Abstract:
Phase detection circuitry is disclosed that can detect phase differences from a quadrature phase relationship, without requiring extensive additional circuitry for driving and correcting the phase detection circuitry. The phase detection circuitry can measure plural or multiple pulse durations consecutively, without interruption to reset the circuit or store the values generated by the circuit, affording a much higher sampling rate and sampling a higher proportion of pulses than is conventional. Averaging of the phase data samples is also provided by measuring multiple pulses, so that phase changes are not instigated based upon a signal from an individual pulse duration that may differ significantly from the average. In addition to detecting quadrature phase relationships, the phase detection circuitry can be adjusted to compensate for a desired offset in one of the signals from quadrature, or can be set to detect other phase relationships. The phase detection circuitry can also be configured to automatically adjust for variations in temperature, voltage and processing conditions, and has a number of advantages for use with high-frequency, low-voltage signals.

Description:
BACKGROUND 
     The present invention relates to phase detectors, phase-locked loops, and delay-locked loops. 
     Phase detectors, which detect a phase relationship between plural signals, may be found in various systems, including communications systems, computers, and memory devices. Phase detectors may be used, for example, to synchronize signals for data transmission or reception by detecting relative phases of signals so that the phases can be adjusted to achieve a desired result. Phase detectors are commonly used, for example, as a part of a phase-locked loop (PLL) or delay-locked loop (DLL). Both PLLs and DLLs generally offer the ability to delay the phase of a signal by an amount that may be in a range between 0° and 360°. Typically, PLLs are employed to recover a clock signal or to multiply a signal from a slow frequency to a high frequency, whereas DLLs are used for precise alignment of signals. 
     A first type of phase detector may be called a zero phase detector, and simply detects and reports whether one signal was early or late compared to another. This can be made with a fairly straightforward circuit, for the phase difference is merely a function of the relative position of the edges of the signals, and is largely independent of the frequency of oscillation. 
     For high-frequency, low-voltage or high-noise signals, it may be desirable to detect a signal at the midpoint of another signal, rather than attempting to align edges for detection. A quadrature phase detector (QPD) attempts to detect a phase difference between a midpoint of one signal and an edge of another. Stated differently, a QPD detects whether two signals are 90° out of phase with each other, and if not, the relative variance of the signals from that 90° phase relationship. A challenge in designing a QPD is that aligning an edge of one pulse with the midpoint of another pulse implies some processing of pulse duration or frequency in order to determine that midpoint. Determining a pulse midpoint becomes more difficult with increasing signal frequency and reduced pulse voltage levels. 
     A prior art QPD circuit is shown in FIG. 1, and clock signals C and D, whose phases are being compared by the QPD, are shown in FIG.  2 . The QPD includes a first current source  20  that provides charge to a first capacitor  22  during a time when signal C is high and signal D is low, such as period T 1 . The amount of charge that has built up on capacitor  22  results in a voltage on that capacitor that is an indication of the duration of period T 1 . A switch  25  can then be closed to reset the voltage across capacitor  22  to zero volts, so that charge build up on the capacitor can be used again to measure the duration of other periods when signal C is high and signal D is low. Similarly, current source  30  provides charge onto a capacitor  33  during a time when signal C is high and signal D is also high, for example period T 2 . The voltage that has built up on capacitor  33  is thus an indication of the duration of period T 2 . A switch  35  can then reset the voltage across capacitor  33  to zero volts, so that the charge build up on the capacitor can be used again to measure the relative duration of other times when both signals C and D are high. A comparator  40  has inputs connected to capacitor  22  and capacitor  33  for determining whether time T 1  is greater or less than time period T 2 , and adjusts the phase of C or D accordingly to attempt to make later pulses have a quadrature phase relationship, which is again measured at times T 3  and T 4 . 
