Abstract:
An apparatus for generating a monocycle comprises an input signal source ( 76 ) for providing an input signal, and a step recovery diode (SRD) ( 80 ) for receiving the input signal and producing an impulse. A shunt inductor ( 102 ) is provided to act as a first differentiator and a capacitor ( 92 ) connected in series to the output of the step recovery diode acts as a second differentiator. The first and second differentiators are arranged to double differentiate the impulse to produce a monocycle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to apparatus and methods for generating a monocycle, for use, for example, in Ultra Wideband (UWB) systems. 
     BACKGROUND OF THE INVENTION 
     Ultra Wideband (UWB) technology, which is useful for both communication and sensing applications, is based on very short pulses and time domain signal processing. A very commonly used pulse in UWB systems is the monocycle and as the monocycle&#39;s width determines the bandwidth, a narrow pulse width is necessary for producing an ultra wideband signal. 
     There are several methods of generating pulses and devices used for pulse generation include, for example, tunnel diodes, avalanche transistors, and step recovery diodes (SRDs). In Ultra Wideband (UWB) applications, each pulse may represent a symbol. In a typical UWB application, the pulses are followed by a silence period (a space). The characteristics of the pulse are changed to represent the data. 
       FIG. 1  shows conventional pulse position modulation where the position of the pulse is either advanced or delayed from its mean position to represent a symbol.  FIG. 2  shows conventional bi-phase modulation of the pulse to represent the symbol. In  FIGS. 1 and 2 , the distance between the peaks of the waveforms represents the pulse repetition period. 
     For high data rate applications, it is imperative that the pulse width is low to permit more pulses to be transmitted in a given period. If only one cycle of a pulse is generated, the energy may be spread over a wide frequency band. Also, the data rate may be improved as the silence period is larger and so more pulses may be transmitted, for a given duty cycle, by multiplexing other channels. 
     One conventional way of generating very narrow pulses is to use Step Recovery Diodes (SRDs). 
     Although there are many fast square wave pulse generators commercially available, there are few high speed monocycle generators. 
     Monocycles may be generated by twice differentiating the rising edge and falling edge of square pulses using differentiators or Impulse Forming Networks. This is described in the Impulse Forming Networks Data Sheet of Picosecond Pulse Labs. This document describes the use of the differentiation of fast rise time signals to generate pulses. Differentiation of the leading edge produces a positive impulse and differentiation of a trailing edge produces a negative impulse. One more differentiation produces a monocycle. Whilst passive resistor and capacitor elements may be used for the differentiation, the amplitude and the pulse width of the resultant monocycle depends, to a large extent, on the rise time and the fall time of the signal. 
     There are a number of further problems with this approach. Firstly, circuits for generating signals with fast rising edges with rise times of the order of tens of picoseconds are needed and such circuits or commercial instruments are generally expensive and not economical for low cost applications. Secondly, for every monocycle generated by the rising edge, a 180 degrees phase shifted monocycle would be generated by the falling edge. This reduces the flexibility of this approach. Thus the generation of sub-nanosecond monocycle pulses with pulse repetition rates of up to 1 GHz using low cost circuitry is very desirable. Most conventional monocycle generators use lumped elements instead of distributed elements and thus are more expensive and less repeatable due to component tolerances. 
     A number of alternative conventional monocycle generators use several active devices in the circuit. For example, in the system described in the document by Jeong Soo Lee, Cam Nguyen and Tom Scullion entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, Step Recovery Diodes (SRDs) are used together with Schottky diodes for generating very narrow pulses. The Schottky diode is included to overcome the ringing effect which tends otherwise to be exhibited as narrower monocycles and higher pulse repetition rates are attempted in systems using SRD circuits for generating sub-nanosecond monocycles. 
     The method described in the document by Jeong Soo Lee, Cam Nguyen and Tom Scullion in the document entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, combines two Gaussian pulses to produce a monocycle. The two Gaussian pulses are 180 degrees out of phase and have a time delay between them. 
