Abstract:
A delay line including analog delay elements each having a selectively adjusted coarse and fine delay portion is described. The coarse delay portion receives an input clock signal and generates a ramp signal having a slope based on a predetermined coarse delay setting. The fine delay portion generates a threshold voltage based on a predetermined fine delay setting. A comparator compares the coarse delay ramp signal voltage with the fine delay threshold voltage and generates an output clock signal when the ramp signal voltage surpasses the fine delay threshold voltage. The coarse delay is linearly adjustable based on a 32-bit binary input signal and the fine delay is binary-weight adjusted based on a 5-bit binary input signal. Both the coarse and fine delay portions are controlled by delay line control circuitry which compares a feedback version of the output clock signal with the input clock signal and provides control signals to increment or decrement coarse and fine delay in the delay line.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation application of U.S. Ser. No. 09/985,972 filed Nov. 7, 2001, now granted, which is itself a continuation application of U.S. Ser. No. 09/514,273 filed Feb. 28, 2000, now abandoned. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of delay locked loops, and in particular to a novel delay element.  
         BACKGROUND OF THE INVENTION  
         [0003]    Delay locked loops are used to control the timing of an internal clock signal, to match that of an input or external clock signal. Typically an external clock signal is passed through a delay line, where the external clock signal is delayed for a controllable time. The output signal of the delay line is applied to a circuit to be clocked via a clock distribution tree. One of the clock signals from the distribution tree (the internal clock signal) is applied with the external clock to a comparator, which determines any phase difference. The difference is used to generate delay line control signals, which are applied to the delay line so as to cause the delay to vary and thus minimize any phase difference between the external clock signal and the internal clock signal.  
           [0004]    Typically the delay line is formed of coarse delay elements and one or more fine delay elements. One or more coarse delay elements are connected in series with a fine delay element. The fine delay element can be adjusted to the maximum time delay of one coarse delay element. A system which uses this structure is described in U.S. patent application Ser. No. 09/106,755 filed Jun. 30, 1998, and entitled “Process, Voltage and Temperature Independent Switched Delay Compensation Scheme”, invented by Gurpreet Bhullar et al, which is incorporated herein by reference.  
           [0005]    In delay lines of this type, plural inverters are connected in series between an input for receiving the input clock and an output. Switches controlled by the delay line control are switched so as to bypass various ones of the inverters, and thus control how many inverters the external clock signal has to pass through.  
           [0006]    However, it has been found that since even the fine delay is controlled in steps, there is some jitter remaining. This is because in attempting to maintain the DLL setting about a lock point, the DLL control circuitry may attempt to add and remove one fine delay element continuously. If one fine control step does not set the delay to cause the internal clock signal to be exactly in phase with the external clock, there will be jitter about the lock point.  
           [0007]    It has also been found that the fine delay line cannot always compensate for one coarse delay element since the coarse element delay can have a longer delay than the maximum that can be provided by the dynamic range of the fine delay control due to temperature and voltage conditions.  
           [0008]    It has also been found that noise on the power supply rails can cause jitter in the output signal of the delay line, especially in the case of RC-based inverter delay lines.  
           [0009]    The digital delay line also takes up significant integrated circuit area, due to the resistors and capacitors required to provide the digital delay line.  
           [0010]    It is also desirable to have as large a dynamic range as possible. This dynamic range is limited in a delay line having fixed coarse and fine delay elements. Furthermore, each delay element of the delay line will experience large variation of delay with variations in temperature and voltage.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention has several advantages over the digital delay line described above. In a comparison between the above described digital delay line, in a prototype which had five delay elements to provide a certain delay variation, only two elements were required using the present invention to achieve approximately the same dynamic range. Thus there is substantial improvement in dynamic range of each element.  
           [0012]    The present invention also takes up smaller integrated circuit chip area than the above-described digital delay line, for approximately the same delay, since plural resistors and capacitors are not required.  
           [0013]    The dynamic range increases with decreasing frequency in the present invention, which is the opposite of that of the digital delay line, for which more and more elements would be required to make up the increased delay time that would be required in a low frequency device and for test purposes.  
           [0014]    The control voltages used in the present invention can be made very accurate and immune to temperature and voltage variations. The delay is thus substantially immune to process variations.  
           [0015]    The present invention uses analog delay elements, instead of digital delay elements of the prior art.  
           [0016]    In accordance with an embodiment of the present invention, A delay line for receiving an input clock signal and delaying the input clock signal to produce an output clock signal, the delay line comprising: at least one analog delay element having a first delay adjustment input and a second delay adjustment input, the first delay adjustment input receiving a first bias voltage and the second delay adjustment input receiving a second bias voltage, wherein the first bias voltage is independently generated from the second bias voltage.  
           [0017]    In accordance with another embodiment, a delay line for receiving an input clock signal and internally delaying the input clock signal to produce an output clock signal, the delay line comprising: at least one analog delay element including a first delay adjustment input and a second delay adjustment input, the first delay adjustment input receiving a first bias voltage and the second delay adjustment input receiving a second bias voltage; a first bias voltage generator generating the first bias voltage in response to a first set of control signals; and a second bias voltage generator generating the second bias voltage in response to a second set of control signals.  
           [0018]    In accordance with another embodiment, a delay line for receiving an input clock signal and internally delaying the input clock signal to produce an output clock signal, the delay line comprising: a plurality of analog delay elements, each analog delay element including a first delay adjustment input and a second delay adjustment input, the first delay adjustment input receiving a first bias voltage and the second delay adjustment input receiving a second bias voltage; a first bias voltage generator generating the first bias voltage in response to a first set of control signals; and a second bias voltage generator generating the second bias voltage in response to a second set of control signals.  
           [0019]    In accordance with another embodiment, a method for delaying an input clock signal to produce an output clock signal, the method comprising: providing a first control voltage to a first control input of a delay element; providing a second control voltage to a second control input of the delay element; providing a first group of delay control signals to a first generating circuit for generating the first control voltage; providing a second group of delay control signals to a second generating circuit for generating the second control voltage; and delaying the output clock signal where the first and second control voltages establish the amount by which the output clock signal is delayed relative to the input clock signal.  
           [0020]    In accordance with another embodiment, a method for delaying an input clock signal to produce an output clock signal, the method comprising: providing a first bias voltage to a first control input of a delay element; providing a second bias voltage to a second control input of the delay element; providing a first group of delay control signals to a first generating circuit for generating the first bias voltage; providing a second group of delay control signals to a second generating circuit for generating the second bias voltage; and delaying the output clock signal where the first and second bias voltages establish the amount by which the output clock signal is delayed relative to the input clock signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    A better understanding of the invention will be obtained by a consideration of the detailed description below, in conjunction with the following drawings, in which:  
         [0022]    [0022]FIG. 1A is a block diagram of a delay element of the present invention in its most basic form,  
         [0023]    [0023]FIG. 1B illustrates waveforms that may be observed in the circuit of FIG. 1A,  
         [0024]    [0024]FIG. 2 is a graph used to illustrate how the circuit of FIG. 1 controls delay,  
         [0025]    [0025]FIG. 3 is a block diagram of a delay line with two analog delay elements constructed in accordance with an embodiment of the present invention,  
         [0026]    [0026]FIG. 4 is a schematic diagram of a delay element constructed in accordance with an embodiment of the present invention,  
         [0027]    [0027]FIG. 5 is a schematic diagram of a coarse delay control signal generating circuit in accordance with an embodiment of the present invention,  
         [0028]    [0028]FIG. 6 is a schematic diagram of a fine delay control signal generating circuit in accordance with an embodiment of the present invention,  
         [0029]    [0029]FIG. 7 is a plot illustrating waveforms at various locations of the circuit of FIG. 4 for a particular coarse and fine setting,  
         [0030]    [0030]FIG. 8 is a plot illustrating waveforms at various locations of the circuit of FIG. 4, showing two different coarse delay settings, and  
         [0031]    [0031]FIG. 9 is a plot illustrating waveforms at various locations of the circuit of FIG. 4, for different fine delay settings. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]    With reference to FIGS. 1A and 1B, a capacitor  101  is connected in series with a constant current source  103 , node N 1  forming their junction. Capacitor  101  can be implemented by using the intrinsic capacitance of node N 1  which includes the input capacitance of a comparator  111  which is also connected to node N 1 . The source-drain circuit of a field effect transistor (FET)  105  is connected in parallel with the capacitor. An external clock signal IN is applied to the gate of the field effect transistor (FET)  105 , shown in FIG. 1B as the signal  107 .  
