Patent Publication Number: US-2023147693-A1

Title: Temperature compensated voltage-controlled oscillator

Description:
BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     This present disclosure generally relates to voltage-controlled oscillators and more particularly to voltage-controlled oscillators with temperature compensation. 
     Description of Related Art 
     A VCO (voltage-controlled oscillator) outputs an oscillatory signal having an oscillation frequency controlled by a control voltage. In this present disclosure, hereafter, “frequency of oscillation” and “oscillation frequency” are the same and interchangeable in a context of VCO. 
     As depicted in a schematic shown in  FIG.  1   , a prior art VCO  100  comprises: a resonant tank  110  configured to determine a frequency of an oscillation in accordance with a control voltage V C ; and a regenerative network  120  comprising two NMOS (n-channel metal oxide semiconductor) transistors  121  and  122  configured in a cross-coupling topology to establish a negative resistance to provide energy to sustain the oscillation. Resonant tank  110  comprises an inductor  111  (of inductance L 111 ), a fixed capacitor  112  (of capacitance C 112 ), and a variable capacitor  115  (of capacitance C 115 ) comprising two varactors  113  and  114  controlled by V C . A center-tap of the inductor  111  is connected to a power supply node denoted by “V SP .” VCO  100  is well known in the prior art and thus not described in detail here. 
     An oscillation frequency f 0  of the VCO  100  is approximately: 
     
       
         
           
             
               
                 
                   
                     f 
                     0 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           
                             L 
                             111 
                           
                           ( 
                           
                             
                               C 
                               112 
                             
                             + 
                             
                               C 
                               115 
                             
                             + 
                             
                               C 
                               120 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, C 120  is a parasitic capacitance of the regenerative network  120 . 
     As used in this present disclosure, a “varactor” is a two-terminal circuit element that comprises a positive terminal marked by “+” and a negative terminal marked by “−”; a capacitance of the varactor increases (decreases) when a voltage at the positive terminal rises (falls) and decreases (increases) when a voltage at the negative terminal rises (falls); when the positive terminal is connected to a control voltage configured to control the capacitance, the varactor is said to be forward connected; when the negative terminal is connected to a control voltage configured to control the capacitance, the varactor is said to be backward connected. Having said that, the two varactors  113  and  114  in  FIG.  1    are clearly backward connected; when V C  rises (falls), the capacitance of the two varactors  113  and  114  decrease (increases) and consequently C 115  decreases (increases), and as a result the oscillation frequency of VCO  100  increases (decreases). 
     In practice, VCO  100  is usually incorporated in a phase lock loop that adjusts the control voltage V C  in a closed-loop manner so that f 0  is equal to target value that is equal to a frequency of a reference clock times a multiplication factor; if f 0  is higher (lower) than the target value, the phase lock loop will lower (raise) the control voltage V C  to lower (raise) f 0 . However, a realistic phase lock loop circuit has a limited range for the control voltage V C  that it can generate. The phase lock loop fails if the control voltage V C  has already reached the minimum (maximum) value that the phase lock loop can generate but f 0  is still higher (lower) than the target value. Therefore, care must be taken to ensure that f 0  can be equal to the target value with a value of the control voltage V C  that is within the range that the phase lock loop can generate. 
     In practice, L 111  is temperature dependent, so are C 112 , C 115 , and C 120 ; given the same control voltage V C , f 0  often varies with the temperature. Consequently, when the temperature changes, f 0  will change, and the phase lock loop must adjust the control voltage V C  to compensate the change of f 0  due to the temperature change. If the control voltage V C  has reached its limit but the change of f 0  due to the temperature change is not yet fully compensated, the phase lock loop fails. 
     What is desired is a VCO that can effectively compensate for a change of oscillation frequency due to a temperature change. 
     SUMMARY OF THE DISCLOSURE 
     In an embodiment, a VCO (voltage-controlled oscillator) comprises: a resonant tank comprising a parallel connection of an inductor, a fixed capacitor, a variable capacitor, a first temperature compensating capacitor, and a second temperature compensating capacitor across a first node and a second node, and configured to establish an oscillation of a first oscillatory voltage at the first node and a second oscillatory voltage at the second node; and a regenerative network placed across the first node and the second node to provide energy to sustain the oscillation, wherein the variable capacitor is controlled by a control voltage, the first temperature compensating capacitor is controlled by a first temperature tracking voltage of a positive temperature coefficient, and the second temperature compensating capacitor is controlled by a second temperature tracking voltage of a negative temperature coefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic diagram of a prior art voltage-controlled oscillator. 
         FIG.  2    shows a schematic diagram of a voltage-controlled oscillator in accordance with an embodiment of the present disclosure. 
         FIG.  3    shows a schematic diagram of a temperature tracking voltage generator. 
     
