Abstract:
An improved RF shielding integrity monitoring system is provided for monitoring the shielding integrity of a shielded enclosure that accurately identifies degraded shielding effectiveness and leakage areas in the shielded enclosure. The RF shielding integrity monitoring system performs self-testing and self-diagnostic routines to assure proper system operation prior to monitoring of the shielded enclosure. The RF shielding integrity monitoring system includes a transmitter for transmitting predetermined transmitted by the shielded enclosure for receiving the signals; a receiver separated from the transmitted predetermined signals and for identifying the shielding effectiveness of the shielded enclosure; and a cable connected to the transmitter and the receiver for communicating signals between the transmitter and the reciever.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio frequency (RF) monitoring systems, and more particularly to RF integrity monitoring systems for shielded enclosures. 
     2. Description of the Prior Art 
     Shielded enclosures are used when sensitive equipment or the like must be isolated from interference due to ambient electromagnetic radiation. An enclosure may also be used to confine radiation within the enclosure. When a shielded enclosure is first installed, the RF shielding effectiveness is at its maximum. After installation shielding effectiveness of the enclosure should be periodically tested to identify degradation of the shielded enclosure and RF energy leakage through the enclosure. 
     Integrity monitoring systems are used for testing the shielding performance of an enclosure to attenuate interference. Integrity monitoring systems typically include an RF transmitter with ar antenna on the outside of a shielded enclosure under test and a receiver with an antenna within the enclosure. Transmitted RF signals are detected by the receiver corresponding to the energy transmitted through the enclosure under test. However, extraneous, interference signals can be detected by the receiver to provide erroneous leakage indications for the shielded enclosure. 
     Another disadvantage of many known integrity monitoring systems is the lack of self-testing or self-diagnostics capability so that any hardware problems, such as in the receiver or the transmitter, tend to introduce inaccuracies. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an improved RF shielding integrity monitoring system for monitoring the shielding integrity of a shielded enclosure that overcomes many of the disadvantages of the prior art systems. Other objects are to provide an improved RF shielding integrity monitoring system for monitoring the shielding integrity of a shielded enclosure that effectively and accurately identifies degraded shielding effectiveness and leakage areas in the shielded enclosure; to provide such RF shielding integrity monitoring system capable of both performing self-testing and self-diagnostics routines and identifying an interference condition resulting from extraneous, interference signals to avoid erroneous leakage indications; and to provide such RF shielding integrity monitoring system including a bidirectional communications link between a transmitter and a receiver of the RF shielding integrity monitoring system. 
     In brief, the objects and advantages of the present invention are achieved by a shielding integrity monitoring system for monitoring the shielding integrity of a shielded enclosure including a transmitter for transmitting predetermined signals; a receiver separated from the transmitter by the shielded enclosure for receiving the transmitted predetermined signals and for identifying the shielding effectiveness of the shielded enclosure; and a cable connected to the transmitter and the receiver for communicating signals between the transmitter and the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the embodiment of the invention illustrated in the drawings, wherein: 
     FIG. 1 is a block diagram of an RF shielding integrity monitoring system according to the present invention; 
     FIG. 2 is a block diagram of a transmitter of the RF shielding integrity monitoring system of FIG. 1; and 
     FIG. 3 is a block diagram of a receiver of the RF shielding integrity monitoring system of FIG. 1; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, in FIG. 1 there is illustrated an RF shielding integrity monitoring system generally designated by the reference numeral 10 for monitoring the shielding integrity of a shielded enclosure according to the invention. As its major components, the RF shielding integrity monitoring system 10 includes a transmitter 12 with an associated antenna 14 for transmitting predetermined test signals, a receiver 16 with an associated antenna 18 for receiving the predetermined test signals and for identifying shielding effectiveness and a direct bidirectional communications link or cable 20 connected between the transmitter 12 and the receiver 16. 
     While the system 10 is illustrated in FIG. 1 with the receiver 16 and antenna 18 mounted inside a shielded enclosure 22 being monitored and the transmitter 12 and antenna 14 mounted outside the shielded enclosure 22, it should be understood that the principles of the invention are not limited to this arrangement. Alternatively, the receiver 16 and antenna 18 can be mounted outside with the transmitter 12 and antenna 14 mounted inside the shielded enclosure 22, depending upon the particular application requirements. 
     A pair of optical fibers or duplex fiber optic cable with a radio frequency nonconductive jacket advantageously is used for the bidirectional communication link 20 to avoid introducing RF interference or leakage signals by the link 20. A conventional waveguide assembly 24 is mounted through the shielded enclosure 22 being monitored for receiving the fiber optic communication link 20. The transmit antenna 14 and the receiving antenna 18 are mounted at selected locations to provide generally uniform RF illumination of a door of the shielded enclosure 22 and are positioned to have the same polarity, such as horizontal polarity. 
     In accordance with important features of the present invention, the RF shielding integrity monitoring system 10 performs self-testing and self-diagnostics routines to identify proper operation of all of the component parts of system 10 prior to testing the shielding effectiveness of enclosure 22. During monitoring operation by the system 10, interference signals are identified so that erroneous leakage indications for the shielded enclosure that otherwise would result from extraneous, interference signals are avoided utilizing the direct bidirectional communications link 20 between the transmitter 12 and the receiver 16. A portable leak locater antenna 18A (FIG. 3) is used with the receiver 16 during an alternative monitoring operation by the system 10 to perform leakage field strength measurements in order to identify a leakage point or leakage points in the shielded enclosure. 
     On start-up for testing, the receiver 16 performs self-testing by injecting an RF signal with a test pulse train signal into its antenna 18. Upon satisfactory self-testing, then the receiver 16 provides a start command via the communications link 20 to activate the RF circuitry of the transmitter 12. Transmitter 12 sends an acknowledge signal to the receiver 12 and performs self-diagnostics and then provides status and test pulse train signals to the receiver 16 via the direct bidirectional communications link 20. When a failure results in either the transmitter 12, receiver 16 or fiber optic link 20 of the system 10, then a particular failure indication is displayed. Otherwise, when correct operation of the component parts of system 10 is established, then testing the shielding effectiveness of the shielded enclosure is performed. During the test monitoring operation by the system 10, the transmitter 12 sends clock data signals to the receiver 16 via the communications link 20. Receiver 16 compares the test signals received by the antenna 18 with the clock data signals from link 20 to identify interference signals. 
     Referring to FIG. 2, there is shown a block diagram representation of the transmitter 12. Transmitter 12 transmits predetermined test signals, for example, of the order of 915 MHz at a constant output level, such as 2 watts, nominal. The predetermined test signals are applied to the transmit antenna 14 via an RF oscillator and modulator 26, an intermediate amplifier 28 and a power amplifier 30. The selected frequency of 915 MHz provides a test signal wavelength of about 33 cm. suitable for detecting small RF leakage locations. An amplifier control circuit 32 is operatively associated with the power amplifier 30. 
     A transmitter logic circuit 34 provides power and control signals and detects the operation of the RF oscillator and modulator 26, the intermediate amplifier 28 and the power amplifier 30. The transmitter logic circuit 34 is coupled to the receiver 16 via a fiber optic module 36 connected to the fiber optic cable 20 for receiving command signals from the receiver 16 and for providing status, clock and data signals to the receiver 16. The transmitter logic circuit 34 applies switched power and level control signals to the amplifier control circuit 32. Switched power control signals also are applied to the intermediate amplifier 28 and the RF oscillator and modulator 26 by the transmitter logic circuit 32. Heater power signals for maintaining uniform temperature are applied by the transmitter logic circuit 32 to the RF oscillator and modulator 26 for providing accurate and consistent transmitted test signals. 
     A mode control module 38 coupled to the transmitter logic circuit 32 provides signals indicating user selections for controlling the operational modes of the transmitter 12. Manual operation of a first push button switch of module 38 provides a signal at line 38A to initiate one timed transmit cycle. A second toggle switch of module 38 provides signals at a line 38B indicating user selections for either an automatic cycle AUTO or a locked continuous transmit cycle CONT. In the selected CONT locked continuous transmit cycle mode, the transmitter 12 sequentially repeats the transmitter operations for the automatic cycle. Ordinarily, the mode control toggle switch of module 38 is provided in the AUTO position. During installation of the system 10, the toggle switch is moved to the CONT position to facilitate adjustment of antennae 14 and 18. During testing operation to perform repeated field strength measurements, for example, for identifying a leakage point, the toggle switch is moved to the CONT position. 
     A display 40 driven by the transmitter logic circuit 32 includes a plurality of status indicators for viewing by the user, such as POWER-ON, STANDBY, TRANSMITTER 0K and TRANSMITTER FAIL. A second internal display 42 coupled to the transmitter logic circuit 32 is used for identifying the cause of the TRANSMITTER FAIL status as indicated by display 40. The second internal display 42 includes a second set of status indicators of monitored operations including HIGH TEMPERATURE of the power amplifier 30, NO CLOCK for the transmitter logic 34 or RF oscillator and modulator 26, LOW RF output from the power amplifier 30, HIGH SWR output connection problems to the antenna 14 and OVER-DRIVE of critical RF amplifier voltages. 
     Referring to FIG. 3, there is shown a block diagram representation of the receiver 16. An antenna selecting switch 50 couples received signals from either the receiving antenna 18 or the portable leak locater antenna 18A being used for the particular monitoring operation. Received signals are applied to a receiver 52 via the antenna selecting switch 50, an input and pre-amplifier 54 and a receiving converter 56 that provide bandpass filtering and voltage clamping protection for the receiver 52. A triple conversion superhetrodyne FM receiver with automatic frequency control can be used for the receiver 52. 
     The received signals are applied to a receiver logic circuit 58 coupled to the receiver 52. The receiver logic circuit 58 is coupled to the transmitter 12 via a fiber optic module 60 connected to the fiber optic cable 20 for sending receiver command signals to the transmitter 12 and for receiving transmitter status, clock and data signals from the transmitter 12. The receiver logic circuit 58 is coupled to the receiving converter 56 for providing level control and switched power control signals. Switched power control signals also are applied to the receiver input and pre-amplifier 54 and the receiver 52. Heater power control signals are applied to the receiver 52 by the receiver logic circuit 58. 
     A receiver self-test source 62 with an associated antenna injection port 63 connection to the antenna 18 is operatively connected to the receiver logic circuit 58 for receiving power and data signals. The receiver self-test source 62 applies predetermined test signals to the antenna 18 for hardware self-tests of the receiver 16 prior to shielding integrity testing. 
     A mode control module 64 coupled to the receiver logic circuit 58 provides signals indicating user selections for controlling the operational modes of line power-off or power-on, warm-up, test and reset at a line 64A, 64B, 64C, 64D, respectively. Manual operation of a toggle switch provides the selected power-off or power-on signal at the line 64A. A control signal for the operational modes of warm-up, test and reset is initiated by the manual operation of a corresponding push button switch of module 64. The warm-up operational mode is initiated by the user to begin routine testing. After a RECEIVER READY indication is displayed, the test mode is initiated by the user to clear the display and to start the self-testing routines normally followed by the automatic test cycle. The reset mode is initiated by the user to clear the display. 
     A leakage meter 66 coupled to the receiver logic circuit 58 provides an analog display or meter reading of the detected leakage signal. The leakage meter 66 provides a relative db leakage indication above an ambient threshold level identified upon installation of the system 10. A display 68 operatively driven by the receiver logic circuit 58 includes a plurality of status indicators for viewing by the user, such as TRANSMITTER FAIL, TRANSMITTER OK, FIBER/OPTIC FAIL, FIBER/OPTIC OK, INTERLOCK OPEN, SYSTEM FAIL, RECEIVER READY/RECEIVER OK, POWER-ON, STAND-BY/WARMUP, INTERFERENCE, SHIELD FAIL, SHIELD OK and TEST IN PROGRESS. An audio tone meter 70 coupled to the receiver logic circuit 58 provides a variable audio tone pitch relative to the detected received signal energy db level. The generated variable audio tone pitch is particular useful during repeated field strength measurements for identifying a leakage point so that viewing of the meter 66 is not required. 
     A sensitivity control module 72 coupled to the receiver logic circuit 58 provides signals indicating user adjustments for sensitivity control and for determining a shield failure alarm relative to the selected sensitivity setting. During installation, the user adjusts the sensitivity control to provide a zero meter indication by the leakage meter 66 corresponding to a first threshold ambient level for the shielded enclosure 22. Then a setting is selected for generating the shield failure alarm, such as in a range between 6 db and 30 db, as desired for the shielded enclosure 22. 
     While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.