# Patent

## Title
Combined lock/out-of-lock detector for phase locked loops

## Abstract

A detector for detecting a locked state and an out-of-lock state of a phase locked loop includes an out-of-lock detector circuit that receives a reference signal and an input signal representing a PLL oscillator signal. The out-of-lock detector detects an out-of-lock state of the PLL and generates an out-of-lock signal indicating whether an out-of-lock state is detected. The detector further includes a lock detector circuit that receives the reference signal and the input signal, detects a locked state of the PLL, and generates a lock signal indicating whether a locked state is detected. A logic circuit receives both the out-of-lock signal and the lock signal and combines both signals to obtain an output signal indicative of whether the PLL is in a locked state or an out-of-lock state.

## Background
FIELD
The present disclosure relates to the field of radio frequency (RF) transceiver circuits.
BACKGROUND
Radio frequency (RF) transceiver can be found in numerous applications, particularly in the field of wireless communications and radar sensors. In the automotive sector, there is an increasing demand for radar sensors used in so-called “adaptive cruise control” (ACC) or “radar cruise control” systems. Such systems may be used to automatically adjust the speed of an automobile so as to maintain a safe distance from other automobiles ahead.
Modern radar systems make use of highly integrated RF circuits, which may incorporate all core functions of an RF front-end of a radar transceiver in one single package (single chip transceiver). Such RF front-ends may include, inter alia, a voltage controlled oscillator (VCO), power amplifiers (PA), mixers, and analog-to-digital converters (ADC). However, not all of these components have to be necessarily in one single chip package. For example, ADCs may be integrated in a separate chip. Particularly in frequency-modulated continuous wave (FMCW) radar systems, the VCO is typically operated in a phase-locked loop (PLL). However, PLLs may be used in many other applications.
Radar applications used in automobiles are subject to various standards concerning road traffic safety, for example the functional safety standard ISO 26262 titled “Road vehicles—Functional safety”. To ensure the functional safety of a radar sensor, it is important to know whether the current state of the radar sensor allows a reliable distance and speed measurement. Particularly underestimating the true distance to a vehicle ahead and its speed may cause dangerous traffic situations.
In a radar system the operating state of the PLL used in the radar transceiver is a critical parameter. For a reliable measurement, the PLL should be in a locked state. That is, the PLL output signal is in phase with (i.e. locked to) a reference signal. If the PLL is in an “out-of-lock” state, any measurement results are unreliable, and therefore it may be important to know, whether the PLL is in a locked state or in an out-of-lock state. To improve functional safety, a lock/out-of-lock detector should provide a fast detection of an out-of lock state as well as precise detection of the locked state, which imposes a conflict of objectives on the circuit designer.
SUMMARY
A detector for detecting a locked state and an out-of-lock state of a phase locked loop (PLL) is disclosed. In accordance with the embodiments described herein, the detector includes an out-of-lock detector circuit that receives a reference signal used by the PLL and an input signal representing a PLL oscillator signal. The out-of-lock detector is configured to detect an out-of-lock state of the PLL and generates an out-of-lock signal indicating whether an out-of-lock state is detected. The detector further includes a lock detector circuit that receives the reference signal and the input signal, is configured to detect a locked state of the PLL, and generates a lock signal indicating whether a locked state is detected. A logic circuit receives both, the out-of-lock signal and the lock signal and is configured to combine both signals to obtain an output signal indicative of whether the PLL is in a locked state or an out-of-lock state.
Furthermore, a method for monitoring a phase locked loop is described, wherein the phase locked loop comprises a frequency divider for providing an input signal based on an oscillator signal. In accordance with one example of the present disclosure the method includes determining whether pulses of the input signal and corresponding pulses of the reference signal coincide within a given range of tolerance. An out-of-lock state of the phase locked loop is signaled, when the corresponding pulses do not coincide. The method further includes counting the pulses of the input signal during a time window. A lock state of the phase locked loop is signaled when the counted number of pulses is within a predefined interval.



BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure can be better understood with reference to the following drawings and descriptions. The components in the figures are not necessarily to scale; instead emphasis is placed upon illustrating the principles of the disclosure. More-over, in the figures, like reference numerals designate corresponding parts. In the drawings:
FIG. 1 is a block diagram one exemplary implementation of a phase-locked loop (PLL);
FIG. 2 is a block diagram illustrating one example of an out-of-lock detector;
FIG. 3 includes a set of timing diagrams illustrating the operation of the out-of-lock detector of FIG. 2;
FIG. 4 is a block diagram illustrating one example of a lock detector;
FIG. 5 includes a set of timing diagrams illustrating the operation of the lock detector of FIG. 4;
FIG. 6 illustrates how to combine the out-of-lock detector if FIG. 2 and the lock detector of FIG. 4 to obtain a combined lock/out-of-lock-detector;
FIG. 7 includes a set of timing diagrams illustrating the operation of the combined lock/out-of-lock-detector of FIG. 6.



DETAILED DESCRIPTION
FIG. 1 illustrates, as an illustrative example, a phase locked loop (PLL) as commonly used, for example, in RF transceivers of FMCW radar systems. However, PLLs and further circuitry described herein may also be employed in other RF applications. The PLL includes a voltage controlled oscillator VCO (or, in a digital implementations, a numerically controlled oscillator, short: NCO) which generates an oscillating output signal SVCO having an frequency, which is denoted as fVCO and set in accordance with a control signal SCTL supplied to a control input of the oscillator VCO. The oscillator output signal SVCO is supplied to a frequency divider MMD having a selectable division ratio N (so-called multi-modulus divider). That is, the frequency divider MMD is configured to divide the frequency fVCO supplied to its input and to generate a divider output signal SDIV having a frequency denoted as fDIV, wherein fVCO=N·fDIV. The division ratio is selectable dependent on a select signal supplied to a select input of the frequency divider MMD. Dependent on the application a fractional-N divider may be used. In this case, the division ratio N may be a fractional number.
The divider output signal SDIV as well as a reference signal SREF, which has a frequency fREF, are supplied to a phase detector PD (also known as phase comparator). Dependent on the implementation a phase-frequency-detector PFD may be employed instead. Phase detectors as well as phase-frequency detectors are commonly used in the field of PLLs. The reference signal SREF may be provided by a reference oscillator, which usually is (but not necessarily has to be) a crystal oscillator denoted herein as XTAL. That is, the frequency fREF may be determined by the resonance frequency of a quartz crystal oscillator.
The phase (-frequency) detector P(F)D usually includes a charge-pump at its output, which generates (as output signal) an error signal, which is denoted as SERR and filtered by a loop filter LF, which determines the band-width of the control loop. The output of the loop filter is used as control signal SCTL to adjust the frequency fVCO and the phase of the oscillator output signal SVCO thus closing the control loop. The closed loop ensures that the frequency fVCO is tuned to such a value that the phases of the divider output signal SDIV and the reference signal SREF match (i.e. are in a defined and fixed relation to each other). That is, the divider output signal SDIV (and thus also the oscillator signal SVCO) is “locked” to the reference signal SREF, and the PLL is in a “locked state”. The oscillator signal SVCO is also the output signal of the PLL. Various implementations of phase (-frequency) detector P(F)D including charge-pumps are as such known in the art and thus not further discussed herein in more detail.
As mentioned above, a PLL operating in a locked state may fall “out-of-lock” (i.e. into an unlocked state) due to various reasons such as, for example, noise, jitter, and other disturbances. As mentioned above, the operating state of the PLL is a critical parameter in many applications such as, for example, in radar transceivers used for measuring distances and velocities. For a reliable measurement, the PLL should be in a locked state. That is, the PLL output signal SVCO is in phase with (i.e. locked to) the reference signal SREF. If the PLL is in an “out-of-lock” state, any measurement results are unreliable and probably have to be discarded. To improve functional safety of the overall system, it may be important to know, whether the PLL is in a locked state or in an out-of-lock state. For this purpose, a PLL may include a lock/out-of-lock detector, which indicates (by generating an appropriate output signal) whether or not the PLL is in a locked state. Generally, such lock/out-of-lock detectors may be regarded as part of the phase detector PD. In accordance with the embodiments described herein, lock detector and out-of-lock detector are separated to allow a fast out-of-lock detections and a precise lock detection. The outputs of the separate lock and out-of-lock detectors may then be combined to form one signal indicative of the operating state (lock or out-of-lock) of the PLL.
