Abstract:
In an embodiment, a field programmable analog array (FPAA) comprises state variable filter engines arranged in parallel, each state variable filter engine comprising at least one variable attenuator and at least one variable integrator configured to operate on a wideband analog signal; and a summer configured to add outputs from the state variable filter engines.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/411,166, filed on Nov. 8, 2010. The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Field Programmable Gate Arrays (FPGAs) provide digital engineers with the ability to rapidly implement digital circuits for a wide range of single-input single-output (SISO) and multiple-input multiple-output (MIMO) applications. At present, analog engineers do not have a comparable tool that operates over a broad frequency range. Attempts have been made to develop an analog equivalent to the FPGA, but the current state of technology is limited to narrowband field programmable analog array (FPAA) applications. 
         [0003]    FPGAs provide programmability to customers to test and revise their designs quickly and with minimum development cost. This minimizes the time to market. In addition, the FPGA can be upgraded in the field which allows the customer to modify the design after the electronic system has been shipped. Recent market trends suggest that customers are increasingly using FPGA not only for prototyping, but also in production designs. The Department of Defense Joint Tactical Radio System (JTRS) is a good example. However, these advantages come at a price. Programmability requires a larger die size which translates to higher per unit cost when compared to application specific integrated circuits (ASICs). FPGAs also have higher power consumption for wideband or high data rate signals because of the high sampling rate required to comply with the Nyquist criteria and contributes to propagation delay. 
         [0004]    Although the equivalency between digital and analog signal processing was long established, the technology development and market acceptance of the former outpaced the latter for at least four reasons. One, the digital signal is less susceptible to interference than an analog one. Two, a digital system is software based and, hence, the implemented functionality can be changed easily. Three, there is an increasing ability to apply complex mathematical techniques to digital signals and, finally, the availability of deep submicron CMOS technology. 
         [0005]    Research on FPAAs began to appear in the academic literature in the early 1990s. Continued research and development over the past 15 to 20 years has led to greater consensus regarding FPAA architectural principles. Standard terms such as CABs (Configurable Analog Blocks) have sprung up, built from common constructions which use op amps and programmable passive components such as switched capacitor and resistor arrays. These CAB sub-components can act as integrators, summers, and attenuators which provide networks of CABs with higher-level functionality. The resultant FPAA has applications in filtering, amplification, signal conditioning, and waveform synthesis, among others. 
         [0006]    FPAA designs have been advancing in the last several years. An FPAA developed at Georgia Tech is an integrated device containing CABs and interconnects between these blocks. It is intended to impact analog signal processing in two ways. First, it performs the function of all rapid prototyping devices in reducing development time. Second, it is a platform for implementing advanced signal processing functions, usually reserved for a digital system, in analog circuits. 
         [0007]    Recently there have been several breakthroughs in the FPAA research. First, there is an increasing trend towards the use of high frequency, small geometry CMOS op amps as the major functional block of a CAB. Madian introduced an FPAA using CMOS Current Feedback Op Amps (CFOA) in a switch matrix specially built for high frequency, programmable filtering applications. A key drawback is low demonstrated bandwidth (order of 1 MHz). Becker created an FPAA in 130 nm CMOS using Operational Transconductance Amplifiers arranged in a hexagonal topology to achieve a routing network which avoids band-limiting switches. Here, the emphasis was on high bandwidth (order of 100 MHz) and low power consumption (&lt;70 mW). The hexagonal structure allows for odd-order feedback in addition to even-order feedback. Unfortunately, this design suffers from poor scalability (single filter at a time with a 7th order maximum). It can be surmised that these new types of FPAAs will soon be used to demonstrate a wide variety of MIMO applications, particularly in advanced radar and wireless communications systems. However, wider bandwidth and sufficiently low power consumption per channel are first required. 
       SUMMARY 
