JPH0832349A - Quartz oscillation circuit - Google Patents

Abstract

PURPOSE:To lower a voltage and to miniaturize a frequency variable type quartz oscillation circuit. CONSTITUTION:The serial connection of a first fixed capacitor 13 and a (p) channel MOS resistor 15 is provided between the output terminal of an oscillation inverter 3 and a power source, a second fixed capacitor 17 is provided between the input terminal of the oscillation inverter 3 and the power source and the gate of the (p) channel MOS resistor 15 is connected to an external input terminal 19. Thus, the adjustment width of a frequency is widened even with a low voltage below 3V and this quartz oscillation circuit is constituted of only elements capable of being easily incorporated in a semiconductor integrated circuit for all the parts except a quartz oscillator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a crystal oscillation circuit having a load capacitance adjusting means.

[0002]

2. Description of the Related Art A crystal oscillation circuit having means for positively adjusting the oscillation frequency by an external voltage is a VCXO (Volt
age Controlled Crystal Osc
It is often used for FM modulation of transmitters, temperature-compensated crystal oscillators, and the like.

In such a VCXO, what is commonly used as means for adjusting the oscillation frequency of the crystal oscillation circuit is to change the load capacitance by a variable capacitance diode.

FIG. 4 is a circuit diagram showing an example of the configuration of a conventional VCXO.

As shown in FIG. 4, a crystal oscillator 1, an oscillation inverter 3 and a feedback resistor 5 are connected in parallel, and a DC cut capacitor 7 and a variable capacitance diode are provided between an input terminal of the oscillation inverter 3 and a power source. 9 is connected in series, and the fixed capacitor 23 is connected between the output terminal of the oscillation inverter 3 and the power supply.

The variable capacitance diode 9 has an input resistance 1
The capacitance value of the variable capacitance diode 9 is changed by a signal coming from the outside of the crystal oscillation circuit.

The capacitance value of the variable capacitance diode 9 becomes maximum when the voltage between both terminals thereof is 0 V, and becomes smaller as the voltage increases in the opposite direction. The degree of change in the capacitance value with respect to the change in voltage varies depending on the type of the variable capacitance diode 9, but regardless of the type, a considerably high voltage is required to bring the capacitance value to around 0 pF.

It is needless to say that this variable capacitance diode 9 is difficult to be built in a semiconductor integrated circuit and is an external component, but the DC cut capacitor 7 is usually too large in capacitance to be built in a semiconductor integrated circuit. Is an external component.

[0009]

In the crystal oscillating circuit having such a conventional structure as described above, due to the nature of the variable capacitance diode, the variation range of the capacitance value cannot be increased when the power supply voltage is 3 V or less, and moreover, Since the capacitance value changes in the vicinity of the maximum value, the rate of change in the capacitance becomes very small.

For this reason, the VCXO mounted on a portable electronic device which requires a low power supply voltage has a problem that the frequency change width cannot be increased.

Further, since the variable capacitance diode is large in size, there is a problem that the miniaturization of the whole oscillator is limited.

An object of the present invention is to provide a crystal oscillating circuit which has a large rate of change in capacitance at a low voltage of 3 V or less and which is composed of only components that can be incorporated in a semiconductor integrated circuit other than a crystal oscillator. is there.

[0013]

In order to achieve the above object, the structure of the crystal oscillation circuit according to the present invention is as described below.

That is, a series connection of a fixed capacitor and a variable resistor is provided between at least one terminal of the crystal unit and a power source on either the high potential side or the low potential side, and the variable capacitance is further provided. The resistor is a MOS resistor.

Furthermore, a series connection of a fixed capacitor and a variable resistor is provided between the two terminals of the crystal unit and the power source on either the high potential side or the low potential side, respectively. The variable resistor is a MOS resistor.

Furthermore, a fixed capacitor and a variable resistor are respectively connected in series between the two terminals of the crystal unit and the power source on either the high potential side or the low potential side, and the resistance of the variable resistor is provided. It is characterized in that the signals for changing the values are the same, and further, those variable resistors are MOS resistors, and their gates are both connected to an external input terminal.

