Abstract:
A method of analysis of a time interval between two selected measurement edges of interest includes locking a plurality of at least three substantially interchangeable oscillators to a common reference frequency, the oscillators containing digital locked-loop (DLL) circuit architecture. The method includes operating one oscillator as a timebase oscillator, and operating the other oscillators as edge-resettable measurement oscillators. The method further includes coupling one oscillator with a switched and physically-immutable parametric variation, producing an offset in the frequency of the coupled oscillator relative to the frequency of the other oscillators. The method includes phase-aligning each of the measurement oscillators to a triggering pulse created by one of selected measurement edges of interest, oscillating each phase-aligned measurement oscillator until its phase matches the phase of the timebase oscillator, and counting the number of oscillation cycles of the phase-aligned measurement oscillator from the time of phase-alignment until the time of phase matching.

Description:
BACKGROUND OF THE INVENTION 
   U.S. Pat. No. 4,164,648 (hereinafter referred to as “HP TIA,” where “TIA” is an abbreviation for “time interval analyzer”) describes an approach to time interval measurement using three analog phase locked loops. Additionally, this patent identifies the shortcomings of a two-ring oscillator time vernier approach, described for example in U.S. Pat. No. 6,295,315 and PCT International Patent WO 01/69328 A2 (hereinafter referred to as “traditional”), currently used almost exclusively. In accordance with HP TIA, all loops are locked to a common reference frequency. Two of the loops differ slightly from a third loop, referred to as the “reference.” Implementation is almost exclusively via analog circuitry. 
   Using analog PLL circuit techniques of HP TIA, three oscillators are locked to a system clock. One is a reference signal oscillator, and the remaining two are measurement oscillators. The measurement oscillators are locked to a frequency that differs from the reference signal by 1+1/N or 1−1/N, where N is an integer. Given an arbitrary phase and frequency start of a measurement oscillator, the phase of the measurement signal gains (or loses) until it matches the phase of the reference signal. Phase alignment requires a number of reference signal clock cycles, wherein the number of cycles is determined by the value of N. 
   The traditional circuit approach uses only two free running ring oscillators, whose slightly differing frequencies are calibrated at the start of the circuit operation, usually just after reset. The phase measurement is made using a measurement edge to start each oscillator and, like the HP TIA circuit above, counts how many clock cycles elapse for the phases to align. The free running oscillators, even if initially calibrated accurately, are unlikely to retain calibration at the time of measurement, if measurement occurs significantly later. The traditional circuits leave the oscillators off before a measurement, such that the precise dynamic bias conditions that existed during calibration will not apply during the measurement, if the oscillators are off for any significant period of time. If the oscillators are left on, the elapsed time between calibration and measurement may allow for a drift in the frequencies in either or both of the oscillators. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with embodiments of the present invention, a circuit operable to perform analysis of a time interval between two measurement edges of interest in a data bit stream is provided. The circuit includes a plurality of at least three substantially interchangeable oscillators configured to be locked to a common reference frequency. The oscillators include digital locked-loop (DLL) circuit architecture. One oscillator of the plurality of oscillators is operable to function in the role of a timebase oscillator, whereas the other oscillators are operable to function in the roles of edge-resettable measurement oscillators. One oscillator of the plurality of oscillators includes a switched and physically-immutable parametric variation operable to produce an offset in the frequency of the oscillator relative to the frequency of the other oscillators. The circuit additionally includes a plurality of phase detectors, each phase detector coupled to one of the plurality of oscillators. 
