Phase-Frequency Lock Detector

The present invention relates in general to integrated circuits, and in particular to a an improved lock-detect circuit for systems using phase-locked loops.

A phase-locked loop (PLL) is a circuit that operates to bring the phase of the output signal of a voltage-controlled oscillator (VCO) in alignment with that of a reference input signal. One common application for PLLs is in frequency synthesizers where a stable reference input clock is used to generate different frequencies. A conventional PLL includes a phase comparator that receives the reference input signal and the output of the VCO, and detects a phase difference therebetween. The error signal that represents the phase difference at the output of the phase comparator is then filtered and applied to the VCO to control its output frequency. By adjusting the phase of the VCO output signal in response to the magnitude of the phase error signal, this servo loop operates to minimize the error signal and to lock the phase of the reference signal with that of the input signal.

A PLL may fail to phase-lock and/or frequency-lock to the reference input frequency under several conditions. Some of these conditions are part of the normal operation and happen at expected times such as during the acquisition, or frequency transition modes. Other conditions are not part of the normal operation and are often unpredictable. These include loss of lock due to loss of input signal or significant degradation of the quality of the input signal (e.g., excessive jitter or modulation).

In many applications, it is crucial for the system to know at all times whether the system clock, which is usually generated by a PLL, is in lock condition in order to ensure data integrity. Hence, a lock-detect circuit becomes necessary.

Of the various lock-detect circuits that have been proposed, some require large frequency counters to monitor the frequency of the reference input signal and the signal at the output of the VCO divider. These schemes consume large area of silicon and power, and are only for frequency lock detection, not phase lock detection. Further, existing lock-detect circuits typically do not address the failure to lock situation when the reference frequency disappears or deviates significantly from the intended frequency.

There is therefore a need for an area and power efficient implementation of a lock-detect circuit that operates reliably under all conditions.

The present invention provides method and circuitry for detecting when a PLL achieves phase- and frequency-lock to a reference frequency with minimal hardware and power dissipation. The invention takes advantage of existing blocks within a PLL to reduce the amount of circuitry required while at the same time reducing error due to mismatch. Broadly, the present invention combines a coarse lock-detect circuit with a fine lock-detect circuit to achieve fast response when the input reference is lost, while filtering occasional minor phase hits due to external or internal noise.

Accordingly, in one embodiment, the present invention provides for use with a phase-locked loop (PLL), a circuit for detecting a lock condition including a coarse lock-detect circuit having an input coupled to an output of the PLL, and a fine lock-detect having an input coupled to the output of the PLL. A combine circuit logically combines an output of the coarse lock-detect with the output of the fine lock-detect to generate the final lock-detect output.

In a more specific embodiment, the input of the coarse lock-detect circuit and the input of the fine lock-detect circuit couple to an output of a phase comparator inside the PLL and receive the phase comparator output error signal. The coarse lock-detect circuit includes a current source that is controlled by the error signal, and a capacitor that is charged by the current source or discharged by a switch. A comparator coupled to the capacitor then decides whether phase/frequency lock has been achieved within a predefined window. The fine lock-detect circuit includes circuitry that generates a predefined window that is smaller than the window used in the coarse lock-detect circuit and checks for phase/frequency lock within the smaller window.

A better understanding of the nature and advantages of the lock-detect circuit of the present invention may be gained with reference to the detailed description and drawings below.

FIG. 1 is a block diagram of a preferred embodiment of the lock-detect circuit of the present invention;

FIG. 2 shows an exemplary circuit implementation for the lock-detect circuit of the present invention;

FIG. 3 is a more detailed schematic diagram of an exemplary circuit implementation of the coarse lock-detector of the present invention; and

FIG. 4 is a more detailed schematic diagram of an exemplary circuit implementation of a fine lock-detector according to the present invention.

Referring to FIG. 1, there is shown a block diagram of a preferred embodiment of the lock-detect circuit 100 of the present invention in combination with a phase-locked loop (PLL) 102. PLL 102 receives the reference input signal Ref.sub.in and after dividing its frequency (104), applies it to one input of a phase/frequency comparator 106. The other input of phase/frequency comparator 106 receives the output of a VCO 108 that has been divided down by a divider 110. Phase/frequency comparator 106 compares phase and frequency of the two signals and generates UP or DN pulses depending on whether the VCO output signal lags or leads in phase compared to the reference signal. The UP and DN pulses control the operation of a charge pump circuit 112 whose output is filtered by filter 114. The output of filter 114 then supplies the control voltage to VCO 108. The frequency of the oscillator in VCO 108 is thus adjusted by the control voltage to bring the phase/frequency of the VCO output signal closer to the reference signal.

