Abstract:
A pulse generating circuit and related method, for producing extremely narrow pulses for use in monolithic microwave integrated circuits (MMICs) for radar, high-speed sampling, pulse radio and other applications. A sinusoidal input signal is supplied to two nonlinear shock wave generators, which are oppositely biased to produce periodic outputs that are mirror images of each other, one with a very steep rising edge and one with a very steep falling edge. The combined outputs would cancel each other completely but for the introduction of a slight time delay in one of them, which results in a narrow peak in the combined signals.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to pulse generators and, more particularly, to pulse generators that can be easily integrated into monolithic microwave integrated circuits (MMICs). Short duration (i.e., narrow) pulses have practical application in time domain measurement in the terahertz region. For reference, a terahertz (THz) is 10 12  hertz, or a thousand gigahertz (GHz). Short pulses are also needed in radar transmitters, high speed sampling applications, and in impulse radio, an emerging technology that requires very narrow pulses with very short rise and fall times. Commercially available pulse generators are less than adequate for advanced microwave, millimeter-wave, radar, or high speed digital sampling applications. 
     The pulse generator described and claimed in U.S. Pat. No. 6,690,247, entitled “Nonlinear Transmission Line Waveform Generator Having an Input Voltage Matched to the C/V Characteristic of the Transmission Line,” issued in the names of Mark Kintis et al. and assigned to the same assignee as the present application, addressed many of the problems of the prior art but presented implementation difficulties. The disclosure of the referenced patent is incorporated by reference into this description. 
     Nonlinear transmission lines are generally known in the art. Examples of such nonlinear transmission lines are disclosed in: “GaAs Nonlinear Transmission Lines for Picosecond Pulse Generation and Millimeter Wave Sampling”, by Rodwell, et al., IEEE Transactions on Microwave Theory and Techniques, vol. 39, no. 7, July 1991, pages 1194-1204; U.S. Pat. Nos. 5,352,994; 5,352,627; 5,023,574; 5,256,996; 5,789,994; 5,014,018; and 5,157,361, all of which are also incorporated by reference into this description. Such nonlinear transmission lines are used in various applications, such as harmonic generators and in the generation of relatively short duration narrow pulses in the hundreds of picosecond range, which provide increased bandwidth in certain applications, such as ultra wideband radio applications, also known as impulse radios. Ultra wideband radios, which are generally known in the art, utilize relatively short-duration pulses with pulse widths measured in the hundreds of picoseconds, for example. These short-duration pulses are pulse-position modulated, to increase the bandwidth of such radios up to, for example, 5 GHz. There is an ever increasing demand to increase the bandwidth of such impulse radios, as well as that of other microwave devices. The bandwidth of ultra-wideband radios is limited by the current technology in which the shortest duration pulses that can be generated are on the order of hundreds of picoseconds. To meet the demand for higher bandwidth impulse radios and other applications, there is a need to provide a system for generating pulses having pulse widths less than hundreds of picoseconds in duration. 
     Accordingly, there is still much room for improvement in the field of time domain pulse generation for MMIC applications. The present invention is directed to that end. 
     SUMMARY OF THE INVENTION 
     The present invention provides a significant improvement in pulse generation for microwave and millimeter-wave applications, over the technique described in U.S. Pat. No. 6,690,247. Briefly, and in general terms, the pulse generator of the invention comprises first and second nonlinear transmission lines (NLTLs) connected to receive a sinusoidal input signal, and configured to produce first and second output signals, respectively, that are mirror images of each other with respect to a selected amplitude level. The pulse generator of the invention also comprises a time delay element interposed in series with one of the first and second NLTLs, whereby one of the first and second output signals is delayed with respect to the other; and means for combining the first and second output signals into a composite output signal. The first and second output signals substantially cancel each other except for a time duration corresponding to the selected time delay, during which the composite output signal is a pulse. 
     Preferably, the selected time delay element is connected in an input line to one of the NLTLs, since the bandwidth of the input signal is much lower than the bandwidth of either of the output signals. The components of the pulse generator are easily implemented as part of a monolithic microwave integrated circuit (MMIC). 
     More specifically, the first NLTL is negatively biased and the first output signal includes steep falling edges that are negative with respect to ground; while the second NLTL is positively biased and the second output signal includes steep rising edges that a positive with respect to ground. Thus, if the time delay is selected to delay the first output signal with respect to the second, the composite signal includes a steep leading edge derived from the second output signal and a steep falling edge derived from the first output signal. 
     The selected time delay element may be an adjustable phase delay circuit or a transmission line providing the selected delay. 
     In terms of a method, the invention comprises the steps of supplying a first nonlinear transmission line (NLTL) with a sinusoidal input signal; supplying a second nonlinear transmission line (NLTL) with an identical sinusoidal input signal; generating in the first NLTL a first periodic output signal that has a steep falling edge; generating in the second NLTL a second periodic output signal that has a steep rising edge and is a mirror image of the first periodic output signal; delaying one of the first and second output signals with respect to the other; and combining the first and second output signal to produce a composite output signal in which the first and second output signals substantially cancel except in the region of a narrow output pulse. 
     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of pulse generation for microwave and millimeter wave applications implemented in MMIC technology which provides very tight parameter matching of the multitude of diodes making up a given NLTL line. Also, an all MMIC implementation where both NLTL&#39;s are on the same chip would result in the very tight parameter matching for both NLTL&#39;s required to create such precise narrow pulse widths via extremely accurate waveform matching and cancellation. Specifically, where In particular, the invention provides for the generation of extremely narrow pulses that are useful in radar, high-speed sampling, pulse radio and other applications. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a preferred embodiment of the present invention. 