FIG. 2 illustrates one example of an out-of-lock detector 2, which allows a very fast detection of an out-of-lock state. This allows a very fast reaction when the PLL falls out-of-lock. As mentioned, the out-of-lock detector may be regarded as part of the phase detector PD. However, it may also be implemented separately from the phase detector. The out-of-lock detector circuit 2 receives the reference signal SREF from the crystal oscillator XTAL and the frequency divider output signal SDIV as input signals. The out-of-lock detector circuit 2 is configured to generate a signal SCMP, which may be a logical conjunction of the two input signals SREF and SDIV. Alternatively, the signal SCMP may be the result of a comparison of the levels of the input signals SREF and SDIV. In the present example, the input signals SREF and SDIV are supplied to the inputs of an AND gate 10, wherein the output of AND gate 10 is inverted by an inverter 11, which provides the signals SCMP. In essence, a logical NAND conjunction is implemented. In the present example of FIG. 2, signal SCMP is the result of the operation SDIV NAND SREF, the signal SCMP′ is the result of the operation SDIV AND SREF.
The resulting signal SCMP is sampled synchronously to the reference signal SREF (sampling unit 12). In the present example, the reference signal SREF has a duty cycle of 50 per cent, and the sampling time instances tS1, tS2, tS3, tS4 for sampling the signal SCMP are a delay time interval TDEL after a corresponding rising edge in the reference signal SREF. This situation is illustrated by the timing diagrams of FIG. 3, in which the time instances, at which rising edge occur in the reference signal SREF, are denoted with t1, t2, t3, and t4. With respect to the periodicity of the reference signal, the sampling time instances tS1, tS2, tS3, tS4 are phase shifted by 90 degrees. However, the delay time TDEL may vary dependent on the actual implementation. Generally, it is determined whether or not corresponding pulses of the reference signal SREF and the second input signal SDIV coincide within a given range of tolerance. That is, corresponding pulses of signals SREF and SDIV need not necessarily be exactly congruent. In the example of FIG. 3, coincidence of corresponding pulses is detected when both pulses are at a high level (first logic level) at the sampling times tS1, tS2, tS3, tS4, etc. Alternatively, coincidence of the low level could be checked. The sampling time instants are determined based on the reference signal. As can be seen from the example of FIG. 3, the mentioned range of tolerance depends on the value of the delay time TDEL. However, other circuits implementing a more complex logic to define the range of tolerance may be used.
The first (top) diagram of FIG. 3 illustrates the reference signal SREF provided by the reference oscillator XTAL. The period TREF of reference signal SREF equals fREF−1 with the rising edges occurring at time instances t1, t2, t3, and t4 (wherein TREF=t4−t3=t3−t2=t2−t1). The oscillation is constant with a comparably low jitter. The second diagram illustrates the frequency divider output signal SDIV. Before time instant t2, the signals SDIV and SREF are in phase. At time instant t1 corresponding pulses occur in both signals. However, the second and the third pulse in signal SDIV are “late” and occur significantly after the corresponding pulses of reference signals SREF at time instants t2 and t3, respectively. Thus, the PLL is out-of-lock at least after time instant t2. The third diagram illustrates the signal SCMP, which is a logical conjunction (AND operation) of signals SREF and SDIV. Due to the sampling (sampling unit 12), the signal SCMP (or SCMP′) is only evaluated at the sampling times tS1, tS2, tS3, tS4, etc. If the pulses of the reference signal SREF and the frequency divider output signal SDIV coincide the signal level should be the same at the sampling times tS1, tS2, tS3, tS4. As soon as the signal levels are different at a sampling time instant (time instant tS2 in FIG. 3) an out-of-lock state is detected. As a result of the AND conjunction the signal SCMP is indicative of whether the signal levels of signals SREF and SDIV match at the time of sampling. The fourth (bottom) diagram