Phase locked loop circuit

Abstract: The circuit uses a reference signal ( r1) and a response control signal ( c1) with a preset transition time, and comprises a voltage-controlled oscillator (58) to produce a signal ( v1) variable in phase, a phase detecting circuit (10, 12, 14) delivering up (S₃) down (54) pulses to a first (16) and a second (32) charge pump circuit. Each of the charge pump circuits has three states. A first first state to advance the phase of the variable-phase signal, a second state to retard the phase of the variable-phase signal, and a third state to maintain the phase of the variable-phase signal. The circuit further comprises a response control circuit (26, 28) responsive to the response control signal for maintaining the second charge pump circuit in the third state at a time subsequent to a phase difference detect period, thereby modifying the rate at which the voltage controlled oscillator responds to the up down pulses.

Description: FIELD OF THE INVENTION The present invention relates to a phase locked loop circuit and, more particularly, to a phase locked loop circuit having variable response characteristics. BACKGROUND OF THE INVENTION A phase locked loop circuit is used to produce a signal synchronized in phase with a periodically varying reference signal such as a pulse signal fixed in phase. A type of such a phase locked loop circuit has response characteristics variable in response to a response control signal. A phase locked loop circuit of this type is used where clock pulse components are to be extracted from signals having synchronizing and data sections such as the signals read out from a flexible memory disc or a compact disc. In such an application of a phase locked loop circuit, the response characteristics of the phase locked loop circuit should be controlled so that the circuit is enabled to respond faster to the response control signal for earlier phase lock for the synchronizing sections of the signals. For the data sections of the signals, it is desirable that the phase locked loop circuit be responsive the reference control signal at retarded timings so that the circuit is enabled to follow only the fluctuations in the wow flutters which may be caused in the rotary system. On the other hand, in extracting horizontal synchronizing pulses from a composite video signal, the phase locked operation is held in abeyance when the equalizer and vertical synchronizing pulses are appearing during vertical retrace-erase-time intervals. The present invention relates to improvements in a phase locked loop circuit having such capabilities. EP-A-0 164 806 discloses a PLL-circuit according to the precharacterizing part of claim 1. This circuit has resistors R1 and R2 for an input of a loop filter, and achieves a frequency lock and a phase lock with a single VCO. The resistors R1 and R2 determines the gain allocation. Nevertheless, when this PLL-circuit as to follow a slow variation after rapidly entering locked state there may arise problems due to jitter. On the other hand, IBM Technical Disclosure Bulletin, Vol. 16, No. 11, April 1974, pages 3719-3722; K.A. Schindler: "RAPID SYNCHRONSATION CIRCUIT FOR A PHASE-LOCK OSCILLATOR" discloses a PLL-circuit comprising a clamp for clamping the output voltage signal of a filter, which is supplied to an VCO, to a certain level. This PLL-circuit controls the dynamic characteristics by limiting the input voltage signal to the filter. In order to rapidly lock the phase a charge pump implemented in this circuit is activated, and an input voltage signal is supplied to the filter. After locking the phase, the charge pump of this circuit only produces the input voltage signal. SUMMARY OF THE INVENTION It is an object of the present invention to improve the phase locked loop circuit, by exactly controlling the frequency controllable oscillator means. Another object of the present invention is to provide a PLL-circuit with variable response characteristics. Still another object of the present invention is to provide a PLL-circuit which is able to follow a slow variation after rapidly entering locked state. These objects are achieved by a PLL-circuit according to claim 1. The dependent claims a related to further advantageous aspects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of a phase locked loop circuit according to the present invention will be more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding units and elements and in which: Fig. 1 is a block diagram showing a preferred embodiment of a phase locked loop circuit according to the present invention; Fig. 2 is a time chart showing the waveforms of various pulse signals which may appear in the phase locked loop circuit shown in Fig. 1; Fig. 3 is a block diagram showing another preferred embodiment of a phase locked loop circuit according to the present invention; and Fig. 4 is a time chart showing the waveforms of various pulse signals which may appear in the phase locked loop circuit which is shown in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 of the drawings, a phase locked loop circuit embodying the present invention comprises a set-reset flip-flop circuit 10 (labeled as FF) having a set terminal S responsive to a reference pulse signal S r1 with a predetermined period T as shown in the time chart of Fig. 2 and a reset terminal R responsive to a variable input pulse signal S v1 . The input pulse signal S v1 is representative of the signal or, more precisely, the inverted version of the pulse signal to be controlled by the loop circuit according to the present invention. The set-reset flip-flop circuit 10 further has an output terminal Q to produce an output signal S 1 in response to the falling edges of the pulse signal S r1 input to the set terminal S . The output pulse signal S 1 from the set-reset flip-flop circuit 10 is supplied to the trigger terminal C of a monostable multivibrator 12 (labeled as MMV) which has an output terminal to produce in response to the input pulse signal S 1 an output pulse signal S 2 with a predetermined pulsewidth T/2 equal to the period T of the reference pulse signal S r1 as shown in Fig. 2. There is further provided a phase comparator 14 which has first and second input terminals V and R responsive to the pulse signals S 1 and S 2 , respectively, from the flip-flop circuit 10 and monostable multivibrator 12 and first and second output terminals UP and DN to produce output pulse signals S 3 and S 4 , respectively. The pulse signal S 3 from the first output terminal UP has a logic "0" level when the pulse signal S v1 to be controlled is backwardly out of phase with the reference pulse signal S r1 and otherwise a logic "1" level, as shown in Fig. 2. On the other hand, the pulse signal S 4 from the second output terminal DN has a logic "0" level when the pulse signal S v1 is forwardly out of phase with the reference pulse signal S r1 and otherwise a logic "1" level, as also shown in Fig. 2. The phase comparator 14 to achieve these functions may be of any of the desired types and/or designs. The signals S 3 and S 4 from the output terminals UP and DN of the phase comparator 14 are supplied to a first charge pump circuit 16 of CMOS (complementary metal-oxide semiconductor) configuration. The charge pump circuit 16 is adapted to discriminate whether or not the pulse signals S v1 and S r1 input to the flip-flop circuit 10 are in phase with each other and, if not, whether the pulse signal S v1 is forwardly or backwardly out of phase with the pulse signal S r1 . Such a charge pump circuit 16 comprises a series combination of p-channel and n-channel field-effect transistors 18 and 20 connected between sources of an appropriate "high" voltage such as a positive supply voltage V DD and an appropriate "low" voltage such as ground potential V SS . The p-channel field-effect transistor 18 has its gate connected to the first output terminal UP of the phase comparator 14 and its source terminal connected to the source of the positive supply voltage V DD . The n-channel field-effect transistor 20 has its gate connected through an inverter 22 to the second output terminal DN of the phase comparator 14 and its source terminal connected to the source of the ground potential V SS . The respective drain terminals of the transistors 18 and 20 are connected together to provide an output node 24 at which is to be produced a first voltage signal S 5 of either a high level or a low level or of a high-impedance state floating between the high and low levels depending on the respective logic states of the pulse signals S 3 and S 4 output from the phase comparator. In the presence of the signal