JPS6048928B2 - Tuning fork crystal oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tuning fork type crystal resonator that can be used under optimal conditions as a watch resonator by appropriately selecting the electromechanical coupling coefficient of the tuning fork type crystal resonator.

Recently, piezoelectric materials, especially quartz crystals, have been used as time standards for wristwatches.

This uses electromechanical conversion of a piezoelectric material to sustain vibration and use it as a time standard, and the larger the electromechanical conversion, the better the efficiency. Therefore, efforts have been made to develop vibrators that should achieve maximum efficiency. However, this idea causes various problems when used as a watch vibrator. Next, we will discuss this problem. 1st
The figure shows a crystal oscillation circuit used in watches, which is a C-MOS
IC, feedback resistance Rf) load capacitance Cg, , Cd
It consists of a tuning fork type crystal oscillator.

When looking at the oscillation circuit side from both connection terminals of the crystal resonator in Figure 1, the oscillation circuit side is represented by the equivalent series capacitance and equivalent series resistance, that is, the load capacitance C and the negative resistance value -R. Ru. Therefore, the oscillation frequency obtained in FIG. 1 matches the resonant frequency obtained from the circuit configuration of FIG. 2. The following relationship exists between the load capacitance C and the load capacitances Cg, . . . Cd. c=12 (1) Cg+Cd Also, as the load capacitance Cg in FIG. 1, a trimmer capacitor with variable capacitance is generally used.

As is clear from equation (1), changing Cg is equivalent to changing the load capacitance C, so the following explanation will be made using FIG. 2. Now, FIG. 3 shows the relationship between the load capacitance C in FIG. 2 and the frequency change Δf/f (based on C). As the load capacitance C becomes smaller, a hyperbola is shown in which the frequency change increases even with a small capacitance change. When using a crystal resonator for a watch, this inclination is very important. That is, the change in frequency per load capacitance IPF becomes a problem. When using a crystal oscillator for a watch, the final frequency adjustment is performed using, for example, the aforementioned trimmer capacitor. In this case, if the frequency change per IPF is too large, the frequency change with respect to the load capacitance J amount will be large, and the aging of the capacitor will become a problem. In other words, even a small change in capacitance causes a large change in frequency, causing the clock to go out of order. On the other hand, if the frequency change per IPF is too small, it is necessary to increase the variable capacitance range of the capacitor or increase the accuracy of tuning the reference frequency of the crystal resonator itself. In this way, it is practically impossible to increase the variable capacitance range, and increasing the load capacitance increases the current consumption (load capacitance C
(proportional to) increases, which is not desirable. Similarly, increasing the tuning precision of the reference frequency of the crystal oscillator itself during manufacturing increases the frequency of the crystal oscillator, which is also undesirable. The present invention facilitates the manufacture of the crystal resonator itself by optimizing the piezoelectric efficiency, that is, the electromechanical coupling coefficient k, of the tuning fork type crystal resonator, and improves the yield. An object of the present invention is to provide a tuning fork type crystal resonator for obtaining a high precision crystal clock.

In particular, when configuring an oscillation circuit, the load capacitance Cg)Cd
Use the lowest possible value to reduce current consumption,
Moreover, it obtains a highly stable oscillation frequency over a long period of time.
k is limited to 0.025 to 0.035. This k is determined by the cutting angle of the crystal resonator, electrode arrangement, etc. Next, it will be specifically described using a mathematical formula that the relationship between the load capacitance C and the frequency change Δf/F changes depending on k. By calculating the impedance of the circuit shown in FIG. 2 and substituting the condition of zero at the series resonance frequency f, the following equation is obtained.

C, f=F, (1+-)”/゜C0+C llCO :fl (1+-・−・−)・・・・・・(2
)2rC0+C, where F, is the resonance frequency when C=ω, given by F,=1C0-, or r is the capacitance ratio r
=-2πVL. It is given by C,C,.

Further, the capacitance ratio r can be expressed as an electromechanical coupling coefficient J, and the relationship between r and k is expressed by the following formula. π1 r: NK...(3) Substituting equation (3) into equation (2) and further using the three frequencies when C=ω as the standard, equation (3) becomes as follows. become.

△Ff-Flv/2C0--nee k-...
(4) FlflπCO+C Therefore, the relationship between the frequency change Δf /F and the load capacity C changes depending on the electromechanical coupling coefficient k.

