Abstract:
A system is disclosed for generating a rectangular pulse with a transmission line, the pulse having a duration of twice the electrical length of the line and a voltage of up to twice the charge voltage. The system includes a voltage source, a switching means, and an output circuit. The voltage source is for providing a voltage potential to a first conductor of a transmission line. The switching means is for controllable coupling the first conductor of the transmission line to a second conductor of the transmission line at a first end of the transmission line, with the second conductor of the transmission line being coupled to a fixed voltage potential at a second end of the transmission line. The output circuit is coupled to the first conductor of the transmission line for providing an output pulse to a load.

Description:
BACKGROUND 
     The invention generally relates to pulse generating systems, and relates in particular to pulse generating systems for providing high voltage and/or high current rectangular pulses with an electrical transmission line. 
     Many analytical fields such as materials analyses, semiconductor testing, and radar and X-ray investigations require the use of rectangular, high speed, high voltage, high current pulses. Generating such pulses generally requires bulky, heavy and/or complex and costly equipment. While several techniques for generating pulses with transmission line pulse systems are available, they are typically limited in that their output voltage is equal to the charge voltage unless specially engineered lines or complex interconnection systems are employed. For example, a conventional technique for generating a quality pulse is with a transmission line pulse generator that charges a line from a source voltage and then connects the transmission line to a load. In most instances the pulse duration of existing transmission line is equal to or less than the electrical length of the line, t L =l/c where l is the physical length of the line and c is the propagation velocity of the transmission line. Transmission line systems, such as a Blumlein Transmission Line for example, typically either halve the voltage or halve the usable electrical length of the cable. 
     Soviet Patent No. SU 723769 discloses a pulse modulator that includes a high voltage source power source, a charger mechanism, a transmission line and a load. This reference discloses that the duration of the pulse on the load is equal to the electrical length of the line and the output voltage equal to the charge voltage. 
     U.S. Pat. No. 2,769,101 discloses a transmission line based pulse generator that can multiply the pulse output voltage into a load of specific impedance with a plurality of transmission lines. This system requires transmission lines of electrical lengths totaling multiple durations of the output pulse to develop higher voltages. 
     U.S. Pat. No. 5,138,270 discloses a pulse generator using a Blumlein circuit, a transmission line having twice the impedance of Blumlein lines, and a matching resistor to generate a pulse of 2 or 4 times the charging voltage. This system requires lines of differing impedances as well as transmission lines of electrical lengths totaling multiple durations of the output pulse. 
     In many applications where size, weight, and cost are concerns, it is desirable to provide a simple transmission line pulse generator using only half the electrical length of cable as its output pulse duration and is capable of driving an arbitrary load at up to twice the charge voltage. In addition, a system that may operate without extensive control and accurate switching is highly desirable, particularly in extreme environments where high reliability is required. 
     There is a need, therefore, for a transmission line pulse generation system that provides increased output voltage and/or pulse duration, yet is efficient and economical to produce, and is robust in relatively hostile environments. 
     SUMMARY 
     A system is disclosed for generating an output pulse, such as a rectangular pulse. The system includes a voltage source, a transmission line, a switching means, and an output circuit. The voltage source is for providing a voltage potential to a first conductor of a transmission line. The switching means is for controllable coupling the first conductor of the transmission line to a second conductor of the transmission line at a first end of the transmission line, with the second conductor of the transmission line being coupled to a fixed voltage potential at a second end of the transmission line. The output circuit is coupled to the first conductor of the transmission line for providing an output pulse to a load. 
     In various embodiments, the transmission line is a coaxial cable conductor, and the second conductor of the transmission line is coupled to ground at a second end of the transmission line. In further embodiments, the output pulse is provided as a voltage potential having an absolute value of greater than the voltage potential of the voltage source, and in further embodiments the output pulse is provided having a pulse width of greater than the electrical length of the transmission line. 
     In accordance with further embodiments, the invention provides a system for generating an electrical pulse that includes a first and second electrical conductors, a voltage source, switching means, reflection means and damping means. The first electrical conductor has a first electrical length, and the second electrical conductor has first and second ends separated by a second electrical length that is substantially the same as the first electrical length. The voltage source establishes a first voltage on the first electrical conductor throughout its first electrical length. The switching means is for initiating propagation of a voltage step having a polarity at a first end of the second conductor. The reflection means is for causing the polarity of the voltage step on the second electrical conductor to be reversed to a reversed polarity and for causing a the first voltage on the first electrical conductor to be shifted when the voltage step reaches the second end of the second conductor. The damping means is for causing reflections at the first end of the second electrical conductor when the voltage step returns to the first end of the second conductor such that the first electrical conductor remains at the shifted voltage until the voltage step again reaches the second end of the second conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
       The following description may be further understood with reference to the accompanying drawings in which: 
         FIG. 1  shows a diagrammatic illustrative view of a pulse line generation system in accordance with an embodiment of the invention; 
         FIG. 2  shows a diagrammatic illustrative view of a pulse line generation system in accordance with another embodiment of the invention; 
         FIG. 3  shows a diagrammatic graphical representation of the voltages of the shield and the core from time t 0  to t L ; 
         FIG. 4  shows a diagrammatic graphical representation of the voltages of the shield and the core from time t L  to  2   t   L ; 
         FIG. 5A  shows a diagrammatic graphical representation of the voltages of the shield and the core from time  2   t   L  to  3   t   L  for a system as shown in  FIG. 1 ; 
         FIG. 5B  shows a diagrammatic graphical representation of the voltages of the shield and the core from time  2   t   L  to  3   t   L  for a system as shown in  FIG. 2 ; 
         FIG. 6A  shows a diagrammatic graphical representation of the voltages of the shield and the core from time  3   t   L  to  4   t   L  for a system as shown in  FIG. 1 ; 
         FIG. 6B  shows a diagrammatic graphical representation of the voltages of the shield and the core from time  3   t   L  to  4   t   L  for a system as shown in  FIG. 2 ; 
         FIG. 7A  shows a diagrammatic graphical representation of the voltage of a pulse in a system as shown in  FIG. 1 ; 
         FIG. 7B  shows a diagrammatic graphical representation of the voltage of a pulse in a system as shown in  FIG. 2 ; 
         FIG. 8  shows a diagrammatic graphical representation of a decoupled voltage pulse in a system as shown in  FIG. 1 ; and 
         FIG. 9  shows a diagrammatic graphical representation of the current associated with a voltage pulse in a system of the invention. 
     
