Patent Abstract:
A differently-tuned voltage controlled oscillator (VCO) and its application in a multi-band VCO tuner are disclosed. In one aspect of the invention, the VCO comprises a plurality of serially connected inductive elements each including inductively coupled inductor elements, a varactor element connected in parallel with the serially connected first inductor elements and means to apply a first and second tuning voltage to elements of the varactor element. In a second aspect, the VCO further comprises a second varactor element connected in parallel with the inductive elements, and means to apply the second tuning voltage elements of the second varactor element.

Full Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit, pursuant to 35 U.S.C. 119(e), of the earlier filing date afforded:  
         [0002]    U.S. Provisional Application Serial No. 60/460,330, entitled “Differentially “Bathtub”—Tuned CMOS VCO Using Inductively Coupled Varactors,” filed on Apr. 4, 2003 and which is incorporated by reference herein. 
     
    
     
       FIELD OF INVENTION  
         [0003]    This application relates to Voltage-Controlled Oscillators and more specifically to oscillators with differential tuning based on inductively-coupled varactors.  
         BACKGROUND OF THE INVENTION  
         [0004]    With the emerging market for the wireless LAN (local area network) standards, the need exists to provide radio solutions that integrate these standards together with the popular 802.11b standard into a single receiver. Preferably, a single VCO in combination with selectable frequency division capable of covering the 802.11a, b and g frequency bands, i.e., 2.4-2.5, 2.4-2.6 and 5.1-5.8 GHz, respectively, is needed. However, the obtainment of a wide VCO tuning range in combination with a low tuning constant K vco  is ever more challenging. A low K vco , is desirable for the PLL (phase-locked-loop) design and for the minimization of the VCO&#39;s sensitivity to noise and supply variations.  
           [0005]    Band switching in addition to differential VCO tuning are known methods used to reduce the K vco  over the extended frequency range. Differential VCO tuning also provides significant reduction in up-converted common-mode (bias) noise into phase noise and in the oscillator&#39;s sensitivity to supply- and bias variations. Several techniques exist to implement differential tuning. As the varactor&#39;s capacitance is determined by the voltage across its terminals, one can decouple a varactor capacitively from the oscillator&#39;s output nodes and bias both its terminals differentially. However, this will reduce the oscillation swing across the varactor, resulting in a highly non-linear tuning curve.  
           [0006]    Alternatively, p-type and n-type varactors can be combined using simple NMOS and PMOS transistors in inversion mode. However, standard MOSFET transistors, used in differentially tuned VCO are not optimized for a maximum C max /C min -ratio or Q. Also, the C(V) curves of NMOS and PMOS devices are not well matched and can cause a loss of CMRR (Common-Mode Rejection Ratio), i.e. the circuit&#39;s ability to reject variations in its common-mode tuning levels that affect the frequency of oscillation.  
           [0007]    Another approach is to use a combination of p- and n-type accumulation-depletion mode varactors. However, this requires a triple well process that adds to the cost. Finally, one could use only PMOS accumulation-depletion varactors, and connect the gates of one set of varactors to the outputs and tune it through the well side, and connect the well sides of a second set of varactors to the outputs and tune this set through the gate sides. However, in that case the oscillator is loaded with the large, low-Q parasitic capacitance between the well and the substrate; this will negatively affect the oscillator&#39;s phase noise and tuning range.  
           [0008]    Hence, there is a need for a VCO differential tuning device, i.e, a tuner, that allows a VCO to be tuned differentially and that preserves maximum oscillation swing across the varactors and thus maximizes the tuning linearity of the VCO.  
