Abstract:
A voltage controlled oscillator and a method of operating a voltage-controlled oscillator are disclosed. The oscillator comprises a current controlled oscillator having a variable frequency current output, a first control path for generating a first control current having a first adjustable gain, and a second control path for generating a second control current having a second adjustable gain. A summer is provided for adding the first and second control currents to obtain a summed control current, and for applying the summed control current as an input current to the current controlled oscillator. A control sub-circuit is used for controlling the gain of the first control current as a function of a defined voltage on the second control path to maintain constant the gain of the current output of the current controlled oscillator over a given operating range of the current controlled oscillator.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to voltage-controlled oscillators, and more specifically, the invention relates to architectures for maintaining constant voltage controlled oscillator gain. Even more specifically, the preferred embodiment of the invention relates to such an architecture that is very well suited for use in phase-locked loops in order to make loop dynamics uniform across the tuning range of the voltage controlled oscillator. 
         [0003]    2. Background Art 
         [0004]    Phase-locked loops (PLLs) are commonly used to generate high-frequency signals in a wide variety of electronic circuit applications. Many important PLL parameters (jitter, settling time) depend on the bandwidth of the PLL. Loop bandwidth in turn depends on, among other parameters, the gain of the oscillator in the PLL. In the case of a voltage-controlled oscillator (VCO), the gain, which is the incremental slope of its output frequency vs. tuning voltage characteristic, has the units Hz/V. For many VCO types (e.g., LC, ring VCOs of the current starved type, ring VCOs of the delay interpolating type, or ring VCOs of a hybrid type), their output frequency vs. tuning voltage characteristic is nonlinear; giving rise to a VCO gain that varies as a function of tuning voltage. For many VCOs, the input voltage is converted to a current (by a voltage to current converter (V-I converter)), which in turn is applied to a circuit and which can be viewed as a current controlled oscillator (ICO). Non-linearity in the overall VCO tuning characteristic can stem from non-linearity in the ICO&#39;s output frequency vs. tuning current characteristic, or from non-linearity in the V-I converter&#39;s current vs. voltage characteristic. Solutions exist to create a VCO with a linear characteristic by introducing a non-linearity into the V-I converter that compensates for the inherent non-linearity of the ICO. 
         [0005]    Single port VCOs do not always enable adequate PLL performance due to tradeoffs associated with VCO gain. High VCO gain helps enable a wide tuning range, while low VCO gain helps reduce noise sensitivity. Architectures have been proposed that split the VCO control path into two (or more) paths. In the case of a two-path control loop, one path has lower gain and high bandwidth, while the other path has high gain and lower bandwidth. This splitting reduces the VCO&#39;s noise sensitivity by allowing for substantial filtering on the high VCO gain path, however, the VCO will still have a nonlinear tuning characteristic. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of this invention is to maintain constant the gain of a voltage-controlled oscillator over the tuning range of the oscillator. 
         [0007]    Another object of the present invention is to provide a voltage-controlled oscillator with two input control paths, and to control the voltage-to-frequency conversion of these two input paths so that the gain of the oscillator is relatively constant over the tuning range of the oscillator. 
         [0008]    These and other objectives are attained with a voltage-controlled oscillator and a method of operating a voltage controlled oscillator. The oscillator comprises a current controlled oscillator having a variable frequency current output, a first control path for generating a first control current having a first adjustable gain, and a second control path for generating a second control current having a second adjustable gain. A summer is provided for adding the first and second control currents to obtain a summed control current, and for applying the summed control current as an input current to the current controlled oscillator. A control sub-circuit is used for controlling the gain of the first control current as a function of a defined voltage on the second control path to maintain constant the gain of the current output of the current controlled oscillator over a given operating range of said current controlled oscillator. 
