Abstract:
A phase-locked loop (PLL) frequency synthesizer capable of being tuned in small step sizes. The PLL frequency synthesizer includes a PLL circuit. A phase-locked loop (PLL) frequency synthesizer includes a PLL core and a feedback frequency divider. The PLL core receives an F(in) signal and generates a plurality of multiphase output signals having an F2 frequency, where F2=(in)(P+Δp). The feedback frequency divider receives the plurality of multiphase output signals and generates a feedback signal having a frequency of F2/(P+Δp), where P is an integer and Δp is a fractional value less than one.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to that disclosed in U.S. patent application Ser. No. 10/320,851, filed on Dec. 16, 2002, entitled “APPARATUS AND METHOD FOR SYNTHESIZING A FREQUENCY USING VERNIER DIVIDES,” and granted as U.S. Pat. No. 6,833,764. U.S. patent application Ser. No. 10/320,851 is commonly assigned to the assignee of the present invention. The disclosure of the related patent application is hereby incorporated by reference for all purposes as if fully set forth herein. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention is generally directed to frequency synthesizers and, more particularly, to a PLL frequency synthesizer that may be finely tuned over a narrow range of frequencies. 
   BACKGROUND OF THE INVENTION 
   There have been great advancements in the speed, power, and complexity of integrated circuits, such as application specific integrated circuit (ASIC) chips, radio frequency integrated circuits (RFIC), central processing unit (CPU) chips, digital signal processor (DSP) chips and the like. These advancements have made possible the development of system-on-a-chip (SOC) devices, among other things. A SOC device integrates in one chip all (or nearly all) of the components of a complex electronic system, such as a wireless receiver (i.e., cell phone, a television receiver, microprocessor, high-speed data transceiver, and the like). 
   In many integrated circuits, a phase-locked loop (PLL) frequency synthesizer generates many of the clock signals that drive the integrated circuit. Phase-locked loops (and delay-locked loops (DLLs)) are well known to those skilled in the art and have been extensively written about. The dynamic performance of the frequency synthesizer that generates clock signals depends on several parameters, including the natural frequency (F n ), the damping factor (D F ), the crossover frequency (F O ) and the ratio of the comparison frequency (F c ) to the crossover frequency. The performance of the frequency synthesizer also depends on the performance of the charge pump located in the PLL. The charge pump pulse timing jitter and pulse amplitude noise both contribute to synthesizer phase noise. 
   A common problem for a conventional PLL based on voltage-controlled oscillator (VCO) is the granularity with which the output frequency may be adjusted. Conventional PLLs contain a frequency divider (i.e., divide-by-N) block in the feedback loop. If the input frequency is F(in), then the output frequency is F(out)=N[F(in)]. However, N is typically an integer, so that increasing or decreasing N results in large increments or large decrements in the output frequency. A fractional N PLL may be implemented that changes in smaller increments. However, a fractional PLL requires complex circuitry and has a much slower response time. 
   Therefore, there is a need in the art for improved frequency synthesizers for use in generating reference frequency signals. In particular, there is a need in the art for a phase-locked loop (PLL) frequency synthesizer that can be finely tuned over a range of frequencies. More particularly, there is a need for a PLL frequency synthesizer that can be finely tuned and that has a fast response time. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an improved phase-locked loop (PLL) frequency synthesizer capable of being tuned in small step sizes. According to an advantageous embodiment of the present invention, the PLL frequency synthesizer comprises: 1) a PLL circuit comprising: i) a PLL core capable of receiving an F(in) signal and generating a plurality of multiphase output signals having an F2 frequency, where F2=F(in) (P+Δp), and ii) a feedback frequency divider capable of receiving the plurality of multiphase output signals and generating a feedback signal having a frequency F2/(P+pΔp), wherein P is an integer and Δp is a fractional value less than 1. 
   According to one embodiment of the present invention, P and Δp are modifiable values. 