       [0008]    Embodiments of the present invention include FPAAs implemented using state variable filter engines arranged in parallel or in series to operate on a wideband analog signal. Each state variable filter engine includes at least one variable attenuator or tunable loss pad and at least one variable integrator configured to operate the wideband analog signal or a signal derived from the wideband analog signal. In some example FPAAs, the state variable filter engines include second-order state variable filters. 
         [0009]    Alternative embodiments include an FPAA implemented with an array of N×M attenuator matrices and a set of K variable integrators, where K, M, and N, are positive integers. Each attenuator matrix includes a set of variable attenuators, where the number of variable attenuators depends on the rank of the attenuator matrix: the matrix in the 11 position includes K×K attenuators; the matrix in the 1M position includes N×K attenuators; the matrix in the N1 position includes K×M attenuators; the matrix in the NM position includes N×M attenuators; and so on. The attenuators in attenuator matrices are each configured to operate on at least one wideband analog signal. The integrators, which may be tuned locally, are operably coupled to each of the attenuator matrices and configured to integrate signals from at least one of the attenuator matrices. Examples of this alternative FPAA may be a SISO FPAA by setting M=1 and N=1; setting M&gt;1 and N&gt;1 makes the FPAA a MIMO FPAA. 
         [0010]    Example FPAAs can be implemented in deep submicron complementary metal oxide semiconductor (CMOS) technology, e.g., in 130 nm CMOS, 65 nm CMOS, 45 nm CMOS, and 22 nm CMOS technology. Specifically, the variable integrators and variable attenuators may be fabricated using deep submicron CMOS technology. Example FPAAs may also include digital controllers or interfaces configured to adjust integration and attenuation values in the digital domain. Example FPAAs may also be combined with other digital signal processors, such as FPGAs, to provide optimum processing performance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0012]      FIGS. 1A and 1B  are block diagrams of first and second order canonical forms, respectively, of a state variable system. 
           [0013]      FIGS. 1C ,  1 D, and  1 E are block diagrams of alternative second order canonical forms of a state variable system 
           [0014]      FIG. 2  is a block diagram of a 2N th  order wideband SISO FPAA based on the summation form of the state variable system transfer function using the block diagram of  FIG. 1B  (alternatively  FIG. 1C ,  1 D, or  1 E). 
           [0015]      FIG. 3  is a block diagram of a 2N th  order wideband SISO FPAA based on the product form of the state variable system transfer function using the block diagram of  FIG. 1B  (alternatively  FIG. 1C ,  1 D, or  1 E). 
           [0016]      FIG. 4  is a block diagram of a 2N th  order wideband SISO FPAA with serial peripheral interface (SPI) or Serial RapidIO control using the block diagram of  FIG. 1B  (alternatively  FIG. 1C ,  1 D, or  1 E). 
           [0017]      FIG. 5  shows plots of pre-layout magnitude (top) and phase (bottom) of an integrator designed in 130 nm CMOS. 
           [0018]      FIG. 6  shows plots of pre-layout magnitude (top) and phase (bottom) of an attenuator designed in 130 nm CMOS. 
           [0019]      FIG. 7A  illustrates state space equations of the generalized FPAA with M inputs and N outputs. 
           [0020]      FIG. 7B  is a generalized block diagram of wideband FPAA with M inputs and N outputs. 
           [0021]      FIG. 8  is a block diagram of a modem solution using a wideband FPAA within a Xilinx Virtex-7 FPGA with a Texas Instruments C6455 digital co-processor. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 1A and 1B  show a first order state variable structure  100  and a second order state variable structure  150 , respectively, suitable for building a wideband SISO FPAA. The first-order state variable filter  100  operates on a wideband analog input u(t) to produce a filtered analog output y(t). A residue block  102  determines the input&#39;s residue R i , which is forwarded to a summer  104  that combines the residue with a pole p i  of the filtered output. The summer  104  forwards the sum of the residue and the pole to an integrator  106 . A pole block  108  produces the pole p i  of the integrator output, which is fed back to the summer  104 . Changing the parameters of the residue block  102  and the pole block  108  (i.e., changing the filter tap weights) changes the filter transfer characteristics, including the filter center frequency and pass bandwidth. 
