Abstract:
The present invention provides a wide tracking range phase locked loop (PLL) circuit that achieves minimal jitter in a recovered clock signal, regardless of the source of the jitter (i.e. whether it is in the source or the transmission media). The present invention PLL has automatic harmonic lockout detection circuitry via a novel lock and seek control logic in electrical communication with a programmable frequency discriminator and a code balance detector. (The frequency discriminator enables preset of a frequency window of upper and lower frequency limits to derive a programmable range within which signal acquisition is effected. The discriminator works in combination with the code balance detector circuit to minimize the sensitivity of the PLL circuit to random data in the data stream). In addition, the combination of a differential loop integrator with the lock and seek control logic obviates a code preamble and guarantees signal acquisition without harmonic lockup. An adaptive cable equalizer is desirably used in combination with the present invention PLL to recover encoded transmissions containing a clock and/or data. The equalizer automatically adapts to equalize short haul cable lengths of coaxial and twisted pair cables or wires and provides superior jitter performance itself. The combination of the equalizer with the present invention PLL is desirable in that such combination permits the use of short haul wires without significant jitter.

Description:
This invention was made with Government support under contract number DE-AC02-98CH10886, awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to an apparatus and method for a wide tracking range phase lock loop (“PLL”) with exceptionally low jitter. In particular, the present invention relates to PLL applications that require low phase noise or zero crossing jitter for encoded transmissions. A PLL of the present invention separates the clock from the encoded data such that the recovered clock and data are both reduced significantly in jitter. This is very helpful in accurate position control applications that require precise phase, spatial or time measurement or control. 
     BACKGROUND OF THE INVENTION 
     It is often necessary in communication, digital audio and similar applications to lock to an external clock or reference while generating a new clock source that is immune to various sources of phase and frequency noise in the incoming reference signal. In order for a receiver to receive and capture data, as referenced to a transmitter&#39;s clock signal, it is necessary for the receiver to have a clock signal that is synchronized or phase aligned to the same clock signal. 
     Phase locked loop (PLL) circuits have been commonly used for the purpose of generating a signal in a preferred phase relationship relative to another signal. A PLL carefully adjusts to its own clock (the “local oscillator”) to bring it into precise alignment with some external signal (the “reference clock”). In serial data communications and similar applications, the reference clock is often embedded in a stream of data bits. It is the PLL&#39;s task in a clock recovery subsystem to align its local oscillator with the reference clock information embedded in the data stream. The PLL circuit within the receiver can adjust its local clock signal frequency to a multiple of the reference signal frequency, thereby phase aligning the two signals together. The phase relationship between the reference signal and the local clock signal is then referred to as being locked. Once properly aligned, the local oscillator can be used to clock bits out of the data stream, sampling each data baud right in the center at the point of maximum noise immunity. 
     As illustrated in FIG. 1, a conventional PLL  10  provides a feedback system combining a voltage-controlled oscillator (VCO)  12  and a phase comparator or detector  14  in electronic communication therewith such that the oscillator frequency or phase accurately tracks that of an applied frequency or phase-modulated signal, respectively. In a PLL, the error signal from the phase comparator is the difference between the input frequency or phase and that of the signal fed back. If f in  does not equal f VCO , phase comparator  14  generates a phase error output signal that is a measure of the phase difference. The phase error signal, after being filtered by loop filter  16  and amplified by amplifier  18 , causes f VCO  to deviate in the direction of f in . The VCO will eventually “lock” to f in  so as to maintain a fixed relationship with the input signal. 
     In this application, any imperfections in a transmit clock that is used to construct a data stream may compromise the ability of the PLL to properly align its local oscillator. Imperfections in the transmit clock are sometimes classified as frequency offsets, wander or, as used herein, “jitter”. Jitter, a common problem associated with PLL circuits, can be defined as abrupt, spurious variations in the phase of successive pulses as referenced to the phase of a continuous oscillator, causing deleterious variations in the output frequency. As the jitter frequency on the input signal increases, the bandwidth requirements of the PLL likewise increase. The precision of the recovered clock, therefore, is a limitation on system performance. 
     With the introduction of more sophisticated services involving the transfer of large amounts of information such as digital data, video or voice data, communication networks must provide predictably superior levels of performance while ensuring the maintenance of data quality. In a digital communication system, for instance, one application of a PLL may be to provide a “clean” clock signal which has the same frequency and phase as a received digital data signal that is to be decoded. A typical digital PLL design uses a reference clock having a frequency that is much higher than that of an incoming data signal to generate the output clock. This approach, however, can only guarantee a phase lock that is within one clock period of the reference clock. This error in the phase lock constitutes jitter. In analog conventional PLL circuits jitter is commonly caused by the errors in the filtering of the control voltage and VCO noise. 
