JPH07106960A - Phase locked loop circuit - Google Patents

Abstract

PURPOSE:To improve the accuracy of the free-running frequency of a voltage control oscillator by using a temperature compensated voltage generating circuit which generates the voltage that varies in response to change of the environmental temperature. CONSTITUTION:In a phase locked loop PLL circuit, a logic circuit 8 outputs the drive signal of a switch 3 when an input break detecting circuit 6 detects the break of an input signal or a step-out detecting circuit 7 detects a step-out state. Receiving the drive signal, the switch 3 switches the output of a loop filter 2 to the output of a temperature compensated voltage generating circuit 9. Thus the output of the circuit 9 is supplied to a voltage controlled oscillator 4. Therefore the voltage that varies to eliminate the temperature characteristic of the oscillator 4 is supplied to the oscillator 4. As a result, the free-running frequency of the oscillator 4 can be accurately controlled and the reliability of the PLL circuit is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase-locked loop circuit, and more particularly to a phase-locked loop circuit capable of obtaining a high free-running frequency accuracy even when the input signal is cut off or the phase is lost.

A phase locked loop circuit is a circuit having many functions such as automatic frequency control, signal tracking, frequency / phase modulation signal demodulation, signal synchronization, narrow band frequency selection, and frequency conversion. It is used at the heart of an extremely wide range of equipment such as transmission equipment, digital exchanges, modems, televisions and frequency synthesizers. In addition, due to recent advances in semiconductor integrated circuit technology,
It has become more economical and easier to use, and its range of application is expanding.

Therefore, improvement of the performance and stability of the phase-locked loop circuit has become an even more important theme, and research by many engineers is continuing.

[0004]

2. Description of the Related Art FIG. 4 shows the configuration of a conventional phase locked loop circuit. In FIG. 4, 1 is a phase comparator, 2 is a loop filter, 3 is a switch, 4 is a voltage controlled oscillator, 5 is a frequency divider, and 6 is an input disconnection detection circuit.

The most basic operation of the phase locked loop circuit is described in, for example, Ken Yanagisawa "PLL (Phase Locked Loop) Application Circuit" (published by Sogo Denshi Publishing Co., Ltd.). Only the points will be explained.

The feature of the configuration of FIG. 4 is that when the input signal is cut off, the switch is driven by the output signal of the input cutoff detection circuit, and the output of the loop filter is switched to a fixed voltage to ensure the free-running frequency accuracy of the voltage controlled oscillator. There is a point in trying.

However, since the voltage applied to the voltage controlled oscillator at the time of input interruption is a fixed voltage, there is a risk that the free-running frequency will deviate from the allowable range of frequency accuracy when the environmental temperature changes greatly. In addition, if the phase synchronization is lost, the feedback loop cannot control the phase-locked loop, and the control voltage applied to the voltage-controlled oscillator becomes the voltage at the end of the control range, and the free-running frequency is allowed. The problem arises that there is a large deviation from the range.

[0008]

SUMMARY OF THE INVENTION The present invention addresses such a problem by providing a phase-locked loop circuit capable of obtaining a highly accurate free-running frequency even when an input disconnection or loss of phase synchronization occurs in the phase-locked loop circuit. The purpose is to provide.

[0009]

FIG. 1 is a diagram showing the principle of the present invention. In FIG. 1, 1 is a phase comparator, 2 is a loop filter, 3 is a switch, 4 is a voltage controlled oscillator, 5
Is a frequency divider, 6 is an input disconnection detection circuit, 7 is an out-of-sync detection circuit, 8 is an OR circuit, and 9 is a temperature compensation voltage generation circuit.

The configuration of FIG. 1 is characterized in that, even if the input signal is interrupted or the phase synchronization is lost, the switch is driven to switch the output of the loop filter to the output of the temperature compensation voltage generating circuit.

[0011]

The temperature-compensated voltage generation circuit can output a voltage that changes according to a change in the ambient temperature. Therefore, by outputting a voltage that changes so as to cancel the temperature characteristic of the oscillation frequency of the voltage-controlled oscillator, the voltage-controlled oscillator The accuracy of the free-running frequency can be increased.

[0012]

FIG. 2 shows an example of a temperature compensation voltage generating circuit in which a thermistor is applied as a temperature sensor element.

In FIG. 2, 911, 912 and 913 are normal resistors, and 914 and 915 are thermistors. The thermistor includes an NTC type having a negative temperature coefficient, a PTC type having a positive temperature coefficient, and a CTR type which has a negative temperature coefficient but whose electric resistance sharply decreases in a certain temperature range. With the relationship between these combinations and the resistance value of normal resistance,
With the configuration of FIG. 2, a voltage having an arbitrary temperature characteristic can be output.