S 3 of the logic "0" level and concurrently the signal S 4 of the logic "1" level with the pulse signal S v1 backwardly out of phase with the reference pulse signal S r1 , the p-channel field-effect transistor 18 is turned on with the n-channel field-effect transistor 20 turned off. The output node 24 of the charge pump circuit 16 is now connected to the source of the positive supply voltage V DD through the p-channel field-effect transistor 18 so that a high voltage approximating the supply voltage V DD appears as the first voltage signal S 5 at the output node 24 of the charge pump circuit 16. When, on the other hand, the pulse signal S v1 is forwardly out of phase with the reference pulse signal S r1 so that the signal S 3 is of the logic "1" level and concurrently the signal S 4 is of logic "0" level, the n-channel field-effect transistor 20 is turned on and the p-channel field-effect transistor 18 is turned off. The output node 24 of the charge pump circuit 16 is now connected to the source of the ground potential V SS through the n-channel field-effect transistor 20 so that a low voltage approximating the ground potential V SS appears as the voltage signal S 5 at the node 24 of the charge pump circuit 16. It will thus be seen that the voltage signal S 5 of a high level as appearing at the output node 24 of the charge pump circuit 16 indicates that the pulse signal S v1 is backwardly out of phase with the reference pulse signal S r1 and, conversely, the voltage signal S 5 of a low level as appearing at the node 24 indicates that the pulse signal S v1 is forwardly out of phase with the reference pulse signal S r1 . In the presence of the pulse signals S 3 and S 4 each of a logic "1" level as appearing when, for example, the pulse signals S v1 and S r1 are in phase with each other, both of the p-channel and n-channel field-effect transistors 18 and 20 are turned off so that the first voltage signal S 5 at the output node 24 assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS . The phase locked loop circuit embodying the present invention further comprises a two-input logic NOR 26 and a delay-latch flip-flop circuit 28. The logic NOR gate 26 has one of its input terminals connected to the output terminal Q of the set-reset flip-flop circuit 10 and the other of the input terminals connected to the output terminal Q of the monostable multivibrator 12. The NOR gate 26 is thus responsive to the pulse signal S 1 from the flip-flop circuit 10 and the pulse signal S 2 from the multivibrator 12 and further has an output terminal to produce an output signal S 6 of a logic "1" level in the presence concurrently of the pulse signals S 1 and S 2 each of the logic "0" level. The delay-latch flip-flop circuit 28 has an input terminal D responsive to a response control pulse signal S c1 and a control terminal C connected to the output terminal of the NOR gate 26. The delay-latch flip-flop circuit 28 is thus responsive to the response control pulse signal S c1 and the pulse signal S 6 from the NOR gate 26 to produce an output pulse signal of a state which the pulse signal S c1 has at an instant the pulse signal S 6 from the NOR gate 26 rises from a logic "0" level to a logic "1" level (or, if desired, falls from the logic "1" level to the logic "0" level), as well known in the art. The signals S 3 and S 4 from the output terminals UP and DN of the phase comparator 14 are further supplied through a logic circuit 30 to a second charge pump circuit 32 which is also of CMOS configuration. The logic circuit 30 comprises a two-input logic OR gate 34 and a two-input logic NOR gate 36. The logic OR gate 34 has one of its input terminals connected to the first output terminal UP of the phase comparator 24 and the other of the input terminals connected to the output terminal Q of the delay-latch flip-flop circuit 28. The logic NOR gate 36 has one of its input terminals connected to the second output terminal DN of the phase comparator 24 and the other of the input terminals also connected to the output terminal Q of the delay-latch flip-flop circuit 28. On the other hand, the second charge pump circuit 32 comprises a series combination of p-channel and n-channel field-effect transistors 38 and 40 connected between the sources of the positive supply voltage V DD and ground potential V SS . The p-channel field-effect transistor 38 of the charge pump circuit 32 has its gate connected to the output terminal of the logic OR gate 34 and its source terminal connected to the source of the positive supply voltage V DD . The n-channel field-effect transistor 40 of the charge pump circuit 32 has its gate connected to the output terminal of the logic NOR gate 36 and its source terminal connected to the source of the ground potential V SS . The respective drain terminals of the transistors 38 and 40 are connected together to provide an output node 42 at which is to be produced a second voltage signal S 7 of either a high level or a low level or a high-impedance state floating between the high and low levels depending on the respective logic states of the pulse signals appearing at the output terminals of the OR and NOR gates 34 and 36. In the presence of a pulse signal of a logic "0" level at each of the output terminals of the OR and NOR gates 34 and 36, the p-channel field-effect transistor 38 is turned on with the n-channel field-effect transistor 40 turned off. The output node 42 of the charge pump circuit 32 is now connected to the source of the positive supply voltage V DD through the p-channel field-effect transistor 38 so that a high voltage approximating the supply voltage V DD appears as the second voltage signal S 7 at the output node 42 of the charge pump circuit 32. When, on the other hand, a pulse signal of a logic "1" level is present at each of the output terminals of the OR and NOR gates 34 and 36, the n-channel field-effect transistor 40 is turned on and the p-channel field-effect transistor 38 turned off. The output node 42 of the charge pump circuit 32 is now connected to the source of the ground potential V SS through the n-channel field-effect transistor 40 so that a low voltage approximating the ground potential V SS appears as the second voltage signal S 7 at the output node 42 of the charge pump circuit 32. Thus, the voltage signal S 7 of a high level as appearing at the output node 42 of the charge pump circuit 32 indicates that both of the pulse signals input to the OR gate 34 are of logic "0" levels and, in addition, at least one of the pulse signals input to the NOR gate 36 is of a logic "1" level. On the other hand, the second voltage signal S 7 of a low level as appearing at the output node 42 of the charge pump circuit 32 indicates that at least one of the pulse signals input to the OR gate 34 is of a logic "1" level and, in addition, both of the pulse signals input to the NOR gate 36 are of logic "0" levels. When at least one of the pulse signals input to the OR gate 34 is of a logic "1" level and, concurrently, at least one of the pulse signals input to the NOR gate 36 is of a logic "1" level, both of the p-channel and n-channel field-effect transistors 38 and 40 are turned off in the presence of a pulse signal (represented by S 3 ′) of a logic "1" level at the output terminal of the OR gate 34 and a pulse signal (represented by S 4 ′) of a logic "0" level at the output terminal of the NOR gate 36. The second voltage signal S 7 at the output node 42 of the charge pump circuit 32 now assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS . The voltage signals S 5 and S 7 thus appearing at the output nodes 24 and 42 of the first and second charge pump circuits 16 and 32 are supplied through resistors 44 and 46, respectively, to a low-pass filter network 48. The low-pass filter network 48 comprises a capacitor 50 connected in parallel with a series combination of a resistor 52 and a capacitor 54 and has an output node 56 at which is to appear a control voltage signal S 8 to result from the first and second voltage signals S 5 and S 7 . The control voltage signal S 8 thus appearing at the output node 56 of the low-pass filter network 48 is supplied to a voltage controlled oscillator circuit 58 (labeled as VCO) which has an output terminal connected to a ripple-carry counter 60. The voltage controlled oscillator circuit 58 is operative to produce a pulse signal with a phase advanced in response to an increase in the control voltage signal S 8 and retarded in response to a decrease in the control voltage signal S 8 . The ripple-carry counter 60 in turn has an output terminal connected through an inverter 62 to the reset terminal R of the flip-flop circuit 10 for supplying thereto the pulse signal S v1 or the inverted version of the pulse signal (S v1 ) to be controlled by the loop circuit under consideration. The ripple-carry counter 60 may be of any of the desired types and/or designs known in the art. Description will now be made with reference to Figs. 1 and 2 in regard to the operation of the phase locked loop circuit thus constructed and arranged. It is assumed by way of example that the reference pulse signal S r1 supplied to the set terminal S of the flip-flop circuit 10 is to fall at times t 1 , t 4 , t 7 , ... while the pulse signal S v1 supplied to the reset terminal R of the flip-flop circuit 10 is to fall at times t 3 , t 5 , t 9 , ... which are respectively subsequent to the times t 1 , t 4 , t 7 , ... as shown in the time chart of Fig. 2. As noted previously, the reference pulse signal S r1 has a predetermined period T so that the times t 1 , t 4 , t 7 , ... are herein assumed to occur at intervals equal to the period of cycle T. The times indicated by t 2 , t 6 and t 8 represent the middle points of the successive periods T each intervening between a falling edge and the immediately subsequent leading edge. From Fig. 2 it will thus be understood that the pulse signal S v1 to be controlled by the phase locked loop circuit shown has a falling edge which is either delayed or advanced with respect to the middle point of each of the periods T. The pulse signal S v1 has a falling edge delayed with respect to the middle point t 2 or t 8 of the period intervening between the times t 1 and t 4 or between the time t 7 and the time (not shown) at which the pulse signal S v1 is to fall thereafter. The pulse signal S v1 further has a falling edge which is advanced with respect to the middle point t 6 of the period intervening between the times t 4 and t 7 . It is further assumed that the reference control pulse signal S c1 supplied to the input terminal D of the delay-latch flip-flop circuit 28 remains at a logic "0" level by time t 10 intervening between the times t 5 and t 6 and swings from the logic "0" level to logic "1" level at such a time t 10 as also shown in the time chart of Fig. 2. The set-reset flip-flop circuit 10 is set at each of the falling edges of the reference pulse signal S r1 and produces an output pulse signal S 1 which rises at times t 1 , t 4 , t 7 , ... and falls at times t 3 , t 5 , t 9 , ... as shown. On the other hand, the monostable multivibrator 12 is triggered at each of the falling edges of the pulse signal S 1 and produces an output pulse signal S 2 which rises at times t 1 , t 4 , t 7 , ... and falls at times t 2 , t 6 , t 8 , ... which occur upon lapse of the delay time equal to one half (T/2) of the period T of the pulse signal S r1 after the times t 1 , t 5 , t 7 , ... as shown. Thus, the pulse signal S 2 produced from the monostable multivibrator 12 has a frequency doubling the frequency of the reference pulse signal S r1 and the times t 2 , t 6 , t 8 , .., occur at the middle points between the successive times t 1 , t 4 , t 7 , ... at which the reference pulse signal S r1 falls. In addition, the falling edges of the pulse signal S 1 are synchronized with the falling edges of the pulse signal S v1 and the falling edges of the pulse signal S 2 synchronized with the middle points between the successive falling edges of the reference signal S r1 . Thus, an advance or a delay in the falling edge of the pulse signal S 1 with respect to the falling edge of the pulse signal S 2 respectively corresponds to the advance or delay in phase of the pulse signal S v1 with respect to the reference pulse signal S r1 . In response to these two pulse signals S 1 and S 2 from the flip-flop circuit 10 and monostable multivibrator 12, the phase comparator 14 produces pulse signals S 3 and S 4 at its first and second output terminals UP and DN . As noted previously, the pulse signal S 3 has the logic "0" level when the pulse signal S v1 to be controlled is backwardly out of phase with the reference pulse signal S r1 and otherwise the logic "1" level and while the pulse signal S 4 has the logic "0" level when the pulse signal S v1 is forwardly out of phase with the reference pulse signal S r1 and otherwise the logic "1" level. Thus, the phase comparator 14 produces the pulse signal S 3 of the logic "0" level during, for example, the period of time intervening between the times t 2 and t 3 and the period of time intervening between the times t 8 and t 9 as shown while the pulse signal S 4 of the logic "0" level during, for example, the period of time intervening between the times t 5 and t 6 . Thus, the logic "0" level of either the pulse signal S 3 or the pulse signal S 4 indicates an advance or a delay in the falling edge of the pulse signal S 1 with respect to the falling edge of the pulse signal S 2 or, in other words, an advance or delay in phase of the pulse signal S v1 with respect to the reference pulse signal S r1 . It may also be noted that each of the periods of time t 2 -t 3 and t 8 -t 9 for which the pulse signal S 3 remains at the logic "0" level and the period of time t 5 -t 6 for which the pulse signal S 4 remains at the logic "0" level is proportional to the amount of phase difference between the pulse signals S v1 and S r1 . This means that each of the pulse signals S 3 and S 4 provides information regarding the phase difference of the pulse signal S v1 advanced or delayed with respect to the reference pulse signal S r1 during a period of time when the pulse signal S 3 or S 4 is maintained at the logic "0" level. Such a period of time will be herein referred to as phase difference detect period and is denoted by t A for an advanced phase and by t D for a delayed phase in the time chart of Fig. 2. During a phase difference detect period t D for which the pulse signal S 3 is of the logic "0" level and concurrently the pulse signal S 4 is of the logic "1" level, the p-channel field-effect transistor 18 of the first charge pump circuit 16 is turned on as at time t 2 or time t 8 with the n-channel field-effect transistor 20 turned off. A high voltage approximating the positive supply voltage V DD thus appears as the first voltage signal S 5 of a high level at the output node 24 of the charge pump circuit 16 as will be seen from Fig. 2. On the other hand, during a phase difference detect period t A for which the pulse signal S 4 is of the logic "0" level and concurrently the pulse signal S 3 is of the logic "1" level, the n-channel field-effect transistor 20 of the charge pump circuit 16 is turned on as at time t 5 with the p-channel field-effect transistor 18 turned off. A low voltage approximating the ground potential V SS appears as the first voltage signal S 5 of a low level at the output node 24 of the charge pump circuit 16 as will be seen from Fig. 2. When both of the pulse signals S 3 and S 4 from the phase comparator 14 are maintained at the logic "1" levels with, for example, the pulse signals S v1 and S r1 in phase with each other, both of the p-channel and n-channel field-effect transistors 18 and 20 are turned off so that the first voltage signal S 5 at the output node 24 assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS as will be also seen from Fig. 2. When both of the pulse signals S 1 and S 2 are concurrently of the logic "0" levels, the NOR gate 26 produces a logic "1" output signal as the pulse signal S 6 and supplies the logic "1" pulse signal S 6 to the control terminal C of the delay-latch flip-flop circuit 28. The pulse signal S 6 from the NOR gate 26 thus falls at times t 1 , t 4 , t 7 , ... and rises at times t 3 , t 6 , t 9 , ... as will be seen from Fig. 2. On the other hand, the reference control pulse signal S c1 appearing at the input terminal D of the delay-latch flip-flop circuit 28 swings from the logic "0" level to the logic "1" level as at time t 10 intervening between the times t 5 and t 6 . At this point of time t 10 , however, the pulse signal S 6 from the NOR gate 26 remains at the logic "0" level with the pulse signals S 1 and S 2 swung to the logic "1" levels as shown so that the pulse signal at the output terminal Q of the delay-latch flip-flop circuit 28 still remains at the logic "0" level. As will be seen, it is at time t 6 when the pulse signal at the output terminal Q of the flip-flop circuit 28 swings to the logic "1" level so that, until the time t 6 , the pulse signal at the output terminal Q of the flip-flop circuit 28 is maintained at the logic "0" level. At time t 2 when the first phase difference detect period t D begins, the pulse signal S 3 from the first output terminal UP of the phase comparator 