Abstract:
A direct conversion ultrawideband transceiver employing three phase locked loops (PLLs). The PLLs are preferably fixed-frequency PLLs that operate continuously, at different frequencies, with a selected frequency determined by selecting the output of one of the three PLLs. The use of three PLLs is suitable for use in a communication system employing frequency hopping across three bands or sub-bands.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/624,891, filed Nov. 3, 2004 which is hereby incorporated by reference as if set forth full herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to wideband transceivers, and more particularly to wideband transceivers using multiple phase-locked loops (PLLs).  
         [0003]     Ultrawideband (UWB) communication has been proposed for high data rate applications. The data may be transmitted, for example, using direct sequence or orthogonal frequency division multiplexing (OFDM) schemes, and may accommodate data transmission as high as 480 Mb/s or more.  
         [0004]     In one such proposal data may be transmitted in the frequency range from 3.168 GHz to 10.560 GHz, subdivided into 5 band groups. Initial efforts for this proposal, however, are generally aimed at operation in a first of these band groups, which provides for data transmission in three sub-bands over the frequency range of 3.168 GHz to 4.752 GHz. The three sub-bands are centered at 3.432 GHz, 3.960 GHz, and 4.488 GHz, and each occupy 528 MHz of spectrum.  
         [0005]     Communication of data on these sub-bands may be performed with a transmitter and a receiver switching from sub-band to sub-band on a periodic basis, and doing so while communicating data. A guard interval may be provided to account for transient effects while the transmitter and receiver switch sub-bands. The sub-band switching time, however, may not be great, for example in the range of 9 nanoseconds, and it may be difficult for the transmitter and receiver to effectively change sub-bands within an allocated time period.  
         [0006]     In view of potentially short sub-band switching time, use of wideband PLLs might be difficult, particularly if the wideband PLL can not quickly lock on to a correct data rate. Similarly, use of single sideband mixers may also generate signals containing excess harmonic distortion and otherwise dissipate excess power, either through filtering of the signals, amplification of data signals, or both.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention provides an Ultrawideband Transceiver. In some aspects the invention provides a receiver with a plurality of phase locked loops (PLLs) with each PLL providing a signal to a corresponding mixer, with each corresponding mixer also receiving a data signal. In some aspects the invention provides from a low noise amplifier, and a summer receiving outputs of the mixers. In some aspects a band select signal selectively couples signals from the PLLs with the corresponding mixers, and in some aspects the corresponding mixers receive a band selected signal from a low noise amplifier amplifying a received signal. In some aspects the invention provides a plurality of PLLs whose outputs are summed and provided to a mixer upconverting an information signal for transmission, and in some aspects a band select signal is used to select a particular PLL signal.  
         [0008]     In one aspect the invention provides a transceiver for use in an ultrawideband communication system, comprising a plurality of phase-locked loops (PLLs), each of the PLLs providing a signal at a different frequency; a plurality of mixers, each of the mixers configured to mix a signal generated from a corresponding PLL of the plurality of PLLs with a radio frequency signal to thereby downconvert the radio frequency signal to baseband; and gate circuitry responsive to a selection signal, the gate circuitry gating the signals provided by the PLLs such that only a signal from a single PLL is provided to a mixer at a given time.  
         [0009]     In another aspect the invention provides a transceiver for use in an ultrawideband communication system communicating data over three frequency sub-bands in a frequency hopping manner, comprising three phase-locked loops (PLLs) each providing a mixing signal at a different frequency; three direct downconvert mixers each receiving an amplified RF signal and a mixing signal from a corresponding one of the three PLLs; and means for gating the mixing signals from the PLLs responsive to a sub-band select signal such that only a single mixer of the three mixers receives a mixing signal from the PLLs at a selected time.  
