Patent Publication Number: US-6985040-B2

Title: Voltage controlled oscillator and method of generating an oscillating signal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application relies for priority upon Korean Patent Application No. 2003-55085 filed on Aug. 8, 2003, the contents of which are herein incorporated by reference in their entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a voltage controlled oscillator and a method of generating an oscillating signal. More particularly, the present invention relates to a temperature independent voltage controlled oscillator and a method of generating an oscillating signal that may generate an oscillating signal having a stable frequency independent of temperature variation. 
   2. Description of the Related Art 
   A voltage controlled oscillator (VCO) generates an oscillating signal having a frequency corresponding to an input voltage, and is used in a phase locked loop (PLL) as to generate oscillating signals. 
     FIG. 1  is a circuit diagram showing a conventional voltage controlled oscillator. 
   As shown in  FIG. 1 , a current mirror  100  of the conventional VCO generates a current  12  corresponding to a control voltage provided from an external source, and a ring oscillator  110  generates an oscillating signal having a frequency corresponding to the current  12 . A buffer  120  stabilizes the oscillating signal and outputs a stabilized oscillating signal Fout. 
   The frequency of the oscillating signal Fout is proportional to the current I 2 . 
   Typically, the current I 2  decreases as temperature increases. Accordingly, the frequency of the oscillating signal Fout decreases as temperature increases. As a result, a gain [Hz/V] of the VCO decreases as the temperature increases. The gain is referred to as a frequency to control voltage ratio. 
   Threshold voltages of the transistors M 2  and M 3  of the current mirror  100  decrease as temperature increases. Thus, the current I 2  generated by the current mirror  100  decreases as the temperature increases. 
     FIG. 2  is a graph showing frequency variations of the voltage controlled oscillator of  FIG. 1  according to temperature variation when the control voltage Vctrl is changed from 0 to 1.8 volts. Curve ‘A’ of  FIG. 2  shows frequency variation when the temperature is −55° C.; curve ‘B’ of  FIG. 2  shows frequency variation when the temperature is 55° C.; and curve ‘C’ of  FIG. 2  shows frequency variation when the temperature is 125° C. 
   Referring to  FIG. 2 , the frequency of the oscillating signal Fout decreases as temperature increases, and thus the gain (slope of the curve in  FIG. 2 ) of the VCO decreases as the temperature increases. 
   Therefore, the frequency of the oscillating signal generated from the conventional VCO may vary depending upon temperature variation. 
   As a result, the conventional VCO may not generate a signal at the desired frequency when the conventional VCO operates in semiconductor chip of a high speed digital system that generates a lot of heat. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is provided to substantially obviate one or more problems due to limitations and disadvantages of the related art. 
   It is a first feature of the present invention to provide a voltage controlled oscillator that is configured to generate an oscillating signal having a stable frequency independent of temperature variation. 
   It is a second feature of the present invention to provide a method of generating the oscillating signal having a stable frequency independent of temperature variation. 
   In one aspect, the invention is directed to a voltage controlled oscillator (VCO). The VCO of the invention includes: a current source that is configured to generate a first current having a first negative temperature coefficient; a current sink that is configured to generate a second current, a current level of the second current varying in response to a first voltage level of a control voltage, the second current having a second negative temperature coefficient; and a frequency generator that is configured to generate an oscillating signal having a frequency corresponding to a difference between the first and second currents. 
   In one embodiment, the current source includes: a reference current source that is configured to generate a reference current; a voltage generator that is configured to receive the reference current to generate a bias voltage based on the reference current; and a current mirror circuit that is configured to generate the first current, the first current being substantially a same current as the reference current. The voltage generator can includes: a first transistor, a second current electrode of the first transistor receiving the reference current, the second current electrode of the first transistor being connected to a control electrode of the first transistor; and a second transistor, a control electrode of the second transistor being connected to the control electrode of the first transistor. The current mirror circuit can include: a third transistor, a second current electrode of the third transistor being connected to the second current electrode of the second transistor, the second current electrode of the third transistor being connected to a control electrode of the third transistor; and a fourth transistor, a control electrode of the fourth transistor being connected to the control electrode of the third transistor. 
