Abstract:
In general, the present invention relates to an adaptive IF filter for a multiband receiver. According to certain aspects, the adaptive IF filter can be dynamically configured as a low-pass architecture or a complex band-pass architecture. According to further aspects, the adaptive IF filter enables a wideband receiver which can simultaneously receive multiple frequency bands, but still protect itself from close-in or in-band jammers by selecting a single frequency band or subset of frequency bands. This retains the multiband functionality of the receiver under nominal conditions, which is traded off dynamically for a single-frequency-band with high jammer resistance under severe jamming conditions. According to still further aspects, the approach of the invention is particularly useful for GNSS receivers, since unlike a cellular transceiver, a temporary loss of signal is not catastrophic to GNSS receivers due to the long integration times.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to multiband RF receivers, and more particularly to methods and apparatuses for multiband GNSS receivers with on-demand RF interference rejection. 
       BACKGROUND OF THE INVENTION 
       [0002]    Global Navigation Satellite Systems (GNSS) available today or in the near future include GPS, GLONASS, BeiDou Satellite Navigation System (BDS, formerly known as Compass) and Galileo. Multiband GNSS receivers that support multiple systems offer a performance advantage due to the increased number of satellites in view. Conventionally, this is done by providing parallel receiver paths for the different bands, which requires more power and hardware. A more efficient way to architect a multiband GNSS receiver, although not typically done today, would be to have a single down-conversion with a large IF bandwidth that includes all the bands of interest, as shown in  FIGS. 1A and 1B . For example,  FIG. 1B  shows bands B 1 , B 2 , B 3 , and B 4  representing four respective frequency bands from different GNSS systems (e.g. GPS, GLONASS, BDS and Galileo) after downconversion to IF by mixer  102 . These are all preserved by a wide passband having bandwidth 2*f c  established by low pass filters  104 -A and  104 -B. The individual bands B 1 , B 2 , B 3  and B 4  are then separated by high-speed filtering and processing in the digital domain, which is very efficient in current digital CMOS technology. 
         [0003]    One problem with a large IF bandwidth is that it leaves the receiver vulnerable to interference close to the GNSS bands, such as that caused by jammers, due to the limited rejection of a wideband IF filter. There are several existing approaches to making RF receivers more resistant to in-band or close-in RF interference. 
         [0004]    Narrower and higher rejection front-end filters can be used in front of the LNA to reject or attenuate the jammer. However, this would only work for a single frequency band receiver. Furthermore, higher rejection and narrower band filters would generally have higher cost and higher insertion loss. Higher insertion loss limits the receiver&#39;s sensitivity. 
         [0005]    Double-conversion RF receivers for one or more frequency bands can be used to separate the IF signal paths of each frequency band. Each frequency band then has its own IF filter that can be tuned to reject the jammer. This approach increases chip area and power consumption due to duplication of the IF filters, VGA and ADC functions for each frequency band. The LO&#39;s used for the second down-conversion mixers are also a source of spurious self-interference in the receiver as their harmonics can mix with the reference frequency and couple to the receiver&#39;s RF input. 
         [0006]    A high dynamic range ADC can be used to reduce the total receiver gain and thus increase headroom for the jammer signal. This increases the ADC power consumption and sampling frequency. For wide-band IF signals the demands on the ADC get increasingly higher, which puts a limit on how much the dynamic range of the ADC can be increased. For a multi-band receiver with a limited power budget, this alone is not sufficient to provide the required jamming immunity. 
         [0007]    Increasing current in the IF filters and amplifier can increase the linearity and compression point of these stages, thus enabling the receiver to withstand a higher jammer level before compression and performance degradation. However, this is only an incremental improvement and signal headroom will ultimately be limited by the supply voltage rails. This method also costs additional current. Adjustable biasing has been proposed to minimize power when linearity is not required. See, e.g., Yoshizawa et al, A channel-select filter with agile blocker detection and adaptive power dissipation, IEEE J. Solid-State Cir, Vol 43, No 5, May 2007. 
         [0008]    Accordingly, a need remains for efficient methods and apparatuses for multiband receivers with effective and flexible interference rejection, such as that caused by in-band or close-in jammers. 
       SUMMARY OF THE INVENTION 
       [0009]    In general, the present invention relates to an adaptive IF filter for a multiband receiver. According to certain aspects, the adaptive IF filter can be dynamically configured as a low-pass architecture or a complex band-pass architecture. According to further aspects, the adaptive IF filter enables a wideband receiver which can simultaneously receive multiple frequency bands, but still protect itself from close-in or in-band jammers by selecting a single frequency band or subset of frequency bands. This retains the multiband functionality of the receiver under nominal conditions, which is traded off dynamically for a single-frequency-band with high jammer resistance under severe jamming conditions. According to still further aspects, the approach of the invention is particularly useful for GNSS receivers, since unlike a cellular transceiver, a temporary loss of signal is not catastrophic to GNSS receivers due to the long integration times. According to still further aspects, embodiments of the invention enable dynamic re-configuration of a receiver&#39;s low-pass IF filter into a complex band-pass filter using minimal additional hardware. 