       FIG. 3  shows the circuit for generating a monocycle according to the above mentioned publication by Jeong Soo Lee, Cam Nguyen and Tom Scullion. The circuit is driven by a local oscillator  1  which supplies 10 MHz square wave signal to the anode of an SRD diode  2 . The cathode of the SRD diode is connected to a 50 Ohm short circuited transmission line  3  and to the anode of a Schottky diode  4 . The cathode of the Schottky diode  4  is connected to a capacitor  6  and to a resistor  8 . The resistor  8  is earthed. The capacitor  6  is connected to two further transmission lines  10 ,  12 , one of which is terminated  12  and the other of which is short circuited  10 . 
     This method has the disadvantage of wider pulse width, as the width of the monocycle is twice the impulse width. Furthermore, the use of Schottky diodes to limit the ringing effect adds to the cost of the pulse generator. 
     The document by Jeong Soo Lee, Cam Nguyen and Tom Scullion in the document entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, also describes a pulse-to-monocycle converter. This differs from the circuit described above in that the SRD  2  is omitted, together with the short circuited transmission line  3 . However, the converter requires a narrow pulse to drive it instead of a square wave and it is not itself a pulse generator. 
     In another prior art document, entitled “A New Ultra-Wideband, Ultra-Short Monocycle Pulse Generator With Reduced Ringing”, Jeongwoo Han and Cam Nguyen, June 2002, IEEE Microwave And Wireless Components Letters, Vol. 12, No. 6, pp 206-208, a system is described and which is illustrated in  FIG. 4 . The circuit includes a square wave generator  14  which is connected to the anode of an SRD  16 . The cathode of the SRD is connected to a short circuited transmission line  18  and also to the anode of a Schottky diode  20 . The cathode of the Schottky diode is connected to a terminated transmission line  22  and to a capacitor  24 . The output of the capacitor  24  is connected to the cathode of a further Schottky diode  26 , the anode of which is earthed. The output of the capacitor  24  is also connected to a resistor  28  and to a further capacitor  30 , the output of which is earthed by a further resistor  32 . The output of the resistor  28  is connected, via a further capacitor  34 , to ground. A voltage source  36  is connected across the capacitor  34 . The SRD  16  produces a Gaussian pulse and the resistor  32  and capacitor  30  form a high pass filter which acts as a differentiator to convert the Gaussian pulse into a monocycle. The width of the monocycle formed after differentiation of the Gaussian pulse is almost the same as that of the pulse itself. The two Schottky diodes  20  and  26  act to reduce the ringing effect. The main disadvantage associated with this system is the use of the Schottky diodes, which adds to the cost of the system. 
     In Jeong Soo Lee and Cam Nguyen, “Novel Low-cost Ultra-Wideband, Ultra-Short-Pulse Transmitter with MESFET Impulse-Shaping Circuitry for Reduced Distortion and Improved Pulse Repetition Rate”, May 2001, IEEE Microwave And Wireless Components Letters, Vol. 11, No. 5, pp 208-210, a system is described which includes, as shown in  FIG. 5 , a generator  37  connected to the cathode of an SRD  38 , the anode of which is connected to a short circuited transmission line  40 . The anode of the SRD  38  is also connected to an earthed resistor  42  and to the gate of a MESFET  44 . The source of the MESFET  44  is earthed. The drain of the MESFET  44  is connected to the anode of a Schottky diode  46 . The cathode of the Schottky diode  46  is connected to an earthed resistor  48  and also to a capacitor  50 . The output of the capacitor  50  is connected to a short circuited transmission line  52  and to the input of an MMIC amplifier  54 . The output of the MMIC amplifier  54  is terminated in a resistor  56  which is connected to ground. The MESFET  44  is used as an impulse-shaping network and it enables the circuit to achieve higher pulse repetition frequencies of up to several hundreds of mega Hertz. However, the use of the MESFET  44 , and the Schottky diode  46  add to the cost of the system. 
     U.S. Pat. No. 4,442,362 describes a short pulse generator using an SRD. A plurality of capacitors are charged in parallel and then connected in series by a plurality of avalanche transistors to obtain a voltage which is substantially equal to the sum of the capacitor voltages when charged. The series coupled capacitors are then coupled, via an output avalanche transistor, to a differentiator which produces a monocycle pulse. This method can generate high peak amplitude pulses. However, the use of the avalanche transistors adds to the cost of the system making it too expensive for low cost systems. Also, the use of avalanche transistors limits the pulse repetition rate. 
     U.S. Pat. No. 3,622,808 describes a pulse shaping circuit for producing high frequency pulses using two step recovery diodes and other lumped components. The circuit is shown in  FIG. 6 . A signal source  58 , for example, a sine wave, is connected to an inductance  60 , the other end of which is connected via a resistor  62  to a voltage source (not shown). The signal source  58  is also connected to the cathode of an SRD  64 . The anode of the SRD  64  is connected via a further inductor  66  to ground. The anode of the SRD  64  is also connected to the cathode of a further SRD  68  and to the output  70  of the system. The anode of the further SRD  68  is connected via a capacitor  72  to ground and via a resistor  74  to a further power supply (not shown). This system produces narrow pulses at high frequency but does not itself produce a monocycle. The main disadvantage of this system is the high cost of the system. 
     Thus, there is a need for a low cost monocycle generator preferably capable of generating sub-nanosecond monocycles with pulse repetition frequencies in excess of 1 GHz. 
     SUMMARY OF THE INVENTION 
     In general terms, the present invention proposes an apparatus and method for generating a monocycle comprising an SRD together with elements for pulse generation, the impulse generated being double differentiated to generate the monocycle. This is particularly advantageous as it makes the apparatus simple and cheap to use and easily reproduceable. 
     Furthermore, the methods of the present invention are easily performed and the apparatus embodying the present invention is easily created. 
     According to a first aspect of the present invention there is provided an apparatus for generating a monocycle comprising:
         an input signal source for providing an input signal;   a step recovery diode (SRD) to receive said input signal and produce an impulse, said step recovery diode having an input and an output; and   one or more differentiators arranged to double differentiate said impulse to produce a monocycle.       

     According to a second aspect of the present invention there is provided an Ultra Wideband system comprising the apparatus defined above. 
     According to a third aspect of the present invention there is provided a system for producing multi-band signals comprising the apparatus defined above, the apparatus having an output, the system further comprising one or more band pass filters having associated inputs and outputs, wherein the output of the apparatus is connected to the inputs of said one or more band pass filters, said system further comprising one or more modulators, each modulator having an associated output, said one or more modulators being arranged to modulate the outputs of the band pass filters, and said one or more modulators being arranged such that said outputs of said one or more modulators are combined to produce a multi-band ultra-wide band signal. 
     According to a fourth aspect of the present invention there is provided a method for generating a monocycle comprising:
         providing an input signal from an input signal source to a step recovery diode;   producing an impulse using said step recovery diode;   differentiating said impulse twice to produce a monocycle.       

     According to a fifth aspect of the present invention there is provided a method for producing multi-band signals comprising:
         (a) generating a monocycle by:
           (i) providing an input signal from an input signal source to a step recovery diode;   (ii) producing an impulse using said step recovery diode; and   (iii) differentiating said impulse twice to produce a monocycle;   
           (b) applying said monocycle as an input to one or more band pass filters, said one or more band pass filters having one or more outputs;   (c) modulating said one or more outputs of said one or more band pass filters using one or more modulators to produce one or more modulated output signals; and   (d) combining said one or more modulated output signals to produce a multi-band ultra-wide band signal.       

     Preferred embodiments of the invention provide a very low cost solution for generating sub-nanosecond monocycles with pulse repetition frequencies in excess of around 1 GHz. 
