Abstract:
A compact GPS receiver includes a GPS antenna composed of a patch and a ground plane to receive high frequency satellite signals, an analog circuit which processes the high frequency satellite signals into a digital signal, a digital circuit which processes the digital signal to compute positional coordinates of the GPS antenna, and an interface connector which outputs the positional coordinates to an external device. A substrate is provided to support the GPS antenna, the analog circuit, the digital circuit, and the interface connector. An EMI (electromagnetic interference) shield is provided to shield at least the digital circuit to block a noise developing at the digital circuit from interfering with the GPS antenna. The GPS antenna is mounted on the EMI shield to share the ground plane with the EMI shield, while keeping the patch insulated electrically from the EMI shield. Thus, the GPS receiver can be made compact by making the use of the EMI shield also as the ground plane of the antenna, while assuring to block the noise from interfering with the antenna and affording sufficient antenna gain with the increased ground plane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a GPS receiver, and more particularly to a compact GPS receiver which has a GPS antenna sharing a ground plane with an EMI shield of an electric circuit which processes satellite signals. 
     2. Description of the Prior Art 
     A GPS receiver is basically composed of an antenna unit for receiving high frequency satellite signals and a processing unit for processing the signals to compute positional coordinates of the GPS receiver. The processing unit includes an analog circuit which is responsible for amplifying the HF satellite signal, reducing the HF signal to an intermediate frequency signal, and converting the signal into a digital signal. Also included in the processing unit is a digital circuit which processes the digital signal to compute the positional coordinates of the GPS receiver. Since the digital circuit handles the digital signal, it is a source of developing a noise which interferes with the antenna and impedes the antenna gain. In order to avoid the interference, the digital circuit has to be surrounded by an EMI (electromagnetic interference) shield. On the other hand, the antenna unit is required to have increased area of ground plane for improving the antenna gain. However, in a prior GPS receiver in which the antenna unit is provided separately from the processing unit, it is difficult to make the whole assembly of the GPS receiver compact enough to be easily carried on, while affording the increased antenna&#39;s ground plane for improved antenna gain in addition to providing the EMI shield of sufficient dimensions for successfully shielding the digital circuit. 
     SUMMARY OF THE INVENTION 
     In view of the above insufficiency, the present invention has been accomplished to provide a GPS receiver which can be made compact to be sufficiently portable. The GPS receiver in accordance with the present invention includes a GPS antenna composed of a patch and a ground plane to receive high frequency satellite signals, an analog circuit which processes the high frequency satellite signals into a digital signal, a digital circuit which processes the digital signal to compute positional coordinates of the GPS antenna, and an interface connector which outputs the positional coordinates to an external device. A substrate is provided to support the GPS antenna, the analog circuit, the digital circuit, and the interface connector. An EMI (electromagnetic interference) shield is provided to shield at least the digital circuit to block a noise developing at the digital circuit from interfering with the GPS antenna. The characterizing feature of the present invention resides in that GPS antenna is mounted on the EMI shield to share the ground plane with the EMI shield, while keeping the patch insulated electrically from the EMI shield. Thus, the GPS receiver can be made compact by making the use of the EMI shield also as the ground plane of the antenna, while assuring to block the noise from interfering with the antenna and affording sufficient antenna gain with the increased area of the ground plane. 
     In a preferred embodiment, the substrate comprises a single double-sided circuit board having a top mount surface and a bottom mount surface. The digital circuit is formed by a plurality of components which are mounted partly on the top mount surface and partly on the bottom mount surface. The EMI shield is composed of a top cover fitted over the top mount surface and a bottom cover fitted over the bottom mount surface. The top cover and the bottom cover are cooperative to surround the components forming the digital circuit, thereby shielding the digital circuit completely. Most preferably, the top cover and the bottom cover are cooperative to surround also components forming the analog circuit for blocking any possible noise developed at the analog circuit. 
     The interface connector may be a universal serial bus (USB) connector for connection with a host computer. A universal serial bus (USB) controller for the USB connector is also included in the digital circuit shielded by the EMI shield so as not to radiate undesired noise towards the antenna. 
