Abstract:
A pulse generator circuit is provided. The pulse generator circuit has an input adapted to receive an input electrical quantity and an output at which an output electrical quantity is made available. A transfer characteristic establishes a relationship between said input and said output electrical quantities. The pulse generator circuit is adapted to provide said output electrical quantity in the form of pulses having a predetermined shape, suitable to be used for UWB transmission. The transfer characteristic has substantially a same shape as that of said pulses. Moreover, a UWB modulating system exploiting the novel pulse generator is proposed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the radio communications field. More specifically, the present invention relates to the Ultra-Wide Band (UWB) radio communications field, and in particular it relates to the generation of UWB electromagnetic pulses. 
       BACKGROUND 
       [0002]    UWB refers to a radio communications technique fundamentally different from all the other radio frequency communications systems, referred to as “narrowband systems”. Without entering into excessive details, well known to those skilled in the art, modulated low-energy pulses of very short duration, typically less than one nanosecond are used to transmit data, whereby the occupied bandwidth has very broad values. According to a definition of the U.S. Federal Communications Commission (FCC), a UWB system is a radio system having a bandwidth greater than 20% of the center frequency measured at the −10 dB points, or, alternatively, having an RF bandwidth greater than 500 MHz. On Feb. 14, 2002, the FCC allocated limited use of spectrum between the interval 3.1-10.6 GHz for signals operating in UWB systems (in short, UWB signals). 
         [0003]    The main concern regarding a UWB system is that it occupies a portion of spectrum wherein other narrowband systems already operate, so a regulation is necessary in order to avoid coexistence (e.g., interference) problems. Therefore, a regulatory authority like the FCC has to set strict limitations in the maximum emission for UWB signals, thus guaranteeing protection to the already existing and deployed radio services. UWB signal transmissions, following the FCC rules, must have a power spectral density below the involuntary electromagnetic emission level. 
         [0004]    A UWB transmitter has to generate UWB pulse signals whose spectrum is compatible with the regulations, both in term of the frequency interval and of the maximum emission. 
         [0005]    U.S. Pat. No. 6,515,622 discloses an antenna system making use of UWB pulse signals, and describes a technique for generating them based on step recovery diodes. However, this technique is not compatible with integrated circuit architectures, and is not adapted to control the shape of the generated pulse signals. 
         [0006]    A typical category of UWB pulse signals that are compatible with the regulatory prescriptions of the national/supernational authorities and that can be easily varied in shape consists of the family of the monocycle wavelets. A Gaussian monocycle wavelet is a short-duration wave having, in the time domain, a shape represented by a Gaussian derivative. 
         [0007]    A first solution known in the art for generating a monocycle wavelet is described by H. Kim, D. Park and Joo Y., in “Design of CMOS Shotlz&#39;s Monocycle pulse generator”, IEEE 7803-8187-4 2003, p. 81. According to this solution, a monocycle wavelet of the second order (i.e., corresponding to the second time derivative of a Gaussian pulse) is generated by differentiating a signal having the shape of a hyperbolic tangent by means of a squarer circuit connected to a high-pass filter. However, this solution is adapted to low-frequency applications only. Moreover, since the generated monocycle wavelet is of the second order, its spectrum does not properly match with the spectral interval allowed by the FCC, unless a further filtering operation is performed. However, the further filtering operation increases the pulse duration, resulting in a degradation of the transmission. 
         [0008]    An alternative solution for generating a monocycle wavelet is described by J. F. M. Gerrits and Farserotu, in “Wavelet generation circuit for UWB impulse radio applications”, Electronics Letters, 5 Dec. 2002, Vol. 38 n. 25, p. 1737. The document describes a circuit capable of approximating a monocycle wavelet of the second order by means of sums and differences of signals with the shape of a hyperbolic tangent. This solution has substantially the same drawbacks as the previous solution. 
         [0009]    Another technique for obtaining a monocycle wavelet, or at least to obtain an approximate version thereof, consists of modulating a sinusoidal signal (the “carrier signal”) with a modulating signal pulse of suitable shape (in the time domain), thereby obtaining a modulated sinusoidal carrier monocycle with envelope corresponding to the shape of the modulating signal pulse. An advantageous method for obtaining a monocycle wavelet having a spectra suitable for a UWB transmission under e.g. the FCC rules, consists of using modulating signal pulses having the shape that is as close as possible to a Gaussian. 
         [0010]    I. Gresham and A. Jenkins describe in “A Fast Switching, High Isolation Absorptive SPST SiGe Switch for 24 GHz Automotive Applications”, 33rd European Microwave Conference, Munich 2003, pag. 903-906, a UWB pulse generator circuit that generates a square-envelope modulated monocycle by multiplying a high frequency sinusoidal wave by a square modulating pulse. This circuit includes a switch circuit having a very short switching time, being based on the E 2 CL architecture. Although this circuit is adapted to operate at high frequencies, the spectrum of a square-envelope modulated monocycle can not be entirely confined in the spectral interval allowed by the FCC, because it is characterized by having (ideally infinite) lateral lobes. 
         [0011]    The International Application WO 0139451 describes a UWB data transmission system that generates low level voltage pulses. The shape of the low level voltage pulses can be varied by means of an adjustable shaping filter. Once shaped, the voltage pulses are used for modulating a sinusoidal signal, in such a way to obtain the UWB pulse signals necessary for the transmission of data. This solution allows modification to some extent the shape of the low level voltage pulses, and consequently modifying the shape of the UWB pulse signals spectrum. However, the shaping capability given by the shaping filter is limited, and the spectrum is scarcely controllable, which is also worsened by the circuit complexity of the shaping filter itself. 
       SUMMARY 
       [0012]    An aspect of the present invention provides a pulse generator circuit. The pulse generator circuit has an input adapted to receive an input electrical quantity and an output at which an output electrical quantity is made available. A transfer characteristic establishes a relationship between said input and said output electrical quantities. The pulse generator circuit is adapted to provide said output electrical quantity in the form of pulses having a predetermined shape, suitable to be used for UWB transmission. The transfer characteristic has substantially a same shape as that of said pulses. 
         [0013]    Another aspect of the present invention relates to a UWB modulating system for modulating at least one carrier signal with predetermined duration enveloping pulses. The UWB modulating system includes a carrier generator adapted to generate the at least one carrier signal at a respective frequency, a pulse generator circuit for generating said enveloping pulses having a predetermined shape, suitable to be used for UWB transmission, and a multiplier circuit adapted to multiply the at least one carrier signal with said enveloping pulses. The pulse generator circuit includes an input adapted to receive an input electrical quantity, an output at which an output electrical quantity is made available in form of said enveloping pulses and a transfer characteristic establishing a relationship between said input and said output electrical quantities. The transfer characteristic has substantially a same shape as that of said pulses. The UWB modulating system further includes a driver circuit adapted to generate the input electrical quantity fed to the input of the pulse generator circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram of a UWB transmitter according to an embodiment of the invention; 
           [0015]      FIG. 2A  is a diagram illustrating characteristics of a UWB signal pulse depending on time; 
           [0016]      FIG. 2B  is a diagram illustrating spectral characteristics of a UWB signal pulse depending on the frequency; 
           [0017]      FIG. 3A  is a circuit diagram of a UWB pulser according to an embodiment of the present invention; 
           [0018]      FIGS. 3B-3D  are circuit diagrams illustrating a bias network for biasing the UWB pulser according to different and alternative embodiments of the present invention; 
           [0019]      FIGS. 3E-3H  are circuit diagrams of the UWB pulser according to different and alternative embodiments of the present invention; 
           [0020]      FIGS. 4A-4D  are diagrams showing the behavior of a circuit having a transfer characteristic with a nearly Gaussian shape; 
           [0021]      FIG. 5  is a block diagram of a driver circuit block included in the UWB transmitter according to an embodiment of the invention; and 
           [0022]      FIG. 6  is an exemplary diagram showing waveforms of the signals involved in the functioning of the driver circuit shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0024]    Referring to  FIG. 1 , a UWB modulating system, in particular a UWB transmitter  100 , is schematically illustrated according to an embodiment of the present invention. Adopting a Pulse Position Modulation (PPM) technique, the UWB transmitter  100  is adapted to receive a data stream, for example in the form of a modulated digital signal SB carrying a stream of bits b i , generated by a binary source included in a control block  105 , and to generate a corresponding train of modulated UWB pulse signals. Each bit b i  can take a high logic value “1” (for example, associated to the value of a supply voltage Vcc), and a low logic value “0” (for example, associated with a ground voltage GND). The train of modulated UWB pulse signals, conveying the information carried by the data stream, i.e. by the modulated digital signal SB, is then radio-transmitted by means of an antenna  110 . The correlation between bit values b i  and UWB pulse signals is established by the modulation technique that is adopted for modulating the digital signal SB. According to the PPM technique, the position of the generic UWB pulse signal depends on the value, “1” or “0”, of the corresponding bit b i  in the data stream. Adopting instead a Pulse Amplitude Modulation (PAM) technique, it is the amplitude of the generic UWB pulse signal that depends on the value assumed by the corresponding bit b i . 
         [0025]    The UWB transmitter  100  includes a UWB pulser  115 , having a first input terminal connected to an output terminal of a driver circuit block  120  fed by the modulated digital signal SB, a second input terminal connected to an output terminal of a sine wave generator block  125 , and an output terminal connected to an input terminal of an output stage circuit  130 , having an output terminal connected to the antenna  110 . The driver circuit block  120  and the sine wave generator block  125  have input terminals connected to the control block  105 . 
         [0026]    When the UWB transmitter  100  has to transmit information, the driver circuit block  120  receives the data stream, i.e., the digital signal SB modulated adopting, for example, a PPM technique, and generates a corresponding signal adapted to drive the UWB pulser  115 . In particular, the driver circuit block  120  generates a corresponding square wave signal RP. Moreover, the sine wave generator block  125  generates a sinusoidal signal Sc of frequency fc; preferably, the sine wave generator block  125  is adapted to generate a sine wave voltage signal having a frequency fc variable (in a continuous or discrete way) within a predetermined frequency range, the frequency value being for example established by the control block  105 . The UWB pulser  115  includes a pulse generator  140 , controlled by the driver circuit block  120 , and adapted to generate a signal pulse of carefully selected shape, for example a nearly-Gaussian pulse Ig, as will be more clear in the following description. The UWB pulser  115  further includes a signal multiplier block  150 , having a first input terminal connected to the output terminal of the sine waves generator block  125  for receiving the sinusoidal signal Sc, and a second input terminal connected to the pulse generator  140  for receiving the nearly-Gaussian pulse Ig. The multiplier block  150  further includes an output terminal for providing a UWB signal pulse Pv, given by the product of the sinusoidal signal Sc by the nearly-Gaussian pulse Ig. An output stage circuit  130  allows coupling the output of the UWB pulser  115  with the antenna  110 , without degrading the spectra of the UWB signal pulse Pv. 
         [0027]    Alternatively, a Binary Phase Shift Keying modulation (BPSK) technique may be adopted: the data stream is provided to the sine wave generator block  125  in the form of a modulated digital signal SB′ carrying a stream of bits b′ i , and the sine wave generator block  125  is driven by the control block  105  in such a way to modify the phase of the sinusoidal signal Sc depending on the values assumed by the bit b′ i  of the modulated digital signal SB′. 
         [0028]    The qualitative trend of a UWB signal pulse Pv generated by the UWB pulser  115  as a function of time is illustrated in  FIG. 2A . The UWB signal pulse is composed by a sinusoidal wave of frequency fc enveloped by a nearly-Gaussian pulse, and lasts some nanoseconds (being its bandwidth higher than 500 MHz). As known in the art, the spectrum of a sinusoidal wave enveloped by a Gaussian pulse is a Gaussian pulse too, having a center frequency (i.e., the frequency corresponding to the maximum amplitude of the Gaussian pulse) that corresponds to the frequency of the sinusoidal wave; neglecting the low-amplitude, side portions of the Gaussian spectrum, the spectral width of the resulting signal can be considered as confined. 
         [0029]    Similarly, the spectrum of the sinusoidal wave signal enveloped by the nearly-Gaussian pulse is given by the spectrum of the nearly-Gaussian pulse, shifted in frequency and centered at the frequency of the sinusoidal wave signal, as illustrated in  FIG. 2B ; the closer the nearly-Gaussian pulse resemble a Gaussian pulse, the more the spectrum Pf of the UWB signal pulse Pv is Gaussian.  FIG. 2B  shows a diagram of the power spectral density of the spectrum Pf versus frequency. Since the duration in time of the UWB signal pulse Pv is less than one nanosecond, the spectrum Pf has a corresponding width of several GHz. For being compatible with the FCC rules, the spectrum Pf has to be restricted within a spectral mask SM that begins at the frequency of 3.1 GHz and ends at the frequency of 10.6 GHz. Moreover, within this spectral mask, the power spectral density must have a higher limit value equal to −41 dBm/MHz. 
         [0030]    By acting on the sine waves generator block  125 , the control block  105  is capable to vary the frequency fc, shifting the entire spectrum Pf. 
         [0031]    In a preferred embodiment of the present invention, the shape of the nearly-Gaussian pulse Ig (i.e., the UWB signal pulse envelope) can be varied, so as to adjust the shape of the spectrum Pf. In particular, according to an embodiment of the present invention, by properly modifying the square wave signal RP the shape of the nearly-Gaussian pulse Ig can be varied; to this end, the control block  105  acts (control line  170 ) on the driver circuit block  120  so as to modify the square wave signal RP. 
         [0032]    The Applicant has found that for generating a pulse of a predetermined shape, an advantageous solution consists of properly stimulating the input of a circuit having a non-linear transfer characteristic, which shape closely approximates, as far as possible, the shape of the desired pulse. For this reason, in order to generate a nearly Gaussian pulse it is expedient to exploit a circuit whose transfer characteristic has a shape that approximates a Gaussian. 
         [0033]    For better understanding the previous statements, reference will be now made to  FIGS. 4A-4D , wherein the behavior of a circuit having a transfer characteristic y=NG(x) (x represents a generic input of the circuit, and y a generic output thereof) with a nearly Gaussian shape is analyzed. As can be seen in  FIG. 4A , the nearly Gaussian shape of the transfer characteristic y=NG(x) is obtained by the overlap of two non-linear transfer characteristics, each one having the shape of a hyperbolic tangent: 
         [0000]        y=NG ( x )= A (tan  h ( x+w )−tan  h ( x−w )),  (1)
 