         FIGS. 2A and 2B  are simplified equivalent circuit diagrams of the respective nonlinear transmission lines (NLTLs) depicted in  FIG. 1 . 
         FIGS. 3A and 3B  are graphs of output signals from the respective NLTLs in  FIG. 1 . 
         FIG. 4  shows the graphs of  FIGS. 3A and 3B  plotted together. 
         FIG. 5  is a graph of a single pulse resulting from combining the two graphs in  FIG. 4 , plotted on a magnified time scale. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for purposes of illustration, the present invention is concerned with the generation of narrow pulses in the time domain, for use in high speed sampling applications, radar and other microwave and millimeter-wave technologies. 
     In accordance with the invention, and as will be described in more detail below, narrow pulses are generated by combining the outputs of two oppositely biased nonlinear transmission line (NLTL) shock wave generators, indicated at  10  and  12  in  FIG. 1 . A sinusoidal input signal is provided on input line  14 , which is split into two input lines  14   a  and  14   b , connected to NLTL shock wave generators  10  and  12 , respectively. A delay  16 , which may be simply a transmission line of selected length or a controllable phase shifter, is interposed in one of the input lines  14   a  and  14   b  to effect a slight time displacement between the two output signals that appear on output lines  18   a  and  18   b , respectively, from the NLTL shock wave generators  10  and  12 . The two output lines  18   a  and  18   b  merge into a single output line  18 . 
     As depicted in  FIGS. 2A and 2B , an NLTL shock wave generator may be represented by an equivalent circuit that includes a series string of inductances L and an approximately equal number of shunt variable capacitors (varactors) C v . It will be understood that in a practical realization of the NLTL the inductances L and varactors C v  have impedances that are distributed uniformly along the transmission line, rather than having the discrete values implied by the depiction of  FIGS. 2A and 2B . Such NLTLs can be formed as monolithic microwave integrated circuits (MMICs), for example as disclosed in U.S. Pat. No. 5,014,018, which is incorporated by reference into this description. 
     The parallel NLTL shock wave generators have identical impedance parameters except that the distributed varactors in each are oppositely directed and oppositely biased. More specifically, in the NLTL  10  the cathode side of the varactor diode is connected to ground and, as shown in the graph of  FIG. 3A , the NLTL  10  is negatively biased. In the other NLTL  12 , the anode side of the varactor diode is connected to ground and, as shown in the graph of  FIG. 3B , the NLTL  12  is positively biased. 
     It is well known in the art of NLTLs that the nonlinearity of this type of transmission line arises because a varactor provides a shunt capacitance value that varies with voltage, and this variation results in a transmission line delay that also varies with voltage. The output signal produced has a steep transition (in one direction) between maximum and minimum values, instead of the more gradual transition of the sinusoidal input. The NLTL shock wave generator  10  produces an output with a steep transition on the falling edge of each signal cycle, as shown in  FIG. 2A , while the generator  12  produces an output with a steep transition on the rising edge of each signal cycle, as shown in  FIG. 2B . In each case, the nonlinearity of the generator characteristic renders the other (non-steep) edge of the output signal even more gradual than the slope of the sinusoidal input. Therefore, the output signal profile in each case is approximately triangular in shape, where the output from generator  10  is characterized by an almost vertical falling edge followed by a gradually sloped rising edge, and the output from generator  12  is characterized by an almost vertical rising edge followed by a gradually sloped falling edge. 
     When these two shock wave generator outputs are combined on line  18 , they appear substantially as depicted in  FIG. 4 , where the lower curve represents the output on line  18   a  from generator  10 , and the upper curve represents the output on line  18   b  from generator  12 . If the two outputs were perfectly in phase they would completely cancel each other, since it is apparent from  FIG. 4  that the two curves are mirror images of each other with respect to the zero voltage axis. However, when the delay or phase shifter  16  is selected (or adjusted) to provide a very small phase difference between the two curves in  FIG. 4 , the resulting composite output is a series of very short pulses, one of which is shown in  FIG. 5 . Whether the pulses are positive going or negative going is determined by which of the two waves is leading with respect to the other. In  FIG. 5 , the rising edge of the depicted pulse is derived from steep rising edge of the signal from generator  12 , which leads the other output signal slightly. The steep falling edge of the pulse is derived from the steep falling edge of the signal from generator  10 . Only one pulse is visible in  FIG. 5  because the time scale along the horizontal axis is magnified so much that the adjacent pulses are off-scale to the left and right of the figure. 
     The delay  16  may be a delay line of a length selected to provide a desired relative time delay between the shock wave generators  10  and  12 , resulting in a desired pulse duration on output line  18 . The delay  16  may also be an adjustable phase shifter. Although  FIG. 1  shows the delay  16  interposed in one of the input lines  14   a  and  14   b , it will be appreciated that the desired time difference could also be effected by interposing a delay in one of the output lines  18   a  and  18   b . Placing the delay in an input line is preferred, however, because the sinusoidal input signal has, of course, a much narrower bandwidth than that of the shock wave generator outputs. Creating a true time delay for a wide-bandwidth signal presents significant design difficulties. 
     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of time domain pulse generators capable of producing extremely short pulses. The pulse generator of the invention employs only three basic components to generate pulse outputs from a sinusoidal input. By combining outputs from two oppositely biased NLTL shock wave generators and delaying one output with respect to the other, very narrow pulses are reliably generated. It will also be appreciated that the pulse generator of the invention can be implemented using monolithic microwave integrated circuit (MMIC) technology, and therefore conveniently integrated with other MMIC functions. Although a specific embodiment of the invention has been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.