     In order to operate the QPD, the frequency of signal C is divided by three, to produce signal C/ 3 , which is then compared with signal C to generate signals R, I and S. Reset signal R is high for a time during which the switches  25  and  35  are closed to dissipate any charge on capacitors  22  and  33 . While integration signal I is high, the switches are open and charge is allowed to build up on capacitors  22  and  33 . While set signal S is high, the voltage on capacitors  22  and  33  is compared and stored for adjusting the phase. 
     A disadvantage of the mechanism shown in FIG.  1  and FIG. 2 is that a phase correction signal is created after looking at only a single pulse cycle, which can cause jitter or oscillations about a desired phase unless additional circuitry is provided for smoothing the shift in phase. Further circuitry is also needed to produce signals R, I and S, and this circuitry also requires additional chip real estate and adds potential sources of error. Moreover, the duration of an individual pulse, such as the time when signal C is high represented by adding T 1  and T 2 , may vary significantly from average, which can cause incorrect adjustments of phase. For high-frequency, low-voltage, or high-noise conditions, or for situations in which the signals being compared differ from perfect square waves, variations of individual pulses from average may be especially pronounced. Further, the phase alignment is only sampled during one-third of the pulses, with the other pulses used for resetting the circuit and measuring or storing the output, thereby ignoring a majority of the data available for phase detection. 
     SUMMARY 
     In accordance with the present invention, phase detection circuitry is disclosed that can detect phase differences from a quadrature phase relationship, without the need for extensive additional circuitry for driving and correcting the phase detection circuitry. Such circuitry can measure plural or multiple pulse durations consecutively, without interruption to reset the circuit or store the values generated by the circuit, affording a much higher sampling rate and sampling a higher proportion of pulses than is conventional. Averaging of the phase data samples is also provided by measuring multiple pulses, so that phase changes are not instigated based upon a signal from an individual sample that may differ significantly from the average. In addition to detecting quadrature phase relationships, the phase detection circuitry can be adjusted to compensate for a desired offset in one of the signals from quadrature, or can be set to detect other phase relationships. The phase detection circuitry can also be configured to automatically adjust for variations in temperature, voltage and processing conditions. Each of these attributes is useful for high-frequency, low-voltage, or high-noise signals, so that the phase detection circuitry has a number of advantages for use with such signals, but is by no means limited by this brief summary of possible features. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a prior art phase detector that is used in conjunction with other circuitry to detect a phase difference between two signals. 
     FIG. 2 is a plot of various signals that may be used in the phase detector of FIG. 1, including control signals generated by some of the other circuitry in order to run the phase detector of FIG.  1 . 
     FIG. 3 is a diagram of a phase detector in accordance with the present invention. 
     FIG. 4 is a plot of signals that may be used in the phase detector of FIG. 3, in accordance with the present invention. 
     FIG. 5 is a diagram of a phase detector in accordance with the present invention including a reference voltage compensation mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows an embodiment of a phase detector  100  in accordance with the present invention that receives signals A and B, which are shown as idealized square waves in FIG.  4 . In operation, signals A and B may differ markedly from that shown in FIG. 4, due to the generation of different forms of signals, noise or other factors. Phase detector  100  includes logic gates  102  and  104 , which in this embodiment are shown to be AND gates, although other logic gates may instead be employed. In other embodiments, the input signals A and B may have more sinusoidal or other non-digital characteristics. In such other embodiments, the logic gates  102  and  104  may be replaced by circuitry that controls the current sources such that: one current source is turned on when the voltage levels of signals A and B are substantially similar (e.g. both are greater than a certain high voltage threshold in one embodiment or both are less than a predetermined low voltage threshold in another embodiment); and the other current source is turned on when the voltage levels of signals A and B are substantially different (e.g. signal A is greater than the high voltage threshold and signal B is less than the low voltage threshold in one case and signal A is less than the low voltage threshold and signal B is greater than the high voltage threshold in another case). 
     In the embodiment illustrated, signals A and B may be clock signals and thus tend to have generally square wave-like characteristics. In such a case, the determination of substantially similar voltage levels is equivalent to detecting a matching logic state and the determination of substantially different voltage levels is equivalent to detecting a non-matching logic state. In the example illustrated, Gate  102  receives signal A at one input and includes a mechanism for receiving the complement of signal B at its other input. Gate  104  receives signal A at one input and signal B at its other input. Gate  102  controls a first current source  106  that is connected to a first capacitor  108  via a first integration line  110 . Gate  104  controls a second current source  112  that is connected to a second capacitor  114  via a second integration line  116 . Line  110  is connected to an input of a first comparator  120 , and line  114  is connected to another input of comparator  120 . A second comparator  122  has an input connected to line  110  and another input connected to a voltage source  124 . Current sources  106  and  112  are substantially matched, as are capacitors  108  and  114 , to provide accurate comparisons to the inputs of the comparators. 
     Logic gate  102  turns on current source  106  during time period T 1 , when signal A is high and signal B is low, causing charge to accumulate on capacitor  108 . Similarly, logic gate  104  turns on current source  112  during time period T 2 , when signal A is high and signal B is also high, causing charge to accumulate on capacitor  114 . Capacitors  108  and  114  are examples of integration mechanisms that may be employed for measuring voltage or current differences over time. Unlike the prior art, phase data is sampled again during the next cycle of signals A and B, during time periods T 3  and T 4 , when charge again accrues on capacitors  108  and  114 , respectively. Capacitors  108  and  114  maybe used to accumulate charge corresponding to a number N of time measurements before the comparator  120  compares the voltage level of capacitor  108  with that of capacitor  114 . Based on the comparison, the comparator outputs a signal indicating which capacitor has a higher voltage level, which indicates whether T 1  was greater or less than T 2 . Once the comparison has been performed, the charge on the capacitors may be dissipated for a subsequent integration and comparison operation. The charge dissipation may be accomplished using switches across the capacitors similar to those shown in FIG.  1 . 
     Integrating charge over a number of measurement periods can increase the accuracy of the determination made by comparator  120 . For example, the charge accumulated during a single measurement period such as T 1  or T 2  may be a few millivolts, which may be less than a minimum voltage needed by comparator  120  to make an accurate comparison. By storing a sum of the charge stored during multiple periods for comparison, moreover, capacitors  108  and  114  offer an averaged value that is less subject to jitter or minor variations. 
     Voltage source  124  provides a reference threshold that ensures that enough voltage has accrued on capacitors  108  and  114  for an accurate measurement by comparator  120 . The reference threshold may be adjustable such that it can respond to changes in operating conditions or to a desired threshold that may be configurable based on a register setting. When the voltage on line  110  passes the threshold established by voltage source  124 , indicating that phase data from a number of time measurements has accrued, comparator  122  switches on, providing a signal to a validity checker  133  that a phase sample is ready for measurement. Because the comparison should be performed when the number of periods that add charge to capacitor  108  is equal to the number of periods that add charge to capacitor  114 , the validity checker  133  also includes circuitry that detects this condition and generates a resulting control signal. The circuitry that performs this function may include a counter that determines whether an even number of time measurements has occurred, so that the number of measurements for which A was high and B was low is equal to the number of measurements for which A and B were both high. In other embodiments, the circuitry may simply include a flip-flop that determines whether the number of time measurements performed thus far is odd or even (if the number of measurements is odd, the number used to add charge to one capacitor cannot equal the number used to add charge to the other). 
     When the comparator  122  indicates that the voltage across capacitor  108  has reached or exceeds the threshold set by voltage source  124 , and the it has been determined that an even or predetermined number of time measurements has occurred, an output valid signal is sent to phase indicator  130 . The phase detector  130  also receives a signal from comparator  120  indicating whether capacitor  108  or capacitor  114  has more voltage. Upon receiving the output valid signal, phase indicator  130  outputs the signal from comparator  120  indicating whether signal B is early or late compared to the quadrature of signal A. This signal may be provided to a conventional digital to analog converter (DAC) that controls a phase adjusting mechanism for adjusting the phase of signal B accordingly. An example of such a phase adjusting mechanism is disclosed in U.S. Pat. No. 5,614,855 to Lee et al., entitled “Delay-Locked Loop,” and incorporated by reference herein. The output valid signal that triggers comparison of the voltages across capacitors  108  and  114  may also be used to control switches, not shown in this figure, that reset the voltages across the capacitors  108  and  110  such that that the capacitors  108  and  110  may begin accumulating phase data for another set of time periods. 
     Note that the phase detector  100  essentially measures a leading edge of pulses for signals A and B, as these edges may be subject to less noise than the trailing edges of those pulses. Straightforward modifications to the logic gates may be made to create a phase detector that instead measures the trailing edges of the pulses. Also note that other forms of signals may be measured for phase relationships. For example, with appropriate settings of logic thresholds, relative phases of sinusoidal signals can be compared with phase detector  100 . The phase detector also can be adjusted to align the relative phases in a non-quadrature relationship, for example by providing unequal current sources  106  and  112 . Aligning the relative phases in a non-quadrature relationship may be useful, for instance, when one of the signals is known to encounter a delay relative to the other signal, so that after encountering the delay the signals are in quadrature. Similarly, an adjustment from quadrature can be provided by setting the validity checker  133  to output a valid signal when an odd instead of an even number N of time measurements has occurred. It is also possible to employ phase detector  100  to compare phases of signals having different frequencies, for example where one frequency is twice the other. 
     Also, circuit elements  102  and  104  need not be logic gates in a strict sense. In a more general sense various circuit elements that regulate the current of lines  110  and  116  can be employed as charge pumps for capacitors  108  and  114 , respectively. Such circuit elements can be found, for example, in emitter-coupled logic (ECL) families that work with signals that swing relative to each other, and analogous complimentary metal-oxide semiconductor (CMOS) differential signal families, such as low voltage differential signal (LVDS) logic families. The circuit elements may for instance be differential amplifiers or other devices that compare input signals. And depending upon the type of device, a threshold for delineating whether input signals such as voltage levels are high or low, and whether plural signals are substantially similar or substantially different, may be arbitrarily set. Thus any charge pump employing an output signal resulting from a logical combination of input signals to control a current source may be implemented according to the present invention. 
     As stated above, the output valid signal may be used to reset the capacitors  108  and  114  for comparing phase data from another series of pulses of signals A and B. The phase indication signal may also be made available to other related elements. For instance, an early or late phase indication may correlate with process, voltage and/or temperature (PVT) conditions throughout an integrated circuit (IC) that includes the phase detector  100 . In this case, the early or late phase indication may be employed for controlling automatic voltage, current or gain control mechanisms in other areas of the IC. 
     The voltage source  124  may be a simple voltage divider that sets the voltage input to comparator  122  at a predetermined level less than Vdd. On the other hand, as shown in FIG. 5, the voltage level of a reference input  200  to comparator  122  can be configured to respond to PVT conditions in a similar manner as current sources  206  and  212  for integration lines  210  and  216 . In this case, transistors  214  and  220  and resistor  218  form a reference-voltage generator which utilizes a current source that matches the current sources uses to generate the integration currents. Transistors  202  and  204  are turned on and off by the output of gates  102  and  104 , respectively, to accrue charge on capacitors  108  and  114  reflecting the phase relationship of signals A and B. A bias voltage Vbias that may relate to various factors such as PVT conditions controls current sources  206  and  212 . The current source  220  is controlled by the same bias signal Vbias, so that the voltage on line  200  can vary, for instance due to PVT conditions, in a similar fashion as the charge accruing on capacitors  108  and  114  may vary. 
     Although this disclosure has focused on teaching the preferred embodiments of a novel phase detector, other embodiments and modifications of this invention will be apparent to persons of ordinary skill in the art in view of these teachings. Therefore, this invention is limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.