Phase-lock-loop circuit

Abstract: A phase-lock-loop circuit for generating a clock signal (CLK) at a frequency higher than a horizontal frequency of a video signal (VIDEO IN) includes a phase detector (101a, 62). The phase detector includes a flip-flop (62) that is set when a horizontal sync pulse (CSI) occurs. An output (Q0---Q9) of a counter (52) that provides frequency division is decoded for resetting the flip-flop in each horizontal period. Other than for the flip-flop, and for the counting stages of the counter, only combinational logic components (101a) are used for producing a phase error indicative signal (OUT) that is coupled via a low-pass filter (54) to a control input (53a) of an oscillator (53) of the phase-lock-loop circuit.

Description: The invention relates to an arrangement for generating a phase-lock-loop clock signal. Digital video signal processing systems with features such as on-screen display of text and picture-in-picture for both television receiver and video tape recorder signal sources may require a clock signal that is phase-locked to a horizontal synchronization signal, referred to as line-locked clock. Typically, a phase-locked loop (PLL) system is formed for line-locked clock generation. Typically, such PLL requires a phase detector and a counter. From GB 1 481 786 a phase-lock-loop circuit is known in which the frequency of a variable frequency generator is divided in frequency, the divided down frequency is compared in phase with a reference frequency to produce positive and negative going signals according to the instantaneous phase relationship between the compared signals and the positive and negative going signals are integrated to provide a control signal for the variable frequency generator. In the disclosed PLL circuit a bistable flip-flop is used which is set by a signal derived from the signal of a voltage controlled oscillator and reset by a stable reference oscillator. According to the switching state of bistable flip-flop respective "go up"- and "go down"-signals are generated which cause the voltage controlled oscillator to increase and decrease its frequency. Two frequency dividers and three monostable flip-flops are required to generate auxiliary signals for the control of the bistable flip-flop and the processing of its states. It may be desirable to produce in a phase detector a phase error indicative signal that is independent of a duty cycle of the horizontal synchronization signal. It may also be desirable to generate the clock signal such that, in phase-lock condition, the edges of the clock and synchronization signals are aligned or occur approximately simultaneously. It may be further desirable to generate the phase error indicative signal such that the horizontal synchronization signal is applied to only one flip-flop in the phase detector and no signal path of the synchronizing signal includes another flip-flop in the phase detector. Thus, only one flip-flop is utilized both when the phase error is positive and negative. Thereby, the circuitry of the phase detector is simplified. These objects are achieved by a phase-lock-loop circuit as defined in claim 1. A phase-lock-loop circuit, embodying an aspect of the invention, includes a controllable oscillator and a source of a synchronizing signal. A flip-flop is responsive to the synchronizing signal for generating a flip-flop output signal at a first state when an edge of the synchronizing signal occurs. The output signal alternates between the first state and a second state. A decoder is used for applying the synchronizing signal to an output of said decoder via a signal path of the synchronizing signal that includes the flip-flop. A phase difference indicative signal is generated at the output of the decoder. The phase difference indicative signal is generated in accordance with a phase difference between the synchronizing signal and an output signal of the oscillator. For phase difference that are both a positive and negative, no other flip-flop is included in any signal path of the synchronizing signal between the source of synchronizing signal and the output of the decoder. A low-pass filter is responsive to the phase difference indicative signal and coupled to a control input of the oscillator for controlling the