JP2024083702A - Crystal Oscillator - Google Patents

Abstract

[Problem] To provide an oven-controlled crystal oscillator using a voltage-variable capacitance element, capable of correcting the downward sloping frequency-temperature characteristic to a flat one by simple means, and a method for manufacturing the same. [Solution] The crystal oscillator includes a constant voltage circuit, an oscillation circuit having an oven circuit and a voltage-variable capacitance element connected to the constant voltage circuit, a bath containing at least the oven circuit and the oscillation circuit, and a ground line connected to the oven circuit and the oscillation circuit, and is characterized in that a compensation resistor is provided between the ground line and a secondary terminal which is the terminal on the ground line side of the oven circuit and the oscillation circuit, and the compensation resistor is a resistor which compensates the potential of the secondary terminal of the voltage-variable capacitance element to a potential which can compensate the inter-terminal voltage of the voltage-variable capacitance element to a predetermined voltage by varying the potential of the secondary terminal of the compensation resistor in accordance with the value of the current flowing through the oven circuit. [Selected Figure] Figure 1

Description

The present invention relates to an oven-controlled crystal oscillator that can correct the frequency-temperature characteristics.

In recent years, 5G has become more widespread. Among them, small base stations have been attracting attention because they overcome the short reach of radio waves, which is a weakness of 5G. Small base stations use oven-controlled crystal oscillators (OCXOs) to withstand rising internal temperatures and severe environmental changes.
Patent Document 1 describes an OCXO that uses a varicap diode to adjust the frequency (paragraph 0011 of Patent Document 1). Figure 8 is a block diagram that shows a simple configuration of this OCXO. This OCXO 20 includes a constant voltage circuit, an oven circuit connected to the constant voltage circuit, an oscillation circuit connected to the constant voltage circuit and having a voltage-variable capacitance element, a tank that contains at least the oven circuit and the oscillation circuit, and a ground line connected to the oven circuit and the oscillation circuit. In this OCXO, the capacitance value of the varicap diode 14a is changed by a control voltage, that is, the load capacitance of the crystal resonator is changed, thereby adjusting the oscillation frequency (paragraph 0023 of Patent Document 1).
In addition, by sharing a common ground line for the oscillator circuit including the quartz crystal oscillator, the oven circuit for keeping the quartz crystal oscillator in the thermostatic chamber at a constant temperature, and the varicap diode, the thermal coupling is strengthened and the frequency-temperature characteristics are improved (paragraph 0021 of Patent Document 1).

Japanese Patent No. 6208472

OCXOs have high frequency stability and frequency-temperature characteristics on the order of ppb (1x10-9). The closer the frequency-temperature characteristic of an OCXO is to 0 ppb, that is, the flatter it is with respect to temperature, the better it is. The frequency-temperature characteristic of an OCXO is generated by the complex interactions of the frequency-temperature characteristic of the crystal unit, the frequency-temperature characteristic of the oscillator circuit, and other factors such as aging and reproducibility. As a result, after manufacture, there are OCXOs that exhibit an almost flat temperature characteristic with respect to temperature, as shown in Figures 9 (A) to (C), those that exhibit a temperature characteristic that declines slightly with increasing temperature (although this occurs less frequently), and those that exhibit a temperature characteristic that rises slightly with respect to temperature. It would be preferable if one of the temperature characteristics of an OCXO that exhibits a temperature characteristic that rises or falls with respect to temperature could be corrected to a flat one by some means.

The present invention has been made in consideration of the above points, and the purpose of this application is to provide a crystal oscillator with a thermostatic oven that uses a voltage-variable capacitance element and that can correct the downward-sloping frequency-temperature characteristic to a flat one using simple means, and a method for manufacturing the same.