         [0033]    The capacitor will charge substantially linearly as long as the FET  105  is nonconductive. For the n-channel type FET shown, while the external clock signal is at low logic level, the source-drain circuit of the FET will be nonconductive. While the external clock signal is at high logic level, the source-drain circuit of FET  105  will be conductive, discharging capacitor  101 . The result is that a continuous ramp signal is generated on every clock cycle, shown in FIG. 1B as signal  109 . The signal  109  is applied from node N 1  to one input of a comparator  111 , which has a reference voltage V T  applied to its other input. As shown in FIG. 1B, the voltage at node N 1  surpasses V T  as the ramp increases at a time t d .  
         [0034]    The value of the constant current controls whether the capacitor will charge either faster or slower thereby controlling the slope of the node N 1  voltage vs time delay curve shown in FIG. 2. The current source therefore controls the coarse delay characteristic of the delay element.  
         [0035]    For example, assuming that the constant current source  103  provides a certain value of current, the capacitor will charge at a particular rate and establish one of the charge rate (slope) curves shown in FIG. 2. When the capacitor discharges due to the input signal to the gate of FET  105  going high and causing FET  105  to conduct, the output to the comparator will go to high logic level. After the input signal goes to low logic level (time t a ) the FET  105  switches off, and capacitor  101  will begin to charge. The ramp signal  109  is applied to the noninverting input of the comparator. The output of the comparator  111  remains at low logic level until the ramp voltage reaches the voltage value applied to its other input, V T , at which time its output signal switches to high logic level. The difference in time between the trailing edge of the signal at IN and the time at which the comparator  111  generates a trailing edge to its output signal on node N 2  represents the coarse delay time t d .  
         [0036]    It may be seen that by controlling the capacitor  101  charging current the time delay t d  can be controlled to a desired coarse delay timing value.  
         [0037]    The voltage V T  applied to one of the inputs of comparator  111  controls the operating point on the curve in FIG. 2, and therefore controls the fine delay characteristic of the delay element.  
         [0038]    Thus, for a fine delay increase, the voltage causes the delay time to increase from t 1  to t 2 , determined by a charge of the operation point on the slope of the particular coarse delay curve. For a coarse delay increase, the coarse delay curve slope changes the time delay thus charging e.g. from t 1  to t 3 . For both a coarse and fine time delay, both the coarse delay curve slope changes and the operating point on the curve changes the time delay charging from t 1  to t 4 .  
         [0039]    The basic concept of the analog delay element which was described above can be embodied in a delay circuit, the block diagram of which is shown in FIG. 3. An analog delay element  304  (more than one can be coupled together as shown in FIG. 3, and as will be readily understood by one skilled in the art, a minimum of two delay elements are needed in order to generate an output clock signal having the same duty cycle as the input clock signal. In general an even number of delay elements is needed, although only one has been described here.) has an external clock applied to it at IN, as well as a coarse delay control signal Pbias generated by a Pbias generator  302 , and a fine delay control signal Vref_fine generated by a Vref_fine generator  305 . The Pbias signal is used to control the constant current value described with reference to FIGS. 1A, 1B and  2 , and is thus a coarse delay control signal. The Vref_fine control signal corresponds to the V T  signal applied to the comparator of FIG. 1A, and thus is a fine delay control signal. The delayed external clock signal is shown at the output of the delay elements as Delayed Clock.  
         [0040]    An Nbias signal generator  301  is used to provide a bias signal for operation of certain FETs in the Pbias generator  302  and in the Vref_fine generator  305 , and will be described in more detail later.  
         [0041]    An analog delay line control circuit  306  outputs coarse delay control signals A 0 -A 31  for controlling the Pbias generator, and also outputs fine delay control signals B 0 -B 4  for controlling the Vref_fine generator, as will be described in more detail later.  
         [0042]    The analog delay line control responds to externally supplied signals (not shown) which designate e.g. a phase mismatch between an external clock and the delay clock. The internal clock is typically derived from the external clock As a result the analog delay line control outputs coarse delay control signals which causes the Pbias generator to input a signal to the coarse delay control input of the analog delay element (or coarse delay control inputs of a series of analog delay elements). This causes the delay time of the delay element or elements to be changed as described above.  
         [0043]    When the time delay is close to the optimum, the analog delay line control causes the fine delay control signal to adjust to time delay further until the external signal to the analog delay line control to indicate that no further adjustments need be done. In practice, in the event the coarse delay control controls the delay too much, this is indicated by the external signal to the analog delay line control, which backs off the coarse delay to a point at which the fine delay control is used to adjust the delay of the analog delay element or series of elements to the optimum delay.  
         [0044]    [0044]FIG. 4 is a schematic diagram of a preferred embodiment of the delay element  304 . Capacitor  101  is repeated from FIG. 1, as is FET  105 . The constant current source  103  is provided by another FET  401 . In the preferred embodiment, FET  105  is an n-channel FET, and FET  401  is a p-channel FET.  
         [0045]    Node N 1  is applied to the input of comparator  413 . The comparator is shown specifically as a current mirror amplifier of well known construction formed of FETs  406 ,  407 ,  404 ,  405  and  410 . The input to the comparator is at the gate of an FET  404 . The junction of FETs  405  and  407  form the comparator output, and the gate of FET  405  forms a second input for the comparator. A bias voltage Va is applied to the gate of FET  410 .  
         [0046]    The output signal of the comparator, at node N 2 , is applied to the input of an inverter  408 , the output of which is applied to one input of NOR gate  409 . The gate of FET  105  receives the CLK_in signal, which signal is also applied to the second input of NOR gate  409 , as well as to the gates of pulldown FETs  412  and  414 , the first of which has its source-drain circuit connected in parallel to the source-drain circuit of FET  410  and the second of which has its source-drain circuit connected between node N 2  and ground.  
         [0047]    In operation, the voltage Pbias is applied to FET  401 , which causes FET  401  to operate in its saturation region, thus operating effectively as a constant current source with the value of the current being controlled by the value of Pbias. Other controlled current source implementations could be envisaged by persons skilled in the art.  
         [0048]    With reference also to the timing diagram shown in FIG. 7, for the high logic level portion of the period of the signal CLK_in, the output signal CLK_out is kept at low logic level due to the action of NOR gate  409 . The high logic level of the signal CLK_in also causes FET  105  to be conductive, thereby discharging capacitor  101 . Node N 1  is thus maintained at a low logic level, as shown in FIG. 7. The amount of constant current flowing through FETs  401  and  105  to ground from the power supply is controlled by the voltage Pbias. However, with CLK_in being at low logic level, substantially no current flows through FET  105 .  
         [0049]    The high logic level voltage applied to FET  414  causes it to conduct, bringing the input of inverter  408  to ground, and the other input to NOR gate  409  to high logic level. The output of the NOR gate is thus at low logic level. The high logic level voltage applied to the gate of FET  412  causes it to conduct, bringing the node N 3  to ground, and thereby disabling the comparator  413 .  
