Phase-locked loop

Abstract: In a phase-locked loop, a phase comparator (1), a low-pass filter (3), and a voltage-controlled oscillator (2) are connected in a loop form. The phase-locked loop includes a switching circuit (6, 7), arranged between the low-pass filter (3) and the voltage-controlled oscillator (2), for switching an output signal (S2) from the phase comparator (1) and an output signal (S5) from the low-pass filter (3) to input the selected output signal to the voltage-controlled oscillator (2), and an input/output signal monitor circuit (4) for controlling a switching timing of the switching circuit (6, 7) in response to a synchronization state between a signal (S1) input to the phase comparator (1) and an output signal (S4) from the voltage-controlled oscillator (2), and frequency components of an input signal (S1).

Description: Background of the Invention The present invention relates to a phase-locked loop (to be referred to as a PLL hereinafter) and, more particularly, to a PLL with improved input jitter characteristics. Fig. 1 is a block diagram showing a conventional PLL. In Fig. 1, reference numeral 1 denotes a phase comparator; 2, a voltage-controlled oscillator; and 3, a low-pass filter. In the conventional PLL of this arrangement, an input signal S1 is compared with an output signal S4 of the voltage-controlled oscillator 2 by the phase comparator 1. An output signal S2 as a comparison result of these signals is input to the low-pass filter 3, and the voltage-controlled oscillator 2 is controlled by an output signal S3 filtered through the filter 3. In the arrangement of the conventional PLL, a jitter pass band (noise band width) is narrowed as much as possible to maintain good output jitter characteristics. In the conventional PLL, however, a hysteresis exists in the synchronization characteristics of the PLL with respect to an input low-frequency jitter amount characteristic to FM modulation. Therefore, input jitter characteristics are degraded. In other words, since hysteresis is present with respect to a change in frequency shift amount in the low-frequency modulation range of the FM modulation, input jitter characteristics are degraded. In the specification of United States patent No. 3,909,735, which was published on September 30 1975, there was proposed a phase-locked loop circuit with a variable impedance which switched smoothly the control of a voltage controlled oscillator from a wide loop filter to a narrow loop filter. In the specification of German patent application No. 86109467.0, which was published on January 14 1987 under number 208328, there was proposed a clock regenerator circuit in which a phase step of a phase discriminator was detected for switching between an output of the discriminator and a low frequency narrow band filter. It is a principal object of the present invention to provide a PLL with improved input jitter characteristics. The following description and drawings disclose, by way of an example, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In the drawings:- Fig. 1 is a block diagram of a conventional PLL; Fig. 2 is a block diagram showing an embodiment according to the present invention; and Fig. 3 is a block diagram showing another embodiment. Fig. 2 is a block diagram showing an embodiment of the present invention. The same reference numerals in Fig. 2 denote the same parts as in Fig. 1. Reference numeral 4 denotes an input/output signal monitor circuit; 5, a first switching control circuit; 6, a first switching circuit; and 7, a second switching circuit. The first switching control circuit 5 outputs a rectangular-wave pulse signal having a duty ratio of 50%. The first switching circuit 6 switches inputs in response to a continuously and periodically changing pulse signal supplied from the first switching control circuit 5. In the PLL of this embodiment, a phase of an input jitter signal S1 characteristic to FM modulation (to be referred to as an FM modulation signal) is compared with a phase of an output signal S4 from a voltage-controlled oscillator 2 by a phase comparator 1. An output signal S2 including a high-frequency signal component is filtered through a low-pass filter 3 to obtain a low-frequency signal component S5. The low-frequency signal component S5 and the output signal S2 are switched by the first switching circuit 6 which is controlled by the first switching control circuit 5 for continuously performing control on the basis of a given synchronization state, thus outputting an output signal S7. The switching cycle in this case is about 0.125 Hz when the modulation frequency of an FM modulation signal is, e.g., 450 Hz. However, the present invention is not limited to these values. The output signal S7 from the first switching circuit 6 and the low-frequency signal component S5 from the low-pass filter 3 are switched by the second switching circuit 7 which is controlled by a second switching circuit control signal S8 from the input/output signal monitor circuit 4, thus outputting a voltage-controlled oscillator control signal S3. The voltage-controlled oscillator control signal S3 controls the voltage-controlled oscillator 2. When the voltage-controlled oscillator output signal S4 is synchronous with the FM modulation signal S1, the input/output signal monitor circuit 4 switches a movable contact 7a of the second switching circuit 7 to a stationary contact 7b side, so that the low-frequency signal component S5 from the low-pass filter 3 is output as the voltage-controlled oscillator control signal S3. When the voltage-controlled oscillator output signal S4 is asynchronous with the FM modulation signal S1, the input/output signal monitor circuit 4 monitors the modulation frequency component of the FM modulation signal S1, and performs the following operation in accordance with the modulation frequency component. More specifically, in a frequency range where a hysteresis is present, the monitor circuit 4 controls to switch the movable contact 7a of the second switching circuit 7 to a stationary contact 7c side, so that the output signal S7 from the first switching circuit 6 is output from the second switching circuit 7. In a frequency region where a hysteresis exists, the first switching circuit 6 selectively outputs the low-frequency signal component S5 of the phase comparator output signal S2 and the phase comparator output signal S2 including the high-frequency signal component for given cycles under the control of the first switching control circuit 5. When the first switching circuit 6 is switched to control the low-frequency signal component S5 of the phase comparator output signal S2, the voltage-controlled oscillator output signal S4 follows the FM modulation signal S1 due to the response characteristics of the PLL, thus eliminating the hysteresis. In a range where no hysteresis is present, the monitor circuit 4 controls the second switching circuit 7 to output the low-frequency signal component S5 without going through the first switching circuit 6. Fig. 3 is a block diagram showing a further embodiment. The same reference numerals in Fig. 3 denote the same parts as in Fig. 2. Reference numeral 8 denotes a switching circuit; 9, a switching control circuit; and 10, a switching pulse generator. The switching pulse generator 10 generates a periodic and continuous pulse signal having a duty ratio of 50% to control the switching circuit 9. For example, the switching cycle in this case is about 0.125 Hz when the modulation frequency of an FM modulation signal is, e.g., 450 Hz. However, the present invention is not limited to these values. In this embodiment, the single switching circuit 8 switches an output signal S2 from a phase comparator 1 and its low-frequency signal component S5 to output a voltage-controlled oscillator control signal S3. The switching circuit 8 is switched by a switching circuit control signal S11 from the switching control circuit 9 which is controlled by a control signal S9 from an input/output signal monitor circuit 4 and a switching pulse signal S10 from the switching pulse generator 10. When an FM modulation signal S1 is synchronous with a voltage-controlled oscillator output signal S4, the input/output signal monitor circuit 4 supplies the synchronous control signal S9 to the switching control circuit 9, and the switching control circuit 9 controls to output the low-frequency signal component S5 of the phase comparator output signal S2 as the voltage-controlled oscillator control signal S3 from the switching circuit 8. When the FM modulation signal S1 is asynchronous with the voltage-controlled oscillator output signal S4, the input/output signal monitor circuit 4 monitors the modulation frequency component of the FM modulation signal S1, and supplies the control signal S9 based on the modulation frequency component data to the switching control circuit 9. The switching control circuit 9 continuously controls the switching circuit 8 to switch the phase comparator output signal S2 and its low-frequency component S5 under the control of the switching pulse signal S10 having a given period and generated by the switching pulse generator 10 in a range where a hysteresis is present. In a range where no hysteresis is present, the switching control circuit 9 controls the switching circuit 8 to output the low-frequency signal component S5 of the phase comparator output signal S2. As described above, the low-frequency signal component of the phase comparator output signal and the phase comparator output signal including the high-frequency signal component are continuously switched for given cycles to control the VCO, thereby providing the PLL circuit which can eliminate a hysteresis existing in the synchronization characteristics of the PLL circuit with respect to an input jitter amount characteristic to FM modulation, and can prevent input jitter characteristics from being impaired.

Claims: A phase-locked loop (PLL) circuit for establishing phase synchronization, including a phase comparator (1) for detecting a phase difference between an input signal (S1) and an output signal (S4) from a voltage-controlled oscillator (2), a first switching circuit (6) for switching between a low-frequency signal component of the output signal from the phase comparator (1) and a signal including a high-frequency signal component of the output signal from the phase comparator (1), and a first switching control circuit (5) for generating a switching control signal (S6) for controlling the operation of the first switching circuit (6) and an input/output signal monitor circuit (4) for monitoring the synchronization/asynchronization state of the voltage-controlled oscillator (2) and the frequency components of the input signal (S1), characterised in that there is provided a second switching circuit (7) for switching between the output signal (S7) from the first switching circuit (6) and the low-frequency signal component output from the phase comparator (1) in accordance with the synchronization/asynchronization state of the voltage-controlled oscillator (2) and the frequency components of the input signal (S1), and in that the input/output signal monitor circuit (41) controls the second switching circuit (7) in such a way that the low frequency signal component from the comparator (1) is connected to control the voltage controlled oscillator (2) when the voltage controlled oscillator (2) output signal (S4) is synchronous with the input signal (S1), the voltage-controlled oscillator (2) being controlled in accordance with the output signal (S3) from the second switching circuit (7).

Inventors: KASHIWABA SATOSHI, NAKAJIMA MASAHIRO, KASHIWABA, SATOSHI, NAKAJIMA, MASAHIRO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1989
Main classification: H03L7
IPC codes: H03L7