In order to achieve this objective, the inventor of this application has conducted intensive research and has come to the conclusion that the above objective may be achieved by actively utilizing a voltage-variable capacitance element in a thermostatic oven-equipped crystal oscillator having a voltage-variable capacitance element.
Therefore, according to the crystal oscillator of the present invention, in a crystal oscillator including a constant voltage circuit, an oven circuit connected to the constant voltage circuit, an oscillation circuit connected to the constant voltage circuit and having a voltage variable type capacitance element, a vessel containing at least the oven circuit and the oscillation circuit, and a ground line connected to the oven circuit and the oscillation circuit,
a compensation resistor is provided between the ground line and a secondary side terminal which is a terminal on the ground line side of the oven circuit and the oscillation circuit,
The compensation resistor is characterized in that the potential of the secondary terminal of the voltage-variable capacitance element is compensated to a potential that can compensate the inter-terminal voltage of the voltage-variable capacitance element to a predetermined voltage by varying the potential of the secondary terminal of the compensation resistor in accordance with the value of the current flowing through the oven circuit.
In carrying out the present invention, the compensation resistor may be either a fixed resistor or a variable resistor, however, it is preferable to use a variable resistor as the compensation resistor, since the potential of the secondary terminal of the voltage variable type capacitance element can be adjusted for each crystal oscillator.

According to the invention of the method for manufacturing a crystal oscillator of this application, when manufacturing a crystal oscillator including a constant voltage circuit, an oven circuit connected to the constant voltage circuit, an oscillation circuit connected to the constant voltage circuit and having a voltage variable type capacitance element, a vessel containing at least the oven circuit and the oscillation circuit, and a ground line connected to the oven circuit and the oscillation circuit,
measuring the frequency temperature characteristics of the crystal oscillator;
selecting a crystal oscillator whose frequency-temperature characteristic in the measurement shows a right-sloping trend with respect to temperature rise; inserting a compensation resistor between the oven circuit, the voltage-variable capacitance element, and a secondary terminal of the oscillation circuit, which is the terminal on the ground line side of the selected crystal oscillator, and the ground line, the compensation resistor fluctuating the potential of the secondary terminal of the compensation resistor in response to the value of the current flowing through the oven circuit, thereby compensating the potential of the secondary terminal of the voltage-variable capacitance element to a potential that can compensate the inter-terminal voltage of the voltage-variable capacitance element to a predetermined voltage;
The present invention is characterized by comprising:

According to the crystal oscillator of the present invention, the potential of the secondary terminal of the compensation resistor varies according to the current flowing through the oven circuit. Here, the current of the oven circuit, for the purpose of this invention, increases as the environment in which the crystal oscillator is placed becomes lower, and decreases as the temperature increases. Therefore, the current flowing through the compensation resistor also increases as the environment in which the crystal oscillator is placed becomes lower, and decreases as the temperature increases, so the potential of the secondary terminal of the compensation resistor, i.e., the potential of the secondary terminal of the voltage-variable capacitance element, increases as the environment in which the crystal oscillator is placed becomes lower, and decreases as the temperature increases. Therefore, the capacitance of the voltage-variable capacitance element changes, and the oscillation frequency of the crystal oscillator changes in a direction that compensates for the above-mentioned right-sloping temperature characteristic. Therefore, a crystal oscillator that previously showed a right-sloping frequency-temperature characteristic can be realized as a crystal oscillator that shows a flat frequency-temperature characteristic with respect to temperature by a simple means of providing a compensation resistor.
Furthermore, according to the manufacturing method of the crystal oscillator of this application, a predetermined process is carried out and an appropriate compensation resistor is added during the process, so that a crystal oscillator that exhibits a downward sloping frequency-temperature characteristic can be corrected to a crystal oscillator that exhibits a flat frequency-temperature characteristic with respect to temperature.

FIG. 2 is a block diagram of an OCXO 10 according to an embodiment of the present invention. A graph of the capacitance value of a varicap diode. 4 is a graph of oven circuit current at temperature for an OCXO of the present invention. 4 is a graph showing the change in potential at the secondary terminal of the OCXO of the present invention. 4 is a graph showing the voltage across the terminals of the varicap diode of the OCXO of the present invention. 4 is a graph showing the inter-terminal capacitance of a varicap diode of the OCXO of the present invention. 13 is a graph showing the frequency temperature characteristics of an OCXO before and after a compensation resistor is provided. FIG. 2 is a block diagram of a prior art OCXO 20. An example of the frequency temperature characteristics of an OCXO.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of a crystal oscillator and a method for manufacturing the same according to the present invention will be described with reference to the drawings.
The drawings used in the description are merely schematic illustrations to the extent that these inventions can be understood. In addition, in the drawings used in the description, similar components are indicated by the same numbers, and their explanation may be omitted. Furthermore, the shapes, materials, etc. described in the following description are merely preferred examples within the scope of this invention. Therefore, the present invention is not limited to the following embodiments.

1 is a block diagram of an OCXO 10 according to an embodiment of the present invention. The OCXO 10 includes a constant voltage circuit 11, an oven circuit 12, an oscillation circuit 14, a thermostatic chamber 13, a compensation resistor 16, and a ground line 17. The oven circuit 12 and the oscillation circuit 14 are included in the thermostatic chamber 13.

The constant voltage circuit 11 converts the power supply voltage Vcc supplied from an external source into a constant voltage and supplies the constant voltage to the oven circuit 12 and the oscillator circuit 14.

The oven circuit 12 is composed of a temperature sensor, a heating element, and other components such as a temperature control circuit. The temperature sensor measures the temperature inside the thermostatic chamber 13, and the heating element controls the oscillation element (described below) inside the thermostatic chamber 13 so that it is kept at a predetermined temperature. The temperature sensor may be, for example, a thermistor or a platinum resistor. The heating element may be, for example, a power transistor or a resistor. The temperature control circuit may be composed of, for example, an operational amplifier.

The oscillator circuit 14 is composed of a voltage-variable capacitance element 14a, an oscillation element 14b, and other components. In the present invention, the voltage-variable capacitance element 14a is a varicap diode. The oscillation element 14b can be, for example, a quartz crystal oscillator.

As a feature of the present invention, a compensation resistor 16 is provided between the secondary side terminal 15, which is the terminal on the ground line 17 side of at least the voltage variable type capacitance element 14a of the oven circuit 12 and the oscillator circuit 14, and the ground line 17.
The compensation resistor 16 has a constant that is determined in advance based on the design content (details will be described later). The potential of the terminal on the secondary side 15 side of the compensation resistor 16 changes depending on the magnitude of the current flowing through the oven circuit 12. By utilizing this, the potential on the secondary side 15 side of the voltage-variable capacitance element 14a can be varied to vary the reverse voltage applied to the voltage-variable capacitance element 14a. The downward sloping frequency temperature characteristic is corrected to become flat by utilizing the potential variation of the compensation resistor 16. This point will be described below using a specific example.

The operation and effect of the present invention will be described below when the power supply voltage Vcc is 2.8 V, a varicap diode is used as the voltage variable capacitance element 14a, and a compensation resistor is 0.1 Ω in the OCXO 10 of the embodiment.
Figure 2 shows the capacitance change versus terminal voltage for the varicap diode we will be using. The capacitance decreases as the terminal voltage increases, which is an inverse proportional relationship. The cubic approximation of this curve is also shown on the graph.