   In accordance with embodiments of the present invention, a method of analysis of a time interval between two selected measurement edges of interest in a data bit stream (or streams) is provided. The method includes locking a plurality of at least three substantially interchangeable oscillators to a common reference frequency, the oscillators containing digital locked-loop (DLL) circuit architecture. The method includes operating one oscillator of the plurality of oscillators as a timebase oscillator, and operating the other oscillators as edge-resettable measurement oscillators. The method further includes coupling one oscillator of the plurality of oscillators with a switched and physically-immutable parametric variation, producing an offset in the frequency of the coupled oscillator relative to the frequency of the other oscillators. The method includes phase-aligning each of the measurement oscillators to a triggering pulse created by one of selected measurement edges of interest, oscillating each phase-aligned measurement oscillator until its phase matches the phase of the timebase oscillator, and counting the number of oscillation cycles of the phase-aligned measurement oscillator from the time of phase-alignment until the time of phase matching. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1A  is a block diagram of a data sampler portion of a digital TIA, in accordance with the present invention; 
       FIG. 1B  is a block diagram of a representative oscillator of at least three substantially identical oscillators of a digital TIA, in accordance with an embodiment of the present invention; 
       FIG. 2  is a flow diagram depicting an operational sequence of vernier time measurement circuitry of a digital TIA, in accordance with an embodiment of the present invention; 
       FIG. 3  is a timing diagram showing processes associated with vernier time measurement using a digital TIA, in accordance with embodiments of the present invention; 
       FIG. 4  depicts an architecture embodiment for RFLAT  141 , which creates latched signal representations of the rising and the falling data edges in a data bit stream preceding the measurement edge of interest; 
       FIG. 5  shows an exemplary architecture for a DataSample block (a 11  Data Sample blocks are identical in design), in accordance with embodiments of the present invention; and 
       FIG. 6  is a timing diagram depicting the order of timing events of a digital TIA in accordance with an embodiment of the present invention, where a rising data edge of a data bit stream is chosen as the beginning of the measurement time interval. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  is a block diagram of data sampler portion  10  of a digital TIA, in accordance with the present invention. Applying digital circuitry to the topology of HP TIA, three or more oscillators  100 - 1 ,  100 - 2 ,  100 - 3 , . . . ,  100 - k  provide the time vernier measurement circuitry to determine the time between two data edges of interest. Any oscillator  100 - 1  through  100 - k  could be used as the reference oscillator. The remaining oscillators would be phase aligned to the data edges. For example, oscillator  100 - 1  is used as a reference, whereas at least two oscillators  100 - 2  through  100 - k  are phase aligned with selected data edges. Phase alignment is accomplished by stopping the oscillator for a duration approximately T/2 (T is the oscillator&#39;s period) and re-starting the oscillator in alignment with the phase of the selected data edge in data bit stream  142 . The data edge itself creates the triggering pulse to stop and re-start the oscillator. 
   In operation in accordance with the methodology of the present invention, oscillator  100 - 2  is phase aligned to the data edge defining the start of a time-interval of interest. However, in accordance with the present methodology, the oscillator roles can and should be commutated (i.e., interchanged) across a series of measurements. Commutating the oscillator roles can help compensate for effects of manufacturing differences, mismatch, or noise effects during the time-interval measurement. 
   Oscillators  100 - 2  through  100 - k  are phase locked to oscillator  100 - 1  until they receive a Trig — In pulse  123 - 2  through  123 - k  from data sample blocks  143 - 2  through  143 - k  respectively. The Trig — In pulses momentarily stop the oscillators and then restart them with a starting phase aligned to data edges of interest to the measurement. Oscillators  100 - 2  through  100 - k  provide counting signals for time interval measurements between the data edges of interest. Data sample blocks  143 - 2  through k also include input signals  142  (data bit stream),  145  (edge#) and  144 - 2  through k (respective rise/fall). Inputs  142 ,  144 ,  145 ,  147 , and  148  are discussed below in more detail with regard to  FIGS. 3 ,  4 , and  5 . 
