Abstract:
Techniques and devices for differential signal repeating are described. A differential signal repeating method may include receiving an input differential signal pair including first and second input signals received at first and second input terminals, respectively, and generating an output signal at an output terminal. Generating the output signal may include: based on a determination, at a first time, that the first and second input signals represent complementary values, setting a level of the output signal to represent an inverse of the value represented by the first input signal, and based on a determination, at a second time, that the first and second input signals do not represent complementary values, placing the output terminal in a high-impedance state.

Description:
FIELD OF INVENTION 
     The present disclosure relates generally to systems and techniques for repeating differential signals. Some implementations relate specifically to differential signal repeater circuits operable to prevent, counteract, and/or correct skewing of differential signals, and/or to equalize differential signals. 
     BACKGROUND 
     Differential signaling circuits can use a pair of complementary signals to transmit data. The pair of signals are generally transmitted using respective electrical conductors. A receiver can decode data carried by the pair of signals by measuring a difference (e.g., a potential difference) between the two signals, instead of measuring a difference between a signal and a reference point (e.g., between the signal&#39;s potential and a reference potential or “ground”). Differential signaling circuits can also be used for transmitting differential clock signals. 
     In an integrated circuit (IC), data and clock signals can be transmitted between different blocks at different locations of the chip using conductive wires (e.g., metal lines and/or vias). To reduce the time delay associated transmitting a signal between two blocks of the chip (e.g., two blocks at opposite ends of the chip), one or more repeaters (e.g., signal buffers and/or inverters) can be inserted into the transmission path, rather than simply inserting a long wire. Each repeater can reproduce its input signal(s) at a higher level (e.g., higher voltage at the output of the repeater than at the input of the repeater), or can restore the input signal(s) to a nominal level. In this way, the repeaters divide the transmission path into several segments of shorter wires separated by repeaters, which together can have a shorter total time delay than the time delay associated with the long wire. 
     When a signal is transmitted over a medium (e.g., conductive wires) from a transmitter to a receiver, sources of distortion (e.g., signal interference, impedance mismatch, or the medium itself) may cause distortion in the received signal at the receiver. For instance, the received signal can have a different frequency profile from the original signal at the transmitter. As another example, when a pair of differential signals are transmitted over a medium from a transmitter to a receiver, sources of distortion may cause skewing between the two signals, such that the two signals do not switch at the same time (or within a specified time window) at the receiver. Signal equalization techniques are used to prevent, counteract, or reduce the distortion introduced into a signal during its propagation through a medium. For example, de-skewing techniques are used to prevent, counteract, or reduce the skew between the switching times of two or more signals. 
     SUMMARY OF THE INVENTION 
     In general, integrated circuits (ICs) continue to increase in size, while the feature sizes of IC components continue to decrease and the frequencies of the clock signals used to synchronize the operation of IC components continue to increase. As a result of these trends, distributing a clock signal throughout an IC or even a portion of an IC has become increasingly difficult. For example, the distorting effects of IC fabrication process variations on the propagation of the clock signal tend to increase as feature sizes decrease, and distortions of the clock signal tend to increase as the ratio of the distance traveled by the clock signal to the clock signal&#39;s period increases. 
     Conventionally, ICs have used long chains of amplifiers (e.g., inverters) to distribute clock signals. However, as clock frequencies and clock signal distortions (e.g., clock skew) increase, the performance of these conventional clock trees tends to suffer. In particular, for high-frequency clock signals in ICs with feature sizes of 28 nm or less, conventional clock trees can produce a very distorted clock signal after a relatively short chain of inverters (e.g., 50 inverters) unless the inverters are very large and powerful. However, a clock tree with large and powerful inverters is generally undesirable, because such a clock tree (1) consumes a significant amount of power, and (2) takes up valuable IC space that could otherwise be devoted to processing circuitry. 
     There is a need for a low-power clock distribution circuit that can propagate a high-frequency clock signal over a relatively long distance (e.g., from one side of an IC to the opposite side of the IC) without introducing significant signal distortion, even in ICs with feature sizes of 28 nm or less. The inventors have recognized and appreciated that a differential clock signal can be distributed using a chain of differential clock repeaters as described in the present disclosure. Each differential clock repeater can provide output differential clock signals (CLKPQ, CLKNQ) based on the values of input differential clock signals (CLKP, CLKN). When the input differential clock signals represent complementary logical values, the differential clock repeater can provide output differential clock signals that also represent complementary values. In some embodiments, the differential signal repeater can switch the output differential signals simultaneously (or within a specified time window) in response to both input differential signals switching, even if the two input signals do not switch simultaneously (or within a specified time window). In some embodiments, a clock tree that includes differential clock repeaters as described herein may be capable of propagating a high-frequency differential clock signal over a relatively long distance without introducing significant signal distortion and without consuming significant power, even in ICs with feature sizes of 28 nm or less. 
     According to an aspect of the present disclosure, a differential signal repeating method is provided. The method includes receiving an input differential signal pair including first and second input signals received at first and second input terminals, respectively, and generating an output signal at an output terminal. Generating the output signal includes, based on a determination, at a first time, that the first and second input signals represent complementary values, setting a level of the output signal to represent an inverse of the value represented by the first input signal, and based on a determination, at a second time, that the first and second input signals do not represent complementary values, placing the output terminal in a high-impedance state. 
     In some embodiments, the output signal is a first output signal, the output terminal is a first output terminal, and the method further includes generating a second output signal at a second output terminal. Generating the second output signal includes, based on a determination that the first and second input signals represent complementary values, setting a level of the second output signal to represent an inverse of the value represented by the second input signal, and based on a determination that the first and second input signals do not represent complementary values, placing the second output terminal in a high-impedance state. In some embodiments, the first and second output signals form an output differential signal pair. In some embodiments, generating the output differential signal pair counteracts skewing of the output differential signal pair. 
     In some embodiments, the levels of the output signals are voltage levels, and the method further includes at least partially equalizing the voltage levels of the first and second output signals. In some embodiments, at least partially equalizing the voltage levels of the first and second output signals is performed based on a determination that the first and second input signals do not represent complementary values. In some embodiments, at least partially equalizing the voltage levels of the first and second output signals includes coupling the first output terminal to the second output terminal. In some embodiments, at least partially equalizing the voltage levels of the first and second output signals counteracts skewing of the first and second output signals. 
     According to another aspect of the present disclosure, a system including a signal repeater circuit is provided. The signal repeater circuit includes a first field effect transistor (FET) of a first type and second and third FETs of a second type. A gate of the first FET is coupled to a gate of the second FET, a drain of the first FET is coupled to a drain of the second FET and to a gate of the third FET, and sources of the second and third FETs are coupled to a first power supply rail. The signal repeater circuit also includes a fourth FET of the second type and fifth and sixth FETs of the first type. A gate of the fourth FET is coupled to a gate of the fifth FET, a drain of the fourth FET is coupled to a drain of the fifth FET and to a gate of the sixth FET, and sources of the fifth and sixth FETs are coupled to a second power supply rail. Sources of the first and fourth FETs are configured to receive a first input signal of an input differential signal pair. The gates of the first, second, fourth, and fifth FETs are configured to receive a second input signal of the input differential signal pair. A drain of the third FET is coupled to a drain of the sixth FET. 