         [0009]    The present invention may be applied to a two path PLL scheme, though it is also applicable to a single charge pump scheme. A preferred PLL, described below in detail, employs two charge pumps, each driving its own loop filter. One charge pump drives a large capacitor and is denoted as the “integral path”. The other charge pump drives a resistor in parallel with a small capacitor. This is denoted as the “proportional path”. The output of each loop filter connects to a V-I converter. The outputs of the V-I converters are added together and applied to the ICO. In this architecture, the VCO gain through the proportional path (K p ) typically defines the PLL bandwidth while the integral path will determine the VCO&#39;s tuning range. The integral path can be filtered without degrading loop stability thereby reducing the PLL&#39;s noise sensitivity. 
         [0010]    During regular operation, the proportional path control voltage is kept small, since the integral path integrates phase errors between the VCO&#39;s output and the reference clock out. The small voltage on the proportional path means that non-linearity in the proportional path V-I converter will not be exercised, and it is only the small-signal gain of the V-I converter on the proportional path that will influence PLL dynamics. However, the small signal gain of the ICO to the proportional path is a function of the voltage on the integral path. As the VCO tunes across its frequency range due to changes in the integral path&#39;s control voltage, the proportional paths gain changes. 
         [0011]    To prevent the proportional path small-signal gain from changing as the VCO is tuned across its operating range by the integral path, the gain of the V-I converter on the proportional path is made to be a function of the voltage on the integral path. Selecting the appropriate function can result in a proportional path VCO gain that is constant across the integral path&#39;s tuning range. 
         [0012]    In this invention, the proportional path V-I converter is decomposed into two V-I converters. The first V-I converter has fixed gain. The second V-I converter has variable gain proportional to 1-x where x is the output of the integral path V-I converter (normalized to having a range from 0 to 1). The outputs of the two proportional path V-I converters are connected together, meaning that their outputs are summed, and applied to the ICO. The gain of the composite proportional path V-I converter varies in a way that largely compensates for the non-linearity in the ICO&#39;s tuning characteristic. This allows for the variation in the VCO&#39;s proportional path gain across the integral path&#39;s tuning range to be reduced. 
         [0013]    Further benefits and advantages of this invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a single path phase-locked loop. 
           [0015]      FIG. 2  shows a dual charge pump phase-locked loop. 
           [0016]      FIGS. 3(   a ),  3 ( b ) and  3 ( c ) illustrate three signaling schemes for delay interpolators that may be used in the practice of the present invention. 
           [0017]      FIG. 4  is a graph showing predicted VCO gain as a delay interpolating VCO is tuned across its integral path. 
           [0018]      FIG. 5  is a circuit diagram showing an implementation of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    The preferred embodiment of the present invention relates to an architecture for maintaining constant the gain of a voltage-controlled oscillator, and the preferred architecture is very well suited for use in phase locked loops (PLLs). As an example,  FIG. 1  illustrates a generic PLL relevant to the field of the invention. 
         [0020]    Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates a conventional PLL. A conventional PLL comprises a Phase-Frequency Detector (PFD)  102 , a charge pump  104 , a Low Pass Filter (LPF)  106 , a VCO  108 , and a frequency divider  110 . The LPF  106  further comprises a capacitor  116  and a resistor  118 . Also, the VCO  108  includes a voltage-to-current converter  112  and a current controlled oscillator  114 . 
         [0021]    The PLL  100  operates by maintaining charge on the first capacitor  116  of the LPF  106 . A reference signal or input signal is input into the PFD  102  along with feedback from the frequency divider  110 . Based on the comparison between the inputted signals, the PFD  102  can activate switches (not shown) of the charge pump  104  to add charge to or to remove charge from the capacitor  116  of the LPF  106 . Also, based on the comparison between the inputted signals, the PFD  102  may not provide an activation signal, which maintains the level of charge on the capacitor  116  of the LPF  106 . 