oscillator in a phase-lock-loop manner. In steady state phase-lock operation, an edge of the oscillator output signal is aligned with the edge of the synchronizing signal. FIGURE 1 illustrates partially in a block diagram a phase-lock loop (PLL), including a phase detector, embodying an aspect of the invention; and FIGURES 2a-2f are waveforms useful for explaining the operation of the PLL of FIGURE 1. FIGURE 1 illustrates a block diagram of a phase-lock loop circuit (PLL) 100, including a phase detector 101, embodying an aspect of the invention. A baseband luminance video signal VIDEO-IN obtained from, for example, a video detector, not shown, of a television receiver is coupled to a conventional sync separator 50 that generates pulses of a horizontal sync signal CSI having a period H at a horizontal deflection frequency f H that is in, for example, the NTSC standard 15,734 Hz. An output signal ClkDiv is produced at an output Q10, the most significant bit of eleven outputs Q0-Q10 of a divide-by-N binary counter 52, by frequency dividing an oscillatory output signal Clk of PLL 100 produced in a voltage-controlled-oscillator 53. Signal Clk may be used in various stages, not shown, of the television receiver for video signal processing. In steady state operation, the frequency of signal Clk is equal to N x f H . The value N denotes a ratio between the frequency of signal Clk and that of signal ClkDiv. The value N may be, for example, equal to 1716. FIGURES 2a-2f illustrate waveforms useful for explaining the operation of the circuit diagram of FIGURE 1. Similar symbols and numerals in FIGURES 1 and 2a-2f indicate similar items or functions. A clear input CLR of a storage element or D-type flip-flop 62 of FIGURE 1 receives a clear signal CLEAR at a LOW state developed at an output of a NAND gate 63. Signal CLEAR is developed, in a given period H of counter 52, when each of the ten least significant inverting signals developed at outputs Q0 - Q9 is at a HIGH state and signal ClkDiv, the most significant bit that is developed at non-inverting output Q10, is at a HIGH state. Thus, at a time TR(1) or TR(2) of FIGURE 2a, signal CLEAR of FIGURE 1 attains the LOW state. When signal CLEAR is at the LOW state, flip-flop 62 is latched to a reset state in which an output Q, where a signal SYN of FIGURE 2c is developed, is established at a LOW state and a signal SYN developed an inverting output Q of flip-flop 62 is at a HIGH state. Signal CLEAR provides initialization of flip-flop 62 in each horizontal period H. When the television receiver is tuned to receive a station, a leading edge LE(1) or LE(2), having a HIGH-TO-LOW transition, of a pulse of horizontal synchronization signal CSI of FIGURE 2b is produced when a horizontal sync pulse occurs in signal VIDEO-IN of FIGURE 1. Between the pulses of signal CSI, signal CSI of FIGURE 2b is at a HIGH state. An input D of flip-flop 62 receives a signal VCC at a HIGH state. When leading edge LE(1) or LE(2) of signal CSI of FIGURE 2b occurs, edge triggered flip-flop 62 of FIGURE 1 is latched to a set state in which output signal SYN of FIGURE 2c, developed at non-inverting output terminal Q of flip-flop 62 of FIGURE 1, attains a HIGH state. Signal ClkDiv has a trailing edge TT(1) or TT(2) of FIGURE 2a that is phase compared to signal SYN of FIGURE 2c. In a first example, leading edge LE(1) shown at the left side of FIGURE 2b occurs when signal ClkDiv of FIGURE 2a is already at the HIGH state. The first example depicts a situation in which the phase of the pulse of signal CSI of FIGURE 2b, as determined by leading edge LE(1), leads the phase of signal ClkDiv of FIGURE 2a, as determined by a trailing edge TT(1) of signal ClkDiv. Consequently, an output signal 64a of FIGURE 1 of an AND gate 64 that receives signals SYN and ClkDiv is HIGH between edge LE(1) of FIGURE 2b and edge TT(1) of FIGURE 2a. Output signal 64a of FIGURE 1 is coupled via an OR gate 65 to an enable input 66a of a tri-state gate 66 to develop an enable signal ENA of FIGURE 2d at input 66a of FIGURE 1. Signal ClkDiv is coupled to an input 66b of gate 66. Signal ENA is at a HIGH state, between the time when edge LE(1) of FIGURE 2b occurs and the time when edge TT(1) of FIGURE 2a occurs. Therefore, signal ENA of FIGURE 1 causes signal