   According to another embodiment of the present invention, the PLL core comprises a phase detector, a charge pump, a loop filter, and a voltage-controlled oscillator (VCO). 
   According to still another embodiment of the present invention, the phase detector receives and compares the F(in) signal and the feedback signal. 
   According to yet another embodiment of the present invention, the VCO generates the plurality of multiphase output signals. 
   According to a further embodiment of the present invention, the feedback frequency divider comprises a counter circuit and a switching circuit capable of receiving the plurality of multiphase output signals and selectively applying selected ones of the plurality of multiphase output signals to an input of the counter circuit. 
   According to a still further embodiment of the present invention, the switching circuit comprises a multiplexer and a multiplexer control circuit. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
       FIG. 1  illustrates a processing system comprising a phase-locked loop (PLL) frequency synthesizer according to an exemplary embodiment of the present invention; 
       FIG. 2  is a flow diagram illustrating the operation of the exemplary PLL frequency synthesizer according to one embodiment of the present invention; 
       FIG. 3  illustrates a phase-locked loop (PLL) frequency synthesizer that implements a fractional frequency divider according to one embodiment of the present invention; 
       FIG. 4  illustrates selected portions of the voltage-controlled oscillator (VCO) and the feedback divider in  FIG. 3  according to one embodiment of the present invention; and 
       FIG. 5  is a timing diagram illustrating the operation of the feedback divider in  FIG. 3  according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged phase-locked loop (PLL) based frequency generator. 
     FIG. 1  illustrates processing system  100 , which comprises phase-locked loop (PLL) frequency synthesizer  130  according to an exemplary embodiment of the present invention. Processing system  100  comprises reference clock generator  110 , clock controller  120  and PLL frequency synthesizer  130 . Under the control of clock controller  120 , reference clock generator  110  generates a reference signal having a reference frequency, F(in). The F(in) signal is the input to PLL frequency synthesizer  130 . PLL frequency synthesizer  130  generates an output signal having a frequency, F(out). As will be explained below in greater detail, the F(out) frequency is a finely tunable multiple of the F(in) frequency, as determined by the values N, Δn, P and Δp, produced by clock controller  120 . According to the principles of the present invention, N and P are integer values and Δn and Δp are fractional value. 
   Processing system  100  is intended to represent any type of electronic device in which a finely tunable reference frequency is required. Thus, for example, processing system  100  may be a wireless communication device (e.g., cell phone), a video processing system (e.g., MPEG decoder), a wireline communication device (e.g., Ethernet card, set-top box, xDSL modem), a digital audio system or digital video system, or the like. 
     FIG. 2  depicts flow diagram  200 , which illustrates the operation of PLL frequency synthesizer  130  according to an exemplary embodiment of the present invention. During routine operation, clock controller  120  may from time to time receive external frequency adjustment control signals (process step  205 ). These control signals may be received from an external device, may be generated by other portions of processing system  100 , or may be generated in response to inputs from an operator of processing system  100 . Clock controller  120  then sets the F(in) reference frequency to an initial value in the correct frequency range (process step  210 ). Thereafter, clock controller  120  monitors the frequency of the F(out) signal and periodically adjusts the values of N, Δn, P and Δp to finely tune to the correct final value of the F(out) frequency (process step  215 ). 
   The present invention provides a fractional-N phase-locked loop (PLL) circuit that is implemented using a voltage-controlled oscillator (VCO) that outputs multiple phases and feedback and output dividers that use the multiple VCO phases to implement fractional frequency division. 
     FIG. 3  illustrates phase-locked loop (PLL) frequency synthesizer  130 , which implements a fractional frequency divider according to one embodiment of the present invention. PLL frequency synthesizer  130  comprises phase detector  306 , charge pump  310 , loop filter  315 , voltage controlled oscillator (VCO)  320 , output divider  325 , and feedback divider  330 . PLL frequency synthesizer  130  receives the input reference signal, F(in) and generates the output reference signal, F(out). According to the principles of the present invention, output divider  325  is a “divide by (N+Δn)” circuit, where N is an integer value and Δn is a fractional value, and feedback divider  330  is a “divide by (P+Δp)” circuit, where P is an integer value and Δp is a fractional value. 
   Phase detector  305  compares the difference in phase/frequency between the output of feedback divider  330  and the input reference signal, F(in). In response to the comparison, phase detection  305  generates a Charge Up signal or a Charge Down signal, depending on whether the output of feedback divider  330  is leading or lagging the F(in) signal. Charge pump  310  receives the Charge Up signal and the Charge Down signal and accordingly either adds charge to, or drains charge from, a large charge capacitor in loop filter  315 . Adding charge to, or draining charge from, the capacitor increases or decreases the voltage on the charge capacitor. The voltage on the charge capacitor in loop filter  315  is the control voltage of VCO  320 . As the VCO control voltage increases or decreases, the frequency of the output of VCO  320  increases or decreases accordingly. 
   According to the principles of the present invention, the output of VCO  320  produces a multiphase output that is applied to the input of output divider  325  and feedback divider  330 . Each one of output divider  325  and feedback divider  330  is capable of rapidly selecting different ones of the multiphase outputs in order to divide the frequency of the VCO  230  output by a fractional value, rather than by an integer value. For the sake of simplicity and brevity, only the operation of feedback divider  330  will be discussed hereafter. However, it will be understood that the following description of feedback divider  330  also applies to output divider  325 . 
     FIG. 4  illustrates selected portions of voltage-controller oscillator (VCO)  320  and feedback divider  330  according to one embodiment of the present invention. VCO  320  comprises S voltage-controlled oscillator (VCO) stages. In the exemplary embodiment, VCO  320  contains S=4 stages, namely VCO stage  401 , VCO stage  402 , VCO stage  403  and VCO stage  404 . VCO  320  is a conventional ring oscillator, but the dual outputs of each VCO stage are brought out and fed to an input of multiplexer (MUX)  331  of feedback divider  330 . MUX controller  410  controls the operation of MUX  331 . 
   The ring oscillator, the interconnects, and MUX  331  are carefully matched so that the phases of the selected phases at MUX  331  output are equally distributed in time within the cycle of VCO  320 . In the four-stage embodiment shown in  FIG. 4 , each stage accounts for one-eighth of the cycle time of the VCO output. In other words, the outputs of each stage are shifted 45° with respect to each other. Also, each VCO stage has two outputs that are 180° out-of-phase with respect to each other. Thus, at steady state, the outputs of VCO stage  401  are the 0° phase output and the 180° phase output, the outputs of VCO stage  402  are the 45° phase output and the 225° phase output, the outputs of VCO stage  403  are the 90° phase output and the 270° phase output, and the outputs of VCO stage  404  are the 135° phase output and the 315° phase output, respectively. 
   Under the control of MUX controller  410 , MUX  331  can rapidly switch from one multiphase output to another multiphase output to thereby achieve a fraction divisor at the output of divide logic  332  in feedback divider  330 . 
   According to an exemplary embodiment of the present invention, divide logic  332  comprises an integral divider and a fractional divider. The integral divider is conventionally designed. It has a counter which counts the desired number of whole cycles of the selected clock, resets itself, and repeats. The counter is typically triggered on each low-to-high transition (i.e., rising edge) of the output of MUX  331 . This desired number of cycles is the integral part of the divider value. For each count sequence, an output pulse is generated. 
   Also during each count sequence, MUX controller  410  retards or retains the selected clock. MUX controller  410  controls MUX  331  to select a new phase output that may be more retarded than the previous one. This phase difference forms the fractional part of the divide value. To simplify divide logic  332 , and to reliably switch between phases without causing clock glitches, the new phase is not selected directly. Instead, on subsequent cycles of the selected clock, the next later phase is selected until the desired phase is reached. It may take as many cycles as there are phases to complete this selection. This imposes a restriction that the integral divider be greater than the number of phases less one. MUX  331  must be carefully designed, since selecting retarded phases may tend to cause glitches. 
     FIG. 5  depicts timing diagram  500 , which illustrates the operation of feedback divider  330  according to one embodiment of the present invention. For the sake of simplicity, it is assumed that VCO  320  is a two-stage VCO producing four outputs that are 90° apart. The outputs are respectively labeled VCO( 0 ), VCO( 90 ), VCO( 180 ), and VCO( 270 ). Timing diagram  500  shows the output of VCO  320  being divided by 5 and 2/4 (i.e., 5.5). 
   From time T 0  to time T 1  (i.e., 0.25 cycles of VCO  320  output), MUX controller  410  switches MUX  331  such that the VCO( 90 ) output is selected by MUX  331  and applied to divide logic  332 . From time T 1  to time T 2  (i.e., 1.25 cycles of VCO  320  output), MUX controller  410  switches MUX  331  such that the VCO( 180 ) output is selected by MUX  331  and applied to divide logic  332 . From time T 2  to time T 3  (i.e., 4.25 cycles of VCO  320  output), MUX controller  410  switches MUX  331  such that the VCO( 270 ) output is selected by MUX  331  and applied to divide logic  332 . From time T 3  to time T 4  (i.e., 1.25 cycles of VCO  320  output), MUX controller  410  switches MUX  331  such that the VCO( 0 ) output is selected by MUX  331  and applied to divide logic  332 . Finally, from time T 4  to time T 5  (i.e., 4.00 cycles of VCO  320  output), MUX controller  410  switches MUX  331  such that the VCO( 90 ) output is selected by MUX  331  and applied to divide logic  332 . 
   The output of MUX  320  is the MUX OUT signal waveform in timing diagram  500 . The value of the counter in divide logic  332  is shown as the value COUNT in timing diagram  500 . The output of feedback divider  330  is shown as the signal waveform FB in timing diagram  500 . As  FIG. 5  illustrates, the value COUNT is incremented on each rising edge of the MUX OUT signal. The value of COUNT is reset to 0 every 5 rising edges. Thus, the divide logic  332  divides by an integer value of 5. However, because of the operation of the present invention, the rising edges of MUX OUT are not spaced evenly apart. During the periods when COUNT equals 2, 3, or 4, the MUX OUT signal has the same spacing between rising edges as the VCO output phases. However, during the period when COUNT equals 0 or 1, an extra quarter cycle is added between rising edges of the MUX OUT signal. Thus, the MUX OUT signal is 1.25 cycles of the VCO  320  output when COUNT IS 0 and when COUNT is 1. Overall, the duration between the rising edge of the first COUNT=0 period and the rising edge of the second COUNT=0 period is equal to (1.25+1.25+1.0+1.0+1.0)=5.5 cycles of the VCO  320  output. Thus, feedback divider  330  effectively divides the output of VCO  320  by 5.5. 
   Those skilled in the art will recognize that PLL frequency synthesizer  130  may be adapted to divide by other fractional amounts. Modifying the number of stages in VCO  320  and altering the pattern of input lines selected by MUX  331  enables PLL frequency synthesizer to divide by other fractional amounts. For example, if VCO  320  comprises eight VCO stages, feedback divider  330  may divide by fractional amounts as small as 1/16 of a cycle of the output of VCO  320 . 
   In an alternate embodiment, the selected phase may be advanced instead of retarded, which has different disadvantages. When advancing the phase, the period of the selected clock is shortened, which places tighter timing requirements on divide logic  332 . The number of whole clock cycles and phase decrements do not correspond to the integral and fractional parts of the divide value, rather the resultant divide value is the number of whole cycles counted minus the number of fractional phase decrements. However, advancing the phase simplifies the design of MUX  331 . 
   Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.