Abstract:
A phase detector for a phase lock loop can detect more than a 2π radian phase difference. The phase detector is modulized so that modules may be stacked to extend the phase detection range by 2Nπ radians, where N equals the number of modules that are stacked together. The modules are combined to obtain an output signal which can be applied to a filter for controlling the voltage of the voltage-controlled oscillator.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to phase detectors that are used in synchronizing systems, and in particular, to phase detectors that may obtain greater than 360° in phase error. 
     Some phase lock loops that utilize phase detectors have large phase excursions at the phase detector input causing a large output frequency change from the phase lock loop. These excursions may greatly exceed the usual 2π or 4π radian phase range of digital phase detectors, such as that disclosed in U.S. Pat. No. 3,989,931 or U.S. Pat. No. 3,431,509. When the phase limit is reached in the prior art detectors, the phase lock loop goes out of lock and considerable time is lost until the reestablishment of phase lock is achieved. Phase detectors with much larger phase range have been tried, but were unacceptable because of disturbances during coincidences or crossover of the two pulses that were used as reference pulses. In the case where ring counters are used, a modulation at a subharmonic of the reference frequency that is used to control the phase lock loop is generated. 
     SUMMARY OF THE INVENTION 
     A phase detector for a phase lock loop can detect more than a 2π radian phase difference. The phase detector is modulized so that modules may be stacked to extend the phase detection range by 2π radians, where N equals the number of modules that are stacked together. The modules are combined to obtain an output signal which can be applied to a filter for controlling the voltage of the voltage-controlled oscillator. 
     It is the object of the invention to provide a phase detector that is made of a plurality of modules with each module capable of detecting up to 2π radians of phase error. 
     It is another object of the invention to provide a modular phase detector in which each of the modules includes a latch and a lock-out circuit. 
     It is another object of the invention to provide a phase detector circuit in which each module includes a latch that provides a first logic indication for every reference pulse that passes through the lock-out means and a second logic indication for every signal pulse that passes through the lock-out means. 
     It is still yet another object of the invention to provide a phase detector circuit having a plurality of modules in which time delays in the paths of the reference pulses and the signal pulses are selected to ensure a smooth transition at coincidence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be readily carried into practice, a number of embodiments will now be described in detail by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a block diagram of a phase lock loop incorporating the phase detector according to the invention; 
     FIG. 2 is a schematic diagram of the phase detector according to the invention; 
     FIG. 3 is a timing diagram illustrating the operation of the phase detector of FIG. 2; and 
     FIG. 4 is a timing diagram illustrating the transition when the detected phase difference is greater than 2π radians. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1, to which reference should now be made, shows a block diagram of a phase lock loop 10 incorporating a phase detector 3. The phase detector 3 compares a reference pulse at its &#34;R&#34; terminal that is provided by a reference frequency source 1, with a signal pulse at its &#34;S&#34; terminal that is the output of a voltage-controlled oscillator (VCO) 7. A mixer 9 obtains the difference between the voltage-controlled oscillator 7 frequency and a second reference frequency provided by the second reference oscillator 12 to obtain a product signal that is filtered by the Low Pass Filter (LPF) 14. This action brings the VCO 7 frequency into the range of the reference frequency R as provided by the reference frequency source 1. The output of the phase detector 3 is applied to a low pass filter (LPF) circuit 5 where it is converted to a voltage to which the voltage-controlled oscillator 7 will respond. 
     The phase detector 3 of FIG. 1 may include a plurality of modules and in the preferred embodiment as shown in FIG. 2 there are three modules shown; a first module 31, a second module 33 and an Nth module 35 where N is a positive integer and in the case of FIG. 2 is equal to 3. Each module is capable of detecting 2π radians or 360° phase errors between a first signal and a second signal that are applied to its &#34;R&#34; and &#34;S&#34; terminals. The first module includes a NAND gate latch 36 that is made up of NAND gates 37 and 39. The &#34;R&#34; terminal is applied to the first stage module via an inverter gate 27 and the &#34;S&#34; terminal is connected to the NAND gate latch 36 via the NAND gate 11. NAND gate 11 constitutes a lock-out circuit. The second module 33 includes a second NAND gate latch 30 which is made up of NAND gates 41 and 43, and a lock-out circuit 34 that comprise the NAND gates 13 and 17. For all modules other than the first module 31 and the Nth module 35, the lock-out should include two NAND gates. The Nth module 35 includes a NAND gate latch 38 configured with NAND gates 45 and 47 and a lock-out circuit of NAND gate 19. The output of the modules are summed by an adder circuit 55 that includes a first output resistor 53, a second output resistor 51 and a third output resistor 49. Additionally to ensure that there are no glitches at coincidences and to compensate for the time delays through each of the NAND gates that are used to make up either the latching circuit or the lock-out circuits, there are delays 23 and 25 between the &#34;S&#34; terminal and inverter 27 with delay 23 also being between the &#34;S&#34; terminal and NAND gate 13. Similarly, delays 15 and 21 are between the &#34;R&#34; terminal and inverter 29 with delay 15 also being between the &#34;R&#34; terminal and NAND gate 17. It should be noted that the phase range of 2π radians per module is reduced by the amount of delay per module, which in practice is usually a negligible amount. 
     FIG. 3, to which reference should now be made, illustrates the operation of the circuit of FIG. 1 and in particular the phase detector 3 of FIG. 1. Waveform S is the output of the mixer 9 as it appears at the &#34;S&#34; terminal of the phase detector 3 and waveform R is the output of the reference frequency source 1 as it appears at the &#34;R&#34; terminal of the phase detector 3. The phase detector 3 compares the phases of the R and S pulses and provides signals that represent the difference between the phases. This is illustrated by waveform 3(out) of FIG. 3. Pulse 60 represents the difference between the corresponding R and S pulses that occur at that point. It should be noted that a transition, as illustrated by balloon 61, occurs where the phase difference between the R and the S pulse is greater than 2π radian, and in which case the signal level is increased by the amount illustrated by dimension lines 62 to show that there is a greater than 2π radian phase error. Balloon 63 illustrates an additional phase transition in which there is a greater than 4π radian phase difference detected by the phase detector 3. Transition 63 illustrates the transition from 6π radian phase difference that is detected by the embodiment illustrated in FIG. 2. The phase detector 3 returns to a 4π phase difference when there is a greater than 6π phase difference, and the process repeats until the circuit achieves lock. Waveform 5(out) illustrates the output signal of the low pass filter 5 which filters the output of the phase detector 3 waveform 3(out) to obtain a smooth voltage signal to which the voltage-controlled oscillator 7 will respond. The transition 65 is illustrated in waveform 5(out) by transition 67. 
     FIG. 4 illustrates the transitional phases more distinctly than FIG. 3, while also illustrating the operation of the circuit of FIG. 2 which should be viewed in conjunction with FIGS. 3 and 4. In FIG. 4, the reference numeral for the waveform corresponds to the circuit numbers on FIG. 2 from which the signal originates. A comparison between the R and S pulses indicates that the S pulse passes through a 2π radian phase difference at approximately point 70. At this stage, the first 2π radian module 31 obtains a solid logic state indicating that there is more than 2π radian error. This is illustrated by waveform 37. Waveform 41 shows that the second 2π radian module 33 is making the comparison between the R and the S pulses and is picking up control from the first 2π radian module 31. Waveform 25 shows the output of the delay 25 which includes a two gate delay in the S pulse. Waveform 21 includes a two gate delay in the R waveform. The output of the inverter 27 is represented by waveform 27 and corresponds at this point to the complement of the S pulse with the appropriate gate delays included therein. Waveform 11 illustrates the lock-out device for the first module and includes the NAND of the R waveform and waveform 43. Comparing two as the transition occurs between the first 2π radian module 31 and the second 2π radian module 33, waveform 11 approaches a logic 1 state, whereas waveform 43 begins to follow the S waveform 41 in a responsive manner, although it is the NAND combination of waveforms 43 and waveform 13. Waveform 47 is combined with the R pulse by the NAND gate 17 as part of the lock-out of the second 2π radian module 30. Waveform 29 essentially at this stage follows the R pulse. However, if the phase difference as detected by the phase detector 3 is greater than 4π then its influence on the circuit would correspond to that of the NAND gate 17. 
     Although the present invention has been described with respect to a particular embodiment thereof, it is not to be so limited as changes might be made therein which fall within the invention as defined in the appended claims.