Patent Publication Number: US-2009224843-A1

Title: Programmable Crystal Oscillator

Description:
FIELD OF THE INVENTION 
     The present invention relates to integrated circuits using standard CMOS technology. More specifically, the present invention relates to a CMOS circuit for adjusting the frequency oscillation of a crystal oscillator. 
     RELATED ART 
       FIG. 1  is a conventional crystal oscillator circuit  100 , which includes inverting amplifier  101 , resistor  102  (which has a resistance RF), crystal  103 , fixed load capacitors  110  and  120 , a first set of programmable load capacitors  111 - 114 , a corresponding first set of switches  131 - 134 , a second set of programmable load capacitors  121 - 124 , and a corresponding second set of switches  141 - 144 . 
     Inverting amplifier  101 , resistor  102  and crystal  103  are connected in parallel between terminals  151  and  152 . Fixed load capacitors  110  and  120  are connected between terminals  151  and  152 , respectively, and ground. The programmable load capacitors  111 - 114  can be selectively connected between terminal  151  and ground by activating switches  131 - 134 , respectively. Similarly, the programmable load capacitors  121 - 124  can be selectively connected between terminal  152  and ground by activating switches  141 - 144 , respectively. Switches  131 ,  132 ,  133  and  134  are controlled by control signals A 1 , A 2 , A 3  and A 4 , respectively. Switches  141 ,  142 ,  143  and  144  are controlled by control signals B 1 , B 2 , B 3  and B 4 , respectively. 
     The fixed load capacitors  110  and  120  have capacitances C 10  and C 20 , respectively, wherein C 10  is typically equal to C 20 . The programmable load capacitors  111 ,  112 ,  113  and  114  have capacitances of C 11 , C 12 , C 13  and C 14 , respectively, and programmable load capacitors  121 ,  122 ,  123  and  124  have capacitances of C 21 , C 22 , C 23  and C 24 , respectively. Capacitances C 11 , C 12 , C 13  and C 14  are typically equal to capacitances C 21 , C 22 , C 23  and C 24 , respectively. 
     Crystal  103  can be modeled by an inductor  104  having an inductance L M , a capacitor  105  having a capacitance C M , a resistor  106  having a resistance R S , and a capacitor  107  having a capacitance C 0 . Inductor  104 , capacitor  105  and resistor  106  are connected in series between terminals  151  and  152 . Capacitor  107  is connected between terminals  151  and  152 , in parallel with the series-connected inductor  104 , capacitor  105  and resistor  106 . 
     The series resonant frequency (fs) of crystal  103  is determined by the inductance L M  of inductor  104  and the capacitance C M  of capacitor  105 . The series resonant frequency (fs) of crystal  103  is specified by the following equation. 
     
       
         
           
             
               
                 
                   fs 
                   = 
                   
                     1 
                     
                       2 
                       * 
                       π 
                       * 
                       
                         
                           ( 
                           
                             
                               C 
                               M 
                             
                             * 
                             
                               L 
                               M 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     The parallel resonant frequency of oscillation (fp) of crystal oscillator circuit  100  can be represented by the following equation, wherein C L  is the capacitance introduced by load capacitors  110 - 114  and  120 - 124 . 
     
       
         
           
             
               
                 
                   fp 
                   = 
                   
                     fs 
                     * 
                     
                       [ 
                       
                         1 
                         + 
                         
                           
                             C 
                             M 
                           
                           
                             2 
                             * 
                             
                               ( 
                               
                                 
                                   C 
                                   0 
                                 
                                 + 
                                 
                                   C 
                                   L 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     If only fixed load capacitors  110  and  120  are connected to terminals  151  and  152  (i.e., switches  131 - 134  and  141 - 144  are all open), the load capacitance C L  can be represented by the following equation. 
         C   L =( C   10   *C   20 )/( C   10   +C   20 )   equation (3) 
     The load capacitance C L  can be adjusted by selectively activating the switches  131 - 134  and  141 - 144 . In general, the load capacitance C L  increases as additional switches are activated (i.e., as additional capacitors are connected to terminals  151  and  152 ). Increasing the load capacitance C L  causes the parallel resonant frequency of oscillation fp to be reduced, as indicated by equation (2). 
     Although crystal oscillator circuit  100  allows the parallel resonant frequency of oscillation fp to be adjusted, the programmable load capacitors  111 - 114  and  121 - 124  require a relatively large layout area on an integrated circuit chip. It would therefore be desirable to have an improved crystal oscillator circuit, which is capable of parallel resonant frequency adjustment, but does not require an excessive layout area. 
     SUMMARY 
     Accordingly, the present invention provides a crystal oscillator circuit having a parallel resonant frequency that is adjustable by switching trim capacitors in parallel with a crystal. 
     The present invention will be more fully understood in view of the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a conventional crystal oscillator circuit having an adjustable frequency. 
         FIG. 2  is a circuit diagram of a crystal oscillator circuit having an adjustable frequency in accordance with one embodiment of the present invention. 
         FIG. 3  is a circuit diagram of a crystal oscillator circuit having an adjustable frequency in accordance with one variation of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a circuit diagram of a crystal oscillator circuit  200  having an adjustable frequency in accordance with one embodiment of the present invention. Crystal oscillator circuit  200  includes inverting amplifier  101 , resistor  102 , crystal  103 , fixed load capacitors  110  and  120 , and terminals  151 - 152 , which are described above in connection with  FIG. 1 . In addition, crystal oscillator circuit  200  includes parallel capacitive trim circuit  250 , which is connected in parallel between terminals  151  and  152 . Capacitive trim circuit  250  includes trim capacitors  201 - 204  and switches  221 - 228 . A pair of switches couples each of the trim capacitors  201 - 204  between terminals  151  and  152 . Thus, switch pairs  221 - 222 ,  223 - 224 ,  225 - 226  and  227 - 228  are configured to couple trim capacitors  201 ,  202 ,  203  and  204 , respectively, between terminals  151  and  152 . Each switch pair is controlled by a corresponding capacitor trim signal. For example, the switch pair  221 - 222  are controlled by the capacitor trim signal CAP_TRIM[ 1 ]. When the capacitor trim signal has a first logic state (e.g., a logic ‘0’ value), both switches of the corresponding switch pair are turned off (non-conductive), thereby disconnecting the corresponding trim capacitor from terminals  151  and  152 . Conversely, when the capacitor trim signal has a second logic state (e.g., a logic ‘1’ value), both switches of the corresponding switch pair are turned on (i.e., conductive), thereby connecting the corresponding trim capacitor between terminals  151  and  152 . 
     In one embodiment, more than one of the capacitor trim signals can be simultaneously activated to the second logic state, such that more than one of the trim capacitors can be simultaneously connected between terminals  151  and  152 . In an alternate embodiment, the four capacitor trim signals CAP_TRIM[ 1 : 4 ] are generated by decoding a 2-bit control signal provided on a pair of pins of an integrated circuit chip. In this embodiment, only one of the capacitor trim signals CAP_TRIM[ 1 : 4 ] can be activated at any given time. Although the illustrated embodiments only implement four trim capacitors and the associated switch pairs, it is understood that other numbers of trim capacitors/switch pairs can be used in other embodiments. Trim capacitors  201 ,  202 ,  203  and  204  have capacitances C 1 , C 2 , C 3  and C 4 , respectively. In one embodiment, capacitances C 1 -C 4  are binary weighted, thereby enabling trimming from a minimum frequency to a maximum frequency with a predefined step. In alternate embodiments, capacitances C 1 -C 4  can have other weightings. 
     The parallel resonant frequency of oscillation (fp) of crystal oscillator circuit  200  can be represented by equation (4) below, wherein C L  is the capacitance introduced by load capacitors  110  and  120 , and C P  is the capacitance of the trim capacitor(s) connected between terminals  151  and  152 . 
     
       
         
           
             
               
                 
                   fp 
                   = 
                   
                     fs 
                     * 
                     
                       [ 
                       
                         1 
                         + 
                         
                           
                             C 
                             M 
                           
                           
                             2 
                             * 
                             
                               ( 
                               
                                 
                                   C 
                                   0 
                                 
                                 + 
                                 
                                   C 
                                   P 
                                 
                                 + 
                                 
                                   C 
                                   L 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     Note that connecting a trim capacitor in parallel with crystal  103  effectively adds the capacitance of the trim capacitor to the capacitance C 0  of the crystal  103 . The trim capacitance C P  can be adjusted by selectively activating the switches  221 - 228 . In general, the trim capacitance C P  increases as additional switches are activated (i.e., as additional capacitors are connected to terminals  151  and  152 ), or as larger trim capacitors are connected between terminals  151  and  152 . Increasing the trim capacitance C P  causes the parallel resonant frequency of oscillation fp to be reduced, as indicated by equation (4). 
     In order for the trim capacitors  201 - 204  of crystal oscillator circuit  200  to produces the same frequency adjustments as the programmable load capacitors  121 - 124  and  121 - 124  of crystal oscillator circuit  100 , the capacitances C 1 -C 4  of trim capacitors  201 - 204  should have the following relationships with respect to the capacitances C 11 -C 14  and C 21 -C 24  of programmable load capacitors. 
         C 1=( C 11* C 21)/( C 11+ C 21)   equation (5) 
         C 2=( C 12* C 22)/( C 12+ C 22)   equation (6) 
         C 3=( C 13* C 23)/( C 13+ C 23)   equation (7) 
         C 4=( C 14* C 24)/( C 14+ C 24)   equation (8) 
     In general, a trim capacitor of the present invention can replace two programmable load capacitors, wherein the trim capacitor is half the size of either of the two programmable load capacitors. For example, suppose that programmable load capacitors  111  and  121  of crystal oscillator circuit  100  ( FIG. 1 ) have capacitances C 11  and C 12  of 10 pf each. In the crystal oscillator circuit  200  of the present invention, these programmable load capacitors  111  and  121  can be replaced with a single trim capacitor  201  having a capacitance C 1  of 5 pf. (See, equation (5)). As a result, the chip area required by capacitors used to adjust the oscillation frequency fp is significantly reduced by the present invention. 
       FIG. 3  is a circuit diagram of a programmable crystal oscillator circuit  300  in accordance with one variation of the present invention. Programmable crystal oscillator circuit  300  includes a series pulling capacitor  301  (having a capacitance C T ) in series with in series with crystal  103 . Series pulling capacitor  301  increases the series resonant frequency (fs) of the oscillator circuit  300 . Trim capacitors  201 - 204  operate in the same manner described above to adjust the parallel resonant frequency (fp) of oscillator circuit  300 . 
     Although the present invention has been described in connection with particular embodiments, it is understood that variations in these embodiments would be apparent to one of ordinary skill in the art. Thus, the present invention is only limited by the following claims.