    
    
     The drawings are shown for illustrative purposes only. 
     DETAILED DESCRIPTION 
     The invention provides a system and method for delivering high voltage pulses of up to twice the charge voltage and a duration of twice the electrical length of the transmission-line with an output impedance equal to that of the transmission line in accordance with an embodiment of the invention. The output voltage V o  is equal to 
     
       
         
           
             
               V 
               o 
             
             = 
             
               
                 - 
                 2 
               
               ⁢ 
               
                 
                   Z 
                   L 
                 
                 
                   
                     Z 
                     C 
                   
                   + 
                   
                     Z 
                     L 
                   
                 
               
               ⁢ 
               
                 V 
                 C 
               
             
           
         
       
     
     where V C  is the charge voltage, Z c  is the characteristic line impedance, and Z L  is the effective load impedance. The system in accordance with a further embodiment includes a length of transmission line with a commonly connected element and one end tied to ground, a switch or semiconductor switching device, a diode, and an output capacitor. 
     A further advantage of the present invention is that the output pulse is offset from the triggering or initiation of the pulse by one electrical length of the line. In instances where the pulse is used for sensitive or delicate measurements, a constant offset from any triggering noise generated to initiate the pulse may be desirable. 
     As shown in  FIG. 1 , a system  10  in accordance with an embodiment of the invention includes a coaxial transmission line having an outer conductor  12  that is coupled to a semiconductor switch  14  (S 1 ) having a trigger node  24 , a voltage source  16  (V C ) via a resistor  18 , and ground via a capacitor  20  (C B ) and the load  30 . An inner conductor  26  of the transmission line  12  is coupled at one end (B) to ground as shown, and is coupled at the other end (A) to the switch  14  (S 1 ), which is coupled in parallel with a reverse diode  28  (D 1 ) to the first end (A) of the outer conductor  12 . A resistor  22  (R B ) may optionally be coupled parallel to the load  30  between the capacitor  20  (C B ) and ground, to bias a capacitive load. 
     In the circuit of  FIG. 1 , current from the voltage source  16  (V C ) flows through the resistor  18  (R C ), charging the outer conductor  12  to voltage Vc relative to the inner conductor  26  which is at ground. The operation of the circuit begins when the semiconductor switch  14  (S 1 ), responsive to a trigger signal at the trigger node  24 , connects the outer conductor  12  to the inner conductor  26 . When the semiconductor switch  14  (S 1 ) closes, a voltage step (V S ) is provided wherein 
               V   S     =         Z   L         Z   C     +     Z   L         ⁢     V   C             
where V C  is the charge voltage, Z c  is the line impedance, and Z L  is the effective load impedance. The voltage step propagates through the inner conductor  26  creating a pulse waveform on the outer conductor  12  that is coupled via the capacitor  20  (C B ) to the load  30 . The reverse coupled diode  28  (D 1 ) captures any pulse reflections induced by the load impedance and prevents current from flowing from the source to the inner conductor  26  when the semiconductor switch  14  (S 1 ) is open. In other embodiments, a stripline over a common plane (which functions as the common element) may be used rather than a coaxial cable.
 
