JPH04280514A - Phase locked loop employing voltage controlled oscillator - Google Patents

Abstract

PURPOSE:To realize a phase locked loop with simple circuit constitution which responds to the state of the loop at a high speed untill the phase locked loop reaches a locking state and operates to enhance the frequency stability when the loop is locked. CONSTITUTION:A voltage controlled oscillator 11 is oscillated with a locked phase at a prescribed frequency when a control voltage VC is in the vicinity of 0V. A voltage detection circuit 15 sends a detection signal SC when the control voltage VC is within a prescribed small range around 0V. A switching circuit 16 is in on/off operation in response to the detection signal SC and increases/decreases the feedback quantity from the output side to the input side of an operational amplifier 12. The operational amplifier 12 decreases/ increases the amplification degree in response to the increase/decrease in the feedback quantity and decreases/increases the loop gain.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a phase-locked loop of a voltage-controlled oscillator, and in particular, it has a DC amplifier in the front stage of the voltage-controlled oscillator, and responds quickly until the phase-locked loop becomes locked. The present invention relates to a phase-locked loop of a voltage controlled oscillator that operates to enhance stability.

0002

2. Description of the Related Art Conventionally, as shown in FIG. 2, for example, a phase-locked loop of this type of voltage controlled oscillator is a voltage controlled oscillator 21 which oscillates at a predetermined frequency when the control voltage VC is around 0V.
, a DC amplifier 22 that outputs a control voltage VC, a loop filter 23, and a phase comparator 24 that compares the phase of the output signal of the voltage controlled oscillator and the reference signal.

[0003] Here, the input/output characteristics of the DC amplifier 22 are as follows:
As shown in FIG. 3, the amplification degree increases as the input voltage moves away from 0V, and is nonlinear so that the amplification degree sharply decreases near 0V. With such characteristics, when the phase-locked loop is in the unlocked state, the control voltage VC moves away from 0V, so the degree of amplification increases and the loop gain also increases, which strengthens the pull-in to the locked state and improves the response speed. becomes faster. On the other hand, as the lock state approaches, that is, as the voltage approaches 0V, the amplification degree decreases and the loop gain also decreases, so the noise bandwidth of the loop narrows and becomes stable in terms of frequency.

0004

In the phase-locked loop of the conventional voltage controlled oscillator described above, the characteristics of the DC amplifier connected before the voltage controlled oscillator are such that as the input voltage moves away from 0V, the degree of amplification increases. Furthermore, the amplification level is nonlinear in that the degree of amplification rapidly decreases near 0V. However, the circuit configuration of such a DC amplifier is not only complicated, but also has problems in that adjustment is troublesome.

[0005] An object of the present invention is to
An object of the present invention is to provide a phase-locked loop of a voltage controlled oscillator that responds at high speed until it reaches a lock state and operates to improve frequency stability when it locks.

[0006]

[Means for Solving the Problems] A phase-locked loop of a voltage-controlled oscillator of the present invention includes a voltage-controlled oscillator that oscillates at a predetermined frequency in phase synchronization when a control voltage is around 0V, and a DC amplifier that outputs the control voltage. a voltage detection circuit that detects whether the control voltage is within a predetermined minute range centered around 0V; a switch circuit that operates on and off according to the detection output of this voltage detection circuit; and a feedback resistor that changes the amount of feedback from the output end to the input end of the DC amplifier according to the operation, and reduces the amplification degree of the DC amplifier when the control voltage is within the predetermined minute range. , the amplification degree of the DC amplifier is increased when the control voltage is outside the predetermined minute range.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, which includes a voltage controlled oscillator 11 that oscillates at a predetermined frequency when the control voltage VC is around 0V, an operational amplifier 12 that outputs the control voltage VC, and a loop. A filter 13, a phase comparator 14 that compares the phase of the output signal of the voltage controlled oscillator and a reference signal, a voltage detection circuit 15 that detects that the control voltage VC is within a predetermined range, a switch circuit 16, and a feedback circuit. It is equipped with resistors 17 and 18.

Here, the voltage detection circuit 15 detects the control voltage V
A detection signal SC is sent out when C is within a predetermined minute range centered around 0V. The switch circuit 16 is turned on upon receiving the detection signal SC. The operational amplifier 12 has a negative feedback amount controlled by feedback resistors 17 and 18 to change the amplification degree.

Next, the operation will be explained.

[0011] When the phase-locked loop is in the unlocked state,
That is, when the control voltage VC is far from 0V, the detection signal SC is not sent out, so the switch circuit 16 is in an off state. Therefore, feedback from the output side of the operational amplifier 12 to the input side is performed via the feedback resistor 17.
Since the amount of feedback is small, the amplification degree of the operational amplifier 12 is large and the loop gain is also increased. In other words, the locking state is strong and the response speed is fast. On the other hand, when the lock state is approached and the control voltage VC enters a predetermined minute range centered around 0V, the voltage detection circuit 15 sends out a detection signal SC, the switch circuit 16 turns on, and the feedback resistor 17
, 18 are connected in parallel, the amount of feedback increases, the amplification degree of the operational amplifier 12 decreases, the loop gain also decreases, the noise bandwidth of the loop is narrowed, and the frequency becomes stable.

Note that the voltage detection circuit 15 is set to detect a sufficiently narrow range centered around 0V so that the change in the control voltage VC is minimized when the amplification degree of the operational amplifier 12 changes. .

[0013]

As explained above, the phase-locked loop of the voltage-controlled oscillator of the present invention detects the control voltage of the voltage-controlled oscillator that oscillates at a predetermined frequency near 0V, and the control voltage is controlled at a predetermined frequency around 0V. When the control voltage is outside the minute range, the amplification of the operational amplifier that outputs this control voltage is increased to increase the loop gain, and when the control voltage is within a predetermined minute range centered around 0V, By reducing the amplification of the operational amplifier and reducing the loop gain,
With a simple circuit configuration, it is possible to operate the phase-locked loop to respond quickly until it is locked, and to improve frequency stability once locked.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a block diagram showing an example of a phase-locked loop of a conventional voltage controlled oscillator.

3 is a diagram showing input/output characteristics of the DC amplifier 22 shown in FIG. 2. FIG.

[Explanation of symbols]

11, 21 Voltage controlled oscillator 12 Operational amplifier 15 Voltage detection circuit 16
Switch circuit 17, 18 Feedback resistor

Claims (1)

[Claims] 1. A voltage controlled oscillator that oscillates at a predetermined frequency in phase synchronization with a control voltage around 0V, a DC amplifier that outputs the control voltage, and a DC amplifier that outputs the control voltage within a predetermined minute range centered around 0V. a voltage detection circuit that detects whether or not the voltage is present, a switch circuit that operates on and off according to the detection output of this voltage detection circuit, and feedback from the output terminal of the DC amplifier to the input terminal according to the operation of this switch circuit. and a feedback resistor that changes the amount of feedback, the amplification degree of the DC amplifier is reduced when the control voltage is within the predetermined minute range, and when the control voltage is outside the predetermined minute range. A phase locked loop for a voltage controlled oscillator, characterized in that the amplification degree of the DC amplifier is increased.