Abstract:
A precision signal delay apparatus and method for introducing time delay to a signal. Precision delay is introduced by a pair of delay locked loops (DLLs) connected in series each with selected delay (i.e., a Vernier-type circuit). Nonuniformity in the precision delay is compensated with a delay compensation circuit. The apparatus and method may be used for phase shifting, data delay, precision pulse width modulation, and precision time windowing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/109,016, filed Mar. 28, 2002, now U.S. Pat. No. 6,580,304. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electronics and, more particularly, to apparatus and methods for introducing signal delay. 
     BACKGROUND OF THE INVENTION 
     Presently, there is a need for signal delay circuits capable of introducing a high precision delay to various signals. For example, in High Range Resolution (HRR) Radar systems, delay circuits are needed to generate intervals between transmit pulses and receive pulses with a resolution finer than 200 pico (p) seconds (s). Due to practical considerations, the delay circuits must be inexpensive, reliable, and small. 
     One well known method for generating a delay involves the use of a resistor and capacitor (RC) circuit. In an RC circuit, as the voltage applied to the RC circuit changes, the voltage across the capacitor gradually changes in a predictable manner to match the new voltage applied to the RC circuit. The time it takes the capacitor to transition from one voltage level to another voltage level in response to a predetermined change in the applied voltage level defines a predictable time interval, which may be used to generate a delay. Although inexpensive, this method is susceptible to noise that adversely affects its reliability. 
     Another method for generating a signal delay is depicted in FIG.  1 . FIG. 1 is a block diagram of a known tapped delay locked loop (TDLL)  10  for shifting the phase of a periodic input signal. The TDLL  10  includes a delay chain  12  with (N) identical delay elements  14 , a comparator  16 , and a multiplexer  18 . Typically, the delay elements are fabricated as a single integrated circuit (IC) on a silicon wafer. Therefore, the delay elements are essentially identical and will each introduce an equal amount of delay to a signal passing through them. 
     In operation, an input signal received at an input port (IN) of the TDLL  10  is passed simultaneously to one input port at the comparator  16  and through the delay chain  12 . After passing through the delay chain  12 , a delayed version of the input signal is fed back to the other input port of the comparator  16  where its phase is compared to the phase of the input signal. The comparator  16  produces an error signal that is fed to each of the delay elements within the delay chain  12  to control the amount of delay provided by each delay element. Since the delay elements are identical, applying the error signal to each delay element will adjust each delay element by an equivalent amount. The comparator  16  adjusts its output (the error signal) until its two inputs are equal, which occurs when the phase of the input signal matches the phase of the input signal as delayed by the delay chain  12 . This occurs when the delayed signal is delayed exactly one period of the input signal. When the phases match, after at least one period of the input signal, the TDLL  10  is “locked,” and the input signal as delayed by the delay chain  12  is identical to the received input signal delayed by one period of the input signal. 
     Each delay element within the delay chain  12  successively introduces delay to the input signal as it passes through the delay chain  12 , where each successive delay is commonly referred to as a “step.” If each of the delay elements within the delay chain  12  is “tapped,” each tap will produce a signal identical to the input signal delayed by a fraction of the period of the input signal. That fraction is given by the number of delay elements the input signal has passed through divided by the total number of delay elements (N) in the delay chain  12 . The multiplexer  18  is coupled to the taps of the individual delay elements and is used to select one of the taps in response to a selection signal received at a selection port (Sel) to provide a desired delay. 
     The TDLL  10  is capable of introducing a delay to a periodic input signal. However, since the TDLL  10  takes at least one period of the input signal to “lock,” it is not suitable for delaying individual (or variable period) pulses. In addition, each step produced within a period of the input signal requires an additional delay element. Accordingly, systems requiring a large number of steps (i.e., high resolution) will require a large number of delay elements. As the number of delay elements increases, however, it becomes increasingly difficult to ensure that the individual delay elements will match, thus adversely affecting linearity. 