         SUMMARY OF INVENTION  
         [0009]    A differently-tuned voltage controlled oscillator (VCO) and its application as a multi-band VCO tuner are disclosed. In one aspect of the invention, the VCO comprises a plurality of inductive elements, each comprising inductively coupled first and second inductor elements wherein corresponding ones of the first inductor elements and second conductor elements are connected in series, a varactor element connected in parallel with the serially connected first inductor elements, the varactor element comprising serially well-to-well connected first and second same-type varactors, each having a well side and a gate side, means to apply a first tuning voltage to a node common to the first inductor elements, wherein the first tuning voltage is applied to the gate-side of each of the first and second varactors through the first inductor elements and means to apply a second voltage to a node common to said well-side of said first and second varactors. In a second aspect, a second varactor element is connected in parallel with the second inductor elements associated with the inductive elements, the second varactor element comprises serially-connected same-type first and second varactors each having a well side and a gate side, and means to apply the second tuning voltage to a node common to the second varactor element first and second varactors, wherein the second tuning voltage is applied to the well-side of each of said first and second varactors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 illustrates a conventional differential tuner using n- and p-type varactors in accumulation mode;  
         [0011]    [0011]FIGS. 2 a - 2   c  illustrate the operating characteristics of the tuner shown in FIG. 1 when tuned in differential mode;  
         [0012]    [0012]FIGS. 3 a - 3   c  illustrate the operating characteristics of the tuner shown in FIG. 1 when tuned in common mode;  
         [0013]    [0013]FIG. 4 illustrates a first exemplary single-type differential tuner in accordance with the principles of the present invention;  
         [0014]    [0014]FIG. 5 illustrates an exemplary application of the single-type tuner shown in FIG. 4;  
         [0015]    [0015]FIG. 6 illustrates a second exemplary embodiment of a single-type tuner in accordance with the principles of the invention;  
         [0016]    [0016]FIGS. 7 a - 7   c  illustrate the operating characteristics of the tuner shown in FIG. 6 operating in differential mode;  
         [0017]    [0017]FIGS. 8 a - 8   c  illustrate the operating characteristics of the tuner shown in FIG. 6 operating in common mode; and  
         [0018]    [0018]FIG. 9 illustrates another exemplary embodiment of a single type tuner in accordance with the principles of the present invention. 
     
    
       [0019]    It is to be understood that these drawings are solely for purposes of illustrating the concepts of the invention and are not intended as a definition of the limits of the invention. The embodiments shown in the figures herein and described in the accompanying detailed description are to be used as illustrative embodiments and should not be construed as the only manner of practicing the invention. Also, the same reference numerals, possibly supplemented with reference characters where appropriate, have been used to identify similar elements.  
       DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 1 illustrates a conventional dual type varactor differential tuner  100  including accumulation-depletion varactor stages  110  and  120 , which are operable to output signals V out1    130  and V out2    140  on nodes  130 ′ and  140 ′, respectively. The frequency of signals V out1    130  and V out2    140 , as is known in the art, is determined by the capacitance value of tuner  100 , in combination with the fixed inductance across the nodes  130 ′ and  140 ′.  
         [0021]    Varactor stage  110  includes, in this illustrated example, dual PMOS accumulation-depletion varactors  112  and  114  and varactor stage  120 , similarly, includes dual NMOS accumulation-depletion varactors  122  and  124 . To illustrate the operation of the conventional dual type varactor, the contribution of PMOS varactor  112  to the single-ended capacitance seen from node  130 ′ to ground, referred to as C 1 , may be determined and adjusted by varying the value of a tuning voltage or potential  116 , referred to as V tunep , that is applied through node  116 ′ to the well-side of varactors  112  and  114 . Similarly, the contribution of NMOS varactor  122  to the single-ended capacitance seen from node  130 ′ to ground, referred to as C 2 , may be determined and adjusted by varying the value of another tuning voltage or potential  126 , referred to herein as V tunen , that is applied at node  126 ′ to the well-side varactors  122  and  124 . A similar analysis may be performed to determine the contributions of PMOS varactor  114  and NMOS varactor  124  to the single-ended capacitance seen from node  140 ′ to ground and need not be discussed in detail herein.  
         [0022]    [0022]FIGS. 2 a  and  2   b  illustrate the small-signal capacitance as a function of voltage, i.e., C(V) curves, as a function of the difference, i.e., V tunediff ., between voltages V tunep  and V tunen . i.e., tuning voltages. In one case, as shown in FIG. 2 a , as V tunediff  increases, the value of the capacitance of C 1    210  and C 2    215  diverges and the width of the minimum in the C(V) curve, i.e., C tot    220 , also increases. In the case shown in FIG. 2 b , as V tunediff  decreases, the value of the capacitance of C 1    210  and C 2    215  diverges and the width of the minimum in the C(V) curve decreases.  
         [0023]    The large-signal output waveform V out1  cycles through these small-signal C(V) curves during each period of oscillation. As a result, the average capacitance experienced by the output V out1  and V out2  which is not shown, determines the frequency of oscillation. In this case, as capacitance decreases the frequency of oscillation increases.  
         [0024]    [0024]FIG. 2 c  illustrates the change in the average capacitance of C tot    220  and consequently of the differential tuner  100 , as a function of V tunediff . As shown, in a “differential” mode of operation as V tunediff  increases, the average capacitance decreases substantially and, hence, the frequency of V out1 ,  130  and V out2    140  increases substantially.  