         [0022]    The act of adding to or subtracting from the charge stored on the capacitor effectively changes the voltage of the LPF  106  because of the capacitive relationship between charge and voltage. The voltage of the LPF  106  is then input into the VCO  108  and is used to control the frequency and phase of the output of the VCO  108 . During steady-state operation (when the PLL is locked), the action of the loop aligns the phase and frequency of the divided feedback signal to the phase and frequency of the reference clock. The output signal from the VCO  108  is input into the frequency divider  110 . Also, the output signal of VCO  108  is used in a variety of circuits to perform a variety of tasks. 
         [0023]    With conventional PLLs, there are some disadvantages. One disadvantage is non-linearity in the overall VCO voltage-to-frequency tuning characteristic. Non-linearity in said tuning characteristic results in variation in VCO gain. The non-linearity can stem from non-linearity in the ICO&#39;s output frequency vs. tuning current characteristic, or from non-linearity in the voltage to current converter&#39;s current vs. voltage characteristic. 
         [0024]    The present invention addresses this voltage-to-frequency tuning characteristic non-linearity and, more specifically, provides an architecture for maintaining constant the gain of the VCO.  FIG. 2  illustrates an embodiment of the present invention. This embodiment employs two charge pumps. It should be noted that the invention is also applicable to single charge pump scheme. 
         [0025]    With reference to  FIG. 2 , one charge pump  212  drives a large capacitor  214  and is denoted as the “integral path”  216 . The other charge pump  220  drives a resistor  222  in parallel with a small capacitor  224 . This is denoted as the “proportional path”  226 . The output of each loop filter connects to a V-I converter  228 ,  230 . The outputs of the V-I converters are added together at  232  and applied to the ICO  234 . In this architecture, the VCO  236  gain through the proportional path (K P ) typically defines the PLL bandwidth while the integral path will determine the VCO&#39;s tuning range. The integral path can be filtered without degrading loop stability thereby reducing the PLL&#39;s noise sensitivity. 
         [0026]    During regular operation, the proportional path control voltage is kept small, since the integral path integrates phase errors between the VCO&#39;s output and the reference clock out. The small voltage on the proportional path means that non-linearity in the V-I converter will not be exercised, and it is only the small-signal gain of the V-I converter on the proportional path that will influence PLL dynamics. However, the small signal gain of the ICO to the proportional path is a function of the total current applied to it. Hence, this gain is dependent on the voltage applied to the integral path V-I converter. As the VCO tunes across its frequency range due to changes in the integral path&#39;s control voltage, the proportional paths gain changes. 
         [0027]    To prevent the proportional path small-signal gain from changing as the VCO is tuned across its operating range by the integral path, the gain of the V-I converter  230  on the proportional path is made to be a function of the voltage on the integral path. This is indicated in  FIG. 2  by the dashed line connecting the proportional path V-I converter  230  to the output of the integral path V-I converter. Selecting the appropriate functional relationship between the two V-I converters can result in a proportional path VCO gain that is constant across the integral path&#39;s tuning range. 
         [0028]    Thus, in this invention, the proportional path V-I converter is decomposed into two V-I converters. The first V-I converter has fixed gain. The second V-I converter has variable gain proportional to 1-x where x is the output of the integral path V-I converter (normalized to having a range from 0 to 1). The outputs of the two proportional path V-I converters are connected together, meaning that their outputs are summed, and applied to the ICO. This allows for the variation in the VCO&#39;s gain across the integral path&#39;s tuning range to be reduced. 
         [0029]    It should be noted that K P  might be dependent on either V i  or I i . If the integral path V-I converter were perfectly linear, the distinction between I i  and V i  would be moot, as I i  would be a linear function of V i , and applying either to K P  would be equivalent. Since the integral path V-I converter might not be perfectly linear, the connection, in the preferred embodiment of the invention, is made to I i . The link is preferably made through I i  in order to undo the non-linearity in the ICO&#39;s tuning curve (due to I i ). The ICO only knows about the current I i  applied to it. This approach is negatively affected by non-linearity in K i . 
         [0030]    Consider an ICO ( FIG. 2 ) whose frequency of oscillation is described by the following: 
         [0000]    f VCO =f (I Total ), where I Total  is the total tuning current for the ICO.
 