     FIG. 2 is a block diagram of a known “Vernier-type” circuit  20  for shifting the phase of a periodic input signal with a high level of precision. The circuit  20  includes a first TDLL identical to the TDLL  10  of FIG. 1 (with like elements being identically numbered) and a second TDLL  22  similar to the TDLL  10  of FIG.  1 . The second TDLL  22  includes a second delay chain  24  with (N−1) identical delay elements  26 , a second comparator  28 , and a second multiplexer  30 . Typically, the first and second delay chains  12 ,  24  are fabricated on a silicon wafer, with the delay elements of the first delay chain  12  being essentially identical to each other and the delay elements of the second delay chain  22  being essentially identical to each other. The output of the first multiplexer  18  is used as the input signal to the second TDLL  22 . Therefore, the first TDLL  10  introduces a first delay, and the second TDLL  22  introduces a second delay to the input signal as delayed by the first TDLL  10 . 
     The Vernier-type circuit  20  of FIG. 2 enables very fine step sizes with fewer delay elements than a single TDLL such as TDLL  10 . By connecting the first and second TDLLs  10 ,  22  in series, a large number of steps can be achieved using a relatively small number of delay elements, thus enabling the delay elements to be more easily matched. The number of steps is given by the product of the number of delay elements (N) in the first delay chain  12  and the number of delay elements (N−1) in the second delay chain  24 . As long as the number of delay elements in the second delay chain  24  is either (N+1) or (N−1), the step size will be the period of the input signal divided by that product. For example, if N=16, the step size will be one input signal period/(16*15)={fraction (1/240)}. Accordingly, if the input signal is 100 MHZ, the delay will be approximately 40 ps per step [({fraction (1/100)} MHZ)/(16*15)=41.7 ps]. In this example, only 31 delay elements are required to achieve 240 steps (15×16=240). A single TDLL such as the TDLL  10  of FIG. 1 would require 240 delay elements to provide 240 steps. 
     FIG. 2A is a table  32  depicting the delay steps associated with a Vernier-type circuit  20  having a first delay chain  12  with N delay elements (vertical axis) and a second delay chain  24  with N−1 delay elements (horizontal axis) where N=5. Thus, this circuit  20  provides 5*4=20 delay steps. Each delay element in the first delay chain  12  will introduce a delay step that is ⅕ (or 0.200) of an input signal period; and each delay element in the second delay chain  24  will introduce a delay step that is ¼ (or 0.250) of the input signal period. Accordingly, the circuit  20  is capable of introducing delays between 0.450 of an input signal period if the minimum delay is introduced by each delay chain  12 ,  24  and 2.000 times the input signal period if the maximum delay is introduced by each delay chain  12 ,  24 . 
     The delay steps of the Vernier-type circuit  20  are spread over two periods of the input signal. Therefore, some of the delay steps are time delays of a fraction of the input signal period and thus the delayed signal appears within one period of the input signal, while other delay steps are time delays of a fraction of the input signal period plus one entire period of the input signal and thus the delayed signal appears between one and two periods of the input signal. For example, as depicted in table  32 , delay steps identified by numerals  9 ,  13 - 14 , and  17 - 19  introduce a delay that is a fraction of the input signal period, while delay steps identified by numerals  1 - 8 ,  10 - 12 ,  15 - 16 , and  20  introduce a delay that is a fraction of the input signal period plus one entire period of the input signal. A description of the delay steps for a Vernier-type circuit  20  can be found in TTCrx Reference Manual: a Timing, Trigger and Control Receiver ASIC for LHC Detectors (Appendix A), Version 3.0, Christiansen et al., CERN-EP/MIC, Geneva, Switzerland, October 1999, incorporated fully herein by reference. 
     For periodic input signals, after at least one period of the input signal, the delays introduced by the Vernier-type circuit  20  will appear to be within one period of the input signal. For example, delay step ( 1 ) will delay a first pulse by {fraction (1/20)}th of the input signal plus one full period of the input signal. Thereafter, however, since the output signal will be periodic and will have the same period as the input signal, the input signal will appear to be delayed by just {fraction (1/20)}th of its period. The bracketed numerals in FIG. 2A represent the apparent sequential delay introduced by the Vernier-type circuit (for N=5). For example, delay step ( 1 ) will produce an apparent delay of {fraction (1/20)}th of the period of the input signal, delay step ( 2 ) will produce an apparent delay of {fraction (2/20)}ths of the period of the input signal, etc. 