         [0050]    At the time t 0  shown in FIG. 7 the falling edge of CLK_in is applied to the gate of FET  105 . As a result, FET  105  ceases conducting, and capacitor  101  begins charging. A ramp voltage will develop at node N 1  as shown in FIG. 7, which will surpass the threshold voltage Vref_fine after a time delay t d , where t d =t 1 −t 0 . The ratio W/L of the gate width W of the FET  105  to the channel length L of FET  105  should be considerably larger than W/L of FET  401 , so that the voltage between the gate and the source of FET  105  is larger than that of FET  401 . It should be noted that although the system is described above with respect to the falling edge of CLK_in, a dual system could be implemented which responds to the rising edge of CLK_in.  
         [0051]    With the CLK_in signal going to low logic level, FET  412  is disabled, allowing FET  410 , having reference voltage Va applied to its gate, to become operative, thereby activating the comparator. FET  414  is also disabled, allowing the output signal on node N 2  to determine the input to inverter  408 .  
         [0052]    With the fine control voltage Vref_fine applied to the gate of FET  405 , when the voltage at the node N 1  reaches and surpasses the Vref_fine level, the comparator  413  conducts more current through the FET branch  406 ,  404 ,  410 , thereby allowing the voltage at the N 2  node to change from low to high logic level, as shown by curve N 2  in FIG. 7. This occurs at time t 1 . Thus the falling edge of the CLK_in voltage is delayed from time t 0  to the time t 1  by the time td. This voltage is inverted in inverter  408 , so that the voltage applied to the NOR gate  409  is the same polarity as that of the CLK_in signal. Note that the output of inverter  408  does not switch until N 2  reaches its switching point at time t 1 . Effectively, inverter  408  transforms the still analog signal at node N 2  into a crisp digital output of CLK_out.  
         [0053]    When the CLK_in signal again reverts to high logic level, FETs  105 ,  412  and  414  are enabled, discharging capacitor  101  (bringing node N 1  to ground), disabling the comparator  413 , bringing node N 2  to ground, and causing the output of NOR gate  409  to go to low logic level. As the CLK_in signal operates at its particular given frequency, the generation of the ramp voltages at under N 1  and N 2 , resulting in the delayed output CLK_out are repeated for every cycle. It is important to note that the delay t d  by which the output signal CLK_out is delayed is determined by the two control voltages Pbias, determining the coarse delay, and by Vref_fine, determining the fine delay. The use of the current mirror comparator allows for the accurate customized control of the delay t d  based on the control voltage levels Pbias and Vref_fine.  
         [0054]    [0054]FIG. 5 is a schematic diagram of a preferred form of a coarse delay control circuit and Pbias signal generator. This is comprised of a current mirror circuit  500 , a current control circuit  501  and a capacitor  510 . The current mirror circuit is comprised of a cascade of three current mirrors connected between a positive power supply rail and ground, formed of PMOS FETs  505  and  506 , NMOS FETs  507  and  508 , and PMOS FET  509 . Capacitor  510  is connected between the output of FET  509  and ground. The control voltage Pbias is obtained from the output of FET  509 .  
         [0055]    The current control circuit  501  is comprised of plural pulldown circuits connected in parallel, each formed of two NMOS FETs  502 A and  503 A- 502 N and  503 N. The source-drain circuits of each pair of FETs are connected in series between the node N 3  and ground, and node N 3  is connected to the positive power rail through a PMOS FET  505 . Decoded select logic signals A 0 -AN are applied to one or more of the gates of FETs  502 A- 502 N, and a bias voltage nbias is applied to the gates of FETs  503 A  503 N.  