     In accordance with another embodiment of the invention as shown in  FIG. 2 , a system  40  includes a coaxial transmission line having an outer conductor  42  that is coupled to a switch  44  (S 2 ), a voltage source  46  (V c ) via a resistor  48  (R C ), and ground via a capacitor  50  (C B ) and the load  60 . An inner conductor  54  of the transmission line is coupled at one end (B) to ground as shown, and is coupled at the other end (A) to the switch  44  via a resistor  56  (R Z ) and diode  58  (D 2 ) that are connected in parallel between the switch  44  and the first end (A) of the outer conductor  42 . A resistor  52  (R B ) may optionally be coupled parallel to the load  60  between the capacitor  50  (C B ) and ground, to bias a capacitive load 
     When the switch  44  (S 2 ) is open, current from the voltage source  46  (V C ) flows through the resistor  48  (R C ) charging the outer conductor  42  to voltage Vc relative to the inner conductor  54 , which is at ground potential. During operation, the switch  44  (S 2 ) closes and current flows from outer conductor  42  to inner conductor  54  through diode  58  (D 2 ), creating the same voltage step Vs discussed above on the inner conductor  54  the propagation of which creates a pulse waveform on the outer conductor  42  that is coupled via the capacitor  50  (C B ) to the load  60 . The diode  58  (D 2 ) blocks reflections induced by the load, forcing them through terminating resistor  56  (R Z ) having a resistance equal to the characteristic impedance of the line. 
     The embodiments show in  FIGS. 1 and 2  differ in the triggering and dampening technique, but both operate in a similar fashion. In each embodiment, a transmission line of electrical length L is assembled such that one element of the line (e.g., the coaxial shield) is employed in the circuit as discussed above, and one end (B) of the other element (e.g., the inner conductor) is connected to ground. In the case of a coaxial transmission line, greater space and weight savings may be achieved by using an un-jacketed cable since the shield element is commonly connected. Alternatively, a strip line above a common plane can yield a very compact embodiment. 
     A significant aspect of this system is that the output terminal of the transmission line pulser comes from the commonly connected element of the line. In operation, this common element is charged up to a voltage V C  via a charging resistor R C . The pulse sequence is then initiated by shorting one end of the transmission line (A) to the commonly connected element. The device may be operated to produce an output pulse of either polarity by simply switching the charging polarity and the diode elements. In the following descriptions it will be assumed that the device is charged to a positive voltage V C  and produces the negative pulse of amplitude V o  as discussed above: 
               V   o     =       -   2     ⁢       Z   L         Z   C     +     Z   L         ⁢       V   C     .             
Further, for illustrative purposes,  FIGS. 3-8  assume a high impedance load giving a voltage V S  that approximates V C .
 