14 shifts from the logic "1" level to logic "0" level. In the presence now of the logic "0" pulse signals at both of its input terminals, the OR gate 34 of the logic circuit 30 is conditioned to produce a logic "0" signal as its output pulse signal S 3 ′ and activates the p-channel field-effect transistor 38 of the second charge pump circuit 32 to turn on as at time t 2 . On the other hand, the pulse signal S 4 from the second output terminal DN of the phase comparator 14 is held at the logic "1" level at time t 2 so that the NOR gate 36 of the logic circuit 30 is producing a logic "0" signal as its output pulse signal S 4 ′ and allows the n-channel field-effect transistor 40 of the charge pump circuit 32 to be turned off. At time t 2 , a high voltage approximating the positive supply voltage V DD thus appears at the output node 42 of the charge pump circuit 32. During the phase difference detect period t D intervening between the times t 2 and t 3 , the high voltage approximating the supply voltage V DD is thus established as the second voltage signal S 7 at the output node 42 of the charge pump circuit 32 as will be seen from Fig. 2. At time t 3 when the phase difference detect period t D terminates, the pulse signal S 3 from the first output terminal UP of the phase comparator 14 shifts from the logic "1" back to the level logic "0" level. It therefore follows that the pulse signal S 3 ' output from the OR gate 34 also swings back to the logic "1" level and accordingly the p-channel field-effect transistor 38 of the charge pump circuit 32 is turned off at time t 3 . At time t 5 when the phase difference detect period t A begins, the pulse signal S 3 from the first output terminal UP of the phase comparator 14 remains at the logic "1". With the logic "0" pulse signal present at one of its input terminals and the logic "0" pulse signal at the other, the OR gate 34 of the logic circuit 30 is now conditioned to produce a logic "1" signal as its output pulse signal S 3 ' and causes the p-channel field-effect transistor 38 of the second charge pump circuit 32 to turn off at time t 5 . On the other hand, the pulse signal S 4 from the second output terminal DN of the phase comparator 14 swings from the logic "1" level to logic "0" level at time t 5 so that the NOR gate 36 of the logic circuit 30 is conditioned to produce a logic "1" signal as its output pulse signal S 4 ' and activates the n-channel field-effect transistor 40 of the charge pump circuit 32 to turn on. At time t 5 , a low voltage approximating the ground potential V SS thus appears at the output node 42 of the charge pump circuit 32. During the phase difference detect period t A intervening between the times t 5 and t 6 , the high voltage approximating the supply voltage V DD is thus established as the second voltage signal S 7 at the output node 42 of the charge pump circuit 32 as will be seen from Fig. 2. At time t 10 immediately subsequent to the time t 5 , the reference control pulse signal S c1 appearing at the input terminal D of the delay-latch flip-flop circuit 28 swings from the logic "0" level to the logic "1" level. When the pulse signal S 2 from the monostable multivibrator 12 then falls to the logic "0" level at time t 6 subsequent to the time t 10 , the pulse signal S 6 from the NOR gate 26 shifts from the logic "0" level to logic "1" level so that the pulse signal at the output terminal Q of the delay-latch flip-flop circuit 28 is now allowed to rise from the logic "0" level to logic "1" level. With the logic "1" pulse signal thus produced at one of its input terminals, the NOR gate 36 of the logic circuit 30 now outputs a logic "0" pulse signal to cause the associated n-channel field-effect transistor 40 of the charge pump circuit 32 to turn off at time t 6 , thereby putting an end to the phase difference detect period t D as will be seen from Fig. 2. When both of the signals S 3 and S 4 output from the phase comparator 14 are of the logic "1" levels with, for example, the pulse signal S v1 held in phase with the reference pulse signal S r1 and/or when the pulse signal output from the delay-latch flip-flop circuit 28 is swung to the logic "1" level, there concurrently are a logic "1" pulse signal S 3 ′ at the output terminal of the OR gate 34 and a logic "0" pulse signal S 4 ' at the output terminal of the NOR gate 36. Both of the p-channel and n-channel field-effect transistors 38 and 40 of the second charge pump circuit 32 are thus turned off so that the voltage signal S 7 at the output node 42 of the second charge pump circuit 32 assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS as will be also seen from Fig. 2. As will have been seen from the foregoing description, the voltage signal S 5 at the output node 24 of the first charge pump circuit 16 assumes a high voltage level approximating the supply voltage V DD during a phase difference detect period t D when the input pulse signal S v1 to be controlled is backwardly out of phase with the reference pulse signal S r1 . During the phase difference detect period t A when the pulse signal S v1 to be controlled is forwardly out of phase with the reference pulse signal S r1 , the first voltage signal S 5 assumes a low voltage level approximating the ground potential V SS . Such a high or low voltage signal S 5 is produced without respect to the logic state of the response control pulse signal S c1 supplied to the delay-latch flip-flop circuit 28. The voltage signal S 7 at the output node 42 of the second charge pump circuit 32 may also assume a high voltage level approximating the supply voltage V DD during a phase difference detect period t D when the pulse signal S v1 is backwardly out of phase with the reference pulse signal S r1 . Likewise, during the phase difference detect period t A when the pulse signal S v1 is forwardly out of phase with the reference pulse signal S r1 , the first voltage signal S 5 may assume a low voltage level approximating the ground potential V SS . Such a high or low voltage signal S 5 is produced in the presence of the logic "0" level at the output terminal Q of the delay-latch flip-flop circuit 28 which is responsive to the response control pulse signal S c1 supplied to the input terminal D thereof. The high or low voltage signals S 5 and S 7 thus produced at the output nodes 24 and 42 of the first and second charge pump circuits 16 and 32, respectively, are fed to the low-pass filter network 48 which thus produces the previously mentioned control voltage signal S 8 at its output node 56. The control voltage signal S 8 resulting from the first and second voltage signals S 5 and S 7 is, in turn, supplied to the voltage controlled oscillator circuit 58 to increase or decrease the control voltage for the oscillator circuit 58. During a phase difference detect periods t D for which the pulse signal S v1 to be controlled is backwardly out of phase with the reference pulse signal S r1 , the voltage controlled oscillator circuit 58 receives the control voltage signal S 8 which is increased in response to the first and second voltage signals S 5 and S 7 approximating the supply voltage V DD if the pulse signal output from the delay-latch flip-flop circuit 28 is of the logic "0" level. In response to such a control voltage signal S 8 , the voltage controlled oscillator circuit 58 is conditioned to generate pulses with an advanced phase so that the pulse signal S v1 to be controlled synchronizes with the reference pulse signal S r1 . On the other hand, during a phase difference detect periods t A for which the pulse signal S v1 to be controlled is forwardly out of phase with the reference pulse signal S r1 , the voltage controlled oscillator circuit 58 receives the control voltage sign S 8 decreased in response to the voltage signals S 5 and S 7 approximating the ground potential V SS if, also, the pulse signal output from the delay-latch flip-flop circuit 28 is of the logic "0" level. In response to such a control voltage signal S 8 , the voltage controlled oscillator circuit 58 is conditioned to generate pulses with a retarded phase so that the pulse signal S v1 synchronizes with the reference pulse signal S r1 . As the pulses generated by the voltage controlled oscillator circuit 58 are thus advanced or retarded in phase, the pulse signal output from the ripple-carry counter 60 and accordingly its inverted version, viz., the pulse signal S v1 as appearing at the output terminal of the inverter 62 subsequent to the counter 60 is also advanced or retarded in phase and will be synchronized with the reference pulse signal S r1 . When the response control pulse signal S c1 swings from the logic "0" level to logic "1" level as at time t 10 as illustrated in Fig. 2, the pulse signal output from the delay-latch flip-flop circuit 28 similarly swings as at time t 6 when the pulse signal input to the control terminal C of the flip-flop circuit 28 rises after time t 10 . As a consequence, the output pulse signal S 4 ′ from the NOR gate 36 swings from the logic "1" level to logic "0" level and causes the n-channel field-effect transistor 40 of the charge pump circuit 32 to turn off at time t 6 . The p-channel field-effect transistor 38 of the charge pump circuit 32 having already been turned off, the second voltage signal S 7 at the output node 42 of the charge pump circuit 32 shifts back to the floating level upon lapse of the phase difference detect period t D at time t 6 . During the phase difference detect period t D intervening between the times t 8 and t 9 , the pulse signal S 3 is of the logic "0" level and concurrently the pulse signal S 4 is of the logic "1" level. The n-channel field-effect transistor 20 of the first charge pump circuit 16 is thus turned off at time t 8 so that a high voltage approximating the positive supply voltage V DD appears as the first voltage signal S 5 of a high level at the output node 24 of the charge pump circuit 16. The output signal from the delay-latch flip-flop circuit 28 being maintained at the logic "1" level, the OR gate 34 of the logic circuit 30 is producing a logic "1" pulse signal S 3 ′ at its output terminal so that the p-channel field-effect transistor 38 of the second charge pump circuit 32 is turned off. In the presence of the logic "1" pulse signal at the output terminal Q of the flip-flop circuit 28, furthermore, the NOR gate 36 of the logic circuit 30 is producing a logic "0" pulse signal S 4 ′ so that the p-channel field-effect transistor 38 of the second charge pump circuit 32 is turned off. The second voltage signal S 7 at the output node 42 of the charge pump circuit 32 is thus maintained at a high-impedance state floating between the supply voltage V DD and ground potential V SS without rising at time t 8 (as indicated in phantom at X in Fig. 2. Under these conditions, only the first voltage signal S 5 at the output node 24 of the first charge pump circuit 16 is increased to the level approximating the supply voltage V DD so that the control voltage signal S 8 resulting from the voltage signals S 5 and S 7 increases at a reduced rate. Such a control voltage signal S 8 is supplied to the voltage controlled oscillator circuit 58 to increase the control voltage for the oscillator circuit 58 now at a reduced rate so that the oscillator circuit 58 is conditioned to generate pulses with a phase advanced at a limited rate. It will thus be understood that the response control signal S c1 used in the phase locked loop circuit embodying the present invention is effective to provide a shift between the rates at which the voltage controlled oscilator circuit 58 responds to the pulse signals S 3 and S 4 from the phase comparator 14 during a phase difference detect period t D or t A . As will have been understood from the foregoing description, the set-reset flip-flop circuit 10, monostable multibivrator 12, phase comparator circuit 14 and logic circuit 30 in the embodiment hereinbefore described with reference to Fig. 1 implement phase difference detecting means responsive to the reference signal S r1 and to the variable-phase signal S v1 for producing output signals (S 3 /S 4 /S 3 ′/S 4 ′) containing information indicative of a difference in phase, if any, between the reference signal S r1 and the variable-phase signal S v1 . Furthermore, the first and second charge pump circuits 16 and 32 implement current supply means responsive to the output signals (S 3 /S 4 /S 3 ′/S 4 ′) from the phase difference detecting means (10/12/14/30/62) for producing a charge current or a discharge current depending upon the information represented by the output signals (S 3 /S 4 /S 3 ′/S 4 ′) from the phase difference detecting means (10/12/14/30/62). On the other hand, the low-pass filter circuit 48 in the embodiment of Fig. 1 implements frequency control means responsive to the charge current or the discharge current from the current supply means (16/32) for producing a frequency control signal (S 8 ) on the basis of which the frequency at which the voltage-controlled oscillator circuit 58 is to oscillate is controlled. In addition, the NOR gate 26 and the delay-latch flip-flop circuit 28 implement response control means responsive to the response control signal S c1 for prohibiting the current supply means (16/32) from producing the charge current or discharge current. It has been assumed in the foregoing description that the response control signal S c1 per se rises at time t 10 which intervenes between the times t 5 and t 6 at which the phase difference detect period t A begins and terminates. It may be noted in this instance that, although the response control signal S c1 per se may thus change its logic state (viz., rise or fall) prior to the termination of the phase difference detect period t A , it is just at the time of termination of the period t D when the pulse signal output from the delay-latch flip-flop circuit 28 changes its logic state as from the logic "0" level to logic "1" level at time t 6 in response to the rise of the pulse signal S 6 supplied to the control terminal C of the flip-flop circuit 28. This means that, if the response control signal S c1 happens to change its logic state during a phase difference detect period t A (or also t A ), the information provided by the pulse signals S 3 and S 4 regarding the phase difference of the pulse signal S v1 advanced or delayed with respect to the reference pulse signal S r1 during such a period of time could not be lost or vanish. It has been assumed that the monostable multivibrator 12 is operative so that the output pulse signal S 2 produced in response to the input pulse signal S 1 has a pulsewidth T/2 equal to the period T of the reference pulse signal S r1 as shown in Fig. 2. This however is not critical, in the present invention and, thus, the pulsewidth of the output pulse signal S 2 to be produced by the monostable multivibrator 12 may be selected to be appropriately with respect to the period T of the reference pulse signal S r1 . Fig. 3 of the drawings shows another preferred embodiment of a phase locked loop circuit according to the present invention. The phase locked loop circuit now comprises a logic circuit comprising a parallel combination of a three-input logic NAND gate 66 and a three-input logic NOR gate 68. The NAND gate 66 has a first input terminal responsive to a reference pulse signal S r2 , a second input terminal responsive to a variable input pulse signal S v2 through a first inverter 70, and a third input terminal to be described. The reference pulse signal S r2 has a predetermined period T′ as shown in the time chart of Fig. 4. The NAND gate 66 further has an output terminal to produce an output pulse signal S 11 of logic "0" or "1" depending upon the logic states of the signals to appear at the input terminals thereof. The NOR gate 68 has a first input terminal responsive to the reference pulse signal S r2 through a second inverter 72, a second input terminal responsive to the pulse signal S v2 through the inverter 70, and a third input terminal to be described. The NOR gate 68 further has an output terminal to produce an output pulse signal S 12 of logic "0" or "1" depending upon the logic states of the signals to appear at the input terminals thereof. The pulse signals S 11 and S 12 respectively output from the NAND and NOR gates 66 and 68 thus arranged are supplied to a charge pump circuit 74. The charge pump circuit 74 per se is constructed similarly to the second charge pump circuit 32 of the phase locked loop circuit described with reference to Fig. 1 and, thus, comprises a series combination of p-channel and n-channel field-effect transistors 38 and 40 connected between the sources of the positive supply voltage V DD and ground potential V SS as shown. The p-channel field-effect transistor 38 of the charge pump circuit 74 has its gate connected to the output terminal of the NAND gate 66 and its source terminal connected to the source of the positive supply voltage V DD . The n-channel field-effect transistor 40 of the charge pump circuit 74 has its gate connected to the output terminal of the logic NOR gate 68 and its source terminal connected to the source of the ground potential V SS . The respective drain terminals of the transistors 38 and 40 are connected together to provide an output node 42 at which