Abstract:
A pyro sensor is for use in a passive infrared motion detector. The pyro sensor includes at least one passive infrared sensor element. A field effect transistor includes a drain, a gate and a source. The gate is connected to the at least one passive infrared sensor element. A first capacitor interconnects the source and ground. The first capacitor has a value of approximately between 47 picoFarads and 1000 picoFarads. A second capacitor interconnects the source and ground. The second capacitor has a value of approximately between 4.7 picoFarads and 47 picoFarads.

Description:
RELATED APPLICATION 
       [0001]    This application is a nonprovisional of, and claims the benefit of, provisional application 61/514,616, filed Aug. 3, 2011, entitled “RF IMMUNITY IMPROVED PYRO SENSOR”, by applicant William DiPoala, which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The patent relates to the field of motion detection and more particularly to pyro sensors used in passive infrared (PIR) motion detectors. 
         [0004]    2. Description of the Related Art 
         [0005]    In the field of surveillance and security systems, the pyro sensor used in passive infrared (PIR) motion detectors is a critical component that determines the overall performance of the detector. False alarms can be caused by radio frequency interference (RFI) in the PIR motion detector if proper measures are not taken when designing the system. 
         [0006]    As an increasing number of radio frequency (RF) devices proliferate the global environment, higher frequency channels are being opened up and used in order to prevent crosstalk or jamming by other devices operating on the same frequency. These higher frequency channels are moving into the microwave spectrum (greater than 1 GHz). 
         [0007]    The standardized RFI/EMI tests such as the EN6100-4-3 are moving to test the RF immunity of devices at frequencies as high as 6 GHz at 10 V/m. The EN50130-4 standard further defines the requirements for a PIR detector to be immune to frequencies as high as 2.7 GHz. The PIR motion detectors sold today need to pass these test requirements in order to be effective in the marketplace. 
         [0008]      FIG. 1  shows a schematic diagram of a typical conventional pyro sensor  20  disposed within a metal can or housing  22 . Sensor  20  includes an optical filter  24 , and sensor elements  26   a - b  detecting optical energy through filter  24 . A field effect transistor (FET)  28  includes a gate  30 , a drain  32  and a source  34 . A gate resistor  36  interconnects gate  30  and electrical ground. An EMI resistor  38  may be connected to drain  32 . A 100 pF capacitor  40  is connected to source  34  in order to reduce noise caused by RF radiation. 
         [0009]    Pyro sensor  20  and other pyro sensors used today are not capable of providing sufficient electro-magnetic immunity at the higher frequencies as needed. Thus, what is neither disclosed nor suggested by the prior art is a surveillance security system including a PIR motion detector which provides the needed level of RFI immunity. 
       SUMMARY 
       [0010]    The invention is directed to a security system including a pyro sensor having improved immunity to electromagnetic interference (EMI). 
         [0011]    In one aspect, the invention includes a pyro sensor for use in a passive infrared motion detector. The pyro sensor includes at least one passive infrared sensor element. A field effect transistor includes a drain, a gate and a source. The gate is connected to the at least one passive infrared sensor element. A first capacitor interconnects the source and ground. The first capacitor has a value of approximately between 75 picoFarads and 470 picoFarads. A second capacitor interconnects the source and ground. The second capacitor has a value of approximately between 10 picoFarads and 25 picoFarads. 
         [0012]    In another aspect, the invention includes a pyro sensor for use in a passive infrared motion detector. The pyro sensor includes at least one passive infrared sensor element. A field effect transistor includes a drain, a gate and a source. The gate is connected to the sensor element. A first capacitor interconnects the source and ground. The first capacitor has a value of approximately between 82 picoFarads and 330 picoFarads. A second capacitor interconnects the source and ground. The second capacitor has a value of approximately between 10 picoFarads and 20 picoFarads. An electrically conductive trace interconnects the second capacitor and the source. The trace has an impedance of less than 40 Ohms. 