     For single pulses, however, the delay steps will appear “nonuniform” in the sense that the delay steps will be spread over two periods of the input signal in a non-sequential manner. For example, the first delay step ( 1 ) will delay a pulse by a fraction of the input period plus one full period of the input signal (i.e., 1 input period+{fraction (1/20)}th of the input period). On the other hand, delay step ( 9 ), although its apparent delay is greater than the apparent delay of delay step ( 1 ), will delay a pulse by only a fraction of the input period (i.e., {fraction (9/20)}ths of the input period). 
     Although the Vernier-type circuit  20  offers very high precision, as with the first TDLL  10  described in reference to FIG. 1, at least one period of the input signal is required to lock each TDLL  10 ,  22 , thereby preventing the circuit  20  from being used to delay single (or variable period) pulses. In addition, although the delay step sizes are small, as described in reference to FIG. 2A, they are spread in a nonuniform manner over two input signal periods. 
     Accordingly, for the reasons discussed above, there is a need for apparatus and methods for introducing high precision delays that are reliable, uniform, and can be used for delaying individual (or variable) pulses. The present invention fulfills this need among others. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for introducing delay to a signal that overcomes the aforementioned problems by using a pair of TDLLs connected in series to introduce a precision delay, and compensating for nonuniformity in the precision delay with a delay compensation circuit. In accordance with certain embodiments, each of the TDLLs in the pair of TDLLs contains a pair of delay chains having identical delay elements, where one of the delay chains is coupled to a periodic clock and is used to set the step size of the other delay chain (as well as its own step size); and the second delay chain is coupled to an input signal to introduce delay to the input signal. Accordingly, since the delay for both delay chains is set by the periodic clock using one delay chain, the other delay chain may be used to introduce precise delay to an input signal without locking to the input signal, thus enabling the circuit to delay individual pulses and variable period signals in addition to periodic signals. 
     One aspect of the invention is an apparatus for introducing delay to a signal. The apparatus includes a first delay circuit configured to introduce a first delay to the signal, the first delay selected from a first set of delays within a first range and a second set of delays within a second range, a delay compensation circuit coupled to the first delay circuit, the delay compensation circuit configured to selectively introduce a second delay to the signal, and a control coupled to the first delay circuit and the delay compensation circuit, the control for configuring the first delay circuit to select the first delay and configuring the delay compensation circuit to introduce the second delay when the first delay is selected from the first set of delays within the first range. 
     Another aspect of the invention is an apparatus for introducing delay to a signal. The apparatus includes a comparator having a first input for receiving a clock signal, a second input for receiving a feedback signal, and an output for producing an error correction signal; a first delay chain having an input port for receiving the clock signal, an output port for producing the feedback signal, and a first set of delay elements, each delay element of the first set of delay elements having an error correction port for receiving the error correction signal; a second delay chain having an input port for receiving the signal and a second set of delay elements having at least as many delay elements as the first set of delay elements, each delay element of the second set of delay elements having an error correction port for receiving the error correction signal and a tap port for producing a delayed version of the signal; a selector having a plurality of inputs coupled to the tap ports of the second delay chain, a control port for receiving a selector signal to select one of the tap ports, and an output for passing one of the delayed versions of the signal. 
     Another aspect of the present invention is a method for producing uniform delay steps in a Vernier-type circuit capable of introducing a first set of delays within a first range and a second set of delays within a second range. The method includes receiving a delay indicator specifying an amount of delay to add to a signal, introducing a first delay to the signal with the Vernier-type circuit, and introducing a second delay to the input signal if the first delay is within the first range, the first and second delays together producing a third delay that is within the second range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art TDLL; 
     FIG. 2 is a schematic diagram of a prior art Vernier-type circuit; 
     FIG. 2A is a table illustrating the delay steps generated by a Vernier-type circuit similar to FIG. 2 with N=5; 
     FIG. 3 is a block diagram of an embodiment of a delay system in accordance with the present invention; 
     FIG. 4 is a schematic diagram of an embodiment of a delay system in accordance with the present invention; 
     FIG. 4A is a schematic diagram of an alternative TDLL for use in the delay system of FIG. 4; and 
     FIG. 5 is a schematic diagram of an alternative embodiment of a delay system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 is a block diagram  100  representing an overview of a circuit for introducing a precision delay into an input signal in accordance with one embodiment of the present invention. The block diagram  100  includes a precision delay circuit  102  to introduce a precision delay to an input signal received at an input port (IN), a delay compensation circuit  104  to selectively add additional delay (i.e., compensation delay) to the input signal, a controller  106  to control the precision delay circuit  102  and the delay compensation circuit  106 , and a clock  108  to produce a periodic signal. The input signal as delayed by the precision delay circuit  102  and the delay compensation circuit  104  is output at an output port (OUT). 
     The precision delay circuit  102  is capable of generating a plurality of different delay steps that are spread in a nonuniform manner over two different ranges. The delay compensation circuit  104  functions to add a compensation delay to the input signal when the precision delay to be added to the input signal falls in a particular one of the ranges such that all delay steps essentially fall within the same range. In a preferred embodiment, the two different ranges of the precision delay circuit  102  cover two consecutive clock periods of the clock  108 , with each of the ranges corresponding to a separate clock period. In this embodiment, the delay compensation circuit  104  adds a one clock period delay if the delay introduced by the precision delay circuit  102  falls within the first clock period, i.e., the first range. Therefore, all of the delay steps introduced by the precision delay circuit  102  will effectively fall within a single clock period, i.e., the second range. As will be illustrated in the following discussion of detailed embodiments, the compensation delay can be added to the input signal prior to the precision delay, after the precision delay, or, by including the delay compensation circuit  104  as part of the precision delay circuit  102 , substantially simultaneously with the precision delay. 
     FIG. 4 depicts an embodiment of a delay system  110  in accordance with the present invention. The delay system  110  includes a precision delay circuit  102 , a delay compensation circuit  104 , a controller  106 , and a clock  108 . In general overview, an input signal is received at an input port (IN). The input signal is passed through the delay compensation circuit  104  where, if needed, a compensation delay is introduced; and through the precision delay circuit  102  where a precision delay is introduced. The controller  106  controls the amount of delay added by the precision delay circuit  102 . In addition, for certain precision delays introduced by the precision delay circuit  102 , the controller  106  will prompt the delay compensation circuit  104  to add a compensation delay to the input signal. 
     The delay system  110  will now be described in greater detail. The clock  108  provides timing and synchronization information. In the illustrated embodiment, the clock  108  is coupled to the precision delay circuit  102 , the delay compensation circuit  104 , and the controller  106 . In a preferred embodiment, the clock  108  is a conventional clock that generates a periodic signal for use in digital circuits. The selection of a suitable clock for use with the present invention will be readily apparent to those skilled in the art. 
     The precision delay circuit  102  introduces precision delay to an input signal received at an input port  102   a  to produce a precision delayed version of the input signal at an output port  102   b . In the illustrated embodiment, the precision delay circuit  102  includes a first TDLL  112  and a second TDLL  114  connected in series, i.e., a Vernier-type circuit. Therefore, as in the Vernier-type circuit  20  (FIG. 2) discussed above, the delay steps introduced by the precision delay circuit  102  will span two ranges, namely, within 1 clock period of the original input signal and between 1 and 2 clock periods of the original input signal. 
     The first TDLL  112  adds a first component of the precision delay to the input signal. In the illustrated embodiment, the first TDLL  112  includes a first delay chain  116  having a first set of delay elements including (N) identical delay elements  118 , a second delay chain  120  having a second set of delay elements including at least (N) identical delay elements  122 , a first comparator  124 , and a first multiplexer  126 . The first TDLL  112  receives an input signal at an input port  112   a  and generates a delayed signal at an output port  112   b  that is a version of the input signal delayed by the first component of the precision delay. In addition, the first TDLL  112  is coupled to the clock  108  through a clock port  112   c  and to the controller  106  through a control port  112   d.    
     As will be described in detail below, the first comparator  124  establishes the amount of delay introduced by each of the delay elements  118  in the first delay chain  116  and also establishes the amount of delay introduced by each of the delay elements  122  in the second delay chain  120 , which is the delay chain that actually introduces the delay to the input signal. In a preferred embodiment, the comparator  124  is a conventional electronic circuit that can be fabricated on a semiconductor wafer and can generate a signal at its output for controlling the delay introduced at each delay element. The selection of a suitable comparator for use with the present invention will be readily apparent to those skilled in the art. 