     Once jitter contaminates a signal, it propagates through a system, often increasing as it passes through various devices. PLLs have therefore traditionally found limited use in those systems requiring very high stability. Systems incorporating radar oscillators, for example, require very high stability that can only be provided by narrowing the loop bandwidth. This in turn degrades the frequency tracking performance of the PLL and detracts from an inherently desirable feature of PLL oscillators. When the loop gain of the PLL is high, the pull-in and lock-in times are short, yet the sensitivity to noise (both internal and external) becomes high, resulting in large phase jitter and frequency jitter in the output. On the other hand, when the loop gain of the PLL is low and the bandwidth is narrow so as to remove noise, the pull-in and lock-in capabilities for the frequency offset and the initial phase difference become low, resulting in narrower pull-in and lock-in ranges. The narrow bandwidth of the loop not only takes a long time to achieve the lock-in state for a large frequency offset but also presents difficulty in holding the lock-in state for a frequency shift. It therefore becomes difficult for the loop to lock, and track and carrier loss is realized. 
     It is therefore desirable to construct a low jitter PLL that has a relatively large bandwidth by controlling harmonics that creep in at higher bandwidths. Good harmonic suppression is important in PLLs because this affects loop characteristics (i.e., stability, phase error, jitter). Although not readily apparent, harmonics with balanced codes exist off of the fundamental that can cause false locks with pseudo-random code streams. A PLL that exhibits optimum harmonic suppression is therefore not only beneficial in reducing jitter on the transmit side, but it is also instrumental in rejecting noise and transmission-media-induced distortions on both of the reception and instrumentation sides. 
     SUMMARY OF THE INVENTION 
     The present invention provides a wide tracking range phase locked loop (PLL) circuit that achieves minimal jitter in a recovered clock signal. The PLL of the present invention is particularly adapted for applications that require excellent stability (i.e., low phase noise and/or low phase drift). The PLL of the present invention is further applicable for encoded transmissions containing clock and/or data such that the recovery of such clock and/or data is effected without the need for signal acquisition preambles, while avoiding harmonic locks in fixed and/or swept frequency systems. 
     The PLL circuitry disclosed herein provides a means to produce a low jitter recovered clock regardless of the source of the jitter (i.e. whether it is in the source or the transmission media) and has automatic harmonic lockout detection circuitry through means of a novel lock and seek control logic in electrical communication with a programmable frequency discriminator and a code balance detector. The frequency discriminator enables preset of a frequency window of upper and lower frequency limits to derive a programmable range within which signal acquisition is effected. The discriminator works in combination with the code balance detector circuit to minimize the potential for false lock. Useable frequency sweeps may thereby extend through several frequency harmonics while avoiding such harmonics during signal acquisition. This allows a large frequency swept range without a preamble to be used while avoiding harmonic lockup during acquisition. 
     In addition, the combination of a differential loop integrator with the lock and seek control logic guarantees swift signal acquisition with a clean noise output signal without harmonic lockup. This combination allows a larger dynamic range by programmable means as compared to other design topologies, thereby allowing greater flexibility, and large swept frequency ranges. 
     An adaptive cable equalizer is desirably used in combination with the present invention PLL to recover encoded transmissions containing a clock and/or data. The adaptive cable equalizer is a low-cost monolithic solution for equalizing data transmitted over cable or any other media having similar dispersive loss characteristics. The equalizer automatically adapts to equalize short haul cable lengths of coaxial and twisted pair cables or wires and provides superior jitter performance itself. The combination of the equalizer with the present invention PLL is desirable in that such combination permits the use of short haul wires with the added advantage of significantly reduced source jitter. 
     Using the PLL circuit of the present invention, no signal acquisition preambles are required to avoid harmonic locks in fixed, intermittent and/or swept frequency systems. This PLL will automatically seek and lock onto fixed and/or varying frequency encoded input signals and reject harmonics of the encoded signal stream. The disclosed technique further permits large frequency sweeps through harmonics without the potential for harmonic lockup on signal acquisition. Digital input data is re-clocked by a low jitter recovery clock and demodulated. The output-recovered clock desirably has a 50% duty factor. The described technique will work for any DC balanced code modulation scheme used in most high capacity digital transmission and storage devices. 
     It is important that the implementation of the present invention PLL generates the least possible switching noise. It is further important that optimum analog and digital noise isolation and shielding are achieved through precise PWB design using isolated solid power planes for both analog and digital circuitry. 