     In some embodiments, the first type of FET is a p-type FET and the second type of FET is an n-type FET. In some embodiments, the first power supply rail is configured to provide a ground potential, and the second power supply rail is configured to provide a supply voltage potential greater than the ground potential. 
     In some embodiments, the first type of FET is an n-type FET and the second type of FET is a p-type FET. In some embodiments, the second power supply rail is configured to provide a ground potential, and the first power supply rail is configured to provide a supply voltage potential greater than the ground potential. 
     In some embodiments, the signal repeater circuit further includes an output terminal coupled to the drains of the third and sixth FETs, and the signal repeater circuit is configured to set a level of an output signal at the output terminal to represent an inverse of a value represented by the first input signal in response to a transition of the first input signal and a complementary transition of the second input signal. In some embodiments, the signal repeater circuit is a first signal repeater circuit, and the system further includes a second signal repeater circuit. In some embodiments, the second signal repeater circuit includes: a seventh FET of the second type and eighth and ninth FETs of the first type, wherein a gate of the seventh FET is coupled to a gate of the eighth FET, a drain of the seventh FET is coupled to a drain of the eighth FET and to a gate of the ninth FET, and sources of the eighth and ninth FETs are coupled to the second power supply rail. In some embodiments, the second signal repeater circuit further includes a tenth FET of the first type and eleventh and twelfth FETs of the second type, wherein a gate of the tenth FET is coupled to a gate of the eleventh FET, a drain of the tenth FET is coupled to a drain of the eleventh FET and to a gate of the twelfth FET, and sources of the eleventh and twelfth FETs are coupled to the first power supply rail, wherein sources of the seventh and tenth FETs are configured to receive the second input signal of the input differential signal pair, wherein the gates of the seventh, eighth, tenth, and eleventh FETs are configured to receive the first input signal of the input differential signal pair, and wherein a source of the ninth FET is coupled to a source of the twelfth FET. 
     In some embodiments, the output signal is a first output signal, the output terminal is a first output terminal, the second signal repeater circuit further includes a second output terminal coupled to the sources of the ninth and twelfth FETs, and the second signal repeater circuit is configured to set a level of a second output signal at the second output terminal to represent an inverse of a value represented by the second input signal in response to a transition of the second input signal and a complementary transition of the first input signal. 
     In some embodiments, the system further includes an equalization circuit coupled to the output terminals of the first and second signal repeater circuits. In some embodiments, the first and second output signals form an output differential signal pair, and the equalization circuit counteracts skewing of the output differential signal pair. In some embodiments, the levels of the output signals are voltage levels, and the equalization circuit is configured to at least partially equalize the voltage levels of the first and second output signals in response to a transition of the first input signal and prior to a complementary transition of the second input signal, and/or in response to a transition of the second input signal and prior to a complementary transition of the first input signal. In some embodiments, the levels of the output signals are voltage levels, and the equalization circuit is configured to at least partially equalize the voltage levels of the first and second output signals based on a determination that the first and second input signals do not represent complementary values. In some embodiments, the levels of the output signals are voltage levels, and the equalization circuit is configured to at least partially equalize the voltage levels of the first and second output signals during a period when the first and second input signals do not represent complementary values. 
     In some embodiments, the equalization circuit includes a switch having a first terminal coupled to the first output terminal of the first signal repeater circuit and a second terminal coupled to the second output terminal of the second signal repeater circuit. In some embodiments, the equalization circuit further includes a driver circuit configured to control operation of the switch. In some embodiments, the driver circuit is configured to activate the switch in response to transitions of the first and second input signals and prior to the signal repeater circuits changing the levels of the first and second output signals in response to the transitions of the first and second input signals. In some embodiments, the driver circuit is configured to activate the switch in response to a transition of the first input signal and prior to a complementary transition of the second input signal, and/or in response to a transition of the second input signal and prior to a complementary transition of the first input signal. 
     In some embodiments, the driver circuit includes: thirteenth and fourteenth FETs of the first type, wherein a gate of the thirteenth FET is coupled to the second output terminal of the second signal repeater circuit, wherein a source of the thirteenth FET is configured to receive the second input signal, wherein a gate of the fourteenth FET is coupled to the first output terminal of the first signal repeater circuit, and wherein a source of the fourteenth FET is configured to receive the first input signal; and fifteenth and sixteenth FETs of the second type, wherein a gate of the fifteenth FET is configured to receive the second input signal, wherein a source of the fifteenth FET is coupled to the first output terminal of the first signal repeater circuit, wherein a gate of the sixteenth FET is configured to receive the first input signal, and wherein a source of the sixteenth FET is coupled to the second output terminal of the second signal repeater circuit, wherein drain terminals of the thirteen, fourteenth, fifteenth, and sixteenth FETs are coupled together and coupled to a control terminal of the switch. 
     In some embodiments, the equalization circuit includes a switch having a first terminal coupled to the first input terminal of the first signal repeater circuit and a second terminal coupled to the second input terminal of the first signal repeater circuit. 
     According to another aspect of the present disclosure, a system is provided. The system includes first and second input terminals configured to receive, respectively, first and second input signals of an input differential signal pair, and means for providing, at an output terminal, an output signal having a level representing an inverse of a value represented by the first input signal in response to a transition of the first input signal and a complementary transition of the second input signal. 
     In some embodiments, the output terminal is a first output terminal. The output signal is a first output signal. The system further includes means for providing, at a second output terminal, a second output signal having a level representing an inverse of a value represented by the second input signal in response to the transition of the first input signal and the complementary transition of the second input signal. 
     In some embodiments, the system further includes means for at least partially equalizing voltage levels of the first and second output signals based on a determination that the first and second input signals do not represent complementary values. 
     Other aspects and advantages of the invention will become apparent from the following drawings, detailed description, and claims, all of which illustrate the principles of the invention, by way of example only. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain advantages of some embodiments of the present disclosure may be understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, similar reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of some embodiments of the invention. 
         FIG. 1  is a block diagram of a signal repeater, in accordance with some embodiments. 
         FIG. 2A  is a schematic of a signal repeater, in accordance with some embodiments. 
         FIG. 2B  is a schematic of another signal repeater, in accordance with some embodiments. 
         FIG. 3  is a block diagram of a differential signal repeater, in accordance with some embodiments. 
         FIG. 4A  is a schematic of a differential signal repeater, in accordance with some embodiments. 
         FIG. 4B  is a schematic of another differential signal repeater, in accordance with some embodiments. 
         FIG. 5  is a block diagram of an equalizing differential signal repeater, in accordance with some embodiments. 
         FIG. 6A  is a schematic of an equalizing circuit, in accordance with some embodiments. 