         [0023]    The second-order state variable filter  150  shown in  FIG. 1B  also operates on a wideband analog input u(t) to produce a filtered analog output y(t). Here, however, fractional gain blocks (also known as attenuators or tap weights)  152 ,  154 , and 156 scale the wideband input by amounts b 0 , b 1 , and b 2 , respectively, and forward the resulting scaled outputs to summers  162 ,  164 , and  166 , respectively. The first summer  162  combines (subtracts) a scaled output from fractional gain block  152  with (from) the output of fractional gain block  182  to provide a combined signal, which is integrated with a first integrator  172 . The output of the first integrator  172  is combined with outputs of fractional gain blocks  154  and  184 , then integrated with a second integrator  174 . The output of the second integrator  174  is combined with outputs of fractional gain block  156  to produce the filtered output y(t). The filtered output is forwarded to fractional gain blocks  182  and  184 , which scale the filtered output by a 0  and a 1 , respectively. 
         [0024]      FIG. 1C  shows a second-order state variable structure  110  that comprises two integration/gain stages, with variable gain attenuators operating within each stage. A combined signal based on a wideband input u(t) is fed to the first integration/gain stage, which, in turn, provides an input to the second integration/gain stage. Variable gain attenuators feed signals forward (b&#39;s)  126 ,  128 ,  130  and backward (a&#39;s)  122 ,  124  from the input and output of each of the two integrators  112 ,  114 . These signals terminate in a first summing block  116  that combines feedback signals and second summing block  118  that combines feed-forward signals. Example second-order state variable filters may also include fractional gain blocks and additional summers. Varying gains of the variable gain blocks changes a center frequency of the embodiment second-order state variable filters. The transfer function coefficients of a wideband signal processing filter constructed from the structure of  FIG. 1C  are determined by the variable attenuator values. 
         [0025]      FIG. 1D  shows a second-order state variable structure  120  that comprise two integration/gain stages, each of which includes three variable gain integrators  132 ,  134 ,  136 ,  138 ,  142 ,  144 , two of which are operably coupled to a summing block  140 ,  148 . A combined signal based on a wideband input u(t) is fed to the first integration/gain stage, which, in turn, provides an input to the second integration/gain stage. Each gain stage consists of an upper and lower path, with two variable integrators in the lower path and one in the upper path. A switch  146  operably couples a third, binary-valued signal to the second stage summing block  148 . Example second-order state variable filters may also include fractional gain blocks and additional summers. Varying gains of the variable gain blocks changes a center frequency of the embodiment second-order state variable filters. The transfer function coefficients of a wideband signal processing filter constructed from the structure of  FIG. 1D  are determined by the integrator gain values. 
         [0026]      FIG. 1E  shows a second-order state variable structure  130  that comprise two integration/gain stages, each of which includes one tunable loss pad  187 ,  190  and two integrators  186 ,  188 ,  192 ,  194 , one of which is operably coupled to a summing block  193 ,  197 . A combined signal based on a wideband input u(t) is fed to the first integration/gain stage, which, in turn, provides an input to the second integration/gain stage. Each gain stage consists of an upper and lower path, with two integrators in the lower path and one tunable loss pad in the upper path. A switch  195  operably couples a third, binary-valued signal to the second stage summing block  197 . Example second-order state variable filters may also include fractional gain blocks and additional summers. Varying gains of the variable gain blocks changes a center frequency of the embodiment second-order state variable filters. The transfer function coefficients of a wideband signal processing filter constructed from the structure of  FIG. 1E  are determined by the tunable loss pads and integrator gain values. 
         [0027]    The first- and second-order state variable filters shown in  FIGS. 1A to 1E  can be generalized to any order by incorporating or removing additional integrators and gain blocks as appropriate. In addition, components in the structures, including the integrators and attenuators (fractional gain blocks) of the second-order structures, can be implemented in 130 nm CMOS. For more on first- and second-order state variable structures, see WO/2009/114123 to Divi Gupta and Dev V. Gupta, which is incorporated herein by reference in its entirety. 
         [0028]    When designing a filter, the goal is to implement an impulse response y(t) or equivalently a transfer function that approximates Y(s), the Laplace transform of y(t): 
         [0000]    
       