    
    
     DETAILED DESCRIPTION OF THIS DISCLOSURE 
     The present disclosure is directed to voltage-controlled oscillators. While the specification describes several example embodiments of the disclosure considered favorable modes of practicing the invention, it should be understood that the invention can be implemented in many ways and is not limited to the particular examples described below or to the particular manner in which any features of such examples are implemented. In other instances, well-known details are not shown or described to avoid obscuring aspects of the disclosure. 
     Persons of ordinary skill in the art understand terms and basic concepts related to microelectronics that are used in this disclosure, such as “voltage,” “signal,” “frequency,” “oscillation,” “voltage-controlled oscillator,” “parallel connection,” “series,” “shunt,” “circuit node,” “ground,” “power supply node,” “MOS (metal oxide semiconductor) transistor,” “CMOS (complementary metal oxide semiconductor) process technology,” “NMOS (n-channel metal oxide semiconductor) transistor,” “PMOS (p-channel metal oxide semiconductor) transistor,” “inductor,” “inductance,” “capacitor,” “varactor,” “capacitance,” “resistor,” “resistance,” and “low-pass filter.” Terms and basic concepts like these, when used in a context of microelectronics, are apparent to those of ordinary skill in the art and thus will not be explained in detail herein. 
     Those of ordinary skills in the art can read and understand schematics of circuits comprising components such as capacitors, NMOS transistors, PMOS transistors, and so on, and do not need a verbose description about how one component connects with another in the schematics. Those of ordinary skill in the art can also recognize a ground symbol, a capacitor symbol, an inductor symbol, a varactor (variable capacitor) symbol, and symbols of PMOS transistor and NMOS transistor, and identify the “source terminal,” the “gate terminal,” and the “drain terminal” thereof. Pertaining to a MOS transistor, for brevity, hereafter, “source terminal” is simply referred to as “source,” “gate terminal” is simply referred to “gate,” and “drain terminal” is simply referred to “drain.” 
     A circuit is a collection of at least a transistor, a capacitor, a resistor, and/or other electronic devices inter-connected in a certain manner to embody a certain function. 
     In this disclosure, a “circuit node” is frequently simply stated as a “node” for short, when what it means is clear from a context. 
     A signal is a voltage of a variable level that carries a certain information and can vary with time. A level of the signal at a moment represents a state of the signal at that moment. 
     As mentioned earlier in this present disclosure, a varactor is a two-terminal circuit element of a variable capacitance that comprises a positive terminal marked by “+” and a negative terminal marked by “−”; a capacitance of the varactor increases (decreases) when a voltage at the positive terminal rises (falls) and decreases (increases) when a voltage at the negative terminal rises (falls); when the positive terminal is connected to a control voltage configured to control the variable capacitance, the varactor is said to be forward connected; when the negative terminal is connected to a control voltage configured to control the variable capacitance, the varactor is said to be backward connected. 
     As shown by a schematic depicted in  FIG.  2   , a VCO  200  in accordance with an embodiment of the present disclosure comprises: a resonant tank  210  comprising an inductor  211  (of inductance L 211 ), a fixed capacitor  212  (of capacitance C 212 ), a variable capacitor  230  (of capacitance C 230 ), a first temperature compensating capacitor  240  (of capacitance C 240 ), and a second temperature compensating capacitor  250  (of capacitance C 250 ), configured in a parallel connection topology across a first node  201  and a second node  202  to establish an oscillation of a first oscillatory voltage V 1  at the first node  201  and a second oscillatory voltage V 2  at the second node  202  in accordance with a control by a control voltage V c , a first temperature tracking voltage V cp  of a positive temperature coefficient, and a second temperature tracking voltage V cn  of a negative temperature coefficient; and a regenerative network  220  comprising two NMOS transistors  221  and  222  configured in a cross-coupling topology to provide a negative resistance across the first node  201  and the second node  202  to sustain the oscillation of V 1  and V 2 . A center-tap of the inductor  211  connects to a power supply node denoted by “V SP .” NMOS transistors  221  and  222  are said to be cross-coupling because a drain of NMOS transistor  221  connects to a gate of NMOS transistor  222 , while a drain of NMOS transistor  222  connects to a gate of NMOS transistor  221 ; that two MOS transistors configured in a cross-coupling topology can provide a negative resistance to sustain an oscillation is well known to those of ordinary skill in the art and thus not described in detail here. 
     The variable capacitor  230  comprises two backward connected varactors  231  and  232  controlled by V c . The first temperature compensating capacitor  240  comprises two backward connected varactors  241  and  242  controlled by V cp . The second temperature compensating capacitor  250  comprises two forward connected varactors  251  and  252  controlled by V cn . V cp  is a voltage that will rise when a temperature of VCO  200  rises and thus is said to have a positive temperature coefficient. V cn  is a voltage that will fall when the temperature of VCO  200  rises and thus is said to have a negative temperature coefficient. 
     VCO  200  of  FIG.  2    is similar to the VCO  100  of  FIG.  1    except that it further includes the first temperature compensating capacitor  240  and the second temperature compensating capacitor  250 . An oscillation frequency f osc  of the first oscillatory voltage V 1  and the second oscillatory voltage V 2  is approximately equal to a resonant frequency of the resonant network  210  and can be written as: 
     