[0017]

In the prior art, when changing the load capacitance of the crystal oscillation circuit, a variable capacitance diode is used to directly change the connected capacitance value itself.

On the other hand, in the present invention, the capacitance is a fixed capacitance, and the variable resistor is connected in series to this fixed capacitance.
And by changing the resistance value of this variable resistor,
The load capacity is variable without changing the capacity value itself.

In the present invention, the reason why the load capacitance of the crystal oscillation circuit can be varied by connecting the fixed capacitance and the variable resistor in series is as follows.

That is, when a capacitor having a certain capacitance value is connected between the crystal unit and the power source without a resistor, there is nothing that hinders the charging and discharging of the electric charge of this capacity. The charge and discharge of the electric charge are completely completed, and the connected capacitance functions as its capacitance value.

If a resistor is connected in series to this capacitance, this resistor interferes with the charging and discharging of electric charges, and discharge begins before the end of charging, and before the end of discharging. Charging will start.

This state means that the amount of charge charged / discharged within the oscillation period of the crystal unit decreases, and it is electrically equivalent to a capacitor having a small capacitance value connected without a resistor. Since the charge / discharge speed of electric charge is determined by the product of the resistance value and the capacitance value of the connected resistor, the connected resistor is made a variable resistor and the resistance value is adjusted even if the capacitance value is fixed. This makes it possible to change the amount of charge to be charged and discharged.

In other words, connecting the fixed capacitor and the variable resistor in series connection to the crystal unit is electrically equivalent to connecting the variable capacitor to the crystal unit. This is the reason why the load capacitance of the crystal oscillation circuit can be changed by the present invention.

In the present invention, the type of variable resistor is not particularly limited, but from the viewpoint that it can be incorporated in a semiconductor integrated circuit and that the resistance value can be greatly changed at a low voltage of 3 V or less, a MOS is used. Resistance is advantageous.

[0025]

Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the structure of a crystal oscillation circuit according to the present invention.

The crystal oscillation circuit shown in FIG.
, The oscillation inverter 3 and the feedback resistor 5 are connected in parallel, and between the connection point on the output terminal side of the oscillation inverter 3 and the power supply,
The first fixed capacitance 13 and the p-channel MOS resistor 15 are connected in series, and the second fixed capacitance 17 is connected between the connection point on the input terminal side of the oscillation inverter 3 and the power supply.

The gate of the p-channel MOS resistor 15 is connected to the external input terminal 19, and the resistance value of the p-channel MOS resistor 15 is controlled by the external voltage.

The capacitance value of the first fixed capacitor 13 is selected so as to obtain a desired adjustment range of the oscillation frequency, but since it should not be so large that oscillation should be stopped, it is usually 6
Set to 0 to 80 pF or less. Also, the second fixed capacitance 1
The capacitance value of No. 7 is about 15 to 30 pF. Capacitances of these sizes differ depending on the manufacturing process, but are about 100 μm □ on a semiconductor integrated circuit. ~ 300μm □ Therefore, it can be easily incorporated in a semiconductor integrated circuit.

The resistance value of the p-channel MOS resistor 15 is naturally proportional to the channel length and inversely proportional to the channel width, but the dependence on the applied gate voltage is most dominant, and 1 GΩ depends on the gate voltage. It is possible to change from the above large value to a small value of 100Ω or less.

The resistance value of the p-channel MOS resistor 15 is 1G.
When it is Ω or more, the time constant of charging / discharging the first fixed capacitance 13 of about 60 to 80 pF is 60 to 80 ms.
Unless it is an oscillator of ec or more and 10 Hz or less,
Since almost no charging / discharging occurs within the oscillation cycle, this is almost equal to the state in which no capacitance is connected to the output terminal side of the oscillation inverter 3.

On the other hand, when the resistance value of the p-channel MOS resistor 15 is 100Ω or less, the first value of about 60 to 80 pF.
The charge / discharge time constant of the fixed capacitor 13 is 6 to 8
It becomes nsec or less, and charge / discharge of charges is almost completed within the oscillation period unless the oscillator is 100 MHz or more. In other words, this is almost equal to the state in which the first fixed capacitance 13 is directly connected to the power supply without passing through the p-channel MOS resistor 15.