         FIG. 6B  is a schematic of another equalizing circuit, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  a block diagram of a signal repeater  100 . The signal repeater  100  includes a first input terminal  102  and a second input terminal  104 . The signal repeater  100  also includes an output terminal  106 . A pair of input differential signals can be provided at the first terminal  102  and the second terminal  104 . The signal repeater  100  operates to provide an output signal at the output terminal  106  based on the input signals at the input terminals  102  and  104 . More particularly, the signal repeater  100  can operate to provide an output signal at the output terminal  106  that represents the logical inverse (complement) of the first input signal, or to place the output terminal  106  in a high-impedance state, depending on the values of the input signals. 
     In some embodiments, when the signal repeater  100  determines that the first and second input signals represent complementary logical values, the signal repeater  100  provides an output signal that represents the logical inverse of the first input signal. In some embodiments, when the signal repeater determines that the first and second input signals represent non-complementary logical values (e.g., the same logical value), the signal repeater  100  places the output terminal  106  in a high-impedance state. 
     One of ordinary skill in the art will understand that a circuit&#39;s output terminal is in a high-impedance state (or “tri-state”) when the circuit is not actively driving a current onto the output terminal or actively driving the potential of the output terminal. For example, a circuit&#39;s output terminal is in a high-impedance state when the circuit provides no conductive path between the output terminal and the circuit&#39;s reference (ground) voltage or power-supply voltage. Thus, when a circuit&#39;s output terminal is in a high-impedance state, any charge on the output terminal will generally not discharge to ground through the circuit, and the circuit will generally not deposit additional charge on the output terminal. One of ordinary skill in the art will understand that when the circuit&#39;s output terminal is in the high-impedance state, the output terminal can be charged or discharged through leakage currents in the circuit, through another circuit coupled to the output terminal, through interaction with an electromagnetic field, etc. Thus, from the circuit&#39;s perspective, the value of the output terminal can generally be undetermined when the output terminal is in the high-impedance state. However, in some embodiments, when the circuit&#39;s output terminal is initially placed in the high-impedance state, the output terminal can initially retain the same value (e.g., potential) that was on the output terminal just prior to the output terminal being placed in the high-impedance state. 
     In some embodiments, the signal repeater  100  provides an output signal at the output terminal  106  in response to a transition of one of the input signals to a complementary logical value of the other input signal. More particularly, after determining that one input signal has transitioned to a complementary logical value of the other input signal, the signal repeater  100  can provide an output signal that represents the inverse of the logical value of one of the input signals. In some embodiments, the signal repeater  100  provides an output signal that is the inverse of the first input signal in response to a logical transition of the first input signal (e.g., from logic “0” to logic “1”, or vice versa) and a complementary logical transition of the second input signal. 
     One of ordinary skill in the art will appreciate that the input signals represent complementary values when one of the input signals represents a logical value of “1” (“L1”) while the other input signal represents a logical value of “0” (“L0”). The input signals do not represent complementary values when both of the input signals represent the same logic value (e.g., both signals represent L0, or both signals represent L1), or when the value represented by at least one of the input signals cannot be reliably determined. Here, L1 can correspond to a power-supply voltage value (e.g., 1.2 Volts) of the signal repeater  100  (or of an IC including the signal repeater  100 ), and L0 can correspond to a reference or ground plane (e.g., 0 Volts) of the signal repeater  100  (or of an IC including the signal repeater  100 ). Other voltage values for L1 and L0 are possible. For example, L1 can correspond to any voltage above a first threshold voltage (e.g., any voltage above a voltage that is 80% of the actual or nominal power-supply voltage), L0 can correspond to any voltage below a second threshold voltage (e.g., any voltage below a voltage that is 20% of the difference between (1) the actual power-supply voltage and the actual reference voltage, or (2) the nominal power-supply voltage and the nominal reference voltage), and the logical value of a signal can be indeterminate if the voltage of the signal is between the first and second thresholds. 
     Here, for purposes of illustration, assume that a first input signal is provided to the input terminal  102  and a second input signal is provided to the input terminal  104 . Assume that at a time t0, the first input signal represents L1 while the second input signal represents L0. Also assume that at the time t0, the signal repeater  100  is providing an output signal at the output terminal  106  representing L0, that is, the inverse of the first input signal. 
     At a time t1 after the time t0, the second input signal transitions from L0 to L1. Because the first and second input signals do not have complementary values (in this example, both signals represent L1), the signal repeater  100  can place the output terminal  106  in a high-impedance state. 
     At a time t2 after the time t1, the first input signal transitions from L1 to L0. In response to determining that the first and second input signals have complementary values (the first input signal at L0 and the second input signal at L1), the signal repeater  100  can place the output terminal  106  at L1 (the inverse of the first input signal) (e.g., at a time t3 after the time t2). 
     In this way, when the first and second input signals correspond to a pair of switching complementary signals, the signal repeater  100  can, in some embodiments, switch the output signal (from L0 to L1, or vice versa) only after both input signals switch, for example, after the first input signal transitions from L0 to L1 and the second input signal transitions from L1 to L0, or after the first input signal transitions from L1 to L0 and the second input signal transitions from L0 to L1. 
     For instance, the first and second input signals can correspond to a pair of differential clock signals CLKP and CLKN. The output signal can correspond to a differential clock signal CLKPQ, which can be an inverse of the input clock signal CLKP. In some embodiments, the signal repeater  100  switches the output clock signal CLKPQ only after both input clock signals CLKP and CLKN switch. If only one of the CLKP and CLKN clock signals switches, such that both signals have the same logic value, the signal repeater  100  can place the output terminal  106  in a high-impedance state. 
     It is thus not required for both clock signals CLKP and CLKN to switch at the same time (or within a particular time window) for the signal repeater  100  to switch the output clock signal CLKPQ. Since it is not required for both clock signals CLKP and CLKN to switch at the same time (or within a particular time window), the signal repeater  100  can tolerate variations (e.g., manufacturing process variations) that may cause either of the input clock signals to switch at a later time than its differential counterpart. 
       FIG. 2A  shows a schematic of a signal repeater  100   a , according to some embodiments. The signal repeater  100   a  is an example of an implementation of the signal repeater  100 . The signal repeater  100   a  includes a first input terminal  102   a , a second input terminal  104   a , and an output terminal  106   a . The signal repeater  100   a  includes a p-type field effect transistor (FET) M 1 , n-type FET (NFET) M 2 , n-type FET M 3 , n-type FET M 4 , p-type FET (PFET) M 5 , and p-type FET M 6 . Here, each FET has a gate terminal (“gate”), a source terminal (“source”) and a drain terminal (“drain”). Source and drain terminals are diffusion terminals corresponding to respective diffusion regions adjacent to the gate structure in the FET. The terms “drain” and “source” are used interchangeably herein and generally describe directions of movement of charge carriers (e.g., electrons or holes) between the two diffusion regions under different voltage bias conditions. 
     As shown in  FIG. 2A , drains of M 1  and M 2  are coupled to a gate of M 3 . Drains of M 4  and M 5  are coupled to a gate of M 6 . Gates of M 1 , M 2 , M 4 , and M 5  are coupled together to receive an input signal at the input terminal  104   a . Sources of M 1  and M 4  are coupled together to receive another input signal at the input terminal  102   a . Drains of M 3  and M 6  are coupled together to provide an output signal at the output terminal  106   a . Sources of M 2  and M 3  are coupled to a reference (ground) voltage. Sources of M 5  and M 6  are coupled to a power supply voltage. 
     In  FIG. 2A , input signals at the input terminals  102   a  and  104   a  can be a pair of differential signals. For instance, the input signals can be a pair of differential clock signals CLKP at the input terminal  102   a  and CLKN at the input terminal  104   a.    
     The clock signals CLKP and CLKN can have complementary values. For instance, when the value of CLKP represents L0 (e.g., CLKP is at the ground voltage), the value of CLKN can represent L1 (e.g., CLKN can be at the power-supply voltage). In this case, M 2  and M 4  are conducting. M 1  and M 5  are not conducting. The gates of M 3  and M 6  are at the ground voltage, because the gate of M 3  is pulled down to ground by M 2 , and the gate of M 6  is coupled to CLKP through M 4 . Thus M 3  is not conducting and M 6  is conducting. The output terminal  106   a  is therefore pulled up to the supply voltage by M 6 , that is, an output clock signal CLKPQ at the output terminal  106   a  is pulled up to the power-supply voltage representing L1, which is the inverse of the logical value of the input clock signal CLKP. 
     As for another example, when CLKP represents L1 (e.g., CLKP is at the power supply voltage), CLKN can represent L0 (e.g., CLKN can be at the ground voltage). In this case, M 1  and M 5  are conducting. M 2  and M 4  are not conducting. The gates of M 3  and M 6  are at the power supply voltage, because the gate of M 3  is coupled to CLKP through M 1 , and the gate of M 6  is pulled up to the power supply voltage by M 5 . Thus M 3  is conducting and M 6  is not conducting. The output terminal  106   a  is therefore pulled down to the ground voltage, that is, the output clock signal CLKPQ at the output terminal  106   a  represents L0, which is the inverse of the value of the input clock signal CLKP. 
     When one of the input clock signals transitions to a value such that the two input clock signals are no longer complementary, the signal repeater  100   a  can place the output terminal  106   a  in a high-impedance state. In some embodiments, the signal repeater  100   a  switches CLKPQ to the inverse of its previous logical value only after both CLKP and CLKN switch and remain complementary. 
     For example, assume that at a time instance t0, the input signals CLKP and CLKN represent complementary logical values L0 and L1, respectively. After time t0, CLKP and CLKN may become non-complementary, for example, CLKP may transition from a value representing L0 to a value representing L1 while CLKN is held at a value representing L1. As described earlier, the gates of M 3  and M 6  are at the ground voltage before CLKP transitions from L0 to L1. As CLKP transitions from a value representing L0 to a value representing L1, M 4  becomes non-conducting (when M 4 &#39;s gate-source voltage drops below M 4 &#39;s threshold voltage). Since M 5  is still not conducting, the gate of M 6  is pulled up to a value approximately equal to the value of CLKP (e.g., the power-supply voltage) minus the threshold voltage of M 4 , and then the gate of M 6  becomes floating when M 4  becomes non-conducting. In this case, M 6  becomes non-conducting, CLKPQ is still at logical value L1, and the output terminal  106   a  is in a high-impedance state. CLKPQ will transition to the inverse value (L0) of CLKP (L1) again after CLKN transitions from L1 to L0, thereby becoming complementary to CLKP (L1), as gates of both M 3  and M 6  settle to the power supply voltage, as described earlier. 
     Similarly, CLKP and CLKN may become non-complementary after time t0, for example, when CLKP is held at a value representing L0 while CLKN transitions from a value representing L1 to a value representing L0. In this case, M 2  becomes non-conducting (because the NFET&#39;s gate-source voltage drops to approximately 0 Volts, which is below the NFET&#39;s threshold voltage) and M 1  remains non-conducting (because the PFET&#39;s gate-source voltage drops to approximately 0 Volts, which is above the PFET&#39;s threshold voltage). Thus, the gate of M 3  becomes floating, with an initial value approximately equal to the ground voltage, and M 3  remains non-conducting. In addition, M 4  becomes non-conducting and M 5  becomes conducting. Thus, the gate of M 6  is pulled up by M 5  to the power-supply voltage, and M 6  becomes non-conducting. In this case. CLKPQ is still at a value representing L1, and the output terminal  106   a  is in a high-impedance state. After CLKP transitions from L0 to L1, thereby becoming complementary to CLKN (L0) again, the signal repeater  100   a  can transition CLKPQ to the inverse value (L0) of CLKP (L1), as the gates of both M 3  and M 6  settle to the power-supply voltage, as described earlier. 
     As another example, assume that at a time t1, CLKP and CLKN represent complementary values L1 and L0, respectively. After time t1, CLKP and CLKN may become non-complementary, for example, CLKP may transition from a value representing L1 to a value representing L0 while CLKN is held at a value representing L0. As described above, the gates of transistors M 3  and M 6  are at L1 just prior to CLKP transitioning to L0. As CLKP transitions from L1 to L0, M 6  remains non-conducting because M 5  continues to pull the gate of M 6  up to the power-supply voltage. Since M 2  is still not conducting, the potential at the gate of M 3  is pulled down to a value approximately equal to the value of CLKP (e.g., the ground voltage) minus the gate-source voltage of M 1 , and then the gate of M 3  becomes floating when M 1  becomes non-conducting. In this case, M 3  is non-conducting, CLKPQ is still at logical value L0, and the output terminal  106   a  is in a high-impedance state. After CLKN transitions from L0 to L1, CLKPQ will transition to L1, thereby becoming complementary to CLKP (L0), as gates of both M 3  and M 6  settle to the ground voltage. 
     Similarly, CLKP and CLKN may become non-complementary after the particular time instance, for example, when CLKP is held at a value representing L1 while CLKN transitions from L0 to L1. As described above, the gates of transistors M 3  and M 6  are at L1 just prior to CLKN transitioning to L1. As CLKN transitions from L0 to L1, M 3  becomes non-conducting because M 2  pulls the gate of M 3  down to the ground voltage. In addition, M 5  becomes non-conducting and M 4  remains non-conducting. Thus, the gate of M 6  becomes floating with the initial value of approximately L1, and M 6  remains non-conducting. In this case, M 3  and M 6  are non-conducting, CLKPQ is still at L0, and the output terminal  106   a  is in a high-impedance state. After CLKP transitions from L1 to L0, thereby becoming complementary to CLKN (L1) again, the signal repeater  100   a  can transition CLKPQ to the inverse value (L1) of CLKP (L0), as the gates of both M 3  and M 6  settle to the ground voltage. 
     In each of the above-described examples, when the input signals represent non-complementary values (e.g., after one input signal transitions to the logical value of the other input signal, and before the other input signal makes a complementary transition), the signal repeater  100   a  places the output terminal  106   a  in a high-impedance state, with the initial value of the output signal at the high-impedance output terminal approximately equal to the previous value of the output signal (e.g., the value of the output signal just prior to the input signals becoming non-complementary, when the input signals most recently represented complementary values). 
       FIG. 2B  shows a schematic of another signal repeater  100   b , according to some embodiments. The signal repeater  100   b  is another example of an implementation of the signal repeater  100 . The signal repeater  100   b  includes a first input terminal  102   b , a second input terminal  104   b , and an output terminal  106   b . The signal repeater  100   b  includes a n-type FET M 7 , p-type FET M 8 , p-type FET M 9 , p-type FET M 10 , n-type FET M 11 , and n-type FET M 12 . 
     As shown in  FIG. 2B , drains of M 7  and M 8  are coupled to a gate of M 9 . Drains of M 10  and M 11  are coupled to a gate of M 12 . Gates of M 7 , M 8 , M 10 , and M 11  are coupled together to receive an input signal at the input terminal  104   b . Sources of M 7  and M 10  are coupled together to receive another input signal at the input terminal  102   b . Drains of M 9  and M 12  are coupled together to provide an output signal at the output terminal  106   a . Sources of M 8  and M 9  are coupled to a power-supply voltage. Sources of M 11  and M 12  are coupled to a reference (ground) voltage. 
     The structure of the signal repeater  100   b  is complementary to the structure of the signal repeater  100   a . For instance, M 1 , M 5 , and M 6  are p-type FETs in the signal repeater  100   a . In comparison, M 7 , M 11 , and M 12  are n-type FETs in the signal repeater  100   b . As for another example, M 4 , M 2 , and M 3  are n-type FETs in the signal repeater  100   a . In comparison, M 10 , M 8 , and M 9  are p-type FETs in the signal repeater  100   b.    
     Similar to the operations of the signal repeater  100   a  described in reference to  FIG. 2A , the signal repeater  100   b  provides an output clock signal CLKNQ at the output terminal  106   b  that represents the inverse of the value represented by an input clock signal CLKN at the input terminal  102   b , when the logical value of CLKN is complementary to the logical value of another clock signal CLKP at the input terminal  104   b . For instance, the signal repeater  100   b  sets CLKNQ to L1 when CLKN is L0 and CLKP is L1, and sets CLKNQ to L0 when CLKN is L1 and CLKP is L0. When CLKN and CLKP are not complementary, for instance, when both CLKN and CLKP have the same logical value (L0 or L1), the signal repeater  100   b  places the output terminal  106   b  in a high-impedance state. In some embodiments, when the input signals represent non-complementary values, the signal repeater  100   b  places the output terminal  106   b  in a high-impedance state, with the initial value of the output signal at the high-impedance output terminal being approximately equal to the previous value of the output signal (e.g., the value of the output signal just prior to the input signals becoming non-complementary, when the input signals most recently represented complementary values). In some embodiments, the signal repeater  100   b  switches CLKNQ only after both CLKP and CLKN switch and remain complementary. 
       FIG. 3  shows a block diagram of a differential signal repeater  300 , according to some embodiments. The differential signal repeater  300  includes a first input terminal  302 , a second input terminal  304 , a first output terminal  306   a , and a second output terminal  306   b . The differential signal repeater  300  operates to provide output signals at the output terminals  306   a  and  306   b  based on the input signals at the input terminals  302  and  304 . More particularly, when the input signals at the input terminals  302  and  304  are a pair of input differential signals with complementary values, the differential signal repeater  300  operates to provide, at the output terminals  306   a  and  306   b , a pair of output differential signals with complementary values. When the input signals represent non-complementary values, the differential signal repeater  300  places the output terminals  306   a  and  306   b  in a high-impedance state. 
     In some embodiments, differential signal repeater  300  provides a differential pair of output signals at output terminals  306   a  and  306   b  that represent the inverse of the input differential signals at the input terminals  302  and  304 , respectively. The differential signal repeater  300  may provide the differential pair of output signals based on a determination that the input differential signals have complementary values (e.g., in response to detecting that the input differential signals have complementary values). For instance, the input signals can correspond to a pair of differential clock signals CLKP (at the input terminal  302 ) and CLKN (at the input terminal  304 ). The output signals can correspond to a pair of output differential clock signals CLKPQ (at the output terminal  306   a ) and CLKNQ (at the output terminal  306   b ). When CLKP and CLKN have complementary values, the differential signal repeater  300  can provide (1) at output terminal  306   a , an output signal CLKPQ with a logical value that is the inverse of the logical value of input signal CLKP, and (2) at output terminal  306   b , an output signal CLKNQ with a logical value that is the inverse of the input signal CLKN. For example, if CLKP and CLKN have values representing L0 and L1, respectively (or L1 and L0, respectively), the differential signal repeater  300  can set the output signals CLKPQ and CLKNQ to values representing L1 and L0, respectively (or L0 and L1, respectively). 
     If the input signals CLKP and CLKN do not represent a pair of complementary values (e.g., when the input signals both represent L0 or both represent L1), the differential signal repeater  300  can place the output terminals  306   a  and  306   b  in a high-impedance state. In some embodiments, when the differential signal repeater  300  places the output terminals  306   a  and  306   b  in the high-impedance state, the initial value of the signal at each high-impedance output terminal is approximately equal to the previous value of that output signal (e.g., the value of the output signal just prior to the input signals becoming non-complementary, when the input signals most recently represented complementary values). In some embodiments, the signal repeater  300  switches CLKPQ and CLKNQ only after both CLKP and CLKN switch and remain complementary. 
     Here, for purposes of illustration, assume that at a time t0, the value of input signal CLKP represents L0 and the value of input signal CLKN represents L1. Since CLKP and CLKN have complementary values, the differential signal repeater  300  sets the output signals CLKPQ and CLKNQ to complementary values (e.g., values representing L1 and L0, respectively). At a time t1 after the time t0, CLKP transitions to a value representing L1, while CLKN remains at L1. Because input signals CLKP and CLKN do not represent complementary logical values, the differential signal repeater  300  places the output terminals  306   a  and  306   b  in a high-impedance state. At a time t2 after the time t1, CLKN transitions to a value representing L0, while CLKP remains at a value representing L1. After determining that the input signals CLKP and CLKN have complementary values, the differential signal repeater  300  can set CLKPQ to a value representing L0 and CLKNQ to a value representing L1 (e.g., at a time t3 after the time t2). 
     In this way, the differential signal repeater  300  can switch the output differential signals CLKPQ and CLKNQ only after both input differential signals CLKP and CLKN switch. Since it is not required for both input differential signals CLKP and CLKN to switch at the same time (or within a specified time window), the differential signal repeater  300  can tolerate variations (e.g., manufacturing process variations) that may cause either of the input differential signals to switch at a later time than its complementary counterpart, or outside a specified time window relative to the switching of its counterpart. Even when one of the input differential signals switches later than the other input differential signal (e.g., outside a specified time window relative to the switching of the other input differential signal), the differential signal repeater  300  can switch the output differential signals at the same time or approximately the same time (e.g., within a specified time window of each other). Thus, the output differential signals of the differential signal repeater  300  can be less skewed than the signal repeater&#39;s input differential signals, because the time period between complementary transitions of the differential signal repeater&#39;s output differential signals can be shorter than the time period between complementary transitions of the differential signal repeater&#39;s input differential signals. Alternatively or in addition, in a circuit in which a differential signal S propagates from a first pair of nodes NA to a second pair of nodes NB, the skewing of the differential signal S can be reduced if the signal propagates from nodes NA to nodes NB through a differential signal repeater, rather than simply propagating along a pair of wires. In other words, the differential signal repeater can, in some embodiments, reduce the skewing of a differential signal S relative to the amount of skewing that would be present in the absence of the differential signal repeater. 
     In some embodiments, a set of differential signal repeaters  300  can be used to propagate differential clock signals throughout an integrated circuit or a region thereof, or to multiple components of an integrated circuit. In some embodiments, the differential signal repeaters  300  can de-skew the differential clock signals (e.g., prevent, counteract, or correct skewing of the differential clock signals). 
     In some embodiments, the delay of the differential signal repeater  300  (e.g., the maximum delay from an input terminal  302 / 304  to an output terminal  306 ) is less than 100 ps (e.g., between approximately 50 ps and approximately 100 ps). 
       FIG. 4A  is a schematic of a differential signal repeater  300   a , according to some embodiments. The differential signal repeater  300   a  is an example of an implementation of the differential signal repeater  300 . The differential signal repeater  300   a  includes two signal repeaters  100 - 1  and  100 - 2 . In the example of  FIG. 4A , the signal repeater  100 - 1  is implemented as a signal repeater  100   a , and the signal repeater  100 - 2  is implemented as a signal repeater  100   b . In some embodiments, both signal repeaters  100 - 1  and  100 - 2  are implemented as signal repeaters  100   a . In some embodiments, both signal repeaters  100 - 1  and  100 - 2  are implemented as signal repeaters  100   b . The integrated circuit layout of the differential signal repeater  300   a  may be more compact when two signal repeaters  100  of different types (e.g., a signal repeater  100   a  and a signal repeater  100   b ) are used, and less compact when signal repeaters  100  of the same type (e.g., two signal repeaters  100   a  or two signal repeaters  100   b ) are used. 
     In the example of  FIG. 4A , the input terminal  302  of the differential signal repeater is coupled to the first input terminal ( 102   a ) of the signal repeater  100 - 1  and the second input terminal ( 104   b ) of the signal repeater  100 - 2 . The input terminal  304  of the differential signal repeater is coupled to the second input terminal ( 104   a ) of the signal repeater  100 - 1  and the first input terminal ( 102   b ) of the signal repeater  100 - 2 . The output terminal  306   a  of the differential signal repeater is coupled to the output terminal ( 106   a ) of the signal repeater  100 - 1 . The output terminal  306   b  of the differential signal repeater is coupled to the output terminal ( 106   b ) of the signal repeater  100 - 2 . 
     Thus, as described above with reference to  FIGS. 1, 2A, and 2B , the signal repeater  100 - 1  provides the output signal at the output terminal  306   a  based on the input signals applied to the input terminals  302  and  304 , with the input signal at the differential signal repeater&#39;s input terminal  302  being applied to the first input terminal  102  of the signal repeater  100 - 1 , and the input signal at the differential signal repeater&#39;s input terminal  304  being applied to the second input terminal  104  of the signal repeater  100 - 1 . Likewise, the signal repeater  100 - 2  provides the output signal at the output terminal  306   b  based on the input signals applied to the input terminals  302  and  304 , with the input signal at the differential signal repeater&#39;s input terminal  304  being applied to the first input terminal  102  of the signal repeater  100 - 2 , and the input signal at the differential signal repeater&#39;s input terminal  302  being applied to the second input terminal  104   b  of the signal repeater  100 - 2 . 
     In some embodiments, a pair of input differential clock signals, CLKP and CLKN, may be provided at the input terminals  302  and  304  of the differential signal repeater  300   a , respectively. The differential signal repeater  300   a  may provide a pair of output differential clock signals, CLKPQ and CLKNQ, at its output terminals  306   a  and  306   b , respectively. 
       FIG. 4B  is another schematic of the signal repeater  300   a , according to some embodiments. More particularly,  FIG. 4B  shows a schematic of the signal repeater  300   a  in which the signal repeater  100 - 1  is implemented using the schematic of the signal repeater  100   a , and the signal repeater  100 - 2  is implemented using the schematic of the signal repeater  100   b . Here, the input terminal  302  (e.g., input signal CLKP) is coupled to the sources of M 1  and M 4  of the signal repeater  100   a , and to the gates of M 7 , M 8 , M 10 , and M 1   l  of the signal repeater  100   b . The input terminal  304  (e.g., input signal CLKN) is coupled to the gates of M 1 , M 2 , M 4 , and M 5  of the signal repeater  100   a , and to the sources of M 7  and M 10  of the signal repeater  100   b . The output terminal  306   a  (e.g., output signal CLKPQ) is coupled to drains of M 3  and M 6  of the signal repeater  100   a . The output terminal  306   b  (e.g., output signal CLKNQ) is coupled to drains of M 9  and M 12  of the signal repeater  100   b.    
       FIG. 5  is a block diagram of an equalizing differential signal repeater  500 . The equalizing repeater  500  includes a differential signal repeater  300  and an equalizer  510 . The input terminals  302  and  304  of the differential signal repeater  300  are coupled to the first and second input terminals  502  and  504  of the equalizer  510 , respectively. The output terminals  306   a  and  306   b  of the differential signal repeater  300  are coupled to the first and second input/output terminals  506   a  and  506   b  of the equalizer  510 , respectively. 
     In some embodiments, a pair of differential signals (e.g., differential clock signals CLKP and CLKN) are applied to the input terminals  302 / 502  and  304 / 504 , respectively. In some embodiments, the differential signal repeater  300  provides a pair of differential signals (e.g., differential clock signals CLKPQ and CLKNQ) at the output terminals  306   a  and  306   b , respectively, as described above with reference to  FIG. 3 . The equalizer  510  operates to equalize the two output signals (CLKPQ and CLKNQ) of the differential signal repeater  300 , at least in part. In some embodiments, equalizing the output signals involves changing the voltage level of one or both output signals so that the voltage levels of the output signals are approximately equal (e.g., in cases where the input signals are severely skewed). In some embodiments, partially equalizing the output signals involves changing the voltage level of at least one output signal toward the voltage level of the other output signal. Other types of equalization are possible, including any operation that reduces the distortion of the differential signal repeater&#39;s output signals relative to the distortion of its input signals, or relative to the amount of distortion that would be present in the output signals in the absence of the equalizer  510 . In some embodiments, the equalization performed by the equalizer  510  conserves power, pre-charges/pre-discharges the differential signal repeater&#39;s output terminals for an upcoming switching of the logical values of the output signals, and/or reduces the switching time and/or stage delay of the differential signal repeater  300 . 
     In some embodiments, the equalizer  510  at least partially equalizes voltage levels of the differential signal repeater&#39;s output signals when the input signals (CLKP and CLKN) do not have complementary values (e.g., both represent L0 or both represent L1). For instance, at a time t0, the input signal CLKP has a value representing L0 and the input signal CLKN has a value representing L1 (complementary to CLKP). In this case, the output signal CLKPQ has a value representing L1 and the output signal CLKNQ has a value representing L0. At a time t1 after the time t0, CLKP transitions from L0 to L1, while CLKN remains at L1. As the input signals CLKP and CLKN do not have complementary values, the differential signal repeater  300  places the output terminals  306   a  and  306   b  in the high-impedance state. At a time t2 after the time t1, but before a time t3 when CLKN transitions from L1 to L0 while CLKP remains at L1, the equalizer  510  can at least partially equalize voltage levels of the output signals CLKPQ and CLKNQ. For example, the equalizer  510  can change the voltage level of CLKPQ from an first value (e.g., at or near the power-supply voltage) to a second value less than the first value, thereby partially equalizing the voltage level of CLKPQ toward the voltage level of CLKNQ (e.g., at or near the ground voltage). In some embodiments, the equalizer  510  performs the equalization by moving charges from the output terminal  306  at the higher potential to the output terminal  306  at the lower potential. Moving charge from one output terminal to the other output terminal, rather than discharging the charge from one output terminal to ground and charging the other output terminal from the power supply, can conserve power. 
     After the time t3 when CLKN transitions from L1 to L0 while CLKP remains at L1, the differential signal repeater  300  sets CLKPQ to L0 (at or near the ground voltage) and CLKNQ to L1 (at or near the power-supply voltage). Thus, the equalizer  510  can “pre-charge” the output terminal  306  that is likely to transition from L0 to L1, and “pre-discharge” that output terminal  306  that is likely to transition from L1 to L0 after the input signals transition to non-complementary values, and before the input signals transition back to complementary values. After pre-charge and pre-discharge, the output signals are at respective voltage levels that are closer to their next expected voltage levels. Thus, the pre-charge and pre-discharge can reduce time needed for the equalizing repeater  500  to switch CLKPQ and CLKNQ from a first pair of complementary values to the opposite pair of complementary values. The equalizer  510  thus can reduce propagation delay (stage delay) of the differential signals across the equalizing differential signal repeater  500 . 
     In some embodiments, the equalizer  510  can at least partially equalize the differential signal repeater&#39;s output signals even in cases where the logical values of the input signals switch simultaneously or nearly simultaneously. Thus, the equalizer  510  may conserve power, facilitate pre-charging/pre-discharging of the output terminals  306 , and/or reduce the propagation delay of the differential output signals, even if the input signals are perfectly differential or very nearly perfectly differential. 
       FIG. 6A  is a schematic of an equalizing circuit  510   a , according to some embodiments. The equalizing circuit  510   a  is an example of an implementation of the equalizer  510 . The equalizing circuit  510   a  includes a switch  620   a . The switch  620   a  can be a p-type FET. Other types of switches for the switch  620   a  are possible. For instance, the switch  620   a  can be a dual-type FET switch. The switch  620   a  has one terminal coupled to the input/output terminal  506   a  (and the output terminal  306   a ), and another terminal coupled to the input/output terminal  506   b  (and the output terminal  306   b ). 
     The equalizing circuit  510  also includes a driver circuit that is configured to control the switch  620   a . In some embodiments, the driver circuit activates the switch  620   a  when either of the input signals (e.g., CLKP or CLKN) of the differential signal repeater  300  makes a logical transition, but before the other input signal (e.g., CLKP or CLKN) makes a corresponding transition to a complementary value of the first input signal. In some embodiments, the driver circuit activates the switch  620   a  at least for a short time period after the first and second input signals make complementary transitions. 
     In the example of  FIG. 6A , the driver circuit includes a p-type FET M 13 , p-type FET M 14 , n-type FET M 15 , and n-type FET M 16 . M 13  has its gate terminal coupled to the second output terminal  306   b / 506   b  of the differential signal repeater  300 , one diffusion terminal coupled to the second input terminal  304 / 504  of the differential signal repeater  300 , and the other diffusion terminal coupled to a gate of the switch  620   a . M 15  has its gate terminal coupled to the second input terminal  304 / 504  of the differential signal repeater  300 , one diffusion terminal coupled to the gate of the switch  620   a , and the other diffusion terminal coupled to the first output terminal  306   a / 506   a  of the differential signal repeater  300 . M 14  has its gate coupled to the first output terminal  306   a / 506   a  of the differential signal repeater  300 , one diffusion terminal coupled to the first input terminal  302 / 502  of the differential signal repeater  300 , and the other diffusion terminal coupled to the gate of the switch  620   a . M 16  has its gate coupled to the first input terminal  302 / 502  of the differential signal repeater  300 , one diffusion terminal coupled to the gate of the switch  620   a , and the other diffusion terminal coupled to the second output terminal  306   b / 506   b  of the differential signal repeater  300 . 
     In some embodiments, when the input signals CLKP and CLKN at the input terminals  302  and  304  have complementary values, the output signal CLKPQ at the output terminals  306   a  has a value that represents the logical inverse of CLKP, and the output signal CLKNQ at the output terminal  306   b  has a value that represents the logical inverse of CLKN, the driver circuit turns off the switch  620   a . Otherwise, the driver circuit facilitates equalization by placing the switch  620   a  in the conducting state. 
     An example of an implementation of the driver circuit is shown in  FIG. 6A . Other embodiments of the driver circuit are possible. In some embodiments, the driver circuit includes one or more exclusive-OR (“XOR”) gates configured to detect whether the input signals CLKP and CLKN have complementary values, and the driver circuit places the switch  620   a  in the conducting state when it detects that the input signals CLKP and CLKN do not have complementary values. 
     For instance, assume CLKP (at terminal  502 ) represents L1, CLKN (at terminal  504 ) represents L0, CLKPQ (at terminal  506   a ) represents L0 and CLKNQ at terminal  506   b  represents L1. M 13 , M 15 , and M 16  are not conducting. M 14  is conducting and therefore pulls up the gate of the switch  620   a  to L1 (CLKP). Thus the switch  620   a  is not conducting, and the equalizer  510  is not performing any equalization. 
     Continuing the example, when CLKP remains at L1 and CLKN transitions from L0 to L1, the differential repeater circuit places terminal  506   a  (CLKPQ) and terminal  506   b  (CLKNQ) in a high-impedance state. In this case, M 13 , M 14 , and M 16  are not conducting. M 15  is conducting, and pulls the gate of the switch  620   a  toward the voltage of CLKPQ, which is approximately the ground voltage. Thus, the switch  620   a  is turned on, enabling the voltage level of CLKNQ to change toward the voltage level of CLKPQ (and vice versa). In some cases (e.g., in cases where CLKN and CLKP are severely skewed), the switch  620   a  can remain turned on until the gate voltage of the switch  620   a  is close to the voltage level of the output signal CLKNQ (e.g., until the difference between the gate voltage of the switch  620   a  and the voltage level of CLKNQ is approximately equal to the threshold voltage of the switch  620   a ). Thus, in some cases, the switch  620   a  can remain turned on until the difference between the voltage levels of CLKPQ and CLKNQ is approximately equal to the threshold voltage of the switch  620   a.    
     As another example, CLKP (at terminal  502 ) represents L0, CLKN (at terminal  504 ) represents L1, CLKPQ (at terminal  506   a ) represents L1 and CLKNQ (at terminal  506   b ) represents L0. M 14 , M 15 , and M 16  are not conducting. M 13  is conducting and pulls the gate of the switch  620   a  to the voltage level of CLKN (e.g., the power-supply voltage). Thus, the switch  620   a  is not conducting, and the equalizer  510  is not performing any equalization. 
     Continuing the example, when CLKN remains at L1 and CLKP transitions from L0 to L1, the differential repeater circuit places terminal  506   a  (CLKPQ) and terminal  506   b  (CLKNQ) in a high-impedance state. M 13 , M 14 , and M 15  are not conducting. M 16  is conducting, and pulls the gate of the switch  620   a  toward the voltage of CLKNQ, which is approximately the ground voltage. Thus, the switch  620   a  is turned on, enabling the voltage level of CLKPQ to change toward the voltage level of CLKNQ (and vice versa). In some cases, (e.g., in cases where CLKN and CLKP are severely skewed), the switch  620   a  can remain turned on until the gate voltage of the switch  620   a  is close to the voltage level of the output signal CLKPQ (e.g., until the difference between the gate voltage of the switch  620   a  and the voltage level of CLKPQ is approximately equal to the threshold voltage of the switch  620   a ). Thus, in some cases, the switch  620   a  can remain turned on until the difference between the voltage levels of CLKPQ and CLKNQ is approximately equal to the threshold voltage of the switch  620   a.    
     As another example, CLKP (at terminal  502 ) represents L0, CLKN (at terminal  504 ) represents L1, CLKPQ (at terminal  506   a ) represents L1 and CLKNQ (at terminal  506   b ) represents L0. M 14 , M 15 , and M 16  are not conducting. M 13  is conducting and pulls the gate of the switch  620   a  to the voltage level of CLKN (e.g., the power-supply voltage). Thus, the switch  620   a  is not conducting, and the equalizer  510  is not performing any equalization. Continuing the example, CLKP and CLKN simultaneously (or nearly simultaneously) switch to L1 and L0, respectively. During the time period after CLKP and CLKN switch but before the differential signal repeater  300  begins to switch the output signals CLKPQ and CLKNQ, M 13 , M 14 , and M 15  are not conducting, but M 16  is conducting. Thus, M 16  pulls down the gate of the switch  620   a  to the voltage of CLKNQ (e.g., approximately the ground voltage), thereby initiating the equalization of CLKPQ and CLKNQ. 
     Some embodiments have been described in which a signal repeater provides an output signal having a value that represents the inverse of the logical value of a first input signal, in response to determining that the first input signal and a second input signal represent complementary logical values. In some embodiments, the value of the signal repeater&#39;s output signal may be regarded as the repeated, non-inverted value of the signal repeater&#39;s second input signal, rather than the repeated, inverted value of the signal repeater&#39;s first input signal. 
     Likewise, some embodiments have been described in which a differential signal repeater provides output signals (e.g., CLKPQ and CLKNQ) that represent the inverses of corresponding input signals (e.g., CLKP and CLKN, respectively) in response to determining that the input signals represent complementary logical values. In some embodiments, the values of the differential signal repeater&#39;s output signals represent the non-inverted values of the corresponding input signals CLKP and CLKN, respectively. For example, referring to  FIG. 4A , if the output terminal  106   a  of the first signal repeater  100 - 1  were coupled to the second output terminal  306   b  (rather than the first output terminal  306   a ) of the differential signal repeater  300 , and the output terminal  106   b  of the second signal repeater  100 - 2  were coupled to the first input terminal  306   a  (rather than the second input terminal  306   b ) of the differential signal repeater  300 , then output terminal  306   a  could be understood to provide the repeated, non-inverted value of CLKP, and the output terminal  306   b  could be understood to provide the repeated, non-inverted value of CLKN. 
     Some embodiments of an equalizer  510  have been described.  FIG. 6B  shows a schematic of an equalizing circuit  510   b , according to some embodiments. The equalizing circuit  510   b  is an example of another implementation of the equalizer  510 . In the example of  FIG. 6B , the equalizing circuit  510   b  includes a switch  620   b , which may be implemented using an NFET. In some embodiments, the equalizing circuit  510   b  is suitable for use as an equalizer  510  in connection with a non-inverting differential signal repeater  300 . In some embodiments, the first input terminal  502  of the equalizing circuit  510   b  may be configured to receive the CLKP signal, the second input terminal  504  of the equalizing circuit  510   b  may be configured to receive the CLKN signal, the first input/output terminal  506   a  of the equalizing circuit  510   b  may be configured to receive the repeated, non-inverted value of the CLKN signal, and the second input/output terminal  506   b  of the equalizing circuit may be configured to receive the repeated, non-inverted value of the CLKP signal. 
     Some embodiments have been described in which the switch  620  of an equalizer  510  is coupled between the output terminals  306  of a differential signal repeater  300 . In some embodiments, the switch  620  of an equalizer  510  may be coupled between the input terminals ( 302 ,  304 ) of a differential signal repeater  300 . 
     Some embodiments have been described in which signal repeaters, differential signal repeaters, and/or equalizers are implemented using FETs. The components described as FETs herein may be implemented using any suitable type of transistor (e.g., MOSFET, FinFET, etc.) or any other suitable type of switch. 
     In some embodiments, two or more differential signal repeaters may be coupled together to form a differential clock distribution tree. The clock distribution tree&#39;s repeaters may be coupled together in any suitable topology including, but not limited to, a star topology, a ring topology, a linear topology, etc. When two differential signal repeaters are coupled together, the output terminals ( 306   a ,  306   b ) of a repeater may be coupled, respectively, to the input terminals ( 302 ,  304 ) of the other repeater. In some embodiments, a differential repeater may drive the inputs of two or more other differential repeaters. 
     In some embodiments, the differential signal repeater and/or differential clock distribution tree may be integrated into any suitable device including, without limitation, a microprocessor, liquid-crystal display (LCD) panel, light-emitting diode (LED) panel, television, mobile electronic device (e.g., laptop computer, tablet computer, smart phone, mobile phone, smart watch, etc.), computer (e.g., server computer, desktop computer, etc.) bitcoin mining device, etc. 
     Terminology 
     The phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     The term “approximately”, the phrase “approximately equal to”, and other similar phrases, as used in the specification and the claims (e.g., “X has a value of approximately Y” or “X is approximately equal to Y”), should be understood to mean that one value (X) is within a predetermined range of another value (Y). The predetermined range may be plus or minus 20%, 10%, 5%, 3%, 1%, 0.1%, or less than 0.1%, unless otherwise indicated. 
     The indefinite articles “a” and “an,” as used in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and/or,” as used in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof, is meant to encompass the items listed thereafter and additional items. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed. Ordinal terms are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term), to distinguish the claim elements. 
     EQUIVALENTS 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.