JPH0533054Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention is a highly stable signal generator that is applied to mobile communication devices such as in-vehicle radios, and its content is a signal generator that uses a crystal resonator. It relates to an oscillator (hereinafter referred to as a crystal oscillator), and more specifically, it provides a means for obtaining long-term stable characteristics of its oscillation frequency.

[Conventional technology]

Since crystal oscillators have excellent frequency stability, they are used as reference signals in many fields. However, extremely high frequency stability is required for crystal oscillators for mobile communications, such as those used in in-vehicle radios.

The frequency stability of a crystal oscillator mainly consists of frequency accuracy (tuning accuracy) and frequency stability with respect to ambient temperature. Therefore, it had built-in means for compensating for frequency-temperature characteristics by combining electrical elements whose characteristics changed. Therefore, in order to reliably obtain these functions, it is necessary to have a mechanically robust and stable structure, which results in a large size.

To solve this drawback, the PEAC element was invented and applied to crystal oscillators. Details of the PEAC element are disclosed in Japanese Patent Publication No. 57-5369.

Figure 2a is a diagram showing the equivalent circuit of the PEAC element, which is composed of a capacitance terminal T _C , a ground terminal T _G , and an injection terminal T _i , and the voltage between the injection terminal T _i and the ground terminal T _G is The dependent frequency stability undergoes a large change as shown in FIG. 2b. Line 1 in the same figure is
This is a graph of the frequency deviation based on V _ssl , and the smaller the slope of this straight line, the more stable the crystal oscillator is.

[The problem that the idea aims to solve]

As mentioned above, by applying the PEAC element, a small crystal oscillator with low power consumption was obtained. However, due to its small internal space capacity, heat dissipation is low, and when exposed to harsh high-temperature environments for long periods of time, V _ssl , which should normally be constant due to the voltage circuit, fluctuates. Therefore, as mentioned above, the PEAC element sensitively follows _Vssl and changes its frequency. Therefore, the present invention adds a fixed capacitor to the capacitive terminal T _C of the PEAC element to reduce the change in frequency over time.

[Means to solve the problem]

The present invention adds a fixed capacitor externally to the PEAC element to increase the total capacitance _CP , thereby reducing the influence of _Vssl and achieving frequency stabilization.

[Effect]

The straight line 1 shown in FIG. 2b is the characteristic of only the PEAC element, and the straight line 2 is the characteristic when a fixed capacitor is attached in parallel to the PEAC element.

In this case, the combined capacitance is C _P +C.

Here, C _P shown in FIG. 4 is the capacitance of the PEAC element, and C is an external fixed capacitor.

Combined capacitance C _O = C _P +C If the change in C _P is ΔC _P , when there is no capacitor C, ΔC _O /C _O = ΔC _P /C _PWhen capacitors C are connected in parallel, ΔC′ _O /C′ _O = With ΔC _P /C _P +C, ΔC′ _O /C′ _O <ΔC/C, and the change in frequency caused by the peac element is reduced.

〔Example〕

Figure 1 shows the crystal oscillation circuit of the present invention, where 1 and 2 are inverters, 3 is a level shifter, and 4 is an inverter.
5 is a buffer, 5 is a constant voltage circuit, 6 is a feedback resistor, and the above components are integrated and incorporated as an IC. 7 is a crystal oscillator, one end of which is 8
Connected to the capacitance terminal T _C of the PEAC element. this
A fixed capacitor 9 is connected in parallel to the PEAC element.
Approximately 1/2 of the voltage between V _DD and V _ssl is applied as a bias voltage V _B to the PEAC element and the fixed capacitor.

As an effective embodiment of the present invention, an example in which the present invention is applied to a temperature compensation circuit is shown in FIG. This operation is explained in FIG. 3, where 16 is a thermistor having a negative temperature coefficient, 15 is a resistor for relaxing the resistance-temperature characteristics of the thermistor, and 14 and 17 are the thermistor and the resistor 15, as well as the bias voltage. Voltage
It is a resistor for properly determining V _B , and R _X is a combined resistance value of the thermistor and the resistors 15 and 17. This causes, as the ambient temperature increases,
The resistance value of the thermistor 16 decreases, and the combined resistance value _R.sub.X decreases. At this time, the bias voltage V _B based on the power supply voltage V _DD increases as the ambient temperature increases, and as a result, the total capacitance C _P of the peac element increases.
decreases and the oscillation frequency increases. Therefore, the frequency-temperature characteristics of the crystal resonator are compensated.

In these operations, the bias voltage V _B of the PEAC element 8 is supplied as V _ssl from the constant voltage circuit 5 even if the power supply voltage V _DD changes. this
V _ssl does not change over a short period of time. However, as mentioned above, when operating for a long time, changes occur due to deterioration of the semiconductor elements, and as mentioned above, the capacitance of the PEAC element changes and the oscillation frequency changes. For this reason, a fixed capacitor 9 is connected in parallel to the PEAC element.
Reduce the contribution rate of changes in PEAC element capacitance. This improves the long-term frequency stability.

[Effect of idea]

As described above, in the crystal oscillator circuit that drives the electrode variable capacitance element PEAC according to the present invention with the output voltage V _ssl of the constant voltage circuit, it is necessary to add a small amount of elements in response to changes in the power supply voltage V _DD due to element deterioration. This makes it possible to easily stabilize the frequency over a long period of time, and has the effect of realizing a highly reliable, high-precision crystal oscillator.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a crystal oscillator according to the present invention, Figure 2a is an equivalent circuit diagram of a PEAC element, and Figure 2b is an equivalent circuit diagram of a PEAC element.
A capacitance/frequency characteristic diagram of the PEAC element, FIG. 3 is a temperature compensation circuit diagram of the present invention, and FIG. 4 is an equivalent diagram of the combined capacitance. 1, 2... Inverter, 5... Constant voltage circuit, 6
...Feedback resistor, 7...Crystal oscillator, 8...peac
Element, 9... fixed capacitance, 16... thermistor.

Claims (1)

[Scope of utility model registration request] This oscillation circuit uses a floating electrode variable capacitance element (PEAC element) having an injection terminal, a capacitance terminal, and a ground terminal, and connects a crystal resonator to the capacitance terminal in an alternating current manner. A crystal oscillator supplied with power, characterized in that a separate fixed capacitor is connected in parallel with the capacitor terminal.