JPH0653824A - Phase locked loop oscillator - Google Patents

Abstract

PURPOSE:To shorten a lock-up time by quickly raising a low pass filter (LPF) by a comparator during a prescribed period after impressing power supply voltage, and separating the LPF after the lapse of the prescribed time. CONSTITUTION:A frequency divider 12 divides the frequency of an output from a variable voltage control type oscillator 11 and a reference signal generator 13 outputs a signal with reference frequency. A phase detector 14 detects a phase difference between an output signal from the frequency divider 12 and a reference signal from a reference signal generator 13 by mutually comparing both signals and generates an output corresponding to the phase difference. The LPF 15 converts the output of the detector 14 into DC control voltage by smoothing the output as control voltage for the oscillator 11. A comparator 16 compares the control voltage obtained from the LPF 15 with reference voltage VR1. When the voltage of a terminal 21 is lower than the reference voltage VR1, a capacitor in the LPF 15 is quickly charged through the comparator 16. After the lapse of the prescribed time, switches SW1, SW2 are switched.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase-locked oscillator, and more particularly to a phase-locked oscillator with a short lock-up time at power-on.

[0002]

2. Description of the Related Art A conventional phase-locked oscillator includes a variable voltage control type oscillator, a frequency divider that divides its output, a reference signal generator that outputs a signal of a reference frequency, and a signal of the frequency divider. And a reference signal of a reference signal generator, and a phase detector that generates an output according to the phase difference, and a variable voltage controlled oscillator that smooths the output of the phase detector and converts it to a DC control voltage. And a low-pass filter as a control voltage.

The lock-up time is the period from when the power of the phase-locked oscillator is turned on until the variable voltage controlled oscillator outputs a stable oscillation frequency. The operation during this period will be described.

When the power is turned on, the frequency divider, the reference signal generator, the phase detector and the variable voltage control type oscillator start up almost instantly with a time delay of several ms, but the low pass filter is a filter. Since there is a capacitor for the capacitor, it takes time to charge the capacitor and a time delay occurs.

[0005]

In this conventional phase-locked oscillator, if the lockup time is shortened,
It is necessary to reduce the time constant of the low pass filter. However, if the time constant is made small, the output of the phase detector cannot be sufficiently smoothed, the output of the variable voltage controlled oscillator is modulated, and when used as the VCO of the receiver, distortion or S /
There was a problem that N deteriorated.

[0006]

A phase-locked oscillator according to the present invention comprises a variable voltage control type oscillator, a frequency divider for dividing the output of the variable voltage control type oscillator, and an output of the frequency divider. A phase detector having an input and a reference signal output as the other input, a low-pass filter provided between the output of the phase detector and the input terminal of the variable voltage controlled oscillator, and the variable voltage controlled oscillator. It has a comparison circuit for comparing the voltage at the input end with the first reference voltage, and a switch provided between the output of the comparison circuit and the input of the low-pass filter.

[0007]

The present invention will now be described in detail with reference to the drawings showing the embodiments of the present invention.

FIG. 1 is a block diagram of a phase locked oscillator according to a first embodiment of the present invention. In the first embodiment, a variable voltage controlled oscillator 11, a frequency divider 12 that divides the output of the variable voltage controlled oscillator 11, a reference signal generator 13 that outputs a signal of a reference frequency, and a divider. A phase detector 14 that compares the phase difference between the output signal of the frequency divider 12 and the reference signal from the reference signal generator 13 to generate an output according to the phase difference, and the output of the phase detector 14 is smoothed to control the direct current. It has a low-pass filter 15 which is converted into a voltage and used as a control voltage of the variable voltage controlled oscillator 11, and a comparator 16 which compares the control voltage from the low-pass filter 15 with the reference voltage VR1.

Next, the operation of the first embodiment will be described. When the power is not turned on, the switches SW1 and S
W2 is in the illustrated state. When the power is turned on in this state, the frequency divider 12, the reference signal generator 13, the phase detector 14, and the variable voltage controlled oscillator 11 immediately start up. The reference voltage (voltage VR1) of the comparator 16 is given,
When the voltage at terminal 21 is less than this value, the output of comparator 16 goes low, charging capacitors C1 and C2 through resistor R2 and capacitors C3 and C4 through resistors R4 and R5. Since the output current capacity of the comparator 16 is sufficient, the capacitor C1
And C2 are rapidly charged, and eventually capacitors C3 and C4 are also charged, and the voltage at terminal 21 rises and voltage VR
When it exceeds 1, the output of the comparator 16 is inverted and becomes high,
A voltage is supplied to the base of the transistor TR1 and the transistors TR1 and TR2 are turned on. Transistor T
Since R2 is turned on and the electric charges of the resistor R2 and the capacitors C1 and C2 are discharged, the collector potential of the transistor TR2 begins to drop and the voltage of the terminal 21 also drops. When the voltage at the terminal 21 falls below the reference voltage VR, the output of the comparator 16 is inverted, and this operation is repeated, and the voltage at the terminal 21 rapidly approaches the reference voltage VR1.

After a lapse of a predetermined time, if the switches SW1 and SW2 are switched to the states opposite to those shown in the figure, the oscillation frequency of the variable voltage controlled oscillator 11 is produced by the output from the phase detector 14 according to the phase difference. Is controlled,
Stable operation starts immediately and lockup time can be shortened.

FIG. 2 is a block diagram of the second embodiment of the present invention. The same reference numerals are attached to the components that operate in the same manner as in the first embodiment shown in FIG. In the second embodiment, a new Schmitt circuit 17 for quick charging is newly added.
Since the reference voltage VR2 thereof is lower than VR1, the output of the comparator 16 becomes low, the output of the Schmitt circuit 17 becomes high, the capacitors C1 and C2 are rapidly charged, and the output of the Schmitt circuit 17 becomes a resistor. The capacitors C3 and C4 are charged through R4 and R5. When the voltage at the terminal 21 exceeds the reference voltage VR2, the output of the Schmitt circuit 17 is turned off. The comparator 16 continues to operate, and the output is inverted when the voltage at the terminal 21 becomes higher than the reference voltage VR1. The subsequent operation is the same as that of the first embodiment. In the case of the second embodiment, the lockup time can be extremely shortened as compared with the first embodiment.

[0012]

As described above, according to the present invention, when the power is turned on, the variable voltage controlled oscillator oscillates at an oscillating frequency close to that of the body actually used, so that the low-pass comparator is used for a predetermined time after the power is turned on. The lock-up time can be shortened by rapidly starting the filter and disconnecting it by a switch after a predetermined time has elapsed.

[Brief description of drawings]

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

FIG. 2 is a block diagram of a second embodiment of the present invention.

[Explanation of symbols]

 11 Variable Voltage Controlled Oscillator 12 Frequency Divider 13 Reference Signal Generator 14 Phase Detector 15 Low Pass Filter 16 Comparison Circuit 17 Schmitt Circuit SW1, SW2, SW3 Switches

Claims (2)

[Claims] 1. A variable voltage controlled oscillator, a frequency divider that divides the output of the variable voltage controlled oscillator, and a phase that uses the output of the frequency divider as one input and the reference signal output as the other input. A low-pass filter provided between the detector, the output of the phase detector and the input terminal of the variable voltage controlled oscillator, and the voltage at the input end of the variable voltage controlled oscillator and the first reference voltage. A phase-locked oscillator comprising: a comparison circuit for comparison; and a switch provided between the output of the comparison circuit and the input of the low-pass filter. 2. A Schmitt circuit to which a voltage at an input terminal of the variable voltage controlled oscillator and a second reference voltage lower than the first reference voltage are input, the output of the Schmitt circuit being the low-pass filter. The phase-locked oscillator according to claim 1, wherein a capacitor in the filter is charged.