   In one embodiment, the current sink includes: a voltage level shifter that is configured to convert the first voltage level of the control voltage into a second voltage level to generate a first voltage having the second voltage level, and is configured to generate the second current corresponding to the first voltage; and a current subtractor that is configured to subtract the second current from the first current. The voltage level shifter can include: a first level shifter that is configured to convert the first voltage level of the control voltage into the second voltage level to generate the first voltage having the second voltage level; and a second level shifter that is configured to convert the second voltage level of the first voltage into a third voltage level to generate a second voltage having the third voltage level. In one embodiment, the first level shifter includes: a first transistor, a control electrode of the first transistor receiving a bias voltage based on a reference current; a second transistor, a control electrode of the second transistor receiving the control voltage, a first current electrode of the second transistor being connected to a second current electrode of the first transistor. The second level shifter includes a third transistor, a control electrode of the third transistor receiving the first voltage from the first level shifter, a first current electrode of the third transistor being connected to the current subtractor. 
   In one embodiment, the frequency generator includes: a ring oscillator that is configured to generate a first oscillating signal having the frequency corresponding to the difference between the first and second currents; and a buffer that is configured to convert a swing width of the first oscillating signal into a full swing width to generate the oscillating signal. 
   In another aspect, the invention is directed to a voltage controlled oscillator that includes: a voltage generator that is configured to generate a bias voltage based on a reference current; a current mirror circuit that is configured to generate a first current, the first current being substantially the same current as the reference current and having a first negative temperature coefficient; a first level shifter that is configured to convert a first voltage level of the control voltage into a second voltage level in response to the bias voltage to generate a first voltage having the second voltage level; a second level shifter that is configured to convert the second voltage level of the first voltage into a third voltage level to generate a second voltage having the third voltage level, and configured to generate a second current corresponding to the second voltage and having a second negative temperature coefficient; a current subtractor that is configured to subtract the second current from the first current to generate a third current; a ring oscillator that is configured to generate an oscillating signal having a frequency corresponding to the third current; and a buffer that is configured to convert a swing width of the oscillating signal into a full swing width. 
   In another aspect, the invention is directed to a method of generating an oscillating signal. The method includes: generating a first current having a first negative temperature coefficient based on a reference current; generating a second current, a current level of the second current varying in response to a first voltage level of a control voltage, and the second current having a second negative temperature coefficient; generating a third current corresponding to a difference between the first and second currents; and generating the oscillating signal having a frequency corresponding to the third current. 
   In one embodiment, generating a second current includes: converting a first voltage level of the control voltage into a second voltage level to generate a first voltage having the second voltage level; converting the second voltage level of the first voltage into a third voltage level to generate a second voltage having the third voltage level; and generating the second current corresponding to the second voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a circuit diagram showing a conventional voltage controlled oscillator. 
       FIG. 2  is a graph showing frequency variations of the voltage controlled oscillator of  FIG. 1  according to temperature variation. 
       FIG. 3  is a circuit diagram showing a voltage controlled oscillator according to one exemplary embodiment of the present invention. 
       FIG. 4  is a graph showing frequency variations of the voltage controlled oscillator of the present invention according to temperature variation. 
       FIG. 5  is a graph showing the variation of the second voltage of the voltage controlled oscillator of the present invention according to temperature variation. 
       FIG. 6  is a flow chart showing a method of generating an oscillating signal according to one exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