     Preferred embodiments of the invention do not require expensive circuitry for generating fast rise/fall time pulses. Furthermore, as the component count in preferred embodiments of the invention is lower than in conventional monocycle generators, and preferably only a single active element (an SRD) is required, the apparatus embodying the invention is economical to use and produce. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the invention will now be described, for the sake of illustration only, with reference to the following Figures in which: 
         FIG. 1  is a waveform illustrating conventional pulse position modulation where the position of the pulse is either advanced or delayed from its mean position to represent a symbol; 
         FIG. 2  is a waveform illustrating a conventional bi-phase modulation of a pulse to represent a symbol; 
         FIG. 3  is a circuit diagram of a conventional system for producing monocycles; 
         FIG. 4  is a circuit diagram an alternative conventional system for producing monocycles; 
         FIG. 5  is a circuit diagram of a further alternative conventional system for producing monocycles; 
         FIG. 6  is a circuit diagram of a conventional pulse generator; 
         FIG. 7  is a circuit diagram of a system for producing monocycles according to a first embodiment of the present invention; 
         FIG. 8   a  is a waveform produced by the SRD of the circuit shown in  FIG. 7 ; 
         FIG. 8   b  is a simulated waveform showing output pulses produced by the circuit of  FIG. 7 ; 
         FIG. 9  is a measured waveform showing output pulses produced by the circuit of  FIG. 7 ; 
         FIG. 10  is a circuit diagram of a system for producing monocycles according to a second embodiment of the present invention; 
         FIG. 11  is a simulated waveform showing output pulses produced by the circuit of  FIG. 10 ; 
         FIG. 12  is a measured waveform showing output pulses produced by the circuit of  FIG. 10 ; 
         FIG. 13  is a circuit diagram of a system for producing monocycles according to a third embodiment of the present invention; 
         FIG. 14  is a waveform produced by the SRD in the circuit shown in  FIG. 13 ; 
         FIG. 15  is a simulated waveform showing output pulses produced by the circuit of  FIG. 13 ; 
         FIG. 16  is a measured waveform showing output pulses produced by the circuit of  FIG. 13 ; 
         FIG. 17  is a circuit diagram of a system for producing monocycles according to a fourth embodiment of the present invention; 
         FIG. 18  is a simulated waveform showing output pulses produced by the circuit of  FIG. 17 ; 
         FIG. 19  is a circuit diagram of a system for producing monocycles according to a fifth embodiment of the present invention; 
         FIG. 20  is a simulated waveform showing output pulses produced by the circuit of  FIG. 19 ; 
         FIG. 21  is a circuit diagram of a system for producing monocycles according to a sixth embodiment of the present invention; 
         FIG. 22  is a simulated waveform showing output pulses produced by the circuit of  FIG. 21 ; and 
         FIG. 23  is a schematic circuit diagram of a system for producing monocycles according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1 to 6  show conventional circuits for producing monocycles and/or pulses and associated waveforms. These circuits have been described above in the Background of The Invention section. 
     The methods and devices which illustrate preferred embodiments of the invention will be explained with reference to  FIGS. 7 to 23 . 
     Preferred embodiments of the invention relate to the generation of narrow monocycles using step recovery diodes (SRDs). These monocycles are suitable for Ultra Wideband applications. Preferably, the embodiments of the invention make use of the reverse recovery phenomenon of the SRD to generate fast transitions, and preferably use distributed microstrip elements to generate very narrow monocycle pulses from these transitions. 
     The SRD has the ability to store charge and to change impedance levels very rapidly. During the forward biased condition, the SRD conducts and stores the charge. When the biasing changes from the forward biased condition to the reverse biased condition, the SRD conducts for a very short duration until the stored charge is removed, after which the diode no longer conducts. This transition from the conducting to the non-conducting state is extremely fast, in the range of a few tens of picoseconds. 
       FIG. 7  is a schematic circuit diagram of a system according to a first preferred embodiment for generating narrow monocycles. The circuit makes use of resistive matching between an SRD and a pulse forming network comprising distributed Microstrip elements to reduce the ringing. An input signal source  76 , which may be any form of bi-polar signal, for example a sine wave, a square wave or a pulse, is connected to a first transmission line  78 , the output of which is connected to the anode of an SRD  80 . The cathode of the SRD  80  is connected to a resistor  88  which is connected to ground. The cathode of the SRD  80  is also connected to a distributed capacitor  92 , the output of which is connected to a short circuited transmission line  96  in the form of a distributed inductor and to a resistor  100 . The distributed inductor  96  is also connected to ground. 
     In the circuit of  FIG. 7 , the SRD  80  generates a fast transition when the input signal changes the biasing of the SRD  80  from forward to reverse. The resistor  88  is used to provide resistive matching and helps to reduce ringing. 
     The resistor  88  also provides a DC path for the SRD  80 . The short circuited transmission line  96  is used as an inductor and this transmission line and the distributed capacitor  92  act as differentiators. The resistor  100  represents the load resistance. 
     The output of the SRD  80  is fed to the pulse-forming network comprising the transmission line  96  and the capacitor  92  which each act as a differentiator to generate impulses and monocycles. 
     The output voltage of a differentiator is given by:
 
V out=T   d V in   /dt  
 
where
 
V out =the output of a differentiator
 
V in =the input voltage, and
 
T=the time derivative coefficient
 
     Thus, the differentiators convert the fast transition of the SRD output into a pulse. The rest of the SRD output after differentiation becomes negligible. 
     The system illustrated in  FIG. 7  may generate highly symmetrical monocycles of widths less than around 300 ps with negligible ringing. 
     The waveform at the output of the SRD  80  in the circuit of  FIG. 7  is shown in  FIG. 8   a , the impulse being formed at a point A as shown in  FIG. 8   a.    
     The circuit shown in  FIG. 7  has been simulated using Agilent Technologies&#39; Advanced Design Systems (ADS) and the pulse obtained after simulation is shown in  FIG. 8   b . The circuit was also fabricated and tested and the measured pulse is shown in  FIG. 9 . The monocycle pulse generator of  FIG. 7  was fabricated on 32 mil Duroid substrate with a dielectric constant of 3.38. The SRD  80  used was MP4023 from M/s Mpulse Microwave. The measurement was made using a 50 GHz Digital Sampling Oscilloscope. It will be seen that the measured pulse obtained from the system of  FIG. 7  has good symmetry on the positive and negative parts and has a width of 260 ps. The amount of ringing is very low and is acceptable for most practical systems. The pulse repetition rate of the monocycle shown in  FIG. 9  is 250 MHz. It was found that the pulse generator is capable of generating monocycles at a repetition frequency in the range of around 10 MHz to around 1 GHz. 
       FIG. 10  is a schematic circuit diagram of a system according to a second preferred embodiment for generating narrow monocycles. The circuit of  FIG. 10  differs from the circuit shown in  FIG. 7  in that it omits the lumped passive resistor  88  and has a single SRD and distributed Microstrip elements. 
     The elements of the circuit illustrated in  FIG. 10  which correspond exactly to elements in the circuit shown in  FIG. 7  are allotted the same reference numerals. 
     The circuit of  FIG. 10  comprises a signal source  76  connected to a first transmission line  78 . The output of the first transmission line  78  is connected to the anode of an SRD  80  and the cathode of the SRD  80  is connected to a first terminal of a second transmission line  102  comprising a distributed inductor which is short circuited. The second terminal of the second transmission line  102  is connected to ground and acts as a DC return for the SRD  80 . The cathode of the SRD  80  is also connected to a distributed capacitor  92  and the output of the capacitor  92  is terminated in a resistor  100 . The other terminal of the resistor  100  is connected to ground. 
     In the circuit of  FIG. 10 , the monocycle generation begins with an impulse that is first formed by the SRD  80 . This impulse is differentiated once by the second transmission line  102  which acts as a shunt inductor and the resultant pulse is differentiated again by the distributed capacitor  92 . The waveform seen by the load (resistor  100 ) is thus a monocycle. 
     The circuit of  FIG. 10  was simulated using Agilent Technologies&#39; ADS. The simulated result is shown in  FIG. 11 . The simulated pulse width was about 250 ps and the pulse repetition frequency was 250 MHz. However, the pulse repetition frequency may be increased to higher frequencies without affecting the performance. By changing the length of the second transmission line  102  which is acting as a shunt inductor, the inductance may be varied to adjust the shape of the monocycle. Thus, in the circuits of  FIGS. 7 and 10 , at least one transmission line is used as an inductor rather than a delay line, which is in contrast to prior art systems. 
     The circuit of  FIG. 10  was also fabricated on 32 mil Duroid substrate with a dielectric constant of 3.38. The measurement result is shown in  FIG. 12 . 
     The pulse repetition frequency was 250 MHz and the measured pulse width was about 290 ps. It was found that the pulse generator  76  of  FIG. 10  is capable of generating monocycles at a repetition frequency in excess of 1 GHz. 
     A comparison of the measured results shown in  FIGS. 12 and 9  (which relate to the circuits of  FIGS. 10 and 7  respectively), shows that there is a little additional ringing in the circuit of  FIG. 10  due to the lack of a matching resistive element which is present in the circuit of  FIG. 7 . This leads to some degradation in performance. However, the circuit of  FIG. 10  is a lower cost alternative to the circuit of  FIG. 7  due to the use of purely distributed elements and may be used when the system requirements allow for a reduction in cost to be traded-off for a small amount of additional ringing. Despite the lack of resistive matching, the generated monocycle from the circuit of  FIG. 10  is comparable to that of the circuit of  FIG. 7 . 
