JP2000188544A - Phase locked loop circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a phase locked loop circuit capable of making frequency followup ability compatible with the frequency stability of an output signal in the case of locking. SOLUTION: A phase comparator 1 multiplies an external reference signal vi with an output signal vo of a voltage controlled oscillator 3 and outputs the resultant, and a low pass filter 2 passes only a low frequency component of this output. A comparator 23 controls a changeover switch 13 so that the changeover switch 13 is thrown to the position of selecting an output of an amplifier 11 whose gain is higher than that of an amplifier 12 when an absolute value of an AC component of an output of the low pass filter 2 is higher than a preset reference voltage VL or to the position of selecting an output of the amplifier 12 whose gain is lower than that of the amplifier 11 when the absolute value of the AC component of the output of the low pass filter 2 is lower than the preset reference voltage VL.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase synchronization circuit for generating a signal synchronized with an external clock signal or a carrier wave.

[0002]

2. Description of the Related Art In recent years, as phase-locked loop (LSI) circuits have become more sophisticated and higher performance, televisions, radios, various digital devices, satellite broadcasts, FM broadcasts, AM broadcasts, etc.
As means for generating a signal synchronized with an external clock or a carrier wave, a phase synchronization circuit is often used.

[0003] A conventional phase locked loop circuit will be described below with reference to the drawings.

FIG. 3 is a diagram showing a configuration of a conventional phase locked loop circuit. In FIG. 3, 1 is a phase comparator, 2 is a low-pass filter, and 3 is a voltage controlled oscillator.

The phase comparator 1 receives a reference signal vi input from the outside and an output signal vo of the voltage controlled oscillator 3. The reference signal vi is a signal such as a clock or a carrier which serves as a reference for the operation of the phase locked loop. The multiplying circuit of the phase comparator 1 multiplies the reference signal vi and the output signal vo of the voltage controlled oscillator 3 to output a signal having the sum and difference of the frequencies of the two input signals as frequency components. Among them, the high frequency component is removed by the low-pass filter 2, and only the low frequency component is input to the voltage controlled oscillator 3. The voltage controlled oscillator 3 generates a signal vo having a frequency corresponding to the voltage of the input signal, and the output vo is input to the phase comparator 1 again. With such a loop, the frequency of the reference signal vi and the frequency of the output signal vo of the voltage controlled oscillator 3 become the same. Two input signals vi and vo to the phase comparator
The fact that the frequencies coincide with each other is said to be locked by the phase locked loop.

[0006]

However, the conventional phase locked loop circuit has the following problems.

When the time constant of the low-pass filter 2 is increased, the frequency of the reference signal vi is substantially constant, and in a steady state where the phase locked loop is locked, the voltage control oscillator 3
The output signal vo is stable in frequency and is not easily affected by the jitter of the reference signal vi. However, in a transient state in which a large frequency fluctuation occurs in the reference signal vi, the lock is easily released. It takes. Also,
Conversely, if the time constant of the low-pass filter 2 is reduced, the circuit follows better, so that the circuit can quickly follow and lock even in the transient state, but also follows the jitter component in the steady state, and the output signal There is a problem that the frequency of vo becomes unstable.

For this reason, when designing the low-pass filter 2, the stability of the circuit in the steady state and the frequency following ability of the circuit in the transient state cannot be compatible, and it is necessary to set the time constant with priority on either one. .

According to the present invention, while the time constant of the low-pass filter 2 is fixed at a constant value, the stability of the circuit in the steady state, that is, the high frequency stability of the output signal vo during locking, and the circuit stability in the transient state An object of the present invention is to provide a phase-locked loop circuit having both high frequency tracking performance.

[0010]

According to a first aspect of the present invention, there is provided a voltage-controlled oscillator for generating a signal having a frequency corresponding to a voltage of an input signal as a phase locked loop circuit. A phase comparator that detects a phase difference between a reference signal input from the VCO and an output signal of the voltage-controlled oscillator, a low-pass filter that receives the output signal of the phase comparator as an input, and amplifies an output signal of the low-pass filter. A gain-adjustable amplifier that supplies the voltage-controlled oscillator as an input signal, and a gain adjuster that changes the gain of the amplifier in accordance with the output signal of the low-pass filter.

According to a second aspect of the present invention, in the phase locked loop circuit according to the first aspect, the gain adjustment section is provided when the amplitude of the AC component of the output signal of the low-pass filter is larger than a predetermined reference voltage. Indicates that the gain of the amplifying unit is set to be large, and when the gain is small, the gain of the amplifying unit is set to be small.