     FIG. 1B  is a block diagram of representative oscillator  100 - 1  of at least three substantially identical oscillators  100 - 1  through  100 - k  of a digital TIA, in accordance with an embodiment of the present invention. At least three of these oscillators are used to provide vernier time measurement circuitry for the digital TIA circuit. One oscillator, without loss of generality designated as oscillator  100 - 1 , is configured to be the timebase clock, and the other two or more oscillators form edge resettable measurement oscillators, such that they are reset by detecting the edge of an incoming signal. In this example embodiment, oscillator  100 - 1  includes filter  101  that employs phase detector output  110  or  111 . It also includes star reset state machine  105 , which handles sequencing of multiplexers  112 – 115  to control a signal T/2 monostable time and oscillator start. Signals  117  and  118  are feedback signals for oscillator  100 - 1 , and signals  119 ,  130 ,  124 , and  128  are input control signals for multiplexers  112 – 115 . In some embodiments, oscillator  100 - 1  may include other components or may omit components from  FIG. 1B . Many alternative embodiments are within the scope of the present invention. 
     FIG. 2  is a flow diagram depicting operational sequence  200  of vernier time measurement circuitry including oscillators  100 - 1  through  100 - k  of a digital TIA, in accordance with an embodiment of the present invention. The discussion of  FIG. 2  frquently refers to components shown in  FIGS. 1A and 1B . After resetting, oscillator  100 - 1  has digital locked-loop (DLL) frequency multiplier constants defined in control registers, and state machines are reset to initiate the lock phase in operation  201 , allowing both digital delays  102  and  103  to be adjusted to reference clock  116  with the relation:
   F   ref     —     clock =1/(2 *T   delay   *N   mult ). 
   Once DLL  100 - 1  locks, the frequency of oscillating delay  102  will be N mult  times that of reference clock  116 . The other delay  103  will have the same T delay  as oscillating delay  102 , but it is configured as a monostable pulse stretcher by star reset state machine  105  and multiplexers  112 — 115 . This monostable delay  103  is triggered by the edge event of Trig — In  123 - 1 . The pulse it produces resets or holds off delay  103  (operating as an oscillator in this example) in operations  203 - 1 ,  203 - 2  and restarts it on the falling edge of monostabled pulse  123 - 1 . The phase of the restarted oscillator is now related to the edge event of Trig — In  123 - 1 . During Oscillator Monostable reset operations  203 - 1 ,  203 - 2 , the frequency of DLL block forming timebase oscillator  102  is changed in operation  202  by a small amount by switching in immutable parasitic load capacitance  133 . 
   Alternatively, capacitance  133  may be added to monostable DLL oscillator  100 - 2  or reference DLL oscillator  100 - 1 . The bold arrows from filter  101  to delay oscillators  102  and  103  represents coarse delay adjustment to tune oscillators  102  and  103 . Similarly, the thin arrows from filter  101  to oscillators  102  and  103  represents an enabler that adds parasitic capacitance for fine adjustment of oscillators  102  and  103 . It should be noted that parasitic adjustment is usually used only when delay  103  is used as an oscillator. The small capacitance amount will be derived in additional measurement operations following output operations  205 - 1 ,  205 - 2 . When the phase aligns in operations  204 - 1 ,  204 - 2 , the number of oscillation cycles N 0  that have accumulated in digital counter  104  are recorded in operations  205 - 1 ,  205 - 2 . In some embodiments, there will be a certain tolerance between the matches which may be evaluated from subsequent measurements even without the addition of the parasitic capacitance. 
   By letting the delay oscillators  102  and  103  continue to run for N additional locks, where N can be any positive integer, indicated by adjacent arrows in  FIG. 2 , iterating operations  204 - 1 ,  205 - 1 , and  204 - 2 ,  205 - 2 , provides additional measurements that produce output data with a known relation. From this larger data set, noise from the initial measurement can be mitigated, and the frequency difference between the oscillators can be assessed. In the event that the frequency difference is too small or zero, the phase alignment may take too long or never happen. In this case, a watchdog timer or counter resets the circuit and indicates a “did not resolve” error state. The control software/hardware makes an adjustment to immutable switchable capacitance  133  so as to change the frequency offset of delay timebase oscillator  102 . This technique can also be used to manipulate the time resolution of the circuit. 
   In a variation of this circuit, timebase delay  102  is a fixed clock source. By using one circuit block  103 , the edges of Trig — In  123 - 1  can be measured against the fixed timebase if immutable switched capacitive load  133  is switched on delay  103 . This configuration allows the edge of Trig — In  123 - 1  to be compared against delay  102 , which is operating as a fixed timebase clock. If enough edges are measured, this forms the basis of an rms jitter measurement. 
     FIG. 3  is a timing diagram showing processes associated with vernier time measurement  200  using a digital TIA, in accordance with embodiments of the present invention. In this example, Edge 1  and Edge 2  represent two instantiations of Sig  1  (of  FIG. 1B ), and f sys  is a reference clock, such as clock  116  ( FIG. 11B ). f pll     —     lo1    306  and f pll     —     lo2    305  are measurement clock signals. N 0 , N 1 , and N 2  are counters. Edge 1   302  rising halts digital delay  103  f pll     —     lo1  at edge  302  for half a period before releasing. It takes 10 cycles before this oscillator&#39;s phase  302  matches digital delay frequency  102  f pll     —     hi    304  (timebase clock) at event  311 . Edge 2   303  rising halts f pll     —     lo2    305  for half a period before releasing. It takes 24 cycles before this oscillator&#39;s phase matches f pll     —     hi    304  (timebase clock)  312 . N rot(ate)    310  is an extension inside digital filter block  101  (of  FIG. 1 ) that counts how many times N 1    308  has rolled over (reached its maximum count and then returned to its minimum count). By recording N 1    308  and N rot    310  for each oscillator match, time differences larger than one timebase clock period can be measured. The timing diagram of  FIG. 3  depicts each of them counting after an event (such as Edge 1  or Edge 2 ) has caused each to start. 
   The counters N 0 , N 1 , and N 2  ( 307 – 309 , respectively) count the number of cycles for the phase alignment of the measurement oscillator to the reference oscillator. The maximum number of cycles is N max , defined as:
 
 N   max   =T   ref /( T   ref   −T   mx ),
         where T ref =1/f pll     —     hi  T mx =1/f pll     —     lox          

   In the above example T is a time measurement, such that T mx  is the time at an edge being measured, and T ref  is the time of a reference edge. The letter “x” denotes which edge is being measured. When an edge ( 1  or  2 ) initiates a counter (N) the first value read multiplied by (T ref −T mx ) gives the time between that edge ( 1  or  2 ) being measured and the next reference edge. The time differences for each edge and the value of N rot    310  gives the total time difference between the edges. If the counters are allowed to run, the subsequent readings will increment by N max  and indicate the fractional mismatch (reference to measurement)in the oscillators. A plurality of these readings may be used to compute an average in order to reduce the effects of noise. Although this example refers to counters  307 – 309  and edges  1  and  2 , there may be more counters and edges in other embodiments. 
   Referring again to  FIG. 1A , rise and fall edge latches (RFLAT)  141  provide n-dimensional open signals rising edge latch open (RElat)  147  and falling edge latch open (FElat)  148  to data sample blocks  143 - 1  through  143 - k .  FIG. 4  depicts an architecture embodiment for RFLAT  141 , which creates latched signal representations  147 ,  148  of the rising and the falling data edges in data bit stream  142  preceding the measurement edge of interest. If a rising edge ‘n’ is the edge of interest, then the preceding falling data edge latches output signals RElat[n], i.e.,  147 ( n ). If a falling edge ‘n’ is the edge of interest, then the preceding rising data edge latches output signal FElat[n], i.e.,  148 ( n ). 