         [0025]    [0025]FIGS. 3 a  and  3   b  illustrate the change in value of capacitances C 1    210  and C 2    215  as the combined value of V tunep  and V tunen , commonly referred to as V tunecomm , increases and decreases, respectively. For example, V tunecomm  may be an average value of V tunen  and V tunep . FIG. 3 c  illustrates the change in the average capacitance of C tot    220  as a function of V tunecomm . In this common mode of operation, as the common voltage V tunecomm  changes, the average capacitance remains substantially constant and, hence, the frequency of V out1    130  and V out2    140  remains substantially constant.  
         [0026]    [0026]FIG. 4 illustrates a first exemplary embodiment  400  of a single-type varactor differential tuner in accordance with the principles of the invention. In this illustrated embodiment, a first n-type varactor stage  410   a , containing PMOS varactors  412   a ,  414   a , is responsive to voltage V tunep    116 , applied through node  116 ′ to the well-side of each of the serially-connected PMOS varactors  412   a ,  414   a . Second varactor stage  410   b , containing PMOS varactors  412   b ,  414   b , is responsive to voltage V tunen    126 , applied through node  126 ′ to the gate-side of PMOS varactors  412   b ,  414   b , through inductors  420   b  and  422   b.    
         [0027]    Inductive elements  420  and  422  electromagnetically couple varactor elements  412   a  and  414   a  in stage  410   a  to corresponding varactor elements  412   b  and  414   b  in varactor stage  410   b . As shown, the windings of inductive elements  420   a ,  420   b  and  422   a ,  422   b  are reversed such that the oscillation signal, present at the gates of the varactor stage  410   b , is inverted with respect to the oscillation signal, present at the gates of varactor stage  410   a.    
         [0028]    Thus, in this embodiment, the contribution of PMOS varactor  412   a  to the single-ended capacitance i.e., C 1  seen from node  130 ′ to ground, and the contribution of PMOS varactor  412   b  to the single-ended capacitance, i.e., C 2 , seen from node  130 ′ to ground operates as discussed with regard to FIGS. 2 a - 2   c  for differential-mode tuning and FIGS. 3 a - 3   c  for common-mode tuning. Hence, as the varactor capacitance C 1  increases due to an increase in V out1  on node  130 ′, the capacitance C 2  decreases, due to the signal inversion performed by the coupled inductors.  
         [0029]    Further illustrated is voltage V bias    430  applied at node  430 ′. Voltage V bias    430  is provided to the common node of inductors  420   a  and  422   a  such that voltage V bias , through inductor elements  420   a  and  422   a , is superimposed on signals V out ,  130  and V out2    140 . V bias    430  also provides a necessary current to a transconductor, as will be discussed with regard to FIG. 5, that is needed to sustain the oscillation of signals V out1    130  and V out2 . To ensure that the varactor stage  410   b  has a similar DC bias point as varactor stage  410   a , the well-side of varactor section  410   b  is connected to the fixed voltage V bias    430 . The application of V bias    430  to V out1    130  and V out2    140  allows a maximum amplitude variation about a non-zero DC-biased reference value equal to substantially one-half the supply voltage.  
         [0030]    [0030]FIG. 5 illustrates a schematic diagram  500  of an exemplary multi-band oscillator in accordance with the principles of the invention. In this exemplary application, the outputs of single-type varactor differential tuner  400 , i.e., nodes  130 ′ and  140 ′, are coupled to bandswitcher circuit  510 , and to negative resistance transconductor  515 . Transconductor  515  is well known in the art to provide a negative resistance that compensates for losses in the circuit to maintain the oscillation of signals V out1    130  and V out2    140 .  
         [0031]    In this exemplary embodiment, the average or DC value of the voltage at the gate-side of varactor section  410   a  and the well-side of varactor section  410   b  are maintained at a fixed voltage determined by V bias    430 . In this illustrated example, V bias    430  is maintained at roughly half the supply voltage V DD  due to the voltage drop across transconductor  515  resulting from a current that is supplied through current mirror  530 . Tuning voltages V tunep    116  and V tunen    126  are applied about a common voltage level equal to half the supply voltage as well. This varactor biasing approach is advantageous as it assures a maximum differential tuning voltage range over which the oscillator may be tuned linearly. Thus, the output signal waveforms V out1    130  and V out2    140  are positioned symmetrical with respect to the sum of the C(V) curves, i.e., C 1  and C 2 , thus giving the largest possible differential tuning voltage range over which the steep transition regions between the maximum and minimum capacitance values of curves C, and C 2  fall within the coverage range of the output waveform V out1 . This is related to the fact that the C(V) curve of the accumulation-depletion varactor used is point symmetrical approximately around the point where the voltage between gate and well is zero volts as shown in FIGS. 2 a  and  2   b ).  
         [0032]    Bandswitcher  510  allows, in this illustrated case, for four switched tuning bands that are binary controlled by voltage signals V sw1    513  and V sw2    515 . Band-switching is implemented, in this case, by applying an appropriate voltage level to the well-side of the varactors in either or both of the varactor stages  512 ,  514 . In this illustrated case, four states of band-switching are achieved by the application of combinations of the supply voltage, e.g., V DD , or ground (e.g., 0 volts) to each of the varactor stages.  
         [0033]    Current mirror  530  provides a bias voltage to tuner  400  as previously described with regard to FIG. 4, i.e., bias voltage  430 . Current mirrors are well-known in the art and need not be described in detail herein.  
         [0034]    [0034]FIG. 6 illustrates a second exemplary embodiment of a single-type varactor in accordance with the principles of the invention. In this illustrated embodiment, the circuit is identical to that of FIG. 4, except that the windings of inductor  620   b  and  622   b  are reversed. In this case, the oscillation signals on the primary and secondary side of the coupled inductors are now in-phase. An analysis of the varactor small-signal capacitances now yields C(V) curves as depicted in FIGS. 7 a - 7   c  for differential-mode tuning and FIGS. 8 a - 7   c  for common-mode tuning.  
         [0035]    With regard to FIG. 7 a , as the voltage V tunediff    130  increases, the capacitance of C 1    210  and C 2    215  shifts at a similar rate to the right in FIG. 7 a  and the total capacitance C tot    220  also shifts to the right in FIG. 7 a . With regard to FIG. 7 b , as the voltage V tunediff    130  decreases, the capacitance curves of C 1    210  and C 2    215  shift at a similar rate to the left in FIG. 7 b  and the total capacitance C tot    220  also shifts to the left in FIG. 7 b . FIG. 7 c  illustrates that the overall capacitance C tot    220 , averaged over one period of the oscillation waveform, yields an average capacitance C avg.  that decreases as the differential tuning voltage increases in a manner similar to that shown in FIG. 2 c.    
         [0036]    [0036]FIGS. 8 a  and  8   b  illustrate the change in total capacitance C tot    220  as the voltage V tunecomm  increases and decreases, respectively. FIG. 8 c  illustrates that the overall capacitance C tot    220  averaged over one period of the oscillation waveform yields an average capacitance C avg.  that remains substantially constants as the voltage V tunecommon  increases in a manner similar to that shown in FIG. 3 c.    
         [0037]    [0037]FIG. 9 illustrates a third exemplary embodiment  900  of a single-type varactor differential tuner in accordance with the principles of the invention. In this embodiment a single varactor stage  910   b  is tuned by applying at node  116 ′ voltage V tunep    116  to the well-side of serially connected varactors  912   b  and  914   b . Electrically connected to the gate-side of varactors  912   b  and  914   b  is one end of inductor elements  920   b  and  922   b , which are electrically connected in series. Voltage V tunen  is applied to a common node of inductors  920   b  and  922   b , and thus applied to the gate-side of varactors  912   b  and  914   b . Inductors  920   b  and  922   b  are electromagnetically coupled to inductors  920   a  and  922   a , which are also connected in series. Voltage V bias    430  is applied to a common node of inductors  920   a  and  922   a  and is thus superimposed on output voltages V out1    130  and V out2    140 . In this illustrated embodiment, the voltage across the varactor, which sets the capacitance value of the varactor, is directly determined by the differential tuning voltage V tunediff , i.e., V tunep −V tunen .  
         [0038]    However, when V tunep    116  and V tunen    126  vary in common-mode manner, the voltage across the varactor does not change and thus the capacitance and frequency of oscillation remain substantially unchanged.  
         [0039]    To ensure a maximum differential tuning voltage range over which the oscillator tunes linearly, again the common mode value of the tune voltages V tunep    116  and V tunen    126  are selected substantially equal to approximately half the supply voltage.  
         [0040]    While there has been shown, described, and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the apparatus described, in the form and details of the devices disclosed, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated.

Technology Classification (CPC): 7