         [0010]    Embodiments of the invention provide a clear advantage in cost, performance and functionality over prior art methods. The invention can be used alone or can be used in conjunction with other conventional methods to significantly improve jamming immunity. Embodiments of the invention invention can adjust current in the LPF, but can actually decrease current consumption instead of increasing current as in prior art methods. When switched to a complex band-pass configuration the LPF filters are set to narrower bandwidth, which requires less current in than the wideband IF filter. According to certain aspects, embodiments of the invention offer the efficiency and versatility of single conversion multi-band receiver with a single wide IF band, which takes maximum advantage of state-of-the-art digital CMOS technology that is available for digital filtering and processing. 
         [0011]    In furtherance of these and other aspects, an IF filter for a multiband GNSS receiver according to embodiments of the invention includes a wideband filter that passes a plurality of signals from different GNSS systems; and cross-coupling elements that convert the wideband filter into a narrowband filter that passes fewer than the plurality of signals. 
         [0012]    In additional furtherance of these and other aspects, an IF filter for a multiband GNSS receiver according to embodiments of the invention includes a first lowpass Op Amp-RC filter coupled to in-phase signals corresponding to a plurality of different GNSS systems; a second lowpass Op Amp-RC filter coupled to quadrature signals corresponding to the plurality of different GNSS systems; and a switched resistor block operatively coupled between the first and second lowpass Op Amp-RC filters, wherein in a first mode of operation, the switched resistor block provides an open circuit between the first and second lowpass Op Amp-RC filters so that they together have a wide passband that passes signals from all of the plurality of GNSS systems, and wherein in a second mode of operation, the switched resistor block provides a resistive coupling between the first and second lowpass Op Amp-RC filters so that they together have a narrower passband that passes signals from fewer than all of the plurality of GNSS systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0014]      FIGS. 1A and 1B  illustrate prior art implementations of a wideband IF filter; 
           [0015]      FIGS. 2A and 2B  illustrate aspects of an adaptive IF filter according to the invention; 
           [0016]      FIGS. 3A and 3B  illustrate further aspects of an adaptive IF filter according to the invention; 
           [0017]      FIG. 4  is a functional block diagram illustrating an example implementation of an adaptive IF filter according to embodiments of the invention; 
           [0018]      FIG. 5  is a functional block diagram further illustrating an example implementation of an adaptive IF filter according to embodiments of the invention; and 
           [0019]      FIG. 6  is a schematic diagram of an example low-pass filter architecture that can be used in an adaptive IF filter according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. 
         [0021]    In general, embodiments of the invention are directed to an adaptive IF filter for a multiband receiver, which can be dynamically be configured as a low-pass architecture or a complex band-pass architecture. The adaptive IF filter enables a wideband receiver which can simultaneously receive multiple frequency bands, but is also able to protect itself from close-in or in-band jammers by selecting a single frequency band or subset of frequency bands. According to certain aspects, embodiments of the invention accomplish this by dynamically re-configuring a receiver&#39;s low-pass IF filters to a complex band-pass filter, using minimal additional hardware. 
         [0022]    Example aspects of an adaptive IF filter according to embodiments of the invention are illustrated in  FIGS. 2A and 2B . From a high-level standpoint,  FIG. 2A  shows a similar multiband receiver architecture as that in  FIG. 1A , except that there are now programmable cross-coupled paths  206  between the I and Q low-pass filters  204 -A and  204 -B. Under nominal conditions (e.g. no destructive interference such as an in-band jammer) the switches in the cross-coupled paths  206  will be open and this receiver will still behave like the receiver in  FIG. 1A , receiving all bands, as shown in  FIG. 2B . In an example where the GNSS signals include GPS, GLONASS, BDS and Galileo, the bandwidth 2*f c  can be around 44 MHz. 
         [0023]    Now assume that a large close-in jammer J 1  is incident at the RF input, and is down-converted to negative IF frequency by mixer  202  along with the desired signals in B 1 , B 2 , B 3  and B 4 , as shown in  FIG. 2B . The jammer causes a large decrease in the AGC gain, which is typically computed by integrating and averaging the I and Q ADC data samples. This sudden large decrease in AGC gain triggers embodiments of the invention to decrease the LPF bandwidth from f C  to f B , and to close the switches in cross-coupling path  206 . These two actions convert the wideband low-pass filters to a narrow-band complex band-pass filter  208 , as illustrated in  FIG. 3A . 
         [0024]      FIG. 3B  shows how the interference from jammer J 1  is greatly attenuated by the complex filter  208 . Note that this is an approximate drawing for illustration purposes only. The actual frequency response and jammer attenuation achieved will depend on the order, bandwidth, and type of filter. Further note that, notwithstanding the illustration in  FIG. 3B , the complex filter  208  can be designed to pass any one or more of the bands, as will be explained in more detail below. Still further, although  FIG. 3B  shows a positive shift in center frequency, similar negative frequency shifts can also be achieved by changing the cross-coupling polarity, or by swapping the I and Q signals in the mixer  202 . 