         [0010]     In another aspect the invention provides a transceiver for use in an ultrawideband communication system, comprising a low noise amplifier receiving an RF signal and providing an amplified RF signal; a plurality of mixers, with a mixer for each sub-band used in the ultrawideband communication system, each of the plurality of mixers receiving a representation of the amplified RF signal and configured to receive a mixing signal for direct downconversion of signals in one of the sub-bands used in the ultrawideband communication system; a plurality of phase-locked loops (PLLs), with a PLL for each sub-band used in the ultrawideband communication system, each of the PLLs generating a mixing signal for direct downconversion of signals in one of the sub-bands used in the ultrawideband communication system; and means for gating the mixing signals responsive to a sub-band selection signal such that only a single mixer downconverts a representation of the amplified RF signal.  
         [0011]     These and other aspects of the invention are more fully comprehended on review of the following description in conjunction with the associated drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of a transceiver architecture in accordance with aspects of the invention;  
         [0013]      FIG. 2  is a semi-schematic diagram of a low noise amplifier in accordance with aspects of the invention;  
         [0014]      FIG. 3  is a semi-schematic of a mixer in accordance with aspects of the invention;  
         [0015]      FIG. 4  is a semi-schematic of a transmitter output stage in accordance with aspects of the invention, additionally showing an antenna and a portion of a low noise amplifier; and  
         [0016]      FIG. 5  shows an example transmitter output. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  is a block diagram of a transceiver architecture in accordance with aspects of the invention. An antenna  111  is coupled to an input of a low noise amplifier (LNA)  113 . The LNA also receives a gain switch signal and a band select signal. In many embodiments it should be noted, however, that the band select signal may be more properly referred to as a sub-band select signal, and in some embodiments as a frequency select signal. The LNA provides as outputs three signals, each of which is coupled to one of three mixers  115   a - c . The LNA output signals provided to the mixers are also coupled to switched resonant networks, illustrated as inductors  117   a - c , which provide, for example, for filtering of the LNA output signals.  
         [0018]     For clarity of discussion, only three mixers, for example, are shown and specifically discussed. In many embodiments, however, signal processing is performed for both in-phase and quadrature signals. Accordingly, it should be recognized that the mixers generally represent sets of mixers. Moreover, in many embodiments circuitry for providing both in-phase and quadrature signal processing is also generally provided, and, although not specifically illustrated in  FIG. 1 , differential signal processing is performed in many embodiments. Further, in various embodiments additional mixers, as part of additional receive and transmit chains, are also provided.  
         [0019]     Returning to  FIG. 1 , three mixing signals are provided to the three mixers, with each mixer receiving a different mixing signal. Three PLLs  119   a - c  source the three mixing signals, with each PLL sourcing a single mixing signal. In some embodiments a first of the PLLs provides a mixing signal approximate 3.432 GHz, a second of the PLLs provides a mixing signal approximate 3.960 GHz, and a third of the PLLs provides a mixing signal approximate 4.488 GHz. The PLLs are preferably fixed-modulus PLLs to reduce noise in the mixing signals, and in many embodiments the PLLs preferably employ ring oscillators. As is understood by those of skill in the art, the PLLs are each provided a reference signal at a fixed frequency generated by a crystal or signal generator (not shown) approximate a fixed frequency, and generate an output signal, in this case a mixing signal, some multiple of the fixed frequency.  
         [0020]     The mixing signals are gated by gates  121   a - c , with operation of the gates controlled by the band select signal. The mixing signal for the selected band is allowed to pass it corresponding gate and reach its corresponding mixer, while the other mixing signals are blocked by their corresponding gates. Accordingly, the mixing signal for the selected band is allowed to reach its corresponding mixer, with resultant downconversion to baseband of the input signal to that mixer.  
         [0021]     The outputs of the mixer are provided to a variable gain summer  123 . The variable gain summer sums the outputs of the mixers, one of which has been downconverted, and an output of the variable gain summer is received by a filter  124 . As illustrated, the filter is a fourth order Sallen-Key (SK) filter. An output of a the filter is received by a programmable gain stage  125 , the output of which is further filtered by a further filter  126 , which as illustrated is a first order RC filter. Further processing of the receive chain signal may thereafter be provided by other components (not shown).  
