Patent Publication Number: US-10333527-B2

Title: Compact high speed duty cycle corrector

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/840,984, filed on Dec. 13, 2017, which is a continuation of U.S. patent application Ser. No. 15/389,830 filed on Dec. 23, 2016, now U.S. Pat. No. 9,882,570. The contents of which are incorporated by reference herein in their entirety. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to data communication. 
     Over the last few decades, the use of communication networks exploded. In the early days of the Internet, popular applications were limited to emails, bulletin board, and mostly informational and text-based web page surfing, and the amount of data transferred was usually relatively small. Today, Internet and mobile applications demand a huge amount of bandwidth for transferring photo, video, music, and other multimedia files. For example, a social network like Facebook processes more than 500 TB of data daily. With such high demands on data and data transfer, existing data communication systems need to be improved to address these needs. For high-data communication applications, duty cycle correction is often needed. 
     Over the past, there have been many types of communication systems and methods. Unfortunately, they have been inadequate for various applications. More specifically, detecting loss of signal can be challenging. Therefore, improved systems and methods are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to data communication. More specifically, embodiments of the present invention provide techniques for duty cycle correction of clock signals. An input clock signal passes through a pair of output transistors, which provides an output clock signal based on the input clock signal. A duty cycle sensor generates a first correction signal based on the output clock signal. The first correction signal is at least partially opposite of the output clock signal. A duty cycle corrector generates a second correction signal based on the first correction signal. The duty cycle corrector includes two or more transistors for generating the second correction signal. The second correction signal is applied to the output clock signal. There are other embodiments as well. 
     According to an embodiment, the present invention provides a duty cycle correction device. The device includes an input terminal for receiving an input clock signal, which is characterized by a first duty cycle and a signal frequency. The device also includes a first transistor comprising a first gate terminal and a first drain terminal. The first gate terminal is coupled to the input terminal. The device further includes a second transistor comprising a second gate terminal and a second drain terminal. The second gate terminal is coupled to the input terminal. The device also includes an output terminal for outputting a corrected clock signal. The output terminal is coupled to the first drain terminal and the second drain terminal. The corrected clock signal is characterized by a second duty cycle, which is closer to 50% than the first duty cycle. The device additionally includes a duty cycle sensor coupled to the output terminal and being configured to generate a first correction signal. The first correction signal is substantially inverted relative to the clock signal. The device also includes a DAC that is configured to generate a control voltage based on a digital control signal. The device includes a duty cycle corrector that is configured to generate a second correction signal using at least the control voltage and the first correction signal. The second correction signal is coupled to the output terminal. 
     According to another embodiment, the present invention provides a communication system. The system includes a communication channel for receiving data signal. The system also includes a signal processing module that is configured to equalize the data signal. The system additionally includes a clock data recovery (CDR) device that is configured to generate a first clock signal based on the processed data signal. The system further includes a duty cycle correction (DCC) device that is configured to generate a corrected clock signal based on the first clock signal. The DCC device comprises an input terminal for receiving the first clock signal. The DCC device also includes a first pair of transistors that is configured to receive the first clock signal via the input terminal and to output a second clock signal. The DCC device further includes an output node that is coupled to the second clock signal. The DCC device also includes a duty cycle sensor that is configured to generate a first correction signal based on the second clock signal. The DCC device further includes a duty cycle corrector that is configured to generate a second correction signal based on the first correction signal. The duty cycle corrector includes a second pair of transistors. The second correction signal is coupled to the output node. 
     According to yet another embodiment, the present invention provides a method for correcting clock duty cycles. The method includes receiving an input clock signal that is characterized by a first duty cycle. The method also includes processing the input clock signal using a first pair of transistors. The method further includes providing an output clock signal using the first pair of transistors. The method additionally includes generating a first correction signal by inverting a portion of the output clock signal. The method also includes a fine digital correction by receiving a duty cycle control code. The method also includes converting the duty cycle control code to a control voltage using a DAC device. The method also includes providing a control signal by a second pair of transistors. The method additionally includes generating a second correction signal by a third pair of transistors using the control signal and the first correction signal. The method also includes applying the second correction signal to the output clock signal, which is characterized by a second duty cycle correction method. The second duty cycle correction is used to correct any duty cycle error which hasn&#39;t been corrected by the first method. 
     It is to be appreciated that embodiments of the present invention provide many advantages over conventional techniques. Among other things, by omitting VCO (as used in existing DCC implementations), input capacitance of duty cycle correction devices according to embodiments of the present invention is reduced and minimized. It is to be appreciated that DCC devices according to the present invention are more energy efficient and have small impact on the clock path bandwidth compared to conventional devices. 
     Embodiments of the present invention can be implemented in conjunction with existing systems and processes. For example, DCC devices according to embodiments of the present invention can be readily incorporated in existing communication devices. Additionally, DCC devices according to the present invention can be manufactured using existing manufacturing equipment and processes. There are other benefits as well. 
     The present invention achieves these benefits and others in the context of known technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following diagrams are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many other variations, modifications, and alternatives. It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this process and scope of the appended claims. 
         FIG. 1  is a simplified diagram illustrating a communication system according to an embodiment of the present invention. 
         FIG. 2  is a simplified diagram illustrating a conventional DCC implementation. 
         FIG. 3  is a simplified diagram illustrating a conventional DCC buffer with a current starving inverter. 
         FIG. 4  is a simplified diagram illustrating a conventional DCC buffer with a self-bias inverter. 
         FIG. 5  is a simplified diagram illustrating a duty-cycle correction device  500  according to an embodiment of the present invention. 
         FIG. 6  is a simplified timing diagram illustrating operation of a duty cycle correction device according to embodiments of the present invention. 
         FIG. 7  is a simplified diagram illustrating DAC correction signals according to embodiments of the present invention. 
         FIG. 8  is a graph illustrating operation of duty cycle correction device  500  according to embodiments of the present invention. 
         FIG. 9  is a graph illustrating a relationship between output duty cycle and input duty cycle for a duty cycle correction device according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to data communication. More specifically, embodiments of the present invention provide techniques for duty cycle correction of clock signals. An input clock signal passes through a pair of output transistors, which provides an output clock signal based on the input clock signal. A duty cycle sensor generates a first correction signal based on the output clock signal. The first correction signal is at least partially opposite of the output clock signal. A duty cycle corrector generates a second correction signal based on the first correction signal. The duty cycle corrector includes two or more transistors for generating the second correction signal. The second correction signal is applied to the output clock signal. There are other embodiments as well. 
     Clock signals are vital in data communication and various other applications. An important characteristic of clock signals and clocking systems is duty cycle. Often, 50% duty cycle is both important and preferred. For example, half-rate transmitters need 50% duty cycle clock, and departure from this could lead to duty cycle distortion that causes horizontal eye opening at the output. For quarter rate transmitters and receivers, quadrature clocks are needed and typically implemented by using a frequency divider circuits that divides the clock signal by 2, followed by a voltage controlled oscillator (VCO) running at double frequency. If the clock input to the frequency divider is not at 50% duty cycle, the quarter rate transmitter cannot function property due to quadrature error, which in turn leads to degraded link timing margin. 
     For a duty cycle corrector to satisfy various performance requirements, there are various desirable characteristics. At the input stage, it is desirable to have low input capacitance and lower power consumption, which can typically be achieved by eliminating the voltage controlled oscillator and/or the VCO clock buffer. At the same time, performance of the duty cycle corrector and the clock path bandwidth should be maintained. 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     Furthermore, any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6. In particular, the use of “step of” or “act of” in the Claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6. 
     Please note, if used, the labels left, right, front, back, top, bottom, forward, reverse, clockwise and counter clockwise have been used for convenience purposes only and are not intended to imply any particular fixed direction. Instead, they are used to reflect relative locations and/or directions between various portions of an object. 
       FIG. 1  is a simplified diagram illustrating a communication system according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 1 , transmitter  110  is coupled to receiver  120  via a pair of communication channels. For example, the communication channels may be serial communication link, optical communication channel, and/or others. In various embodiments, transmitter  110  performs signal modulation to generate analog signal for transmission over a high-speed communication network (e.g., communication channels as shown). For example, communication signals transmitted from transmitter  110  to receiver  120  are transmitted in pairs (i.e., a positive signal and a negative signal as a differential pair). Once incoming signals are received, receiver  120  processes the received signals accordingly. In certain implementations, receiver  120  includes a pair of input inductors. To process the incoming signal, continuous time linear equalizer (CTLE) and variable gain amplifier (VGA) components are used, which is shown as functional block  121 . Depending on the application, CTLE, VGA, and/or other components may be used to adjust the received analog signals for further processing. A clock and data recovery (CDR) module  122  is configured to generate clock signal based on the signal processed by the CTLE/VGA module  121 . For example, the CDR module  123  generates clock signal based on the received signal. 
     In various embodiments, duty-cycle correctors are provided both for the transmitter and the receiver. On the transmitter send, duty-cycle corrector (DCC)  111  is coupled to transmitter  110 . Additional, DCC  11  is couple to a phase-lock loop (PLL)  130 . PLL  130  includes a divider  131  and a VCO. For example, DCC  111  is implemented to provide clock signal “CLK” as shown for data sampling. Depending on the implementation, such as full-rate architecture, transmitter  110  may not need DCC  111 . 
     DCC  124  of receiver  120  is coupled to the CDR  122 . For example, recovered clock signals (at different phases) are coupled to divider  123 , and are then processed by DCC  124 . The clock signals from the CDR  122  are in four phases ( 0 ,  90 ,  180 , and  270 ), and different types of clock signals can be used as well. As shown in  FIG. 1 , DCC  124  is coupled to a voltage-controlled oscillator, but it is understood that other configurations are possible as well. 
     As mentioned above, conventional duty-cycle corrector implementations have been inadequate.  FIG. 2  is a simplified diagram illustrating a conventional DCC implementation. At the input of the DCC buffer, a VCO is coupled to the DCC through a VCO buffer. As mentioned above, the use of VCO introduces capacitance at the input stage, which undesirable for power consumption and potential signal degradation. The output of the DCC buffer is coupled to an operation amplifier via a resistor and a capacitor. It is to be noted that in conventional implementation such as the one shown in  FIG. 2 , both the resistor and the capacitor are large both in size and value. As a part of the DCC implementation, the resistor and the capacitor effectively provide an RC filter. When operating, the portion of clock signal that is outside of the 50% duty cycle is feed into the DCC buffer, which receives a correction signal from the operation amplifier. The operation amplifier itself is also a relatively large module. 
       FIG. 3  is a simplified diagram illustrating a conventional DCC buffer with a current starving inverter. Duty cycle correction (i.e., a correction voltage) is applied at node C, as supplied by a PMOS transistor and an NMOS transistor. For example, a high voltage at node C leads to a long duty cycle; a low voltage at node C leads to a short duty cycle. Since both the PMOS transistor and the NMOS transistor are current starving, their operation reduces the speed of the inverter. 
       FIG. 4  is a simplified diagram illustrating a conventional DCC buffer with a self-bias inverter. The input signal is received through capacitor C F , which adds a bottom plate capacitance at the input terminal. For the input, the VCO or the buffer driving self-bias inverter would need to drive the high capacitance attributed to C F . Additionally, the inverter is coupled to resistors RF and R C . Duty cycles are corrected by applying a correction voltage at node C. To decrease the duration of duty cycle, voltage applied to node C is increased; to increase the duration of duty cycle, voltage applied to node C is decreased. 
       FIG. 5  is a simplified diagram illustrating a duty-cycle correction device  500  according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 5 , input clock signal is received at terminal  501 . It is to be understood that the input clock signal received at terminal  501  is not at the desired 50% duty cycle, and thus duty cycle is needed, and is performed by the DCC device  500 . The input signals are processed by duty cycle sensor  505 , and correction signal is generated by the duty cycle corrector and applied to output node X  504  as shown. When the input clock signal is received at terminal  501  and then passes through transistors  502  and  503 , duty cycle error (i.e., deviation from the desired 50% duty cycle) is detected by the duty cycle sensor  505  that includes one or more inverters. Duty cycle correction signal is injected by transistor  509  and transistor  520 . Relative to transistors  502  and  503 , transistors  509  and  520  are much smaller (and “weaker”), and the correction signal that opposes the duty cycle error is provided by transistors  509  and  520 . 
     As can be seen in  FIG. 5 , there is additional load configured at the input (e.g., coupled to transistors  502  and  503 ), DCC device  500  imposes no additional load on the circuit (not shown in  FIG. 5 ) that drives the duty cycle correction buffer. Further, DCC device  500  does not include a filter (e.g., RC elements) or operational amplifier, and as a result DCC device  500  can be made very compact. 
     In various embodiments, transistors  509  and  520  are configured much smaller compared to transistors  502  and  503 . For example, transistors  509  and  520  may be an order of magnitude smaller (e.g., 6 times) than transistors  502  and  503 . It is to be appreciated that the relative small transistor size (i.e., smaller in size and electrical characteristics) of transistors  509  and  520  reduces and minimizes latching effects of these transistors on the signal. 
     When input clock signal is received at input terminal  501 , the input clock signal is processed coupled to the gate terminals of transistors  502  and  503 . More specifically, source terminal of transistor  502  is coupled to a supply voltage, while the drain terminal is coupled to output node X  504 . Transistor  502  is implemented using a PMOS transistor. Transistor  503  is implemented using an NMOS transistor. The source terminal of transistor  503  is grounded, and the drain terminal of transistor  503  is coupled to output node X  504 . 
     Output node X  504  is coupled to duty cycle sensor  505 , which uses the output clock signal at node X  504  as an input signal. Duty cycle sensor  505  comprises inverters  506  and  507  as shown. For example, “inverted” clock signals provides an opposite waveform that can be used to correct the output clock signal as needed. Depending on the implementation, duty cycle sensor  505  can be implemented in other ways as well. 
     Correction signal for the output clock signal is generated by duty cycle corrector  511 . The output of duty cycle sensor  505  is coupled to node Y  510 . Node Y  510  is coupled to the gates of transistors  509  and  520 . In various implementations, transistor  509  comprises a PMOS transistor, and transistor  520  comprises an NMOS transistor. Transistors  509  and  520  are much smaller in size compared to transistors  502  and  503 , as explained above. Since the input signal to the gates includes inverted output clock signal, the output of transistors  509  and  520  at their respective drain terminals are “opposite” relative to the output clock signal at node X  504 , thereby applying a correction to the output clock signal. Additionally, source terminals of transistors  509  and  520  are respectively coupled to transistors  508  and  530 . Transistors  508  and  530  provides control signal received from DAC  540 . As shown, drain terminal outputs of transistors  508  and  530  are respectively coupled to the source terminals of transistors  509  and  520 . Implemented with a PMOS transistor, the source terminal of transistor  508  is coupled to the supply voltage (e.g., VDD). Implemented with an NMOS transistor, the source terminal of transistor  530  is coupled to the ground. 
     DAC  540  receives duty cycle control signal (i.e., “Duty_cycle”) in digital format and generates a duty cycle control voltage is applied to the gate terminals of transistors  508  and  590 . In various implementations, duty cycle control signal may be received from an external control module (not shown) that is part of a feedback control loop. For example, a large value of the duty cycle control signal is converted by DAC  540  to a high duty cycle control voltage. A high voltage at the gate terminal of transistor  508  increases the rise time. Additionally, a high voltage at the gate terminal of transistor  530  decreases the fall time. By both increasing rise time and decreasing fall time, a high duty cycle control voltage thus increases the duty cycle. And vice versa. Depending on the implementation, “duty_cycle” signal may be determined using a feedback mechanism. More specifically, based on the duty cycle of the output clock signal, a control module (not shown) generates the “duty_cycle” signal for DAC  540 , which in turn generates the duty cycle control voltage. 
       FIG. 6  is a simplified timing diagram illustrating operation of a duty cycle correction device according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, waveforms  601  and  602  illustrates operation of duty cycle corrector  500  illustrated in  FIG. 5 . More specifically, waveform  601  corresponds to the input clock signal voltage at input terminal  501 , which is coupled to the gate terminals of transistors  502  and  503 . The input clock signal is off from the desired 50% duty cycle. The “high” state of waveform  501  is longer than its “low” state. For example, “high” state at  601 A is longer than the “low” state at  601 B. To correct waveform  601 , an “opposite” voltage at corresponding time is needed. Waveform  602  corresponds to the signal at node Y  510  in  FIG. 5 . As described above, the voltage at node Y  510  is based on the output of duty cycle sensor  505 . Waveform  602  is used as input voltage (applied at the gate terminals) for transistors  509  and  520 , which provide a correction signal to the output clock signal at node X  504 . Being “opposite” relative to the input clock signal, waveform  602  provides the control voltage (to transistors  509  and  520 ) for correcting the output clock signal that is based on the input clock signal (waveform  601 ). For example, the “high” state at  601 A of waveform  601  can be corrected by the “low” state at  602 A of waveform  602 . Similarly, the “low” state  601 B of waveform  601  can be corrected by the “high” at  602 B of waveform  602 . In various embodiments, the “opposite” waveform  602  can be generated by using one or more inverters, but it is to be understood that other implementations are possible as well. 
       FIG. 7  is a simplified diagram illustrating DAC correction signals according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, waveforms  701  and  702  illustrate operation of duty cycle corrector  500  illustrated in  FIG. 5 . More specifically, waveform  701  corresponds to the outputs of transistors  509  and  520 . As explained above, a high voltage at the gate terminal of transistor  508  increases the rise time T r  for output of transistor  509 . Additionally, a high voltage at the gate terminal of transistor  530  decreases the fall time T f  of output of transistor  520 . More specifically, an increase in the DAC  540  output causes an increase in resistance of transistor  508 . Also, the increase in the DAC  540  output causes a decrease in resistance of transistor  530 . The changes in clock duty cycle are evident by comparing waveforms  701  and  702 . Waveform  702  shows the input signal (from the duty cycle sensor  505 ) that is applied to gate terminals of transistors  508  and  530 . By increasing the rise time T r , waveform  701  reaches “high” state at time  710 , which is later than the time that waveform  702  reaches its corresponding “high” stage. By decreasing the fall time T f , waveform  701  starts falling from its “high” state at time  720 , which is earlier than the time that waveform  702  starts falling from its corresponding “high” state. It is to be appreciated that transistors  509  and  520  are much smaller than transistors  502  and  503 , and the relative small size ensures that bandwidth reduction on in the clock path is minimized. 
       FIG. 8  is a graph illustrating operation of duty cycle correction device  500  according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For the purpose of illustration, an input clock signal with a duty cycle of 40% is provided to the input terminal  501  of the duty cycle correction device  500 . Transistors  509  and  520  provide correction voltage to the output clock signal at node X  504 , and the output waveform is shown on graph  801 . Voltage waveform at node Y  510  (as provide by duty cycle sensor  505 ) is shown on graph  802 . For example, voltage waveform at node Y  510  is the output of duty cycle sensor  505 , and it is applied to the gate terminals of transistors  509  and  520 . Graph  803  shows duty cycle of the output clock signal (i.e., corresponding to the waveform  801  for the output clock signal voltage). The output voltage on graph  803  is corrected by the outputs of transistors  509  and  520 . Starting at 40% duty cycle, the output clock signal is at about 43% duty cycle at about 1 ns (about 10 clock cycles later). By 4 ns, the duty cycle for the output clock signal is already at 49%, and at about 7 ns mark, the duty cycle is at about 49.7%, which is very close to the desired 50% duty cycle. 
       FIG. 9  is a graph illustrating a relationship between output duty cycle and input duty cycle for a duty cycle correction device according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 9 , a duty cycle correction device is capable of correct clock signal that is way off. For example, for an input clock with duty cycle of 15%, the DCC device can bring the output clock duty cycle to over 47.5%. For an input clock with duty cycle of 85%, the DCC device can bring the output clock duty cycle to over 47%. For input clock duty cycle between 20% and 70%, the DCC device is capable of bring the output clock duty cycle to over 49%. Depending on the system configuration, a high level of duty cycle accuracy can be achieved. 
     It is to be appreciated that DCC techniques according to embodiments of the present invention can be implemented in various ways. For example, one or more inverters can be used in conjunction with transistors to generate the correction signal as needed to adjust the output clock signal. Depending on the application, duty cycle waveform and adjustment speed can be specifically implemented. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.