Some OCXOs have a frequency-temperature characteristic that declines with increasing temperature. For example, some OCXOs measured by the inventor of this application have the frequency-temperature characteristic shown in FIG. 7A. Specifically, as shown in the figure with temperature on the horizontal axis and frequency fluctuation rate (ppb) on the vertical axis, some OCXOs have a frequency-temperature characteristic that declines with increasing temperature, with a frequency fluctuation rate of about 16 ppb on the low temperature side and about -15 ppb on the high temperature side.
On the other hand, in the case of the OCXO 10 shown in the embodiment, even if the environmental temperature changes, it is necessary to control the internal temperature of the thermostatic chamber 13 (the temperature of the crystal unit 14b) to a predetermined temperature, for example, 85° C. For this reason, in the case of the OCXO 10 of the embodiment, in order to control the internal temperature of the thermostatic chamber 13 to be constant, the current flowing through the oven circuit 12 with respect to the change in the environmental temperature exhibits a current behavior in which, as shown in FIG. 3, the current increases to about 600 mA on the low temperature side and decreases to about 50 mA on the high temperature side. In FIG. 3, the horizontal axis represents temperature and the vertical axis represents the current flowing through the oven circuit. Therefore, since a current according to the current behavior of the oven circuit described above also flows through the compensation resistor 16, which is a feature of the present invention, the potential on the secondary side terminal 15 side of the compensation resistor 16 becomes a potential determined by the resistance value 0.1Ω of the compensation resistor 16 and the current flowing through the compensation resistor 16, as shown in FIG. 4. That is, as shown in FIG. 4, the potential fluctuation is about 0.06 V on the low temperature side and decreases to about 5 mV on the high temperature side. In FIG. 4, the horizontal axis represents temperature, and the vertical axis represents the potential on the secondary terminal 15 side of the compensation resistor 16 .
The above-mentioned potential fluctuation occurring in the compensation resistor 16 can be used to compensate for the above-mentioned downward sloping frequency temperature characteristic. That is, the above-mentioned potential fluctuation occurring in the compensation resistor 16 becomes a potential fluctuation at the terminal 15 on the secondary side of the varicap diode 14a, and in this embodiment, it causes a fluctuation in the terminal voltage of 2.8 V applied between the capacitance elements of the varicap diode 14a. This state is shown in Fig. 5, with the horizontal axis representing temperature and the vertical axis representing the terminal voltage of the varicap diode 14a. From Fig. 5, it can be seen that when the compensation resistor 16 is provided, the terminal voltage of the varicap diode 14a changes linearly with temperature due to the influence of the potential fluctuation at the compensation resistor 16 shown in Fig. 4, becoming approximately 2.74 V on the low temperature side and approximately 2.795 V on the high temperature side.
The variation in the terminal voltage of the varicap diode 14a shown in Fig. 5 directly varies the terminal capacitance of the varicap diode 14a. This variation in capacitance is shown in Fig. 6, with temperature on the horizontal axis and the terminal capacitance of the varicap diode 14a on the vertical axis. It can be seen from Fig. 6 that when the compensation resistor 16 is provided, the terminal capacitance of the varicap diode 14a varies linearly with temperature due to the influence of the potential variation at the compensation resistor 16 shown in Fig. 5, becoming approximately 7.6 pF on the low temperature side and approximately 7.45 pF on the high temperature side.
The variation in the capacitance between the terminals of the varicap diode 14a as shown in FIG. 6 corresponds to the variation in the load capacitance of the oscillator circuit 14, so that the frequency output from the oscillator circuit 14 varies.

The provision of the compensation resistor 16 causes changes in the potential of the secondary side terminal 15, the terminal voltage of the varicap diode 14a, and the terminal capacitance of the varicap diode 14a shown in FIGS. 4 to 6, making it possible to compensate the frequency-temperature characteristic that declines to the right with increasing temperature shown in FIG. 7(A) into a nearly flat frequency-temperature characteristic with respect to temperature as shown in FIG. 7(B).
In contrast, in the case of the conventional OCXO 20 shown in FIG. 8 which does not include the correction resistor 16, potential fluctuations due to the correction resistor 16 do not occur in the first place, so the voltage between the terminals of the varicap diode 14a cannot be varied, and therefore the frequency-temperature characteristics cannot be corrected.

2. Manufacturing method of crystal oscillators A number of crystal oscillators before sealing are prepared, and the characteristics of the crystal oscillators, such as the frequency temperature characteristic, and, if necessary, the phase noise characteristic and short-term frequency stability, are inspected. After the characteristic inspection, it is confirmed whether each characteristic meets the specifications of the crystal oscillator. Among them, crystal oscillators whose frequency temperature characteristic shows a right-leaning trend at high temperatures and does not meet the specifications are selected. From the deviation between the frequency temperature characteristic of the selected crystal oscillator and the specifications, the constant of the compensation resistor 16 (see FIG. 1) to correct the deviation is considered. The compensation resistor 16 with the considered resistance constant is inserted between the secondary terminal 15 and the ground line 17. After inserting the compensation resistor 16, the frequency temperature characteristic is measured again, and it is confirmed that the frequency temperature characteristic has been corrected to an almost flat state within the temperature range of the oscillator specifications. Next, the oscillator is resistance-welded and sealed.

The compensation resistor 16 may be a variable resistor. In the case of a variable resistor, if the compensation resistor 16 is inserted and the frequency-temperature characteristic is measured and the compensation is not as expected, the resistance value is adjusted again and compensation is performed, so that a flat frequency-temperature characteristic with respect to temperature can be obtained.

In OCXOs, which require frequency-temperature characteristics on the order of ppb, it is possible to manufacture crystal oscillators with flat frequency-temperature characteristics over temperature by the simple measure of providing a compensation resistor. In addition, since the deviation from the specifications differs for each crystal oscillator, it is possible to manufacture crystal oscillators that can be compensated for to suit each individual crystal oscillator.

10: OCXO of the present invention 20: Prior art OCXO
11: constant voltage circuit 12: oven circuit 13: thermostatic chamber 14: oscillation circuit 14a: voltage variable capacitance element 14b: oscillation element 15: secondary side terminal 16: compensation resistor 17: ground line

Claims (3)

A crystal oscillator comprising: a constant voltage circuit; an oven circuit connected to the constant voltage circuit; an oscillation circuit connected to the constant voltage circuit and having a voltage variable type capacitance element; a vessel containing at least the oven circuit and the oscillation circuit; and a ground line connected to the oven circuit and the oscillation circuit,
a compensation resistor is provided between the ground line and a secondary side terminal which is a terminal on the ground line side of the oven circuit and the oscillation circuit,
The compensation resistor is
A crystal oscillator comprising: a resistor that corrects the potential of the secondary terminal of the voltage-variable capacitance element to a potential that can correct the inter-terminal voltage of the voltage-variable capacitance element to a predetermined voltage by fluctuating the potential of the secondary terminal of the compensation resistor in accordance with the value of the current flowing through the oven circuit. The crystal oscillator according to claim 1, characterized in that the compensation resistor is a variable resistor. In manufacturing a crystal oscillator including a constant voltage circuit, an oven circuit connected to the constant voltage circuit, an oscillation circuit connected to the constant voltage circuit and having a voltage variable type capacitance element, a vessel containing at least the oven circuit and the oscillation circuit, and a ground line connected to the oven circuit and the oscillation circuit,
measuring the frequency temperature characteristics of the crystal oscillator;
selecting a crystal oscillator whose frequency-temperature characteristic shows a right-sloping slope with respect to an increase in temperature in the measurement;
a step of inserting a compensation resistor between a secondary terminal, which is a terminal on the ground line side of the oven circuit, the voltage variable capacitance element, and the oscillation circuit of the selected crystal oscillator, and the ground line, the compensation resistor varying a potential of the secondary terminal of the voltage variable capacitance element in response to a current value flowing through the oven circuit, thereby compensating the potential of the secondary terminal of the voltage variable capacitance element to a potential that can compensate the inter-terminal voltage of the voltage variable capacitance element to a predetermined voltage;
A method for manufacturing a crystal oscillator comprising the steps of:

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