Patent Publication Number: US-6657500-B1

Title: Method and system of characterization and behavioral modeling of a phase-locked loop for fast mixed signal simulation

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the formation of simplified representations of electronic circuits to model responses of the electronic circuits to input stimuli. More particularly this invention relates to the simplified representations of circuits such as phase-locked loops to efficiently predict the behavior of the circuits to input stimuli. 
     2. Description of Related Art 
     Today, there are new mixed-signal and multi-level simulation languages, which can accurately represent the analog portion of the mixed circuits with behavioral models. Behavioral models are “if-then-else” structured programs, which describe the analog circuit&#39;s behavior. The “if-then-else” structured program is based on the basis that an input signal to a known system will produce a predictable response signal. Thus, if a function for the input signal vs. output signal is derived, the behavior of the circuit can be predicted without having a full circuit or device level model. The function may be a simple table lookup of input signal levels with attendant response signal levels or a complex mathematical function having multiple parameters of the input signal necessary to derive the output response. 
     FIG. 1 shows a system diagram for a phase-locked loop (PLL). A PLL circuit causes a given system to track with another system. A PLL typically causes one electrical signal with a given frequency to track, follow, or become synchronized with another electrical signal. In FIG. 1, the PLL synchronizes a Voltage Controlled Oscillator (VCO  185 ) signal with a reference signal (Vref  180 ). The PLL synchronizes frequency as well as phase. When Vref and VCO are synchronized, the PLL is in a “locked” state. In this state, the phase error between VCO and Vref is zero or close to zero. If a finite phase error builds up, the PLL circuit in FIG. 1 uses a feedback control mechanism, which causes the phase error to approach, zero once again. 
     The phase frequency detector  120  compares the phase of VCO to Vref and produces an output signal, which is proportional to the phase error. This output signal has AC and DC components. In FIG. 1, the loop filter  150 , which is usually a first-order low-pass filter, filters out the AC component. The voltage controlled oscillator (VCO) produces an output signal, which has a frequency, which is a function of the VCO input, which is the control voltage. FIG. 2 shows a standard VCO transfer function where the output signal frequency is plotted vs. the VCO input control voltage. The divider circuit  170  in FIG. 1 is an option, which is needed only when the VCO output frequency is a multiple N times greater than the frequency of Vref. In this case, the divider would be a Divide-by-N circuit. Similarly, the divider in FIG. 1 could be replaced by a multiplier if the VCO output frequency is a multiple N times less than the frequency of Vref. In order for the phase frequency detector  120  to work, the frequencies of Vref and VCO must be close to each other. The charge pump circuit  140  in FIG. 1 takes the up/down voltage output of the phase frequency detector  120  and outputs a proportional current which goes to the loop filter  150 . A state diagram in FIG. 3 a  shows the three states of the phase frequency detector  120  and the three phase detector output voltage states which control the action of the charge pump circuit  140 . The phase angle diagram in FIG. 3 b  shows two counterclockwise rotating vectors, which represent the VCO and Vref signals. The input control for the state diagram in FIG. 3 a  can be understood by following the two rotating vectors as they rotated past the trigger axis shown in FIG. 3 b . This mechanism is the feedback control system, which causes the VCO signal to “lock on to”, the Vref signal after several oscillation cycles via the charge pump current source/sink function of FIG. 3 a . The speed of reaching the equilibrium state of “locking on to” the Vref signal by the VCO signal is a figure of merit of phase locked loops. 
     Simulation of the PLL blocks described consists of the following models. The phase frequency detector is modeled as a behavioral model with two inputs, VCO and Vref. The model determines the lead-lag relationship and the frequency/phase difference. It then delivers two signals to direct the charge pump circuitry to source current, sink current or turn the charge pump current off. 
     A state machine similar to FIG. 3 a  is included in the phase frequency detector model. The current and voltage values used in the behavioral model tables are obtained from simulation of computer aided circuit design models. The VCO  185  is modeled as a behavioral model. Circuit simulation provides the output frequency vs. control voltage transfer curve, such as in FIG.  5 . The charge pump  140  is modeled with a behavioral where the relationship between the output voltage and pump current is modeled with tables, which were constructed from circuit analysis results. The divider  170  in FIG. 1 is basically a counter that outputs a clock signal whose frequency in the input clock divided or multiplied by a parameter, N. The voltage relationship between input and output signals is derived from circuit analysis and can be described with digital event-driven behavioral statements. Finally, the loop filter  150  in FIG. 1 is modeled as an RC low-pass network where each component is represented by an analytical I-V transfer function. 
     U.S. Pat. No. 5,392,227 (Hiserote) provides a system and a method for translating gate level design data into behavioral level simulation models that are accurate and have high simulation performance. It also provides accurate simulation models for complex integrated circuits prior to the availability of the circuits. 
     U.S. Pat. No. 5,987,238 (Chen) provides a method for making a phase-locked loop circuit. The method includes a method of simulating the phase-locked loop circuit. 
     Smedt et al., “Models for Systematic Design and Verification of Frequency Synthesizers,” IEEE Transactions on Circuit and Systems—II: Analog and Digital Signal Processing, Vol. 46, No. 10, October 1999, IEEE, provides models for the design and verification of frequency synthesizers. It provides for insight into both the top-down high level design and the bottom-up low-level verification phase of frequency synthesizer development. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a piecewise linear phase locked loop frequency generator to generate a voltage signal controlled by an input voltage. 
     In addition, it is also an objective of this invention to provide a model of a phase locked loop to be simulated within a simulator to determine the function of a phase locked loop within an electronic system. 
     These and other objects of this invention are accomplished by a piecewise linear phase locked loop frequency generator. The piecewise linear phase locked loop frequency generator has a variable static frequency generator, whose frequency is selected by a control voltage and whose transfer function between control voltage and output frequency is further selected by a frequency selection parameter, n. This frequency selection parameter, n, designates the number, which identifies which portion of the frequency spectrum is being represented by the control voltage to frequency transfer function being selected. Second, a set of low pass filters is used to work in conjunction with the corresponding control voltage to output frequency transfer functions, which are selected by a frequency selection parameter, n. The number of low pass filters is equal to the frequency selection parameter n. Finally, a switch circuit is employed to select as a function of the frequency selection parameter, n, which low pass filter output should be presented as the dynamic frequency which corresponds to the original control voltage input. 
     The simulation model would use the behavioral model of a variable static frequency generator. The frequency of the variable static fuse generator is selected by a control voltage, a set of RC network models of n low pass filters, LPF 1 , LPF 2 , . . . LPFn, and a behavioral model of a selector switch which selects between the output of n low pass filters, LPF 1 , LPF 2 , . . . LPFn, using the same control voltage which was used to select the frequency in the variable static frequency generator. 
     A simulation model of a phase locked loop employs a function description of the piecewise linear phase locked loop frequency generator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system diagram of the prior art of a phase locked loop. 
     FIG. 2 shows a plot of the frequency vs. control voltage transfer function graph of a voltage controlled oscillator (VCO) of the prior art. 
     FIG. 3 a  shows a state diagram of a phase frequency detector and how its output controls a charge pump circuit in a PLL of the prior art. 
     FIG. 3 b  shows a phase graph relationship between Vref and VCO signals of a PLL of the prior art. 
     FIG. 4 shows a schematic diagram the PLL structure technique of this invention. 
     FIG. 5 shows graphically the piece-wise linear PLL simulation method of this invention. 
     FIGS. 6 a - 6   i  shows the individual frequency vs. voltage graphs obtained from the piece-wise approximation of FIG.  5 . 
     FIGS. 7 a - 7   i  shows the individual gain vs. frequency graphs for frequencies sampled in FIG.  5 . 
     FIG. 8 is a flowchart for the method for behaviorally simulating a phase-locked loop of this invention. 
     FIG. 9 is a block diagram of an implementation of the static frequency generator using the interpolation approximation. 
     FIG. 10 is a block diagram of an implementation of the static frequency generator using the slope/intercept approximation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Refer now to the system diagram in FIG. 4 for a discussion of the piecewise linear phase locked loop frequency generator of this invention. The input to the system of FIG. 4 is a control voltage Vctrl of the phase-locked loop. The output of the system is a dynamic frequency Fout. The function of the system is a real-time dynamic frequency Fout generated as a function of the input control voltage Vctrl. The frequency, Fout, vs. control voltage, Vctrl, static frequency generator curves  410  shown in FIG. 4 represent a number (n) of transfer curves. 
     The static frequency generator  410  produces a static frequency Fs as a function dependent on the control voltage Vctrl. The control voltage Vctrl is the output voltage of the loop filter  150  of FIG. 1 as described above. The static frequency generator  410  contains a set of arithmetic descriptions. Each arithmetic description provides an algebraic function that describes a section of the VCO transfer curve of FIG.  2 . Refer to FIG. 5 for the construction of the algebraic descriptions. FIG. 5 illustrates a VCO transfer curve  500 . In the preferred embodiment of this invention, the VCO transfer curve  500  is divided into multiple segments. An algebraic function of the form: 
     
       
           f ( x )= mx+b   Eq.1 
       
     
     is fitted to each segment of the VCO transferee curve  500 . The segments as fitted to Eq. 1 each form a straight line as shown in the piecewise approximation  510  to the VCO transfer curve. The number of straight line segments of the piecewise approximation  510  determines the accuracy of the piecewise approximation versus the actual VCO transfer curve. 
     The algebraic parameters, in this case, slope m and zero crossing b, are determined for each segment and retained within the static frequency generator. Referring to FIGS. 6 a - 6   i , the slope m and zero crossing b are determined using well understood classic algebraic methods for each segment. Each of the FIGS. 6 a - 6   i  shows the function of the static frequency Fs vs. the control voltage Vctrl for each segment of the piecewise approximation  510  of FIG.  5 . The static frequency generator  410  of FIG. 4 retains each of the segment functions of FIGS. 6 a - 6   i.    
     A number of techniques are possible for retaining the functions for each segment within the static frequency generator. Table 1 tabulates the control voltage Vctrl and the corresponding static frequency Fs. The magnitude of the input control voltage Vctrl is determined. The region into which the input control voltage Vctrl falls is determined. The static frequency Fs is then determined by interpolating between the two boundaries of the region. 
     Alternately, the static frequency generator  410  contains Table 2 containing the slope m and zero crossing b for each segment as shown in FIGS. 6 a - 6   i . In this instance the magnitude of the input control voltage Vctrl is used to point to a range within the table and the static frequency Fs is determined according to Eq. 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Vctrl 
                 Fs 
               
               
                   
                   
               
             
            
               
                   
                 V0 
                 F0 
               
               
                   
                 V1 
                 F1 
               
               
                   
                 V2 
                 F2 
               
               
                   
                 | 
                 | 
               
               
                   
                 Vmax 
                 F8 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 RANGE Vctrl 
                 m 
                 b 
               
               
                   
                   
               
             
            
               
                   
                 V0-V1 
                 m1 
                 b1 
               
               
                   
                 V1-V2 
                 m2 
                 b2 
               
               
                   
                 V2-V3 
                 m3 
                 b3 
               
               
                   
                 | 
                 | 
                 | 
               
               
                   
                 Vn-1-Vn 
                 mn 
                 bn 
               
               
                   
                   
               
            
           
         
       
     
     Referring back to FIG. 4, the output of the static frequency generator Fs is the input to multiple low pass filters LPF 1   420 , LPF 2   430 , . . . LPFn  440 . The low pass filters LPF 1   420 , LPF 2   430 , . . . LPFn  440  are uniquely designed to have cutoff frequencies that match the upper frequencies of each piecewise segment of segment  510  of FIG.  5 . 
     The output of each low pass filter LPF 1   420 , LPF 2   430 , . . . LPFn  440  is connected to an input of the switch  450 . The magnitude of the control voltage Vctrl is used to determine which output of the low pass filters LPF 1   420 , LPF 2   430 , . . . LPFn  440  are to be transferred through the switch  450  to form the dynamic frequency Fout at the output of the piecewise linear phase locked loop frequency generator of this invention. 
     The static frequency generator is implemented as a lookup table structured after either Table 1 or Table 2 above. An input variable to the static frequency generator is the magnitude of the control voltage Vctrl. The appropriate segment of the piecewise linear approximation of the VCO transfer curve is determined by the magnitude of the control voltage Vctrl. From the function of the appropriate segment, the static frequency Fs is calculated as described above. 
     FIG. 9 is a block diagram for an implementation of the static frequency generator  410  of FIG.  4 . FIG. 9 implements the static frequency generator function using the method as described above for Table 1, where the frequency Fs for a control voltage range (Vctrl) is obtained by interpolating the frequency at the endpoints of the range. In FIG. 9, the analog control voltage Vctrl goes into the analog-to-digital converter  900  and comes out as a digital coded signal. The digital signal goes into selector  910  where a piecewise segment of the frequency vs. control voltage curve is selected based on the control voltage endpoints listed in Table 1. Each piecewise segment is assigned a word or address in a function table implemented in a memory circuit  920 . When the appropriate memory word in the function table  920  is selected, the output of the memory word which includes the digital coding of the frequency and control voltage of the two endpoints of the selected piecewise segment is sent to the interpolator  930 . The interpolator  930  finds the middle or average value of frequency between the given endpoint frequencies. The digital representation of the interpolated frequency result is then sent to the digital-to-analog converter  940  where the digital frequency coding is converted to an analog frequency Fs. 
     FIG. 10 is a block diagram of a second implementation of the static frequency generator  410  of FIG. 4 using the method as described above for table 2. The frequency Fs for a control voltage range (Vctrl) is obtained by piecewise approximations of the Frequency vs. Control Voltage curves. The slope &amp; y-intercept method of Table 2 is used. In FIG. 10, the analog control voltage Vctrl goes into the analog-to-digital converter  1000  and comes out as a digital coded signal. The digital signal goes into selector  1010  where a piecewise segment of the frequency vs. control voltage curve is selected based on the slope and intercept points listed in Table 2. Each piecewise segment is assigned a word or address in a function table implemented in a memory circuit  1020 . When the appropriate memory word in the function table  1020  is selected, the output of the memory word which includes the digital coding of the frequency and control voltage of the slope and intercept of the selected piecewise segment is sent to the calculator  1030 . The calculator  1030  generates straight line approximation for the piecewise segment using a function of the form f(x)=mx+b. The digital representation of the approximated frequency result is then sent to the digital-to-analog converter  1040  where the digital frequency coding is converted to an analog frequency Fs. 
     The piecewise linear phase locked loop frequency generator of FIG. 4 can be built as a hardware simulator of a PLL. In addition, the system can be used to construct a behavioral model of a PLL in a software simulation program executed on a digital computing system. The use of single pole low pass filter RC network representations of piecewise linear sections of the curve in FIG. 5 is an important item of this invention. FIGS. 7 a - 7   i  represent nine low-pass filter approximations for the nine frequency samples shown in FIG.  5 . The gain labeled on the nine figures in FIG. 7 a - 7   i  are obtained from curve fitting the samples in FIG.  5 . The use of a switch to select from a bank of n low pass filters allows the complex PLL to be simulated using standard mixed signal simulation models/programs such as VHDL-AMS and Verilog-AMS. These simulators support multi-level and mixed signal simulation. The outputs of the low pass filter  420 ,  430 ,  440  are the inputs to the switch  450 . The calculated static frequency Fs is the input variable to the low pass filters  420 ,  430 ,  440 . The low pass filters  420 ,  430 ,  440  of FIG. 4 are modeled using Laplace transform filters. The pole position of each filter is found using Spice computer aided circuit analysis. The use of the standard mixed simulation programs with the piecewise linear representation of the PLL results in accurate results for PLL outputs and locking time with far fewer simulation cycles than an exhaustive Spice circuit level simulation of the PLL circuitry. 
     FIG. 8 shows a method diagram for the simulation of a phase-locked loop. The frequency vs. control voltage curve of FIG. 5 is sub-divided  810  into n piecewise linear sections. Next, these n piecewise linear curve sections are compiled  820  into n frequency vs. control voltage curves. The control voltage Vctrl input is used index to  830  into a table of control voltage Vctrl vs. n. The number, n selected by the table is used to select  840  the corresponding Output frequency vs. Control voltage graph. The output of the graph section  840  is filtered by a bank of low pass filters,  850 . The output of the appropriate low pass filter is selected  860  by the same number, n, indexed by the control voltage Vctrl input above.