Therefore, by varying the resistance value of the p-channel MOS resistor 15 by the gate voltage, the capacitance value connected to the output terminal side of the oscillation inverter 3 is substantially the capacitance value of the first fixed capacitance 13. Can be changed.

Since the minimum capacitance value that can be changed is approximately 0 pF, the capacitance change rate becomes extremely large, and the adjustment range of the oscillation frequency also becomes very large.

The p-channel MOS resistor 15 can be easily incorporated in a semiconductor integrated circuit, and when it is incorporated, it is usually formed at the same time as other MOS transistors in the semiconductor integrated circuit. Therefore, the absolute value of the threshold voltage is 1V. Is
Usually, it is about 0.5 to 0.7V.

Therefore, although the resistance value of the p-channel MOS resistor 15 depends on its shape, it generally becomes 1 GΩ or more when the absolute value of the gate voltage is 0.3 V or less, and the absolute value of the gate voltage is 2.5 V or more. Then 10
It becomes less than 0Ω.

That is, the resistance value of the p-channel MOS resistor 15 can be sufficiently controlled even when the power supply voltage is 3 V or less. Therefore, even if the power supply voltage is low, it is connected to the output terminal side of the oscillation inverter 3. The capacitance value can be changed substantially by the capacitance value of the first fixed capacitance 13.

In the crystal oscillation circuit shown in FIG. 1, the first fixed resistor 13 and the p-channel MOS resistor 15 are connected in series to the output terminal side of the oscillation inverter 3, but the input terminal of the oscillation inverter 3 is not changed. You may connect to the side.

However, when the series connection of the first fixed resistor 13 and the p-channel MOS resistor 15 is connected to the input terminal side of the oscillation inverter 3, compared to the case where it is connected to the output terminal side of the oscillation inverter 3. Since the change in the oscillation frequency becomes about half, if it is connected to either one, it is more advantageous to connect to the output terminal side of the oscillation inverter 3 as shown in FIG.

FIG. 2 is a graph showing the relationship between the absolute value of the gate voltage of the p-channel MOS resistor 15 of the crystal oscillation circuit shown in FIG. 1 and the rate of change of the oscillation frequency. The capacitance values of the first fixed capacitance 13 and the second fixed capacitance 17 are set to 30 p, respectively.
F and 15 pF, the channel length and the channel width of the p-channel MOS resistor 15 are both 30 μm, the absolute value of the threshold voltage of the p-channel MOS resistor 15 is 0.7 V, and the fundamental frequency of the crystal unit 1 is 12.8M
This is the case of Hz.

As shown in FIG. 2, when the absolute value of the gate voltage exceeds 0.7 V of the threshold voltage, the oscillation frequency changes rapidly, and when the absolute value of the gate voltage changes from around 2 V, the oscillation frequency changes. Tends to be slightly saturated.

As shown in FIG. 2, it is apparent that the present invention can provide a crystal oscillation circuit having a large frequency change rate even when the power supply voltage is 3 V or less.

In the example shown in FIG. 2, p-channel MO
The threshold voltage of the S resistor is 0.7 V, but the power supply voltage can be further lowered by lowering the threshold voltage.

For example, if the threshold voltage is 0.2V, the curve shown in FIG. 2 shifts to the low voltage side by about 0.5V, so that the power supply voltage can be set to about 2V.

Next, another embodiment of the present invention will be described in detail with reference to the drawings. FIG. 3 is a circuit diagram showing the configuration of a crystal oscillation circuit according to another embodiment of the present invention.

In another embodiment of the crystal oscillation circuit shown in FIG. 3, the crystal oscillator 1, the oscillation inverter 3 and the feedback resistor 5 are connected in parallel, and the connection point on the output terminal side of the oscillation inverter 3 and the power supply are connected. Between the first fixed capacitance 13 and the first p
A series connection with the channel MOS resistor 15 is connected, and a second fixed capacitor 17 and a second p-channel MOS resistor 21 are connected in series between the connection point on the input terminal side of the oscillation inverter 3 and the power supply. There is.

The gates of the first p-channel MOS resistor 15 and the second p-channel MOS resistor 21 are both connected to the external input terminal 19, and the resistance values of both p-channel MOS resistors are controlled by an external voltage. ing.

The crystal oscillating circuit shown in FIG. 3 is provided with a frequency adjusting mechanism on both sides of the oscillating inverter 3, so that the frequency adjusting range can be increased by about 1.5 times compared with the example shown in FIG. Therefore, it is suitable for a VCXO that requires a very large frequency change and a VCXO that is used at an extremely low voltage.

In the crystal oscillation circuit shown in FIG. 3, the gates of the first p-channel MOS resistor 15 and the second p-channel MOS resistor 21 are driven by the same external voltage, but they are driven by different signals. It goes without saying that it is okay.

For example, the first p-channel MOS resistor 1
It is also possible to use 5 as a VCXO and use the second p-channel MOS resistor 21 as a trimmer for canceling the change with time of the crystal unit 1.

The present invention has been specifically described based on the embodiments as described above, but the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Needless to say.

For example, as shown in FIG. 1 or FIG. 3, a MOS resistor is generally used as a variable resistor by using a gate as a resistance adjusting terminal. The resistance value may be changed by utilizing the back gate effect due to the change.

Although the fixed capacitor is connected to the oscillation inverter 3 side and the p-channel MOS resistor is connected to the power source side in FIG. 1 or 3, the connection order may be reversed.

However, a p-channel M is formed on the n-type semiconductor substrate.
The connection order shown in FIG. 1 or 3 is advantageous in that the influence of the back gate effect can be eliminated only when forming the OS resistance.

Further, in FIG. 1 or 3, the p channel M
Although the OS resistance is used, it may be an n-channel MOS resistance.

In that case, however, it is advantageous to set the power source of the connection destination to the low potential side in order to eliminate the influence of the back gate effect.

Further, it is not always necessary to use the MOS resistance as the variable resistance, and for example, a bipolar transistor or a field effect transistor may be used.

[0057]

As described above, by connecting the variable resistance whose resistance value is changed by the external voltage and the fixed capacity in series between the crystal oscillation circuit and the power supply, the operation is performed at a low voltage, and It is possible to provide a frequency tunable crystal oscillation circuit that can achieve miniaturization of the oscillator, and the effect is very large. VCXO mounted on portable electronic devices
If applied to, the effect is enormous.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a crystal oscillation circuit according to an embodiment of the present invention.

FIG. 2 is a graph showing characteristics of a crystal oscillation circuit according to an example of the present invention.

FIG. 3 is a circuit diagram showing a configuration of a crystal oscillation circuit according to another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of a crystal oscillation circuit in a conventional example.

[Explanation of symbols]

 1 Crystal Resonator 3 Oscillation Inverter 5 Feedback Resistor 13 First Fixed Capacitance 15 p-Channel MOS Resistor 17 Second Fixed Capacitance 19 External Input Terminal

Claims (6)

[Claims] 1. A crystal oscillating circuit comprising a fixed capacitor and a variable resistor connected in series between at least one terminal of the crystal unit and a power source on either the high potential side or the low potential side. . 2. A crystal oscillation circuit comprising a fixed capacitor and a MOS resistor connected in series between at least one terminal of the crystal unit and a power source on either the high potential side or the low potential side. . 3. A crystal oscillating circuit comprising a fixed capacitor and a variable resistor connected in series between two terminals of the crystal unit and a power source on either the high potential side or the low potential side. 4. A fixed capacitor and a MOS are provided between two terminals of the crystal unit and a power source on either the high potential side or the low potential side.
A crystal oscillating circuit, characterized in that each is provided with a series connection with a resistor. 5. A fixed capacitor and a variable resistor are connected in series between the two terminals of the crystal unit and a power source on either the high potential side or the low potential side, and the resistance values of both variable resistors are set. A crystal oscillation circuit characterized in that the variable signals are the same. 6. A fixed capacitor and a MOS are provided between the two terminals of the crystal unit and a power source on either the high potential side or the low potential side.
A crystal oscillating circuit, characterized in that each of them has a series connection with a resistor and the gates of both MOS resistors are connected to an external input terminal.