     The first delay chain  116  includes an input port  116   a , an output port  116   b , and a plurality of error correction ports  116   c  (one for each delay element  118  in the first delay chain  116 ). A signal received at the input port  116   a  will result in a signal at the output port  116   b  that is delayed by all of the delay elements in the delay chain  116 . The error correction ports  116 c are all coupled to an output port  124   c  of the comparator  124  to receive an identical error correction signal generated by the comparator  124 . Since each individual delay element receives the same error correction signal and the delay elements are essentially identical, the error correction signal changes the delay period associated with each delay element by an equal amount. In a preferred embodiment, the delay elements are made essentially identical by fabricating them in a known manner as a single integrated circuit on a silicon wafer. As will be readily apparent to those in the art, the delay elements could be fabricated using other semiconductor materials such as Galium Arsenide to achieve delay elements with quicker response times. 
     In the illustrated embodiment, the clock  108  is coupled to the input port  116   a  of the first delay chain  116  and the first input port  124   a  of the comparator  124 . The delayed clock signal at the output port  116   b  of the first delay chain  116  is coupled to a second input port  124   b  of the comparator  124 . The comparator  124  compares the phase of the clock signal to the phase of the delayed signal out of the first delay chain  116  and generates the error correction signal. The comparator  124  modifies the error correction signal based on feedback from the output  116   b  of the first delay chain  116  until the phase of the delayed signal out of the first delay chain  116  matches the phase of the clock signal, thereby indicating that the delayed signal is delayed by one full clock period and that the delay through the first delay chain  116  is one clock period. 
     The second delay chain  120  introduces the first component of the precision delay to the input signal. The second delay chain  120  includes an input port  120   a , tap ports  120   b  for each delay element, and error correction ports  120   c  for each delay element. An input signal received at the input port  120   a  will result in delayed versions of the input signal at each tap port with each tap port producing a version of the input signal delayed proportionally to the number of delay elements the signal travels through to reach a particular tap port. 
     The second delay chain  120  includes at least as many delay elements as the first delay chain  116  and, preferably, includes the same number of delay elements. If the second delay chain  120  includes more delay elements than the first delay chain  116 , the first TDLL  112  will be capable of introducing delays that are longer than a clock period. In a preferred embodiment, the delay elements of the second delay chain  120  are made essentially identical to one another and to the delay elements of the first delay chain  116 . The delay elements are made essentially identical by fabricating both of the delay chains  116 ,  120  in a known manner on a single integrated circuit on a semiconductor wafer. 
     In the illustrated embodiment, the input port  120   a  of the second delay chain  120  is coupled to receive the input signal through the delay compensation circuit  104 . The error correction ports of the delay elements in the second delay chain  120  are all coupled to the output port  124   c  of the comparator  124  to receive an identical error correction signal generated by the comparator  124 . 
     In accordance with this arrangement, the individual delay elements of the first delay chain  116  provide delay periods of equally spaced fractions of the clock period, as opposed to the equally spaced fractions of the period of the input signal provided in the prior art circuit of FIGS. 1 and 2. The same error correction signal (the output signal of the comparator  124 ) is also used to change the delay period associated with each delay element  122  of the second delay chain  120 . Hence, the delay period added to the input signal by each delay element of the second delay chain  120  is the same as for each element of the first delay chain  116 . That is, the delay elements of the second delay chain  120  add delay to the input signal in equal fractional increments of the clock period, not the input signal period. 
     The tap ports of the delay elements of the second delay chain  120  are coupled to the multiplexer  126  to pass delayed versions of the input signal. The multiplexer  126  is a selector used to select the version of the input signal as delayed by the second delay chain  120  with the appropriate amount of delay. The multiplexer  126  is coupled to the taps of each of the delay elements in the second delay chain  120  through a plurality of input ports. In addition, the multiplexer  126  is coupled to the controller  106  to receive a selection signal. Based on the selection signal, the multiplexer  126  selects one of the delayed versions of the input signal from the second delay chain  120  and passes it to the output port  112   b  of the first TDLL  112 . The signal on the output port  112   b  is the input signal delayed by the first component of the precision delay. 
     In a preferred embodiment, the multiplexer  126  is fabricated on a semiconductor wafer using conventional technology. The multiplexer  126  is designed such that the signal propagation time through the multiplexer  126  from each one of its inputs to its output is matched, i.e., essentially identical. The design of a suitable multiplexer  126  will be readily apparent to those skilled in the art of integrated circuits. 
     By using the first delay chain  116  to lock to a periodic signal from the clock  108  and establish the delay for the individual elements in the first and second delay chains  116 ,  120  and using the second delay chain  120  to introduce delay to the input signal, the TDLL  112  is capable of introducing delay to individual (or variable period) pulses. This is accomplished by effectively removing the need for a locking period to the input signal since the first and second delay chains  116 ,  120  are essentially pre-locked to the clock signal of the clock  108 . 
     The second TDLL  114  introduces a second component of the precision delay to the delayed version of the input signal from the first TDLL  112 . The second TDLL  114  includes a third delay chain  130 , a fourth delay chain  132 , a second comparator  134 , and a second multiplexer  136 . The second TDLL  114  is identical to the first TDLL  112  with the exception that the number of delay elements in the third and fourth delay chains  130 ,  132  are different from the number of delay elements (N) in the first and second delay chains  116 ,  120 . Accordingly, the components and their functionality in the second TDLL  114  are essentially identical to those of the first TDLL  112  and will be described only where necessary. 
     The second TDLL  114  adds the second component of the precision delay to the input signal such that the input signal is delayed by the first and second components of the precision delay (and the compensation delay, if added) to produce an output signal at the output port  102   b  of the precision delay circuit  102  that is delayed by the precision delay of the precision delay circuit  102 . In a preferred embodiment, the number of delay elements in the first delay chain  116  is N and the number of delay elements in the third delay chain  130  is either N+1 elements or N−1 elements, which enables the delay system  110  to introduce delay in equal fractional steps of the clock signal. 
     The delay compensation circuit  104  is a conventional delay circuit for introducing delay to a signal. The delay compensation circuit  104  is coupled to the precision delay circuit  102  for introducing a compensation delay to the input signal to accommodate nonuniformity in the precision delay introduced by the precision delay circuit  102 . The delay compensation circuit  104  is coupled to the clock  108  to receive the clock signal for synchronization. The delay compensation circuit  104  may be a known counter or shift register. Other circuits suitable for use in the present invention will be readily apparent to those skilled in the art. 
     In a preferred embodiment, the delay compensation circuit  104  is controlled by the controller  106  to either introduce a one clock period delay to the input signal or to do nothing to the input signal. In the illustrated embodiment, the precision delay circuit  102  receives the input signal through the delay compensation circuit  104 . Accordingly, the delay compensation circuit  104  will introduce a compensation delay to the input signal prior to the precision delay circuit  102  introducing a precision delay to the input signal. Although the delay compensation circuit  104  in the illustrated embodiment is coupled to the input of the precision delay circuit  102 , it will be readily apparent to those skilled in the art that the delay compensation circuit  104  may be coupled to the output of the precision delay circuit  102 . 
     The controller  106  controls the amount of delay introduced by the precision delay circuit  102  and the delay compensation circuit  104 . In the illustrated embodiment, the controller  106  is coupled to the selection ports (Sel) of the first and second multiplexers  126 ,  136  of the precision delay circuit  102 . The controller  106  selects the amount of delay introduced by the precision delay circuit  102  by selecting a tap port in the second delay chain  120  using the first multiplexer  126  and selecting a tap port in the fourth delay chain  132  using the second multiplexer  136 . Based on the selected tap ports of the second and fourth delay chains  120 ,  132 , the controller  106  will instruct the delay compensation circuit  104  to add compensation delay to the input signal. The controller  106  may be a microcontroller, microprocessor, digital signal processor, state machine, or essentially any device for processing signals. 