     The interface connector is surrounded by a connector shield which has a top end portion projecting above a top plane of the EMI shield. The top portion is electrically connected to the top plane so as to have an electrical potential substantially equal to the top plane, i.e., the ground plane of the EMI shield for assuring a stable antenna characteristic free from the interface connector. In order to assure the reliable and easy electrical coupling of the EMI shield to the connector shield, the EMI shield is formed with a lug or lugs which project above the top plane of the EMI shield and come into surface contact with the top end portion of the connector shield 
     Preferably, the circuit board has a ground conductor embedded between the top mount surface and the bottom mount surface. The ground conductor is electrically connected to a plurality of pads arranged around a circumference of the circuit board. The EMI shield has a periphery corresponding to the circumference of the circuit board and has a plurality of anchors which are arranged around the periphery and bonded to the pads of the circuit board, respectively. The anchors are spaced from each other by a distance of one-fourth (¼) or less of a wavelength (λ) of the GPS signal. The distance is selected in order to block the noise from escaping through a gap between the circuit board and the EMI shield. Thus, the antenna can be completely kept intact from the noise developed at the digital and the analog circuits. 
     The top plane or the ground plane of the EMI shield is preferably configured into a regular polygon having four sides or more with the patch being mounted on the top plane at a location offset from the center of the regular polygon. This offset arrangement is advantageous for minimizing a loss in antenna gain when the GPS receiver is in use to be placed on a metal base. In this condition, the creepage distance from the patch to the metal base is an important factor with regard to the loss of the antenna gain. In principle, when the creepage distance from the patch to the metal base becomes nearly equal to one-fourth (¼) of the wavelength (λ) of the GPS signal, the antenna will suffer from a certain loss in the antenna gain. Therefore, as the number of such creepage distances of (λ/4) measured from various edge portions of the patch to the metal base increases, there will be a considerably increased loss in the antenna gain. This can be avoided by the offset arrangement of the patch on the ground plane of the regular polygon. That is, the patch give different creepage distances extending from various edge portions of the patch to the metal base through corresponding edges of the regular polygon and therefore can exclude a possibility where nearly all of the creepage distances would be λ/4. For this reason, it is possible with the offset arrangement of the patch to reduce the loss in the antenna gain when. the GPS antenna is placed on the metal base. 
     In this connection, a radome covering the GPS antenna is designed to have a bottom surface which is spaced from the patch by a distance longer than a critical distance below which a substantial loss in antenna gain appears. 
     Further, the EMI shield may be formed with a positioning structure for accurately positioning the patch on the top plane of the EMI shield, and therefore assuring reliable antenna characteristics as intended. 
     Also, when a feed cable is utilized to connect the patch to the analog circuit, the EMI shield may be formed with a retainer for retaining the feed cable in a fixed position for keeping the antenna characteristics as intended. 
     In another embodiment, a battery is incorporated in the GPS receiver to back up data stored in the memory included in the digital circuit. The battery is mounted on the EMI shield with a negative electrode of the battery being directly connected to the EMI shield, thereby simplifying a structure of connecting the battery in circuit. 
     Further, when the patch is connected to the analog circuit by a feed pin depending from the patch, the patch is preferably mounted at a located immediately upwardly of the analog circuit on the circuit board to minimize a connection path of the patch antenna to the analog circuit. 
    
    
     These and still other advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taking in conjunction with the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a disk-type GPS receiver in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is an exploded perspective view of the GPS receiver; 
     FIG. 3 is a perspective view of the GPS receiver shown with a radome removed therefrom; 
     FIG. 4 is a bottom view of a top cover of an EMI shield utilized in the GPS receiver; 
     FIG. 5 is a bottom view of a circuit board utilized in the GPS receiver; 
     FIG. 6 is a circuit diagram of the GPS receiver; 
     FIG. 7 is a diagram illustrating the feature of the GPS receiver; 
     FIG. 8 is a view of the GPS receiver showing a distance (D) between the antenna and a metal base on which the receiver is placed in use; 
     FIG. 9 is a graph illustrating the relation between an antenna gain (dB) and the distance (D); 
     FIG. 10 is an exploded perspective view of a stick-type GPS receiver in accordance with another preferred embodiment of the present invention; 
     FIG. 11 is a perspective view of the GPS receiver shown with a radome removed therefrom; and 
     FIG. 12 is a circuit diagram of the stick-type GPS receiver. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 to  6 , there is shown a disk-type GPS receiver in accordance with a preferred embodiment of the present invention. The GPS receiver  1  includes a GPS antenna  10  receiving high frequency (HF) satellite signals from a plurality of satellites orbiting around the earth, and a signal processing unit for computing positional coordinates of the GPS antenna in a known manner based upon the received satellite signals including location data as well as time data of the respective satellites. 
     As shown in FIG. 6, the processing unit is basically composed in function of an analog circuit  60 , and a digital circuit  70 . The analog circuit  60  has a pre-amplifier  61  which amplifies the HF satellite signals, a downconverter  62  which reduces the HF signals into intermediate frequency (IF) signals, and an A/D converter or digitizer  63  which converts the IF signals into digital signals. The digital circuit  70  comprises a navigation processor  71  for processing the digital signals to compute positional coordinates of the GPS receiver, ROM  72  storing a program of computing the positional coordinates, RAM  73  storing the computed positional coordinates, and EEPROM  74  storing orbital data of the satellites as well as the updated positional coordinates. Also included in the digital circuit  70  are a microprocessor  76  and a universal serial bus (USB) controller  76  for intercommunication with a host computer  90  through a universal serial bus (USB) connector  80 . The USB controller  76  is provided for transferring the data and instructions under control of the microprocessor  75  between the digital circuit and the host computer  90  in a prescribed format. The USB connector  80  receives a USB plug  92  at one end of a cable  91  leading to the computer. The computer  90  obtains the positional coordinates from the digital circuit  70  of the GPS receiver  1  for mapping or the like processing. 
     As best shown in FIGS. 2 and 5, the processing unit, i.e., the analog and digital circuits are formed by a plurality of electric components which are mounted on a single circuit board  20 . The circuit board  20  is a double-sided board with a top mount surface, a bottom mount surface, and an intermediate layer of a ground conductor  21  which is embedded in the board  20  to serve as a ground for the electrical circuits, i.e., the analog and digital circuits  60  and  70 . The components includes as major components, IC chips  31  to  37  which are partly mounted on the top mount surface and partly on the bottom mount surface. The components forming the digital circuit include chips  32  to  37  and the associated circuitry on the opposite sides of the circuit board  20 , while components forming the analog circuit include the chip  31  and the associate circuitry on the opposite sides of the circuit board  20 . The board  20  is shaped into a regular octagon and carries the USB connector  80  at a center of one side of the octagon. The USB connector  80  is surrounded by a connector shield  81  which is also connected to the ground conductor  21 . 
     The circuit board  20  is covered by an EMI (electromagnetic interference) shield  40  to shield the analog and digital circuits for blocking noises developing at the circuits from interfering with the GPS antenna  10 . The EMI shield  40  is composed of a top cover  41  and a bottom cover  51  which are respectively shaped to have flat planes of regular octagons in conformity with the circuit board  20 . The top cover  41  includes a peripheral wall  43  depending from a periphery of the top octagonal flat plane  42  to abut against the circumference of the circuit board  20 , as shown in FIG. 3. A plurality of anchors  44  extend integrally from the peripheral wall  43  and engage respectively into corresponding notches  24  in the circumference of the circuit board  20  for fixing the top cover  41  to the circuit board  20 . The bottom cover  51  also includes a peripheral wall with a plurality of joints  52  which extend through corresponding recesses  25  in the circumference of the circuit board  20  for soldering connection to the peripheral wall  43  of the top cover  41 . Although the EMI shield  40  is designed to shield the analog and digital circuits in this embodiment, the shield  40  may be sufficient to shield only the digital circuit which are a major source of develops the noises interfering with the receiving characteristic of the antenna. 