     It is therefore an advantage of the present invention to provide a PLL that is highly immune to input signal jitter. 
     It is another advantage of the present invention to provide a PLL that recovers the accuracy of the original clock in a noise transmission environment with the least possible jitter. 
     It is an additional advantage of the present invention to provide such a low jitter PLL that achieves wide range tracking with low phase error and drift for data and clock recovery. 
     It is yet another additional advantage of the present invention to provide a low jitter PLL in a compact and inexpensive design that uses commercially available technology. 
     It is still another advantage of the present invention to implement a low jitter PLL for both fixed and swept frequency systems. 
     It a further advantage of the present invention to provide a PLL having an output that maintains a constant phase relationship with its input, even over a large range of input frequencies. The PLL, having one reference clock input and multiple buffered clock outputs, continuously monitors the relation between the phase of the input reference clock and the phase of the outputs. The PLL then adjusts the output transitions so that they land precisely on top of the input transitions within a tight timing window. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a conventional phase lock loop circuit. 
     FIGS. 2A and 2B are a functional diagram of a low jitter phase lock loop circuit of the present invention. 
     FIGS. 3A and 3B are a circuit diagram of a desired embodiment of a frequency discriminator used with a phase lock loop circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides for a phase lock loop (PLL) that recovers a reference clock and reduces jitter from a serial data stream that is transmitted thereto. The low jitter PLL output clock is used to clock counters for accurate delays and then buffered for further distribution. Sub-nanosecond jitter is achieved with the combination of an adaptive cable equalizer at the PLL input and the use of a digital frequency discriminator that avoids harmonic lock-up on signal acquisition. Implementing a differential loop integrator further attenuates jitter. The integrator that is holding the last lock-on voltage then only has to slew over a narrow range to track. This limits the control voltage swing and hence the probability of crossing over and finding a nearby harmonic. 
     Now referring to the figures, wherein like elements are identically numbered, a functional block diagram of a phase lock loop (“PLL”) circuit  20  of the present invention is shown in FIGS. 2A and 2B. PLL  20  and components in electrical communication therewith (further described hereinbelow) are supported by a printed circuit board (PCB)  23 . The architecture for PLL  20  is similarly sustained on a field programmable gate array (FPGA)  25 , that enables fast prototyping of circuits as is commonly known in the art. 
     At least one short haul transmission medium  27  furnishes a conduit for transfer of an incoming data stream from a transmitter source (not shown) to PLL  20 . Transmission medium  27  is desirably selected from short haul cable lengths of coaxial and twisted pair cables, wires and the like desirably having a maximum length of about 320′. A balanced isolation transformer  29  is desirably coupled in electrical communication with transmission medium  27  for translating the incoming data stream onto PCB  23 . 
     An adaptive cable equalizer  31  is provided that is coupled between balanced isolation transformer  29  and PLL  20  for reducing jitter inherent in the incoming data stream. Adaptive cable equalizer  31  allows short runs on inexpensive twisted wire pairs such as those found on fanouts to local devices. Adaptive cable equalizer  31  further adjusts for the lowest incoming jitter independent of cable length as a signal pre-conditioner to PLL  20 , reducing system jitter thereby. Adaptive cable equalizer  31  may be selected from any of a plurality of commercially available embodiments that are well known in the art. 
     PLL  20  implements an “autoranging” technique that enables preprogramming of a discrete frequency window within which an incoming signal is identified to which lock is effected. Such autoranging capability is executed by a lock and seek control logic  33 . Control logic  33  has a first input that is connected to an output of a digital programmable frequency discriminator  35  (further described hereinbelow) in electrical communication with a balance code detector circuit  36 . Balance code detector fault is the first indication of a possible harmonic lockup but is not sufficient alone since the received data may contain errors. Frequency discriminator  35  verifies re-acquisition to enable precise programming of a wide frequency tracking range; this sets the window frequency frame within which a signal shall be “locked” to derive data from the data stream and further provide harmonic lockup protection over the prescribed range. An exemplary model of a frequency discriminator for use with the present invention is illustrated in FIG.  3  and further described hereinbelow. 
     Frequency discriminator  35  uses dividers and frequency-phase comparators to set an upper and lower window around a local clock signal generated by a local oscillator or clock  37 . Frequency discriminator  35  produces an output voltage that depends on an instantaneous signal frequency generated in response to the local clock signal and an input reference signal generated by a reference clock  39 . Its signal is also utilized as a feedback signal within the loop. Local oscillator  37  is shown in FIG. 2A as a VCO or VCM that is well established in the art as an electronically tunable oscillator in which the output frequency is dependent on the value of an applied tuning voltage. Improved jitter performance can be easily attained by the use of low phase noise differential signal VCOs. It is understood, however, that the type of tunable oscillator implemented with the present invention may be modified without departing from the scope thereof. 
     Further referring to FIG. 2A, reference clock  39  provides a baseline reference frequency to frequency discriminator  35  and is selected from an external fixed clock such as a crystal reference clock or an external dynamic clock such as a direct digitally synthesized (DDS) clock. Reference clock  39  eliminates any analog tuning components, thereby eliminating the drift and aging characteristics associated with such devices. A DDS reference input may be used to quickly acquire and track a known frequency/time profile within a narrow window while avoiding harmonics with low signal to noise ratio (S/N) inputs. Reference clock  39  therefore provides a “ball park” clock reference to frequency discriminator  35  and a basis for creating an exact frequency window using programmable counters. This window will not drift or vary with age as occurs with conventional control voltage threshold detectors. 
     Frequency discriminator  35  further uses programmable counters that can make the frequency window as small as needed to ensure that no harmonics with sufficient power to cause lock with the selected filter network will occur. As further shown in FIGS. 3A and 3B, an illustrative frequency discriminator  35 ′ conducive to operation of the present invention PLL  20  is provided wherein frequency discriminator  35 ′ includes an upper frequency threshold counter  45  and a lower frequency threshold counter  47  for establishing threshold settings around the local clock signal. Each of upper counter  45  and lower counter  47  divides the reference clock signal by a distinct predetermined divisor. Frequency discriminator  35 ′ further includes a reference clock frequency counter  49  that divides the local clock signal by a predetermined divisor to generate a local reference clock signal. 
     Frequency discriminator  35 ′ includes at least one frequency phase comparator  51  having an input from one of lower frequency threshold counter  45  and upper frequency threshold counter  47 , and an input from reference clock frequency counter  49 . Comparator  51  establishes at least one of a minimum frequency and a maximum frequency that will not cause reacquisition of the reference clock signal. In practice it may be necessary to inhibit the output of local oscillator  37  many times to determine if the window is narrow enough to avoid harmonics, however, the tracking range will be hard limited by this window. 
     Referring back to FIG. 2, a frequency/phase comparator  53  is provided in electrical communication with reference clock  39 . Comparator  53  detects the phase difference between the reference frequency input and the signal frequency input from the VCO. Frequency/phase comparator  52  locks on to the input reference signal to effect the locking step. 
     An adaptive phase only comparator  55  is provided in electrical communication with lock and seek control logic  33  wherein phase only comparator  55  determines a phase difference between the input reference signal and the local clock signal when lock is effected therebetween. Phase comparator  55  then generates a phase error signal corresponding to this phase difference and in proportion to the period so as to adapt to changing input frequency and thereby reduce phase lead and/or lag over a wide frequency spectrum. Phase comparator  55  minimizes phase drift by varying output pulse width proportionally with frequency to accommodate a wide dynamic and swept frequency range with low phase error. The integration of phase comparator  55  therefore results in equal jitter specifications over the entire control voltage, translating to a full frequency range over a large frequency span. 
     A differential loop integrator or filter  57  is also provided in PLL  20  wherein filter  57  not only reduces frequency-dependent phase shift error, but also reduces temperature induced phase drift. Differential loop filter  57  is in electrical communication with each of adaptive phase comparator  55  and frequency/phase comparator  53 . Differential loop filter  57  generates a voltage control signal that corresponds to the phase error signal generated by phase comparator  55  and delivers such control voltage signal to local oscillator  37 . Upon achievement of said locking step, PLL  20  transmits this control voltage into the programmable range of frequency discriminator  35  so as to repeatedly lock the circuit to the input reference signal. Differential loop filter  57  lowers control voltage noise, increases sensitivity and dynamic range, increases frequency tracking range, increases common mode noise rejection, compensates for component variations and reduces thermal drift and jitter. Filter  57  thereby adapts to component variations so as to minimize the effects of thermally induced phase shift and noise. 
     PLL  20  further includes a divider  59  (shown herein as a divide-by-2 divider) in electrical communication with local oscillator  37  to provide a low jitter recovered clock  61  having 50% duty factor. PLL  20  additionally has a synchronous modulation decoder circuit  63  having inputs from local oscillator  39  and the data stream for demodulating re-clocked digital input data from the data stream and recovering the re-clocked data  65  therefrom. Demodulation of a signal takes place at a receiver located some distance from the transmitter. The process whereby demodulation is carried out by a second multiplication requires that the carrier available at the receiver be precisely synchronous with the carrier used in the modulation process. Synchronous modulation decoder circuit  63  may be selected from a plurality of modulation methods that are known within the art (i.e., this can be “IP” or other logic implemented in the FPGA, although any type conducive to operation of the present invention will suffice). 