         [0034]    where A is an amplitude parameter and w is a width parameter. 
         [0035]    The amplitude parameter A establishes the amplitude of the transfer characteristic y=NG(x). The width parameter w establishes the shape and the width of the transfer characteristic y=NG(x), as illustrated in  FIG. 4B , wherein a family of transfer characteristics y=NG(x) is depicted, depending on different values of the width parameter w: the higher the value of the width parameter w, the wider the shape of the transfer characteristic y=NG(x). 
         [0036]    Turning now to  FIG. 4C , the effects of an input x variation on the output y are illustrated according to a first example, where it is assumed that the input x varies depending on time t x=x(t)) within an interval of values Δx. According to this first example, the input x(t) is a periodic generically rectangular signal of frequency 1/T between a lower value xl and a higher value xh, wherein the difference between the higher value xh and the lower value xl is equal to the interval Δx. Moreover, the input x(t) has a rise time Tr (i.e., the time it takes for x(t) to rise from xl to xh) equal to the fall time Tf (i.e., the time it takes for x(t) to fall from xh to xl). 
         [0037]    The output y varies over time, i.e. y=y(t), and is in particular a periodic signal having a period T/2 (i.e., half the period of the input x(t)). The output y(t) consists of a train of nearly Gaussian pulses each having the same shape as the transfer characteristic y=NG(x), but, in general, a different duration. More particularly, the output y(t) comprises nearly Gaussian pulses in correspondence of the rising and falling edges of the input x(t); said nearly Gaussian pulses thus have, a time duration equal to the rise/fall times Tr/Tf. By varying, particularly increasing the rise/fall times Tr/Tf, as is illustrated in  FIG. 4D , the time durations of the nearly Gaussian pulses of the output y(t) are accordingly varied, particularly increased. 
         [0038]    Referring to  FIG. 3A , a detailed circuit diagram of the UWB pulser  115  is illustrated. As previously described, the UWB pulser  115  consists of a pulse generator  140  and a multiplier block  150 . 
         [0039]    The pulse generator  140  comprises a first and a second NPN differential pairs, one having the output terminals cross-coupled to the output terminals of each other. More particularly, the first differential pair comprises two NPN bipolar transistors Q 1 , Q 2 , and the second differential pair comprises two NPN bipolar transistors Q 3 , Q 4 . The transistors Q 1  and Q 2  have the emitter terminals connected to each other, and further connected to a first biasing current generator, supplying a continuous current Iee 1 . The transistors Q 3  and Q 4  have the emitter terminals connected to each other, and further connected to a second biasing current generator supplying a continuous current Iee 2 . The base terminals of the transistors Q 1  and Q 3  are connected to the output terminal of the driver circuit block  120 , schematized in the drawing as a voltage signal generator generating an input voltage signal Vin series-connected to a bias voltage generator generating a continuous (DC) voltage Vb; the collector terminal of the transistor Q 1  is connected to the collector terminal of the transistor Q 4 , forming a circuital node N 1 . The transistor Q 2  has the base terminal connected to a bias voltage generator supplying a second DC voltage Vb 1 , and the collector terminal connected to the collector terminal of the transistor Q 3 , forming a circuital node N 2 . The base terminal of the transistor Q 4  is connected to a bias voltage generator generating a third DC voltage Vb 2 . 
         [0040]    The multiplier block  150  comprises a third and a fourth NPN differential pairs coupled to each other. The third differential pair comprises two NPN bipolar transistors Q 5 , Q 6 , and the fourth differential pair comprises two NPN bipolar transistors Q 7 , Q 8 . The transistors Q 5  and Q 6  have the emitter terminals connected to each other and further connected to the circuital node N 1 ; the transistors Q 7  and Q 8  have the emitter terminals connected to each other and further connected to the circuital node N 2 . Moreover, the base terminal of the transistor Q 5  is connected to the base terminal of the transistor Q 8 , and the base terminal of the transistor Q 6  is connected to the base terminal of the transistor Q 7 . Both the third and the fourth differential pairs are driven by the sinusoidal voltage signal Sc, provided by the sine wave generator block  125  in a differential way. More particularly, the sinusoidal voltage signal Sc is applied between the base terminal of the transistor Q 5  (positive input terminal) and the base terminal of the transistor Q 6  (negative input terminal). Consequently, the sinusoidal voltage signal Sc is also applied between the base terminal of the transistor Q 8  (positive input terminal) and the base terminal of the transistor Q 7  (negative input terminal). The transistors Q 5  and Q 7  have the collector terminals connected to each other, defining a first output node No 1  of the UWB pulser  115 . In a similar way, the transistors Q 6  and Q 8  have the collector terminals one connected to each other, defining a second UWB pulser output node No 2 . 
         [0041]    A current-to-voltage converter  155  is further provided, attached to the output nodes No 1  and No 2 , including a first and a second resistors R 1  and R 2 , both having a resistance value Rc. The first resistor R 1  is connected between the first output node No 1  and a terminal providing the supply voltage Vcc, the second resistor R 2  is connected between the second output node No 2  and a terminal providing the supply voltage Vcc. 
         [0042]    A differential pair of NPN bipolar transistors exhibits a non-linear transfer characteristic (expressing the differential output current Id as a function of the differential input voltage Vd), having the shape of a hyperbolic tangent: 
         [0000]    
       
         
           
             
               
                 
                   
                     Id 
                     = 
                     
                       α 
                       · 
                       Ibias 
                       · 
                       
                         tanh 
                          
                         
                           ( 
                           
                             Vd 
                             
                               2 
                                
                               Vt 
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0043]    wherein Ibias is the current biasing the differential pair, α is a proportionality parameter including the saturation current of the transistors, and Vt is the thermal voltage. It is pointed out that for small input voltages Vd (in particular, sufficiently smaller than 2Vt), the transfer characteristic (2) is almost linear, while for large values of Vd the non-linearities of the NPN bipolar transistors reduce the gain of the differential pair and cause the transfer characteristic to bend, thereby obtaining the hyperbolic tangent shape. 
         [0044]    The behavior of the pulse generator  140  of  FIG. 3A  is adapted to generate nearly-Gaussian (current) pulses Ig. In fact, taking account of equation (2) above, defining with Ig 1  the current flowing from the emitter terminals of the transistors Q 5  and Q 6  to the node N 1 , and defining with Ig 2  the current flowing from the emitter terminals of the transistors Q 7  and Q 8  to the node N 2 , the differential output current Ig=Ig 1 −Ig 2  of the pulse generator  140  is equal to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Ig 
                         = 
                         
                           
                             Ig 
                              
                             
                                 
                             
                              
                             1 
                           
                           - 
                           
                             Ig 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           ( 
                           
                             
                               Ic 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               Ic 
                                
                               
                                   
                               
                                
                               4 
                             
                             - 
                             
                               ( 
                               
                                 
                                   Ic 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 + 
                                 
                                   Ic 
                                    
                                   
                                       
                                   
                                    
                                   3 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               Ic 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Ic 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             
                               Ic 
                                
                               
                                   
                               
                                
                               3 
                             
                             + 
                             
                               Ic 
                                
                               
                                   
                               
                                
                               4 
                             
                           
                           = 
                         
                       
                     
                   
                   
                     
                       
                         
                           = 
                           
                             
                               
                                 α 
                                 · 
                                 Iee 
                               
                                
                               
                                   
                               
                                
                               
                                 1 
                                 · 
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         
                                           Vid 
                                            
                                           
                                               
                                           
                                            
                                           1 
                                         
                                         , 
                                         2 
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             - 
                             
                               
                                 α 
                                 · 
                                 Iee 
                               
                                
                               
                                   
                               
                                
                               
                                 2 
                                 · 
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         
                                           Vid 
                                            
                                           
                                               
                                           
                                            
                                           3 
                                         
                                         , 
                                         4 
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                         
                         , 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0045]    wherein Ic 1 , Ic 2 , Ic 3 , Ic 4  are the collector currents of the transistors Q 1 , Q 2 , Q 3 , Q 4 , respectively. Vid 1 , 2  is the differential input voltage of the first differential pair, and Vid 3 , 4  is the differential input voltage of the second differential pair. Since: 
         [0000]        Vid 1,2 =Vin+Vb−Vb 1 ; Vid 3,4 =Vin+Vb−Vb 2,  (4)
 
         [0046]    wherein the input signal Vin (representing the square wave signal RP generated by the driver circuit block  120 ) is a square wave signal of period T having rise times Tr and fall times Tf, the equation (3) becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     Ig 
                     = 
                     
                       
                         
                           α 
                           · 
                           Iee 
                         
                          
                         
                             
                         
                          
                         
                           1 
                           · 
                           
                             tanh 
                              
                             
                               ( 
                               
                                 
                                   Vin 
                                   + 
                                   Vb 
                                   - 
                                   
                                     Vb 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                                 
                                   2 
                                    
                                   Vt 
                                 
                               
                               ) 
                             
                           
                         
                       
                       - 
                       
                         
                           α 
                           · 
                           Iee 
                         
                          
                         
                             
                         
                          
                         
                           2 
                           · 
                           
                             tanh 
                              
                             
                               ( 
                               
                                 
                                   Vin 
                                   + 
                                   Vb 
                                   - 
                                   
                                     Vb 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                 
                                 
                                   2 
                                    
                                   Vt 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0047]    which resembles equation (1). The value of the width parameter w of equation (1) depends on how much the biasing of the transistors Q 1 , Q 2 , Q 3 , Q 4  unbalances the corresponding two differential pairs. The value of the width parameter w is established in equation (5) by properly setting the voltages Vb, Vb 1  and Vb 2  according to the following relationships: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Vb 
                           - 
                           
                             Vb 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                         
                           2 
                            
                           Vt 
                         
                       
                       = 
                       w 
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         Vb 
                         - 
                         
                           Vb 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       
                         2 
                          
                         Vt 
                       
                     
                     = 
                     
                       - 
                       
                         w 
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0048]    Since equation (5) resembles equation (1), the pulse generator  140  has a transfer characteristic having a nearly Gaussian shape. Thus, the pulse generator  140  is adapted to generate a nearly Gaussian pulse. 
         [0049]    The multiplier block  150 , having a “Gilbert cell” circuital architecture, is characterized by the following transfer characteristic: 
         [0000]    
       
         
           
             
               
                 
                   Pi 
                   = 
                   
                     
                       
                         Io 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         Io 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     = 
                     
                       α 
                       · 
                       Ig 
                       · 
                       
                         
                           tanh 
                            
                           
                             ( 
                             
                               Sc 
                               
                                 2 
                                  
                                 Vt 
                               
                             
                             ) 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0050]    Io 1  and Io 2  are the output currents of the multiplier block  150 , given by Ic 5 +Ic 7  and Ic 6 +Ic 8 , respectively; Ic 5 , Ic 6 , Ic 7  and Ic 8  are the collector currents of the transistors Q 5 , Q 6 , Q 7  and Q 8 , respectively; the differential output current of the multiplier block  150 , i.e., Io 1 -Io 2 , corresponds to the UWB (current) signal pulse. 
         [0051]    Defining with Vo 1  and Vo 2  the voltages at the first and second output nodes No 1 , No 2 , respectively, and thanks to the presence of the first and second resistors R 1 , R 2 , the differential output voltage of the UWB pulser  115 , taken between the first output node Not (positive terminal) and the second output node No 2  (negative terminal) results equal to (supposing that R 1 =R 2 =Rc): 
         [0000]        Vo 1 −Vo 2 =Vcc−Rc·Io 1−( Vcc−Rc·I 02)= Rc ·( Io 2− Io 1),  (8)
 
         [0052]    Substituting equation (7) in equation (8), the following relationship is obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Vo 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       Vo 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       - 
                       α 
                     
                     · 
                     Rc 
                     · 
                     Ig 
                     · 
                     
                       tanh 
                        
                       
                         ( 
                         
                           Sc 
                           
                             2 
                              
                             Vt 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0053]    Substituting now equation (5) in equation (9), the previous equation becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Vo 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       Vo 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       α 
                       2 
                     
                     · 
                     Rc 
                     · 
                     
                         
                       
                         
                           [ 
                           
                             
                               Iee 
                                
                               
                                   
                               
                                
                               
                                 2 
                                 · 
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         Vin 
                                         + 
                                         Vb 
                                         - 
                                         
                                           Vb 
                                            
                                           
                                               
                                           
                                            
                                           2 
                                         
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             - 
                             
                               Iee 
                                
                               
                                   
                               
                                
                               
                                 1 
                                 · 
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         Vin 
                                         + 
                                         Vb 
                                         - 
                                         
                                           Vb 
                                            
                                           
                                               
                                           
                                            
                                           1 
                                         
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                         · 
                         
                           tanh 
                            
                           
                             ( 
                             
                               Sc 
                               
                                 2 
                                  
                                 Vt 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0054]    By imposing Iee 1 =Iee 2 =Iee, equation (10) can be rewritten in the following way: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Vo 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       Vo 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       α 
                       2 
                     
                     · 
                     Rc 
                     · 
                     Iee 
                     · 
                     
                         
                       
                         
                           [ 
                           
                             
                               tanh 
                                
                               
                                 ( 
                                 
                                   
                                     Vin 
                                     + 
                                     Vb 
                                     - 
                                     
                                       Vb 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                   
                                   
                                     2 
                                      
                                     Vt 
                                   
                                 
                                 ) 
                               
                             
                             - 
                             
                               tanh 
                                
                               
                                 ( 
                                 
                                   
                                     Vin 
                                     + 
                                     Vb 
                                     - 
                                     
                                       Vb 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   
                                     2 
                                      
                                     Vt 
                                   
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                         · 
                         
                           tanh 
                            
                           
                             ( 
                             
                               Sc 
                               
                                 2 
                                  
                                 Vt 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0055]    As can be seen observing equation (11), the differential output voltage of the UWB pulser  115  depends both on the input voltage signal Vin (representing the square wave signal RP generated by the driver circuit block  120 ) and on the sinusoidal voltage signal Sc. 
         [0056]    Moreover, when the sinusoidal voltage signal Sc has a low amplitude, where by “low amplitude” there is intended sufficiently lower than the thermal voltage Vt, the previous equation can be simplified. In fact, assuming that: 
         [0000]        Sc=Vm ·sin(2 π·fc·t ),  (12)
 
         [0057]    where Vm is the amplitude of the voltage signal Sc, and assuming that: 
         [0000]        Vm ·sin(2 π·fc·t )&lt;&lt;2 Vt,   (13)
 
         [0058]    equation (11) can be approximated in the following way: 
         [0000]    
       
         
           
             
               
                 
                   Pv 
                   = 
                   
                     
                       
                         Vo 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         Vo 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     ≅ 
                     
