Abstract:
Apparatus for protecting a device from transients. The apparatus includes a switching network and a transmission line electrically connecting an input to an output. The switching network includes a detector, a switch, and a communication path therebetween. The detector, such as an electrical-to-optical converter, detects a transient at the input and communicates with the switch The switch then actuates to place a low impendence across the output of the transmission line, thereby attenuating the transient. The switching network has a switching time that equals the sum of the times to detect the transient at the input, transmit a signal corresponding to the detection to the switch, and actuate the switch. The input signal travels from the input to the output along the transmission line, which has a propagation delay. The propagation delay is greater than the switching time of the switch network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/754,778, filed Jan. 21, 2013. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under HQ0147-11-C-7654 awarded by the U.S. Missile Defense Agency. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field of Invention 
     This invention pertains to a protection system that attenuates a transient in a signal before the signal is delivered to a driven device that is sensitive to transients. More particularly, this invention pertains to a protection system that generates a detection signal upon detection of a transient in an input signal and the detection signal operates a fast switch that attenuates the transient before it reaches the driven device. 
     2. Description of the Related Art 
     As our understanding of physics increases there has been a concerted effort to move away from mechanical type projectile weapons. Directed energy weapons (DEW) are being investigated for non-projectile weaponry. Generally, directed energy weapons project a beam of energy toward a target, thereby transferring energy from the weapon to the target. Types of directed energy weapons being investigated include those that use fast ultra wideband (UWB) pulses and high power microwave (HPM) signals. 
     When the target of a directed energy weapon is a radar system, the radar system can be damaged or taken out of service from the blast of energy. Recent advances in directed energy weapons require radar systems to implement front door protection against such high power signals. 
     Nonlinear protection elements such as plasma limiters have been successfully employed to protect against electrostatic discharge and near electromagnetic pulse (EMP) transients. But such devices have a finite turn-on time that does not permit them to fully block the first nanosecond or more of transients from some directed energy weapons. Fast ultra wideband pulses and high power microwave signals can inject significant transients into the radar front door within a short window of time. It is desirable to attenuate or block such damaging signals from sensitive electronic equipment. 
     BRIEF SUMMARY 
     According to one embodiment of the present invention, a protection system having an input and an output with a switch network for attenuating or blocking transients from the output is provided. In the protection system, a detection signal is generated upon detection of a transient in an input signal at the input. The detection signal operates a fast switch that attenuates or blocks the transient before the transient reaches the output. In this way, potentially damaging transients at the input are prevented from reaching the output. 
     The protection system includes a detector adjacent the input to the system. The detector senses when the input signal exceeds a threshold value, thereby indicating a transient. The detector is connected to a switch that is adjacent the output of the system. The switch attenuates or blocks the input signal before it reaches the output. In one embodiment the switch puts a low impedance across the output upon actuation by the detector. In another embodiment the switch is in series with the conductor carrying the input signal and creates a high impedance upon actuation by the detector. An electrical transmission line is electrically connected between the detector and the switch. The electrical transmission line has an associated propagation delay that is greater than the operating delay introduced by the switch network. The switch network includes the detector, the connection from the detector to the switch, and the switch. 
     In one embodiment, the detector is an electrical-to-optical (E-O) converter, such as a light emitting diode (LED), and the switch is a photoswitch that is responsive to the optical signal or light from the E-O converter. The E-O converter is optically connected to the photoswitch, such as through an optical fiber. The electrical transmission line has an associated delay that is greater than the operating delay introduced by the E-O converter, the optical connection, and the photoswitch. 
     The protection system protects against both transients and signals that last longer, including those that extend to DC. The protection system is a quasi-passive, solid state electro-optic terminal protection system (EOTPS) that blocks high power transient and extended signals from the front end of sensitive electronic equipment. Such transients include fast ultra wideband (UWB) and high power microwave (HPM) signals. One advantage to the protection system is that it is capable of handling multiple transients. That is, the protection system is configured to dissipate the energy in the transient without being destroyed, thereby leaving the protection system ready to handle a second transient. 
     In one embodiment, the protection system uses power from the incoming transient to drive the detectors, such as laser diodes, that activate the switches, such as silicon photoswitches. The switches short the signal line to ground, thereby attenuating the transient. The protection system is passive because no external power other than the power in the transient is required. In another embodiment, the switch network is excited or externally powered. In this embodiment the detectors and/or switches have external excitation that allows the devices to respond in less time than if the devices were powered solely from the signal being monitored. 
     The delay in the switch network is less than the propagation delay in the electrical path, permitting the switch to become fully conductive before the electrical signal with the transient arrives at the switch. With the switch turned on and conductive, the transient is significantly attenuated before it reaches the output device, which may be another sensor or a low noise amplifier (LNA) of a radar system. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above-mentioned features will become more clearly understood from the following detailed description read together with the drawings in which: 
         FIG. 1  is a block diagram of one embodiment of the terminal protection system. 
         FIG. 2  is a simplified schematic diagram of one embodiment of the protection system. 
         FIG. 3 a    is a graph of the signal over time at a first test point. 
         FIG. 3 b    is a graph of the signal over time at a second test point. 
         FIG. 3 c    is a graph of the signal over time at a third test point. 
         FIG. 4  is a plan view of the protection system of  FIG. 2  that is implemented with a coplanar waveguide. 
         FIG. 5  is a partial plan view of another embodiment of a protection system with a coplanar waveguide. 
     
    
    
     DETAILED DESCRIPTION 
     Apparatus for a protection circuit  100  is disclosed. The terminal protection system  100  attenuates a transient  322  in a signal  312 . The protection system  100  is generally indicated as  100  with particular embodiments and variations shown in the figures and described below having an alphabetic suffix, for example,  100 -A. Other elements are described generically below and are uniquely identified when pertinent to the discussion, for example, the photoswitches  106  are generally indicated as  106  with particular embodiments and variations shown in the figures and described below having a suffix, for example,  106 -A,  106 -B,  106 -A 1 , etc. 
       FIG. 1  illustrates a block diagram of one embodiment of the terminal protection system  100 . A source  102  is connected to the input  112  and provides a signal  312  to the protection system  100 . The signal  312  is generally a low level signal  310  that is subject to transients  322 . For example, the source  102  in one embodiment is the front end of a radar system with the signal  312  including fast ultra wideband (UWB) and high power microwave (HPM) signals as transients. The transient  322  is of the type such as resulting from directed energy weapons (DEW) and/or electrostatic discharge and near electromagnetic pulse (EMP) transients. 
     An output device  110  is connected to the output  114  of the protection system  100 . For example, the output device  110  in one embodiment is a low noise amplifier (LNA) of the radar system. The protection system  100  has a transmission line  118  connecting the input  112  to the output  114 . That transmission line  118  has an electrical propagation delay  108 . In various embodiments, the electrical delay  108  is implemented by a conductor such as a trace  118 -A on a coplanar waveguide or a coaxial cable. The propagation delay of the electrical signal  312  resulting from the length and propagation speed of the conductor  118 -A provides the electrical delay  108 . For copper transmission lines, the propagation speed is generally 0.59 to 0.77 times the speed of light c. The conductor  118  has a length sufficient to provide the requisite delay  108 . 
     The source  102  is connected to the input  112  of the protection system  100 . Connected physically close to the input  112  are detectors or sensors  104 , such as electrical-to-optical (E-O) converters. In various embodiments the detectors  104  are light emitting diodes, such as laser diodes, that are powered by the energy in the transient  322  or are separately excited. The detectors  104  have a fast response time when detecting when the input signal  312 -A exceeds a setpoint value  324 . 
     In one embodiment, one detector  104 -A monitors the negative going signal portion at the input  112 . When the absolute value of the negative going portion of the input signal  312  exceeds a threshold value, a signal  116 -A is generated and transmitted to a switch  106 -A that places a low impedance across the output  114  to ground. Another detector  104 -B monitors the positive going signal portion at the input  112 . When the positive going portion of the input signal  312  exceeds a threshold value, a signal  116 -B is generated and transmitted to a switch  106 -B that places a low impedance across the output  114  to ground. In one embodiment, each detector  104  triggers multiple switches  106  to ensure that the power of the transient  322  is adequately dissipated to ground. In another embodiment, multiple detectors  104  trigger multiple switches  106  to ensure that the power of the transient  322  is adequately dissipated to ground. In these various embodiments, the detectors  104  and switches  106  form a switching network. 
     In another embodiment, the detectors  104  have an output that varies based on the input signal  312 -A intensity. The switches  106  have a setpoint and the switches  106  actuate when the output from the detectors  104  passes the setpoint value. For example, in the embodiment where the detectors  104  are LEDs, the emitted light has a characteristic that varies based on voltage. The switches  106  are configured to actuate when the varying characteristic of the LED light reaches a specified value. 
     The actuation time of the detectors  104 , the transmission time of the detection signal  116 , and the actuation time of the switches  106  is less than the electrical delay  108  connecting the input  112  to the output  114 . In this way, when a transient  322  is detected at the input  112 , the output  114  is attenuated before the transient  322  reaches the output  114 . 
       FIG. 2  illustrates a simplified schematic diagram of one embodiment of a terminal protection system or circuit  100 -A. In the illustrated embodiment, the detectors  104 -A,  104 -B are E-O converters, such as LEDs, each with a optical fiber  206  connecting the detector  104 -A,  104 -B to a corresponding switch  106 -A,  106 -B. The switches  106 -A,  106 -B are photoswitches responsive to the optical signals  116 -A,  116 -B from the detectors  104 -A,  104 -B. The electrical delay  108  is implemented by selecting a length and/or propagation speed for the transmission line  118  between the input  112  and the output  114 . In the illustrated embodiment, the connection between the detectors  104 -A,  104 -B and switches  106 -A,  106 -B operate in the optical domain through the fiber optic connections  206 . In other embodiments, the connection between the detectors  104 -A,  104 -B and switches  106 -A,  106 -B operate in other domains, such as the electrical domain or by way of a direct physical connection such as would be found on semiconductors sharing a common substrate. For the embodiment in which the connection is in the electrical domain, the length of the connection is substantially less than the transmission line  118  such that the propagation delay of the detector to switch connection  206  is substantially less than the electrical delay  108  of the transmission line  118 . In such an embodiment, the detector response time and the switching time, when added to the propagation delay, must be less than or equal to the electrical delay  108  of the transmission line  118  between the detector  104  and the switch  106 . 
     Electrically, the protection system  100 -A has the circuit equivalence of a transmission line. In one embodiment, the protection system  100 -A has a 50 ohm impedance. The E-O converters  104 -A,  104 -B and the photoswitches  106 -A,  106 -B do not present a significant impedance mismatch to the circuit, thereby minimizing the losses in the protection system  100 -A. 
       FIGS. 3 a , 3 b , and 3 c    illustrate graphs showing how the protection system  100  affects the input signal  312  having a transient  322  as the signal  312  travels from the input  112  to the output  114  of the system  100 . The graphs plot signal amplitude  304  over time t  302 . The embodiment of the protection system  100 -A illustrated in  FIG. 2  includes various test points  212 ,  214 ,  216  that correspond to the signals  312 -A,  312 -B,  312 -C illustrated on the graphs of  FIGS. 3 a , 3 b , and 3 c   . Each of the signals  312 -A,  312 -B,  312 -C includes the effects of the transient  322 -A,  322 -B,  322 -C. 
       FIG. 3 a    illustrates a graph of the signal  312 -A over time at the first test point  212 . The first test point  212  is located at the input  112  and provides information on the input signal  312 -A, including any transients  322 -A as received by the protection system  100 . The input signal  312 -A includes a steady state portion  310  and a transient portion  322 -A. Before the start time t 0    306  of the transient  322 -A, the signal  312 -A at the input  112  has a steady state  310  corresponding to the normal signal from the source  102 . The transient  322 -A starts at time t 0    306  to produce the composite input signal  312 -A. The transient  322 -A has a fast rise time, typically on the order of nanoseconds. The waveform of the transient  322 -A varies based on the type and characteristics of the transient  322 -A, plus any changes due to circuit impedance before reaching the input  112  of the protection system  100 . 
     The chart of  FIG. 3 a    also shows the threshold trigger level  324  of the detector  104 . When the amplitude  326  of the transient  322 -A equals or exceeds the threshold trigger level  324 , each of the detectors  104  are triggered to produce a signal  116  sent over the line  206  to a corresponding switch  106 . The threshold trigger level  324  is at a level that the output device  110  is capable of withstanding. That is, the output device  110  has a maximum input level  328  that can be applied to it without causing damage or otherwise adversely affecting the output device  110 . The threshold trigger level  324  is equal to or less than the maximum input level  328  of the output device  110 . In this way the protection system  100  is not triggered for levels of the signal  312 -A that the output device  110  can safely handle. 
     It bears noting that the graphs show amplitude  304 , not polarity. The pair of detectors  104 -A,  104 -B, one for each polarity, have the same threshold trigger level  324  and will trigger the switches  106 -A,  106 -B, respectively, when the absolute value of the signal amplitude  326  reaches the threshold trigger level  324 . 
       FIG. 3 b    illustrates a graph of the amplitude  304  of post-detection signal  312 -B over time t  302  as measured at the second test point  214 , which is on the opposite side of the detectors  104 -A,  104 -B than the input  112 , that is, it is representative of the signal  312  as it enters the transmission line  118 . The post-detection signal  312 -B includes a slightly diminished transient portion  322 -B due in part from the draw by the detectors  104 -A,  104 -B of their operating power from the transient  322 . 
