Abstract:
A variable delay element includes first and second input stages, each input stage comprising a charge pumping circuit and a discharging circuit, each charge pumping circuit and each discharging circuit associated with the first and second input stages configured to operate on opposite phases of an input signal, and an output stage comprising at least two transistors. The transistors are independently controlled by the first and second input stages and produce an output signal which is a delayed version of the input signal.

Description:
BACKGROUND 
       [0001]    A delay element is used in many electronic circuits to delay one or more signals. In a typical implementation one or more delay elements are arranged serially in what is referred to as a delay line to serially delay a signal. A typical clock signal is provided as a signal that varies over a 360 degree phase. Typical delay lines can delay the output phase of a clock signal by any amount within the 360 degree phase over a band of different frequencies. One typical implementation of a delay line, or a delay lock loop, for use in a phase detector employs an exclusive OR (XOR) logic element. Unfortunately, when implementing a delay locked loop phase detector using an XOR element, allowing the output phase of a clock signal to be delayed more than 180 degrees allows such a phase detector to lock into multiple and possibly non-optimal modes. Implementing a single delay element may be able to compensate for this deficiency. However, a conventional delay element can only delay an input clock signal from the minimum intrinsic gate delay value to a maximum delay of 90 degrees. Further, a conventional delay element attenuates the input signal as the delay is increased and typically requires that the output signal be amplified to a useful level. This attenuation effect limits the lower bandwidth of the circuit. 
         [0002]    Therefore, it would be desirable to have a variable delay element that overcomes these shortcomings. 
       SUMMARY 
       [0003]    In an embodiment, a variable delay element comprises first and second input stages, each input stage comprising a charge pumping circuit and a discharging circuit, each charge pumping circuit and each discharging circuit associated with the first and second input stages configured to operate on opposite phases of an input signal, and an output stage comprising at least two transistors which are independently controlled by the first and second input stages producing an output signal which is a delayed version of the input signal. Other systems and methods will also be described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0005]      FIG. 1  is a block diagram illustrating an embodiment of a variable delay element. 
           [0006]      FIG. 2  is a timing diagram illustrating the operation of an embodiment of the variable delay element of  FIG. 1 . 
           [0007]      FIG. 3  is a schematic diagram illustrating an embodiment of the variable delay element of  FIG. 1 . 
           [0008]      FIG. 4  is a schematic diagram illustrating two instances of the variable delay element of  FIG. 3 . 
           [0009]      FIG. 5  is a timing diagram illustrating the operation of the embodiment of the variable delay element of  FIG. 3 . 
           [0010]      FIG. 6  is a flowchart showing the operation of an embodiment of the variable delay element of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Embodiments of the variable delay element to be described below will be described in the context of a voltage controlled variable delay element. However, the delay provided by the variable delay element can be controlled using signals other than a voltage signal. 
         [0012]    More than one variable delay element can be implemented to form a variable delay line. All such implementations are within the scope of this disclosure. 
         [0013]      FIG. 1  is a block diagram illustrating an embodiment of a variable delay element  100  illustrated as a buffer. The variable delay element  100  includes a buffer  110  having differential inputs  102  and  104  and differential outputs  106  and  108 . The buffer  110  has a control input via connection  112 . The buffer  110  receives a control signal, V CTRL , via connection  112 . In an embodiment, the control signal is a voltage signal, but it may be another type of signal, such as a control current signal. The level of the control signal, V CTRL , determines the amount of delay provided by the variable delay element  100 . 
         [0014]      FIG. 2  is a timing diagram  200  illustrating the operation of an embodiment of the variable delay element  100  of  FIG. 1 . The timing diagram includes an input signal  202 , illustrated as a square wave signal. The input signal is typically a clock signal that is part of an integrated circuit. A full 360° cycle of the input signal  202  is illustrated at  212 . However, any input signal can be delayed using the variable delay element  100 . In an embodiment, the variable delay element  100  can delay an input signal by an amount between zero delay and 180° delay. The trace  204  illustrates a first delay that is indicated as delay, d 1 . The trace  206  illustrates a second delay that is indicated as delay, d 2 . The trace  208  illustrates that the variable delay element  100  ceases delaying the input signal  202  at point  210  and provides no delay after the delay reaches 180 degrees. The delays d 1  and d 2  are used for illustration purposes only. Any amount of delay between zero delay and 180° can be provided by the variable delay element  100 . As will be described below, at the point of 180° phase of the input signal, which is shown as point  210 , the variable delay element ceases providing delay. 
         [0015]      FIG. 3  is a schematic diagram illustrating an embodiment of one of the differential circuits of the variable delay element  100  of  FIG. 1 . One of the differential circuits is shown for simplicity. A complete variable delay element includes two instances of the circuit shown in  FIG. 3 . The variable delay element  300  is constructed using field effect transistor (FET) technology and is implemented using the complementary metal oxide semiconductor (CMOS) manufacturing process. However, the variable delay element  300  can be formed using other switching and semiconductor manufacturing process technology. 