                       
                         
                           
                             α 
                             2 
                           
                           · 
                           Rc 
                           · 
                           Iee 
                           · 
                           Vm 
                         
                         
                           2 
                            
                           Vt 
                         
                       
                       · 
                       
                           
                         
                           
                             
                               [ 
                               
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         Vin 
                                         + 
                                         Vb 
                                         - 
                                         
                                           Vb 
                                            
                                           
                                               
                                           
                                            
                                           2 
                                         
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                                 - 
                                 
                                   tanh 
                                    
                                   
                                     ( 
                                     
                                       
                                         Vin 
                                         + 
                                         Vb 
                                         - 
                                         
                                           Vb 
                                            
                                           
                                               
                                           
                                            
                                           1 
                                         
                                       
                                       
                                         2 
                                          
                                         Vt 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               ] 
                             
                             · 
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   2 
                                    
                                   
                                     π 
                                     · 
                                     fc 
                                     · 
                                     t 
                                   
                                 
                                 ) 
                               
                             
                           
                           , 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where the differential output voltage Vo 1 -Vo 2  of the UWB pulser  115  corresponds to the UWB voltage signal Pulse Pv. In fact, as can be seen by equation (14), the UWB voltage signal pulse Pv generated by the UWB pulser  115  is a sinusoidal wave enveloped by a nearly-Gaussian pulse. 
         [0059]    As previously mentioned, for being adapted to be exploited in a transmission system, the UWB voltage signal pulse Pv has to be compatible with the strict limitations imposed by the regulatory authorities like the FCC. In this case, the extension of its spectrum Pf has to be restricted within the spectral mask SM. By neglecting possible aliasing effects, an approximated expression of the envelope of the Fourier transform of the module of the UWB voltage signal pulse Pv is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         ( 
                         
                            
                           Pv 
                            
                         
                         ) 
                       
                     
                     = 
                     
                       Pf 
                       ≅ 
                       
                         
                           
                             Tr 
                             T 
                           
                           · 
                           
                             
                               
                                 α 
                                 2 
                               
                               · 
                               Iee 
                               · 
                               Rc 
                               · 
                               Vm 
                             
                             
                               V 
                                
                               
                                   
                               
                                
                               max 
                             
                           
                         
                          
                         
                           tanh 
                            
                           
                             ( 
                             w 
                             ) 
                           
                         
                          
                         
                           
                             
                               
                                 2 
                               
                                
                               
                                 π 
                                  
                                 
                                   ( 
                                   
                                     w 
                                     + 
                                     
                                       1 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           · 
                           
                              
                             
                               
                                 - 
                                 
                                   2 
                                 
                               
                                
                               
                                 ( 
                                 
                                   w 
                                   + 
                                   
                                     1 
                                     2 
                                   
                                 
                                 ) 
                               
                                
                               
                                 
                                   ( 
                                   
                                     
                                       π 
                                       · 
                                       Tr 
                                       · 
                                       Vt 
                                       · 
                                       
                                         ( 
                                         
                                           f 
                                           - 
                                           fc 
                                         
                                         ) 
                                       
                                     
                                     
                                       V 
                                        
                                       
                                           
                                       
                                        
                                       max 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0060]    wherein Vmax is the highest voltage that the square wave signal RP assumes. 
         [0061]    From the previous equation, an inverse proportionality relation exists between the −10 dB (in respect with the frequency fc) band BW of the UWB voltage signal pulse Pv and the rise times Tr of the input voltage signal Vin (i.e., the rise time Tr of the rectangular voltage pulses of the square wave signal RP): 
         [0000]    
       
         
           
             
               
                 
                   BW 
                   = 
                   
                     
                       
                         2 
                          
                         V 
                          
                         
                             
                         
                          
                         max 
                       
                       
                         π 
                         · 
                         Vt 
                         · 
                         Tr 
                       
                     
                     · 
                     
                       
                         
                           
                             ln 
                              
                             
                               10 
                             
                           
                           
                             
                               2 
                             
                              
                             
                               ( 
                               
                                 w 
                                 + 
                                 
                                   1 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0062]    By making explicit the depending of Vmax on Tr and w, the following relationship is obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     BW 
                     = 
                     
                       
                         
                           2 
                            
                           V 
                            
                           
                               
                           
                            
                           max 
                         
                         
                           π 
                           · 
                           Vt 
                           · 
                           Tr 
                         
                       
                       · 
                       
                         
                           
                             ln 
                              
                             
                               10 
                             
                           
                           
                             
                               2 
                             
                              
                             
                               ( 
                               
                                 w 
                                 + 
                                 
                                   1 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                       · 
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               
                                 b 
                                 - 
                                 a 
                               
                               2 
                             
                             + 
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         b 
                                         - 
                                         a 
                                       
                                       2 
                                     
                                     ) 
                                   
                                   2 
                                 
                                 - 
                                 1 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       a 
                       = 
                       
                         
                           
                              
                             
                               2 
                                
                               w 
                             
                           
                           - 
                           
                              
                             
                               
                                 - 
                                 2 
                               
                                
                               w 
                             
                           
                         
                         
                           p 
                           · 
                           
                             tanh 
                              
                             
                               ( 
                               w 
                               ) 
                             
                           
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       b 
                       = 
                       
                         
                            
                           
                             2 
                              
                             w 
                           
                         
                         + 
                         
                            
                           
                             
                               - 
                               2 
                             
                              
                             w 
                           
                         
                       