is to be produced a voltage signal S 13 of either a high level or a low level or a high-impedance state floating between the high and low levels depending on the respective logic states of the pulse signals appearing at the output terminals of the NAND and NOR gates 66 and 68. In the presence of a pulse signal of a logic "0" level at each of the output terminals of the NAND and NOR gates 66 and 68, the p-channel field-effect transistor 38 is turned on with the n-channel field-effect transistor 40 turned off. The output node 42 of the charge pump circuit 74 is now connected to the source of the positive supply voltage V DD through the p-channel field-effect transistor 38 so that a high voltage approximating the supply voltage V DD appears as the voltage signal S 13 at the output node 42 of the charge pump circuit 74. When, on the other hand, a pulse signal of a logic "1" level is present at each of the output terminals of the NAND and NOR gates 66 and 68, the n-channel field-effect transistor 40 is turned on and the p-channel field-effect transistor 38 turned off. The output node 42 of the charge pump circuit 74 is now connected to the source of the ground potential V SS through the n-channel field-effect transistor 40 so that a low voltage approximating the ground potential V SS appears as the voltage signal S 13 at the output node 42 of the charge pump circuit 74. Thus, the voltage signal S 13 of a high level as appearing at the output node 42 of the charge pump circuit 74 indicates that all of the pulse signals input to the NAND gate 66 are of logic "1" levels and, in addition, at least one of the pulse signals input to the NOR gate 68 is of a logic "1" level. On the other hand, the voltage signal S 13 of a low level as appearing at the output node 42 of the charge pump circuit 74 indicates that at least one of the pulse signals input to the NAND gate 66 is of a logic "0" level and, in addition, all of the pulse signals input to the NOR gate 68 are of logic "0" levels. When at least one of the pulse signals input to the NAND gate 66 is of a logic "0" level and, concurrently, at least one of the pulse signals input to the NOR gate 68 is of a logic "1" level, both of the p-channel and n-channel field-effect transistors 38 and 40 are turned off in the presence of a signal of a logic "1" level at the output terminal of the NAND gate 66 and a signal of a logic "0" level at the output terminal of the NOR gate 68. The voltage signal S 13 at the output node 42 of the charge pump circuit 74 now assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS . The voltage signal S 13 thus appearing at the output node 42 of the charge pump circuit 74 is supplied through a resistor 46 to a low-pass filter network 76. The low-pass filter network 76 is similar to its counterpart in the phase locked loop circuit described with reference to Fig. 1 and comprises a capacitor 50 connected in parallel with a series combination of a resistor 52 and a capacitor 54. The low-pass filter network 76 is operative to smooth out the input voltage signal S 13 and has an output node 56 at which is to appear a control voltage signal S 14 which thus results from the voltage signal S 13 . The control voltage signal S 14 thus appearing at the output node 56 of the low-pass filter network 76 is supplied to a voltage controlled oscillator circuit 78 (VCO) which has an output terminal connected to a frequency divider 80. The voltage controlled oscillator circuit 78 is also similar to its counterpart in the phase locked loop circuit described with reference to Fig. 1 and is operative to produce a pulse signal with a phase advanced in response to an increase in the control voltage signal S 14 and retarded in response to a decrease in the voltage signal S 14 . The frequency divider 80 in turn has an output terminal connected through the inverter 70 to the second input terminal of each of the NAND and NOR gates 66 and 68 and further through a third inverter 82 to the control terminal C of a delay-latch flip-flop circuit 84. From the frequency divider 80 to the second input terminal of each of the NAND and NOR gates 66 and 68 is thus supplied the inverted version, S v2 , of the pulse signal S v2 to be controlled by the loop circuit under consideration. The frequency divider 80 may be of any of the desired types and/or designs known in the art. The delay-latch flip-flop circuit 84 has, in addition to the control terminal C, an input terminal D responsive to a response control pulse signal S c2 . The delay-latch flip-flop circuit 84 is thus responsive to the response control pulse signal S c1 and the pulse signal S v2 from the inverter 82 to produce at its non-inverted output terminal Q a pulse signal of a state which the pulse signal S c2 has at an instant the pulse signal S v2 from the inverter 82 rises from a logic "0" level to a logic "1" level (or, if desired, falls from the logic "1" level to the logic "0" level). The pulse signal thus appearing at the non-inverted output terminal Q of the delay-latch flip-flop circuit 84 is supplied to the third input terminal of the NAND gate 66 of the logic circuit 64. The delay-latch flip-flop circuit 84 is further operative to produce the inverted version of such a pulse signal at its inverted output terminal Q . The pulse signal which appears at the inverted output terminal Q of the flip-flop circuit 84 is supplied to the third input terminal of the NOR gate 68 of the logic circuit 64. Description will now be made with reference to Figs. 3 and 4 in regard to the operation of the phase locked loop circuit thus constructed and arranged. It is assumed by way of example that the reference pulse signal S r2 herein used is to rise at times t 11 , t 15 , t 19 , ... and to fall at times t 13 , t 17 , t 21 , ... while the pulse signal S v2 is to rise at times t 12 , t 16 , t 20 , ... which are respectively subsequent to the times t 11 , t 15 , t 19 , ... and to fall at times t 14 , t 16 , t 18 , ... which are respectively subsequent to the times t 13 , t 17 , ... as shown in the time chart of Fig. 4. As noted previously, the reference pulse signal S r2 has a predetermined period T so that the times t 11 , t 15 , t 19 , ... are herein assumed to occur at intervals equal to the period of cycle T. From Fig. 4 it will be further seen that the pulse signal S v2 to be controlled by the phase locked loop circuit shown has a leading edge either advanced or delayed with respect to the middle point of a period for which the reference pulse signal S r2 remains at a logic "1" level. The pulse signal S v2 has a leading edge advanced with respect to the middle point of the period intervening between the times t 1 and t 4 or between the time t 7 and the time (not shown) at which the pulse signal S v2 is to fall thereafter. The pulse signal S v1 further has a leading edge which is delayed with respect to the middle point t 6 of the period intervening between the times t 4 and t 7 . It is further assumed that the reference control pulse signal S c2 supplied to the input terminal D of the delay-latch flip-flop circuit 28 remains at a logic "1" level by time t 22 intervening between the times t 15 and t 16 and swings from the logic "1" level to a logic "0" level at such a time t 22 as also shown in the time chart of Fig. 4. As will be seen as the description proceeds, it is at time t 18 when there appear logic "0" and "1" pulse signals at the non-inverted and inverted output terminals Q and Q , respectively, of the delay-latch flip-flop circuit 84 in response to the fall of the pulse signal S v2 for the first time after the reference control pulse signal S c2 has fallen from the logic "1" level to logic "0" level at time t 22 . As long as the logic "1" and "0" pulse signals are maintained at the non-inverted and inverted output terminals Q and Q , respectively, of the delay-latch flip-flop circuit 84, the conditions of the NAND and NOR gates 66 and 68 of the logic circuit 64 depend on the logic states of the reference pulse signal S r2 and the pulse signal S v2 to be controlled. In the presence of the reference pulse signal S r2 of a logic "1" level and the pulse signal S v2 of a logic "0" level, the pulse signals at all of the input terminals of the NAND 66 are maintained at the logic "1" levels so that there appears a logic "0" signal as the pulse signal S 11 at the output terminal of the NAND gate 66 as during the period intervening the times t 11 and t 12 or the period intervening the times t 15 and t 17 as shown in Fig. 4. Under these conditions, the pulse signal S 12 output from the NOR gate 68 remains at a logic "0" level in the presence of a logic "1" pulse signal appearing at the second input terminal of the NOR gate 68 in response to the pulse sign S v2 of the logic "0" level which is inverted by the first inverter 70. Under other conditions, the pulse signal at least one of the input terminals of the NAND 66 is maintained at the logic "0" level so that there appears a logic "1" signal as the pulse signal S 11 at the output terminal of the NAND gate 66. If, in this instance, both of the pulse signal S r2 and S v2 are at the logic "1" levels, the pulse signals at all of the input terminals of the NOR gate 68 are maintained at the logic "0" levels so that there appears a logic "1" signal as the pulse signal S 12 at the output terminal of the NOR gate 68 as during the period intervening the times t 12 and t 13 or the period intervening the times t 16 and t 17 as shown in Fig. 4. Under these conditions, the pulse signal S 11 output from the NAND gate 66 remains at the logic "1" level in the presence of the logic "0" pulse signal appearing at the second input terminal of the NAND gate 66 in response to the pulse signal S v2 of the logic "1" level which is inverted by the inverter 70. It will thus be seen that the pulse signal S 11 from the NAND gate 66 assumes the logic "0" level in the presence of the reference pulse signal S r2 of the logic "1" level and the pulse signal S v2 of the logic "0" level while the pulse signal S 12 from the NOR gate 68 assumes the logic "1" level in the presence of the reference pulse signal S r2 and pulse signal S v2 both of the logic "1" levels. In the presence of the pulse signals S 11 and S 12 of the logic "0" levels at the output terminals of the NAND and NOR gates 66 and 68, respectively, the p-channel field-effect transistor 38 of the charge pump circuit 74 is turned on as at time t 11 with the n-channel field-effect transistor 40 turned off. A high voltage approximating the positive supply voltage V DD thus appears as the voltage signal S 13 of a high level at the output node 42 of the charge pump circuit 74 as will be seen from the bottom section of Fig. 4. In the presence, on the other hand, of the pulse signals S 11 and S 12 of the logic "1" levels at the output terminals of the NAND and NOR gates 66 and 68, respectively, the n-channel field-effect transistor 40 of the charge pump circuit 74 is turned on as at time t 12 with the p-channel field-effect transistor 38 turned off. A low voltage approximating the ground potential V SS thus appears as the voltage signal S 13 of a low level at the output node 42 of the charge pump circuit 74 as will also be seen from Fig. 4. The high or low voltage signal S 13 thus produced at the output node 42 of the charge pump circuit 74 are fed to the low-pass filter network 76 which smoothes out the voltage signal S 13 and produces at its output node 56 the previously mentioned control voltage signal S 14 resulting from the voltage signal S 13 . The control voltage signal S 14 supplied from the low-pass filter network 76 generally represents the mean value of the high and low levels of the voltage signal S 13 produced at the node 42 of the charge pump circuit 74 as during the period intervening between the times t 11 and t 13 or the period intervening between the times t 15 and t 17 . The difference between such a mean value of the voltage signal S 13 and the arithmetic mean value (V DD + V SS )/2 of the supply and ground voltages is proportional to the amount of phase difference between the leading edges of the pulse signals S v2 and S r2 . This means that the difference between the mean value of the voltage signal S 13 and the arithmetic mean value of the voltages V DD and V SS provides information regarding the phase difference of the pulse signal S v2 advanced or delayed with respect to the pulse signal S r2 during a period of time for which the voltage signal S 13 is first increased to the high level and thereafter reduced to the low level as during the period of time t 11 -t 13 or t 15 -t 17 . In respect of the embodiment shown in Fig. 3, such a period of time will be herein referred to as phase difference detect period and is also denoted by t A for an advanced phase and by t D for a delayed phase in the time chart of Fig. 4. The control voltage signal S 14 produced at the output node 56 of the low-pass filter network 76 as above described is supplied to the voltage controlled oscillator circuit 78 to increase or decrease the control voltage for the oscillator circuit 78. During the phase difference detect periods t A for which the pulse signal S v2 to be controlled is forwardly out of phase with the reference pulse signal S r2 as during the period t 11 -t 13 , the voltage controlled oscillator circuit 78 receives the control voltage signal S 14 which is decreased in response to the voltage signal S 13 . In response to such a control voltage signal S 14 , the voltage controlled oscillator circuit 78 is conditioned to generate pulses with a delayed phase so that the pulse signal S v2 to be controlled synchronizes with the reference pulse signal S r2 . On the other hand, during the phase difference detect periods t D for which the pulse signal S v2 to be controlled is backwardly out of phase with the reference pulse signal S r2 as during the period t 15 -t 17 , the voltage controlled oscillator circuit 78 receives the control voltage signal S 14 increased in response to the voltage signal S 13 . In response to such a control voltage signal S 14 , the voltage controlled oscillator circuit 78 is conditioned to generate pulses with an advanced phase so that the pulse signal S v2 synchronizes with the reference pulse signal S r2 . As the pulses generated by the voltage controlled oscillator circuit 78 are thus advanced or retarded in phase, the pulse signal output from the frequency divider 80 is also advanced or retarded in phase and will be synchronized with the reference pulse signal S r2 . The resultant pulse signal S v2 is inverted by each of the inverters 70 and 82 and the inverted version S v2 of the pulse signal S v2 is passed from the inverter 70 to the second input terminal of each of the NAND and NOR gates 66 and 68 and from the inverter 82 to the control terminal C of the delay-latch flip-flop circuit 84. When the pulse signal S 11 from the NAND gate 66 remains at the logic "1" level with both of the pulse signa S v2 and S r2 at the logic "0" or logic "1" levels, the pulse signal at one of the first and second input terminals of the NAND gate 66 is maintained at the logic "0" level so that the associated p-channel field-effect transistor 38 of the charge pump circuit 74 remains turned off. Under these conditions, the pulse signal at one of the first and second input terminals of the NOR gate 68 is maintained at the logic "1" level so that the associated n-channel field-effect transistor 40 of the charge pump circuit 74 also remains turned off. The voltage signal S 13 at the output node 42 of the charge pump circuit 74 therefore assumes a high-impedance state floating between the supply voltage V DD and ground potential V SS as will be also seen from Fig. 4. The pulse signal S v2 produced from the inverter 70 is supplied to the control terminal C of the delay-latch flip-flop circuit 84 which is receiving the response control pulse signal S c2 at its input terminal D. The pulse signal S v2 thus appearing at the control terminal C of the flip-flop circuit 84 falls at times t 12 , t 16 , t 20 , ... and rises at times t 14 , t 18 , ... as will be seen from Fig. 4. On the other hand, the reference control pulse signal S c2 appearing at the input terminal D of the flip-flop circuit 84 swings from the logic "1" level to the logic "0" level as at time t 22 intervening between the times t 15 and t 16 . At this point of time t 22 , however, the pulse signal S v2 at the control terminal C of the flip-flop circuit 84 remains at the logic "1" level so that the pulse signals at the non-inverted and inverted output terminals Q and Q of the delay-latch flip-flop circuit 28 still remain at the logic "1" and "0" levels, respectively. At time t 18 subsequent to the phase difference detect period t D which has already terminated at time t 17 , the pulse signal S v2 appearing at the control terminal C of the delay-latch flip-flop circuit 84 rises from the logic "0" level to logic "1" level. The pulse signal at the non-inverted output terminal Q of the flip-flop circuit 28 is now allowed to fall from the logic "1" level to logic "0" level and the pulse signal at the inverted output terminal Q of the flip-flop circuit 28 allowed to rise from the logic "0" level to logic "1" level. With the logic "0" pulse signal thus produced at its third input terminal, the NAND gate 66 of the logic circuit 64 now outputs a logic "1" pulse signal to maintain the associated p-channel field-effect transistor 38 of the charge pump circuit 32 turned off after time t 18 as will be seen from Fig. 4. Likewise, the NOR gate 68 of the logic circuit 64 receives the logic "1" pulse signal at its third input terminal and outputs a logic "0" pulse signal to maintain the associated n-channel field-effect transistor 40 of the charge pump circuit 32 also turned off after the time t 18 . During the period intervening between times t 19 and t 20 , the NAND gate 66 receives logic "1" pulse signals at both of its first and second input terminals. If a logic "1" pulse signal were present also at its third input terminal, the NAND gate 66 would then produce a logic "0" signal as the pulse signal S 11 as indicated in phantom at Y in Fig. 4 if a logic "0" pulse signal were present at its third input terminal. Likewise, the NOR gate 68 would produce a logic "0" signal as the pulse signal S 12 as indicated in phantom at Z in Fig. 4 during the period intervening between times t 20 and t 21 . In the presence, actually, of a logic "0" pulse signal at the third input terminal of the NAND gate 66 and a logic "1" pulse signal at the third input terminal of the NOR gate 68, there could be no such pulses as indicated at Y and Z produced at the output terminals of the NAND and NOR gates 66 and 68, as will be seen from the bottom section of Fig. 4. After time t 18 , the p-channel and n-channel field-effect transistors 38 and 40 of the charge pump circuit 74 thus remain in non-conduction states so that the output node 42 of the charge pump circuit 74 is maintained at a high-impedance state flaoting between the supply voltage V DD and ground potential V SS . As will have been understood from the foregoing description, the logic circuit 64 and inverters 70 and 72 in the embodiment hereinbefore described with reference to Fig. 3 implement phase difference detecting means responsive to the reference signal S r2 and to the variable-phase signal S v2 for producing output signals (S 11 /S 12 ) containing information indicative of a difference in phase, if any, between the reference signal S r2 and the variable-phase signal S v2 . Furthermore, the charge pump circuit 74 implements current supply means (74) responsive to the output signals (S 11 /S 12 ) from the phase difference detecting means (64/70/72) for producing a charge current or a discharge current depending upon the information represented by the output signals (S 11 /S 12 ) from the phase difference detecting means (64/70/72). On the other hand, the low-pass filter circuit 76 in the embodiment of Fig. 3 implements frequency control means responsive to the charge current or the discharge current from the current supply means (74) for producing a frequency control signal S 14 on the basis of which the frequency at which the voltage-controlled oscillator circuit 78 is to oscillate is controlled. In addition, the inverter 82 and the delay-latch flip-flop circuit 84 implement response control means responsive to the response control signal S c2 for prohibiting the current supply means (74) from producing the charge current or discharge current.

Claims: A phase locked loop circuit for causing a variable-phase signal (Sv1; Sv2) to synchronize with a reference signal (Sr1; Sr2) comprising: a) a phase detecting means (10/12/14/22;66/68/70/72) operative to compare said variable-phase signal with said reference signal for producing a first detecting pulse signal (S3; S11) during a phase difference detect period indicative of advance of said variable-phase signal and a second detecting pulse signal (S4; S12) during a phase difference detect period indicative of delay of said variable-phase signal; b) a charge pump means (18/20/38/40/;38/40) responsive to said first and second detecting pulse signals for producing a composite voltage control signal indicative of both advance and delay of said variable-phase signal (S5/S7; S13); c) a low-pass filter means (44/46/52/50/54;46, 52/50/54) responsive to said composite voltage control signal for producing a frequency control signal (S8; S14); d) a frequency controllable oscillator means (58/60/62;78/80) responsive to said frequency control signal for producing said variable-phase signal; and e) a response control means (34/36; 66/68) responsive to a predetermined change of logic state of a response control signal (Sc1; Sc2) for changing said charge pump means from a promptly responsive state to said first and second detecting pulse signals to an irresponsive state to said first and second deteting pulse signals, characterized by f) a masking means (26/28; 82/84) coupled between a source of response control signal and said response control means, and keeping said charge pump means in said promptly responsive state until the end of the first phase difference detect period which ends after the occurence of said predetermined change of logic state of the response control signal. A phase locked loop circuit as set forth in claim 1, in which said phase detecting means (10/12/14/22) further produces a first intermediate signal (S1) indicative of a first time interval between a predetermined edge of said reference signal (Sr1) and a corresponding edge of said variable-phase signal (Sv1) and a second intermediate signal (S2) indicative of a half of pulse period of said reference signal (Sr1); said first and second detecting pulse signals being produced from said first and second intermediate signals, said masking means (26/28) comprising a NOR gate (26) responsive to said first and second intermediate signals for producing a first output signal (S6) indicative of concurrent-absence of said first and second detecting pulse signals, and a delay-latch flip flop circuit (28) having an input node (D) supplied with said response control signal and a clock node (C) supplied with said first output signal for producing a disable signal at an output node (Q) thereof, said response control means (34/36) comprising a NOR gate (34) responsive to said first detecting pulse signal in the absence of said disable signal for producing a first gate control signal (S3'), and a NOR gate (36) responsive to said second detecting pulse signal in the absence of said disable signal for producing a second gate control signal (S4'), said charge pump means (16/32) comprising a first charge pump circuit (18/20) responsive to said first and second detecting pulse signals for producing a first sub-signal (S5) of said composite voltage control signal, and a second charge pump circuit (38/40) responsive to said first and second gate control signals for producing a second sub-signal of said composite voltage control signal. A phase locked loop circuit as set forth in claim 1, in which said phase detecting means (66/68/70/72) comprises a first inverter (70) for producing a complementary variable-phase signal from said variable-phase signal (Sv1), a second inverter (72) for producing a complementary reference signal from said reference signal (Sr2), a first NAND gate (66) responsive to said reference signal and said complementary variable-phase signal in the absence of a disable signal for producing a first gate control signal (S11), and a second NAND gate (68) responsive to said complementyry reference signal and said complementary variable-phase signal in the absence of said disable signal for producing a second gate control signal (S12), said first and second NAND gates further forming in combination said response control means (66,68), said masking means comprising a third inverter (82) for producing a complementary variable-phase signal and a delay-latch flip flop circuit (84) having an input node (D) supplied with said response control signal (Sc2), a clock node (C) supplied with said complementary variable-phase signal (Sv2) and complementry output nodes for producing said disable signal, said charge pump means (74) comprising a frist enhancement type field effect transistor (38) of one channel conductivity type coupled between a first source of power voltage level (VDD) and an input node of said low-pass filter means (76) and gated by said first gate control signal, and a second enhancement type field effect transistor (40) of the opposite channel conductivity type coupled between said input node of said low-pass filter means (76) and a second source of power voltage level (Vss) and gated by said second gate control signal.

Inventors: TATEISHI HISAO, TATEISHI, HISAO, TATEISHI, HISAO C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1987
Main classification: H03L7
IPC codes: H03L7, G11B20