     PLL  20  optionally includes a reference override  67  in electrical communication with lock and seek control logic  33 . Reference override  67  will force the lock on the ball park reference  39  when no input signal is available. 
     PLL  20  may also include at least one indicator (not shown) for indicating when PLL  20  has not attained signal acquisition and lock. The indicator, which may be an LED indicator or the like, is desirably pulse-stretched so that intermittent problems can be readily visualized. The indicator can therefore illuminate when out of lock. 
     At the initiation of signal transmission, then, there are two frequencies applied to phase comparator  55 . As the loop acquires lock, PLL  20  must deal with two different ranges: one in the area in which frequency lock is acquired and, only after frequency lock has been accomplished, one in which phase lock occurs. In operation, frequency/phase comparator  53  first establishes lock-in with reference clock  39 . Once frequency/phase lock is established and the phase error is minimized as the control voltage stabilizes, phase comparator  55  switches in for phase encoded data separation. Phase comparator  55  determines the phase differences between the input signals and the local reference. The rising edge of local oscillator clock  37  is measured against the pseudo-random code generator crystal output as a trigger to make jitter measurements. The detected phase error signal is filtered so as to reduce noise and is sent thereafter to local oscillator  37 . Oscillator  37  adjusts the frequency of the locally generated signal accordingly to approach lock. 
     A “ball park” frequency is selected by producing a digital clock frequency that will center the window established by frequency discriminator  35 . Frequency lock must be established first, then phase lock. Frequency discriminator  35  will therefore force frequency comparator  53  to be used, and when this locks, phase comparator  55  can be used. Thus, when stable lock is attained, lock and seek control logic  33  switches over to adaptive phase comparator  55 , a frequency compensated phase detector for swept frequency systems. Since filter  57  will not allow a step response when switching away from reference clock  39  over to the input signal, the output of filter  57  is initially within the ball park VCO control voltage to achieve the desired frequency for any given set of initial conditions, even with component variations from unit to unit. PLL  20  then, upon frequency lock, confines the rather arbitrary control voltage now to the ball park frequency. Only a small adjustment to the control voltage now will lock the output of filer  57  to the desired frequency every time. In this configuration, frequency/phase detector  53  determines initial point frequency so that no tuning is necessary even with component variations between filter  57  and local oscillator  37 . In this manner, hard limits can be precisely set without deleterious effects from component aging or variation as is found with analog frequency comparators. 
     Lock and seek logic  33  will automatically attempt to re-acquire if no lock is detected after a programmable frequency dependent input delay. Logic  33  will then return to locking on the reference frequency and the cycle repeats itself. If the input signal does not get kicked out by frequency discriminator  35 , then an accurate and harmonic-free lock has been achieved. A pulse stretched indicator as described hereinabove desirably indicates successful phase lock on the input signal. 
     Frequency discriminator  35 , in combination with PLL  20 , can restrict operation to a precise frequency range having its accuracy determined solely by the high accuracy of reference clock  39 . This can be useful in deciphering frequency division multiplexed signals over transmission media without the need for preambles. Discriminator  35  may alternatively be used to hold or constrain the output frequency of local oscillator  37  when a signal is lost or interrupted. In fixed or periodic swept frequency systems, a fixed frequency reference may be used to quickly and accurately set the initial local oscillator control voltage for fast signal acquisition, and to establish a programmable and precise fixed frequency tracking window. A narrow window can be established for fixed frequency or a wider window established for swept frequency inputs. 
     The control signals generated by phase comparator  55  are proportional to the input signal period to keep phase offset minimized. Phase comparator  55  therefore allows a very wide range of input frequencies as the period is divided in half before the control voltage outputs are produced. This reduces and nearly eliminates phase shift for swept frequency systems, such as those that are commonly found in high velocity systems (i.e., missile, aircraft and satellite systems, as well as particle accelerators). Precise timing applications for these systems require low jitter and precise aperture windowing. 
     An illustrative example of the operation of the present inventive PLL is provided in the example below. It is emphasized that the values provided herein merely constitute an example of the efficiency with which the present process can be completed, and that these values can be affected as described thoroughly hereinabove. It is therefore understood that any or all steps may be modified in sequence or duration to adapt to different applications. 
     Example 
     Frequency discriminator window threshold settings: 
     *The upper frequency (UF) threshold counter is set to divide a recovered 2X reference clock=56 MHz by 52 
     *The lower frequency (LF) threshold counter is set to divide a recovered 2X reference clock=56 MHz by 60 
     *The crystal reference clock=14.318 MHz is divided by 14. 
     56 MHz/52=1.08 MHZ (high threshold) 
     56 MHZ/60=0.938 MHz (low threshold) 
     14.318 MHz/14=1.023 MHz (reference). 
     When the reference is between the high and low thresholds 
     
       
         (0.938 MHz&lt;=1.023 MHz&lt;=MHz &lt;=1.08 MHz), 
       
     
     the frequency discriminator will not attempt to reacquire the signal. 
     Using the above settings with a 14.318 reference crystal clock oscillator: 
     1.) The minimum frequency that will not cause reacquisition is: 
     
       
         LF/52=1.023 MHz thus LF=53.196 MHz (low threshold) 
       
     
     2.) The maximum frequency that will not cause re-acquisition is: 
     
       
         HF/60=1.023 thus HF=61.38 MHz (high threshold) 
       
     
     The following stable harmonics have been identified using a 14.318 MHz reference oscillator with a desired 57.272 MHz fundamental PLL frequency using the phase only comparator: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 MHz 
                 Harmonic Ratio 
                 Decimal 
               
               
                   
                   
               
             
             
               
                   
                 32.7274 
                 4/7 
                 0.571428 
               
               
                   
                 34.3637 
                  6/10 
                 0.6 
               
               
                   
                 38.1819 
                 2/3 
                 0.666666 
               
               
                   
                 40.0910 
                  7/10 
                 0.7 
               
               
                   
                 40.9092 
                 5/7 
                 0.714285 
               
               
                   
                 42.9547 
                 3/4 
                 0.75 
               
               
                   
                 45.8183 
                  8/10 
                 0.8 
               
               
                   
                 47.7274 
                 5/6 
                 0.833333 
               
               
                   
                   
               
             
          
         
       
     
     Other integer ratio harmonics are possible, but these lacked sufficient power to cause a harmonic lock in the test set over the tracking range (28 MHz through 80 MHz). For example, the 7/9 harmonic did not occur. The frequency discriminator prevents these harmonics. 
     Using the same ratios for the 2X reference clock at +0%, −0.3% for an approximately 56 MHz operation, the following possible harmonics were predicted using the same harmonic ratios as above. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 −0.3% (MHz) 
                 0% (MHz) 
                 Harmonic Ratio 
               
               
                   
               
             
             
               
                 32.07 
                 32.17 
                 4/7 
               
               
                 33.68 
                 33.78 
                  6/10 
               
               
                 37.42 
                 37.53 
                 2/3 
               
               
                 39.29 
                 39.41 
                  7/10 
               
               
                 40.09 
                 40.21 
                 5/7 
               
               
                 42.10 
                 42.23 
                 3/4 
               
               
                 44.90 
                 45.04 
                  8/10 
               
               
                 46.78 
                 46.92 
                 5/6 
               
               
                   
               
             
          
         
       
     