       FIG. 3 c    illustrates a graph of the amplitude  304  of the output signal  312 -C over time t  302  at the third test point  216 , which is located at the output  114 . The output signal  312 -C is the signal that goes to the output device  110 . The output signal  312 -C includes a transient remnant  322 -C that has a maximum amplitude less than the maximum input level  328  of the output device  110 . The reduction in amplitude of the transient  322 -C is due to the switches  106  being triggered by the sensors  104  before the transient  322 -C reaches the switches  106 , thereby resulting in an attenuated signal  312 -C 
     The transient start time t 0    306  is shown at the point  326  where the amplitude of the input signal  312 -A crosses the threshold trigger level  324 . The output signal  312 -C is shifted in the time domain from the input signal  312 -A by the delta  330  between the transient start time t 0    306  and output time t′ 0    308 . This time delay  330  corresponds to the electrical propagation delay  108  of the protection system  100 . For example, the time delay  330  is implemented by a length of transmission line  118  that has sufficient length to produce the electrical delay  108 . In one test the delay  330  was on the order of 17 nanoseconds, which is sufficient to attenuate energy spikes induced from a directed energy weapon. 
     The transient start time t 0    306  at the point  326  is where the detector  104  can first react to or detect the transient  322 -A. The detector  104  has a response time before it provides an output or detected signal  116  indicating that a transient  322 -A has been detected. That detected signal  116  has a propagation time to travel between the detector  104  and the corresponding switch  106 . Then the switch  106  has an actuation time before it causes a low impedance connection to be made between the transmission line  118  and ground. The sum of the detector response time, the detected signal propagation time, and the switch actuation time is the switching time delay  316 . The switch  106  is actuated at switch time t s    314  such that the switch  106  attenuates and/or reflects the input signal to produce the waveform  312 -C illustrated in  FIG. 3 c   . The attenuation and/or reflection occurs because the switching time delay  316  is less than or equal to the delta  330  representative of the electrical propagation delay  108  of the transmission line  118 . In this way the low impedance connection across the output  114  attenuates the input signal  312  such that it has the waveform  312 -C illustrated in  FIG. 3   c.    
       FIG. 4  illustrates a plan view of the terminal protection system  100 -A shown in  FIG. 2  that is implemented with a coplanar waveguide  402 . The coplanar waveguide  402  has a conductor  118  flanked by gaps exposing the dielectric  406  between the conductor  118  and the coplanar ground plane  408 . The conductor  118  terminates at the input  112  at one end and at the output  114  at the other end. In order to keep the length of the connection  206  between the detectors  104  and the switches  106  as short as possible, the input  112  and output  114  are located adjacent to each other. The illustrated conductor  118  has a serpentine layout so that the conductor  118  has a length sufficient to create the electrical delay  108 . The physical length of the conductor  118  forming the illustrated embodiment of the electrical delay  108  is sufficient to create an electrical delay  108  that is greater than the switch network operating time. The switch network operating time includes the time for the detectors  104  to sense the transient  322 -A at the input, the time for propagation of the signal  116  indicating the presence of the transient  322 -A at the detectors  104  to the switches  106 , and the time for the switches  106  to operate to block the transient  322 -C at the output  114 . Having the length of the conductor  118  long enough for a delay  108  greater than the switch network operating time allows for the transient  322  to propagate to the output  114  after the switches  106  are actuated and attenuating the signal  312 -C. 
     In the illustrated embodiment, the portions of the conductor  118  proximate the input  112  and the output  114  are parallel and a short distance apart. One detector  104 -A is positioned over the conductor  118  and an adjacent ground plane  408  with the detector  104 -A leads making electrical contact with the conductor  118  and the adjacent ground plane  408 . Extending from the detector  104 -A is the optical fiber  206  that connects to the switch  106 -A. The switch  106 -A is positioned over the conductor  118  and both adjacent ground plane  408  with the switch  106 -A leads making contact with the conductor  118  and the adjacent ground plane  408 . Adjacent the first detector  104 -A is a second detector  104 -B that is positioned over the conductor  118  and the opposite adjacent ground plane  408 . The detector  104 -B leads make electrical contact with the conductor  118  and the adjacent ground plane  408 . Extending from the detector  104 -B is the fiber  206  that connects to the switch  106 -B. The switch  106 -B is positioned over the conductor  118  and both adjacent ground planes  408  with the switch  106 -B leads making electrical contact with the conductor  118  and the adjacent ground planes  408 . In the illustrated embodiment, the switches  106 -A,  106 -B each provides two paths to ground  408  when actuated by the detection signal  116 -A,  116 -B from the corresponding detectors  104 -A,  104 -B. 
       FIG. 5  illustrates a partial plan view of another embodiment of a protection system  100 -B with a coplanar waveguide  402 ′ having a tapered waveguide structure  506  at the output  114 .  FIG. 5  also illustrates in phantom a pair of switches  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2  for each detector  104 -A,  104 -B. Each pair of switches  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2  corresponds to one of the switches  106 -A,  106 -B shown in  FIG. 4 . Each switch  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2  is electrically connected to the conductor  118 -A and to one of the two ground planes  408  adjacent the conductor  118 -A. 
     The geometry of the tapered structure  506  minimizes the gap between the conductor  118 -A and the ground plane  408  at the switch location. The tapered structure  506  with the switches  106  being connected to the conductor  118 -A and to one of the two adjacent ground planes  408  allows for the use of a smaller, more compact switch  106  with a comparably shorter actuation time. The switches  106  are dependent upon optical energy to actuate. The reduced size of the switch  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2  to accommodate the tapered structure  506  reduces the amount of energy required for actuation of each switch  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2 . In one embodiment, a single detector  104 -A,  104 -B is optically connected to a pair of switches  106 -A 1  &amp; -A 2 ,  106 -B 1  &amp; -B 2 . In another embodiment, each switch  106 -A 1 ,  106 -A 2 ,  106 -B 1 ,  106 -B 2  is connected to a single detector  104 -A,  104 -B. 
     In one prototype, the tapered structure  506  has a 25 micrometer width. During testing, the off-state attenuation with one photoswitch was 2-3 dB, depending upon frequency. The attenuation was primarily due to the line resistance. The tested on-state attenuation with one photoswitch was 22 dB. In the illustrated embodiment, multiple switches increases the attenuation when the switches  106 -A 1 ,  106 -A 2 ,  106 -B 1 ,  106 -B 2  are actuated. 
     The terminal protection system  100  includes various functions. The function of delaying the electrical signal  312 -A is implemented, in one embodiment, by a transmission line having a length and propagation speed such that the electrical propagation time along the transmission line is greater than the switch network time, which is the sum of the detection time of the detector  104 , the propagation delay of the detection signal  116 , and the actuation time of the switch  106 . 
     The function of attenuating a transient  322 -A is implemented, in one embodiment, by at least one switch  106  that shorts the output  114  to ground. In one such embodiment, the switch  106  is a photoswitch that is actuated to a conductive state by a signal  116  from a sensor  104 . The switch  106  shorts the transmission line to ground, thereby causing attenuation and reflection of the transient  322 -A. In another embodiment the switch  106  is in series with the transmission line  118  with a normal low impedance that changes to a high impedance when the switch  106  is actuated, thereby attenuating the transient  322 -A. 
     The function of detecting a transient is implemented, in one embodiment, by a detector  104  connected at the input  112 . In one such embodiment, the detector  104  is a LED that emits light when a sensed voltage threshold  324  is reached. The emitted light is directed into a light tube or optical fiber that is connected to one or more photoswitches  106 . 
     The function of minimizing the time to actuate the switch  106  is implemented, in one embodiment, by positioning the detector  104  proximate the switch  106  with a short connection  206 . In another embodiment, the function of minimizing the time to actuate the switch  106  is implemented by electrically connecting the switch  106  across a tapered structure  506  or similar structure where the distance between the conductor  118 -A and the ground plane  408  proximate the switch  106  is substantially less than the distance away from the switch  106 . In this way the switch size is reduced to accommodate the shorter distance between the conductor  118 -A and the ground plane  408 . 
     The function of attenuating a transient  322  before it reaches the output  114  is implemented, in one embodiment, by the switch network operating time being less than or equal to the electrical propagation delay  108 . The switch network operating time is determined by the detector response time, the detector signal propagation time from the detector  104  to the switch  106 , and the switch  106  actuating time. 
     From the foregoing description, it will be recognized by those skilled in the art that a terminal protection system  100  has been provided. The protection circuit  100  prevents a high voltage transient  322 -A from propagating from a source  102 , such as a front end of a radar system, to an output device  110 , such as a low noise amplifier (LNA) that is susceptible to damage from such high voltage transients  322 -A. The protection circuit  100  has an electrical domain propagation delay  108  that is longer than the delay in sensing the transient  322 -A, the propagation delay of the detection signal  116 , and the actuation time of the device  106  attenuating the transient  322 -B. In this way transients that have a very fast rise time or that are not attenuated by conventional means are prevented from propagating to the output device  110 . 
     While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.