JP2008187427A - Piezoelectric oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric oscillator whose frequency can be adjusted with high precision. <P>SOLUTION: The piezoelectric oscillator having a piezoelectric vibrator and an oscillation circuit includes a first capacity array including a fixed capacity element and a plurality of capacity elements, a first on/off means of individually making and breaking series connections between the plurality of capacity elements and the fixed capacity element, and a control means of controlling making and breaking of the first on/off means, where a series circuit of the fixed capacity element and first capacity array is connected to the piezoelectric vibrator or oscillation circuit. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a piezoelectric oscillator having a highly accurate frequency adjustment function.

Since a piezoelectric oscillator using an inverter (inverter-type piezoelectric oscillator) can easily form an oscillation circuit, it is widely used as a frequency and time reference source using a crystal resonator as an oscillator.
FIG. 1 of Japanese Patent Application Laid-Open No. Sho 62-200804 discloses an inverter type piezoelectric oscillator having a frequency adjusting function. In this piezoelectric oscillator, seven capacitive elements (capacitor arrays) are arranged at the input end of the inverter. Each capacitive element is weighted with a capacitance value of 0.5 pF, 1 pF, 2 pF, 4 pF, 8 pF, 16 pF, 32 pF, and the connection of each capacitive element to the inverter input terminal is individually connected to the analog switch. Control.
As a result, it is possible to adjust the oscillation frequency by switching the combined capacitance value of the capacitance array in the range of 0 pF to 63.5 with a minimum resolution of 0.5 pF and changing the load capacitance value of the piezoelectric oscillator.
In recent years, it has become possible to obtain an IC in which these capacitor arrays are integrated together with electronic components such as inverters. By incorporating the capacitance array in the IC, it is possible to reduce the size by eliminating the need for external components for frequency adjustment.
JP-A-62-200804

  However, the conventional piezoelectric oscillator disclosed in Patent Document 1 has the following problems. That is, for those that require high frequency accuracy, such as a temperature compensated piezoelectric oscillator, the minimum resolution of the capacitive array needs to be further refined. However, in order to reduce the capacitance value of the capacitive element that determines the minimum resolution, it is necessary to make the electrode area extremely small. However, a small-capacitance capacitor tends to vary in capacitance value. Therefore, it is difficult to make a small-capacitance capacitor in an IC with high accuracy, and there is a problem that high-accuracy frequency adjustment cannot be performed for those requiring high frequency accuracy.

  In view of such problems, an object of the present invention is to provide a piezoelectric oscillator capable of highly accurate frequency adjustment.

In order to solve such a problem, the present invention provides a piezoelectric oscillator including a piezoelectric vibrator and an oscillation circuit, a fixed capacitor element, a first capacitor array including a plurality of capacitor elements, and each of the plurality of capacitor elements. A first connecting / disconnecting unit for connecting / disconnecting the serial connection with the fixed capacitive element individually; and a control unit for controlling the connecting / disconnecting of the first connecting / disconnecting unit, the fixed capacitive element and the first capacitor. A series circuit with an array is connected to the piezoelectric vibrator or the oscillation circuit.
As a result, the load capacity of the piezoelectric oscillator can be changed with a desired resolution, and high-accuracy frequency adjustment is possible.

  In addition, the present invention according to claim 1, further comprising a second capacitor array including a plurality of capacitor elements, wherein any one of the plurality of capacitor elements constituting the second capacitor array, and the first capacitor. And a second connecting / disconnecting unit that individually connects and disconnects the parallel connection with any one of the plurality of capacitive elements constituting the array. The control unit connects and disconnects the second connecting / disconnecting unit. Controlled. Thus, by correcting the change in the load capacitance by the second capacitance array, it is possible to correct the frequency adjustment step so as to be substantially uniform in the adjustment of the oscillation frequency.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the constituent elements, types, combinations, and the like described in this embodiment are merely illustrative examples rather than the main purpose of limiting the scope of the present invention only to those unless otherwise specified.
FIG. 1 is a circuit diagram of a piezoelectric oscillator according to a first embodiment of the present invention.
The voltage controlled piezoelectric oscillator includes an oscillation circuit 2 having an inverter 1, a feedback resistor R1, and capacitors C3 and C4, a piezoelectric vibrator X1 connected to the oscillation circuit 2, and a capacitor C1 connected to the oscillation circuit 2. (First fixed capacitance element), a capacitor C2 connected in series to the capacitor C1, and a switch 3 (Swa1, Swa2,... Swan-1, Swan) having one end connected to a connection point of the capacitors C1 and C2. : First connecting / disconnecting means), capacitance array 4 (Ca1, Ca2,..., Can-1, first capacitance array) connected to the switch 3, and opening / closing of the switch 3 (capacitor 1). And a control unit 5 for controlling the connection / disconnection of the device.

The piezoelectric oscillator shown in FIG. 1 operates as follows.
First, an n-bit capacitance array setting signal is input to the control unit 5 from the outside.
Then, the control unit 5 sends an open / close control signal to the switch 3 with reference to the setting information stored in advance in the memory. The switch 3 sets each switch element Swa1, Swa2,... Swan-1, Swan to an open state or a closed state in accordance with the open / close control signal.
Capacitance elements Ca1, Ca2,..., Can-1, and Can constituting the capacitor array 4 are connected to a capacitor C2 according to the state of each of the switch elements Swa1, Swa2,. Either the state or the disconnected state is selected.

Here, the capacitance values of the capacitive elements Ca1, Ca2,..., Can-1, and Can are weighted so that Ca1>Ca2>...>Can-1> Can. Therefore, according to the n-bit capacity array setting signal given to the control unit 5, the combined capacity of the capacity array 4 can be changed from the minimum 0 pF to the maximum (Ca1 + Ca2 +... + Can-1 + Can) with a resolution Ca1 step.
However, since the capacitor C1 is connected to the capacitor array 4 in series, the combined capacitance including the capacitors C1 and C2 and the capacitor array 4 has a minimum step of capacitance change smaller than Ca1. This will be described using a specific example.

FIG. 2 shows a simulation result with the number of elements of the capacitive array 4 being 4 elements (that is, 2 ⁴ = 16 steps). Here, Ca1 = 0.2 pF, Ca2 = 0.4 pF, Ca3 = 0.8 pF, Ca4 = 1.6 pF, C1 = 1 pF, C2 = 0 pF, 0.5 pF, 1 pF.
FIG. 2A shows the combined capacitance values of C1, C2, Ca1, Ca2, Ca3, and Ca4. FIG. 2B shows the amount of change (resolution) of the capacity per step.
For example, referring to the simulation result when C2 = 0.5 pF, the combined capacitance changes from 0.333 pF to 0.778 pF, and the amount of change in capacitance for each step is 0.078 pF to 0.011 pF. That is, it means that the resolution of the combined capacitance is apparently improved by about 2.5 to 18 times despite the minimum value Ca1 of the capacitive element being 0.2 pF.

FIG. 3 similarly shows the result of simulating the frequency change of the piezoelectric oscillator when the number of elements of the capacitive array 4 is four. Ca1, Ca2, Ca3, Ca4, C1, and C2 are the same as the conditions in FIG. Note that the oscillation frequency in step 15 is 38.4 MHz, which is the reference value for the frequency.
FIG. 3A shows the frequency deviation from the reference value, and FIG. 3B shows the amount of change (resolution) for each step. For example, referring to the simulation result when C2 = 0.5 pF, it was found that the resolution of the oscillation frequency can be secured from 0.1 ppm to 4.2 ppm.

Thus, the present invention improves the apparent resolution of the combined capacitance by combining and switching fixed capacitors. In the prior art, fine frequency adjustment is often impossible only by switching the composite capacity. Therefore, in the prior art, when high-accuracy frequency adjustment is required, voltage adjustment must be performed using a voltage-controlled variable capacitance element such as a varactor diode, and the signal-to-noise ratio is likely to deteriorate. There was a drawback.
On the other hand, the present invention eliminates the need for a voltage-controlled variable capacitance element such as a varactor diode for high-accuracy frequency adjustment, so that it is possible to prevent deterioration of the signal-to-noise ratio as much as possible.

FIG. 4 is a circuit diagram of a piezoelectric oscillator according to the second embodiment of the present invention.
4 further includes a switch 6 (Swb1, Swb2,... Swbn-1, Swbn: second connecting / disconnecting means) and a capacitor array 7 (Cb1,. Cb2,... Cbn-1: second capacity array) are added. Here, it should be noted that the number of elements of the capacitor array 7 is n−1, which is one less than the number of elements of the capacitor array 4.
This capacity array 7 is inserted for the purpose of correcting the capacity change in steps 0-7. That is, in steps 0 to 7, the connection between the capacitor array 4 and the capacitor array 7 is disconnected. In steps 8 to 15, all the elements of the capacitor array 7 are connected in parallel to the capacitor array 4. It becomes a state. Therefore, compared with the embodiment shown in FIG. 1, the change from step 0 to 7 is completely the same, and only the change from step 8 to 15 is different. Therefore, by appropriately setting the capacitance values of the capacitive elements Cb1, Cb2,... Cbn-1, the capacitance change characteristics in steps 8 to 15 can be corrected to be substantially equivalent to those in steps 0 to 7. .
It is also possible to correct an arbitrary step range. At this time, the switch 4 and the switch 7 may be controlled independently so that the capacitor array 7 is connected to the capacitor array 4 only in a range where correction is desired.

In FIG. 1, the insertion position of the switch 3 may be changed between the capacitor array 4 and the ground. Similarly, in FIG. 4, the insertion positions of the switches 3 and 6 can be changed between the capacitor array 4 and the ground, or between the capacitor array 7 and the ground, respectively. In short, as long as the capacitor array 4 and the capacitor C1 can be connected and disconnected, the switches 3 and 6, the capacitors C1 and C2, and the capacitor arrays 4 and 7 may be arranged in any manner.
Further, the oscillation circuit is not limited to the inverter type, and any oscillation circuit such as a Colpitts type oscillation circuit or a Pierce type oscillation circuit may be used.
As described above, since the load capacity of the piezoelectric oscillator can be adjusted with high accuracy, the present invention is effective in providing a piezoelectric oscillator that requires high frequency accuracy.

1 is a circuit diagram of a piezoelectric oscillator according to a first embodiment of the present invention. The simulation result of the piezoelectric oscillator which concerns on the 1st Embodiment of this invention. The simulation result of the piezoelectric oscillator which concerns on the 1st Embodiment of this invention. The circuit diagram of the piezoelectric oscillator which concerns on the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Inverter 2 ... Oscillation circuit 3, 6 ... Switch 4, 7 ... Capacitance array 5 ... Control part C1, C2, C3, C4 ... Capacitor R1 ... Feedback resistance X1 ···Crystal oscillator

Claims (2)

  In a piezoelectric oscillator including a piezoelectric vibrator and an oscillation circuit, a fixed capacitance element, a first capacitance array including a plurality of capacitance elements, and a series connection of each of the plurality of capacitance elements and the fixed capacitance element are individually provided. First connecting / disconnecting means for connecting / disconnecting and control means for controlling connecting / disconnecting of the first connecting / disconnecting means, and a series circuit of the fixed capacitance element and the first capacitor array is connected to the piezoelectric vibrator or A piezoelectric oscillator connected to the oscillation circuit.   A second capacitive array including a plurality of capacitive elements is further provided, and a plurality of capacitive elements constituting the second capacitive array and a plurality of capacitive elements constituting the first capacitive array are individually connected in parallel. 2. The piezoelectric oscillator according to claim 1, further comprising a second connection / disconnection means for connecting / disconnecting, wherein the control means controls the connection / disconnection of the second connection / disconnection means.

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