which closely approximates the quality of the waveform that would be generated and transmitted by pre-existing analog techniques, but with significant reductions in size, weight, power and cost for the relevant circuitry.

A further advantage results from the fact that a separate transmitter/receiver circuit is provided for each antenna element, whereas the traditional analog approach uses a single transmitter/receiver to handle a single signal which is subjected to respective phase shifts or time delays between the transmitter/receiver and respective antenna elements. Consequently, the disclosed embodiment provides capabilities which are not present in pre-existing analog configurations. For example, it would be possible to use only a subset of the antenna elements in the antenna array. Alternatively, different subsets of the antenna elements could be used at the same time to transmit different waveforms.

Still another advantage results in the receive mode, when a respective phase shift is introduced into the analog signal from each antenna element before it is digitized, and then the same phase shift is removed after digitization. When the signals derived from different antenna elements are then combined, the components for the fundamental add coherently whereas the components for harmonics add non-coherently, thereby greatly reducing harmonics introduced by the digitization. This is particularly advantageous where the digitization process involves the use of one-bit samples.

Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.