Abstract:
A bit synchronizer for Miller-encoded data includes a phase-locked loop for synchronizing the Miller-encoded data to the clock signal necessary for proper decoding thereof. The phase-locked loop includes a monostable multivibrator that is triggered on each transition of the Miller-encoded data. The monostable multivibrator controls the operation of two flip-flops that produce time-varying signals when the clock is leading or lagging the Miller-encoded data. The flip-flop output signals are constant when the clock is in phase with the Miller-encoded data. The flip-flop output signals are integrated, and the resultant signal controls a voltage-controlled oscillator so that the clock signal is phase coherent with the Miller-encoded data signal.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a method and apparatus for synchronizing a clock signal to an encoded data signal for use in decoding thereof, and more specifically, to a method and apparatus for synchronizing a clock signal to Miller-encoded data for accurate decoding thereof. 
     BACKGROUND OF THE INVENTION 
     There are many well-known techniques for encoding digital data for transmission or recording. One such popular technique is called Miller encoding, where digital ones and zeros are represented by transitions at different locations during a bit cell. A bit cell is the time interval occupied by each data bit. The placement of the transition within the bit cell conveys information regardless of the polarity of the transition, i.e., a high-to-low transition or a low-to-high transition. In Miller encoding, a one bit is encoded with a transition (from the previous state) at the middle of the bit cell. Consecutive zero bits are encoded by a transition at the beginning of each bit cell. Single, isolated zero bits are ignored, i.e., there are no transitions for single isolated zeros. Miller encoding permits regeneration of the clock signal from the Miller-encoded data signal; however, for proper decoding the clock signal must have a rate of twice the bit cell time. That is, two clock pulses must occur during each bit cell. 
     One disadvantage associated with Miller-encoding is that the encoded data does not have a regularly-repetitive transition edge for use in synchronizing a clock signal; that is, transitions may occur at one bit cell interval, one-and-one half bit cell intervals, or two bit cell intervals depending on the data pattern. The data is said to be &#34;missing transitions&#34; and hence it is not possible to employ a simple phase detector to compare the Miller-encoded data signals and the clock signal to synchronize them (i.e., to make the Miller-encoded data signal and the clock signal phase coherent). A phase detector looks at signal transitions or edges; with missing transitions for certain data patterns, the phase detector output would be erroneous. 
     One technique for synchronizing Miller-encoded data to a clock signal is disclosed in U.S. Pat. No. 4,124,778, entitled &#34;Digital Frame Synchronizing Circuit.&#34; This patent discloses a phase-locked loop (including a phase detector and a voltage-controlled oscillator) to provide bit synchronization and clock signal regeneration. The well-known Miller encoding scheme is modified to provide a synchronization signal having a duration of three bit cells, thus generating a lower frequency signal than the Miller-encoding scheme. The phase-locked loop includes a feedback gate that is controlled by the synchronization signal. The feedback gate permits the clock signal from the voltage-controlled oscillator to be received as an input to the phase detector only when the synchronization signal is high. The phase detector compares the Miller-encoded signal and the clock signal to control the voltage-controlled oscillator so that a clock signal at twice the bit cell rate, which is necessary for decoding of the Miller-encoded data, is provided. 
     SUMMARY OF THE INVENTION 
     Although Miller encoding has various advantages as discussed above, the primary disadvantage is the characteristic missing transitions in the data. The missing transition occurs in a 1-0-1 data sequence, because in Miller encoding, there is no transition for a single isolated zero. One advantage of Miller encoding is the ability to generate a clock signal from the data signal, but it is necessary to compensate for the missing transitions when producing the clock signal. A clock signal with a frequency of twice the Miller-encoded signal is required for decoding. 
     The present invention is a bit synchronizer employing a phase-locked loop in a unique configuration that allows regeneration of the clock signal from the data signal, despite the missing transitions in the latter. According to the inventive principles, the phase-locked loop includes a phase comparator, integrator, and voltage-controlled oscillator. The phase comparator produces two output signals, and when the regenerated clock signal is phase-locked to the Miller-encoded signal, the two signals from the phase comparator are continuously in a high state. When the clock signal is either leading or lagging the Miller-encoded data, short pulses appear in one of the two output signals from the phase comparator. These pulses are integrated by the integrator and control the voltage-controlled oscillator in such a manner to bring the clock signal back into phase synchronization with the Miller-encoded signal. 
     Further according to the inventive principles, when the Miller-encoded data is not being received, the bit synchronizer is locked to an oscillator, which produces a signal at the clock frequency. When Miller-encoded data is received, the bit synchronizer is switched to receive the Miller-encoded data and regenerate the clock signal based thereon. Locking the bit synchronizer to the oscillator when no Miller data is received allows the bit synchronizer to lock quickly onto the transitions in the Miller encoded data when received. Thus, the capture time of the bit synchronizer (or phase-locked loop) is decreased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be better understood, and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings in which: 
     FIG. 1 is a block diagram of a bit synchronizer constructed according to the teachings of the present invention; 
     FIG. 2 is a block diagram of the phase comparator illustrated in FIG. 1; 
     FIG. 3 is a timing diagram showing the operation of the phase comparator of FIG. 2; and 
     FIG. 4 is a schematic diagram of the integrator of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a bit synchronizer 10, which is part of a receiver (not shown), and is responsive to a signal from the receiver demodulator (not shown) on a conductor 11. In the preferred embodiment, the receiver receives a carrier signal modulated by a Miller-encoded data signal, and demodulates it to reproduce the Miller-encoded data signal. A phase comparator 14 is coupled to receive the Miller-encoded data signal via a switch 12 when the latter is in the position shown in FIG. 1. The phase comparator 14 produces two output signals, designated Q U  and Q D  that are supplied as inputs to an integrator 16. An output signal from the integrator 16 is supplied as an input to a voltage-controlled oscillator 18. The voltage-controlled oscillator 18 produces the clock signal, having a frequency of twice the Miller-encoded data. The clock signal is fed as an input signal to the phase comparator 14 and also to a decoder, not shown in FIG. 1. The decoder uses the clock signal to decode the Miller-encoded data and reproduce the one and zero bits containing the information. Such Miller decoders are well-known in the art. The phase comparator 14, the integrator 16, and the voltage-controlled oscillator 18 constitute a phase-locked loop 19. 
     The carrier detect circuit 13 controls the position of the switch 12. The carrier detect circuit 13 is responsive to the signal input to the receiver for detecting the presence of a carrier signal. When a carrier signal is present, the switch 12 is in the position shown in FIG. 1 so that the phase comparator 14 is responsive to the received demodulated signal for use in generating the clock signal. When carrier is absent, the carrier detect circuit 13 places the switch 12 in the upper position such that the phase comparator 14 is responsive to a signal from a crystal oscillator 20. 
     According to the inventive principles, the crystal oscillator 20 is necessary to overcome a disadvantage associated with the use of a phase-locked loop when no signal is being received and therefore there is no signal on the conductor 11 from the demodulator. When the receiver is not receiving a signal, no signal will be present on the conductor 11 and the output signals Q U  and Q D  from the phase comparator 14 will not contain useful information. This causes the voltage-controlled oscillator 18 to drift away from the desired clock frequency. It is also possible that without useful information from the phase comparator 14, the integrator 16 may integrate to the supply voltage, thus forcing the phase-locked loop 19 into a non-linear state. To overcome this disadvantage, the carrier detect circuit 13 moves the switch 12 into a position such that the phase comparator 14 is responsive to the signal from the crystal oscillator 20. The signal therefrom has a frequency approximating the frequency of the clock signal, thereby giving the phase comparator 14 useful information and causing the phase-locked loop 19 to lock onto a frequency reasonably close to the desired clock frequency. The phase-locked loop 19 thus operates in a linear region and avoids the problems associated with the non-linearities created when a signal is not present on the conductor 11. 
     FIG. 2 is a block diagram of the elements of the phase comparator 14, according to the inventive principles. An input terminal of an edge detector 22 is connected to the wiper of the switch 12; a signal from the edge detector 22 is input to a monostable multivibrator 24. An output signal from the monostable multivibrator 24 is input to a D and a set input terminals of a D flip-flop 26. The output signal from the mono-stable multivibrator 24 is also input to a clock input terminal of a D flip-flop 28. The clock signal from the voltage-controlled oscillator 18 is supplied as an input to a clock input terminal of the D flip-flop 26, to a D input terminal of the D flip-flop 28, and to a set input terminal of the D flip-flop terminal 28. The D flip-flop 26 produces the Q U  signal, and the D flip-flop 28 produces the Q D  and Q D  signals. 
     In operation, when the clock signal is in phase with the Miller-encoded data signal, the signals Q U  and Q D  are in a high state (Q D  is low). This causes the output signal from the integrator 16 to be constant, and the constant input signal to the voltage-controlled oscillator 18 causes no change in the frequency of the clock signal. When the clock signal is leading or lagging the Miller encoded data signal, pulses appear in the Q U , Q D  and Q D  signals. These pulses are integrated by the integrator 16, causing the voltage-controlled oscillator 18 to change the frequency of the clock signal to synchronize the clock signal and Miller-encoded data signal. 
     The edge detector 22 detects positive and negative transitions in the signal from the switch 12 and converts these to short pulses, which trigger the monostable multivibrator 24. 
     Referring to FIG. 3, there is shown a sample bit pattern and the Miller-encoded data signal for that bit pattern. As previously discussed, for each one bit there is a transition in the Miller-encoded data signal at approximately the midpoint of the bit cell. For two consecutive zeros, there is a transition between each zero. There is no transition in the Miller-encoded data signal for a zero immediately following a one. The monostable multivibrator output signal is also shown in FIG. 3. There is a transition in the monostable multivibrator output signal for each transition of the Miller-encoded data signal. The duration of the monostable output pulse is controlled by a capacitor not shown in FIG. 2; in the preferred embodiment the monostable multivibrator pulse has a duration of approximately 0.25 of a bit period. The clock-in-phase segment of FIG. 3 shows the signals Q U  and Q D  and a clock signal in phase with the Miller-encoded data signal. Before explaining the operation of the phase comparator 14, it should be noted that in the preferred embodiment the set input terminal of the D flip-flops 26 and 28 is a level/asynchronous input terminal that overrides the D and clock input terminals. Also, in the preferred embodiment the D flip-flops 26 and 28 trigger on positive-going transitions of the signal input to the clock terminals thereof. Further, due to the hold time of the D flip-flops 26 and 28, the D flip-flops 26 and 28 see the signal at each respective D input terminal just prior to a positive going transition in the clock signal. 
     Refer to FIG. 2 and the clock-in-phase segment of FIG. 3. Because the set input terminal of the D flip-flop 26 is responsive to the monostable multivibrator signal, the signal Q U  is high whenever the monostable multivibrator signal is high. Even when the monostable multivibrator signal goes low, (see FIG. 3), the Q U  signal remains high because the D input terminal sees the signal immediately prior to the positive-going transition of the in-phase clock signal. Thus the D input terminal sees a high state and Q U  remains high. When the next positive going transition of the clock occurs, the monostable multivibrator signal has returned to a high state and thus through the action of the set input, Q U  remains high. 
     With respect to the D flip-flop 28, the monostable multivibrator signal is the clock input thereto. At every positive-going transition of the monostable multivibrator signal the clock signal, which is input to the D input terminal, is high (considering the hold time of the D flip-flop 28) and therefore Q D  remains high. Thus, when the clock signal from the voltage-controlled oscillator 18 is in-phase with the Miller-encoded data signal, the signals Q U  and Q D  remain high. The voltage-controlled oscillator 18 therefore sees a constant input signal and the frequency of the clock signal is not changed. 
     The clock-leading segment of FIG. 3 shows that when the clock is leading the Miller encoded data signal pulses are produced in the signal Q D . These pulses are integrated in the integrator 16, supplying a signal to the voltage-controlled oscillator 18 that causes the voltage-controlled oscillator 18 to bring the clock back into phase with the Miller encoded data signal. The pulses occur in Q D  because when the mono-stable multivibrator signal (which is the clock input to the D flip-flop 28) goes high, the D input terminal sees a low state of the clock signal. Thus, Q D  goes low. When the clock signal from the voltage-controlled oscillator 18 (provided as an input to the set input terminal) goes high, Q D  returns to the high state. 
     Q U  is always high when the clock is leading. When the mono-stable multivibrator signal is high the set input terminal of the D flip-flop 26 is high and Q U  is high. When the monostable multivibrator signal goes low, this does not effect Q U  because positive-going transitions of the clock signal occur only when the monostable multivibrator signal is high. See FIG. 3 for this timing relationship. 
     Referring to the clock lagging-segment of FIG. 3, pulses appear in the signal Q U  that cause the voltage-controlled oscillator 18 to bring the lagging clock signal in phase with the Miller encoded data signal. The pulses appear in Q U  because during each negative data pulse in the monostable multivibrator signal, there is a positive going transition in the lagging clock signal. Thus the D input terminal of the D flip-flop 26 sees a low state on a positive-going transition of the clock signal and Q U  goes low. Q U  goes high when the monostable multivibrator signal goes high, driving the set input terminal, and thus Q U  high. For the lagging clock case, Q D  remains high because for each positive-going transition of the monostable multivibrator signal, the signal at the D input terminal of the D flip-flop 28 is also high. 
     FIG. 4 shows a schematic diagram of a preferred embodiment for the integrator 16 of FIG. 1. The signal Q U  from the phase comparator 14 is supplied as an input to a base terminal of a transistor 32. The emitter terminal thereof is connected to ground, and the collector terminal thereof is connected to a power supply (designated V C ) via a resistor 34. A diode 36 is connected between the collector terminal of the transistor 32 and a terminal 38, with the cathode terminal connected to the terminal 38 via a resistor 37. The signal Q D  from the phase comparator 14 is supplied as an input to a base terminal of a transistor 46. Q D  has a waveform of opposite polarity to that of Q D , illustrated in FIG. 3. The emitter terminal thereof is connected to ground, and the collector terminal thereof is connected to the power supply (designated V C ) via a resistor 44. The cathode terminal of the diode 42 is also connected to the collector terminal of the transistor 46. The anode terminal of the diode 42 is connected to the terminal 38 via a resistor 40. The terminal 38 is connected to ground via a capacitor 48, and is also connected to ground via a series combination of a resistor 50 and a capacitor 52. The terminal 38 is connected to the input terminal of the voltage-controlled oscillator 18, as illustrated in FIG. 1. 
     In the preferred embodiment, the integrator 16 is designed to source between two and three volts to a high impedance load, i.e., the voltage-controlled oscillator 18. If the clock signal is in-phase with the Miller-encoded data signal and has been for some time, the capacitors 48 and 52 are charged to a constant potential. The transistor 32 is normally on and the transistor 46 is normally off, therefore diodes 36 and 42 are reverse biased and no charge is being added to or subtracted from the capacitors 48 and 52. If the clock signal is lagging the Miller-encoded data signal, pulses appear in the signal Q U . See FIG. 3. These pulses turn off the transistor 32, forward biasing the diode 36; current flow into the capacitors 48 and 52 is approximately given by the following equation. ##EQU1## Wherein V OUT  is the voltage at the terminal 38, V 36  is the voltage drop across the diode 36, R 34  is the ohmic value for the resistor 34, and R 37  is the ohmic value for the resistor 37. V OUT  thus increases, causing the voltage-controlled oscillator 18 to increase the clock frequency so that the clock signal and the Miller-encoded data signal approach an in-phase condition. 
     Similarly, if the clock signal is leading the Miller-encoded data signal, pulses appear in the signal Q D , which turn on the transistor 46. The action forward biases the diode 42 and current flows out of the capacitors 48 and 52 to ground via the transistor 46. The current is approximately given by the following equation. ##EQU2## Wherein V 42  is the voltage drop across the diode 42, V sat  is the saturated collector to emitter voltage drop of the transistor 46, and R 40  is the ohmic value for the resistor 40. This lowers V OUT  causing the voltage-controlled oscillator 18 to decrease the clock frequency so that the clock signal and the Miller-encoded data signal approach an in-phase condition. 
     In another embodiment of the present invention the signals Q U  and Q D  are modified to have equal on/off duty cycles when the clock is in phase with the Miller-encoded data signal. With this technique, an equal quantity of charge is being continuously added to and substracted from the capacitors 48 and 50 during in-phase conditions, thus providing more stable jitter-free operation of the phase-locked loop 19. 
     While several embodiments in accordance with the present invention have been shown and described, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.