According to the first or second aspect of the present invention, when the frequency of the reference signal greatly changes and the amplitude of the AC component of the output signal of the low-pass filter becomes larger than a predetermined reference voltage, the gain of the amplifying unit is increased. Becomes larger, so that the phase locked loop can follow the frequency fluctuation of the reference signal better.

According to a third aspect of the present invention, in the phase locked loop circuit according to the second aspect, the amplifying section has a plurality of amplifiers having different gains having a common input and an instruction from the gain adjusting section. A signal selector for selecting one from the outputs of the plurality of amplifiers.

According to the third aspect of the invention, by configuring the amplifying unit using a plurality of amplifiers having different gains, it is possible to realize an amplifying unit capable of adjusting the gain in a wide range.

According to a fourth aspect of the present invention, in the phase locked loop circuit according to the second aspect, the amplifying section has an amplifier whose gain can be adjusted, and a part of a circuit element of the amplifier is the gain adjusting section. , The gain changes.

According to the fourth aspect of the present invention, since a plurality of amplification factors can be set for one amplifier, the configuration of a phase locked loop having an amplifying unit that can adjust the gain can be simplified.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a phase locked loop circuit according to one embodiment of the present invention.

In FIG. 1, a phase comparator 1 receives a reference signal vi such as a clock and a carrier wave input from the outside and an output signal vo of a voltage controlled oscillator 3. These signals are multiplied by the phase comparator 1, and a signal having the sum and difference of the frequencies of the two signals as frequency components is output.

The output signal of the phase comparator 1 is input to the low-pass filter 2. The low-pass filter 2 passes and outputs only a signal having a component of a frequency difference between the two signals vi and vo. The time constant of the low-pass filter 2 is fixed and is set to a large value in advance.

Amplifiers 11 and 12 and changeover switch 13
Constitute the amplifying unit 10 which can adjust the gain. Also,
The capacitor 21, the rectifier 22, and the comparator 23 constitute a gain adjustment unit 20. The output of the low-pass filter 2 is input to the amplifiers 11 and 12 and the capacitor 21.
The amplifiers 11 and 12 amplify this signal and output it to the switch 13. It is assumed that the gain of the amplifier 11 is larger than the gain of the amplifier 12.

In the capacitor 21, the low-pass filter 2
Of the output, only the AC component is passed. Therefore, the signal output from the capacitor 21 has a waveform that fluctuates around 0V. The rectifier 22 performs full-wave rectification on this signal and outputs the signal to the comparator 23. The comparator 23 has a predetermined reference voltage VL set in advance, and compares the reference voltage VL with the output signal voltage of the rectifier 22. Here, if the output signal voltage of the rectifier 22 is higher than the reference voltage VL, the amplifier 11 having a large gain
The switch 13 as a signal selector is controlled so that the output of the amplifier 12 is selected to be smaller when the output signal voltage is smaller. The selected signal is input to the voltage controlled oscillator 3 as an oscillation frequency control voltage, and the voltage controlled oscillator 3 generates a signal vo having a frequency corresponding to the voltage and uses the signal vo as an input to the phase comparator 1.

The operation of the phase locked loop configured as described above will be described below.

First, until the phase locked loop is locked for the first time, or during locking, the input reference signal vi is input.
At a time when there is a large frequency fluctuation, that is, when the operation of the phase locked loop is in a transient state.

In such a case, the output signal of the low-pass filter 2 is not a constant value, but changes toward a new value so that the reference signal vi and the output signal vo of the voltage controlled oscillator 3 have the same frequency. At this time, the low-pass filter 2
Is converged to a constant value while repeating the oscillation.

FIG. 2A shows the waveform of the output signal of the low-pass filter 2 at this time. FIG. 2 (b)
Is the waveform of the AC component of this output signal that has passed through the capacitor 21, and FIG.
2 shows a waveform of a signal converted to take an absolute value by full-wave rectification.

In the comparator 23, a reference voltage VL as shown in FIG. Only when the output voltage of the rectifier 22 exceeds the reference voltage VL, the comparator 23 selects the output of the amplifier 11 having the larger gain.
Control. For this reason, a greatly amplified signal as shown in FIG. 2D is input to the voltage controlled oscillator 3 as an oscillation frequency control voltage.

Therefore, by selecting the output of the amplifier having a large amplification factor, the loop gain of the phase locked loop becomes high. Therefore, even though the time constant of the low-pass filter 2 is set to be large, an external reference Even if the frequency of the signal vi fluctuates, it is possible to quickly follow and lock.