       
         
           
             
               
                 
                   
                     f 
                     OSC 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           
                             L 
                             211 
                           
                           ( 
                           
                             
                               C 
                               212 
                             
                             + 
                             
                               C 
                               230 
                             
                             + 
                             
                               C 
                               240 
                             
                             + 
                             
                               C 
                               250 
                             
                             + 
                             
                               C 
                               220 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Here, C 220  is a parasitic capacitance of the regenerative network  220  across the two nodes  201  and  202 . 
     Without the first temperature compensating capacitor  240  and the second temperature compensating capacitor  250 , L 211  increases (decreases) when the temperature of VCO  200  rises (falls) and will result in a decrease (increase) of f osc . When the temperature rises (falls), V cp  rises (falls) and causes C 240  to decrease (increase) thanks to the two backward connected varactors  241  and  242 , while at the same time V cn  falls (rises) and causes C 250  to decrease (increase) thanks to the two forward connected varactors  251  and  252 ; as a result, the effect of the increase (decrease) of L 211  can be compensated by the effect of the decrease (increase) of C 240  and C 250 . This way, a change of the oscillation frequency of VCO  200  due to a temperature change can be reduced. 
     Both the first temperature compensating capacitor  240  and the second temperature compensating capacitor  250  can fulfill a purpose of temperature compensation; therefore, using merely one of the two is sufficient, as far as temperature compensation is concerned. However, using both of the two has an advantage that, the circuit becomes more balanced and more immune to noise. For instance, an increase of a voltage level of V 1  due to a noise will cause an increase of C 240  but at the same time will cause a decrease of C 250  that can offset the increase of C 240 . 
     V cp  is a voltage that rises (falls) when the temperature of VCO  200  rises (falls), while V cn  is a voltage that falls (rises) when the temperature of VCO  200  rises (falls). A schematic diagram of a temperature tracking voltage generator  300  that can be used to generate V cp  and V cn  is shown in  FIG.  3   . The temperature tracking voltage generator  300  comprises: a first resistor R 1  configured to receive a first PTAT (proportional to absolute temperature) current I ptat1  and establish a first reference voltage V r1 , and a first low-pass filter  310  comprising a series resistor  311  and a shunt capacitor  312  configured to receive the first reference voltage V r1  and output V cp ; a second resistor R 2  configured to receive a second PTAT (proportional to absolute temperature) current I ptat2  and establish a second reference voltage V r2 , and a second low-pass filter  320  comprising a series resistor  321  and a shunt capacitor  322  configured to receive the second reference voltage V r2  and output V cn . Here, “V DD ” denotes a power supply node. A PTAT current is a current of a level proportional to an absolute temperature. A PTAT current and how to generate a PTAT current are well known in the prior art and well understood by those of ordinary skills in the art and thus not described in detail here. I ptat1  and I ptat2  are both PTAT currents, and thus increase (decrease) when the temperature rises (falls). 
     We can derive the following two equations: 
         V   cp   ≅V   r1   =I   ptat1   R   1   (3)
 
         V   cn   ≅V   r2   =V   DD   −I   ptat2   R   2   (4)
 
     It is clear that V cp  rises (falls) when the temperature rises (falls), whereas V cn  falls (rises) when the temperature rises (falls). Therefore, V cp  has a positive temperature coefficient, whereas V cn  has a negative temperature coefficient. 
     The principle of this present disclosure can apply to other VCO circuit embodiments. For instance, the regenerative network  220  can be embodied using two cross-coupling PMOS transistors or using a combination of two cross-coupling NMOS transistors and two cross-coupling PMOS transistors, instead of using two cross-coupling NMOS transistors as shown in the case of VCO  200  of  FIG.  2   . 
     The two varactors  231  and  232  are backward connected, therefore an increase of V c  will lead to a decrease of C 230  and thus an increase of f osc . This is just by way of example but not limitation. In an alternative embodiment not explicitly shown in  FIG.  2   , varactors  231  and  232  are forward connected, i.e., the “+” terminal of varactor  231  connects to V c , the “−” terminal of varactor  231  connects to node  201 , the “+” terminal of varactor  232  connects to V c , and the “−” terminal of varactor  232  connects to node  202 . In this alternative embodiment, an increase of V c  will lead to an increase of C 230  and thus a decrease of f osc . This is workable, provided the control voltage is adapted in a correct direction in a closed-loop control manner. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the disclosure. Accordingly, the above disclosure should not be construed as limited only by the metes and bounds of the appended claims.