     The top flat plane  42  of the top cover  41  is utilized also to define a ground plane of the GPS antenna  10 , in addition to the function of shielding the analog and digital circuits. That is, as shown in FIGS. 2 and 3, the GPS antenna  10  is composed of a patch  11  supported on a dielectric ceramics  12  and the ground plane  42 . The patch  11  is shaped into a rectangular having lengths of about 20 to 22 mm which are approximately equal to one-half of a wavelength of 1,575.42 MHz, a center frequency of the high frequency satellite signals. A plurality of positioning flaps  45  project on the top flat plane  42  to engage the exterior of the dielectric ceramics  12  for positioning the patch  11  at an accurate position on the plane  42 . The ceramics  12  is then secured to the plane  42  by means of an adhesive tape  13 . The patch  11  is connected to the component forming the analog circuit by means of a coaxial cable  15  having a core and a shield conductor, As shown in FIG. 4, the core and the shield conductor are connected at one end of the coaxial cable  15  respectively to a feed pin  14  and soldering catches  46  on the back of the ground plane  12 . The coaxial cable  15  is retained at a fixed position on the back of the top cover  41  by means of a retainer flap  47  integrally formed on the top cover  41  to keep uniform antenna characteristics. 
     The ground plane  42  has a cut out portion  48  for receiving the connector shield  81  which has a greater height than the top cover  41 . A pair of upstanding lugs  49  are formed to extend from the cut out portion for electrical connection with a top end of the connector shield  81  of the USB connector  80 , giving the same electrical potential to the top surface of the connector shield and the ground plane  42  for assuring consistent antenna characteristics irrespective of the existence of the connector shield  81  projecting on the ground plane  42 . 
     The GPS antenna  10  thus integrated on the EMI shield  40  is accommodated within a radome  100  of an electrically insulating material. The radome  100  is disk-shaped to have a circular contour and is composed of a top case  101  and a bottom case  102  which are fastened by means of screws  103  extending also through the circuit board  20  and the EMI shield  40 . Whereby, the antenna  10  and the processing unit are assembled into a single structure. The bottom case  102  is provided with a plurality of rubber feet  104  by which the GPS receiver  1  is placed in use on a supporting structure. 
     In the GPS receiver  1  thus assembled, the EMI shield  40  is best utilized also to define the ground plane  42  of the antenna  10  in addition to the circuit ground, as shown in FIG.  7 . Therefore, the GPS receiver  1  including the antenna and the signal processing unit can be made compact enough to be sufficiently portable, while leaving the antenna free from the noises developed at the signal processing unit. 
     The EMI shield  40  is electrically connected to the ground conductor or the circuit ground  21  by soldering the anchors  44  respectively to a corresponding number of pads  25  which are provided at the bottom circumference of the circuit board  20 , as shown in FIG. 5, and connected to the ground conductor  21 . It is noted in this connection that, the distance between the adjacent anchors  44  or the pads  25  are equal to or shorter than a distance which is one-fourth (¼) of the wavelength of the satellite signal in order to effectively minimize noise leakage outside of the EMI shield  40 . 
     Further, as shown in FIG. 3, the patch  11  of the GPS antenna  10  is offset from a center of the octagonal ground plane  42  in order to give different creepage distances L from various edge portions of the patch  11  to a metal base  2  on which the GPS receiver  1  is possibly placed in an actual condition of use, for minimizing a loss in antenna gain in such use. In principle, when the creepage distance from the patch  11  to the metal base  2  becomes nearly equal to one-fourth (¼) of the wavelength (λ) of the GPS signal, the antenna will suffer from a certain loss in the antenna gain. Therefore, as the number of such creepage distances of (λ/4) measured from various edge portions of the patch  11  to the metal base  2  increases, there will be a considerably increased loss in the antenna gain. This can be avoided by the offset arrangement of the patch  11  on the ground plane of the regular octagon. That is, the patch  11  give different creepage distances L extending from various edge portions of the patch  11  to the metal base  2  through corresponding edges of the octagon and therefore can exclude a possibility where nearly all of the creepage distances would be λ/4. For this reason, it is possible with the offset arrangement of the patch to minimize the loss in the antenna gain when the GPS receiver is placed on the metal base. 
     FIG. 9 illustrates the antenna gain [dB] with varying distance D from the patch  11  to the metal base  2  for comparison between the embodiment (indicated by round dots) in which the patch  11  is offset from the center of the octagonal ground plane  42  and a case (indicated by square dots) in which the patch is centered on the ground plane. As demonstrated in FIG. 9, a drop in the antenna gain below 0 dB appears over a longer range of the distance D (18 mm&lt;D&lt;42 mm) for the patch centered on the ground plane, while the like drop appears only over a shorter range of the distance D (25 mm&lt;D&lt;32 mm) for the offset patch. This means that the GPS receiver  1  of the present embodiment can have sufficient antenna gain only by spacing the patch away from the metal base by at least 32 mm, in contrast to the antenna with the centered patch where sufficient gain is promised by spacing the patch away from the metal base by as long as 42 mm. Therefore, the GPS receiver  1  of the present embodiment can be made into a low-profile structure while assuring the sufficient antenna gain even placed on the metal base. In practice, the distance D between the patch  11  and the bottom of the radome  100  is selected to be as less as 32 mm which is a critical distance below which a undesired antenna gain loss appears. 
     It is noted in this connection that, although the EMI shield  40  having the octagonal ground plate  42  is utilized in the illustrated embodiment, the present invention is not limited to the use of the octagonally shaped EMI shield and the correspondingly shaped circuit board  20 , and may use the EMI shield  40  and the circuit board of regular polygon having at least four sides or of circular configuration. 
     FIGS. 10 to  12  show a stick-type GPS receiver  1 A in accordance with another preferred embodiment of the present invention. The GPS sensor  1 A is identical in function to the previous embodiment except for the shape of an EMI shield  40 A and the associated components and for an inclusion of a back-up battery  110 . Like parts are designated by like reference numerals with a suffix letter of ‘A’. Components  31 A to  34 A forming the analog circuit  60 A as well as the digital circuit  70 A and a USB connector  80 A are mounted on a double-sided circuit board  20 A of an elongated rectangular configuration. The EMI shield  40 A is also shaped into a rectangular configuration in conformity with the circuit board  20 A and is composed of a top cover  41 A and a bottom cover  51 A. The top cover  41 A has a rectangular flat top plane  42 A which carries thereon a patch  11 A and defines a ground plane for the antenna  10 A. The patch  11 A is supported on a dielectric ceramics  12 A which is secured to one end of the ground plane  42 A by means of an adhesive tape  13 A, and which is connected to the analog circuit by means of a feed pin  14 A depending from the patch  11 A. The analog circuit is formed on one end of the circuit board  20 A at a location immediately below the patch  11 A for easy connection thereto while avoiding an extra connection line which would impede the antenna characteristics. 
     The top cover  41 A covers the entire top mount surface of the circuit board  20 A and is electrically connected to a like ground conductor (not shown) embedded in the circuit board  20 A by means of anchors  44 A depending from a peripheral wall  43 A of the top cover  41 A. The bottom cover  51 A also covers the entire bottom mount surface of the circuit board  20 A and is electrically connected to the top cover  41 A by means of joints  52 A. The top cover  41 A is formed at its one lengthwise end with an extension  49 A which overlaps on a top end of a connector shield  81 A of the USB connector  80 A for electrical interconnection therebetween. The circuit board  20 A and the EMI shield  40 A carrying the antenna are assembled into a single structure together with a radome  100 A composed of a top case  101 A and a bottom case  102 A. 
     The back-up battery  110  is provided to back-up the data, i.e., the positional coordinates computed at the navigation processor  71 A and stored in RAM  72 A, as well as to supply an operating voltage to a real time clock (RTC)  77  included in the digital circuit  70 A to compute the positional coordinates. The battery  110  is retained in a holder  120  which is detachable to the top case  101 A. When the holder  120  is attached to the top case  101 A, a negative electrode of the battery  110  comes into a direct contact with ridges  55  on the top surface  42 A of the EMI shield  40 A, while a positive electrode of the battery comes into electrical contact with a terminal  122  provided on the back of the top case  101 A. The terminal  122  is connected to the digital circuit on the circuit board  20 A through a lead  124 , thus establishing the electric path of feeding the voltage of the battery to the digital circuit. Thus, the battery can be electrically connected to the digital circuit by utilizing the EMI shield also as a conductor of connecting the negative electrode of the battery to the circuit ground.