The structure of FIG. 2 uses a thermistor as a temperature sensor element and utilizes the change in resistance thereof, but it is also possible to utilize the temperature characteristic of the forward voltage of the junction diode. In this case, the fact that the temperature characteristic of the forward voltage is about −2 mV / ° C. is utilized.

FIG. 3 shows a second example of the temperature compensation voltage generating circuit. In FIG. 3, 91 is a temperature sensor circuit, 92 is an analog / digital conversion circuit, 93 is a memory, and 94 is a digital / analog conversion circuit.

The temperature sensor circuit can be constructed by using a thermistor as shown in FIG. 2 or by utilizing the temperature characteristic of the forward voltage of the junction diode. analog·
The digital conversion circuit converts the voltage output from the temperature sensor circuit into a digital code having a predetermined number of bits to form address information of the memory. The memory outputs the code stored at this address, the digital-analog conversion circuit converts this code into a voltage, and supplies this voltage to the voltage controlled oscillator.

When the oscillation frequency of the voltage controlled oscillator is controlled by the voltage output from the circuit of FIG. 2, it is difficult to completely compensate the temperature characteristic of the oscillation frequency of the voltage controlled oscillator, and it is necessary to have a high compensation accuracy. There is a problem that the circuit configuration must be complicated.

On the other hand, the advantages of the configuration of FIG. 3 are as follows. That is, the voltage to be supplied to the voltage controlled oscillator for each temperature can be known from the test data of the voltage controlled oscillator. Therefore, the voltage output from the temperature sensor circuit at a certain temperature is digitally encoded and used as a signal for designating the address of the memory, and at that address, the voltage to be supplied to the voltage controlled oscillator at that temperature is obtained by the test. If digitally encoded and stored, the free-running frequency of the voltage controlled oscillator can be accurately controlled within the required temperature range.

The above-mentioned control of the free-running frequency is effective even when the input of the phase locked loop circuit is cut off or when the phase synchronization is lost. Further, according to the circuit of the present invention, a stability of about ± 1 ppm can be obtained with respect to the allowable value of the deviation of the free-running frequency of about ± 50 ppm. That is, a phase-locked loop circuit that realizes a sufficiently stable free-running frequency even if the input is disconnected or the phase synchronization is lost is realized.

[0020]

As described above, according to the present invention, the free-running frequency of the voltage-controlled oscillator provided in the phase-locked loop circuit is always maintained regardless of the disconnection of the input of the phase-locked loop circuit and the loss of the phase synchronization. Can be controlled accurately, and the reliability of the phase-locked loop circuit widely applied to communication devices and the like can be significantly improved.

[Brief description of drawings]

FIG. 1 is a principle of the present invention.

FIG. 2 shows an example of a temperature compensation voltage generation circuit.

FIG. 3 shows a second example of the temperature compensation voltage generating circuit.

FIG. 4 is a conventional phase locked loop circuit.

[Explanation of symbols]

 1 Phase Comparator 2 Loop Filter 3 Switch 4 Voltage Controlled Oscillator 5 Frequency Divider 6 Input Loss Detection Circuit 7 Out-of-Sync Detection Circuit 8 Logical OR Circuit 9 Temperature Compensation Voltage Generation Circuit

Claims (4)

[Claims] 1. A phase locked loop circuit, wherein a temperature compensation voltage generation circuit (9) is output from an output of a loop filter (2) by a signal output by an input disconnection detection circuit (6) detecting a disconnection of an input signal. The phase-locked loop circuit is characterized in that the output voltage of the temperature-compensated voltage generation circuit is supplied to the voltage controlled oscillator (4) by switching to the output of. 2. A phase locked loop circuit, wherein an output of a loop filter is switched to an output of a temperature compensation voltage generating circuit in accordance with a signal output from an out-of-synchronization detection circuit, and a voltage controlled oscillator is provided to the temperature compensation voltage generating circuit. A phase-locked loop circuit that supplies the output voltage of the. 3. The phase-locked loop circuit according to claim 1, wherein the temperature-compensated voltage generating circuit is a temperature-compensated voltage generating circuit configured by a temperature sensor circuit that outputs a voltage according to a temperature change. A phase-locked loop circuit characterized by: 4. The phase locked loop circuit according to claim 1, wherein the temperature compensation voltage generating circuit includes a temperature sensor circuit and an analog / digital conversion circuit.
A phase-locked loop circuit characterized by applying a temperature-compensated voltage generation circuit including a memory and a digital / analog conversion circuit.