Patent Publication Number: US-7715502-B2

Title: Digital low-power CMOS pulse generator for ultra-wideband systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a nonprovisional application claiming priority to U.S. Provisional Patent Application No. 60/714,709, filed Sep. 7, 2005, the specification of which is incorporated herein by reference. 

   BACKGROUND 
   This invention relates to ultra-wide bandwidth circuits and systems and in particular to a low power CMOS pulse generator device for use in ultra-side bandwidth circuits and systems. 
   In ultra-wideband (hereinafter referred to as “UWB”) applications, such as communication systems, radar applications, and radio frequency identification tagging, ultra-short pulses are used for transmitting an information signal. UWB applications are emerging as a useful way to provide high speed, low power communications with resistance to multi-path interference. Advantages of UWB systems over conventional radio frequency (hereinafter referred to as “RF”) systems in appropriate applications include low power consumption and simple architecture, as described by Win and Scholtz in IEEE Commun. Lett. Vol 2, No. 2, pp 10-12 (1998), and incorporated herein by reference. UWB signals generally consist of a train of extremely narrow pulses, often on the order of 0.2-2.0 nanoseconds. Because the pulse width is short compared to the pulse period, UWB signals occupy a broad communication band at low power levels. 
   Federal Communications Commission (hereinafter referred to as “FCC”) regulations for UWB technology require that transmitted UWB pulses should observe strict limitations in terms of a pulse bandwidth and amplitude. The emissions of radio frequency devices generally are regulated by Part 15 of Title 47 of the Code of Federal Regulations (“C.F.R.”). Subpart F, in particular, entitled “Ultra-Wideband Operation,” and found at 47 C.F.R. §§15.501-15.525, recites regulations that specifically restrict the emissions of UWB devices. Among those regulations, the FCC sets forth frequency masks for UWB devices in particular applications, namely “ground penetrating radars and wall imaging systems” (§15.509); “through-wall imaging systems” (§15.510); “surveillance systems” (§15.511); “medical imaging systems” (§15.513); “vehicular radar systems” (§15.515); “indoor UWB systems” (§15.517); and “hand held UWB systems” (§15.519). These frequency masks are incorporated herein by reference. Further limitations and measurement requirements are set forth in §15.519, “Technical requirements applicable to all UWB devices,” also incorporated herein by reference. 
   Given the tight regulations of UWB devices, designing a simple and low power UWB pulse generator that can meet FCC limits is challenging. The first or second derivative of a Gaussian pulse is generally used for a UWB pulse, because it is easily expressed by a mathematical form. Several methods have been proposed to generate these signals using integrated circuits, and are described by Stoica et al., in Proc. 2004 IEEE Conf. on Ultra Wideband Systems and Tech, pp 258-262 (2004), by Bagga et al., in Proc. 2004 IEEE Conf. on Ultra Wideband Systems and Tech, pp 130-134 (2004), by Gerrits et al., in IEE Electron. Lett., Vol. 38, No. 25, pp 1737-1738 (2002), and by Kim et al., in Proc. 2003 IEEE Conf. on Ultra Wideband Systems and Tech, pp 258-262 (2003), all of these articles being incorporated herein by reference. A disadvantage of these pulse generators is that the output signals generated must further be filtered in order to satisfy FCC regulations. Furthermore, many of these previous approaches consume constant powers for biasing. 
   Considering the various FCC-imposed frequency masks, the fifth derivative of the Gaussian pulse is in some ways more effective as single UWB pulse than the first or second derivative of the Gaussian, as described by Sheng et al., in Conf. Rec. 2003 IEEE Int. Conf. Communications, pp. 738-743 (2004), incorporated herein by reference. The spectrum of this pulse is close to maximally occupied under the FCC limitation floor, and this pulse can be transmitted without any extra filtering. However, although the fifth-derivative of a Gaussian pulse can be implemented with complicated digital signal processing circuits, such circuits typically consume too much power, which can make these circuits unsuitable for low power applications such as radio frequency identification tagging, as described by Carr et al., in IEEE Commun. Lett., Vol 7, No. 5, pp 219-221 (2003) and incorporated herein by reference, and other applications where power consumption is a concern. 
   It is generally desirable to design pulse generators with a simple and power efficient architecture, preferably without the need to use additional filtering to meet FCC specifications. It would be advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. 
   SUMMARY 
   A low-power pulse generator is provided for use in UWB systems. In one embodiment, the UWB pulse generator includes four Gaussian-like pulse generators that generate pulses at different time offsets. The resulting four Gaussian-like pulses are combined to generate a UWB pulse that approximates the fifth derivative of a Gaussian pulse. 
   In another embodiment, an ultra-wideband pulse generator includes a sequence control stage, a pulse generation stage, and an output stage. The sequence control stage receives a pulse enable signal and generates output signals with different time offsets at a plurality of output branches. The pulse generation stage includes a plurality of pulse generators, wherein each pulse generator is coupled to an output branch of the sequence control stage and generates a Gaussian-like pulse at its respective time offset. The output stage combines the generated pulses into an ultra-wideband pulse. Preferably, the pulses are timed in such a way as to approximate a derivative of first or higher order of the Gaussian pulse. In a preferred embodiment, four Gaussian-like pulses are combined to approximate the fifth derivative of the Gaussian pulse. 
   Preferred embodiments of the invention presented here are described below in the Brief Description of the Drawings and in the Detailed Description. Unless specifically noted, it is intended that the words and phrases in the specification and the claims be given the ordinary and accustomed meaning to those of ordinary skill in the applicable arts. If any other special meaning is intended for any word or phrase, the specification will clearly state and define the special meaning. In particular, most words have a generic meaning. If it is intended to limit or otherwise narrow the generic meaning, specific descriptive adjectives will be used to do so. Absent the use of special adjectives, it is intended that the terms in this specification and claims be given their broadest possible, generic meaning. 
   Likewise, the use of the words “function” or “means” in the Detailed Description is not intended to indicate a desire to invoke the special provisions of 35 U.S.C. 112, Paragraph 6, to define the invention. To the contrary, if it is intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6, to define the inventions, the claims will specifically recite the phrases “means for” or “step for” and a function, without also reciting in such phrases any structure, material or act in support of the function. Even when the claims recite a “means for” or “step for” performing a function, if they also recite any structure, material or acts in support of that means or step, then the intention is not to provoke the provisions of 35 U.S.C. 112, Paragraph 6. Moreover, even if the provisions of 35 U.S.C. 112, Paragraph 6 are invoked to define the inventions, it is intended that the inventions not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function, along with any and all known or later-developed equivalent structures, materials or acts for performing the claimed function. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings: 
       FIG. 1A  is a graph showing a pulse shape of the fifth derivative of a Gaussian Pulse; 
       FIG. 1B  a graph showing a power spectral density of the fifth derivative of a Gaussian Pulse; 
       FIG. 1C  is a graph showing a pulse shape of an approximation of a fifth derivative of a Gaussian Pulse; 
       FIG. 1D  is a graph showing a power spectral density of an approximation of a fifth derivative of a Gaussian Pulse; 
       FIG. 2  is a schematic of a circuit design of a pulse generator; 
       FIG. 3A  is a graph showing sumulation results of the pulse shape of a pulse generator designed; 
       FIG. 3B  is a graph showing simulated power spectral density of a pulse generator; 
       FIG. 4  is an image illustrating the visual appearance of a pulse generator circuit; 
       FIG. 5A  is a graph showing a simulation of a pulse shape; 
       FIG. 5B  is a graph showing a generated pulse shape; 
       FIG. 6  is a graph showing a synchronized output pulse with a 10 MHz input signal; 
       FIG. 7  is a schematic of a circuit design of another pulse generator; 
       FIG. 8  is a graph showing simulation results of a pulse with bi-phase modulation; 
       FIG. 9  is a graph showing simulation results of a pulse with different amplitudes; 
       FIG. 10  is a graph showing a simulation of a power spectral density of pulses; 
       FIG. 11  is an image illustrating the visual appearance of another pulse generator circuit; 
       FIG. 12  is a graph showing bi-phase modulation of the pulse generator shown in  FIG. 11 ; and 
       FIG. 13  is a graph showing amplitude control of the pulse generator shown in  FIG. 11 . 