     In either embodiment, the switching device shorts the common element of the transmission line to end A. Due to the realities of high speed switching and the costs and limitations of semi-conductor and relay devices, alternate methods of switching and terminating the pulse may be achieved. In the embodiment of  FIG. 1  a high-speed semiconductor switch, such as a silicon controlled rectifier (SCR), thyristor, or a very high speed relay device is used to initiate the pulse (S 1 ). This device has the requirement that it must be able to switch off in  2   t   L  where t L  is the electrical length of the line. The switching device does not have to interrupt current, and depending on the overall length of the pulse and desired cost of the device, it may be made of SCRs or other high speed semiconductor devices. 
     In the embodiment of  FIG. 2 , switching is accomplished with a fast-make switch S 2  via diode D 2 . The switch S 2  does not have a requirement of turning off within a specific timeframe. This shorting action creates a positive step of amplitude V S  on the line that propagates from end A to end B. For example,  FIG. 3  shows that while the voltage of the shield (V SHIELD ) is maintained at V C  (as shown at  70 ) a voltage pulse travels along the core (V CORE ) as shown at  72  for time t 0 ≦t≦t L  where t L  is the time for the pulse to travel the electrical length of the transmission line (A to B). 
     After time t L  the voltage step reaches end B which is un-terminated, giving a reflection coefficient of 1. Normally, the step wave would be reflected back on itself. If this were the case, the voltage of the core V CORE  would have a wave reflecting back at 2V S  or a step of amplitude V S −V C  over the shield, which would remain at V C . Since the end B is tied to ground however, it not possible for end B to deviate from ground potential, yet the reflected wave must have a positive going step of 2V S . In order to satisfy these conditions, at the reflection of the wave V SHIELD  and V CORE  are both instantaneously forced down by 2V S  so that end B remains at ground potential.  FIG. 4  shows that while the voltage pulse travels back along the core at −V S  as shown at  74  for time t L &lt;t&lt; 2   t   L  the voltage of the shield (V SHIELD ) is driven down by −2V S  as shown at  76 . When the positive going step from −V S  to 0 propagates all the way back to end A at time  2   t   L  it may be terminated or reflected as discussed below. 
     In the embodiment of  FIG. 1 , the returning wave will encounter the conducting diode (D 1 ) creating a short and giving a reflection coefficient of −1. Thus, the reflected wave is a negative going step from 0 to −V S  propagating again toward end B as shown at  80  in  FIG. 5A . The voltage of the shield V SHIELD  is then V C −2V S  as shown at  82 . During this time, the current into the line passes through D 1  as shown at  116  in  FIG. 9 , and the switch element S 1  can transition to the off position. 
     In the embodiment shown in  FIG. 2 , the wave is terminated by the resistor R Z  having a resistance equal to the impedance of the line. Upon termination of the wave, however, there still exists a potential difference between the shield (V SHIELD ) and the core (V CORE ). Thus, a new step wave begins as shown at  84  in  FIG. 5B , only this time, the diode D 2  is reverse biased and the current flows from the shield to the core via R Z . The amplitude of this new wave is now halved as the line is charging in series with R Z , doubling the charging impedance. This −V S /2 step wave propagates toward end B as shown at  84  in  FIG. 5B , and the voltage of the shield (V SHIELD ) is at V C −2V S  as shown at  86  in  FIG. 5B . 
     When the returning wave encounters end B, it is reflected in a fashion similar to that shown in  FIG. 4 , only this time the polarity is reversed to positive. In the embodiment of  FIG. 1 , this reflection forces the shield up by 2V S , returning it to V C  as shown at  90  in  FIG. 6A , and sends a negative going step to 0 volts propagating back to end A as shown at  92 . In the alternate embodiment, the shield potential is only forced up to 0 volts (2×(V S /2) moves it up only +V S  volts) and a negative going step of 0 volts propagates toward end A as shown at  94  in  FIG. 6B , while the voltage of the shield is at zero volts as shown at  96 . 
     When the negative step wave reaches end A in the embodiment of  FIG. 1 , it flows to the shield through D 1 . Normally the short circuit produced by D 1  would produce a pulse of −1 times the negative going pulse, a positive reflection. However, as the diode D 1  blocks positive current, and the switch S 1  has shut off, the cable core cannot be charged and at time  4   t   l  the operation of the system has ended producing the waveform on the shield that is show at  100  in  FIG. 7A . 
     In the embodiment of  FIG. 2 , when the negative going V S /2 step reaches the resistor R Z  it is terminated by the characteristic impedance of the transmission line and leaves a voltage differential of 0 between the shield and the core. At this time  4   t   l , the pulse event has ended and produces the shield waveform shown at  102  in  FIG. 7B . 
     Finally, in either embodiment, the output pulse may be decoupled from the +V C  biased shield with capacitor C B , giving a mono-polar negative going voltage pulse of −2V S  as shown at  110  in  FIG. 8 . The current into end A of the transmission line is shown at  112  in  FIG. 9 , and includes a first portion  114  where positive current passes through the switching diode S 1 , followed by a second portion  116  where negative current passes through the diode D 1 . The value of the capacitor C B  will depend on the load being driven and the duration of the pulse and must be of sufficient size to convey the pulse signal with minimal distortion. C B  can be varied to meet the desired quality of output pulse, but in general, C B  will be on the order of: 
               C   B     =     10   ⁢     t   L     ⁢       Z   L         (       Z   L     +     Z   C       )     2               
where C B  is in Farads, t L  is the electrical length of the line in seconds, Z C  is the line impedance, and Z L  is the effective load impedance in ohms. The resistor R B  is chosen to be sufficient to adequately bias a capacitive load, but not unnecessarily load the system as R B  factors into the effective load impedance Z L , effecting V S  and C B .
 
     In accordance with various embodiments, therefore, the invention provides a transmission line pulse generator that uses a commonly connected shield, plane, or line element as the output terminal. In further embodiments, the system provides that one end of the other element is directly grounded, and that the line uses reflections off of a grounded element to shift the voltage of the whole system. In accordance with further embodiments, the invention provides a transmission line pulse generator that uses a capacitor to decouple a bi-polar signal from above terminal. In accordance with still further embodiments, the invention provides a transmission line pulse generator that is switched by switching the common terminal to the core or other line element, and a transmission line pulse generator that uses a diode with the switch element above to bloc oscillations, and further uses terminating resistor in parallel with diode. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.