JP2001136030A - Crystal oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem of a conventional crystal oscillation circuit whose power line is in common to those of other electronic circuits where its phase noise characteristic is deteriorated due to noise superimposed on the power line. SOLUTION: A low pass filter 4 is provided between power line 2 and a crystal oscillation circuit 1 to eliminate noise that cannot be eliminated by a conventional bypass capacitor 3 thereby enhancing a phase noise characteristic. This invention also includes prevention of a delayed start time of the crystal oscillation circuit by providing an analog switch in parallel with the low pass filter and a CR differentiation circuit, which turns on the analog switch when the voltage is established on the power line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal oscillation circuit in which a power supply line is the same as another electronic circuit, and more particularly to a crystal oscillation circuit in which phase noise characteristics are prevented from deteriorating due to noise intrusion from a power supply line.

[0002]

2. Description of the Related Art In recent years, a crystal oscillator circuit has been integrated into an LSI by a technique of miniaturizing components and integrating semiconductors. Along with this, the size of the crystal oscillator has been rapidly reduced, and this has greatly contributed to downsizing of equipment using the crystal oscillator.

[0003] On the other hand, as a harmful effect of the LSI, there is a problem that a crystal oscillation circuit is easily affected by external circuits or other circuits on the LSI, and noise is superimposed on an oscillation frequency. That is, the phase noise characteristic which is a purity index of the oscillation frequency is reduced.

For example, in the field of mobile communication, high-precision frequency stability is required. However, a decrease in phase noise characteristics leads to a deterioration in communication quality, so that it may not be possible to cope with higher functionality in the future.

[0005]

The cause of the phase noise characteristic deterioration is considered to be that the crystal oscillation circuit receives interference from other circuits on the LSI. This is mainly affected through a power supply line shared by each circuit. Since power supply fluctuations lead to frequency fluctuations, phase noise characteristics deteriorate.

In the prior art, as shown in FIG. 3, measures are taken to cut high frequency noise by adding a bypass capacitor 3 to the connection point between the crystal oscillation circuit 1 and the power supply line 2. Noise levels that affect noise cannot be removed.

An object of the present invention is to provide a crystal oscillation circuit in which phase noise characteristics are prevented from deteriorating due to noise from a power supply line.

[0008]

SUMMARY OF THE INVENTION According to the present invention, a low-pass filter is provided between a power supply line and a crystal oscillation circuit to remove the noise to a level which cannot be eliminated by a bypass capacitor. By improving the characteristics and providing a circuit that supplies power to the crystal oscillation circuit by bypassing the low-pass filter when the voltage of the power supply line is established, the start-up time of the crystal oscillation circuit is prevented from being delayed. The configuration is characterized.

In a crystal oscillation circuit having the same power supply line as another electronic circuit, a noise superimposed on the power supply line between the power supply line and the crystal oscillation circuit is prevented from entering the crystal oscillation circuit. A low-pass filter is provided.

An analog switch is provided in parallel with the low-pass filter, and a CR differentiating circuit is provided for turning on the analog switch when the voltage of the power supply line is established.

[0011]

FIG. 1 is a power supply circuit diagram of a crystal oscillation circuit showing an embodiment of the present invention. While the conventional crystal oscillation circuit 1 has a circuit configuration in which power is directly supplied from the power supply line 2, in the present embodiment, the crystal oscillation circuit 1
An LPF (low pass filter) 4 is interposed between the power supply line 2 and the power supply line 2. The LPF 4 can be composed of, for example, a T-type or π-type filter.

With this configuration, of the noise superimposed on the power supply line 2, even the low-level noise that cannot be completely removed by the bypass capacitor 3 is removed by the LPF 4, thereby stabilizing the operation of the crystal oscillation circuit 1 and its phase noise characteristic. Can be improved.

FIG. 2 is a power supply circuit diagram of a crystal oscillation circuit showing another embodiment of the present invention. 1 differs from FIG. 1 in that an analog switch 5 and a CR differentiating circuit 6 are provided.

The analog switch 5 is connected in parallel with the LPF 4, and its on / off control is performed by a CR differentiating circuit 6 that responds to the establishment of the voltage of the power supply line 2. That is, when the power is turned on, when the voltage of the power supply line 2 rises, an instantaneous pulse is generated in the CR differentiating circuit 6 in response to the change, and the analog switch 5 is turned on only during this pulse period.

As a result, when the analog switch 5 is turned on, the crystal oscillation circuit 1 is directly supplied with power from the power supply line 2, and when the analog switch 5 is turned off, a circuit equivalent to FIG.

The CR differentiating circuit 6 for controlling the analog switch 5 turns on the analog switch 5 when the power is turned on, and turns off after a time corresponding to the time constant of the CR. C
The R differentiating circuit 6 can adjust the time for bypassing the LPF 4 after the power is turned on, for example, by making the value of R (resistance) variable.

In the case of FIG. 1, the power supply voltage applied to the crystal oscillation circuit 1 is delayed even if the power is turned on due to the influence of the LPF 4, and the startup time may be delayed. In this regard,
In the present embodiment, when the power is turned on, the power is supplied directly from the power supply line 2 to the crystal oscillation circuit 1, so that the startup time is not delayed. After startup, the LPF 4 is interposed to remove high frequency noise, so that an effect of improving the phase noise characteristic of the crystal oscillation circuit 1 can be expected.

[0018]

As described above, according to the present invention, since the low-pass filter is provided between the power supply line and the crystal oscillation circuit, it is possible to prevent the noise from the power supply line from deteriorating the phase noise characteristic of the crystal oscillation circuit. it can.

Further, since a circuit for supplying power to the crystal oscillation circuit by bypassing the low-pass filter when the voltage of the power supply line is established is provided, it is possible to prevent the startup time of the crystal oscillation circuit from being delayed.

Further, since the cutoff frequency of the low-pass filter can be varied, it is possible to cope with power supply noise whose frequency component changes depending on the environment around the LSI.

[Brief description of the drawings]

FIG. 1 is a power supply circuit diagram of a crystal oscillation circuit according to an embodiment of the present invention.

FIG. 2 is a power supply circuit diagram of a crystal oscillation circuit according to another embodiment of the present invention.

FIG. 3 is a power supply circuit diagram of a conventional crystal oscillation circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Crystal oscillator circuit 2 ... Power supply line 3 ... Bypass capacitor 4 ... Low pass filter (LPF) 5 ... Analog switch 6 ... CR differentiation circuit

Claims (2)

[Claims] 1. A crystal oscillation circuit having the same power supply line as another electronic circuit, wherein between the power supply line and the crystal oscillation circuit, noise superimposed on the power supply line is prevented from entering the crystal oscillation circuit. A crystal oscillation circuit provided with a low-pass filter for removing. 2. The crystal oscillation circuit according to claim 1, wherein an analog switch is provided in parallel with said low-pass filter, and a CR differentiating circuit is provided for turning on said analog switch when a voltage of said power supply line is established.