JPS5936444B2 - crystal oscillation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a crystal oscillation circuit that can oscillate at a constant frequency regardless of fluctuations in power supply voltage.

2. Description of the Related Art A crystal oscillation circuit 1, an example of which is shown in FIG. 1, is used in electronic watches such as electronic watches and electronic clocks.

The basic configuration of the crystal oscillation circuit 1 includes an inverter 2, a crystal resonator 3, a feedback resistor 4, an input capacitor 5, an output capacitor 6, etc., and an output signal is obtained at an output terminal 7.

Furthermore, VDD? VSS indicates a power supply.

There are two types of crystal oscillators 3 with different physical properties.
A crystal resonator used in a crystal oscillation circuit with a low oscillation frequency of about Hz is hereinafter abbreviated as an A-type resonator, and a crystal with a high oscillation frequency of about 4.19 MHz, such as in a crystal oscillation circuit for electronic clocks. The crystal resonator used in the oscillation circuit is hereinafter abbreviated as a B-type resonator.

Generally, this type of crystal oscillation circuit 1 has a drawback in that when the power supply voltage fluctuates, the oscillation frequency also fluctuates.

Specifically, in the case of an oscillation circuit using an A-type resonator, as the power supply voltage increases, the oscillation frequency tends to increase as shown in FIG. 2A, and in the case of an oscillation circuit using a B-type resonator, As the power supply voltage increases, the oscillation frequency tends to decrease as shown in FIG. 2B.

On the other hand, in this type of crystal oscillation circuit 1, an input side or output side capacitor (hereinafter referred to as an oscillation frequency determining capacitor) is used.

) The oscillation frequency can be changed by the capacitance of
Specifically, it is known that the oscillation frequency decreases when the capacitance of the oscillation frequency determining capacitor is increased, regardless of the physical properties of the crystal resonator.

Therefore, by utilizing the relationship between the capacitance and the oscillation frequency, it is possible to suppress fluctuations in the oscillation frequency caused by fluctuations in the power supply.

To put this into practice, for a crystal oscillator circuit using an A-type resonator, whose characteristics are shown in Figure 2A, as the power supply voltage increases, as shown in Figure 3A, it becomes static. A capacitive element (hereinafter referred to as a positive characteristic element) has a characteristic of increasing capacitance.

) may be used as the capacitor for determining the oscillation frequency.On the other hand, for a crystal oscillation circuit using a B-type resonator whose characteristics are shown in FIG. 2B, a capacitor whose characteristics are shown in FIG. 3B is used. Second, a capacitive element has a characteristic that its capacitance decreases as the power supply voltage increases (hereinafter referred to as a negative characteristic element).

) can be used in the same way. Therefore, in conventional crystal oscillation circuits using type B resonators, negative characteristic elements such as variable capacitance diodes are used as capacitors for determining the oscillation frequency, and the oscillation effect is maintained at a constant frequency regardless of fluctuations in the power supply voltage. It had been possible to do so.

However, for a crystal oscillator circuit using an A-type resonator, it is not easy to obtain a suitable positive characteristic element, so it is necessary to relate the fluctuation of the power supply voltage to the capacitance of the capacitor for determining the oscillation frequency. Depending on the method, frequency correction is difficult.

An object of the present invention is to stabilize the oscillation frequency regardless of fluctuations in the power supply voltage, particularly in a crystal oscillation circuit using an A-type resonator.

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

In the following description of the drawings, the same parts as in the conventional example shown in FIG. 1 are given the same reference numerals as in FIG. 1, and the description thereof will be omitted.

In FIG. 4, reference numeral 8 designates a crystal oscillation circuit of the present invention used in electronic watches and the like, and as the crystal oscillator 3, the above-mentioned A type oscillator is used.

Also, code 2 is impark, usually CMO8, FE
It is constituted by a T-inverter or the like, and therefore, as the power supply voltage increases, the output voltage obtained at the output terminal 7 also tends to increase.

A feature of the crystal oscillation circuit 8 according to the present invention is that a frequency correction capacitor 9 is connected in parallel with the output side capacitor 6 which is the oscillation frequency determining capacitor, and a current path 10 flowing through the frequency correction capacitor 9 This is because a switching element 11 is connected which closes when a voltage higher than a certain voltage value is applied.

Here, the frequency correction capacitor 9 may be provided in parallel with the input capacitor 5 instead of the output capacitor 6.

Let us consider the operation of the embodiment having the above configuration.

FIG. 6 is a diagram showing the output waveform of the crystal oscillation circuit 8, and waveform a is the oscillation signal output waveform before the power supply voltage rises.

In the figure, VTR is the voltage at which the switching element 11 closes, and in the waveform a, the switching element 11 is closed for a period tl exceeding the VTR.

Now, the power supply voltage rises and the voltage level of the output signal becomes ■1
to V2, and the output signal becomes as shown in the waveform, the period during which the switching element 11 is closed in the waveform is t2, and between the length of time t1 and t2, tl<t2. The relationship holds true.

In other words, as the power supply voltage rises and the output signal rises, the period during which the switching element 11 is closed becomes longer.
The oscillation frequency is prevented from increasing by increasing the capacitance value in a certain amount of time.

That is, by continuously changing the ratio of time during which the frequency correction capacitor 9 is disconnected during one period of the oscillation signal, an increase in the oscillation frequency due to a rise in the power supply voltage is suppressed.

Next, FIG. 5 shows a more specific embodiment of the present invention.

FIG. 5 shows only the oscillation circuit in detail in the electronic circuit diagram of the electronic watch.

The part surrounded by the dotted line 12 in FIG. 5 is a part to be integrated into an IC chip.

The feature of this circuit is that a frequency correction capacitor 9 is connected in parallel to the output side capacitor 6, and a current path 10KMO8 flowing through this frequency correction capacitor 9,
Connect FET13 to this MOS.

The gate terminal of FET13 is connected to the output terminal 7 via the resistor 14.
It is in that it is connected to.

Here, MOS. The connection position of the gate terminal of the FETI 3 is not limited to the illustrated example, but may be any position as long as the output voltage of the oscillation signal increases as the power supply voltage increases.

According to this embodiment having the above configuration, when the oscillation frequency attempts to increase due to a rise in the power supply voltage, the output voltage of the inverter 2 rises, causing the voltage at the output terminal 7 to rise, and this voltage causes the resistor 14 to rise. Through MOS, FE
By applying the voltage to the gate terminal of T13, the MOS
, the FET 13 is periodically turned on as shown in the previous stage and in FIG. 6, and the frequency correction capacitor 9 functions.

As a result, in the same way as in the embodiment described above, as the output voltage increases, the time during which the frequency correction capacitor 9 functions becomes longer, and the capacitance of the output side capacitor increases within a certain period of time, so that the power supply voltage increases. The increase in the oscillation frequency of the crystal oscillator circuit due to the increase in oscillation frequency is suppressed.

As is clear from the above description, according to the present invention, the oscillation frequency determining capacitor is provided, and as the capacitance of the oscillation frequency determining capacitor increases, the oscillation frequency decreases and the power supply voltage increases. In the crystal oscillation circuit having the characteristic that the oscillation frequency increases as a result, a frequency correction capacitor is connected in parallel with the oscillation frequency determining capacitor, and the oscillation signal of the oscillation circuit is connected to the current path flowing through the frequency correction capacitor. By providing a switching element that is closed during a period when the output voltage of the oscillation circuit is equal to or higher than a predetermined value, the oscillation frequency of this type of oscillation circuit can be significantly stabilized regardless of fluctuations in the power supply.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an example of a conventional oscillation circuit, Figure 2A is a graph showing the relationship between power supply voltage and oscillation frequency in a crystal oscillation circuit using an A-type resonator, and Figure 2B is ,
A graph showing the relationship between power supply voltage and oscillation frequency in a crystal oscillation circuit using a B-type resonator, Figure 3A is a graph showing the relationship between power supply voltage and capacitance in a positive characteristic element, Figure 3B is a graph showing the relationship between power supply voltage and capacitance in a negative characteristic element, FIG. 4 is a circuit diagram showing an example of the oscillation circuit of the present invention, and FIG. 5 is a more specific example of the oscillation circuit of the present invention. FIG. 6 is a waveform diagram for explaining the operation of the oscillation circuit of the present invention. 5.6...Capacitor for determining oscillation frequency, 8.
...Crystal oscillation circuit, 9...Frequency correction capacitor, 10...Current path, 11...
・Switching element, 13...MOS, FE
T0

Claims (1)

[Claims] 1. An oscillation frequency determining capacitor is provided, and as the capacitance of the oscillation frequency determining capacitor increases, the oscillation frequency decreases, and as the power supply voltage increases, the oscillation frequency increases. In the crystal oscillator circuit having the above characteristics, a frequency correction capacitor is connected in parallel with the oscillation frequency determining capacitor, and a switching element is connected to the current path flowing through the frequency correction capacitor, and the switching element is configured such that the oscillation signal voltage is A crystal oscillation circuit characterized by being closed for a period when the value exceeds a predetermined value. 2. The crystal oscillation circuit according to claim 1, wherein the switching element is a MOSFET whose gate terminal is connected to a portion of the crystal oscillation circuit where the oscillation signal voltage increases as the power supply voltage increases. .