     In use, the controller  106  controls the delay compensation circuit  104  such that precision delay introduced to the input signal by the precision delay circuit  102  are in a single range, e.g., a single clock period. For example, referring to table  32  (FIG.  2 A), assume the precision delay circuit  102  is capable of generating the depicted delay steps. If the controller  106  configures the precision delay circuit  102  to generate delay step  9  (i.e., 0.450), the delay step falls within the first clock period, e.g., between 0 delay and a one clock period delay. However, if the controller  106  configures the precision delay circuit  102  to generate delay step  1  (i.e., 1.050) the delay step falls within the second clock period, e.g., between a one clock period delay and a two clock period delay. Accordingly, the controller  106  will instruct the delay compensation circuit  104  to introduce a compensation delay, e.g., one clock period, to the signal when the precision delay circuit  102  is configured to generate a delay step that is in the first clock period, such as step  9 , thereby effectively shifting the precision delay for delay step  9  to the second range. By introducing the compensation delay whenever the precision delay circuit  102  will introduce a delay in the first clock period, all twenty delay steps can be effectively placed within one clock period, namely, the second clock period. 
     In an alternative embodiment, depicted in FIG. 4A, the first and/or second TDLL is  112  (FIG. 4) are configured as ring oscillators TDLL  112   a . The TDLL  112   a  is similar to the TDLL  112  described above, with like elements labeled identically in the Figures, except that the clock  108  is coupled within the TDLL  112   a  solely to the first input port  124   a  of the comparator  124 , rather than to both the input port  116   a  of the first delay chain  116  and the first input port  124   a  of the comparator  124 . Also, the delayed signal at the output port  116 b of the first delay chain  116  is coupled to the input port  116   a  of the first delay chain  116  in addition to the second input port  124   b  of the comparator  124 . In this embodiment, known oscillation initiation circuitry  125  initiates oscillations in the TDLL  112   a  that result in an oscillation signal out of the first delay chain  116 . The comparator  124  then compares the clock signal to the oscillation signal to generate the error correction signal used to establish the amount of delay introduced by each of the delay elements in the first and second delay chains  116 ,  120 . The comparator  124  modifies the error correction signal based on feedback from the output  116   b  of the first delay chain  116  until the phase of the oscillation signal out of the first delay chain  116  matches the phase of the clock signal, thereby indicating that the delayed signal is delayed by one full clock period and that the delay through the first delay chain  116  is one clock period. 
     FIG. 5 depicts an embodiment of an alternative delay system  148  for delaying an input signal. The delay system is identical to the delay system  110  of FIG. 4 with the exception that the second TDLL  114  (FIG. 4) in the precision delay circuit  102  is replaced with an extended TDLL  150 . Accordingly, only the relevant differences will be discussed in detail below, with like elements labeled identically in the Figures. The extended TDLL  150  includes an extended delay chain  156  that allows it to perform the function of the delay compensation circuit  104  (FIG. 4) in addition to introducing the second component of the precision delay. Therefore, a separate component to introduce compensation delay is not necessary in the alternative delay system  148 . 
     The extended TDLL  150  includes a first delay chain  130  with (N−1) identical delay elements  154 , an extended delay chain  156  with at least [(N−1)+(N−1)] identical delay elements  158 , a comparator  134 , and an extended multiplexer  162 . The second delay chain  156  includes a base delay chain  164  and an extension delay chain  166 , the extension delay chain  166  having at least as many delay elements at the base delay chain  164 . The delay elements in the first and second delay chains  130 ,  156  are essentially identical. In a preferred embodiment, the delay elements are made essentially identical by fabricating the delay chains  130 ,  156  adjacent to one another in a known manner on a semiconductor wafer as a single integrated circuit (IC). The number of delay elements in the extended delay chain  156  is at least twice the number of delay elements in the first delay chain  152  of the extended TDLL  150 . 
     The comparator  134  establishes the amount of delay introduced by each of the delay elements in the first and the extended delay chains  130 ,  156  as described above in reference to FIG.  4 . The extended delay chain  156  introduces the second component of the precision delay to the input signal and, if needed, the extended delay chain  156  introduces compensation delay to the input signal. 
     In the illustrated embodiment, the extended delay chain  156  includes twice as many delay elements as the first delay chain  130 . Accordingly, the extended delay chain  156  is capable of delaying the input signal by up to twice the delay through the first delay chain  130 , i.e., two clock periods. The base delay chain  164  is used to add the second component of the precision delay if delay compensation is not necessary. If delay compensation is necessary, the extension delay chain  166  is used to add the second component of the precision delay. It will be readily apparent to those skilled in the art that if the number of delay elements in the extended delay chain includes more than twice as many delay elements as the first delay chain  130 , the extended delay chain  156  will be capable of introducing delays that are greater than two clock periods. 
     The delay elements in the extension delay chain  166  can be used to add a delay that is the equivalent of the corresponding delay element in the base delay chain  164  plus one clock period. For instance, between an input port  156   a  of the extended delay chain  156  and a tap port  156   b  of the third delay element  158   a  within the base delay chain  164  a delay of 3/N−1 of a clock period is introduced, whereas between the input port  156   a  and a tap port  156   c  of the third delay element  158   b  within the extension delay chain  166  a delay of (1+3/N−1) clock cycles is introduced. Thus, the extended delay chain  156  serves the function of both the second precision delay component and delay compensation. For example, assume each delay element of the extension delay chain  166  introduces a 0.1 clock period delay. If the second component of the precision delay requires a 0.1 clock period delay and delay compensation is needed, the first delay element of the extension delay chain  166  may be used to introduce both the 0.1 clock period precision delay and the 1.0 clock period delay required for delay compensation. If no delay compensation is required, the first delay element of the base delay chain  164  is used to introduce solely the 0.1 clock period delay. 
     The extended multiplexer  162  is a selector used to select the version of the input signal as delayed by the extended delay chain  156  with the appropriate amount of delay. The multiplexer  162  is coupled to the tap ports of each of the delay elements in the extended delay chain  156  through a plurality of input ports. In addition, the multiplexer  162  is coupled to the controller  106  to receive a selection signal at a selection port (Sel). Based on the selection signal, the multiplexer  162  selects one of the delayed versions of the input signal from the extended delay chain  156  and passes it at an output port. The multiplexer  162  is a conventional multiplexer such as multiplexer  126  described above in reference to FIG.  4 . 
     In use, an input signal is received at an input port (IN). The input signal passes through the first TDLL  112 , which is configured by the controller  106  via the first multiplexer  126  to add a first component of the precision delay to the signal. The input signal then passes through the extended TDLL  150  where a second component of the precision delay is added. If the controller  106  identifies that the combination of delays introduced by the first TDLL  112  and the extended TDLL  150  will result in the total delay falling within a first range, the controller  106  controls the extended multiplexer  162  to select the appropriate tap port from the extension delay chain  166  rather than the corresponding tap port from the base delay chain  164  (which are offset from each other by exactly one clock period), thereby effectively placing the total delay in the second range. If on the other hand, the total delay will fall within the second range without delay compensation, the controller  106  selects the appropriate tap port from the base delay chain  164 . Accordingly, the total delay will always occur in a single range, e.g., the second clock cycle after the original input signal. 
     Although the extended TDLL  150  is described and illustrated as a replacement for the second TDLL  114  in the circuit of FIG. 4, it will be readily apparent to those skilled in the art that a similar extended TDLL  150  could be used to replace the first TDLL  112  or both TDLLs  112 ,  114  in the circuit of FIG.  4 . 
     The present invention may be used for a wide range of applications. The following applications are an illustrative, but by no means exhaustive, list of potential uses for the present invention. 
     The present invention may be used in periodic delay applications, which involve the delay of a periodic signal. For example, the present invention may be used to shift the phase of a periodic signal and to compensate for propagation delay across a circuit, i.e., de-skew. In addition, the present invention may be used for high frequency phase shifting (e.g., for clock frequencies of up to one (1) GHz and beyond), phased array delay generation, and frequency measurement. 
     The present invention also may be used to delay a non-periodic signal, such as might be needed for data skewing and data storage. 
     The present invention may be used with a digital controller for generating pulses with precision widths, generating long pulses with precision widths, and generating a synchronous pulses. In addition, the present invention may be used as a precision frequency divider or multiplier, or as an arbitrary pulse train generator for random number and pattern generators. 
     The present invention may be used in process control systems such as stimulus/response systems, e.g., HRR radar, where a stimulus is applied to a system and a precise time later a data sample is collected. In addition, the present invention may be used for multiple sample collection and for feedback control. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.