Next, consider a case where the input reference signal vi does not have a large frequency variation and is in a steady state in which the phase locked loop maintains a locked state.

At this time, the voltage of the signal output from the phase detector 1 and passed through the low-pass filter 2 is almost constant, and there is no large fluctuation. Therefore, this signal passes through the capacitor 21, and the waveform obtained by full-wave rectification by the rectifier 22 is small and does not exceed the reference voltage VL set by the comparator 23. In such a case, the comparator 23 controls the changeover switch 13 so as to select the output of the amplifier 12 having the smaller gain. For this reason,
As shown in FIG. 2E, a signal having a small degree of amplification is input to the voltage controlled oscillator 3 as an oscillation frequency control voltage.

Therefore, by selecting the output of the amplifier having a small amplification factor, the loop gain of the phase locked loop becomes low, and a stable locked state can be maintained.

Further, since the time constant of the low-pass filter 2 is set to be large, it is hardly affected by the jitter in both the transient state and the steady state, and the output signal v
The frequency stability of o increases.

In this embodiment, the amplifying unit 10 capable of adjusting the gain includes two amplifiers 11 and 12 having different gains.
However, three or more amplifiers having different gains may be used. In this case, the number of reference voltage setting values in the comparator, the number of selectable input signals of the changeover switch, and the like need to correspond to the number of amplifiers. Further, in this case, since the gain range of the amplifying unit can be expanded by using a large number of amplifiers, the stability of the phase locked loop at the time of locking and the frequency tracking performance of the phase locked loop with respect to the reference signal vi are improved. And it is easy to widen the operating frequency range of the phase locked loop.

Further, a plurality of amplifiers may be replaced with a single amplifier whose gain can be adjusted. In this case, since the number of amplifiers is small, the circuit configuration is simplified. This configuration can be realized by, for example, adopting an amplifier that can perform gain adjustment by switching a circuit element such as a resistor, and allowing the gain adjustment unit to perform the switching.

[0034]

As described above, according to the present invention, the gain of the amplifying unit is adjusted in accordance with the output signal of the low-pass filter, so that the reference signal is quickly followed and locked even if there is a large frequency fluctuation. It is possible to maintain a stable lock state when there is no large frequency fluctuation in the reference signal, and in any case, to reduce the influence of jitter and to keep the frequency stability of the output signal of the voltage controlled oscillator high. A phase-locked loop circuit that can be implemented can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a phase locked loop circuit according to an embodiment of the present invention.

FIGS. 2A to 2C are graphs illustrating an example of a signal waveform in a gain adjustment unit, and FIGS. 2D and 2E are graphs illustrating an example of a signal waveform of an output of an amplifier.

FIG. 3 is a block diagram illustrating a configuration of a conventional phase locked loop circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 phase comparator 2 low-pass filter (LPF) 3 voltage-controlled oscillator (VCO) 10 amplifying unit 11, 12 amplifier 13 changeover switch (signal selector) 20 gain adjusting unit 21 capacitor 22 rectifier 23 comparator vi reference signal vo voltage-controlled oscillator Output signal VL Reference voltage

Claims (4)

[Claims] A voltage-controlled oscillator that generates a signal having a frequency corresponding to a voltage of an input signal; a phase comparator that detects a phase difference between a reference signal input from outside and an output signal of the voltage-controlled oscillator; A low-pass filter that receives the output signal of the phase comparator as an input; a gain-adjustable amplifier that amplifies the output signal of the low-pass filter and supplies the input signal to the voltage-controlled oscillator; and an output of the low-pass filter. A phase adjustment circuit comprising: a gain adjustment unit that changes a gain of the amplification unit according to a signal. 2. The phase-locked loop according to claim 1, wherein the gain adjustment section increases the gain of the amplification section when the amplitude of the AC component of the output signal of the low-pass filter is larger than a predetermined reference voltage. A phase-locked loop, wherein the gain is set to a small value when the gain is small. 3. The phase-locked loop according to claim 2, wherein the amplifying unit comprises: a plurality of amplifiers having a common input and different gains; and an output of the plurality of amplifiers according to an instruction of the gain adjusting unit. And a signal selector for selecting one. 4. The phase-locked loop according to claim 2, wherein the amplifying section has an amplifier whose gain can be adjusted, and the amplifier has a gain by selecting a part of the circuit element by the gain adjusting section. The phase-locked loop is characterized in that: