Self-calibrating phase-lock loop

Abstract: A phase-lock loop (PLL) has an oscillator having a plurality of operating curves. During PLL auto-trim operations, the oscillator is automatically trimmed to an appropriate oscillator operating curve for use during normal PLL operations. In particular embodiments, the PLL is a charge-pump PLL having a phase/frequency detector (PFD) that generates error signals based on comparing an input signal and a PLL feedback signal; a charge pump that generates amounts of charge corresponding to the error signals; a loop filter that accumulates the amounts of charge to generate a loop-filter voltage; and a voltage-controlled oscillator (VCO), where the VCO output signal is used to generate the PLL feedback signal. During normal PLL operations, the loop-filter voltage is applied to the voltage input of the VCO. During the PLL auto-trim operations, a state machine applies a sequence of digital control input values to the VCO to select different VCO operating curves until an appropriate operating curve for the present PLL application is found. In different embodiments, the state machine uses different signals to determine whether the center frequency of each operating curve in the sequence is above or below the desired nominal operating frequency for the VCO, and select one such operating curve for use in normal operations.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to electronics, and, in particular, to phase-lock loops. Description of the Related Art A phase-lock loop (PLL) is a circuit that generates a periodic output signal that has a constant phase relationship with respect to a periodic input signal. PLLs are widely used in many types of measurement, microprocessor, and communication applications. One type of phase-lock loop is the charge-pump PLL, which is described in Floyd M. Gardner, "Charge-Pump Phase-Lock Loops" IEEE Trans. Commun., vol. COM-28, pp. 1849-1858, Nov. 1980. Fig. 1 shows a block diagram of a conventional charge-pump phase-lock loop 100 . Phase/frequency detector (PFD) 102 compares the phase  IN of the input signal F IN to the phase  FB of the feedback signal F FB and generates an error signal: either an up signal U (when  IN leads  FB ) or a down signal D (when  FB leads  IN ), where the width of the error signal pulse indicates the magnitude of the difference between  IN and  FB . Charge pump 104 generates an amount of charge equivalent to the error signal (either U or D ) from PFD 102 . Depending on whether the error signal was an up signal or a down signal, the charge is either added to or subtracted from the capacitors in loop filter 106 . For purposes of this explanation, loop filter 106 has a relatively simple design, consisting of a capacitor C S in parallel with the series combination of a resistor R and a relatively large capacitor C L . As such, loop filter 106 operates as an integrator that accumulates the net charge from charge pump 104 . Other, more-sophisticated loop filters are of course also possible. The resulting loop-filter voltage V LF is applied to voltage-controlled oscillator (VCO) 108 . A voltage-controlled oscillator is a device that generates a periodic output signal ( F OSC in Fig. 1), whose frequency is a function of the VCO input voltage ( V LF in Fig. 1). In addition to being the output signal from PLL 100 , the VCO output signal F OSC is used to generate the feedback signal F FB for the closed-loop PLL circuit. Optional input and feedback dividers ( 110 and 112 ) are placed in the input and feedback paths, respectively, if the frequency of the output signal F OSC is to be either a fraction or a multiple of the frequency of the input signal F IN . If not, the input and feedback dividers can both be considered to apply factors of 1 to the input and feedback signals, respectively. Due to the effect of the feedback path in PLL 100 , the steady-state output signal F OSC will have a fixed phase relationship with respect to the input signal F IN . Unless some phase offset is purposely added, the phases of the input and output signals will be synchronized will minimal offset. Voltage-controlled oscillators, such as VCO 108 of Fig. 1, are devices that are often designed for a wide range of applications (e.g., signal frequencies from 40 KHz to 400 MHz) Such VCOs are often designed with a number of operating curves (i.e., voltage in vs. frequency out), where the frequency range of any one curve is only a fraction of the total operating range of the VCO. Fig. 2 shows a hypothetical set of eight operating curves for a VCO. A special digital control input N is used to select one of the operating curves. The process of selecting a VCO operating curve is called trimming. For low-noise PLL applications, it is important for VCO 108 in Fig. 1 to have a relatively low gain. This implies that the slope of the selected VCO operating curve should be relatively low, such as those shown in Fig. 2. A particular PLL application may have a specific desired frequency or desired frequency range for the VCO. For example, in one application, the PLL may be needed to generate a nominal 100-MHz output signal. To achieve the desired PLL operations, the VCO is trimmed by selected the operating curve (e.g., N =3 in Fig. 2) whose center frequency F CTR is close to the desired nominal PLL output frequency. Under ideal circumstances, corresponding operating curves (i.e., those having the same digital control input value) in all VCOs of the same design would have the same center frequencies and slopes. In this case, for a particular PLL application, the same VCO operating curve could be selected for each and every PLL instance. However, in the real world, due to variations during device fabrication, the characteristics of the operating curves will vary from VCO to VCO. For example, the operating curves shown in Fig. 2 could shift up or to the right, and even have differing slopes. Nor are they all necessarily linear. As a result, for some applications, the VCOs in different PLL instances may need to be trimmed with different digital control input values N to select the appropriate VCO operating curve for the desired output frequency. Conventionally, each VCO is tested in the factory to characterize its set of operating curves to pre-determine which digital control input values are appropriate for different desired output frequencies. When a particular VCO is selected for a particular application, such as PLL 100 of Fig. 1, the appropriate trim setting (i.e., the particular digital control input value N that corresponds to the desired output frequency) is permanently burned into the device (e.g., by blowing fuse links). This factory testing and hard-wiring of the VCO adds to the costs of manufacturing the PLLs. It also limits the operating frequency range of each PLL to the permanently selected operating curve. EP-A-0598260, US 5,257,294, WO 97 09786 and EP-A-0442461 each disclose voltage controlled oscillator circuits. According to this invention there is provided an integrated circuit having a charge-pump phase-lock loop (PLL) according to claim 1. The loop-filter may be an integrating loop filter that generates a loop-filter voltage based on the PFD error signals. The VCO output signal may be used to generate the PLL feedback signal; and during normal PLL operations, the loop-filter voltage may be applied to the voltage input of the VCO. The PLL may comprise a state machine; and during PLL auto-trim operations, the state machine may apply a sequence of digital control input values to the VCO to select different VCO operating curves and the state machine may select an appropriate trim setting for the VCO for normal PLL operations. The PLL may comprise a timer that controls how long the state machine applies each of the digital control input values during PLL auto-trim operations. During PLL auto-trim operations, the state machine may receive and use a voltage signal corresponding to the loop-filter voltage to select the appropriate trim setting. .The PLL may comprise a switch configuration in the loop filter that reduces the effective capacitance of the loop filter during PLL auto-trim operations to accelerate the PLL auto-trim operations. The PLL may comprise an inverter, connected between the loop filter and the state machine, which provides the voltage signal to the state machine. The PLL may comprise a second PFD, a second charge pump, and a capacitor that are used to generate a voltage input to the state machine during PLL auto-trim operations. The second PFD may be a frequency-only detector. The PLL may comprise a second phase/frequency detector and a digital accumulator, wherein the digital accumulator accumulates error signals from the second phase/frequency detector, and generates a digital input to the state machine. The second PFD may be a frequency-only detector. The digital accumulator may apply upper and lower limits to the accumulated frequency error signals from the second PFD to accelerate PLL auto-trim operations. The state machine may apply either a linear search algorithm or a binary search algorithm in selecting the sequence of digital control input values for the VCO. The present invention is directed to a self-trimming or self-calibrating phase-lock loop that automatically selects an appropriate VCO operating curve when the PLL is powered up. In general, the present invention is an integrated circuit having a phase-lock loop, where the PLL comprises an oscillator having a plurality of operating curves. During auto-trim operations, the oscillator is automatically trimmed to an appropriate oscillator operating curve for use during normal PLL operations. In particular embodiments of the present invention, the PLL is a charge-pump PLL having a phase/frequency detector (PFD) that generates error signals based on comparing an input signal and a PLL feedback signal; a charge pump that generates amounts of charge corresponding to the error signals; a loop filter that accumulates the amounts of charge to generate a loop-filter voltage; and a voltage-controlled oscillator (VCO), where the VCO output signal is used to generate the PLL feedback signal. During normal PLL operations, the loop-filter voltage is applied to the voltage input of the VCO. During the PLL auto-trim operations, a state machine applies a sequence of digital control input values to the VCO to select different VCO operating curves until an appropriate operating curve for the present PLL application is found. In different embodiments, the state machine uses different signals to determine whether the center frequency of each operating curve in the sequence is above or below the desired nominal operating frequency for the VCO. The present invention is not limited to charge-pump PLLs. As long as the PLL has a VCO with multiple operating curves and a frequency detector, the PLL can be automatically calibrated to select a VCO operating curve by breaking the PLL feedback loop, applying a reference signal to the VCO input, and adjusting the VCO digital control values. Brief Description of the Drawings Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: Fig. 1 shows a block diagram of a conventional charge-pump phase-lock loop; Fig. 2 shows a hypothetical set of operating curves for the voltage-controlled oscillator of the phase-lock loop of Fig. 1; and Figs. 3-5 show block diagrams of charge-pump phase-lock loops, according to different embodiments of the present invention. Detailed Description Fig. 3 shows a block diagram of a charge-pump phase-lock loop 300 , according to one embodiment of the present invention. In PLL 300 , phase/frequency detector 302 , charge pump 304 , voltage-controlled oscillator 308 , input divider 310 , and feedback divider 312 are analogous to the corresponding components of PLL 100 of Fig. 1. Loop filter 306 is similar to loop filter 106 of Fig. 1, except that a switch SW3 has been added between the resistor R and the large capacitor C L . In addition, PLL 300 has switches SW1 and SW2, inverter INV1, timer 314 , and state machine 316 . These components are designed to enable PLL 300 to automatically select an appropriate trim setting whenever PLL 300 is powered on. In one embodiment, this auto-trim phase is implemented after the reset analog phase, but before the acquisition phase of a conventional PLL power-on sequence. During the auto-trim phase, switches SW I and SW3 are open and switch SW2 is closed. As a result, the reference voltage V REF is applied to the voltage input of VCO 308 rather than the loop-filter voltage V LF , which is instead applied to the input of state machine 316 through inverter/comparator INV1. The reference voltage V REF is preferably the nominal center of the range of input voltages over which VCO 308 is designed to operate. During the auto-trim phase, state machine 316 generates a sequence of digital control input values N that are input to VCO 308 to sequentially select different VCO operating curves. For each VCO operating curve, with reference voltage V REF applied, VCO 308 generates an output signal F OSC having a constant frequency. With the loop broken at switch SW1, charge pump 304 will eventually drive the loop-filter voltage V LF either to the negative supply rail (e.g., ground) or to the positive supply rail (e.g., V DD ), depending on whether the frequency of the feedback signal F FB is greater than or less than the frequency of the input signal F IN as adjusted, if at all, by input divider 310 . The ultimate loop-filter voltage for a given VCO operating curve (i.e., either ground or V DD ) is referred to as the quiescent loop-filter voltage. This definition applies only during the auto-trim phase. During normal operations, the term may have another meaning. Each digital control input value N in the sequence is applied for a period of time designated by timer 314 . Preferably, each VCO operating curve is selected for a period of time sufficiently long for the loop-filter voltage V LF to converge substantially to the appropriate quiescent loop-filter voltage. If the frequency of the feedback signal F FB is greater than the frequency of the input signal (adjusted by input divider 310 ), then the loop-filter voltage V LF will eventually reach ground. Similarly, if the frequency of the feedback signal F FB is less than the frequency of the adjusted input signal, then the loop-filter voltage V LF will eventually reach V DD . The desired frequency of the VCO output signal F OSC is the frequency of the input signal F IN (ignoring for the time being any frequency multiplication or division resulting from dividers 310 or 312 ). By purposefully selecting digital control input values N, state machine 316 can eventually identify the two VCO operating curves that have center frequencies just above and just below the frequency of the input signal F IN . Either one of these two operating curves can then be selected for use during normal PLL operations. In one embodiment, state machine 316 performs a linear search algorithm in which the values for the digital control input N are selected linearly starting at one end of the range of possible values (e.g., 0) and proceeding towards the other end of the range until the quiescent loop-filter voltage V LF flips from one side (e.g., ground) to the other (e.g., V DD ) . In an alternative embodiment, state machine 316 performs a binary search algorithm in which each new digital control input value is selected midway between two previously selected values that yielded opposing quiescent loop-filter voltages, until two consecutive digital control values are found that yield opposing quiescent loop-filter voltages. By opening switch SW3 during the auto-trim phase, the large capacitor C L is temporarily removed from the loop-filter operations and the loop-filter voltage V LF will settle to the appropriate quiescent voltage (i.e., either ground or V DD ) at a faster rate than when C L is present. This effectively reduces the time that each digital control input value N needs to be maintained to test each of the VCO operating curves in the search sequence, thereby reducing the overall duration of the auto-trim phase. In addition, increasing the value of the charge pump current during the auto-trim phase can further speed up the convergence of the loop-filter voltage to the appropriate quiescent voltage level. Once the auto-trim phase is complete, switches SW1 and SW3 are closed and switch SW2 is opened to enable normal PLL operations, which proceed analogously to PLL 100 of Fig. 1, in this case, using the digital control input value corresponding to the VCO operating curve selected by state machine 316 during the auto-trim phase as the trim setting for VCO 308 . Inverter INV1 helps isolate state machine 316 from the loop-filter voltage V LF during normal PLL operations. In an alternative implementation of PLL 300 of Fig. 3, PFD 302 is implemented as two separate detectors: a phase-only detector and a frequency-only detector, each of which has its own charge pump tied to the loop-filter voltage node. In this implementation, only the frequency-only detector needs to be used during the auto-trim phase. Fig. 4 shows a block diagram of a charge-pump phase-lock loop 400 , according to an alternative embodiment of the present invention. In PLL 400 , PFD 402 , charge pump 404 , loop filter 406 , VCO 408 , input divider 410 , and feedback divider 412 are analogous to the corresponding components of PLL 100 of Fig. 1. Furthermore, timer 414 and state machine 416 are analogous to timer 314 and state machine 316 of PLL 300 of Fig. 3, in that timer 414 controls the timing of state machine 416 , which applies a sequence of digital control input values N to VCO 408 to identify the two closest VCO operating curves for the desired application. Similarly, switches SW1 and SW2 of PLL 400 operate analogously to switches SW1 and SW2 of PLL 300 to apply the reference voltage V REF to the voltage input of VCO 408 during the initial auto-trim phase and the loop-filter voltage V LF during normal PLL operations. PLL 400 differs from PLL 300 in a number of respects. First of all, PLL 400 has a separate PFD, charge pump and capacitor for generating an input voltage to state machine 416 . In particular, PFD 418 generates up and down error signals ( U and D ) based on the phase and frequency relationships between the adjusted input signal and the feedback signal F FB . Charge pump 420 generates amounts of charge based on the error signals from PFD 418 and capacitor C S2 accumulates the net charge, which affects the input voltage to state machine 416 . Because PLL 400 does not rely on the loop-filter voltage V LF during the auto-trim phase, PLL 400 can be implemented without switch SW3 of Fig. 3, since there is no advantage in PLL 400 to removing capacitor C L during the auto-trim phase. In addition, PLL 400 can be implemented without inverter INV1, since state machine 416 is already isolated from the normal loop-filter voltage path. In fact, one of the motivations for the design of PLL 400 is to avoid inverter INV1, which can adversely affect the loop-filter voltage V LF during normal PLL operations. Fig. 5 shows a block diagram of a charge-pump phase-lock loop 500 , according to yet another alternative embodiment of the present invention. In PLL 500 , PFD 502 , charge pump 504 , loop filter 506 , switches SW1 and SW2, VCO 508 , input divider 510 , and feedback divider 512 are analogous to the corresponding components of PLL 400 of Fig. 4. In the digital embodiment of PLL 500 , digital accumulator 522 replaces charge pump 420 and capacitor C S2 of PLL 400 . Digital accumulator 522 accumulates the digital up and down error signals from PFD 518 and generates a digital input to state machine 516 . In that sense, digital accumulator 522 is the digital equivalent of charge pump 420 and capacitor C S2 of PLL 400 . In addition, PLL 500 is preferably implemented without a timer, such as timer 414 of PLL 400 . In this case, upper and lower limits are placed on the value contained within digital accumulator 522 , so that the correct frequency error polarity is known when the accumulator value reaches either of the hard limits. The limits are preferably set at magnitudes such that each limit can be reached only under the appropriate corresponding condition (i.e., output frequency either too high or too low for an appropriate length of time). The use of these upper and lower limits eliminates the need for a separate timer, and appropriate selection of the limits can further reduce the overall duration of the auto-trim phase. In general, the duration of the timer used in the analog implementations of Figs. 3 and 4, and magnitudes of the upper and lower limits used in the digital implementation of Fig. 5 are preferably set based on the minimum frequency difference between the input and output signals that the PLL needs to resolve. In possible implementations of PLL 400 and 500 , the second phase/frequency detector ( 418 and 518 ) can be replaced with a frequency-only detector, thereby potentially reducing cost, layout, and power requirements for the PLL. The present invention provides advantages over the PLLs of the prior art. Since the VCO is trimmed automatically to the appropriate operating curve at power up, there is no need to trim the VCO in the factory. Nor is there a need to keep an inventory of different VCOs for different applications, since each VCO will be automatically trimmed to the appropriate trim setting for the particular application. In addition, since the VCO is not permanently trimmed, the PLL can be used and then re-used for different applications operating at different nominal frequencies. Each time the PLL is powered up, the VCO will be trimmed to the current appropriate trim setting. In addition, the PLL auto-trim operations may be repeated whenever an appropriate reset signal is applied to the PLL. Another advantage of the present invention is that very few additional components need to be added to the conventional design of PLL 100 of Fig. 1 to achieve an instance of the auto-calibrating PLL of the present invention. For example, many conventional PLL designs already have a digital accumulator that can be used for digital accumulator 522 of Fig. 5. Similarly, most conventional PLLs already have switches SW1 and SW2 present. In addition, in PLLs where a separate frequency loop is used to aid acquisition, an extra frequency detector, charge pump, and capacitor may be present for use in PLL 400 of Fig. 4. Moreover, although timers and state machines may not already be present in conventional PLLs, because they are low-speed logic, the added cost is negligible compared to the savings. Although the embodiments of Figs. 3-5 show charge-pump PLLs, the present invention is not limited to charge-pump PLLs. In general, the present invention can be implemented for almost any PLL having a phase/frequency detector, an integrating loop filter, and a voltage-controlled oscillator. Furthermore, in alternative embodiments, the functions of switches SW1 and SW2 in Figs. 3-5 can be incorporated inside the VCO as controlled by one of the digital control input values. It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the principle and scope of the invention as expressed in the following claims.

Claims: An integrated circuit having a charge-pump phase-lock loop (PLL) (300, 400, 500) comprising: (a) a phase/frequency detector (PFD) (302, 402, 502); (b) a charge pump (304, 404, 504) connected to the PFD; (c) a loop filter (306, 406, 506) connected to the charge pump; (d) a voltage-controlled oscillator (VCO) (308,408, 508) connected to the loop filter; and (e) a controller (316,416,516) connected to the VCO, wherein: during normal PLL operations: the PFD generates error signals based on comparing an input signal and a PLL feedback signal generated from a VCO output signal generated by the VCO; the charge pump generates amounts of charge corresponding to the error signals; the loop filter accumulates the amounts of charge to generate a loop-filter voltage that is applied to a VCO voltage input of the VCO; and the VCO generates the VCO output signal based on the loop-filter voltage using an appropriate one of a plurality of VCO operating curves; and during PLL auto-trim operations, the controller automatically trims the VCO to the appropriate VCO operating curve for use during the normal PLL operations CHARACTERISED IN THAT : during the PLL auto-trim operations, the normal operating characteristics of one or more of the PLL components are adjusted to accelerate the PLL auto-trim operations. The integrated circuit of claim 1, wherein, during the PLL auto-trim operations, the charge pump current is increased to accelerate the PLL auto-trim operations. The integrated circuit of claims 1-2, wherein, during the PLL auto-trim operations, the effective capacitance of the loop filter is reduced to accelerate the PLL auto-trim operations. The integrated circuit of claim 3, wherein the PLL further comprises a loop filter switch configuration (SW3) adapted to change the effective capacitance of the loop filter. The integrated circuit of claims 1-4, wherein, during the PLL auto-trim operations, a reference voltage is applied to the VCO voltage input instead of the loop-filter voltage. The integrated circuit of claim 5, wherein the PLL further comprises a switch configuration (SW1, SW2) adapted to selectively control whether the loop-filter voltage or the reference voltage is applied to the VCO voltage input. The integrated circuit of claims 1-6, wherein the PLL further comprises an inverter (INV1) connected between the loop filter and the controller and adapted to provide an input signal used by the controller to generate digital control signal values for the VCO to select different VCO operating curves; and the controller limits the time that each digital control signal value is applied to the VCO to accelerate the PLL auto-trim operations. The integrated circuit of olaims 1-6, wherein the PLL further comprises a second detector (418) connected to second charge pump (420) connected to a second loop filter (C S2 ) connected to the controller, wherein, during the PLL auto-trim operations: the second detector generates secondary error signals based on comparing the input signal and the PLL feedback signal; the second charge pump generates secondary amounts of charge corresponding to the secondary error signals; the second loop filter accumulates the secondary amounts of charge to generate an input signal used by the controller to generate digital control signal values for the VCO to select different VCO operating curves; and the controller limits the time that each digital control signal value is applied to the VCO to accelerate the PLL auto-trim operations. The integrated circuit of claims 1-6, wherein the PLL further comprises a second detector (518) connected to a digital accumulator (522) connected to the controller, wherein, during the PLL auto-trim operations: the second detector generates secondary error signals based on comparing the input signal and the PLL feedback signal; and the digital accumulator accumulates the secondary error signals and generates a digital input for the controller, wherein the digital accumulator applies upper and lower limits to the accumulated secondary error signals to accelerate the PLL auto-trim operations. The integrated circuit of claims 1-9, wherein the controller is a state machine that, during the PLL auto-trim operations, applies a sequence of digital control input values to the VCO to select different VCO operating curves and selects an optimal trim setting for the VCO for use during the normal PLL operations.

Inventors: MOON UN-KU, WILSON WILLIAM B, MOON, UN-KU, WILSON, WILLIAM B.
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1998
Main classification: H03L7
IPC codes: H03L7