ClkDiv that is also at the HIGH state to be coupled to an output 66c of gate 66 for developing an output signal OUT of FIGURES 1 and 2e at the HIGH state. Thus, a pulse of phase difference indicative signal OUT having a peak at the HIGH state is developed from signal ClkDiv. Immediately prior to and after the interval between edge LE(1) of FIGURE 2b and edge TT(1) of FIGURE 2a, that establish the pulse width of signal ENA of FIGURE 2d, output 66c of FIGURE 1 is at a high impedance state. The pulse width of signal OUT is proportional to the phase error or difference. Signal OUT is coupled via a conventional low-pass filter 54 that forms the loop filter of PLL 100 to control input terminal 53a of oscillator 53 for controlling the phase and frequency of signal Clk. In a second example, leading edge LE(2) shown at the right side of FIGURE 2a, occurs when signal ClkDiv of FIGURE 2a is already at the LOW state. The second example depicts a situation in which the phase of signal CSI of FIGURE 2b, as determined by leading edge LE(2), lags the phase of signal ClkDiv of FIGURE 2a as determined by a trailing edge TT(2). Consequently, an output signal 67a of FIGURE 1 of an AND gate 67 that receives signals SYN and ClkDiv is HIGH between edge TT(2) of FIGURE 2a and edge LE(2) of FIGURE 2b. Output signal 67a of FIGURE 1 is coupled via OR gate 65 to enable input 66a of tristate gate 66 for developing enable signal ENA of FIGURE 2d at input 66a of FIGURE 1. Signal ClkDiv is coupled to input 66b of gate 66. Signal ENA is at a HIGH state, between the time when edge TT(2) of FIGURE 2a occurs and the time when edge LE(2) of FIGURE 2b occurs. Therefore, signal ENA of FIGURE 1 causes signal ClkDiv, that is at the LOW state, to be coupled to output 66c of gate 66. Consequently, output signal OUT of FIGURES 1 and 2e is developed at the LOW state. Thus, a pulse of phase difference indicative signal OUT having a peak at the LOW state is developed. Immediately prior to and after the interval between edge TT(2) of FIGURE 2a and edge LE(2) of FIGURE 2b, that establish the pulse width of signal ENA of FIGURE 2d, output 66c of FIGURE 1 is at the high impedance state. The pulse width of signal OUT is proportional to the phase error or time difference between leading edge LE(2) of FIGURE 2b and edge TT(2) of FIGURE 2a. Advantageously, signal OUT is independent of the duty cycle of each of signal CSI and signal ClkDiv. Therefore, advantageously, PLL 100 of FIGURE 1 is not affected by any variations of the duty cycle of signal CSI that may occur as a result of, for example, noise or signal reception variation. In phase-lock condition, edge TT(2) or TT(1) of FIGURE 2a occurs immediately after or almost simultaneously with a clocking edge CE of signal Clk of FIGURE 2f. Thus, the edges of signals Clk and CSI are aligned in phase-lock operation. Having the clocking edge CE of signal Clk of FIGURE 2f aligned with edge TT(1) or TT(2) of FIGURE 2a may facilitate signal processing in other stages of a video apparatus, not shown, that utilize signal Clk. In accordance with an inventive feature, signal CSI of FIGURE 1 that is coupled to flip-flop 62 is not applied to any other signal storage stage in the course of generating signal OUT. Gates 64, 65, 67 and 66 form a decoder 101a of the phase detector. Decoder 101a is formed entirely from combinational logic stages. Thus, no flip-flop, other than flip-flop 62, is coupled to any signal path formed between terminal 61a where signal CSI is generated and terminal 66c, where signal OUT is generated both when the phase difference is positive and negative. The result of using only one flip-flop is that the circuitry of the phase detector is simplified. In a third example, the television receiver is not tuned to receive any video signal, resulting in missing signal VIDEO-IN of FIGURE 1. The operation of sync separator 60 is such that when no sync pulse is present, such as when the television receiver is not tuned to receive a transmitting station, signal CSI is continuously at a LOW state. Because of the operation of signal CLEAR, flip-flop 62, is at a reset state and signal SYN is at a LOW state. Signal CSI at the LOW state inhibits through gate 67 the generation of signal ENA. When gate 67 is not enabled, as a result of signal ENA