   It should also be noted that in some alternate implementations, the functions/acts noted in the steps may occur out of the order noted in the flowcharts. For example, two steps shown in succession may in fact be executed substantially concurrently or the steps may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     FIG. 3  is a circuit diagram showing a voltage controlled oscillator according to one exemplary embodiment of the present invention. 
   As shown in  FIG. 3 , the voltage controlled oscillator (VCO) includes a reference current source  300 , a current mirror circuit  310 , a voltage level shifter  320 , a current subtractor  330 , a ring oscillator  340  and a buffer  350 . 
   The reference current source  300  generates a reference current Iref. The current mirror circuit  310  generates a first current I 1 . The first current is substantially the same current as the reference current Iref, and has a first negative temperature coefficient. The voltage level shifter  320  converts a first voltage level of the control voltage Vctrl into a second voltage level to generate a first voltage V 1  having the second voltage level, and generates a second current I 2 . A current level of the second current I 2  corresponds to the first voltage V 1 . The second current I 2  has a second negative temperature coefficient. The current subtractor  330  subtracts the second current I 2  from the first current I 1 , and generates a third current I 3  corresponding to the difference between the first and second currents. Thus, the third current I 3  is independent of temperature variation. The ring oscillator  340  generates a first oscillating signal having a frequency corresponding to the difference between the first and second currents. The buffer  350  converts a swing width of the first oscillating signal into a full swing width. 
   The current mirror circuit  310  includes a voltage generator  312  and a current mirror  314 . The voltage generator  312  receives the reference current Iref, and generates a bias voltage Vbias based on the reference current Iref. The current mirror  314  generates the first current I 1  substantially the same as the reference current Iref input from the voltage generator  312 . 
   The voltage generator includes first and second transistors M 1  and M 2 . A drain electrode of the first transistor M 1  receives the reference current Iref from the reference current source  300 . The drain electrode of the first transistor M 1  is connected to a gate electrode of the first transistor M 1 . A gate electrode of the second transistor M 2  is connected to the gate electrode of the first transistor M 1 . The current mirror  314  includes third and fourth transistors M 3  and M 4 . For example, the first and second transistors M 1  and M 2  are NMOS transistors, and the third and fourth transistors M 3  and M 4  are PMOS transistors. 
   The voltage level shifter  320  includes first and second level shifters  322  and  324 . The first level shifter  322  converts the first voltage level of the control voltage Vctrl into the second voltage level to generate the first voltage V 1  having the second voltage level. The second level shifter  324  converts the second voltage level of the first voltage V 1  into a third voltage level to generate a second voltage V 2  having the third voltage level. 
   The first level shifter  322  includes fifth and sixth transistors M 5  and M 6 . A gate electrode of the fifth transistor M 5  receives a bias voltage Vbias, and a gate electrode of the sixth transistor M 6  receives the control voltage Vcr 1 . 
   The second level shifter  324  includes a seventh transistor M 7 . A gate electrode of the seventh transistor M 7  is connected to a source electrode of the sixth transistor M 6 . For example, the fifth and sixth transistors M 5  and M 6  are NMOS transistors, and the seventh transistor M 7  is PMOS transistor. 
   The ring oscillator includes first, second and third inverters INV 1 , INV 2  and INV 3 . The third current I 3  output from the current subtractor  330  is applied to the first, second and third inverters INV 1 , INV 2  and INV 3 . For example, the ring oscillator includes an odd number of inverters such as 5 or 7 inverters, etc. 
   Hereinafter, the operation of the VCO is described. 
   The voltage generator  312  of the current mirror circuit  300  generates the bias voltage Vbias based on the reference current Iref provided from the reference current source  300 . The voltage generator  312  outputs the bias voltage Vbias to the gate electrode of the fifth transistor M 5  of the first level shifter  322 . 
   In addition, the current mirror  314  generates the first current I 1  that is a mirror current of the reference current Iref provided from the voltage generator  312 , and outputs the first current I 1  to the current subtractor  330 . The first current I 1  has a negative temperature coefficient. 
   The first level shifter  322  shifts the first voltage level of the control voltage Vctrl into the second voltage level of the first voltage V 1 . The fifth transistor M 5  of the first level shifter  322  is turned on by the bias voltage Vbias, and the sixth transistor M 6  shifts the first voltage level of the control voltage Vctrl into the second voltage level to generate the first voltage V 1  having the second voltage level. 
   The first voltage V 1  is shown in expression 1.
 
 V 1 =V   ctrl −( V   th6   +ΔV   6 )  &lt;Expression 1&gt;
 
   , wherein the V th6  represents a threshold voltage of the sixth transistor M 6 , and ΔV 6  represents a saturation voltage between the drain electrode and the source electrode of the sixth transistor M 6 . 
   The seventh transistor M 7  of the second level shifter  324  receives the first voltage V 1  generated by the sixth transistor M 6  via the gate electrode of the seventh transistor M 7 , and the seventh transistor M 7  shifts the second voltage level of the first voltage V 1  into the third voltage level to generate the second voltage V 2  having the third voltage level. 
   The second voltage V 2  is shown in expression 2.
 
 V 2 =V   1 +( V   th7   +ΔV   7 )  &lt;Expression 2&gt;
 
   , wherein the V th7  represents a threshold voltage of the seventh transistor M 7 , and ΔV 7  represents a saturation voltage between the drain electrode and the source electrode of the seventh transistor M 7 . 
   The third voltage V 3  is expressed using the control voltage Vctrl in expression 3.
 
 V   2 = V   ctrl +( V   th7   −V   th6 )+( ΔV   7   −ΔV   6 )  &lt;Expression 3&gt;
 
   Expression 4 shows the voltage variation with respect to the temperature by applying partial difference to the expression 3. 
                 ∂   V2       ∂   T       =         ∂     V   ctrl         ∂   T       +       ∂     (       V   th7     -     V   th6       )         ∂   T       +       ∂     (       Δ   ⁢           ⁢     V   7       -     Δ   ⁢           ⁢     V   6         )         ∂   T                 〈     Expression   ⁢           ⁢   4     〉             
 
   Since the control voltage Vctrl is independent of the temperature, 
         ∂     V   ctrl         ∂   T         
 
is zero. 
         ∂     (       V   th7     -     V   th6       )         ∂   T         
 
has very small value and is almost zero. 
         ∂     (       Δ   ⁢           ⁢     V   7       -     Δ   ⁢           ⁢     V   6         )         ∂   T         
 
is proportional to the current mobility difference between the seventh and sixth transistors M 7  and M 6  with respect to the temperature change, since 
             ∂   Δ     ⁢           ⁢     V   7         ∂   T       ⁢           ⁢   and   ⁢           ⁢         ∂   Δ     ⁢           ⁢     V   6         ∂   T           
 
has negative value, respectively, 
         ∂     (       Δ   ⁢           ⁢     V   7       -     Δ   ⁢           ⁢     V   6         )         ∂   T         
 
is almost zero. As a result, 
         ∂   V2       ∂   T         
 
s almost zero. Therefore, the second voltage V 2  has substantially a constant independent of temperature variation and is proportional to the control voltage Vctrl.
 
   The current subtractor  330  subtracts the first current I 1  generated from the current mirror  314  from the second current I 2  applied to the source electrode of the seventh transistor M 7 , generates the third current I 3 , and outputs the third current I 3  to the ring oscillator  340 . 
   The third current I 3  is shown in expression 5.
 
 I   3 = I   1 − I   2 = I   1 −β( V   1   −V   2 ) 2   &lt;Expression&gt;5
 
   The third current I 3  is rewritten using expression 2 in the expression 6.
 
 I   3 = I   1 −β( V   th7   +ΔV   7 ) 2   &lt;Expression 6&gt;
 
   , wherein β represents a proportional constant. 
   Expression 7 shows the current variation with respect to the temperature by applying partial difference to the expression 6. 
                 ∂   I3       ∂   T       =         ∂   I1       ∂   T       -       ∂       β   ⁡     (       V   th7     +     Δ   ⁢           ⁢     V   7         )       2         ∂   T       +         ∂   2     ⁢     β   ⁡     (       V   th7     +     Δ   ⁢           ⁢     V   7         )           ∂   T                 〈     Expression   ⁢           ⁢   7     〉             
 
   Since 
         ∂     (       V   th7     +     Δ   ⁢           ⁢     V   7         )         ∂   T         
 
is almost zero, 
         ∂   I3       ∂   T         
 
is determined by 
                   ∂   I     ⁢           ⁢   1       ∂   T       ⁢           ⁢   and   ⁢           ⁢         ∂   β       ∂   T       .                           
 
Since 
         ∂   β       ∂   T         
 
has a negative value depending upon physical property, 
           ∂   I     ⁢           ⁢   3       ∂   T         
 
has almost zero when 
           ∂   I     ⁢           ⁢   1       ∂   T         
 
has a negative value.
 
   Since the first current I 1  is the mirror current of the reference current Iref, 
           ∂   I     ⁢           ⁢   1       ∂   T         
 
has a negative value when the Iref variation with respect to the temperature 
       (       ∂     I   ref         ∂   T       )       
 
has a negative value. Thus, the I 3  variation with respect to the temperature 
       (         ∂   I     ⁢           ⁢   3       ∂   T       )       
 
is almost zero. Therefore, the ring oscillator  340  generates the oscillating signal having a stable frequency corresponding to the third current I 3  that is independent of the temperature.
 
   The buffer converts the swing width of the oscillating signal generated from the ring oscillator into a full swing width to output an oscillating signal Fout. 
   As described above, the first current I 1  output from the current mirror circuit  310  has a first negative temperature coefficient, and the second current I 2  output from the voltage level shifter  320  has a second negative temperature coefficient. Thus, the third current generated from the current subtractor  330  by subtracting the second current I 2  from the first current I 1  is independent of temperature variation. That is, the third current I 3  has substantially constant value independent of temperature variation and is provided to the ring oscillator  340 . 
     FIG. 4  is a graph showing frequency variations of the voltage controlled oscillator of the present invention according to temperature variation when the control voltage Vctrl varies from 0 volt to about 1.8 volts. Curve ‘A’ of  FIG. 4  shows frequency variation when the temperature is about −55° C., curve ‘B’ of  FIG. 4  shows frequency variation when the temperature is about 55° C., and curve ‘C’ of  FIG. 4  shows frequency variation when the temperature is about 125° C. 
   Referring to  FIG. 4 , the variation ratio of the gain (slope of the curve in  FIG. 4 ) with respect to the temperature is reduced by about two times compared with that of the conventional VCO of  FIG. 2 . 
     FIG. 5  is a graph showing the variation of the second voltage of the voltage controlled oscillator of the present invention according to temperature variation when the control voltage Vctrl varies from 0 volt to about 1.8 volts. Curve ‘A’ of  FIG. 5  shows variation of the second voltage V 2  when the temperature is about −55° C., curve ‘B’ of  FIG. 5  shows variation of the second voltage V 2  when the temperature is about 55° C., and curve ‘C’ of  FIG. 5  shows variation of the second voltage V 2  when the temperature is about 125° C. 
   As shown in  FIG. 5 , the variation ratio (slope of the curve in  FIG. 5 ) of the second voltage V 2  has substantially constant value, and the second voltage V 2  is independent of temperature variation. 
     FIG. 6  is a flow chart showing a method of generating an oscillating signal according to one exemplary embodiment of the present invention. 
   Referring to  FIG. 6 , the current mirror circuit  310  generates a first current I 1  that is substantially the same as a reference current Iref input from the reference current source  300  (step S 600 ). The first current I 1  has a first negative temperature coefficient. 
   The first level shifter  322  of the voltage level shifter  320  shifts a first voltage level of the control voltage Vctrl into a second voltage level to generate a first voltage V 1  having the second voltage level, and the second level shifter  324  of the voltage level shifter  320  shifts the second voltage level of the first voltage V 1  into a third voltage level to generate a second voltage V 2  having the third voltage level (step S 602 ). The first and second voltage V 1  and V 2  are independent of temperature variation. 
   The second level shifter  324  generates a second current I 2  corresponding to the second voltage V 2  (step S 604 ). The second current I 2  has a second negative temperature coefficient. 
   The current subtractor  330  subtracts the second current I 2  from the first current I 1  to generate a third current I 3  (step S 606 ). Since the first and second currents I 1  and I 2  both have negative temperature coefficients, the third current I 3 , generated by subtracting the second current I 2  from the first current I 1 , is independent of temperature variation. 
   Then, the ring oscillator  340  generates an oscillating signal having a frequency corresponding to the third current I 3  (step S 608 ). The oscillating signal has substantially constant swing width. The buffer  350  changes the swing width of the oscillating signal into a full swing width. 
   According to above exemplary embodiments of the present invention, the VCO generates the oscillating signal based on the third current that is independent of temperature variation, and thus the oscillating signal has a stable frequency that is independent of temperature variation. Therefore, the stability of the system using the VCO according to above exemplary embodiments of the present invention may be enhanced. 
   In addition, when the VCO according to above exemplary embodiments of the present invention is used in a semiconductor chip of a high speed digital system that generates a lot of heat, the VCO may operates stably even though a lot of heat is generated. 
   While the exemplary embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the scope of the invention.