JPH0520407U - Temperature compensated crystal oscillator - Google Patents

Abstract

(57) [Summary] [Structure] First switch 27 connected to closed signal 25
A second switch 29 connected to the temperature signal 12, a switched capacitor circuit 11 connected to the first switch 27 and the second switch 29, and a first switch 2
7 and the second switch 29, a first inverter 31 and a second inverter 33, and a first inverter 3
The time measuring circuit 41 for controlling 1 is provided, and all or part of the switch 11a constituting the switched capacitor circuit 11 is closed by the closing signal 25 when the power is turned on or within a short time after the power is turned on, and the time is kept for a predetermined time. After that, the temperature signal 12 is used to control the switch of the switched capacitor circuit 11.
Switch to. [Effect] By matching the potential of the lower electrode of the capacitor that constitutes the switched capacitor circuit with the potential of the semiconductor substrate, the effective capacitance value can be stabilized within a short time after power-on, and the oscillation frequency is stabilized. be able to.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a temperature-compensated crystal oscillator configuration using a switched capacitor circuit.

[0002]

[Prior Art]

 In recent years, the development of the field called mobile communications such as car phones and mobile phones has been remarkable. Along with this, demands for downsizing and weight reduction of these communication devices have become stronger and stronger. In order to reduce the size and weight of mobile communication devices, it is effective to reduce the number of parts by integrating individual parts such as resistors and transistors into integrated circuits as much as possible.

There is an increasing demand for miniaturization of the temperature-compensated crystal oscillator, which is a main component of mobile communication devices, and it is possible to use a switched capacitor circuit as one means for promoting the integration of the temperature-compensated crystal oscillator. It is reported in the following documents.

IEICE Technical Report US82-6 p. 41-48 "One configuration method of digital control type TCXO" Takehiko Uno 1982

The structure of a conventional temperature-compensated crystal oscillator using a switched capacitor circuit will be described below with reference to the drawings.

FIG. 2 is a circuit diagram showing a structure of a conventional temperature-compensated crystal oscillator using a switched capacitor circuit, and FIG. 3 is a sectional view showing a region of a 1-bit portion in the switched capacitor circuit. However, in FIG. 2, the temperature signal 12 is represented as a circuit block.

As shown in FIG. 2, the crystal unit 1, the oscillation inverter 3, and the feedback resistor 5 are connected in parallel. Further, a capacitor 7, a capacitor 9, and a switched capacitor circuit 11 having a plurality of columns of a capacitor 11b and a switch 11a are connected between these connection points and a power source, respectively.

The opening / closing operation of the plurality of switches 11 a forming the switched capacitor circuit 11 is controlled by the temperature signal 12. By the opening / closing operation of the plurality of switches 11a, the capacitance value of the entire switched capacitor circuit 11 is changed to perform temperature compensation of the oscillation frequency of the crystal oscillator.

Next, the structure of the switched capacitor circuit 11 will be described with reference to FIG. 3 showing a cross section of a region of a 1-bit portion in the switched capacitor circuit 11 shown in FIG.

The capacitor 11 b forming the switched capacitor circuit 11 has a first diffusion region 13 having a conductivity type different from that of the semiconductor substrate 24, and an insulating film provided in contact with the first diffusion region 13. This is a so-called MIS type capacitor consisting of 15 and a conductor 17 provided in contact with the insulating film 15.

The switch 11 a forming the switched capacitor circuit 11 is formed of a MOS transistor in which a gate insulating film made of the insulating film 15 and a gate made of the conductor 17 are stacked.

The switch 11 a formed of a MOS transistor and the first diffusion region 13 forming a lower electrode of the capacitor 11 b are connected by a metal wiring 20.

Further, an oxide film 16 is provided to isolate the capacitor 11b and the switch 11a from each other. The oxide film 16 is formed by a selective oxidation method in which an oxidation resistance film such as a silicon nitride film is used as a mask to perform an oxidation process.

Further, a source region of a MOS transistor forming the switch 11a and a second diffusion region 21 having a conductivity type opposite to that of the semiconductor substrate 24 which is a drain region are provided.

Furthermore, a third diffusion region 23 is provided so as to contact the second diffusion region 21. The third diffusion region 23 has the same conductivity type as the semiconductor substrate 24 and is connected to the metal wiring 20.

Further, an intermediate insulating film 18 is provided to insulate the metal wiring 20 from the conductor 17 which is the upper electrode of the capacitor 11b and the gate electrode of the switch 11a composed of a MOS transistor.

The capacitance value of the capacitor 11b, which is a MIS type capacitor, has voltage dependence, and it is common to connect the capacitor 11b so that the capacitance value is maximized from the viewpoint of increasing area efficiency.

As shown in FIG. 3, of the two electrodes of the capacitor 11b, the lower electrode connected to the switch 11a is the electrode on the side of the first diffusion region 13 because the capacitance value of the capacitor 11b is This is due to the reason for increasing.

As shown in FIG. 3, the capacitor 11b and the switch 11a forming the switched capacitor circuit have similar cross-sectional structures. Therefore, the switch 11a and the capacitor 11b can be manufactured in almost the same manufacturing process. From this, it can be said that the switched capacitor circuit is an effective means for integrating the temperature-compensated crystal oscillator into an integrated circuit.

[0020]

[Problems to be solved by the device]

 However, in the conventional temperature-compensated crystal oscillator using the switched capacitor circuit, there is a phenomenon that the oscillation frequency gradually decreases after the power is turned on. As a result, there is a problem that it takes several tens of seconds at room temperature and several minutes at low temperature for the oscillation frequency to stabilize.

In a crystal oscillator that does not use a switched capacitor circuit, such a phenomenon that it takes a long time for the oscillation frequency to stabilize does not occur. Therefore, the phenomenon that it takes a long time to stabilize the oscillation frequency means that it takes time to stabilize the effective capacitance value of the switched capacitor circuit.

An object of the present invention is to solve the above problems and to stabilize the oscillation frequency of a temperature-compensated crystal oscillator integrated into a circuit using a switched capacitor circuit within a short time after power-on. Another object of the present invention is to provide a temperature-compensated crystal oscillator.

[0023]

[Means for Solving the Problems]

 The structure of the temperature-compensated crystal oscillator according to the present invention for achieving the above object is as described below.

A first switch connected to the closed signal, a second switch connected to the temperature signal, a switched capacitor circuit connected to the first switch and the second switch, a first switch and a first switch A second inverter for controlling the second switch and a second inverter, and a time measuring circuit for controlling the first inverter are provided, and the switched capacitor circuit of the switched capacitor circuit is turned on when the power is turned on or within a short time after the power is turned on. It is characterized in that all or most of the switches are closed by a closing signal, and the control of the switches in the switched capacitor circuit is switched to a temperature signal after a certain period of time.

The switched capacitor circuit used in the configuration of the temperature-compensated crystal oscillator of the present invention comprises a switch connected to a power source having the same potential as the semiconductor substrate, and a conductive-type diffusion region connected to the switch and different from the semiconductor substrate. The capacitor is composed of an insulating film in contact with the diffusion region, a conductor in contact with the insulating film, and a metal wire connecting the capacitor and the switch.

[0026]

According to the temperature-compensated crystal oscillator of the present invention, all or most of the switches of the switched capacitor circuit are closed when the power is turned on or immediately after the power is turned on. This allows the potential of the first diffusion region, which is the lower electrode of the capacitor of the switched capacitor circuit, to quickly match the potential of the semiconductor substrate, stabilizing the effective capacitance value of the switched capacitor circuit in a short time. is there.

That is, when the power source of the crystal oscillator is turned on, the potential of the first diffusion region, which is the lower electrode of the capacitor forming the switched capacitor circuit, is about the middle of the power source voltage due to the capacitive coupling effect or the like. Therefore, the pn junction capacitance between the first diffusion region and the semiconductor substrate is smaller than the maximum value.

However, the pn junction has a leak current determined by the impurity concentration and the temperature. Therefore, in the bit in which the switch of the switched capacitor circuit is in the open state, the potential of this first diffusion region gradually approaches the potential of the semiconductor substrate with the passage of time, and with this, the potential of this first diffusion region The pn junction capacitance with the semiconductor substrate gradually increases and approaches the maximum value.

When the potential of the first diffusion region matches the potential of the semiconductor substrate, the pn junction capacitance between the first diffusion region and the semiconductor substrate reaches a maximum value and stabilizes.

The pn junction capacitance is a kind of stray capacitance in the switched capacitor circuit and affects the effective capacitance value of the switched capacitor circuit.

Therefore, immediately after the crystal oscillator is powered on, the potential of the first diffusion region, which is the lower electrode of the capacitor forming the switched capacitor circuit, is made equal to the potential of the semiconductor substrate to stabilize the pn junction capacitance. If this is done, the effective capacitance of the switched capacitor circuit will be stabilized, and the oscillation frequency will also be stabilized.

After that, if the control of the switches forming the switched capacitor circuit is switched to the temperature signal, the temperature compensation operation of the oscillation frequency is performed as in the conventional case.

[0033]

【Example】

 An embodiment of a temperature-compensated crystal oscillator according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment in which the present invention is realized by a time measuring circuit using time constants of resistors and capacitors. The same elements as those of FIG. 2 showing a conventional example are designated by the same reference numerals and the description thereof will be omitted. To do. However, in FIG. 1, the closed signal 25 and the temperature signal 12 are shown as a circuit block.

The configuration of the switched capacitor circuit is the same as FIG. 3 which is a sectional view showing a 1-bit portion of the switched capacitor circuit, and detailed description thereof will be omitted.

As shown in FIG. 1, a first switch 27 and a second switch 29, which are connected to the switch 11 a constituting the switched capacitor circuit 11, are provided. Furthermore, the first switch 27 is connected to the closed signal 25 and the second switch 29 is connected to the temperature signal 12 respectively.

The closed signal 25 and the temperature signal 12 of the switch 11 a forming the switched capacitor circuit 11 control the switch 11 a of the switched capacitor circuit 11 via the first switch 27 and the second switch 29, respectively. To do.

A first inverter 31 and a second inverter 33 are connected to the first switch 27 and the second switch 29. The control of the first switch 27 and the second switch 29 is performed by the first inverter 31 and the second inverter 33.

Regarding the first inverter 31 and the second inverter 33, when one of the first switch 27 and the second switch 29 is in the closed state, the other switch is in the open state. Connected to.

The operation of the second inverter 33 is controlled by the first inverter 31, and the operation of the first inverter 31 is controlled by the time measuring circuit 41 composed of the resistor 35 and the capacitor 37. ..

Hereinafter, the operation of the temperature-compensated crystal oscillator of the present invention shown in FIG. 1 will be described by referring to the potential of the higher power supply as level high and the potential of the lower power supply as level low.

The potential at the connection point 39 between the resistor 35 and the capacitor 37 becomes high when the crystal oscillator is powered on because the impedance of the capacitor 37 is lower than the impedance of the resistor 35.

After that, however, the potential of the connection point 39 between the resistor 35 and the capacitor 37 gradually shifts to the level low with the time constant of the product of the sizes of the resistor 35 and the capacitor 37.

Since the connection point 39 between the resistor 35 and the capacitor 37 is the input of the first inverter 31, the output of the first inverter 31 is level low and constant when the crystal oscillator is powered on. After time, move to level high.

On the other hand, since the second inverter 33 operates in the opposite manner to the first inverter 31, the output of the second inverter 33 is at level high when the power is turned on and shifts to level low after a certain period of time.

The first switch 27 and the second switch 29 are connected to the first inverter 31 and the second inverter 33 as follows.

When the output of the first inverter 31 is at level low, the first switch 27 is closed, and when the output of the first inverter 31 is at level high, the second switch 27 is open.

Further, the second switch 29 is closed so that the output of the second inverter 33 is low level, and the second switch 29 is open when the output of the second inverter 33 is high level. deep.

Then, as described above, when the power of the crystal oscillator is turned on, the output of the first inverter 31 is level low, and the output of the second inverter 33 is level high. Therefore, when the power of the crystal oscillator is turned on, The first switch 27 is closed and the second switch 29 is open.

When the outputs of the first inverter 31 and the second inverter 33 are inverted after a lapse of a certain time after the crystal oscillator is powered on, the first switch 27 is opened and the The second switch 29 is closed.

As described above, the first switch 27 is connected to the closed signal 25 of the switch 11 a of the switched capacitor circuit 11, and the second switch 29 is connected to the temperature signal 12.

Therefore, the switched capacitor circuit 11 is controlled by the closed signal 25 until a certain time after the crystal oscillator is powered on, and is controlled by the temperature signal 12 after a certain time.

As described above, when the switch 11a constituting the switched capacitor circuit 11 is once closed, the first diffusion region shown in FIG. 3, which is the lower electrode of the capacitor 11b of the switched capacitor circuit 11, is formed. The potential of 13 matches the potential of the semiconductor substrate 24. As a result, the pn junction capacitance between the first diffusion region 13 and the semiconductor substrate 24 becomes stable, and the effective capacitance of the switched capacitor circuit 11 also becomes stable.

Therefore, according to the temperature-compensated crystal oscillator of the present invention shown in FIG. 1, in the temperature-compensated crystal oscillator using the switched capacitor circuit, the problem that the oscillation frequency is gradually lowered after the power is turned on is solved. is there.

In the embodiment described with reference to FIG. 1, all of the switches 11a forming the switched capacitor circuit 11 are closed for a certain period of time after the power is turned on, but the oscillation frequency of the crystal oscillator is affected. If it is within the range that does not exist, the object of the present invention can be achieved even if the switch 11a that does not close for several bits remains.

That is, of the switches 11a forming the switched capacitor circuit 11, a part thereof may be closed once when the crystal oscillator is powered on.

Further, in the embodiment described with reference to FIG. 1, the time when the switch 11a constituting the switched capacitor circuit 11 is closed is the power-on time. Is enough for several msec.

Furthermore, it is practically sufficient if the temperature-compensated frequency is output within about 1 second after the crystal oscillator is powered on. Therefore, the switch 11a of the switched capacitor circuit 11 is closed. The time may be delayed for a short time after the power is turned on, for example, about several 100 msec.

Further, in the embodiment of FIG. 1, the time for which the switch 11a forming the switched capacitor circuit 11 is kept closed is calculated by multiplying the size of the resistor 35 forming the time measuring circuit 41 by the size of the capacitor 37. Although the time constant is so-called CR, any means can be used as long as a desired time can be obtained.

For example, if the output of the crystal oscillator is guided to the frequency dividing circuit and time is measured and the output of this frequency dividing circuit is input to the latch circuit, only when the frequency dividing circuit first counts up after the power is turned on. It is possible to create a signal whose electric potential changes.

[0061]

As described above, the switches constituting the switched capacitor circuit are closed within a short time after the power is turned on, and after a certain period of time, the control of each switch of the switched capacitor circuit is switched to the temperature signal. By doing so, the problem of the oscillating frequency gradually decreasing after the power is turned on can be solved in the water crystal oscillator using the switched capacitor circuit. As a result, in a temperature-compensated crystal oscillator using a switched capacitor circuit, it is possible to stabilize the oscillating frequency within a short time after the power is turned on, and the effect is very large.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a temperature-compensated crystal oscillator according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a temperature-compensated crystal oscillator in a conventional example.

FIG. 3 is a sectional view showing a region of a 1-bit portion of a switched capacitor circuit applied to the present invention and a conventional example.

[Explanation of symbols]

 1 Crystal Resonator 3 Oscillation Inverter 11 Switched Capacitor Circuit 11a Switch 11b Capacitor 12 Temperature Signal 13 First Diffusion Region 15 Insulating Film 16 Oxide Film 17 Conductor 20 Metal Wiring 24 Semiconductor Substrate 25 Closed Signal 27 First Switch 29 Second Switch 31 First inverter 33 Second inverter 41 Time measurement circuit

Claims (1)

[Scope of utility model registration request] 1. A first switch connected to a closed signal, a second switch connected to a temperature signal, a switched capacitor circuit connected to the first switch and a second switch, and the first switch. A first inverter for controlling the switch and the second switch; a second inverter; and a time measuring circuit for controlling the first inverter, wherein the switched capacitor circuit is a power supply having the same potential as the semiconductor substrate. A switch connected to the switch, a capacitor connected to the switch, the diffusion region having a conductivity type different from that of the semiconductor substrate, an insulating film in contact with the diffusion region, and a conductor in contact with the insulating film; and the switch and the capacitor. And a metal wiring for connecting the switch, and all or a part of the switches that form the switched capacitor circuit at the time of power-on or within a short time after power-on. A temperature-compensated crystal oscillator, which is in a closed state and switches a control of a switch constituting the switched capacitor circuit to a temperature signal after a predetermined time.