not being generated, terminal 66a is not driven and a high impedance is developed at terminal 66c. Therefore, advantageously, filter 54 and oscillator 53 are not disturbed when, for example, a short duration interruption in signal CSI occurs. Furthermore, when video signal VIDEO-IN is continuously missing, oscillator 53 will operate at a nominal free running frequency. In accordance with another inventive feature, signal ClkDiv of FIGURE 2a has a LOW-TO-HIGH ratio of greater than 1:1, for example, 2:1. Therefore, signal CLEAR of FIGURE 1, that occurs when edge TR(1) or TR(2) of FIGURE 2a occurs, will cause flip-flop 62 of FIGURE 1 to reset following the occurrence of equalizing pulses EQ of FIGURE 2b in signal CSI. Consequently, equalizing pulses EQ that occur during vertical retrace will not disturb the operation of PLL 100 because they will have no effect on signal SYN following edge TR(1) or TR(2) of FIGURE 2a. Except for the flip-flop counting stages of counter 52 that are required for frequency division purposes, only one additional flip-flop, flip-flop 62, is required. All the other logic stages of decoder 101a are memory-less or combinational logic stages. As explained before, advantageously, PLL 100 is constructed without additional storage elements, other than flip-flop 62, in any signal path between a terminal 61a where signal CSI is developed and terminal 66c where signal OUT is developed. Thus, flip-flop 62 is the only flip-flop in any signal path of signal CSI both when the phase difference is positive and negative.

Claims: A phase-lock-loop circuit, comprising: a controllable oscillator (53) for generating an oscillatory signal (CLK); a counter (52, 63) responsive to said oscillatory signal for frequency dividing said oscillatory signal, said counter (52, 63) generating first (ClkDiv) and second (CLEAR) signals having a time difference therebetween and that are synchronized to said oscillatory signal, each having a lower frequency than that of said oscillatory signal; a source of a synchronizing signal (CSI); characterized in that an edge triggered flip-flop (62) is provided responsive to said synchronizing signal (CSI) for generating a flip-flop output signal (SYN) at a first state in accordance with a transition of said synchronizing signal (CSI), said first state being generated when a phase difference between said synchronizing signal (CSI) and said first signal is positive and when said phase difference is negative, said flip-flop being responsive to said second signal for generating said flip-flop output signal at a second state in accordance with said second signal, said flip-flop output signal alternating between said first and second states within a given period of said synchronizing signal; a decoder (101a) is provided for generating a decoder output signal (OUT) that is indicative of said phase difference at a first state, when said flip-flop output signal (SYN) is at said first state and said first signal (ClkDiv) is at a first state, for generating said decoder output signal (OUT) at a second state, when said flip-flop output signal (SYN) is at said second state and said first signal (ClkDiv) is at a second state and for generating said decoder output signal at a third state during the remainder of a given period of said synchronizing signal; and a low-pass filter (54) is provided responsive to said phase difference indicative signal and coupled to a control input (53a) of said oscillator (53) for controlling said oscillator in a phase-lock-loop manner. A phase-lock-loop circuit according to claim 1, wherein both when said phase difference is positive and negative, only one of a rising edge and a falling edge of a given pulse of said synchronizing signal (CSI) affects said phase difference indicative signal (OUT) and only one of a rising edge and a falling edge of a given pulse of said first signal (ClkDiv) affects said phase difference indicative signal. A phase-lock-loop circuit according to claim 2, wherein said edges of said synchronizing (CSI) and first (ClkDiv) signals are aligned in steady state phase-lock operation.

Inventors: DEN HOLLANDER WILLEM, DEN HOLLANDER, WILLEM
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1995
Main classification: H03L7
IPC codes: H03L7, H04N9, H04N5, G11B20