of FIG. 3 illustrates the output signal (out-of-lock-signal SOLK) of the sampling unit 12, wherein the signal level sampled at one time instant is held until the subsequent sampling time instant. Assuming the PLL is in a lock state before time instant t1, the out-of-lock-signal SOLK is low before sampling time instant tS1. At sampling time instant tS1 the signal SCMP′ is at a high level (and SCMP is low) indicating that the levels of the signals SREF and SDIV match and the PLL is still in a locked state. At the next sampling time tS2 the pulses in signals SREF and SDIV do not coincide and thus the level of signal SCMP′ is low indicating that the levels of signals SREF and SDIV do not match and the PLL is in an out-of-lockstate. As a result, the out-of-lock-signal SOLK changes to a high level for signaling the detected out-of-lock state. The situation is the same at time instant tS3, and thus signal SOLK remains high. At time instant t4 the pulses in signals SREF and SDIV again coincide and thus their level match at sampling time instant tS4. Signal SCMP′ is at a high level at sampling time instant tS4 (and SCMP is low) and the out-of-lock-signal SOLK is set back to a low level, indicating that no out-of-lock state is detected (anymore).
As shown in FIG. 3, the out-of-lock detector is able to detect an out-of-lock state of the PLL very quickly, and in the best case within a single period fREF−1 of the reference signal SREF. However, a locked state is detected with high precision. As can be seen from FIG. 3, the temporal position of the pulses in the frequency divider output signal SDIV (relative to the reference signal SREF) may vary up to a time TDEL without an out-of-lock state being detected. As long as the signal levels of signals SREF and SDIV are the same at the time of sampling, an out-of-lock state is not detected. That is, the out-of-lock detector may be used for quickly detecting an out-of-lock state but the precision is not sufficient for detecting when the PLL is again in a locked state.
To accomplish a precise detection of a locked state of a PLL (such as the PLL shown in FIG. 1), the lock detector circuit of FIG. 4 may be used. Basically, the lock detector circuit 4 is configured to count the number of pulses present in the frequency divider output signal SDIV within a defined time window (window length TCNT, see FIG. 5) and to check whether or not the counted number of pulses is within a given range, which is, in the present example, defined by an upper and a lower counter limit CUL, CLL. The frequency divider output signal SDIV is supplied to an input of counter 41, which counts the pulses in the frequency divider output signal SDIV. In the present example, the counter value CNT is increased each time a rising edge is detected at the counter input of counter 41 during the mentioned time window. When the counter value is valid (i.e. after the time window) the counter value is compared with the mentioned upper and lower limits CUL and CLL using a digital comparator 42. A locked state is detected when the counter value CNT lies within the range defined by the upper and lower limit CUL and CLL. The counter 41 and the comparator 42 may be controlled by a finite state machine 45, which provides an enable signal EN and a reset counter signal RES for counter 41. The time window length TCNT is determined based on the reference signal SREF. In the present example, the time window length TCNT is an integer multiple of the period TREF=fREF−1 of the reference signal SREF. It may be determined using a further counter 40 which receives the reference signal SREF. The counter 40 is started by a respective start signal generated by the state machine 45, and the counter 40 supplies a stop signal to the state machine 45 indicating that a predefined counter value N has been reached (e.g. N=512). In the present example, the time window length TCNT is thus N/fREF. The state machine 45 may be clocked by the reference signal SREF.
The function of the lock detector circuit 4 can be better understood with the help of the timing diagrams illustrated in FIG. 5. A lock state detection is triggered by the state machine 45 by generating, at time instant tA, a start command, which is represented by a rising edge in a binary start signal generate by the state machine (see first (top) diagram of FIG. 5). The start command starts the counter 40. Simultaneously with the start command, the other counter 41 is enabled by a respective enable signal EN (see fourth diagram of FIG. 5) thereby “opening” the mentioned time window. Counter 40 counts the rising edges in the reference signal SREF and signals a stop command to the state machine 45 when a defined counter value is reached. For example, a counter overflow signal may be used to signal the stop command to the state machine 45 (e.g. when a counter overflow occurs after a maximum counter value N). When receiving the stop command a time instant tB, the time window “closes”, the state machine 45 disables counter 41 (see fourth diagram of FIG. 5) and generates a valid flag for the digital comparator 42 (see fifth (bottom) diagram of FIG. 5). Between time instants tA and tB counter 41 counts the rising edges in the frequency divider output signal SDIV. The frequency fDIV can be calculated as fDIV=CNT·fREF/N for a time window length TCNT equaling N/fREF. As fREF and N are known (and generally fixed) parameters the counter value CNT may be used as a measure of the frequency fDIV. It should be noted that the lock detector circuit 4 allows determining the frequency fDIV with a resolution Δf=fREF/N. That is, the resolution and thus the precision of the lock detection increases with an increasing time window length N/fREF. A lock signal SLCK indicating a precisely locked state of the PLL is generated by the digital comparator if the following condition is fulfilled, CUL<CNT<CCOL. Generally, a locked state is detected when the counter value CNT lies within an interval defined by CUL and CLL.
The lock detector circuit is able to detect a locked state of a PLL very precisely and the precision depends on the time window length TCNT (proportional to N). Therefore, an out-of-lock state can not be detected sufficiently fast. In accordance with the one embodiment, an out-of-lock detector circuit 2 (see FIG. 2) and a lock detector circuit 4 (see FIG. 4) can be combined to make use of both, a fast detection of an out-of-lock state and a precise (but slower) detection of a locked state of the PLL. FIG. 6 illustrates one example of how to combine the out-of-lock detector circuit 4 of FIG. 2 and the lock detector circuit of FIG. 4 to obtain an improved PLL lock detector. Accordingly, a SR latch (set-reset latch) 50 is used to generate the final PLL lock/out-of-lock signal SLOCK-PLL, wherein the SR latch 50 is set by lock signal SLOK generated by lock detector circuit 4 of FIG. 4 and reset by out-of-lock signal SOLK generated by out-of-lock detector circuit 2 of FIG. 2.
The operation of the combined lock/out-of-lock detector is illustrated by the timing diagrams of FIG. 7. The first (top) diagram illustrates the output signal SOLK of the out-of-lock detector circuit 2 of FIG. 2, the second (middle) diagram illustrates the output signal SLCK of the lock detector circuit 4 of FIG. 4, and the third (bottom) diagram illustrates the PLL lock/out-of-lock signal SLOCKPLL of the combined lock/out-of-lock detector of FIG. 6. Is should be noted that the waveform shown in the Figures are merely illustrative and idealized examples. The actual waveforms may look different dependent on the actual application. Furthermore, propagation delays are neglected in the present discussion. For the present example it is assumed that the PLL is in a locked state before time instant ta and an out-of-lock state is signaled by the out-of-lock detector circuit 2 (see FIG. 2) at time instant ta. Therefore the out-of-lock signal SOLK changes from a low level to a high level at time instant ta thus resetting the SR latch 50 (see FIG. 6). The reset is triggered by the rising edge of signal SOLK, which remains at a high level as long as the out-of-lock detector circuit 2 detects an out-of lock state. As the lock detector is much slower, the output signal SLCK of the lock detector circuit 4 (see FIG. 4) changes from a high level (indicating locked state) to a low level at time instant tb which is after time instant ta. The closed loop control of the PLL drives the VCO in order to reestablish a locked state. At time instant tb the out-of-lock detector circuit 2 does not detect an out-of-lock state anymore and the signal SOLK falls back to a low level. As mentioned, this does not necessarily mean that a locked state is established with the desired precision. It takes until time instant td until the lock detector circuit 4 detects (and confirms) the locked state of the PLL. As a result, signal SLCK is set back to a high level thus triggering a set of the SR latch 50 at time instant td. At time instant te an out-of-lock state is again detected and the SR latch is reset again (analogous to time instant ta)
Although the disclosure has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. For example, signal SDIV may be taken as reference signal in the described circuits and methods while signal SREF may be taken as second input signal. In particular regard to the various functions per formed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the disclosure. In addition, while a particular feature of the disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

## Claims

1. A detector for detecting a locked state and an out-of-lock state of a phase locked loop (PLL), the detector comprising:
an out-of-lock detector circuit configured to receive a reference signal used by the PLL and an input signal representing a PLL oscillator signal, the out-of-lock detector further configured to detect an out-of-lock state of the PLL and generate an out-of-lock signal indicating whether an out-of-lock state is detected;
a lock detector circuit configured to receive the reference signal and the input signal, and further configured to detect a locked state of the PLL, and generate a lock signal indicating whether a locked state is detected; and
a logic circuit configured to receive the out-of-lock signal and the lock signal and further configured to combine the out-of-lock signal and the lock signal to obtain an output signal indicative of whether the PLL is in a locked state or an out-of-lock state.

2. The detector of claim 1, wherein the out-of-lock detector circuit comprises:
a further logic circuit configured to receive the reference signal and the input signal, and configured to generate an output signal based on the input signal and the reference signal; and
a sampling unit configured to receive the output signal of the further logic circuit, and configured to sample the output signal to provide the out-of-lock signal.

3. The detector of claim 2, wherein the further logic circuit includes at least one logic gate, which is configured to provide, as the output signal, a logical conjunction of the input signal and the reference signal.

4. The detector of claim 3, wherein the logic conjunction is an AND conjunction or a NAND conjunction.

5. The detector of claim 2, wherein the sampling unit is configured to sample the output signal synchronously to the reference signal or to the input signal.

6. The detector of claim 2, wherein the sampling unit is configured to sample the output signal synchronously to the reference signal, and wherein the output signal is sampled using a sampling period which is delayed with respect to a corresponding period of the reference signal.

7. The detector of claim 1, wherein the lock detector circuit comprises:
a first counter configured to count cycles of the input signal during a time window thus providing a counter value,
wherein the time window has a length equal to a pre-defined number of cycles of the reference signal.

8. The detector of claim 7, wherein the lock detector circuit comprises:
a digital comparator configured to evaluate whether the counter value of the first counter is within a desired range, which is defined by a lower counter limit and an upper counter limit.

9. The detector of claim 8, wherein the digital comparator is configured to provide a comparator signal indicative of whether the counter value of the first counter is within the desired range.

10. The detector of claim 8, wherein the digital comparator is configured to provide a comparator signal indicative of whether the counter value of the first counter is within the desired range, and wherein the lock signal is generated based on the comparator signal.

11. The detector of claim 7 wherein the lock detector circuit further comprises:
a second counter configured to count cycles of the reference signal,
wherein the length of the time window depends on a counter value of the second counter.

12. The detector of claim 7 wherein the lock detector circuit further comprises:
a state machine coupled to the first counter and configured to control the operation of the first counter.

13. The detector of claim 1, wherein the logic circuit includes a latch.

14. The detector of claim 1, wherein the logic circuit includes a SR-latch, which is set in response to a pulse in the lock signal and reset in response to an out-of-lock signal.

15. A method for monitoring a phase locked loop (PLL), which comprises a frequency divider for providing an input signal based on an oscillator signal, comprising:
determining whether pulses of the input signal and corresponding pulses of a reference signal coincide within a given range of tolerance;
signaling an out-of-lock state of the PLL, when the corresponding pulses do not coincide;
counting the pulses of the input signal during a time window; and
signaling a lock state of the PLL when the counted number of pulses is within a predefined interval.

16. The method of claim 15, wherein the length of the time window is determined based on the reference signal.

17. The method of claim 15, wherein determining whether pulses of the input signal and corresponding pulses of the reference signal coincide comprises:
comparing the corresponding pulses and detecting coincidence when two corresponding pulses are at a first logic level at one of a series of sampling time instants.

18. The method of claim 17, wherein the sampling time instants are determined based on the reference signal.