Piezoelectric oscillator

Details Piezoelectric oscillator Piezoelectric oscillator

2000-10-05

2004-05-11

Kinkead, Arnold (Department: 2817)

Oscillators

Electromechanical resonator

Crystal

C331S1160FE, C331S1160FE, C331S17700V, C331S03600C

Reexamination Certificate

active

06734747

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric oscillator and, more particularly, to a piezoelectric oscillator that uses a MOS construction type capacitance element.
2. Description of the Related Art
Various forms of circuits have hitherto been proposed and put to practical use as an oscillator that uses a piezoelectric resonator that is represented by a crystal resonator. They are being used in a wide variety of electronic appliances, for example, as signal sources of portable telephones, computers, etc.
On the other hand, in each of such oscillators, for achieving various objects that include making adjustments of the frequency at the time of the manufacture, realizing the channel-frequency-adjusting function or AFC (automatic frequency control) function, etc. and that further include making compensations for the frequency in connection with the temperatures, conforming the oscillator to a large number of channel frequencies, etc., it is indispensable to use a variable-capacitance element.
As a circuit that can realize such objects, there is generally used a circuit that is illustrated in, for example, FIG.
6
.
The oscillation circuit illustrated in the figure is an ordinary circuit of the crystal oscillator using an inverter amplifier. In this oscillation circuit, between an input and an output of an inverter amplifier
101
there is inserted a parallel circuit comprised of a crystal resonator
102
and a feedback resistor R
1
. Also, between the input and the output thereof and the ground there are inserted a capacitor C
1
and a capacitor C
2
, respectively. Simultaneously, to either one of the capacitor C
1
and the capacitor C
2
(in this example to the capacitor C
1
) there is connected a variable-capacitance diode D
1
serving as a variable-capacitance element. And, a cathode of the variable-capacitance diode D
1
and a control terminal Vcont are connected to each other via a resistor R
2
for preventing the flow of direct currents.
Because the operation of this oscillation circuit is well known, it is thought unnecessary to newly give an explanation of it. Briefly explaining, however, in this oscillation circuit, according to a direct current voltage that is applied to the control terminal Vcont, the capacitance value of the variable-capacitance diode D
1
varies. For this reason, by controlling this control voltage, it is possible to perform various kinds of adjustments of the frequency, for example, AFC and so forth as mentioned above.
On the other hand, in view of the recent demands for miniaturization, and reduction in the power consumption, of various kinds of electronic appliances, there has been also a demand for conversion of the above-described oscillator to an IC version.
However, in a case where converting the oscillation circuit including the variable-capacitance diode D
1
, such as that illustrated in
FIG. 6
, to an IC version, this diode cannot but be formed using a process step different from that for forming other semiconductor circuits. Therefore, this diode has been an obstacle to cheaply converting the oscillation circuit to an IC version.
Namely, the variable-capacitance diode D
1
which is a bipolar type of semiconductor must be formed using a process step separate from that for forming the inverter amplifier
101
which is generally a C-MOS type of semiconductor. Therefore, the process for such formations is not only complex but the IC product was high in cost.
On the other hand, as a variable-capacitance element suited to conversion to IC version, there is known a MOS construction type capacitance element, the utilization of that has been in expectation.
As a crystal oscillator using such a MOS construction type capacitance element, there is the one that is disclosed, for example, in Japanese Patent Application Laid-Open No. 10-13155 entitled “Crystal Resonator with Frequency-Adjusting Function”.
This crystal oscillator is constructed as follows. As illustrated in
FIG. 7
, between an input terminal and an output terminal of an inverter amplifier
101
there is inserted a parallel circuit comprised of a crystal resonator
102
and a feedback resistor R
1
. Additionally, a capacitor C
2
is connected to the output terminal of the inverter amplifier
101
, and a MOS construction type capacitance element
103
is connected to the input terminal of the inverter amplifier. And, simultaneously, an electric-charge injection terminal TI of the MOS construction type capacitance element
103
and a control terminal Vcont are connected to each other.
As the MOS construction type capacitance element
103
, although it is only a mere one example, there is known the one illustrated in FIG.
8
. Namely, in this element
103
, a positive or negative voltage is applied to the control terminal Vcont by using the N type substrate as a basis to thereby cause the flow of a tunnel current through the interior of SiO
2
to thereby cause electrons to inject into or come out of a floating electrode
104
.
Namely, for example, in a case where having applied a positive voltage to the control terminal Vcont, electrons flow out of the floating electrode
104
. Therefore, the thickness of a depletion layer
105
situated near the floating electrode
104
becomes narrow, with the result that with a decrease in that thickness the depletion layer capacitance increases.
Also, in a case where having applied a negative voltage to the control terminal Vcont, the operation reverse to that mentioned above occurs. So, an explanation thereof is omitted.
However, as will be explained below, fundamentally, the MOS construction type capacitance element can have its capacitance value varied over a wide range only with use of a positive power supply or negative power supply. For this reason, there was the drawback that almost no change in the capacitance value occurred when merely using only either a positive, or a negative, single-polarity power supply alone.
This will hereafter be explained in a little more detail.
FIG. 9
is a graph illustrating an example of the relationship between an inter-electrode voltage and a capacitance value of the MOS construction type capacitance element.
As clear from this figure, in this example, when the terminal-to-terminal voltage varies within a range of from −1.5V to +0.5V including therein 0V therebetween, the capacitance value changes over a range covering approximately 80 pF.
However, on the other hand, in the crystal oscillator, whereas the range within that the frequency is variable need generally be wide, it is more preferable that the capacitance value be gently varied with respect to the change in the control voltage than that sharply in order to perform high-precision frequency control. For this reason, there has been a demand for a variable-capacitance element whose capacitance value varies over a wide range of control voltage.
Accordingly, in the case of the crystal oscillator such as that illustrated in
FIG. 7
, in order to obtain a wide range of variable capacitance with use of the MOS construction type capacitance element
103
, it is necessary to use control voltage sources for applying both positive and negative voltages to the control terminal Vcont. Therefore, there was the problem that the construction of the system making control of the frequency became complex.
SUMMARY OF THE INVENTION
The present invention has been made in order to solve the above-described problems and has an object to provide a small-sized piezoelectric oscillator which, while using a MOS construction type capacitance element suited to conversion to an IC version, enables obtaining a wide range of changes in the variable capacitance even with use of either a positive, or a negative, single-polarity power supply, and which facilitates the frequency control.
To attain the above object, according to the first aspect of the invention, there is provided a piezoelectric oscillator wherein, in an oscillator including a piezoelectric resonator, an amplifier, and a variable-capacitance element, the v

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