         [0016]    The variable delay element  300  comprises an adjustable current source  302  having a variable input  304 . The control signal, V CTRL , is provided to the adjustable current source  302  via the variable input  304 . In an embodiment, the control signal, V CTRL , is a variable level signal that varies between 0 and 3.5 volts. The output of the adjustable current source  302  is provided to transistors  306  and  308 . The output of the adjustable current source  302  is supplied to the source terminal  312  of the transistor  306 . The gate terminal  316  of the transistor  306  and the gate terminal  318  of the transistor  308  are coupled to an input signal on connection  320 . The input signal on connection  320  can be, for example, the input clock signal described in  FIG. 2 . The drain terminal  314  of the transistor  306  is coupled to the drain terminal  324  of the transistor  308 . The source terminal  326  of the transistor  308  is coupled to common terminal  328 . A capacitance  324  is coupled between connection  322  and the common terminal  328 . 
         [0017]    The connection  322  between the drain terminal  314  of the transistor  306  and the drain terminal  324  of the transistor  308  is coupled to the gate terminal  336  of the transistor  332 . The transistors  332  and  342  form an inverter  330 . The source terminal  334  of the transistor  332  is coupled to common terminal  328 . The drain terminal  344  of the transistor  342  is coupled to the drain terminal  338  of the transistor  332 . The connection  340  between the drain terminal  344  of the transistor  342  and the drain terminal  338  of the transistor  332  forms the output of the variable delay element  300 . 
         [0018]    The source terminal  346  of the transistor  342  is coupled to supply terminal  384 . The gate terminal  348  of the transistor  342  is coupled between the drain terminal  364  of the transistor  356  and the drain terminal  378  of the transistor  366 . A capacitance  352  is connected between the connection  348  and the supply terminal  384 . The source terminal  358  of the transistor  356  is coupled to the supply terminal  384 . 
         [0019]    The gate terminal  362  of the transistor  356  and the gate terminal  368  of the transistor  366  are coupled to an input signal on connection  382 . The input signal on connection  382  is the same input signal provided on connection  320 . 
         [0020]    An adjustable current source  374  having a variable input  376  is coupled to the source terminal  372  of the transistor  366 . The control signal, V CTRL , is provided to the adjustable current source  374  via the variable input  376 . In an embodiment, the control signal, V CTRL , is a variable level signal that varies between 0 and 3.5 volts. The adjustable current source  374  is coupled to the common terminal  386 . 
         [0021]    For purposes of the description to follow, the signal on connection  322  will also be referred to as “n pump” and the signal on connection  348  will also be referred to as “p pump.” 
         [0022]      FIG. 4  is a schematic diagram illustrating two instances of the variable delay element  300  of  FIG. 3 . The variable delay element  400  constitutes a variable delay element that operates on both phases of an input signal. The variable delay element  400  comprises a circuit portion  406  and a circuit portion  418 . The circuit portion  406  corresponds to the variable delay element  300  described above. The circuit portion  418  operates on the opposite phase of the input signal than the portion  406 . A first phase of the input signal is supplied on connection  402  and an opposite phase of the input signal is supplied on connection  422 . The input signal on connection  402  is delayed by the circuit portion  406 , as described above, and the output is provided on connection  408 . The connection  408  corresponds to the connection  340  in  FIG. 3 . 
         [0023]    The output signal on connection  408  is provided to an inverter  412 . The output of the inverter  412  is a signal that is opposite in phase from the signal on connection  408 . The opposite phase input signal on connection  422  is delayed by the circuit portion  418 , as described above, and the output is provided on connection  414 . The output signal on connection  414  is provided to an inverter  416 . The output of the inverter  416  is a signal that is opposite in phase from the signal on connection  414 . 
         [0024]    The two inverters  412  and  416  across outputs of circuit portions  406  and  418  improve the duty cycle of the variable delay element  400  by operating the circuit portions  406  and  418  on opposite phases of the input clock signal. This arrangement causes rise and fall time behavior to be effectively averaged creating an output with a duty cycle characteristic that closely approximates the duty cycle of the input clock signal. 
         [0025]      FIG. 5  is a timing diagram  500  illustrating the operation of the embodiment of the variable delay element of  FIG. 3 . The input signal is shown using the trace  506 , the signal on connection  322  (n pump) is shown using trace  508  and the signal on connection  348  (p pump) is shown using trace  512 . The supply voltage, V DD  is shown using traces  514  and  516 . System ground is shown using traces  526  and  528 . The output signal on connection  340  is shown using trace  520 . At the time, t 0 , the input signal transitions from a logic low to a logic high. It should be mentioned that the transition from logic low to logic high is arbitrary and the operation of the variable delay element is similar on either a logic low to logic high transition or on a logic high to logic low transition. 
         [0026]    At time t 0 , the signal on connection  322  (trace  508 ) remains unchanged and the signal on connection  348  (trace  512 ) begins to fall from V DD  to a minimum value determined by the adjustable current source  374  and the size of the capacitance  352  on node  354  at the time t 1 . During the time after t 0  and prior to t 1 , the observed output  340  (trace  520 ) remains low until the threshold voltage (V THRESHOLD P ) of the transistor  342  is exceeded. When the threshold voltage (V THRESHOLD P ) of the transistor  342  is exceeded, the transistor  342  causes a low to high transition to be quickly made at the output node  340 . At time t 1 , the input signal  506  transitions from logic high to logic low. At time t 1 , the signal on connection  348  (trace  512 ) is quickly pulled to V DD  where it remains unchanged until the next input transition. At this time the signal on connection  322  (trace  508 ) begins to rise from a ground level to the maximum level. During the time after t 1  and prior to t 2 , the observed output  340  (trace  520 ) remains high until the threshold voltage (V THRESHOLD N ) of the transistor  332  is exceeded. When the threshold voltage (V THRESHOLD N ) of the transistor  332  is exceeded, the transistor  332  causes a high to low transition to be quickly made at the output node  340 . At time t 2 , which time is equal to 360° of the input signal  506 , the node  322  (trace  508 ) is quickly pulled low and the process repeats from time t 0 . 
         [0027]    The transistors  306  and  308  and the adjustable current source  302  form a first input stage  392  that operates on one phase of the input signal. The transistor  306  acts as a charge pump circuit and the transistor  308  acts as a discharging circuit. The transistors  356  and  366  and the adjustable current source  374  form a second input stage  394  that operates on the opposite phase of the input signal. The transistor  366  acts as a charge pump circuit and the transistor  356  acts as a discharging circuit. The transistors  332  and  342  form an inverter  330  in which the transistors  332  and  342  are independently controlled by the first input stage  392  and the second input stage  394 , respectively, thus producing an output signal which is a delayed version of the input signal. The transistors  332  and  342  in the output stage alternately pull the output  340  to a logic high value and a logic low value. 
         [0028]    A wide bandwidth is achieved because the output of the transistors  342  and  332  swings between the level of the supply voltage (V DD ) and ground regardless of the delay. The output will always be full swing, regardless of delay, until a delay of 180 degrees is reached, in which case no switching will occur. A conventional delay line produces an increasingly smaller swing as delay is increased. Thus, at low frequencies the output signal of a conventional delay line is not capable of driving a circuit that requires full swing drive such as standard CMOS logic. Further, because the output is typically amplified some degree of signal corruption to accommodate standard CMOS logic is encountered using a conventional delay line. 
         [0029]    With reference to  FIG. 3  and  FIG. 5 , the transistor  306  forms a current pumping circuit and the transistor  308  forms a current dumping circuit in which the transistor  308  is sized to quickly pull up the node  322  when switched on. Similarly, the transistor  366  forms a current pumping circuit and the transistor  356  forms a current dumping circuit in which the transistor  366  is sized to quickly pull up the node  348  when switched on. In this manner, the variable delay element  300  will quickly transition the output  340  from V DD  to ground without attenuating the output signal as the delay increases from zero delay to a delay of 180°. The control voltage, V CTRL , controls the length of the delay by increasing or decreasing the amount of current supplied by the adjustable current sources  302  and  374 . This current charges the capacitances  324  and  352  at a rate determined by the current supplied by the adjustable current sources  302  and  374 . Increasing the current supplied by the adjustable current sources  302  and  374  decreases the time required to charge the pump nodes  322  and  348  and decreases the time required for the transistors  342  and  332  to reach their respective threshold voltages, V THRESHOLD P  and V THRESHOLD N . Therefore, more current supplied by the adjustable current sources  302  and  374  corresponds to a shorter delay. The pump node capacitances  352  and  324  are charged in a controlled manner on alternating phases of the input signal on connection  320 . The pump nodes  348  and  322  are discharged quickly on the opposite phase of the input signal via transistors  308  and  356 . These transistors should be sized in a manner to maximize discharge time relative to charge time. The result is shown in  FIG. 5  using traces  508  and  512  and appears as a voltage ramp followed by a quick edge during the discharge cycle. 
         [0030]    The two inverters  412  and  416  across outputs of circuit portions  406  and  418  improve the duty cycle of the variable delay element  400  by operating the circuit portions  406  and  418  on opposite phases of the input clock signal. This arrangement causes rise and fall time behavior to be effectively averaged creating an output with a duty cycle characteristic that closely approximates the duty cycle of the input clock signal. 
         [0031]      FIG. 6  is a flowchart showing the operation of an embodiment of the variable delay element  300  of  FIG. 3 . In block  602 , an input signal is provided to a first stage of the variable delay element  300 . In block  604 , the input signal is provided to a second stage of the variable delay element  300 . The first input stage and the second input stage operate on opposite phases of the input signal. In block  606 , a variable level control signal, V CTRL  is provided to the variable delay element  300 . In block  608 , the variable delay element  300  delays the input signal by an amount between zero delay and a delay of 180°. The delay is determined by the level of the variable level control signal. 
         [0032]    This disclosure describes the invention in detail using illustrative embodiments. However, it is to be understood that the invention defined by the appended claims is not limited to the precise embodiments described.