         [0022]     For the transmit chain, a signal for transmission is received by a filter  127 , also a fourth order SK filter as illustrated in  FIG. 1 . An output of the filter is received by a mixer  129 . The mixer  129  also receives a mixing signal from a summer  131 . The summer receives gated signals from the three previously mentioned PLLs, with the signals from each of the PLLs being gated by corresponding gates  133   a - c . Operation of the gates  133   a - c  is also controlled by the band select signal, with a signal from only one of the PLLs being allowed to pass through its corresponding gate at any time. Accordingly, the mixer receives as a mixing signal a signal approximate 3.432 GHz, 3.960 GHz, or 4.488 GHz as selected by the band select signal. The output of the mixer is provided to an output stage  135 , with the output stage performing, for example, amplification of the signal as appropriate for transmission by the antenna.  
         [0023]      FIG. 2  illustrates a semi-schematic of a low noise amplifier in accordance with aspects of the invention. The LNA of  FIG. 2  is used, in some embodiments, as the LNA of the system of  FIG. 1 . The LNA includes switched cascode drivers M 3 , M 4 , and M 5 . Gates of the switched cascode drivers are each coupled to a band select signal. When a first band is selected cascode driver M 5  is set to an on state, when a second band is selected cascode driver M 4  is set to an on state, and when a third band is selected cascode driver M 3  is set to an on state.  
         [0024]     Each of the cascode drivers provide outputs at their drains, and each of the drains is also coupled to a resonant tank  211   a - c , illustrated schematically as, and in some embodiments comprising, inductors. Each of the resonant tanks preferably have a resonant frequency centered at the resonant frequency of their respective band. In many embodiments the Q of the tanks are selected to reduce droop near band edges, with the Q being set to or approximate 3 in many embodiments, with the droops canceled by slight peaking in the baseband filters in some embodiments.  
         [0025]     Sources of the cascode drivers are coupled to a cascode common-gate stage. The cascode common gate stage includes gate coupled transistors M 1  and M 2 . The gates of M 1  and M 2  are coupled to a voltage bias source. Drains of M 1  and M 2  are coupled to sources of transistors M 3 , M 4 , and M 5 . Sources of M 1  and M 2  are coupled to an RF input and, by way of a source inductance  213 , to ground. Preferably the source inductance is approximate 20 nH, resonating, with the capacitance at this node, approximate 3.5 GHz, thereby presenting a relatively high impedance across all three bands.  
         [0026]     The gate of M 1  is switchably coupled to either the gate of M 2  or to ground by switches  215   a,b  controlled by a gain signal, allowing M 1  to be set to an off state. Turning M 1  off reduces the gain of the LNA. The magnitude of the gain reduction may be chosen through selection of width/length (W/L) ratios of M 1  and M 2 , with the W/L of M 1  approximately 8 times the W/L of M 2 . In many implementations length is common to transistors on a substrate, and the W/L ratio is modified merely by adjusting width.  
         [0027]     Setting M 1  to the off state, however, may result in an increase to input impedance. Accordingly, a transistor M 6  is coupled in parallel to the source inductance, and is turned on when M 1  is turned off. The on-resistance of M 6  varies with process and temperature, but generally provides for a magnitude of the S 11  parameter to be greater than 10 dB.  
         [0028]      FIG. 3  is a semi-schematic of a mixer in accordance with aspects of the invention. In some embodiments the mixer of  FIG. 3  is used for the mixers of the system of  FIG. 1 . The mixer of  FIG. 3  mixes an RF signal, preferably amplified by a low noise amplifier, with a mixing signal sourced by a local oscillator (LO), such as in a PLL.  
         [0029]     The oscillator includes a differential pair M 2  and M 3 , which receive a differential LO signal at their gates. Sources of M 2  and M 3  are coupled to a drain of a driving transistor M 1 . The gate of M 1  receives an RF signal for downconversion, and has a source coupled to ground. In various embodiments additional bias transistors may be interposed between the driving transistor M 1  and ground, or between the differential pair and the driving transistor, providing bias current.  
         [0030]     Drains of the differential pair, from which a differential output signal are taken, are each coupled to Vdd by a selectable resistive network  311   a,b . Taking the resistive network coupled to the drain of M 2  as an example, the resistive network includes a plurality of resistances coupled in series, with nodes between each of the resistances switchably coupled by gates  313   a - g  to the drain of M 2 . As illustrated in  FIG. 3 , the resistances are provided by resistors, although in various embodiments the resistances are provided through use of transistors, and may make use of selectable resistances provided by transistors operating in their linear range.  
         [0031]     In somewhat more detail, the resistances of the resistive network form a resistive ladder, with taps along the ladder switchably coupled to the drain of M 2 . As illustrated, the resistive ladder is a binary scaled ladder, with a first resistance  315   a  coupled to Vdd, a second resistance  315   b  twice the magnitude of the first resistance coupled to the first resistance, a third resistance  315   c  twice the magnitude of the second resistance coupled to the second resistance, etc. In the embodiment illustrated seven such resistances are so coupled, with each resistance in the ladder having a resistance double the magnitude of the prior resistance in the ladder, and taps between each resistance. The resistive ladder provides high linearity, gain steps substantially linear in dB, and a substantially constant output impedance.  
         [0032]      FIG. 4  is a semi-schematic in accordance with aspects of the invention showing a portion of an output stage, antenna, and portion of a low noise amplifier. An upconverted differential signal is provided to the portion of the output stage. The upconverted differential signal is ac-coupled to a differential to single ended converter  411  comprised of transistors M 1 , M 2 , and M 3 . More specifically, as shown in  FIG. 4 , M 2  is coupled between M 1  and M 3 , with the gate of M 2  provided a bias signal. Also coupled between the source of M 3  and the drain of M 2  is an inductor  413 , preferably resonating with a low Q so as to improve bandwidth, particularly over 4 GHz. The differential pair is coupled to the gates of M 1  and M 3 , with a single ended output taken between the drain of M 2  and the inductor.  
         [0033]     The single ended output is ac-coupled to a gate of a driver transistor M 4 , which delivers an output to an antenna  415 . The gate of M 4  is also coupled to enable circuitry, which allows M 4  to deliver the output signal when an enable signal is high. In some embodiment the enable signal is also used to disable a low noise amplifier  417  also coupled to the antenna.  
         [0034]     In one embodiment circuitry in accordance with the foregoing is implemented in 0.13 um CMOS technology, on, for example, a 0.9 mm×0.8 mm die provided a 1.5 V power supply. In such an embodiment, using three bands (or more properly, sub-bands), each in use approximately one-third of the time, noise in sub-bands 1 and 2 (the lower frequency sub-bands) is approximately 5.5 dB, with noise in sub-band 3 approximately 8.4 dB. Table 1 summarizes some aspects relating to the embodiment.  
                                             TABLE 1                                       Voltage Gain   69-73   dB           Noise Figure   5.5-8.4   dB           In-Band 1-dB Compression Point           High LNA Gain   −27.5-−29.5   dBm           Low LNA Gain   −9.5-−12.5   dBm           S11           High LNA Gain   −12   dB           Low LNA Gain   −11   dB           TX Output 1-dB Comp.   −10   dBm           Phase Noise @ 1-MHz Offset   −104-−108   dBc/Hz           Power Dissipation   105   mW           Supply Voltage   1.5   V                Technology   0.13-um CMOS                      
 
         [0035]     In addition,  FIG. 5  shows an example transmitter output with a 4 MHz tone applied to the baseband  
         [0036]     Accordingly, the invention provides in some aspects an ultrawideband transceiver. Although the invention has been described with respect to certain embodiments, it should be recognized that the invention may comprise the claims and their insubstantial variations supported by this disclosure.