         
           
             
               
                 Y 
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         [0029]    Equation 1: Transfer function of state variable system 
         [0000]    The approximation in Equation 1 can be made to an arbitrary degree of accuracy by adding additional terms in both numerator and denominator polynomials. It can be implemented in the second-order filter state variable structure  150  by using the fractional gain blocks to represent the coefficients in the numerator and denominator (i.e., a 0 , a 1 , a 2 , b 0 , b 1 , and b 2 ) of the transfer function. Like variable gain blocks, the fractional gain blocks attenuate or amplify signals according to a corresponding coefficient. Typically, the variable and fractional gain blocks scale signals to a value within a normalized range of −1 to 1, inclusive. 
       The Summation Form 
       [0030]    Equation 1 can be transformed to two forms, namely, the summation form and the product form. To arrive at the summation form, a partial fraction expansion of Equation 1 is followed by an inverse Laplace transform to provide a temporal function, given below in Equation 2, which is a linear combination of complex sinusoids. In this equation, pi and Ri are the ith pole and its corresponding residue, respectively: 
         [0000]    
       
         
           
             
               
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         [0031]    Equation 2: Temporal transfer function 
         [0000]    The real pole/residue pairs in Equation 2 can be realized using the first-order canonical form structure in  FIG. 1A . The complex pole/residue pairs occur as complex conjugates and are combined as in Equation 3 where all coefficients (a&#39;s and b&#39;s) are real. 
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         [0032]    Equation 3: Transfer function with complex pole/residue pairs 
         [0000]    These conjugate pairs can be realized using the second-order observer canonical form structure in  FIG. 1B  or alternatively  FIG. 1C ,  1 D, or  1 E. The first-order structure is essentially a special case of the second-order structure with b 0 =a 0 =0. Such second-order structures can then be combined and their outputs cascaded to realize T mn (s). 
         [0033]      FIG. 2  shows a wideband FPAA  200  that implements the transfer function T mn (s) of Equation 3. The FPAA  200  includes a plurality of engines  202 , each of which includes a second-order state variable structure  150  ( FIG. 1 ). The engines  202  operate in parallel on a wideband input u(t), and a summer  204  combines the outputs of the engines  202  to produce a processed analog output y(t). 
         [0034]    An integrator followed by a gain stage G has frequency scaling effects. The gain stage G transforms the transfer function T mn (s) to T(s/G). Therefore, if T mn (s) is implemented as a bandpass filter, the center frequency of the filter can be swept across the band merely by changing the gain. This technique is applicable both in the summation form and in the product form. 
       The Product Form 
       [0035]    To arrive at the product form, Equation 1 can be rewritten as in Equation 4. 
         [0000]    
       
         
           
             
               
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         [0036]    Equation 4: Transfer function in product form 
         [0000]    The second equality in Equation 4 is obtained by grouping conjugate zero pairs with proper pole pairs. Any real zero can be grouped with a real pole, two real poles, or a conjugate pole pair. The last equation indicates the transfer function is a product of many second-order structures as described earlier. 
         [0037]      FIG. 3  shows a SISO FPAA  300  that includes a plurality of engines  302  cascaded to realize T mn (s). Each engine  302  includes a second-order structure  150  as shown in  FIG. 1B  or alternatively  FIG. 1C ,  1 D, or  1 E. As described above, blocks a and b are attenuators whose values characterize the transfer function of the FPAA. 
         [0038]      FIG. 4  shows a top level digital wiring diagram of a SISO FPAA  400  that includes a plurality of cascaded second-order structures  410  based on  FIG. 1B , alternatively  FIG. 1C ,  1 D, or  1 E, that operate in series on a wideband analog input u(t) to produce a processed analog output y(t). (For simplicity,  FIG. 4  shows only the first and N th  structures  410 - 1  and  410 -N.) The integrators and attenuators (fractional gain blocks) of the second-order structures can be implemented in 130 nm CMOS. A Serial Peripheral Interface (SPI) or Serial RapidIO  402  coupled to each second-order structure  410  controls the FPAA  400  by changing the attenuator values and the integrator gains of the second-order structures  410 . The signal processing is performed in the analog domain and the digital signals are only used to change the transfer function. 
         [0039]      FIG. 5  shows the performance of the integrator. The integrator magnitude has a 20 dB per decade slope (in log-log scale) and its phase is −90°±10° from 1.2 MHz to 5.6 GHz, corresponding to a transfer function of 1/s. It has a unity gain frequency of 47 GHz. 
         [0040]      FIG. 6  shows the performance of the attenuator. The frequency is 3.8 GHz at −10° phase. 
       Analytical Framework for a Generalized SISO and MIMO System 
       [0041]    The goal in this section is to develop a generalized framework for a wideband adaptive FPAA in state variable structure.
     {dot over (x)} =A x +B u  Where  x  represents the state of the FPAA system and it has size of K     y =C x +D u  x 1, where K defines the order of the FPAA
         u  is the input vector of size M×1     y  is the output vector of size N×1   
       
 
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         [0000]    is a vector size K×1
       A, B, C and D are matrices determining the state of the FPAA system of size (K×K), (K×M), (N×K) and (N×M) respectively       
 
         [0047]      FIG. 7A  illustrates state space equations of the generalized FPAA with M inputs and N outputs. 
         [0048]      FIG. 7B  is the implementation block diagram of a wideband FPAA  700  using the equations above. The elements in matrices A, B, C and D are attenuators  702  and the integrator vector includes integrator/gain blocks  704 . The FPAA  700  has K integrator/gain blocks  704  and (K+N)(K+M) attenuators  702 . The performance of example integrators  704  and attenuators  702  can be found in  FIGS. 5 and 6 , respectively, for implementation in 130 nm CMOS. The transfer function and frequency scaling is controlled and changed similar to the SISO FPAA  400  in  FIG. 4 . The generalized block diagram is a wideband SISO FPAA  700  of K th  order when M=1 and N=1. It is a MIMO FPAA  700  of K th  order when M&gt;1 and N&gt;1. Several features of this architecture are especially significant. First, the integrators  704  are locally tuned, which allows for individual tuning This can be useful when multiple, differing inputs are present, each dedicated to a distinct integrator  704 . Second, the attenuators  702  use randomization algorithms to create a finer range of values, making them extremely precise. This is important in many MIMO radar systems, where individual antenna elements see slightly delayed versions of an input signal. Capturing this fine delay and varying the circuit response appropriately is often critical to the application. Finally, the connections between summation points are nearest neighbor, minimizing routing parasitic and improving bandwidth through the switch matrix. As in the SISO case, this architecture allows for a programmable and dynamic implementation of any specified MIMO filter and transfer function. 
       Combination FPGA/FPAA Architectures for Wideband Modems 
       [0049]    The wideband FPAAs described above have a broad range of applications in commercial and defense markets. This can be explained by considering a radar application of Xilinx&#39;s Virtex-7 855T listed in the Xilinx FPGA technology roadmap. The Xilinx radar application makes use of twenty-four 8- to 16-bit ADCs to supply digital inputs to the FPGA. Each ADC is followed by a digital downconversion (DDC) to create baseband in-phase (I) and quadrature (Q) channels. This type of high-end radar application can have an input frequency of 3 GHz or more, with multi-Gbps data being acquired and processed. However, the ADC power consumption in this example is astronomical, as it requires 2.4 W minimum at 3 Gbps. 
         [0050]      FIG. 8  shows a hybrid modem architecture  800  that uses an FPGA  804  with wideband FPAA technology for baseband processing of radar signals. Analog I/Q data is fed directly to an analog interface  808  coupled to a wideband FPAA  802 , replacing the ADC and internal DDC stage. The FPAA  802 , which is based on state variable technology, perform wideband signal processing chores in continuous time, reducing the signal processing load on an FPGA  804 . Signals from each antenna element in the radar antenna are independently processed through the individual baseband channel level, and digital beamforming is done at baseband. In one example of the hybrid modem architecture  800  shown in  FIG. 8 , the wideband FPAA  802  is integrated into the Xilinx Virtex-7 FPGA  804 , with a TI C6455 digital signal processor  806  acting as a co-processor. Broadband processing is done by the wideband FPAA while the narrowband processing is performed elsewhere in the FPGA and in the DSP. 
         [0051]    The hybrid FPAA/FPGA modem architecture  800  is a much smaller and cheaper design than other architectures and realizes tremendous power savings by eliminating the ADCs and DDC stage. Moreover, the FPAA  802  enables designers to implement programmable filters of arbitrary order. Wideband analog signal processing performed by FPAA filters makes tasks such as interference rejection significantly more robust, as they are performed over greater bandwidth than currently achievable. This approach is facilitated by an eventual integration of the wideband FPAA into FPGAs made by companies such as Xilinx and Altera. 
         [0052]    Wideband FPAAs can be implemented in deep submicron CMOS, e.g., 130 nm, 65 nm, 45 nm, and 22 nm CMOS nodes. Each node offers the advantage of high cut-off frequency, but developing analog circuits in these nodes is challenging owing to non-systemic mismatch problems. 
         [0053]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.