                     
                     , 
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
         [0000]    with p that is a ratio term determining the value of Vmax that allows generating a Gaussian pulse having a precision p on the side portions thereof. 
         [0063]    In this way, by varying the rise times Tr of the rectangular voltage pulses of the square wave signal RP generated by the driver circuit block  120 , it is possible to vary the bandwidth BW of the UWB voltage signal pulse Pv in a reliable way. 
         [0064]    Referring now to  FIG. 3B , an unbalancing circuit for providing the DC Vb, Vb 1  and Vb 2  to the pulse generator  140  in such a way to unbalance the differential pairs Q 1 , Q 2  and Q 3 , Q 4  according to equation (6) is depicted. More particularly, the input voltage signal Vin (representing the square wave signal RP) is provided to the base terminals of the transistors Q 1  and Q 3  by means of a first coupling capacitor Cc 1 , having a first terminal receiving the input voltage signal Vin and a second terminal connected both to the base terminal of the transistor Q 1  and to the base terminal of the transistor Q 3 . A terminal providing the DC voltage Vb is connected to the second terminal of the first coupling capacitor Cc 1  by means of a biasing resistor Rb. In the same way, a terminal providing the DC voltage Vb 1  is connected to the base terminal of the transistor Q 2  by means of a further first biasing resistor Rb 1 , and a terminal providing the DC voltage Vb 2  is connected to the base terminal of the transistor Q 4  by means of a further second biasing resistor Rb 2 . Moreover, a second and a third coupling capacitors Cc 2 , Cc 3  are included. The second coupling capacitor Cc 2  has a first terminal connected to the base terminal of the transistor Q 2  and a second terminal connected to a terminal providing the ground voltage GND; The third coupling capacitor has a first terminal connected to the base terminal of the transistor Q 4  and a second terminal connected to a terminal providing the ground voltage GND. 
         [0065]    In case the input voltage signal Vin is provided to the to the pulse generator  140  in a differential way, as depicted in  FIG. 3C , and according to an embodiment of the present invention, the unbalancing circuit is the same as the one depicted in  FIG. 3B , but with the second terminals of the second and third coupling capacitors that are connected to each other, and with the input voltage signal Vin that is applied between the first terminal of the first coupling capacitor Cc 1  (positive input terminal) and the second terminals of the second and third coupling capacitors Cc 2 , Cc 3  (negative input terminal). 
         [0066]    A further embodiment of the unbalancing circuit is depicted in  FIG. 3D , in which, as in the previous case, the input voltage signal Vin is provided in a differential way. The input voltage Vin is applied between the base terminal of a NPN bipolar transistor Q 9  (positive input terminal) and the base terminal of a further NPN bipolar transistor Q 10  (negative output terminal). The transistor Q 9  has the collector terminal connected to a terminal providing the supply voltage Vcc and the emitter terminal connected to the first terminal of a biasing resistor Rb 3 . The biasing resistor Rb 3  has a second terminal connected to the base terminals of the transistors Q 1  and Q 3 , forming a circuital node NB 1 . A further biasing resistor Rb 4  has a first terminal connected to the node NB 1 , and a second terminal connected to a biasing current generator providing a continuous current Iee 3 . The transistor Q 10  has the collector terminal connected to a terminal providing the supply voltage Vcc and the emitter terminal connected to the base terminal of the transistor Q 4 , forming a circuital node NB 2 . A biasing resistor Rb 5  has a first terminal connected to the node NB 2 , and a second terminal connected to a first terminal of a further biasing transistor Rb 6 . The biasing resistor Rb 6  has the second terminal connected to the base terminal of the transistor Q 2  and to a biasing current generator providing a continuous current Iee 4 . The input signal Vin is provided to the inputs of the two differential pairs Q 1 , Q 2  and Q 3 , Q 4  by means of the transistors Q 9  and Q 10 , acting as emitter followers. The unbalancing of the differential pairs is accomplished by the voltage drops generated by the passage of the continuous currents Iee 3 , Iee 4  through the biasing resistors. 
         [0067]    Although the UWB pulser  115  previously described has been implemented using NPN bipolar transistors, alternative solutions are possible. For example, similar results can be achieved if each transistor Q 1 -Q 8  in  FIG. 3A  is replaced by a corresponding voltage-controlled current generator G 1 -G 8 , as depicted in  FIG. 3E . From a practical viewpoint, the voltage-controlled current generators may be implemented by MOSFETs, as depicted in  FIG. 3F . As can be seen, the circuital architecture is the same as that illustrated in  FIG. 3A , with the NPN bipolar transistors Q 1 -Q 8  replaced by n-channel MOSFETs M 1 -M 8 . 
         [0068]    Mixed solutions are also possible: for example, in  FIG. 3G  the UWB pulser  115  comprises a pulse generator  140  realized with NPN bipolar transistors and a multiplier block  150  realized with MOSFET transistors; in  FIG. 3H  the UWB pulser  115  comprises a pulse generator  140  realized with MOSFET transistors and a multiplier block  150  realized with NPN bipolar transistors. 
         [0069]    As previously mentioned, the UWB pulser  115  converts each transition of the square wave signal provided to its input into a corresponding UWB voltage pulse Pv. Moreover, the time duration of the UWB voltage pulse Pv is uniquely determined by the duration of the rise/fall times Tr/Tf of the square wave signal. Since the −10 dB bandwidth BW of the UWB voltage pulse Pv is inversely proportional to the time duration of the UWB voltage pulse Pv, i.e., to Tr or Tf, the performance in terms of speed and temporal coherence of the driver circuit block  120  needs to be carefully controlled; in particular, it is to be observed that the circuit performances are affected by the fabrication process tolerances. 
         [0070]    In  FIG. 5 , the driver circuit block  120  is depicted according to an embodiment of the present invention, in which the duration of the rise/fall times Tr/Tf is constant. 
         [0071]    The driver circuit block  120  includes a shift-register  510 , a summing network  520  and a low-pass filter  530 . The shift register  510  is capable to store a number N of bits b i . It receives from the control block  105  a clock signal clk having a repetition period Tc, necessary for timing all the operation performed by the driver circuit block  120 , and the modulated digital signal SB. The shift register  510  provides N output bits Q 1 , Q 2 , . . . , QN carried by corresponding output terminals (for convenience, the bits and the corresponding terminals providing them are denoted with the same references) connected in sequence to N input terminals S 1 , S 2 , . . . , SN of the summing network  520 . 
         [0072]    Data bits b i  are fed by the modulated digital signal SB with a frequency 1/T. The shift register  510  is capable to store an ordinate sequence of N bits, and includes N bistable elements (for example, D-latches implemented with E 2 CL technology) timed by the same clock signal clk, one bistable element per bit. Moreover, each bistable element of the shift register  510  is connected to a corresponding one of the output terminals Q 1 , Q 2 , . . . , QN. The bistable elements are connected in such a way that the output of a generic bistable element (except the last) is connected to the input of the subsequent bistable element. The bits stored in the shift register  510  moves from the first bistable element (having the output connected to the output terminal Q 1 ) to the last bistable element (having the output connected to the output terminal QN), passing from a generic bistable element to a subsequent one at each half period Tc/2 of the clock signal clk. 
         [0073]    Since, according to this example, the shift register  510  is implemented with E 2 CL technology, the D-latches included therein have a differential circuit structure, and the logic values “1”, “0” are associated with a high logic voltage Vh (e.g., equal to 275 mV) and a low logic voltage Vl (e.g., equal to −275 mV), respectively. Consequently, also the modulated digital signal has to be properly adapted, by means of a voltage shifter circuit not shown in the Figure, before being provided to the input of the shift register  510 . In the starting condition, it is supposed that the modulated digital signal SB and the output bits Q 1 , Q 2 , . . . , QN are at the low logic voltage Vl. When the modulated digital signal SB assumes the high logic voltage Vh during a half period Tc/2, at the subsequent half period the output bit Q 1  assumes the high logic voltage Vh (i.e., it assumes the “1” logic value). If the modulated digital signal SB is maintained at the high logic voltage Vh for at least N/2 periods Tc, the input variation is transferred to all the N output terminals; consequently, at the end of N/2 periods Tc, all the output bits Q 1 , Q 2 , . . . , QN are at the high logic voltage Vh (i.e. they are all at the “1” logic value). 
         [0074]    The summing network  520  includes an output terminal providing a sum signal SS to the low-pass filter  530 . The sum signal SS is an analog voltage signal which value is proportional to the number of output bits Q 1 , Q 2 , . . . , QN that are at the high logic voltage Vh: 
         [0000]        SS=k ·( Q 1 +Q 2 + . . . +QN ),  (19)
 
         [0075]    wherein k is a constant parameter. For example, for implementing the function expressed in equation (19) a number N of NPN differential pairs connected to a same pair of resistors can be used. The sum signal SS takes the highest value when all the output bits Q 1 , Q 2 , . . . , QN are at the “1” logic value, and is equal to: 
         [0000]        SS   MAX   =k·N·V   h .  (20)
 
         [0076]      FIG. 6  illustrates the time trends of all the signals involved in the generation of a single rectangular voltage pulse of the square wave signal RP, in the exemplary case of a 4-bit shift register  510 . In this case, the sum signal SS is a rectangular voltage pulse having staircase-like rising/falling edges with rise/fall times Tr/Tf equal to two times the period Tc. 
         [0077]    The low pass-filter  530  (that will not be described in detail, because not relevant to the scope of the present invention) includes an output terminal, for providing the square wave signal RP to the UWB pulser  115 . In fact, by providing the sum signal SS to the low-pass filter  530 , the rising/falling edges of the rectangular voltage pulse are smoothed, and their trends become nearly linear, as required for properly driving the UWB pulser  115 . 
         [0078]    According to a further embodiment of the present invention, the driver circuit block  120  is adapted to be controlled by the control block  105  in such a way to vary the duration of the rise/fall times Tr/Tf and, consequently, to adjust the width of the UWB voltage pulses Pv. Since the sum signal SS is a rectangular voltage pulse having staircase-like rising/falling edges with rise/fall times Tr/Tf that depend on the period Tc of the clock signal clk, a method for varying the rise/fall times Tr/Tf consists of directly adjusting the period Tc. 
         [0079]    Moreover, the rising/falling edges of the signal RC may be non linear. In fact, referring back to  FIGS. 4C and 4D , and considering again the generic input x and the generic output y related by the nearly-Gaussian transfer characteristic y=NG(x), a non-linear variation of the input x allows to change the shape of the output y(t). Since the shape variation of a pulse in the time domain implies a corresponding shape variation of its spectrum in the frequency domain, the possibility of having non linear rising/falling edges can be very useful for adjusting the spectrum Pf of the UWB voltage pulses Pv in a carefully controlled way. For example, a driver circuit block  120  adapted to generate rectangular voltage pulses with non linear rising/falling edges can be implemented by means of a multivibrator circuit, or by properly modifying the contributions of the bits provided by the shift register. 
         [0080]    According to a further embodiment of the present invention, the sine wave generator block  125  ( FIG. 1 ) may generate a signal Sc that is the sum of a plurality of (at least two, more generally) P of sinusoidal waves Sc 1 , Sc 2 , . . . ScP, of different frequencies fc 1 , fc 2 , . . . , fcP. In this way, the spectrum of the signal Sc has a corresponding plurality P of harmonics. If the frequencies fc 1 , fc 2 , . . . , fcP are sufficiently spaced from each other, the spectrum of the UWB signal pulse Pv consists of P replicas of the spectrum Pf of the nearly-Gaussian pulse Ig, each replica having a center frequency equal to a corresponding one among the frequencies fc 1 , fc 2 , . . . , fcP. Conversely, when the frequencies fc 1 , fc 2 , . . . , fcP are sufficiently close, said P replicas of the spectrum Pf are mutually influenced: the resulting spectrum has a wider width with respect to spectrum Pf of the nearly-Gaussian pulse Ig. 
         [0081]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to preferred embodiment(s) thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice. 
         [0082]    The UWB transmitter  100  of  FIG. 1  may be utilized in a variety of different types of electronic communications systems such as wireless communications systems contained in a variety of different types of electronic devices such as consumer electronic devices like telephones and portable digital assistants (PDAs).