     Thus, the circuits of  FIGS. 7 and 10  provide two low cost, high performance circuits for monocycle generation. The circuit of  FIG. 7  provides very good performance whilst using two lumped elements in the circuit, namely an SRD and a shunt resistor. The circuit of  FIG. 10  allows for an even lower cost implementation by using only one lumped element, an SRD, in return for a very small sacrifice in performance. 
     The circuits of  FIGS. 7 and 10  may be used to achieve pulse repetition frequencies in excess of 1 GHz. The circuit of  FIG. 7  may enable the generation of highly symmetrical monocycles of widths less than around 300 ps with negligible ringing. The circuit of  FIG. 10  sacrifices a very small amount of performance in return for a cut in the cost of fabrication. The use of purely distributed components in the circuit of  FIG. 10  also increases the repeatability in the performance of the circuit. This will be a major advantage in mass production. 
     Sub-nanosecond pulse width monocycle generators with the kind of high pulse repetition frequencies demonstrated by preferred embodiments of the invention are currently not available commercially. Furthermore, preferred embodiments of the invention may be fabricated using very low cost components. Also, preferred embodiments of the invention preferably do not make use of any other active device to reduce the ringing, unlike conventional systems. 
       FIG. 13  is a schematic circuit diagram of a system according to a further preferred embodiment of the present invention. The circuit of  FIG. 13  differs from the circuit of  FIG. 7  in that the distributed capacitor  92  and distributed inductor  96  of the circuit of  FIG. 7  are replaced with lumped passive elements in the form of a lumped capacitor  110  and a lumped inductor  112 . The elements of the circuit illustrated in  FIG. 13  which correspond exactly to elements in the circuit shown in  FIG. 7  are allotted the same reference numerals. 
     The circuit of  FIG. 13  comprises a signal source  76  connected to the anode of an SRD  80  and the cathode of the SRD  80  is connected to a first terminal of a resistor  88  which provides a DC path for the SRD  80 . The other terminal of the resistor  88  is connected to ground. The cathode of the SRD  80  is also connected to the input of a lumped capacitor  110  and the output of the capacitor  110  is connected to a first terminal of a lumped inductor  112  and to a first terminal of a load resistor  100 . The other terminal of the inductor  112  is connected to ground, as is the other terminal of the load resistor  100 . 
     In the circuit of  FIG. 13 , the monocycle generation begins with an impulse that is first formed by the SRD  80 . This impulse is differentiated by a second order inductor capacitor differentiator comprising the capacitor  110  and the shunt inductor  112  to produce the monocycle. The waveform seen by the load resistor  100  is thus a monocycle. The waveform produced by the SRD  80  is shown in  FIG. 14 . 
     The circuit of  FIG. 13  was simulated using Agilent Technologies&#39; ADS. The simulated result is shown in  FIG. 15 . The simulated pulse width was about 250 ps and the pulse repetition frequency was 250 MHz. 
     The circuit of  FIG. 13  was also fabricated on 32 mil Duroid substrate with a dielectric constant of 3.38. The measurement result is shown in  FIG. 16 . The pulse repetition frequency was 250 MHz and the measured pulse width was about 290 ps. 
       FIG. 17  is a schematic circuit diagram of a system according to a further preferred embodiment and is similar to the circuit of  FIG. 10 . However, in the circuit of  FIG. 17 , the distributed capacitor  92  of the circuit of  FIG. 10  is replaced by a lumped capacitor  114  and a lumped shunt inductor  116  replaces the distributed inductor  102 . The lumped shunt inductor  116  provides a DC return path for the SRD  80 . 
     The circuit of  FIG. 17  comprises a signal source  76  connected to the anode of an SRD  80 , the cathode of which is connected to a first terminal of a lumped inductor  116  and the input of a lumped capacitor  114 . The second terminal of the inductor  116  is connected to ground and the output of the capacitor  114  is connected to a load resistor  100 , the other end of the resistor  100  being connected to ground. 
     In the circuit of  FIG. 17 , the SRD  80  generates a sharp voltage transition which is differentiated by a second order inductor-capacitor (L-C) differentiator comprising the capacitor  114  and the inductor  116 , to produce a monocycle across the load resistor  100 . 
