Abstract:
A low-noise amplifier (LNA) filter for use with global navigation satellite system (GNSS) devices is disclosed. A first LNA stage, which is configured to connect to an antenna configured to receive GNSS signals, includes an LNA. A second LNA stage, which is connected to the output of the first LNA stage, has a surface acoustic wave (SAW) filter and an LNA. A third LNA stage, which is connected to the output of the second LNA stage, also has a SAW filter and an LNA.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/557,847, filed Nov. 9, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates generally to low-noise amplifier (LNA) filters and, more specifically, to an antenna LNA filters for global navigation satellite system (GNSS) devices. 
     2. Description of Related Art 
     Navigation receivers that use GNSS, such as GPS or GLONASS (hereinafter collectively referred to as “GNSS”), enable a highly accurate determination of the position of the receiver. The satellite signals may comprise carrier harmonic signals that are modulated by pseudo-random binary codes and which, on the receive side, may be used to measure the delay relative to a local reference clock. These delay measurements are used to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges are not true geometric ranges because the receiver&#39;s local clock is different from the satellite onboard clocks. If the number of satellites in sight is greater than or equal to four, then the measured pseudo-ranges can be processed to determine the user&#39;s single point location as represented by a vector, as well as to compensate for the receiver clock offset. 
     A more detailed description of determining a position based on signals from GNSS satellites and base stations is available in U.S. patent application Ser. No. 12/070,333, filed Feb. 15, 2008, published as US Patent Publication No. 2008/0208454 and U.S. Ser. No. 12/360,808, filed Jan. 27, 2009, published as US Patent Publication No. 2009/0189804 assigned to the assignee of the present application, and each of which are incorporated herein by reference in their entirety for all purposes. 
     Positioning accuracy of GNSS technology is directly dependent on the accuracy of the delay measurements. Transmissions from non-GNSS frequencies that are near GNSS frequencies can cause degraded performance of GNSS services, such as less positional accuracy, by interfering with the delay measurements. 
     For example, a GPS based GNSS device may use timing information transmitted on the L1 band from 1563.42 MHz to 1587.42 MHz. Without filtering, transmissions from the next lower band from 1525 MHz to 1559 MHz could interfere with accurate measurement of the timing signals in the L1 band. Many GPS-based devices employ antenna LNA filters to remove the unwanted frequencies in bands outside of the L1 band. 
     However, many of these GPS-based devices were designed with filters built based on assumptions about the signal strength of the transmissions in the 1525 MHz to 1559 MHz band. Specifically, as this portion of the spectrum was originally designated for satellite transmission, filters for some GPS-based devices assumed a weak signal strength for transmission in this spectrum. This assumption may not longer be accurate. 
     LightSquared is a wireless broadband company that is proposing to use the above frequency spectrum, which is just below the L1 band, to provide a nationwide high-speed wireless network. As discussed above, this spectrum was previously assigned for satellite based communication. Accordingly, many GPS-based devices were designed to only filter out inferences from this neighboring spectrum based on relatively weak signals that are transmitted from space. 
     However, part of LightSquared&#39;s network may involve ground based transmissions that are many orders of magnitude stronger than those that originate in space. Accordingly, filters designs based on an assumption about the power levels of signals in this neighboring spectrum may not be able to sufficiently filter out interference from LightSquared&#39;s network. 
     BRIEF SUMMARY 
     An embodiment of an LNA filter for use with a GNSS has a first LNA stage, which is configured to connect to an antenna configured to receive GNSS signals and includes an LNA. A second LNA stage, which is connected to the output of the first LNA stage, has a surface acoustic wave (SAW) filter and an LNA. A third LNA stage, which is connected to the output of the second LNA stage, also has a SAW filter and an LNA. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts a first antenna LNA filter for use with a GNSS device. 
         FIG. 2  depicts a second antenna LNA filter for use with a GNSS device according to one embodiment of the present disclosure. 
         FIG. 3  depicts a table listing several parameters and specifications for a SAW filter that may be used to implement embodiments of the present disclosure. 
         FIG. 4  depicts several response curves for a SAW filter that may be used to implement some embodiments of the present disclosure. 
         FIG. 5  depicts a third antenna LNA filter for use with a GNSS device according to another embodiment of the present disclosure. 
         FIG. 6  is a graph showing the group delay of the first, second, and third antenna LNA filters. 
         FIG. 7  is a graph showing the frequency response of the first, second, and third antenna LNA filters. 
         FIG. 8  depicts another antenna LNA filter according to an embodiment of the present disclosure. 
         FIG. 9  depicts yet another antenna LNA filter accordingly to an embodiment of the present disclosure 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific devices, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims. 
       FIG. 1  depicts a first LNA antenna filter having two signal paths: signal path  102  and signal path  104 . Signal path  102  is electrically coupled to antenna  106 , and signal path  104  is electrically coupled to antenna  108 . While  FIG. 1  depicts antenna  106  and antenna  108  as being distinct and separate antennas, these two antennas could also be the same antenna (e.g., signal path  102  and signal path  104  may be coupled to the same antenna). 
     Signal path  102  may be configured to receive GNSS signals on the L1 band (1563.42 MHz to 1587.42 MHz) whereas signal path  104  may be configured to receive GNSS signals on the L2 band (1215.6 to 1239.6 MHz). Signal path  102  and signal path  104  may couple together at junction  122 , which may then couple to circuitry to convert the signals in each path to an intermediate frequency that is easier to handle when extracting information from the GNSS signals. 
     Signal path  102  may be implemented with filters  110  and  114  and LNA  112 . Filters  110  and  114  reduce interference from frequencies outside of the L1 band. Filters  110  and  114  maybe implemented with, for example, ceramic filters. LNA  112  amplifies the GNSS signals received by antenna  106 . LNA  112  may be selected to amplify the received GNSS signals while limiting the amount of noise introduced into the signals. 
     Similarly, signal path  104  may be implemented with filters  116  and  120  and LNA  118 . Filters  116  and  120  reduce interference from frequencies outside of the L1 band. Filters  116  and  120  may be implemented with, for example, ceramic filters. LNA  118  amplifies the GNSS signals received by antenna  108 . LNA  118  may be selected to amplify the received GNSS signals while limiting the amount of noise introduced into the signals. 
     Antenna LNA filter  100  may work well when filters  110 ,  114 ,  116 , and  120  can sufficiently attenuate frequencies outside of the L1 and L2 bands so that those external frequencies do not affect the timing data transmitted in the L1 and L2 bands. However, if transmissions in portions of the spectrum neighboring the L1 or L2 bands increase in intensity beyond what filters  110 ,  114 ,  116 , and  120  were designed to attenuate, then antenna filter  100  may be insufficient to prevent those transmissions from inferring with or degrading the GNSS signals. 
       FIG. 2  depicts an embodiment of a second antenna LNA filter  200  according to an embodiment of the present disclosure. Antenna LNA filter  200  has two signal paths: signal path  202  and signal path  204 . Signal path  202  is electrically coupled to antenna  206 , and signal path  204  is electrically connected to antenna  208 . While  FIG. 2  depicts antenna  206  and antenna  208  as being distinct and separate antennas, these two antennas could also be the same antenna (e.g., signal path  202  and signal path  204  may be coupled to the same antenna). 
     Signal path  202  may be configured to receive GNSS signals on the L1 band (1563.42 MHz to 1587.42 MHz) whereas signal path  204  may be configured to receive GNSS signals on the L2 band (1215.6 to 1239.6 MHz). Signal path  202  and signal path  204  may couple together at junction  228 , which may then couple to circuitry to convert the signals in each path to an intermediate frequency that is more manageable for the processing necessary to extract data from the GNSS signals and determine a position based on those signals. 
     Signal path  204  may be implemented with filters  222  and  226  and LNA  224 . Filters  222  and  226  reduce interference from frequencies outside of the L2 band. Filters  222  and  226  maybe implemented with, for example, ceramic filters. LNA  224  amplifies the GNSS signals received by antenna  208 . LNA  224  may be selected to amplify the received GNSS signals while limiting the amount of noise introduced into the signals. For example, LNA  224  may be a pseudomorphic high electron mobility transistor (PHEMT). 
     For ease of discussion, signal path  202  will be explained with respect to three cascaded stages. However, division of signal path  202  in this manner should not be construed as limiting on the structure of the claims. 
     The first LNA stage of signal path  202  includes a filter and an LNA, specifically filter  210  and LNA  212 . As the first LNA in the signal path, first LNA  212  may be selected to amplify the received GNSS signals while limiting the amount of noise introduced into the signals. In some cases, LNA  212  may have a lower noise figure than other LNAs in the  202  signal path. Additionally, LNA  212  may be selected to have a higher dynamic range than other LNAs in signal path  202 . For example, LNA  212  may be a PHEMT. 
     The second LNA stage of signal path  202  includes another filter and another LNA, specifically filter  214  and LNA  216 . Filter  214  may reject more of the signals from outside of the L1 band than filter  210  does alone. In one example, filter  214  is a SAW filter.  FIG. 3  depicts a table containing several parameters for a SAW filter that could be used to implement filter  214 . For example, filter  214  may have a 3 db bandwidth of less than 60 MHz, a center frequency of 1574-1577 MHz, and an insertion loss of less than 2 db.  FIG. 4  depicts several response curves for the same SAW filter. Implementing filter  214  with a SAW filter may have the added benefit of using a minimal amount of space. 
     While the addition of filter  214  may help reduce interference from other portions of the frequency spectrum, it may also degrade the GNSS signals due to the insertion loss of the filter. LNA  216  may counteract the insertion loss of filter  214  by amplifying the GNSS signals. LNA  216  may be implemented using the same or a different LNA as LNA  214 . 
     The third LNA stage of signal path  202  includes another filter and another LNA, specifically filter  218  and LNA  220 , which serve a similar purpose as filter  214  and LNA  216  of the second LNA stage. Filter  218  may be implemented using the same or a different filter as filter  214 . Similarly, LNA  220  may be implemented using the same or a different LNA as LNA  216 . 
       FIG. 5  depicts an embodiment of third antenna LNA filter  500  according to another embodiment of the present disclosure. Antenna LNA filter  500  is similar to antenna LNA filter  200  except the second and third LNA stages each have an additional filter, filter  502  and filter  504 , respectively. Filters  502  and  504  may be implemented using the same or a different filter as filter  214 . 
     The added filters of LNA filter  500  may further reduce the negative impact of signals from outside of the L1 band. However, the added filters may also increase the ripple group delay across the L1 band, thereby reducing timing accuracy. For GNSS signals that are transmitted on different channels within the L1 band, the inter-channel bias that is partially dependent on the group delay across the L1 Band, can be accounted for. This is discussed in U.S. Pat. No. 8,022,868, which is herein incorporated by reference for all purposes. 
     While  FIGS. 1 ,  2 , and  5  depict the components of the respective antenna LNA filter as being directly connected to each other, it should be understood that the claims are not limited in this manner. Rather, those skilled in the art will understand that additional components, such as matching networks and bias circuits are required for the antenna LNA filters depicted in  FIGS. 1 ,  2 , and  5 . 
     Similarly, the depiction of the various components of the above described antenna LNA filters should not be construed to mean that each component must be in a separate package. For example, multiple filters could be packaged together. As another example, one or more filters could be packaged with one or more LNAs. 
       FIG. 6  is a graph of the frequency response for the above described antenna LNA filters. The frequency response of antenna LNA filter  100  ( FIG. 1 ) is represented by curve  602 . The frequency response of antenna LNA filter  200  ( FIG. 2 ) is represented by curve  604 . The frequency response of antenna LNA filter  500  ( FIG. 5 ) is represented by curve  606 . As can be seen, the addition of the filters in the second and third cascaded stages of the antenna LNA filters  200  and  500  greatly reduce out of band interference while maintaining the same level of gain for the in-band GNSS signals. For example, a gain of LNA filter  200  below frequencies of 1540 MHz and above 1640 MHz is less than −40 dB. Additionally, the addition of four filters and two LNAs in antenna LNA filter  500  has improved the out of band interference for the frequency just above the L1 band as compared to antenna LNA filter  200 . For example, a gain of LNA filter  500  below frequencies of 1540 MHz and above 1625 MHz is less than −40 dB. Processors operating at these frequency ranges may generate interference in this portion of the spectrum. Accordingly, if a GNSS device is using a processors or other electrical components (e.g., communication modules such as a LightSquared module) that operate in this frequency range, then antenna LNA filter  500  may provide some benefits over antenna LNA filter  200  by better attenuating noise from this portion of the spectrum. 
       FIG. 7  is a graph showing the group delay for the above described antenna LNA filters. The group delay of antenna LNA filter  100  ( FIG. 1 ) is represented by curve  702 . The group delay of antenna LNA filter  200  ( FIG. 2 ) is represented by curve  704 . The group delay of antenna LNA filter  500  ( FIG. 5 ) is represented by curve  706 . Ripple group delay for antenna LNA filter  500  is less than 40 ns (e.g., about 33 ns) in the GPS band and less than 60 ns (e.g., 43 ns) in the GLONASS band. Ripple group delay for antenna LNA filter  200  is about 23.4 ns in the GPS band and 16.8 ns in the GLONASS band. Ripple group delay for antenna LNA filter  100  is about 2 ns in the GPS band and 2 ns in the GLONASS band. All of these ripple group delays are within the limit of 40 ns for the GPS band and 60 ns for the GLONASS band. 
       FIG. 8  depicts an embodiment of fourth antenna LNA filter  800  according to another embodiment of the present disclosure. Antenna LNA filter  800  is similar to antenna LNA filter  500  except that filter  800  further includes splitter  805  coupled to receive a calibration signal from coupling  803 . Splitter  805  may be configured to selectively couple the calibration signal to either path  202  or path  204 . 
       FIG. 9  depicts an embodiment of fourth antenna LNA filter  900  according to another embodiment of the present disclosure. Antenna LNA filter  900  is similar to antenna LNA filter  800  except that antennas  206  and  208  are replaced with a single antenna  901  operable to receive GNSS signals on both the L1 and L2 bands.