I Total  is given by:
 
       I Total =I P (V P )+I I (V I ), 
       [0031]    where I P (V P ) is the current generated by the proportional path V-I converter due to the proportional path voltage V P , and I I (V I ) is the current generated by the integral path V-I converter due to the integral path voltage V I . 
         [0032]    The VCO gain on the proportional path is given by: 
         [0000]    
       
         
           
             
               
                  
                 
                   f 
                   VCO 
                 
               
               
                  
                 
                   V 
                   P 
                 
               
             
             = 
             
               
                 
                    
                   
                     f 
                      
                     
                       ( 
                       
                         I 
                         Total 
                       
                       ) 
                     
                   
                 
                 
                    
                   
                     I 
                     Total 
                   
                 
               
                
               
                 
                    
                   
                     I 
                     P 
                   
                 
                 
                    
                   
                     V 
                     P 
                   
                 
               
             
           
         
       
     
         [0033]    Due to the integration action of the integral path, V P  is usually kept small, limiting our interest to: 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     
                       f 
                       VCO 
                     
                   
                   
                      
                     
                       V 
                       P 
                     
                   
                 
                  
                 
                   | 
                   
                     
                       V 
                       P 
                     
                     = 
                     0 
                   
                 
               
               = 
               
                 
                   
                      
                     
                       f 
                        
                       
                         ( 
                         
                           I 
                           Total 
                         
                         ) 
                       
                     
                   
                   
                      
                     
                       I 
                       Total 
                     
                   
                 
                  
                 
                   G 
                   
                     m 
                     , 
                     P 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where G m,P  is the transconductance of the proportional path V-I converter. Clearly, to maintain a constant VCO gain, G m,P  should have the following functional form: 
         [0000]    
       
         
           
             
               G 
               
                 m 
                 , 
                 P 
               
             
             = 
             
               
                 ( 
                 
                   
                      
                     
                       f 
                        
                       
                         ( 
                         
                           I 
                           Total 
                         
                         ) 
                       
                     
                   
                   
                      
                     
                       I 
                       Total 
                     
                   
                 
                 ) 
               
               
                 - 
                 1 
               
             
           
         
       
     
         [0000]    generally, this will require G m,P  to be a function of I Total    
         [0034]    The following analysis applies to the particular case of a delay-interpolating ring VCO. Such a VCO is composed of at least three delay interpolators connected in DC negative feedback.  FIG. 3   a  shows a delay interpolator. The output voltage of the interpolator, V OUT , is a weighted average of signals V 1  and V 2 . As I TOTAL  tunes from −I MAX  to +I MAX , the output voltage of the interpolator tunes from V 1  to V 2  in the following fashion: 
         [0000]    
       
         
           
             
               V 
               OUT 
             
             = 
             
               
                 
                   V 
                   1 
                 
                  
                 
                   ( 
                   
                     1 
                     - 
                     
                       
                         I 
                         TOTAL 
                       
                       
                         I 
                         MAX 
                       
                     
                   
                   ) 
                 
               
               + 
               
                 
                   V 
                   2 
                 
                  
                 
                   
                     I 
                     TOTAL 
                   
                   
                     I 
                     MAX 
                   
                 
               
             
           
         
       
     
         [0000]    This leads to an overall delay from V IN  to V OUT  is given by: 
         [0000]    
       
         
           
             
               
                 τ 
                 DI 
               
               = 
               
                 
                   τ 
                   MIN 
                 
                 + 
                 
                   
                     τ 
                     D 
                   
                   2 
                 
                 + 
                 
                   
                     
                       τ 
                       D 
                     
                     2 
                   
                    
                   
                     
                       I 
                       TOTAL 
                     
                     
                       I 
                       MAX 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    Where τ MIN  is the minimum delay of one stage, τ D  is the delay of the extra delay added to the lower path in the interpolator. It is assumed that the input signal, V IN , has a rise time longer than τ D . 
         [0035]    The period of a delay-interpolating ring VCO (composed of interpolators depicted in  FIG. 3   c ) is: 
         [0000]    
       
         
           
             
               τ 
               VCO 
             
             = 
             
               2 
                
               
                 N 
                  
                 
                   ( 
                   
                     
                       τ 
                       MIN 
                     
                     + 
                     
                       
                         τ 
                         D 
                       
                       2 
                     
                     + 
                     
                       
                         
                           τ 
                           D 
                         
                         2 
                       
                        
                       
                         ( 
                         
                           
                             
                               
                                 I 
                                 I 
                               
                                
                               
                                 ( 
                                 
                                   V 
                                   I 
                                 
                                 ) 
                               
                             
                             + 
                             
                               
                                 I 
                                 P 
                               
                                
                               
                                 ( 
                                 
                                   V 
                                   P 
                                 
                                 ) 
                               
                             
                           
                           
                             I 
                             MAX 
                           
                         
                         ) 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where N is the number of stages in the ring. I MAX  is the maximum value of the sum of I I  and I P . I I  and I P  are assumed to be differential quantities. The overall delay of each delay interpolating stage can range between τ MIN  (when the sum of I I  and I P  is −I MAX ) and τ MIN +τ D  (when the sum of I I  and I P  is +I MAX ). The frequency of oscillation is given by: 
         [0000]    
       
         
           
             
               f 
               VCO 
             
             = 
             
               1 
               
                 2 
                  
                 
                   N 
                    
                   
                     ( 
                     
                       
                         τ 
                         MIN 
                       
                       + 
                       
                         
                           τ 
                           D 
                         
                         2 
                       
                       + 
                       
                         
                           
                             τ 
                             D 
                           
                           2 
                         
                          
                         
                           ( 
                           
                             
                               
                                 
                                   I 
                                   I 
                                 
                                  
                                 
                                   ( 
                                   
                                     V 
                                     I 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   I 
                                   P 
                                 
                                  
                                 
                                   ( 
                                   
                                     V 
                                     P 
                                   
                                   ) 
                                 
                               
                             
                             
                               I 
                               MAX 
                             
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    and the gain of the VCO through the proportional path is: 
         [0000]    
       
         
           
             
               
                  
                 
                   f 
                   VCO 
                 
               
               
                  
                 
                   I 
                   P 
                 
               
             
             = 
             
               
                 - 
                 
                   
                     τ 
                     D 
                   
                   
                     2 
                      
                     
                       I 
                       MAX 
                     
                   
                 
               
               
                 2 
                  
                 
                   
                     N 
                      
                     
                       ( 
                       
                         
                           τ 
                           MIN 
                         
                         + 
                         
                           
                             τ 
                             D 
                           
                           2 
                         
                         + 
                         
                           
                             
                               τ 
                               D 
                             
                             2 
                           
                            
                           
                             ( 
                             
                               
                                 
                                   
                                     I 
                                     I 
                                   
                                    
                                   
                                     ( 
                                     
                                       V 
                                       I 
                                     
                                     ) 
                                   
                                 
                                 + 
                                 
                                   
                                     I 
                                     P 
                                   
                                    
                                   
                                     ( 
                                     
                                       V 
                                       P 
                                     
                                     ) 
                                   
                                 
                               
                               
                                 I 
                                 MAX 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                   2 
                 
               
             
           
         
       
     
         [0036]    To gain more insight into the above expression, assume τ MIN  and τ D  are equal and consider the behavior for I P =0. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                          
                         
                           f 
                           VCO 
                         
                       
                       
                          
                         
                           I 
                           P 
                         
                       
                     
                      
                     
                       | 
                       
                         
                           I 
                           P 
                         
                         = 
                         0 
                       
                     
                   
                   = 
                   
                     
                       - 
                       1 
                     
                     
                       4 
                        
                       
                         I 
                         MAX 
                       
                        
                       N 
                        
                       
                           
                       
                        
                       
                         
                           
                             τ 
                             D 
                           
                            
                           
                             ( 
                             
                               
                                 3 
                                 2 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       I 
                                       I 
                                     
                                     
                                       I 
                                       MAX 
                                     
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                         2 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       - 
                       1 
                     