     The circuit of  FIG. 17  was simulated using Agilent Technologies&#39; ADS. The simulated result is shown in  FIG. 18 . Simulated pulse width was about 250 ps and the pulse repetition frequency was 250 MHz. 
       FIG. 19  is a further alternative embodiment of the present invention for monocycle generation using two first order differentiators. The circuit of  FIG. 19  comprises a signal source  76  connected to the anode of an SRD  80 . The cathode of the SRD  80  is connected to a resistor  88  which provides a DC return path to ground. The cathode of the SRD  80  is also connected to the input of a lumped capacitor  118 , the output of the lumped capacitor  118  being connected to a first terminal of a resistor  120 . The other terminal of the resistor  120  is connected to ground. The output of the capacitor  118  is connected to the input of a further capacitor  122 . The output of the further capacitor  122  is connected to a first terminal of a load resistor  100 , the other terminal of the load resistor  100  being connected to ground. 
     The circuit of  FIG. 19  was simulated using Agilent Technologies&#39; ADS. The simulated result is shown in  FIG. 20 . The SRD  80  generates a sharp voltage transition and this is differentiated by the first differentiator comprising the capacitor  118  and the resistor  120  to produce the impulse and by the second differentiator comprising the capacitor  122  and the resistor  100  to produce a monocycle. 
     A further preferred embodiment of the invention is shown in  FIG. 21 . In this embodiment, two first order differentiators are used but the lumped capacitors  118  and  122  of the circuit of  FIG. 19  are replaced by distributed capacitors  124  and  126 . 
     The circuit of  FIG. 21  comprises a signal source  76  connected to the anode of an SRD  80 , the cathode of the SRD  80  being connected to a resistor  88  which provides a DC return to ground. The cathode of the SRD  80  is further connected to the input of a distributed capacitor  124 , the output of the capacitor  124  being connected to a first terminal of a resistor  120 . The second terminal of the resistor  120  is connected to ground. The output of the capacitor  124  is connected to the input of a further distributed capacitor  126 . The output of the distributed capacitor  126  is connected to a first terminal of a load resistor  100 , the second terminal of the load resistor  100  being connected to ground. 
     The SRD  80  generates a sharp voltage transition which is doubly differentiated by the resistor-capacitor (R-C) networks formed by capacitor  124  and resistor  120 , and capacitor  126  and resistor  100  to produce a monocycle at the output. 
     The circuit of  FIG. 21  was simulated using Agilent Technologies&#39; ADS. The simulated result is shown in  FIG. 22 . The simulated pulse width was about 250 ps and the pulse repetition frequency was 250 MHz. 
       FIG. 23  is a schematic of a system for multi-band operation which may include any of the pulse generator systems shown in the circuits of  FIGS. 7 ,  10 ,  13 ,  17 ,  19  and  21 . In the embodiment of  FIG. 23 , a pulse generator system  128  according to any of the embodiments shown in the circuits of  FIGS. 7 ,  10 ,  13 ,  17 ,  19  and  21  is connected to a number (1 to n) of band pass filters  130  and the output of each band pass filter  130  is connected to the input of a respective modulator  132 . The outputs of the modulators  132  are combined to produce a multi-band ultra wideband signal  134 . The output of the pulse generator  128  is a monocycle which covers the frequency band defined by the band pass filters  130 . The output may therefore be de-multiplexed in the frequency domain using the band pass filters  130 . Each of the de-multiplexed signals may be modulated in a respective modulator  132  to produce a range of modulated signals in different wavebands which may be combined to produce the multi-band ultra wideband signal output  134 . 
     The input signal source  76  from which the monocycles are to be generated using the circuits of  FIGS. 7 ,  10 ,  13 ,  17 ,  19  and  21  may be, for example, a sinusoidal waveform, a square wave, a pulse or any other bi-polar signal. 
     The systems and methods according to the present invention may be particularly useful in the production of devices for use, for example, in the fields of communications, radar, ranging, imaging, depth measurement, and position locating. 
     Various modifications to the embodiments of the present invention described above may be made. For example, other components, materials and method steps can be added or substituted for those described above. Thus, although the invention has been described using particular embodiments, many variations are possible within the scope of the claims, as will be clear to the skilled reader, without departing from the spirit and scope of the invention.