     The ±5% discriminator threshold at 53.196 MHz is well above the next nearest stable harmonic (5/6). 
     Testing reveals lock-up without harmonic lock every time, illustrating the predictability and repeatability of PLL  20  performance. Such predictable operation is important in guaranteeing that a plurality of PLLs will all achieve lock in critical systems, as there is no practical way to “bump” any PLL that does not gain harmonic-free lock-up on encoded data in a broadcast system. Each PLL unit must achieve lock the first time without any re-tries when signal cables are attached to a running system and without preambles. This allows clock and data to be sent on a single transmission medium without compromising jitter performance. The advantages are large when compared to dual media systems, as small thermal shifts will change the phase of the clock of the data, not only resulting in irretrievable data loss but also requiring precise media length matching and thermal compensation. 
     In addition, the temperature performance of PLL  20  is excellent due to the implementation of differential filter  57  in combination therewith. Filter  57  resolves to nanovolt control voltages for picosecond resolution and precision in phase. This frequency sweep range performance goes well beyond that of VCOs commonly used to recover data in high bit rate (i.e. fixed frequency) communications systems. PLL  20  thereby separates clock from data such that the recovered clock and data are reduced significantly in jitter. This is very beneficial for accurate position control systems. 
     In the present embodiment, PLL  20  has a desirable jitter of about 50 ps, a maximum RMS jitter of 125 ps and a maximum peak-peak jitter of 1 ns when used in lengthy cable distribution systems with swept frequency inputs and phase modulated data. It can be used in numerous applications with data rates ranging from a few Hz to several GHz. For simple clock multiplication applications, the jitter is better than 25 ps RMS in the 10 to 50 MHz range. These ranges, however, are not hard limited by the scope of this disclosure and merely serve as an illustration of the jitter attenuation that is attainable by the inventive circuit topology. 
     The PLL of the present invention therefore exhibits differential locking capability over a larger range of frequencies than those attainable by conventional PLLs. The present invention PLL works on the data stream itself, exhibiting an “autoranging” capability wherein the PLL locks onto different frequencies without harmonic lockup. This technology could be used to advance transmission electron microscope images, MRI equipment, telescopes, electronic lenses, filters, any instrument that accepts wavelength information and it is desired to stabilize the image of that wavelength under varying input conditions. The result is a sharper image and more precise positions for fixed or moving point targets. 
     The PLL disclosed herein, especially in combination with an adaptive cable equalizer, can solve many imaging, transmission and position measurement problems involving noise-limited precision due to transmission media property shifts. Consequently, it is also possible to stabilize the image of rapidly moving targets with PLL  20 . Implementation of the present invention is advantageous in a myriad of applications, which includes but is not limited to those applications requiring precise space and time measurement (i.e., telemetry, trajectory, radar, Doppler radar, phased array radar and guidance systems; global positioning satellite equipment, satellite communications equipment, survey equipment, HDTV and other precise positioning or measuring equipment). The present invention is also desirable for picosecond-adjustable passive fiber optic delays: high baud rate receivers: range and velocity finding or trajectory measurement equipment; gyroscopes; precise high speed rotation measurement and control such as is used in scanners, printers, scanning image surveillance satellites, or attitude correction electronics; communications systems; and systems with large inertial damping requirements. The above described combination of features consistently yields incrementally better results relative to conventional PLL circuits. Most PLL systems are designed to operate over a relatively narrow frequency range, and preambles are commonly used to prevent harmonic locks. The present invention PLL is performance oriented and many of its enhancements are performance related with its large swept frequency range and large frequency span. The PLL design disclosed herein can therefore “seek” over encoded data harmonics without lock-up on those harmonics during signal acquisition. This design requires no preamble to attain its harmonic-free lock performance. 
     Swept systems require a large tracking range that may pass through several harmonics of the source signal. This design provides all of the necessary detection circuitry to prevent any harmonic lockups upon signal acquisition with encoded data without any false locks due to power up or signal interruption. This design has no analog comparator that could drift with temperature and range 
     The signal information processing techniques achieved by the present invention PLL can further produce a new level of clarity or certainty at many points in space at the same time if the distributed signal aperture is adequately stabilized in time to picoseconds or better. These techniques can pass data along with the signal such that many distributed devices can share the same broadcast information. With loopback techniques, the timing can be equalized over great distances through a noisy transmission media that may experience variations due to temperature, Doppler effect or physically curved trajectories. The resultant signal can be automatically trimmed to noise floor accuracy, in both resolution and precision. 
     Various changes to the foregoing described and shown methods and corresponding structures would now be evident to those skilled in the art. The matter set forth in the foregoing description and accompanying figures is therefore offered by way of illustration only and not as a limitation. Accordingly, the particularly disclosed scope of the invention is set forth in the following claims.