                     
                       K 
                        
                       
                         ( 
                         
                           
                             9 
                             4 
                           
                           + 
                           
                             
                               3 
                               2 
                             
                              
                             
                               ( 
                               
                                 
                                   I 
                                   I 
                                 
                                 
                                   I 
                                   MAX 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                 ( 
                                 
                                   
                                     I 
                                     I 
                                   
                                   
                                     I 
                                     MAX 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             K 
             = 
             
               4 
                
               
                 I 
                 MAX 
               
                
               N 
                
               
                   
               
                
               
                 τ 
                 D 
               
             
           
         
       
     
         [0037]    If the PLL is designed so that I P  is limited to be much smaller than I I , the above derivative can vary over a 1 to 4 range as I I  is tuned from −I MAX  to +I MAX  and so would the VCO gain if the proportional path V-I converter were to have a constant gain. However, the VCO gain can be made constant if (referring to the equation given in paragraph 29): 
         [0000]    
       
         
           
             
               G 
               
                 m 
                 , 
                 P 
               
             
             = 
             
               
                 
                    
                   
                     I 
                     P 
                   
                 
                 
                    
                   
                     V 
                     P 
                   
                 
               
                
               
                 | 
                 
                   
                     V 
                     P 
                   
                   = 
                   0 
                 
               
                
               
                 ∝ 
                 
                   
                     9 
                     4 
                   
                   + 
                   
                     
                       3 
                       2 
                     
                      
                     
                       ( 
                       
                         
                           I 
                           I 
                         
                         
                           I 
                           MAX 
                         
                       
                       ) 
                     
                   
                   + 
                   
                     
                       1 
                       4 
                     
                      
                     
                       
                         ( 
                         
                           
                             I 
                             I 
                           
                           
                             I 
                             MAX 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
           
         
       
     
         [0038]    Making G m,P  have a quadratic relationship with I I  may be impractical. However, an adequate approximation can be achieved by using only a constant and linear dependency, as given below: 
         [0000]    
       
         
           
             
               G 
               
                 m 
                 , 
                 P 
               
             
             ∝ 
             
               
                 5 
                 2 
               
               + 
               
                 
                   3 
                   2 
                 
                  
                 
                   ( 
                   
                     
                       I 
                       I 
                     
                     
                       I 
                       MAX 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0039]      FIG. 4  shows predicted small-signal proportional path VCO gain as a delay interpolating VCO is tuned across its integral path. Clearly, implementing only a linear scheme significantly reduces across band fluctuations in gain as compared to the uncompensated scheme. 
         [0040]      FIG. 5  shows an implementation of the described linear scheme. The integral path V-I converter  502  is shown on the left (denoted by K I  and enclosed within a dash line) while the proportional path V-I converter  504  is shown on the right (denoted by K P  and enclosed within a dot-dash line). The outputs of these two V-I converters are summed and applied to the delay interpolators  512 ,  514  and  516 . The converter  502  includes a group of transistors M 1a , M 1b , M 2a , M 2b , M 2c , M 2d  and M 3a . The converter  504  includes a second group of transistors M 1c , M 1d , M 1e , M 1f , M 2e , M 2f , M 2g , M 2h , M 3b , M 3c , M 3d , M 2aa , M cma  and M cmb . 
         [0041]    The proportional path V-I converter is further composed of two V-I converters  520  AND  522 . Transistors M 1c/d  form a constant transconductance while M 1f/g  form a variable transconductance dependent on the current I I+ . If M 1f/g  operate in weak inversion, their transconductance will be proportional with the current I I+ . Through a common-mode feedback network (not shown) transistors M cma/b  ensure that the common-mode current in transistors M 2e/f  is regulated. Through appropriate sizing of the transistors in the V-I converters, the desired transconductance can be achieved giving rise to significantly reduced fluctuations in the small-signal proportional path VCO gain. 
         [0042]    While it is apparent that the invention herein disclosed is well calculated to fulfill the objects stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.