         [0013]    In yet another aspect, the invention includes a pyro sensor for use in a passive infrared motion detector. The pyro sensor includes at least one passive infrared sensor element. A field effect transistor includes a drain, a gate and a source. The gate is connected to the sensor element. A first capacitor interconnects the source and ground. The first capacitor has a value of approximately between 82 picoFarads and 220 picoFarads. A second capacitor interconnects the source and ground. The second capacitor has a value of approximately between 10 picoFarads and 20 picoFarads. An impedance of the first capacitor is at least five times greater than an impedance of the second capacitor at a frequency of 2 GHz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a block diagram of an example pyro sensor; 
           [0016]      FIG. 2  is a block diagram of one embodiment of an example pyro sensor of the present invention; 
           [0017]      FIG. 3  is a pair of plots of impedance as a function of frequency for the two source capacitors of the pyro sensor of  FIG. 2 ; and 
           [0018]      FIG. 4  illustrates a microscopic plan view of the pyro sensor  120  of  FIG. 2 . 
       
    
    
       [0019]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the invention. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
       DETAILED DESCRIPTION 
       [0020]    The embodiments hereinafter disclosed are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following description. Rather the embodiments are chosen and described so that others skilled in the art may utilize its teachings. 
         [0021]    Referring to  FIG. 2 , there is shown one embodiment of a pyro sensor  120  of the invention. Pyro sensor  120  may be disposed within a metal can or housing  122 . Sensor  120  includes an optical filter  124 , and sensor elements  126   a - b  detecting optical energy through filter  124 . A field effect transistor (FET)  128  includes a gate  130 , a drain  132  and a source  134 . A gate resistor  136  interconnects gate  130  and electrical ground. An EMI resistor  138  may be connected to drain  132 . As in the example of  FIG. 1 , a 100 pF capacitor  140  may be connected to source  134 . However, in order to further reduce noise caused by RF radiation (e.g., reduce electro-magnetic interference), a 15 pF capacitor  142  is connected to source pin  134  in parallel with capacitor  140 . 
         [0022]    In one embodiment, both capacitors  140 ,  142  are in 0402 surface mount two-terminal packages. In one embodiment, 100 pF capacitor  140  is in the form of a monolithic ceramic capacitor no. GRM1555C1H101JZ01, and 15 pF capacitor  142  is in the form of a monolithic ceramic capacitor no. GRM1555C1H150JZ01, both marketed by Murata Manufacturing Co., Ltd. 
         [0023]    The 15 pF capacitor  142  may provide a lower impedance than 100 pF capacitor  140  at frequencies above 1 GHz. The lower impedance capacitor  142  provides a better path to direct the RF energy to ground, bypassing the sensitive pyro circuitry (not shown) which may be connected to drain  132  and source  134 . 
         [0024]    The impedances of the 100 pF capacitor  140  and the 15 pF capacitor  142  are plotted in  FIG. 3 . The 15 pF capacitor  142  may provide a bypass impedance of less than five Ohms between 1.5 GHz and 2.5 GHz. The impedance of the 100 pF capacitor  140  connected to source  134  may be over five times larger than the impedance of the 15 pF capacitor  142  between 1.5 GHz and 2.5 GHz. More particularly, as shown in the plot of  FIG. 3 , the impedance of the 100 pF capacitor  140  may be about ten times larger than the impedance of the 15 pF capacitor  142  at 2.0 GHz. As also shown in the plot of  FIG. 3 , the impedance of the two source capacitors  140 ,  142  may be non-linear and/or may have non-ideal characteristics at operating frequencies that are this high (e.g., above about 500 MHz). Because of the non-linearity and/or non-ideal characteristics of the capacitors  140 ,  142  at these high frequencies, it has been found that the advantages of the invention are better achieved by arranging the two capacitors in parallel rather than providing a single 115 pF capacitor. A 115 pF capacitor may be equivalent to the 100 pF and 15 pF parallel capacitor combination only in the linear frequency region wherein the characteristics of the capacitors are more linear and/or ideal. 
         [0025]    It may be beneficial for the 15 pF capacitor  142  to have a low impedance connection to source pin  134  of FET  128  and to ground. This low impedance connection may be achieved by making the connecting copper PCB traces short and wide on both sides of capacitor  142 . It may also be beneficial to provide the ground connection to the can  122  directly below the negative terminal of capacitor  142 . 
         [0026]    The width of the traces used in most known pyro sensors is about 0.01 inch. The impedance of a known 0.01 inch wide trace may be calculated to be about 60 Ohms. In one embodiment, the impedance of the trace of the invention is less than 50 Ohms at a frequency of 1.5 GHz. Because the impedance of the trace is largely resistive, the resistance of the trace is also less than 50 Ohms. The calculated impedance of a 0.05 inch wide and 0.01 inch thick trace of the invention may be about 24 Ohms at a frequency of 1.5 GHz. The resistance of the 0.05 inch wide trace of the invention may also be about 24 Ohms. 
         [0027]    An electro-magnetic interference capacitor  144  may be connected to drain pin  132  as a bypass capacitor. The value of this drain capacitor  144  can be in the range of 10 pF to 470 pF. In a particular embodiment, the value of the drain capacitor is 100 pF. 
         [0028]    Any or all of capacitors  140 ,  142 ,  144  may have low impedance traces and/or low impedance connections on one or both of its two terminals. For example, any or all of these six traces may have a width of about 0.05 inch and a length of less than 0.05 inch. In some embodiments, any or all of these six traces may have a length of about 0.03 inch and a thickness of about 0.01 inch. 
         [0029]      FIG. 4  illustrates a microscopic image of pyro sensor  120  of the invention. The circuit board  145  on which sensor  120  is mounted may include a ground via  146  that is disposed close to capacitors  140 ,  142 . An optional second ground via  148  may also be disposed close to capacitors  140 ,  142 . For example, capacitor  142  may be disposed about 0.03 inch from ground via  146 , and capacitor  140  may be disposed about 0.03 inch from ground via  148 . Ground vias  146 ,  148  may be electrically connected to can  122  on the side of circuit board  145  opposite that shown in  FIG. 4 . Ground via  146  may have conductive epoxy applied thereto on the side of circuit board  145  opposite that shown in  FIG. 4 . 
         [0030]    Capacitors  140 ,  142  share a common low impedance connection  150  to ground vias  146 ,  148 . In one embodiment, a width  152  of connection  150  is about 0.05 inch, and a length  154  of connection  140  is about 0.03 inch. Thus, a ratio of width to length of connection  150  is about 5 to 3, or about 1.67. Similarly, capacitors  140 ,  142  share a common low impedance connection  156  to source via  134   a.  In one embodiment, a width of connection  156  is about 0.05 inch, and a length of connection  156  between capacitor  142  and source via  134   a  is about 0.03 inch. Thus, a ratio of width to length of connection  156  is about 5 to 3, or about 1.67. 
         [0031]    Circuit board  145  may include a ground via  158  that is disposed close to drain capacitor  144 . For example, capacitor  144  may be disposed about 0.02 inch from ground via  158 . Ground via  158  may be electrically connected to can  122  on the side of circuit board  145  opposite that shown in  FIG. 4 . Capacitor  144  has a low impedance connection  160  to ground via  158 . In one embodiment, a width of connection  160  is about 0.03 inch, and a length of connection  160  is about 0.02 inch. Thus, a ratio of width to length of connection  160  is about 3 to 2, or about 1.5. Similarly, capacitor  144  has a low impedance connection  162  to drain via  132   a.  In one embodiment, a width of connection  162  is about 0.05 inch, and a length  164  of connection  162  between capacitor  144  and drain via  132   a  is about 0.05 inch. Thus, a ratio of width to length of connection  162  is about 1.0. 
         [0032]    Although the drain resistor, gate resistor and FET are not shown in  FIG. 4  in order to simplify the illustration, it is to be understood that any or all of these components may be visible from the viewpoint depicted by  FIG. 4 . Further, any or all of these components may be mounted on the same circuit board  145  that other components of  FIG. 4  are mounted on. 
         [0033]    Although the capacitors of the present invention have been described herein as ceramic, it is to be understood that other types of capacitive elements may be used within the scope of the invention. For example, any or all of capacitors  140 ,  142 ,  144  may be Mylar capacitors, polystyrene capacitors, and/or polypropylene film capacitors, for example. 
         [0034]    While this invention has been described as having an exemplary design, the invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.