Abstract:
A voltage controlled oscillator, includes a tank circuit including an inductor having a value L, interconnected with first and second variable capacitors, having values C VAR1  and C VAR2 , and a fixed capacitor C FIXED , to cause oscillation of the oscillator at a controlled frequency 
     
       
         
           
             
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     C VAR1  controls coarse frequency tuning of the oscillator, and C VAR2  may control fine tuning of the oscillator. The variable capacitors may be formed using accumulation-mode MOS varactors

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to oscillator circuits, and more particularly to voltage controlled oscillator circuits that allow for fine and coarse tuning. 
       BACKGROUND OF THE INVENTION 
       [0002]    A variety of oscillator circuits are known. Examples include the Colpitts oscillator as described in U.S. Pat. No. 1,624,537, the contents of which are hereby incorporated by reference. A Colpitts oscillator uses a “tank” circuit having an inductance with two capacitors that determine the frequency of oscillation. The feedback signal to drive the oscillator is taken from a voltage divider made of the two capacitors, connected in series. 
         [0003]    Another example oscillator is the Clapp oscillator. The Clapp oscillator is basically a Colpitts oscillator that has an additional capacitor placed in series with the inductor. 
         [0004]    Many applications use oscillators for tuning purposes. As such, Clapp and Colpitts oscillators are often formed as variable frequency oscillators, whose frequencies of oscillation are controlled using variable capacitors, whose capacitance(s) vary with an applied control voltage. Such oscillators are referred to as voltage controlled oscillators (VCOs). 
         [0005]    In variable frequency, radio applications a Colpitts VCO is often preferred because of its relatively low phase noise. 
         [0006]    Nevertheless, to tune a relatively wide frequency range, requires a capacitor having significant variability, and/or relatively large VCO control voltages. In many micro-electric and integrated circuits, required voltages and variable capacitors of such size and range are not available or impractical. 
         [0007]    Accordingly, there is a need for an improved VCO that provides a relatively wide tuning range, with a desired precision, while using components that may be integrated and whose size may be limited for certain desired frequency ranges. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with an aspect of the present invention, there is provided a voltage controlled oscillator, comprising: a transistor; a tank circuit interconnecting the transistor in feedback, the tank circuit including an inductor having a value L, interconnected with first and second variable capacitors, having values C VAR1  and C VAR2 , and a fixed capacitor C FIXED , to cause oscillation of the oscillator at a controlled frequency 
         [0000]    
       
         
           
             
               f 
               osc 
             
             = 
             
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                   π 
                    
                   
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                       L 
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                               VAR 
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                                
                               
                                 C 
                                 
                                   VAR 
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                                 FIXED 
                               
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         [0000]    wherein adjustment of C VAR1  and C VAR2  control tuning of the oscillator. 
         [0009]    In accordance with another aspect of the present invention, there is provided a method of producing an oscillating voltage at a desired frequency, comprising: providing a transistor; providing a tank circuit interconnecting the transistor in feedback, the tank circuit including an inductor having a value L, interconnected with first and second variable capacitors, having values C VAR1  and C VAR2 , and a fixed capacitor C FIXED , to cause oscillation of the transistor at a controlled frequency 
         [0000]    
       
         
           
             
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             = 
             
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                    
                   
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         [0000]    adjusting C VAR1  and C VAR2  to cause oscillation at the desired frequency. 
         [0010]    Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the figures which illustrate by way of example only, embodiments of the present invention, 
           [0012]      FIG. 1  is a schematic diagram of a conventional Colpitts oscillator circuit; 
           [0013]      FIGS. 2A and 2B  are simplified schematic diagrams of a VCO circuit, exemplary of embodiments of the present invention; 
           [0014]      FIG. 3  is further schematic diagram of the VCO circuit of  FIG. 2B ; and 
           [0015]      FIGS. 4 ,  5  and  6  are schematic diagrams of differential VCO circuits, exemplary of embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a simplified schematic diagram of a Colpitts oscillator  10 . As illustrated, a transistor  12  is interconnected with a tank circuit  14 . Analysis of this circuit is detailed in  Analysis of Common - Collector Colpitts Oscillator.  H R Pota. May 20, 2005, the contents of which are hereby incorporated by reference. As shown therein, the frequency of oscillation may be closely approximated as the resonant frequency of the tank circuit  14 , namely 
         [0000]    
       
         
           
             
               
                 
                   
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         [0017]    Unfortunately, the frequency range of operation of the circuit of  FIG. 1  is practically limited by the available sizes of C 1  and C 2 . This is particularly so, where the circuit of  FIG. 1  is formed on an integrated circuit that is miniaturized. 
         [0018]      FIG. 2A  is a therefore a schematic diagram of a VCO circuit  50 , that allows for more flexible frequency tuning, exemplary of an embodiment of the present invention. Circuit  50  includes a transistor  52 , interconnected with tank circuit  54 . Tank circuit  54  includes an inductor  56  interconnected in parallel with a capacitor C VAR2    58 . Inductor  56  is further interconnected in parallel with two series capacitors C FIXED , C VAR1    60 ,  62 . C FIXED  and C VAR1    60 ,  62  provide a voltage divider, interconnecting the transistor  52  in feedback. 
         [0019]    It may be shown that the oscillation frequency of VCO circuit  50  may be determined by 
         [0000]    
       
         
           
             
               
                 
                   
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         [0020]    Of note, this oscillation frequency substantially equals the resonant frequency of the tank circuit  54 . Both C VAR1  and C VAR2  may be used to tune VCO circuit  50  to a frequency of interest. This allows for greater tuning flexibility of VCO circuit  50 , particularly when the values of C VAR1  and C VAR2  are constrained, for example by size. 
         [0021]      FIG. 2B  is a further schematic diagram of a VCO circuit  100 , exemplary of an embodiment of the present invention. As illustrated VCO circuit  100  includes a bipolar junction transistor  102  connected in a common collector configuration to tank circuit  104 . As illustrated, the collector of transistor  102  is connected to ground, and its base is connected to ground through a parallel inductor L  106  and capacitor C VAR2    108  of tank circuit  104 . A further static capacitor C FIXED    110  connects the base to the emitter. The emitter is further coupled to ground through a variable capacitor C VAR1    112 . Inductor, L  106 , C FIXED    110 , C VAR2 , C VAR1    108 ,  112  thus form tank circuit  104  for VCO circuit  100  providing a frequency of oscillation as set out in equation (2), supra. 
         [0022]    Two separate variable capacitors, C VAR2  and C VAR1  allow VCO circuit  100  to achieve a broad range of frequencies that may be tuned with precision. In particular, C VAR1  may be used for coarse frequency tuning, while C VAR2  may be used for fine tuning. 
         [0023]    To that end,  FIG. 3  schematically illustrates the circuit of  FIG. 2B , in which C VAR1    112  is a formed by a bank  120  of multiple tuneable capacitors  124 , arranged in parallel. Capacitors  124  are connected in parallel, while C VAR1    112  is also formed as a tuneable capacitor. The capacitance of each capacitor  124  may be adjusted, through an applied voltage, CONTROL j . 
         [0024]    Each of capacitors  124  of C VAR1  may be formed in numerous ways, for example as varactor diodes, varactor transistors or other tuneable variable capacitors. Practically, as illustrated in  FIG. 3 , each capacitor  124  may be formed as a varactor transistor, whose capacitance is controlled through an applied voltage. Specifically, an applied analog control voltage CONTROL j  will control the effective capacitance of the i th  varactor transistor/capacitor  124  forming bank  120 . 
         [0025]    To further simplify overall control, the control voltages applied to each node may be chosen as 0 or a fixed control voltage V CONTROL . In this way each capacitor  124  in bank  120  will assume one of two values—one with CONTROL j =0, the other with CONTROL j =V CONTROL . The number of possible values for C VAR1    112  will therefore depend on the number, n, of capacitors  124  used, and the capacitance of each capacitor  124  (as dictated by the varactor used to form that capacitor). As capacitors  124  are effectively connected in parallel, their capacitance will sum. Theoretically, for n capacitors  124 , each formed as varactor, and two possible control voltages applied to CONTROL j , 2 n−1  capacitance values for bank  120  are possible. However, if the varactors are identical, n values for bank  120  will be possible. 
         [0026]    As further illustrated in  FIG. 3 , C VAR2    106  may also be formed as a varactor transistor  126 . An analog voltage V CONTROL     —     ANALOG  may control the effective capacitance of transistors  126 . 
         [0027]    Then, coarse frequency tuning may be effected through insertion of capacitors  124  of bank  120  defining C VAR1    112  using binary control signals CONTROL j , while fine tuning may be accomplished by varying analog control voltage V CONTROL     —     ANALOG , and thus the value of C VAR2 . 
         [0028]    As may now be appreciated, VCO circuit  100  has been illustrated as a common collector bipolar junction transistor (BJT) oscillator. However, tank circuit  104  suitably modified, may be used in numerous oscillators, including for example BJT oscillators connecting in common emitter or common base configurations. Likewise tank circuit  104  may be used with FET oscillators. As well, tank circuit  104  may be used in differential oscillators. 
         [0029]    To that end, a further differential VCO circuit  200  embodying tank circuit  104 ′ providing coarse and variable frequency control, in the manner described above, is depicted in  FIG. 4 . Here two single transistor VCOs, formed of bipolar junction transistor  202 ,  204  are arranged in differential mode, back to back. 
         [0030]    Each transistor  202 ,  204  is connected with capacitor C FIXED  connecting its base to its emitter. A variable capacitor of tank  104 ′ governs the value of C VAR2    208  for both transistor  202 ,  204  of both VCOs. Likewise, a common capacitor controls the value of C VAR1    212  for transistors  202 ,  204  both VCOs. 
         [0031]    The frequency of oscillation of VCO circuit  200  is governed by equation (2). Coils of value L connected to the base of transistors  202 ,  204  provide the inductance value L. Current sources  206  along with a bias to the base of transistors  202 ,  204  power oscillator  200 . 
         [0032]      FIG. 5  illustrates a further specific VCO circuit  200 ′, like VCO circuit  200 . In VCO circuit  200 ′, C VAR2    208  is formed using back-to-back varactor transistors  214 , and C VAR1    212  may be formed as a bank of back-to-back transistor pairs  216 . The node between each transistor in the pair is connected to virtual ground, as a result of the differential configuration of VCO circuit  200 . 
         [0033]    The bank forming C VAR1    212  includes three MOS varactor pairs  216  (although more varactor pairs could be used). An applied control voltage DIGITAL 13  CONTROL_BIT_ 0 , DIGITAL_CONTROL_BIT_ 1 , DIGITAL_CONTROL_BIT_ 2 , having a defined value will insert a respective capacitance of a varactor pair  214  into the bank forming C VAR1    212 . In the depicted embodiment, the values of varactor pairs  214  differ from each other, with one having a value CBASE=7 or 14 fF; the second having a value of 2*CBASE=14 to 28 fF; and the third having a value of 4*CBASE=28 to 56 fF. With values so chosen, eight (8) values of C VAR1  may be selected, having total capacitance between 7*C BASE  (49 fF) and 14*C BASE  (98 fF). 
         [0034]    Current source  206  (illustrated in  FIG. 4 ) may be formed as a current mirror  220 , and passive components  222 . 
         [0035]    The net capacitance of the two sets of varactors that form C VAR2    208  is controlled by a differential analog signal applied, ANAOG CONTROL+, and ANALOG_CONTROL-applied respectively to nodes between each varactor pair  214 . In the depicted embodiment, the capacitance of each varacator pair  214  may be adjusted continuously between 22 fF and 44 fF, allowing CVAR2 to assume a value between 22 fF and 88 fF. 
         [0036]    Of note, the applied differential analog signal control further reduces VCO phase noise of VCO circuit  200 ′. Varactors  214  are conveniently placed at the base node of transistors  202 ,  204  (and not at their emitters), whose voltage may also be close to the centre voltage of an external control source, such as a PLL charge pump, thereby extending the usable voltage control range of the VCO circuit  200 ′. 
         [0037]    VCO circuit  200 ′ is biased with a current mirror  220  from a 1.8V source. Transistors  202 ,  204  may be formed as SiGe heterojunction bipolar transistors. Accumulation-mode MOS varactors may be used as varactors in varactor pairs  216  forming C VAR1    212 , and as varactors in varactor pairs  214  forming C VAR2    208 . Example component values are also illustrated in  FIG. 5 . These component values are selected to tune the VCO circuit  200 ′ to a center frequency of around 60 GHz. The tunable bandwidth of VCO circuit  200 ′ with the specified components is about +/−4 GHz 
         [0038]    In the depicted embodiment of  FIG. 5 , the applied analog fine tune voltage causes the oscillation frequency of VCO circuit  200 ′ to vary up to 1.5 to 2 GHz. At the same time, each increment of C VAR1  controls the frequency of oscillation by 2 GHz. 
         [0039]    Choice of appropriate components such as the accumulation-mode MOS varactors, allow the supply voltage for VCO circuit  200  to be kept low. As noted, the frequency tuning is split into coarse digital controls for C VAR1  and fine analog control for C VAR2 . The digital controls perform frequency band selection; the control voltages (DIGITAL_CONTROL_BIT_i) are single-ended and are fixed at either ground or at the supply voltage of 1.2V using CMOS inverters. The analog control performs fine frequency tuning within the selected band. 
         [0040]    The dual tuning allows the VCO circuit  200 ′ to operate from a low supply voltage of only 1.2V without extra current, thus reducing the power consumption. 
         [0041]    As well, VCO circuit  200 ′ may be formed as a single integrated circuit, with all of the aforementioned components, or a subset thereof formed on-chip. 
         [0042]    In an alternate embodiment depicted in  FIG. 6 , VCO circuit  200 ′ may be further modified to add additional capacitance to C VAR2  capacitor to form VCO circuit  200 ″. In particular C VAR2    208 ′ may be formed a bank, including varactor pairs  214 ′ (like varactor pairs  214  of C VAR2    208 — FIG. 5 ), in parallel with additional varactor pairs  236 . This provides additional range to the value of C VAR2    208 ″. 
         [0043]    To simplify control, each varactor pair  236  may be controlled with the same control voltage used to control a varactor pair  216  of C VAR1 . The value of each varactor pair  236  may take on one of two states, depending on the presence of absence of a control voltage at DIGITAL_CONTROL_BIT 13  i. CMOS inverter/followers  232  may be interposed between the control voltage source and the varactor pair  236  and varactor pair  216 . So arranging C VAR2  and C VAR1  allows for an even greater range of capacitance values of C VAR2  and greater range of frequencies to which VCO circuit  200 ″ may be tuned. 
         [0044]    Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.