Instead of having separate circuitry (external to the PLL) to compare the phase/frequency of the reference input signal with the VCO output signal for lock detection purposes, the lock-detect circuit 100 of the present invention utilizes the UP and DN pulses from the phase/frequency comparator 106. This not only reduces the amount of circuitry, it eliminates potential error introduced by mismatches between an external phase/frequency detector and that of the PLL. Lock-detect circuit 100 relies on the ability of phase/frequency comparator 106 to generate UP and DN pulses when the inputs of the phase comparator are phase and frequency aligned. Many of today's PLL circuits continue to generate UP and DN pulses of predefined duration even after there has been a phase/frequency alignment to reduce or eliminate what is commonly referred to as the `dead-band` effect. This is accomplished by generating UP and DN pulses with a predefined duration that is longer than the response time of the phase comparator and the charge-pump in the PLL, after phase/frequency alignment has been achieved. By combining the UP and DN pulses and comparing the duration of the resulting pulse with a predefined window, the present invention detects whether a lock condition has been achieved.

FIG. 1 shows one embodiment of the present invention that takes advantage of the UP and DN pulses generated by the phase/frequency comparator of the PLL. Lock-detect circuit 100 as shown in FIG. 1 first performs a logic OR function (116) on the UP and DN pulses from phase/frequency comparator 106 and then feeds the resultant pulse NETP to inputs of a coarse lock-detector 118 and a fine lock-detector 120. A combine circuitry 122 combines the outputs of both coarse and fine lock-detectors to generate the final output signal LOC.sub.-- VAL that indicates a valid lock.

Referring to FIG. 2, there is shown an exemplary circuit implementation for lock-detect circuit 100. As discussed above, coarse detector 118 detects gross frequency/phase mismatch between the inputs of the phase/frequency comparator 106 by checking the duration of the NETP pulse against a predefined window. Coarse detector 118 is primarily active during the phase capture and loss of reference signal. Under either one of these conditions, one of the pulse signals UP or DN has a much wider (positive) pulse width indicating gross mismatch in the phase/frequency of the two signals. As the PLL operates to bring the phases in alignment, the pulse width of UP or DN narrows until it reaches its minimum duration.

The principals of operation of coarse detector 118 will be described hereinafter. Referring to FIG. 2, the pulse NETP resulting from the OR of the UP and DN signals, is inverted and then applied to a current source 200 that charges a capacitor C1, and a switch S1 that, when closed, discharges capacitor C1. Switch S1 provides for a very fast discharge of capacitor C1 when current source 200 is turned off. The state of NETP controls when and for how long current source 200 is turned on and charging capacitor C1 with a predefined current Ic. A comparator 202 compares the voltage on the top plate of capacitor C1, node NCAP, with a predefined threshold voltage Vth. Therefore, under the loss of reference signal condition, for example, OR gate 116 generates a relatively wider positive pulse at NETP. Current source 200 is activated in response to NETP having a high logic level. Capacitor C1 is thus charged by current Ic for as long as NETP remains high. If NETP remains high long enough to charge cap C1 to a voltage above Vth of comparator 202, the output of comparator 202, C.sub.-- OUT will change state from logic low to logic high. The assertion of C.sub.-- OUT indicates that the inputs of the phase/frequency comparator (106 in FIG. 1) are grossly out of phase.

Figure 3 shows an exemplary circuit implementation for coarse detector 117. The combination of resistor R and transistors M1 and M2 generates the current Ic. Transistors M3 and M4 operate in response to the inverse of NETP signal to either charge C1 (when M4 is OFF and M3 is ON), or discharge C1 (when M4 is ON and M3 is OFF) as described above. An inverter INV1 performs the compare function of comparator 202 (in FIG. 1). In this particular embodiment, a feedback circuit including transistors M5 and M6 provide a degree of hysteresis to avoid accidental toggling of the C.sub.-- OUT signal.

Depending on the specific requirement of the PLL, the magnitude of Ic and capacitance of C1 can be adjusted, to change the duration of time Tc needed for Ic to charge C1 up to Vth. That is, the combination of Ic and C1 adjust the size of the window for coarse detection. For example, a duration of about 10% of the output period may be used for Tc.

The output C.sub.-- OUT of coarse detector 118 also controls fine detector 120. The signal C.sub.-- OUT is used to reset the state of fine detector 120 and hence prohibit the fine detector 120 to operate. Thus, as long as coarse detector 118 has not qualified the signal, fine detector 120 remains in a reset state. This ensures that in case of gross errors that may not be detected by fine detector 120, the output of coarse detector 118 overrides the operation of the circuit. For example, because fine detector 120 integrates the results from several cycles before it changes state, it will not detect a sudden loss of one cycle and will thus erroneously stay in the lock state. Such a gross phase error, however, would be detected by coarse detector 118 whose output shuts off fine detector 120 so that the circuit correctly detects the failure to lock condition.

After coarse detector determines that the phase error is less than Tc (i.e., less than the coarse window), it releases fine detector 120. Fine detector 120 checks the phase error represented by NETP against a tighter window. In addition, it digitally integrates the results from several cycles before deciding on fine lock or loss thereof. Fine detector 120 is primarily activated when the PLL is in lock condition to detect occasional minor phase errors. As discussed above, under lock condition, phase/frequency comparator 106 generates UP and DN signals having a minimum pulse width Tmin. Fine detector 120 utilizes these pulses to perform its detect operation.

Referring back to FIG. 2, fine detector 120 includes a pulse stretch circuit 204 that receives NETP that in combination with a programmable delay circuit 206 operates to widen the duration of the high level of the NETP pulse. The amount of delay Tf introduced by delay circuit 206 sets the finer window for detection and can be digitally adjusted. The stretched NETP pulse, or D.sub.-- NETP, is then fed to the input of a FINE.sub.-- LOCK circuit 208. FINE.sub.-- LOCK circuit 208 includes a D-type flip flop circuit (described in greater detail in connection with FIG. 4) that receives D.sub.-- NETP at its clock input and NETP at its D input. The widening of the NETP pulse ensures that the clock signal for the flip flop inside FINE.sub.-- LOCK circuit 208 is wide enough to satisfy the setup and hold time requirements.

With NETP at its D input and D.sub.-- NETP applied to the clock input, a logic low level at the output of the flip flop indicates that NETP is shorter than the delay Tf, indicating fine lock. Conversely, a logic high level at the output of the flip flop indicates that NETP is wider than delay Tf, indicating loss of fine lock. The amount of delay Tf is adjusted such that it is larger than Tmin but is considerably smaller than Tc in coarse detector 118. In this fashion, coarse detector 118 uses a larger window Tc to detect gross phase/frequency mismatches, while fine detector 120 uses a tighter window Tf to detect the minor phase/frequency drifts.

Combine circuit 122 can be implemented by, for example, a NOR gate 210. Therefore, the final output signal LOCK.sub.-- VAL is at logic high level indicating a valid lock only when both C.sub.-- OUT and F.sub.-- OUT are at logic low level. That is, only when both coarse detector 118 and fine detector 120 agree that a lock condition has been achieved, is the signal LOCK.sub.-- VAL asserted.

To avoid chattering of the output signal LOCK.sub.-- VAL, fine detector 120 integrates the results from several cycles before changing the state of the F.sub.-- OUT signal. The present invention accomplishes this by using several (e.g., four) flip flops in FINE.sub.-- LOCK circuit 208. Referring to FIG. 4, there is shown an exemplary circuit implementation of FINE.sub.-- LOCK circuit 208. The circuit includes four D-type flip flops 400-1, 400-2, 400-3, 400-4 connected in a shift register fashion. Each flip flop 400 receives the D.sub.-- NETP signal (buffered by two inverters) at its clock input. First flip flop 400-1 receives NETP at its D input, while each of the other flip flops receives the Q output of the preceding flip flop. The outputs of all four flip flops are respectively connected to inputs of a four-input AND circuit 402 and a four-input NOR circuit 404. To eliminate glitching, the outputs of AND 402 and NOR 404 are resynchronized with the falling edge of D.sub.-- NETP by flip flops 406 and 408, respectively. The resynchronized output of the AND 402, F.sub.-- LOC, resets a latch 410, while resynchronized output of NOR, F.sub.-- NLOC, sets latch 410. Each flip flop 400 also receives the output of coarse detector, signal C.sub.-- OUT, at a set input. As discussed above, this deactivates fine detector 208 for as long as coarse detector 118 has not detected a lock condition (i.e., C.sub.-- OUT is at logic high level).

Accordingly, four consecutive samples of 1's or 0's are needed for F.sub.-- OUT to change state. This provides integration and hysteresis for fine detector 120. The amount of integration and hysteresis in FINE.sub.-- LOCK circuit 208 can be easily adjusted by changing the number of flip flops 400. In one embodiment, the present invention includes additional flip flops 400 with wider AND 402 and NOR 404 circuits, where the flip flops can be digitally set (or reset) to reduce the amount of integration, or enabled to increase the amount of integration.

In conclusion, the present invention provides an improved lock detect circuit that requires less circuitry and consumes less power, while operating more reliably. By utilizing the output of the phase/frequency comparator inside the PLL, the circuit of the present invention reduces the circuit size and eliminates potential error caused by mismatches. Further, the circuit includes separate coarse and fine detector blocks to allow for a faster and more accurate detection of lock or loss of lock conditions. While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents.