Abstract:
Apparatus, systems, and methods implementing techniques for converting clock signals are described. A voltage-based input clock signal is received and converted into a current-based clock signal. An electrical current of the current-based clock signal is varied in response to the input clock signal while a voltage of the current-based clock signal remains substantially constant.

Description:
BACKGROUND 
     The following disclosure relates to electrical circuits and signal processing. 
     Digital circuits can use a central clock whose signal is distributed to multiple circuit components and allows the circuit components to operate synchronously. The clock signal is typically generated by an oscillator circuit, for example, a quartz crystal or RC oscillator circuit. In a wireless communications circuit (which may include analog and/or digital components), a clock signal from a local oscillator can be used to mix signals up to intermediate frequency (IF) and radio frequency (RF) bands. 
     Clock signals are conventionally distributed as voltages. A conventional clock signal typically alternates between a low voltage and a high voltage. A quick transition between the low voltage and the high voltage, which results in a sharp clock edge, is typically desirable. The frequency of the transitions between voltages determines the clock frequency. 
     In large integrated circuits, between integrated circuit chips, or in communications systems, the conduits along which a clock signal is transmitted can be long. A conduit typically has a parasitic capacitance that increases with the length of the conduit. The parasitic capacitance on long clock distribution conduits can be significant at the frequencies at which a modern clock can operate. As clock frequencies rise, detrimental effects of parasitic capacitance typically become more severe. The parasitic capacitance typically requires power to charge and discharge as the voltage on the conduit varies, thereby increasing the power used by a circuit. Charging and discharging the parasitic capacitance on a conventional clock distribution conduit typically slows the transition of the clock signal between the low voltage and the high voltage, resulting in a lower-quality clock signal. Voltage swings on a long conduit can capacitively couple to parts of a circuit to which the conduit is not connected, thereby introducing noise into the circuit from a clock signal on a conduit and into the clock signal from the circuit. 
     To mitigate the increased power consumption caused by parasitic capacitance and to sharpen clock edges, inductance can be added to a conventional clock distribution circuit. The amount of added inductance can be chosen so that the inductance in the conventional clock distribution circuit resonates with the parasitic capacitance at the clock frequency. When the inductance and the parasitic capacitance resonate, a high impedance is presented between the conduit and ground, reducing the power loss through the parasitic capacitance and sharpening the clock edge. 
     Monolithic inductors typically require a significant amount of area on an integrated-circuit chip. In addition, when process variations affect the amount of inductance or parasitic capacitance in a conventional clock distribution circuit, the resonant frequency of the inductance and parasitic capacitance can shift away from the clock frequency, degrading the clock signal and increasing power loss. The introduction of inductance into the clock distribution circuit typically decreases the bandwidth of the clock signal. 
     SUMMARY 
     In one aspect, the invention features an apparatus for converting clock signals. A driver circuit receives a voltage-based input clock signal and converts the input clock signal into a current-based clock signal. The driver circuit varies an electrical current of the current-based clock signal in response to the input clock signal while keeping a voltage of the current-based clock signal substantially constant. 
     In another aspect, the invention features an apparatus for converting clock signals. A driving means receives a voltage-based input clock signal and converts the input clock signal into a current-based clock signal. The driving means varies an electrical current of the current-based clock signal in response to the input clock signal while keeping a voltage of the current-based clock signal substantially constant. 
     In one aspect, the invention features a wireless transceiver that includes a communications receiver, which receives a modulated carrier signal. The communications receiver includes a clock signal conversion circuit, which includes a driver circuit that receives a voltage-based input clock signal and converts the input clock signal into a current-based clock signal. The driver circuit varies an electrical current of the current-based clock signal in response to the input clock signal while keeping a voltage of the current-based clock signal substantially constant. 
     In another aspect, the invention features a wireless transceiver that includes a receiving means for receiving a modulated carrier signal. The receiving means includes a clock signal conversion circuit, which includes a driving means for receiving a voltage-based input clock signal and converting the input clock signal into a current-based clock signal. The driving means varies an electrical current of the current-based clock signal in response to the input clock signal while keeping a voltage of the current-based clock signal substantially constant. 
     In yet another aspect, the invention features a method for processing a clock signal. A voltage-based input clock signal is received and converted into a current-based clock signal. An electrical current of the current-based clock signal varies substantially in response to the input clock signal, and the current-based clock signal has substantially no voltage variation. 
     Particular implementations may include one or more of the following features. The driver circuit can receive the input clock signal from an oscillator. A frequency of the current-based clock signal can be different than a frequency of the input clock signal. The driver circuit can convert the input clock signal into a differential current-based clock signal. The driver circuit can include an input section including at least one input transistor and a conversion section including at least one conversion transistor. 
     A receiver circuit can receive the current-based clock signal and convert the current-based clock signal into a voltage-based output clock signal. A mixer can mix the voltage-based output clock signal with an information signal. A digital circuit that performs a synchronous operation can be driven with the voltage-based output clock signal. The driver circuit can be included on a first integrated circuit, and the receiver circuit can be included on a second integrated circuit. 
     An intermediate circuit can be connected between the driver circuit and the receiver circuit, and the intermediate circuit can transform the current-based clock signal. The intermediate circuit can filter a spurious signal included in the current-based clock signal or the intermediate circuit can amplify the current-based clock signal. The driver circuit can be connected to a plurality of receiver circuits. An input impedance of the receiver circuit can be low relative to an impedance of a conduit coupling the driver circuit to the receiver circuit. 
     Implementations can include one or more of the following advantages. A method, apparatus, and system are disclosed that can be used to distribute a clock signal on a conduit with substantially no voltage swing. The method, apparatus, and system can reduce capacitive coupling between the clock distribution conduit and other parts of a circuit. The method, apparatus, and system can reduce power usage in a clock distribution circuit, sharpen clock edges, and increase clock signal bandwidth. On-chip area used by the clock distribution circuit can be reduced. 
     These general and specific aspects may be implemented using an apparatus, a system, a method, or any combination of apparatus, systems, and methods. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a current-based clock distribution system. 
         FIG. 2A  is a circuit for converting between a voltage signal and a current signal. 
         FIG. 2B  shows a differential voltage waveform. 
         FIG. 2C  shows a differential current waveform. 
         FIG. 3  is a block diagram of a wireless transceiver. 
         FIG. 4  is a flowchart of a process for current-based clock distribution. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a clock distribution system that distributes a clock signal as a differential current along a conduit  170  and a conduit  180 . A voltage-based clock signal in the form of a differential voltage is applied to a voltage-to-current converter  130  between an input terminal  110  and an input terminal  120 . Voltage-to-current converter  130  converts the voltage-based clock signal to a corresponding current-based clock signal and applies the current-based clock signal differentially between conduits  170  and  180 . In other implementations, the voltage-based and/or current-based clock signals may be single ended instead of differential. When the input impedance of current-to-voltage converter  140  is low relative to the impedance of any parasitic capacitance on conduits  170  and  180 , the voltages of the clock signal on conduits  170  and  180  do not vary substantially as the currents of the clock signal in conduits  170  and  180  are varied substantially; the clock signal transmitted on conduits  170  and  180  is current-based. Since the voltages of conduits  170  and  180  do not vary substantially, the parasitic capacitance of conduits  170  and  180  does not charge or discharge substantially, resulting in little power loss due to the parasitic capacitance. 
     Voltage-to-current converter  130  can be any circuit that takes a voltage-based clock signal as an input and outputs a corresponding current-based clock signal. The output signal can be at a same frequency as the input signal, or the frequency of the output signal can be a multiple or a fraction of the frequency of the input signal. The output signal can also have a non-linear relationship with the input signal, for example, an exponential or squared relationship. 
     The current-based clock signal on conduits  170  and  180  is applied differentially to current-to-voltage converter  140  to produce a differential output voltage between an output terminal  150  and an output terminal  160 . Current-to-voltage converter  140  can be any circuit that takes a current-based clock signal as an input and outputs a corresponding voltage-based clock signal. When current-to-voltage converter  140  has a low input impedance, the voltage on conduits  170  and  180  is substantially constant, reducing the effects of the parasitic capacitance on conduits  170  and  180 . Like voltage-to-current converter  130 , the relationship of the input frequency of current-to-voltage converter  140  to the output frequency of current-to-voltage converter  140  can be linear or nonlinear, and the input signal and/or output signal can be single ended instead of differential. 
     In some implementations, voltage-to-current converter  130  is connected to several current-to-voltage converters. In other implementations, other circuits may be connected to conduits  170  and  180 , either in parallel or in series with current-to-voltage converter  140 . Other circuits connected to conduits  170  and  180  have impedance characteristics that keep the voltage on conduits  170  and  180  substantially constant. The current-based clock signal may be modified by other circuits connected to conduits  170  and  180  between voltage-to-current converter  130  and current-to-voltage converter  140 . For example, the current-based clock signal can be filtered, can be amplified, or can undergo a frequency change caused by a circuit connected to conduits  170  and  180 . 
       FIG. 2A  shows an implementation of a circuit  200  that can be used for voltage-to-current or current-to-voltage conversion. When circuit  200  is used as a voltage-to-current converter (e.g., voltage-to-current converter  130 ), a differential voltage-based input clock signal is applied between the gates of a transistor  210  and a transistor  220 , and a differential current-based output clock signal is produced between a terminal  230  and a terminal  240 . When circuit  200  is used as a voltage-to-current converter, the output impedance at terminals  230  and  240  is high because of the resistance between the sources and the drains of transistors  210  and  220 . Transistors  210  and  220  can be designed to have a high resistance between source and drain so that the output impedance of the voltage-to-current converter is high. 
     Referring to  FIG. 2A  and  FIG. 2B , the voltage-based input clock signal switches between a low differential voltage and a high differential voltage. When referenced to ground, the voltage input at transistor gate  210  swings between two voltages, yielding a waveform  212 . The voltage input at transistor gate  220  swings between approximately the same two voltages approximately 180 degrees out of phase with waveform  212 , yielding a waveform  214 . When the input clock signal is referenced to one of the waveforms, for example waveform  212  at transistor gate  210 , the other waveform appears to switch polarity from positive to negative, yielding a differential waveform  216 . 
     Referring to  FIG. 2A  and  FIG. 2C , the current-based output clock signal switches between a low differential current and a high differential current. The current output at terminal  230  swings between two currents, yielding a waveform  232 . The current output at terminal  240  swings between two currents approximately 180 degrees out of phase with waveform  232 , yielding a waveform  234 . When the output clock signal is referenced to one of the waveforms, for example waveform  232  at terminal  230 , the alternating-current (AC) current flow in terminal  240  appears to switch direction, yielding a differential waveform  236 . 
     Referring again to  FIG. 2A , when circuit  200  is used as a current-to-voltage converter (e.g., current-to-voltage converter  140 ), transistors  210  and  220  are biased with a DC bias voltage. A differential current-based input clock signal is applied between terminal  230  and terminal  240  and a differential voltage-based output clock signal is produced between a terminal  250  and a terminal  260 . When circuit  200  is used as a current-to-voltage converter, the input resistance seen at terminals  230  and  240  is low because of the common-gate configuration of transistors  270  and  280 . 
     A current-based clock distribution system can be used in a wide range of applications. Referring to  FIG. 3 , the current-based clock distribution system can be used in a wireless transceiver  300  (hereafter referred to as transceiver  300 ). The receive path of transceiver  300  includes an RF amplifier  310  for amplifying an RF input signal. A mixer  320  modulates the amplified RF input signal from the output of RF amplifier  310  with a clock signal generated by local oscillator  330  to create a baseband signal. The baseband signal is filtered by filter circuit  340  to attenuate undesired frequencies in the baseband signal. The filtered baseband signal is then amplified by gain stage  350  and is converted into a digital signal by an analog-to-digital converter  360 . The transmit path of transceiver  300  is represented as a transmitter  370 . 
     In transceiver  300 , the clock signal from local oscillator  330  is distributed as a current. The voltage-based output clock signal of local oscillator  330  is converted to a current-based clock signal by voltage-to-current converter  130 . The local oscillator clock signal is distributed as a current to current-to-voltage converter  140  and to transmitter  370 . Transmitter  370  includes a separate current-to-voltage converter to convert the current-based local oscillator clock signal to a voltage-based clock signal. Transceiver  300  can be IEEE 802 compliant with the following standards: 802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, 802.11i, 802.11n, and 802.16. 
       FIG. 4  shows a method  400  for distributing a current-based clock signal. A voltage-based clock signal is generated (step  410 ) and is converted to a current-based clock signal (step  420 ). The current-based clock signal is distributed (step  430 ). For example, the current-based clock signal can be distributed to several receivers. The current-based clock signal is converted (e.g., at a receiver) to a voltage-based clock signal (step  440 ), and the voltage-based clock signal is used (step  450 ). 
     Various implementations have been described. These and other implementations are within the scope of the following claims. For example, the described system and method can be used to distribute a current-based clock signal within a single integrated circuit or between separate integrated circuits.