   For example, rise/fall 1  input  144 - 1  selects whether the beginning of the measurement edge is rising or falling. Rise edge latch (RL) 1   401  or fall edge latch (FL) 1   411  output  402  or  403  (depending on rise/fall 1  at MUX 1   412  output  404 ) is then used to prepare RL 2   421  and FL 2   422  for triggering. The remaining latches RL 2  . . . ‘n’  421 – 431  and FL 2  . . . ‘n’  422 – 432  are cascaded. RL 2 &#39;s output resets RL 3  to prepare it for the next falling edge in data bit stream  142 . Similarly, FL 2 &#39;s output resets FL 3  to prepare it for the next rising edge on the data bit stream. Once RL 1  . . . ‘n’ and FL 1  . . . ‘n’ are latched, they are cleared and reset by MeasRS signal  146  in order to be re-latched. The MeasRS pulse immediately precedes the time interval measurement. 
   Referring to  FIG. 1A , important conditions for the operation of the Oscillator Triggering Block (composed of data sample trigger blocks  143 - 2  through  143 - k  and edge latch block  141 ) are:
         1. Allow only the data edges of interest to flow to oscillator Trig — In nodes  123 - 1  through  123 - k.      2. The oscillator triggering pulses (stop and re-start) received at Trig — In nodes  123 - 1  through  123 - k  follow as directly as possible from the data edges in data bit stream  142 , i.e., no latches and as few gates as possible between data bit stream  142  and oscillator Trig — In  123 - 1  through  123 - k.      3. Trigger pulse  123 - 1 , . . . , 123 - k  for an oscillator  100 - 1  through  100 - k  disables a running oscillator and then re-enables the oscillator at a time T/2 (T is the oscillator&#39;s period) after the disable edge.   4. Include the flexibility to phase align the oscillators to either rising or falling data edges.   5. Phase align a multiplicity of oscillators  100 - 1  through  100 - k  with data edges.   6. Create trigger pulses  123 - 2  through  123 - k  based on selected data edges  1 , . . . , ‘n.’       

   In order to provide the optimum implementation conditions, pass gates can be used between the data and the oscillator Trig — In node. The use of pass gates minimizes alterations to the timing and shape of the data edges as they propagate to oscillator Trig — In. In one example, the pass gates are opened by edges immediately preceding the data edges of interest. The pass gates are closed immediately following the data edges of interest. Pass gate open signals for successive measurement edges  1  . . . n are created by a cascade of latches. 
   The combination of Edge Latch Block  141  and Data Sample blocks  143 - 2  through  143 - k  creates triggering pulses for oscillators  100 - 2  through  100 - k , as depicted in  FIG. 1A . Data Sample blocks  143 - 2  through  143 - k  are identical, which allows oscillators  100 - 2  through  100 - k  to be triggered in any chronological order. The oscillators themselves are identical in design and are well matched. RFLAT block  141  provides pass gate open signals for any desired data edge  1  . . . ‘n’ to every Data Sample block  143 - 2  through  143 - k . The Data Sample selection signals, namely edge#  145  and rise/fall 2  . . . ‘k’  144 - 2  through  144 - k  are used to choose any edge  1  . . . ‘n’ of either type (rise or fall) to be used in the time interval measurement 
     FIG. 5  shows an exemplary architecture for DataSample 2  block  143 - 2  (in this example, all Data Sample blocks are identical in design), in accordance with embodiments of the present invention. Data Sample blocks receive bit stream  142 , RElat  147 , FElat  148 , edge#  145 , and rise/fall signals  144 - 2  through  144 - k  as inputs, and provide oscTRIG signals  123 - 2  through  123 - k  to Trig — In nodes of oscillators  2  through k (respectively) of  FIG. 1 . To accomplish this function in DataSample 2  block  143 - 2 , pass gate  502  is opened and closed surrounding the desired measurement edge in bit stream  142 . The leading edge of triggering pulse  123 - 2  (output from NOR gate  503  using input  501 ) stops the oscillator, and the trailing edge restarts the oscillator in phase with the data edge of interest. 
   Pass gate open signals coming from RElat  147  and FElat  148  are selected with multiplexers M 1 , M 2 , and M 3 . Edge# input  145  to n-way multiplexers M 1  and M 2  selects which data edge  1  . . . ‘n’ is to be used as the pass gate open signal. Rise/fall 2  signal  144 - 2  to rise/fall multiplexer M 3  selects whether pass gate open signal  508  precedes a rising or falling data edge. 
   Pass gate close signals  506 ,  507  are generated using DSRL 2  and DSFL 2  (wherein “DSRL” stands for “data sample rise edge latch,” and “DSFL” stands for “data sample fall edge latch”) by latching on the data edge of interest passing through pass gate  502 . If the data edge of interest is a rising edge, then DSRL 2  latches on it. If the data edge of interest is a falling edge, then DSFL 2  latches on it. DSRL 2  and DSFL 2  latches are cleared and reset by MeasRS pulse  146  sent immediately prior to a time interval measurement. Pass Gate Control blocks PC 1  and PC 2  receive open and close input signals and generate the proper logic signals to open and close pass gate  502 . PC 1  and PC 2  may each, for example, be as simple as a single logic gate. 
   The data edge of interest is thus passed through pass gate  502  to XNOR (or XOR) gate  503 , which generates oscillator triggering pulse TRIG — In  123 - 2 . T/2 timing delay  505  creates the proper enable, disable, re-enable triggering pulse shape for oscillator TRIG — In pulse  123 - 2 . 
     FIG. 6  is a timing diagram depicting the order of timing events  600  of a digital TIA in accordance with an embodiment of the present invention, where rising data edge  603  of data bit stream  142  is chosen as the beginning of the measurement time interval. The third falling edge  604  following rising edge  603  is selected as the end of the measurement time interval. The order of events is similar if a falling edge is selected as the first measurement edge. Timing events  600  include:
         1. Select rising edge  603  as first measurement edge (set rise/fall  144 - 1  and rise/fall 2   144 - 2  signal lines to rise). Precharge block  504  in DataSample 2  charges the inputs to XNOR  503  to logic 0 to prepare for a rising edge propagating through pass gate  502 .   2. Select edge#  145 =1 for DataSample 2   143 - 2 .   3. Select falling edge as the second measurement edge (set rise/fall 3  signal line  144 - 3  to fall). Precharge block  504  in DataSample 3   143 - 3  charges the inputs to XNOR  503  to logic 1 to prepare for a falling edge propagating through pass gate  502 .   4. Select edge#  145 =4 for DataSample 3   143 - 3 .   5. Pulse and release MeasReset line  146  to allow the latches to look for edges.   6. RL 1   401  in RFLAT  141  latches on the falling edge preceding the rising data edge  603  of interest, generates RElat[1]  147 - 1 , which opens pass gate  502 , in DataSample 2   143 - 2 . M 3 &#39;s rise/fall 2   144 - 2  signal selects PC 1 &#39;s output  605  to control pass gate  502 .   7. The rising edge of interest propagates through pass gate  502  and arrives at XNOR (or XOR)  503  and OSC T/2 delay block  505 .   8. Osc2TRIG  123 - 2  steady state enable level goes to disable at this time point.   9. Latch DSRL 2   606  in DataSample 2   143 - 2  finds the rising measurement edge propagating through pass gate  502 , and sends a signal to PC 1  to close pass gate  502 .   10. Osc2TRIG&#39;s level  607  goes back to enable T/2 after the rising measurement edge reaches XNOR (or XOR) gate  503 , re-enabling oscillator 2   100 - 2  in T/2 delayed synchronization with rising measurement edge  603 .   11. Concurrently with operations  6 – 10  listed above, FL 3  in RFLAT latches on the rising edge preceding the third falling data edge after the rising beginning of measurement edge.   12. FElat[3] signal  148  is selected by M 2  in DataSample 3   143 - 3 , and pass gate  502  is opened. M 3 &#39;s rise/fall 3  signal  144 - 3  selects PC 2 &#39;s output  608  to control pass gate  502 .   13. The falling edge of interest  604  propagates through pass gate  502  and arrives at XNOR (or XOR)  503  and OSC T/2 delay block  505 .   14. Osc3TRIG  610  steady state enable level goes to disable at this time point.   15. Latch DSFL 2   609  in DataSample 2  finds the falling edge propagating through pass gate  502  and sends a signal to PC 2  to close pass gate  502 .   16. Osc3TRIG level  610  goes back to enable T/2 after rising measurement edge  603  reaches XNOR (or XOR) gate  503 , re-enabling oscillator 3   100 - 3  in T/2 delayed synchronization with the third falling edge  604  following rising beginning of measurement edge  603 .       