Figure 4 shows the relationship between load capacitance C and frequency change Δf /F, and shows that the characteristics of Δf /F and C, that is, the slow and fast characteristics, differ depending on the k of the crystal oscillator. There is.
k of the tuning fork crystal resonator used in conventional oscillation circuits
was generally 0.04. This is because, as described above, in a region where the load capacitance C is small, the frequency change Δf /F, with respect to the load capacitance C becomes large, and a small capacitance change causes a large frequency change. Also, the frequency change Δf /
In a region where F is small, the load capacitance C automatically increases, resulting in an increase in current consumption. Therefore, the present invention provides a tuning fork type crystal resonator that can obtain a stable oscillation frequency over a long period of time even when using a small load capacity, and actually improves the electromechanical coupling coefficient k. This achieves the object of the present invention. FIG. 4 shows the optimum k) of the present invention, that is, Δ when k=0.025 and 0.035.
7 shows the relationship between f/F and C. It can be easily understood from the figure that as k becomes smaller, Δf /fl becomes smaller within the same load capacity range. Next, we will explain how to actually change k and the reason for limiting the value. Formula (3
1c1, and as is clear, k varies depending on the capacitance ratio r.

That is, the equivalent capacitance C of the equivalent circuit constant of the crystal resonator and the parallel capacitance C. It varies depending on. Therefore, since C, CO is determined by the electrode arrangement and cut angle of the crystal resonator,
k actually varies and is determined by the electrode placement and cut angle. FIG. 5 shows the positional relationship between the coordinate axes and the crystal when cutting out the tuning fork type crystal resonator of the present invention.

That is, the XNYNZ axes indicate the electrical axis, mechanical axis, and optical axis of the crystal, respectively, and the tuning fork crystal resonator is first considered to be in a position perpendicular to the X axis. Next, FIG. 5 is a diagram when this crystal resonator is rotated by θ degrees with the Y axis as the rotation axis. And X'
, the Z' axis is the new axis after rotation of the X) and Z axes. FIG. 6 shows a cross-sectional view of the crystal resonator of FIG. 5, and also shows the electrodes arranged on the resonator. The electrode structure is a two-terminal electrode structure as shown in FIG. Oscillation can be maintained by connecting these terminals as shown in the circuit shown in FIG. No.? The figure shows the relationship between the cutting angle θ and the electromechanical coupling coefficient k of a tuning fork crystal resonator. As can be seen from this figure, as the cutting angle θ increases, k decreases, and when θ=90°, the electric field component in the X-axis direction is zero, making it impossible to excite. In this way, by changing the cut angle, k of the present invention can be obtained. Here, if the value of k is less than the upper limit of 0.035, then
In a crystal oscillator circuit, even if the load capacitance is small (C
By using a capacitor with a capacitance (gNCd), frequency stability can be sufficiently maintained even if there is a slight change in capacitance over time, and at the same time, the current consumption can be reduced because the load capacitance is small. Also, if k is too small, the efficiency will deteriorate, so
As the equivalent circuit constant R of a crystal resonator increases, it is almost impossible to continue stable oscillation with such a crystal resonator in an oscillation circuit, and in extreme cases, it may not oscillate. . If we try to forcefully excite this, the amplification rate of the IC must be made extremely large. That is, the current consumption becomes significantly large, making it impossible to use it particularly as a watch vibrator. For this reason, 0.025 is obtained as the lower limit value of k. As mentioned above, optimizing k has the following effects. (i)
Even if the load capacity C is small, the frequency change Δ per IPF
f /F is smaller than that of the conventional device, and even if the capacitor changes slightly over time, only a small change in frequency is required, so oscillation with a stable frequency can be maintained.

(Ii) Since the load capacitance C (or Cg, .Cd) is small, current consumption can be reduced.

As described above, the present invention satisfies both the reduction in current consumption and the stability of the crystal oscillation circuit by limiting k (the shaded area in FIG. 4), thereby making it possible to obtain a high-precision crystal clock.
Brief explanation of the drawings Figure 1 is a diagram of a crystal oscillation circuit used in watches, Figure 2 is a diagram of a crystal oscillation circuit used in watches.
The figure is a circuit diagram when a load capacitance C is connected in series with a crystal resonator, Figure 3 is a diagram showing the relationship between load capacitance C and frequency change Δf /F, and Figure 4 is a diagram with k as five parameters. Figure 5 shows the cut-out angle of the crystal resonator, Figure 6 shows the electrode configuration of the crystal resonator, and Figure 7 shows the relationship between the load capacitance C and the frequency change Δf /F. FIG. 3 is a diagram showing the relationship between the cutting angle of a crystal resonator and k.

Claims (1)

[Claims] 1. A tuning fork type crystal resonator, characterized in that the electromechanical coupling coefficient k of the resonator satisfies 0.025≦k0.035≦.