         [0056]    The sizes of FETs  503 A- 503 N are selectively different from each other so that with a common nbias voltage applied to their gates, the FETs provide different resistances. The sizes of FETs  502 A- 502 N should be such that they all provide minimal resistance, and with an enabling voltage applied to their gates, they act as switches. The overall sizes should be such that the currents, passing from the positive voltage rail (VDD) to ground via FET  505 , the switches formed by FETs  502 A- 502 N through respective FETs  503 A- 503 N, should vary (increase) linearly through successive paths through FETs  503 A- 503 N. In a preferred embodiment, there were  32  pairs of FETs  502  and  503 . The reference voltage nbias was received from a conventional DC reference voltage generator (not shown).  
         [0057]    The signals A 0 -AN can be derived from a decoder  512  which decodes a signal provided by a counter (e.g. a 5 bit counter  514  for AN= 32 ). The counter receives up and down (UP/DN) control signals from the delay line control  306  (FIG. 3) which compares the phase of a feedback clock signal with the external (clock) signal and generates the UP and DN signals in a well known manner.  
         [0058]    In operation, because the gate bias nbias is a constant DC voltage, the gate biases of the NMOS FETs  503 A- 503 N are constant. The currents passing through these FETs are determined by their sizes. The gates of FETs  502 A- 502 N receive the coarse delay control signal A 0 -AN referred to with reference to FIG. 3. One (or more, if necessary) of FETs  502 A- 502 N are switched on, which causes one (or more) of FETs  503 A- 503 N to be connected between the node N 3  and ground. As an example, if only one of the A 0 -AN control signals is activated, then a constant current controlled by the size of a corresponding one of FETs  503 A- 503 N passes from the positive voltage rail, through PMOS  505  to node N 3  and through one of FETs  502 A- 502 N activated by the A 0 -An control signal, and a corresponding one of FETs  503 A- 503 N to ground. Depending on the ratios of FETs  505 ,  506 ,  507 ,  508  and  509  in FIG. 5 and FET  401  shown in FIG. 4, a constant current is supplied to the node N 1  of the circuit of FIG. 4. As a result, the ramp slope of the node N 1  is determined. In effect, this ramp slope is determined by the control signals A 0 -AN.  
         [0059]    The ramp slope at node N 1  also affects the ramp slope at the N 2  node, as shown in FIG. 8. FIG. 8 is similar to FIG. 7 with more detail added, and in particular illustrates the effect of varying the ramp slope at node N 1  with coarse control. In FIG. 8, the dotted line SL 1  shows the steeper slope of the signal at node N 1  when the effective resistance of one of FETs  503 A- 503 N is smaller (a greater constant current), and the shallower slope when the effective resistance of one of FETs  503 A- 503 N is larger (a smaller constant current).  
         [0060]    The number of FETs  503 A- 503 N (and corresponding FETs  502 A- 502 N and control signals A 0 -AN) to be used in the circuit will be determined by the required resolution and amount of coarse delay. More than one FET  503 A- 503 N can be switched in parallel to provide different resistances in order to cause a particular slope which may be intermediate or steeper than those that may be provided by switching only a single current path (a single pair of FETs  502  and  503 ).  
         [0061]    The fine delay control voltage Vref_fine is preferably generated in a circuit such as is shown in FIG. 6. A current control circuit  611  is formed similarly to that of current control circuit  501  in FIG. 5, except as described below. In the present case, the serially connected FET pairs are  605 A and  605 N- 606 A and  606 N, the drains of FETs  606  being connected to the sources of FETs  605 . The sizes of FETs  606 A- 606 N are preferably binary weighted; in a preferred embodiment 5 bits being used so as to provide 32 steps. Thus the width to length ratio of these FETs (W/L) of FET  606 A was 1, the next was 2W/L, the next was 4W/L, the next was 8W/L and the last of the 5 was 16W/L. The bias voltage nbias is applied to the gates of FETs  606 A- 606 N.  
         [0062]    Fine delay control voltages B 0 -BN are applied to one or more of the gates of switch FETs  605 A- 605 N.  
         [0063]    Current mirror  613  is comprised of PMOS FETs  601  and  602  and has a pulldown path through resistor  603  and NMOS FET  604 . The output of the current mirror provides the output signal Vref_fine. The level of Vref_fine is controlled by the bias current control circuitry  611 , which sets the current flowing from the output of the current mirror to ground. The bias current control circuit  611  is formed of a plurality of series connected to FETs  605 A and  606 A- 606 A and  606 N, each series pair being connected in parallel between the output carrying the signal Vref and ground, as shown in FIG. 6. A resistor  612  is also connected in parallel with the series pairs to provide a load to the output of the current mirror when none of the FETs in  611  are enabled.  
         [0064]    In operation, a constant current passes from the positive voltage rail, through FET  601 , resistor  603  and FET  604 . Due to the current mirror action, a proportional constant current flows from the positive voltage rail, through FET  602  and the control circuit  611  according to the size ratio between FETs  601  and  602 . The FETs  606 A- 606 N function as binary-weighted resistors. Upon enabling of one or more FETs  605 A- 605 N by the control signals B 0 -BN, the constant current flowing through FET  602  is conducted through one or more corresponding FETs  606 A- 606 N from the positive voltage rail. The voltage Vref_fine at the output is determined by the ratio of the resistances of FET  602  and the single or parallel resistances of FETs  606 A- 606 N, times the voltage at the positive voltage rail, e.g. VDD.  
         [0065]    Thus in the 5-bit example shown in FIG. 6, the voltage Vref_fine can have 32 different levels by the 5 bit binary combination of the B 0 -B 4  control signals. The control signals B 0 -B 4  (or BN) can be generated in a manner similar to control signals A 0 -AN, using a counter and decoder in delay line control  306 , the counter being driven by UP/DN signals from a comparison of the phase of the feedback clock with that of the external (input) signal.  
         [0066]    [0066]FIG. 9 illustrates the enlarged voltage curves of FIGS. 7 and 8, with the effect of variation of the fine delay control voltage Vref_fine. As previously noted, at node N 1 , a particular ramp slope is determined by a particular coarse control signal A 0 -AN. To illustrate the effects of varying the fine delay, superimposed on this curve are three values of Vref_fine, VF 1 , VF 2  and VF 3 . The intersections of VF 3 , VF 2  and VF 1  with the ramp constitutes delay operating points t 3 , t 2  and t 1  of the circuit (shown on the CLK_out curve), which provide delays td 3 , td 2  and td 1  respectively.  
         [0067]    The corresponding timing of the signal at node N 2  is also shown, with the rising edges at LE 1 , LE 2  and LE 3  respectively. The respective rising edges of different time delayed output clock signals are also shown as LE 1 , LE 2  and LE 3  in the curve CLK_out.  
         [0068]    Thus the coarse delay signal Pbias applied to FET  401  of the delay element or series of elements (FIGS. 3 and 4), which varies the slope of the ramp at the node N 1  (FIG. 4), is controlled by the signals A 0 -AN, and the fine delay signal Vref_fine applied to FET  405  in comparator  413  (FIG. 4) which varies the operating point on the slope of the ramp, combine to vary the time delay in the analog delay element or elements to a wide degree, avoiding the problems encountered with the prior art structures described earlier.  
         [0069]    The above has thus described a system which controls a digital clock signal by means of variable analog delay elements, to produce a digital output clock signal with controllable delay.  
         [0070]    While the description of the preferred embodiment described above has indicated the use of particular conductivity types of FETs for various purposes and a power supply having the positive polarity and ground, it will be recognized that opposite conductivity FETs can be used instead, with a corresponding change in the polarity of the power supply, within the scope of the invention.  
         [0071]    A person understanding this invention may now conceive of alternate embodiments and enhancements using the principles described herein. All such embodiments and enhancements are considered to be within the spirit and scope of this invention as defined in the claims appended hereto.