   

   DETAILED DESCRIPTION 
   A. Properties of the Fifth-Derivative of the Gaussian Pulse 
   Low power pulse generator circuits for UWB systems are described herein. In one embodiment, the pulse generator is a complementary metal oxide semiconductor (hereinafter referred to “CMOS”) pulse generator designed to generate four pulses. The individual pulse shape has an approximately Gaussian-like pulse or triangular pulse shape. The combined output pulse shape is similar to the fifth derivative of a Gaussian pulse which complies with FCC emission limits. 
   The fifth-derivative of a Gaussian pulse is represented by the equation, 
   
     
       
         
           
             
               
                 
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   where f(t) is a function representing the pulse output, t represents time and A and σ are variables which are chosen to fit the pulse f(t) to FCC limits. For this pulse shape,  FIG. 1A  shows the transient response in the time domain and  FIG. 1B  shows the corresponding power spectral density (hereinafter referred to as “PSD”) in the frequency domain, illustrated together with a UWB frequency mask. As seen in  FIG. 1B , the PSD of the Gaussian fifth derivative occupies a substantial portion of the spectrum available under the UWB frequency mask. 
   As is visible from the waveform illustrated in  FIG. 1A , the fifth derivative of an ideal Gaussian pulse has five finite roots (or zero-crossings)—a consequence of the fifth-order polynomial of equation (1)—and two roots at infinity. The fifth derivative of a Gaussian pulse further has six local extrema, or peaks: three positive and three negative. In general, the N th  derivative of an ideal Gaussian pulse has N finite roots and N+1 peaks. For example, the Gaussian pulse is its own 0 th  derivative, and it has no finite roots and one peak. 
   A Gaussian pulse may be processed using an analog differentiator to generate the fifth-derivative of the Gaussian pulse. However, this procedure requires many analog and digital circuit stages and therefore is not suited for low power transceiver design. 
   To implement or approximate the above-described function, or another function that meets FCC regulations, with a power-efficient architecture, alternative approaches are required. 
   B. Description of Applicable Frequency Masks 
   For the purposes of example, the frequency mask illustrated herein is based on the FCC frequency mask for indoor “indoor UWB systems,” which limits a device&#39;s equivalent isotropically radiated power, or “EIRP,” defined in the regulations as “the product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna.” 47 C.F.R. §15.503(c). This frequency mask, as set forth in 47 C.F.R. §15.517(c), is summarized in Table 1, below. 
                               TABLE 1                       Frequency in MHz   EIRP in dBm                           960-1610   −75.3           1610-1990   −53.3           1990-3100   −51.3            3100-10600   −41.3           Above 10600   −51.3                        
It should be understood that this particular frequency mask summarizes only one of many FCC regulations that either directly or indirectly restrict the operation of UWB devices, and that skilled practitioners of the art would, in designing a UWB device, take other applicable regulations into consideration, including, but not limited to, any other applicable radiated emissions limits.
 
   Although in a preferred embodiment a UWB device described herein is designed to comply with the frequency mask of Table 1 for indoor UWB systems, other devices prepared according to the present specification may be designed to comply with different UWB frequency masks, whether they be frequency masks corresponding to other UWB applications set forth in Subpart F of the FCC regulations, such as hand held applications, or masks corresponding to not-yet-developed UWB applications. It is further understood that regulations concerning UWB applications may change over time and may vary among different jurisdictions, and it is contemplated that the UWB devices disclosed herein are not limited to UWB applications set forth in the current FCC regulations. 
   C. Efficient Generation of a Gaussian Fifth Derivative Pulse 
   As can be seen from  FIG. 1A , the fifth derivative of a Gaussian pulse includes six peaks. The four central peaks-those with extrema ranging between approximately 2.0 and 2.2 ns—have prominent amplitudes compared with the two found at approximately 1.9 and 2.3 n. Each of the four central peaks itself has a generally Gaussian shape. 
   To generate a pulse that approximates the shape of the Gaussian fifth derivative, four consecutive Gaussian-like pulses are consecutively combined with predetermined delay times and alternating polarities to generate a new pulse shape as shown in  FIG. 1C . Each of the four constituent Gaussian-like pulses can be designed with different coefficients of width and amplitude, such that their combination approximates the Gaussian fifth derivative. The resulting waveform, as seen in  FIG. 1C , differs from the ideal Gaussian fifth derivative in that it has only three finite roots and two roots at infinity. However, as illustrated in  FIG. 1D , the main envelope of the PSD, centering around approximately 6 GHz and spanning from approximately 3 GHz to 10 GHz, is similar to the PSD of the ideal Gaussian fifth derivative. Like the ideal Gaussian fifth derivative, the approximated Gaussian fifth derivative of  FIGS. 1C and 1D  occupies a substantial portion of the spectrum available below the frequency mask. Therefore, it is possible to generate an FCC-compliant UWB pulse by combining four consecutive Gaussian-like pulses. 
   It should be noted that the term “Gaussian-like pulse,” as used herein, is not limited to a waveform that strictly follows the equation of the Gaussian distribution. Instead, as would be understood by those skilled in the art, a Gaussian-like pulse may be a practical approximation to a strictly Gaussian waveform including but not limited to, a Lorentzian, triangle or square pulse. In accordance with—and not in place of—the understanding of those in the art, a Gaussian-like pulse is generally characterized as having a prominent primary peak whose amplitude dominates over any secondary peaks, while the amplitude of the pulse generally diminishes with increased separation from the primary peak. Similarly, it should further be understood that pulses characterized as “triangle” or “square” pulses are not expected, in practical applications, to take precisely the forms from which they are named, but may instead take on various trapezoidal or rounded characteristics or may display artifacts from technical limitations in their generation, such as attenuation of high-frequency components. It is further to be expected that triangle, square, and other Gaussian-like pulses will demonstrate at least some temporal asymmetry due to, among other causes, residual capacitance and inductance of the electronic components used to implement the pulse generator and to convey and combine any generated pulses. 
   A Gaussian-like pulse may itself be formed by combining other pulses, which may be, but need not be, Gaussian-like pulses themselves. For example, two or more Gaussian-like pulses generated in sufficient temporal proximity to one another and combined with like polarity can themselves form a single Gaussian-like pulse with a greater pulse width and greater amplitude than the original pulses. Superimposing two or more Gaussian-like pulses that are separated with little or no temporal difference and that have different amplitudes and/or polarities can be used as a technique of varying the pulse amplitude with little or no change in the pulse width. 
   D. Generalization to Higher Derivatives of the Gaussian Pulse 
   Pulse shapes other than a fifth order derivative of Gaussian pulse that meet FCC requirements can also be generated in UWB devices as described herein. In consequence, the discussion of a fifth derivative of a Gaussian pulse should be understood as a preferred but not limiting embodiment. 
   In light of the observations of Section A, above, the N th  derivative of a Gaussian pulse may be approximated by combining a series of N+1 or fewer Gaussian pulses of alternating polarities. In particular, it has been observed, as discussed in Section C, above, that although the ideal N th  derivative includes N+1 peaks, the PSD of the N th  derivative can be approximated with the combination of fewer than N+1 Gaussian-like pulses. In a preferred embodiment, the number of Gaussian-like pulses used to approximate the N th  derivative is N−1, such that, as discussed in Section C, the fifth derivative is approximated by combining four Gaussian-like pulses. 
   As illustrated by Sheng et al., in Conf. Rec. 2003 IEEE Int. Conf. Communications, pp. 738-743 (2004), higher-order derivatives of the Gaussian generally result in a narrower PSD. In consequence, a particular Gaussian derivative may be selected as the best fit for a targeted UWB frequency mask, and a pulse generator may be designed to approximate that selected derivative by using a combination of Gaussian-like pulses as described herein. If, for example, the N th  derivative of the Gaussian provides a PSD that is an acceptable fit for the appropriate frequency mask, then a circuit that generates a series of up to N+1 Gaussian-like peaks with alternating polarities may be designed and simulated to approximate the selected Gaussian N th  derivative. The PSD of the simulated circuit may then be compared with the frequency mask to determine an appropriate fit. 
   While the use of N−1 pulses in a simulated circuit may be advantageous as a starting point for simulation, the number of pulses may be reduced further if simulation and/or actual measurements indicate that the resulting PSD will fit under the frequency mask, as the use of fewer pulses can simplify the resulting circuit. Preferably, at least two Gaussian-like pulse generators are used to generate at least two pulses of opposite polarity, thereby approximating at least a first derivative of the Gaussian pulse. A series of Gaussian-like pulses approximates the N th  derivative of a Gaussian pulse when the relative timing and relative amplitudes of the most prominent peaks in the series of Gaussian-like pulses corresponds to the relative timing and relative amplitude of the Gaussian N th  derivative. 
   E. An Exemplary UWB Pulse Generator 
     FIG. 2  is a circuit diagram of a pulse generator. The pulse generator generates an approximated fifth-derivative Gaussian pulse as described by Kim et al., in IEE Electron. Lett., 40(24), pp. 1534-1535, 2004, and herein incorporated by reference. The circuit includes a pulse generator stage and an output stage. In the embodiment of  FIG. 2 , the pulse generator is a digital triangular pulse generator and the output stage is designed to drive a 50 ohm load. However, it will be understood that other digital pulse generators can also be used that generate other shapes such as Lorentzian shaped pulses and the like. It will also be understood that the output stage can be designed to drive a different impedance load. 
   In the digital triangular pulse generator stage, a Gaussian-like triangular pulse is generated at each of the nodes A, B, C, and D, and the output stage combine four consecutive outputs from the digital triangular pulse generators. At nodes A and C, a triangle pulse is generated by a circuit consisting of a NAND gate and a corresponding inverter, while at nodes B and D, the triangle pulse is generated by a circuit consisting of a NOR gate and a corresponding inverter. Each of the triangular pulse generator circuits is triggered through a sequence control stage comprising a series of inverters. A pulse enable signal propagating through the sequence control stage passes through a different number of inverters for different branches of the sequence control stage. As each inverter delays the signal propagation by a known amount, the inverters determine the relative time at which each triangular pulse generator circuit receives an input and, thus, the delay time at which it generates its respective pulse. 
   Since the NAND gate is low when both inputs are high, and NOR gate is high when both inputs are low, the outputs of NAND and NOR gates become low and high within a very short time period, respectively. As shown in  FIG. 2 , four digital triangular pulse generators carry out the above procedures with pre-determined delay time, where the delay time at which each pulse is generated is determined by the number and properties of inverters disposed between the input and each respective NAND or NOR gate. 
   The voltage variations on nodes A and C follow the negative-peak triangular pulse shape from a power rail voltage, denoted V DD  to the ground, and voltage variations on nodes B and D are the positive-peak triangular pulse shape from the ground to V DD . The NOR gate generates positive-peak triangular pulse, and the negative-peak triangular pulse is constructed with NAND gate. In a preferred embodiment, each triangular pulse is designed to have the same peak-to-peak amplitude. To organize four successive signals correctly, different size and number of inverter blocks are used to control the delay times. 
   It will be understood that a pulse generator such as a digital triangular pulse generator can be designed using different circuits and approaches, including other types of digital logic circuits or analog circuits. 
   Four triangular pulses generated in the previous block successively enter the output stage. The output current magnitude is controlled by the size of output transistor (M 1 -M 4 ). The node voltage in front of the capacitor should be kept in consideration, because it may affect the operating region of the output transistors. In general, only one of the MOS transistors in the output stage is turned on at any one time, so that the static power consumption is minimized. Additionally, a bi-phase pulse can be easily implemented by changing the order of phase, as will be described shortly. 
   Pulse generator of  FIG. 2  has been simulated with standard 0.18-μm CMOS technology using Spectre, a circuit simulation tool. The results of the simulation are illustrated in  FIGS. 3A and 3B .  FIG. 3A  shows the pulse waveform, illustrating that the simulated output pulse width is 380 ps. The PSD of the pulse is illustrated in  FIG. 3B  and is seen to be in compliance with the FCC frequency mask. 
   F. Implementation of the Exemplary UWB Pulse Generator Circuit 
   A circuit designed in accordance with the present invention has been fabricated and tested using a standard 0.5 μm CMOS process, where the dimension 0.5 μm relates to a feature size of the CMOS process. Fabrication using a 0.5 μm CMOS process is used for purposes of illustration only. It will be understood that other CMOS processes exist with other feature sizes and processing capabilities, such as a 0.18 μm CMOS process, a 0.11 μm CMOS process, or the like, and that circuits can be fabricated using these processes.  FIG. 4  illustrates the appearance of a UWB pulse generator fabricated using a 0.5 μm CMOS process. 
   As shown in  FIG. 5A , the simulated pulse width for a circuit fabricated using a 0.5 μm process is 1.947 ns and the peak-to-peak amplitude is 211.12 mV.  FIG. 5B  shows the output pulse for the circuit shown in  FIG. 4 , measured using an Agilent Infiniium DCA 86100A wide-bandwidth oscilloscope. The pulse width is 2.1 ns and peak-to-peak amplitude is 198.0 mV, in close agreement with the simulation result. The average power consumption is 85 μW at a pulse repetition frequency (hereinafter referred to as “PRF”) of 1 MHz. Since the proposed pulse generator is designed with digital circuitry, average power consumption is proportional to PRF. The measured data indicates that all-digital UWB pulse generators are suitable for low-cost, low-power UWB transmitter design. 
     FIG. 6  shows the output pulse synchronized with a digital input signal which is operated with a PRF of 10 MHz. When an input pulse is applied with a PRF of 20 MHz, 10 MHz, and 1 MHz, average power consumption values are 1.715 mW, 860 μW, and 85 μW, respectively. The average power consumption is proportional to PRF. 
   G. An Exemplary Bi-Phase UWB Pulse Generator with Amplitude Control 
   In another embodiment, a UWB pulse generation circuit can include two additional features. First, the circuit can be modified to generate a bi-phase signal which has the same shape but opposite phase. Second, output pulse amplitude can be controlled. With the same pulse amplitude, a lower PRF reduces PSD. With the same PRF, an increased pulse amplitude increases PSD. Since the amplitude of UWB pulse directly influences PSD, which represents average power emission as described in Revision of Part 15 the Commission&#39;s rules regarding ultra-wideband transmission systems: FCC, ET Docket 98-153, 2002, and incorporated herein by reference, UWB pulse amplitude is preferably controlled for effective PSD control, although amplitude does not have to be controlled. 
   A simplified schematic of a circuit incorporating the above two features is shown in  FIG. 7 . When a pulse enable signal, denoted V IN  is high, the pulse generator is activated. At the first stage, a pulse enable signal is entered into a sequence control stage. In this stage, the pulse enable signal is divided into four branches where each branch is composed of inverter strings. Since the number of inverter strings is different in each branch, four pulse enable signals have different delay times. Thus, the sequence control stage controls time delay to arrange four subsidiary pulses. Then four signals are transferred to digital triangular pulse generators. As in the example of  FIG. 2 , each digital triangular pulse generator is composed of either a NAND or a NOR gate together with a corresponding inverter. Based on a logic status of NAND or NOR gates, triangular pulses are generated in each logic output stage. The outputs of digital triangular pulse generator have the same amplitude, but polarities are different due to the characteristics of NAND and NOR gates. 
   Four triangular pulses from the NAND and NOR gates flow into an output stage. Each triangle pulse generator is connected to two output MOS transistors in the output stage. The MOS transistors, P 11 , N 11 , P 12 , N 12 , P 21 , N 21 , P 22 , and N 22 , combine the outputs of the triangle pulse generators. After four output pulses are combined at the output stage, the approximated fifth derivative of a Gaussian pulse is delivered into a 50Ω output termination resistor. 
   To produce a bi-phase pulse, the order of four branches in sequence control stage has to be arranged in accordance with each phase. As shown in  FIG. 7 , sequence control stage is extended into two parts. The left side is for the positive phase and the right side is for the negative phase. The order of inverter strings is different and leads to two opposite phases. Switches which are located on ahead and behind of inverter strings are used to choose each phase. The transmission gate consisting of NMOS and PMOS is used to operate as a switch. 
   The output amplitudes of the final UWB pulse are decided by the sizes of MOS transistors in output stage. If the size of output transistor can be regulated, an amplitude of a UWB pulse can be controlled. After the maximum amplitude which is available to be generated is decided, it is divided into reasonable step sizes or increments. Each transistor is a controlled MOS switch with the same size as output transistor. As shown in  FIG. 7 , MOS switches in output stage play a role in selecting output transistors which are connected to NAND and NOR gates. 
   In one embodiment, three different amplitudes are generated. They are distinguished by 1 st  and 2 nd  enable signals. It will be understood that more than two enable signals can be used to generate more than three different amplitudes, but two enable signals with three different pulse amplitudes is described by way of example only, and is not intended to limit the scope of the invention. In a system with two enable signals, the largest amplitude is generated when both signals are enabled. The bi-phase UWB pulse generator is designed and simulated with standard 0.18-μm CMOS technology. However, it can also be implemented using other CMOS technologies, as previously discussed. 
   H. Simulation of the Bi-Phase UWB Pulse Generator with Amplitude Control 
     FIG. 8  shows the simulation result of the bi-phase UWB pulses generator of  FIG. 7 . The UWB pulse is generated and synchronized by an edge of a pulse enable signal. In one embodiment, a rising edge of a pulse signal enable is used. As shown in  FIG. 7 , the phase of the UWB pulse is controlled by positive phase enable signal. After the positive phase enable signal changes, the UWB pulse is generated with the same amplitude but opposite phase. 
   Three distinguished UWB pulses are generated as shown in  FIG. 9 . They are distinguished by 1 st  and 2 nd  step enable signals. The former is worked for lowest amplitude and the latter is operated for medium amplitude. The highest amplitude is intentionally designed to sum value of outputs in the 1 st  and 2 nd  step. Each MOSFET switch forms 2-digital binary weighted array and are digitally selected. Peak-to-peak voltage swings are 924-mV, 665-mV, and 402-mV on 50-Ω output termination resistor. 
   The pulse generator shown schematically in  FIG. 7  consumes very low power. Only digital circuits are used, and the connection between VDD and GND exists only when a UWB pulse is generated in output stage. Thus power consumption of the proposed pulse generator is proportionally reduced with decreased PRF. 
   The highest amplitude is used to simulate PSD, although other amplitudes can also be used. As shown in  FIG. 10 , using a PRF of 77 MHz as an example PRF, the UWB pulse which is generated with bi-phase modulation and the highest amplitude complies with the FCC frequency mask. Total power consumption is 1.88-mW for a 1.8V supply voltage. When the lowest amplitude is used, the power consumption is 807.6 μW. This power consumption is proportionally decreased when PRF is decreased. 
   I. Implementing a Bi-Phase UWB Pulse Generator with Amplitude Control 
     FIG. 11  illustrates the appearance of another pulse generator circuit formed in accordance with the present invention, corresponding to the schematic design shown in  FIG. 7 .  FIG. 12 , analogous to the simulated  FIG. 8  of the simulation, shows bi-phase modulation from this circuit.  FIG. 13 , analogous to  FIG. 9  of the simulation, shows amplitude control from the circuit. 
   Fine tuning circuits can also be incorporated into circuit designs. A purpose of a fine tuning circuit is to provide control of a pulse width. Yet another purpose of a fine tuning circuit is to provide control of a pulse delay time. Fine tuning circuits can be used to account for mismatch between simulation and test results, which can arise from non-perfect circuit modeling as well as errors introduced during fabrication. 
   Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of embodiments of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description. The scope of the present invention includes any other applications in which embodiment of the above structures and fabrication methods are used. The scope of the embodiments of the present invention should be determined with reference to claims associated with these embodiments, along with the full scope of equivalents to which such claims are entitled.