   Previously described HP TIA was implemented using analog PLL circuitry. The locked PLLs were made open loop before a measurement was made. Opening an analog control loop was not trivial and employed some analog design skill. Embodiments of the present invention use the topology described in HP TIA, except implemented with digital locked loop (DLL) circuitry to integrate more easily with digital processes and to provide enhanced noise assessment and cancellation. The control loop of the DLL according to embodiments of the present invention is simpler to keep stable and to make open loop when a measurement is made. 
   One of the operations of the traditional TIA technology is to offset the frequencies of the oscillators relative to one another by a small, but known, frequency. In the case of the HP TIA PLLs, the slightly offset PLL frequencies severely compromise the loop bandwidth of the circuit and hence its ability to keep the phase locked in the presence of noise. Traditional techniques, in which the ring oscillators are calibrated, suffer from time taken to calibrate and the lack of control after the calibration. 
   Embodiments of the present invention lock all of the oscillators to the same low multiple of the system clock and use only a physically immutable control (e.g., capacitance change) of the reference DLL to produce a small offset in the frequency of the reference clock. The control of all oscillators is suspended during measurement to allow noise to affect them all in a substantially identical way. The small frequency offset induced in the reference DLL does not need to be known, as two or more sequential phase measurements provide a measure of the count to align phase (i.e., 0 radians) and the count to align phase 2τ radians later (slip or advance in phase of one DLL relative to the other). The second count relates to the actual frequency difference of the two DLLs. It also serves to provide noise-averaging statistical information concerning the accuracy of the first measurement count. Phase detectors  110 ,  111  in each oscillator  100 - 1  through  100 - k  indicate when the phase of either of the two DLLs aligns with the reference clock. The first time this occurs is defined as 0 radians. If the oscillations are allowed to continue, the phase will eventually align again at phase 2τ radians relative to the previous alignment at 0 radians. 
   There may still be error incurred by slight offsets of the frequencies. With use of identical oscillator layouts, a commutation process is used to interchange the roles of reference and measurement oscillators. The mathematics of the time commutation process interchangeably designates different circuits for the same function to compensate for error offsets in frequency and delay. The pulse conditioning circuitry and the DLLs are intended to be of identical layout in most embodiments. This enables, for example, three-way commutation among three nominally identical oscillators for pulse steering and three-way commutation for the DLL circuitry. Processing of the acquired sets of commutated data compensates for noise and systematic offsets in the measurements. 
   Most traditional TIAs make the assumption that any noise processes on the oscillation or phase detection is gaussian. However, power supply noise is known to contain systematic noise from the dominant system clock, which is not gaussian if the signals being measured have any temporal relation to this system clock. Embodiments of the present invention use control over the input signals to search for this clock noise and its effects on the measurement data and to select the optimum time to make the phase measurement. Additionally, whether it is from the power supply or through the substrate, the oscillators have power supply and substrate noise suppressed by using standard layout techniques used by analog designers. Namely, all oscillators use the same power supply lines, and all oscillators are laid out substantially symmetrically to maximize sharing the same noise sources identically with all elements of the oscillators. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.