         [0025]      FIG. 4  is an example functional block diagram for implementing an adaptive IF filter according to embodiments of the invention. 
         [0026]    As shown, it includes a jam detect and band select block  402 , an adaptive IF filter  404 , a digital filter and processing block  406  and a SNR detect block  408 . 
         [0027]    In normal operation, adaptive IF filter  404  is configured as conventional wideband low-pass filters, with digital filter and processing block  406  operating to separate out the digital baseband signals B 1 , B 2 , B 3  and B 4 . Jam detect and band select block  402  receives real-time feedback signals from digital filter and processing block  406  (e.g. digital part of the AGC loop) and SNR detect block  408 . For example, feedback from block  406  can include AGC gain control information and feedback from block  408  can include band-specific SNR information. This information can be used by block  402  to determine the presence and frequency of destructive interference, such as from an in-band or close-in jammer. In response, block  402  can send appropriate signals to adaptive filter  404  to cause it to be reconfigured as a narrowband band pass filter, with a center frequency set to greatly attenuate the destructive interference, as described above. 
         [0028]    Jam detect and band select block  402  can be implemented by any combination of software, hardware, firmware, etc. In one example where adaptive IF filter  404  is included in a GNSS receiver, block  402  can be implemented by firmware for execution by a GNSS receiver processor such as a CPU core. Those skilled in the art will understand how to implement the functionality of block  402  after being taught by the present disclosure, and depending on the actual filter design, etc. 
         [0029]    Digital filter and processing block  406  can be implemented in various ways known to those skilled in the art, and can include functional components such as CW detectors, digital filters, digital down converters for the separate baseband signals, FIR filters, etc. These can be implemented using any combination of software, hardware, firmware, etc., as appreciated by those skilled in the art. Further details thereof will be omitted here for sake of clarity of the invention. SNR detect block  408  can also be implemented by any combination of software and hardware to monitor the SNR for each individual band B 1  to B 4  and/or various other portions of the overall IF passband. 
         [0030]      FIG. 5  is a functional block diagram further illustrating an example implementation of adaptive IF filter  404  according to embodiments of the invention. 
         [0031]    As shown, filter  404  in this example includes low pass filters  504 -A and  504 -B operatively coupled to switched resistor block  506 . 
         [0032]    In normal operation, switched resistor block  506  is configured to create an effective open circuit between low pass filter  504 -A and  504 -B, causing them to operate independently as wide-band IF filters. Depending on signals from jam detect/band select block  402 , switched resistor block  506  and filters  504 -A and  504 -B can be dynamically reconfigured to operate as a complex narrowband filter  510 . 
         [0033]    In embodiments, switched resistor block  506  includes a plurality of resistors in series with switches implemented as NMOS transistors, representing the multiple cross-coupled paths. The number of cross-coupled paths is proportional to the order of the filters. The resistance is split into two parts so that both filters  504 -A and  504 -B see the same load. The switches are simultaneously opened and closed depending on control signals from block  402 , thereby enabling or disabling the resistive cross-coupling paths between filters  504 -A and  504 -B. The resistors can be made variable to set to the center frequency of the complex bandpass filter, and may be programmed by a digital code from block  402 . 
         [0034]    The particular design and values of components in filters  504 -A and  504 -B and switched resistor block  506  will depend on the mixer LO frequency, the frequency and bandwidth of the RF signal bands, anti-aliasing requirements, and the desired interference rejection. Example implementations will be described in more detail below. Those skilled in the art will be able to implement the invention for various signal bands such as different GNSS signals after being taught by the examples below. 
         [0035]    In embodiments, the design of filters  504 -A and  504 -B is started from the passive LC ladder prototype of the desired filter function (e.g. with the required cutoff frequencies, order, ripple, etc.). The type of filter function or frequency response may be Butterworth, Chebyshev, or other. The coefficients of the inductor (L) and capacitor (C) components of the LC prototype can be obtained from published tables, filter design programs, or derived analytically. Kirchoff&#39;s Current Law (KCL) and Kirchoff&#39;s Voltage Law (KVL) equations are then used to calculate the R and C values of the low-pass filters  504 -A and  504 -B, which in embodiments are implemented in Leapfrog topology. 
         [0036]    The complex filter  510  is realized by first shifting the frequency of the low-pass filters  504  to be centered on the frequency of the desired baseband signal(s). If HLP(jω) is the transfer function of the low-pass filter  504 , then the transfer function of the Complex Band-Pass filter  510 , HCBP(jω) is given by: 
         [0000]        H   CBP ( jω )= H   LP   [j (ω−ω 0 )]
 
         [0000]    where ω 0  is the frequency shift and ω is the frequency of the original wideband IF filter. 
         [0037]    In the embodiment shown in  FIG. 5 , this shift in center frequency is achieved by cross-coupling the two low-pass filters  504 -A and  504 -B using switched resistor block  506 . For OpAmp-RC filters this frequency shift can be achieved by resistors in the cross-coupling paths given by: 
         [0000]    
       
         
           
             
               R 
               C 
             
             = 
             
               1 
               
                 
                   ω 
                   0 
                 
                  
                 
                   C 
                   0 
                 
               
             
           
         
       
     
         [0000]    where C 0  is the capacitance of the integrator. 
         [0038]    For simplicity, C 0  is identical for all the stages in the filter so that the coupling resistors are the same for all the cross-coupled paths through block  506 . Note that the frequency shift can be made variable by making the cross-coupled resistors R C  programmable. Digitally programmable variable resistors can be implemented using a combination of series or parallel resistors coupled to MOS switches, using methods known to those ordinarily skilled in the art. These variable resistors within block  506  may be controlled by signals from block  402 . The direction of the frequency shift can be made positive or negative by inverting the I and Q cross coupling, or by swapping the I and Q phases in the mixer. 
         [0039]    According to certain aspects, the bandwidth of complex filter  510  is also adjusted. In embodiments, this is accomplished by adjusting the capacitance of the integrator C 0 . Variable capacitors may be easily implemented using multiple capacitors coupled to switches. 
         [0040]    Embodiments of the invention use a Leapfrog OpAmp-RC topology for the low-pass filters  504  which offers high linearity. This topology is suitable for any odd order filter with three or more poles (third order, fifth order, seventh order, etc.). 
         [0041]      FIG. 6  is a schematic diagram of one example third order Leapfrog OpAmp-RC low-pass filter  504  according to embodiments of the invention. Note that the input and feedback resistors  602 ,  604  in this filter have been made programmable in this embodiment order to integrate the variable gain amplification (VGA) function shown in  FIGS. 2A and 3A  into the low-pass filter. The integrating capacitors C 0  are also variable for RC calibration of the low-pass filter, and for varying the complex filter bandwidth, as mentioned previously. 
         [0042]    The adaptive IF filter using the example filter shown in  FIG. 6  is designed for two wideband LPF modes with different bandwidths. For example, one wider bandwidth can be designed to capture all baseband signals of interest, while another narrower bandwidth can be used to capture only a subset of the baseband signals. In these modes, the cross-coupling paths between filters  504 -A and  504 -B through block  506  are disabled by signals from block  402 . The bandwidth of the LPF  504  is adjusted by varying the capacitance C 0  under control of signals from block  402 . 
         [0043]    To implement a narrowband complex BPF  510 , the cross-coupling paths between filters  504 -A and  504 -B through block  506  are enabled by signals from block  402 . In embodiments, the inputs and outputs of each respective op-amp  606 - 1 ,  606 - 2  and  606 - 3  between filters  504 -A and  504 -B are cross-coupled. For example, the positive input of op-amp  606 - 1  in filter  504 -A is coupled to the positive output of op-amp  606 - 1  in filter  504 -B, the negative input of op-amp  606 - 1  in filter  504 -A is coupled to the negative output of op-amp  606 - 1  in filter  504 -B, the positive output of op-amp  606 - 1  in filter  504 -A is coupled to the negative input of op-amp  606 - 1  in filter  504 -B and the negative output of op-amp  606 - 1  in filter  504 -A is coupled to the positive input of op-amp  606 - 1  in filter  504 -B. Similar cross-coupling is performed for op-amps  606 - 2  and  606 - 3 . The center frequency of the narrowband mode is further adjusted for the desired baseband signal by selecting appropriate resistance values in the switched resistance block  506 , controlled by signals from block  402 . 
         [0044]    In embodiments, the current for each mode is optimized to meet the OpAmps gain bandwidth requirement for a given filter bandwidth. According to aspects of the invention, the narrower filter used to form the complex band-pass filter  510  requires less current than the wideband low-pass filter  504  options. 
         [0045]    Although the filter implementation described above is a Opamp-RC topology, the invention could also be implemented with Gm-C filter or other filter architectures. The implementation of the low-pass to complex band-pass translation and cross-coupling method will differ by topology. Opamp-RC offers good linearity, and so is preferred in implementations where linearity is desired. 
         [0046]    Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications.