JPH0846427A - Voltage controlled crystal oscillator - Google Patents

Abstract

PURPOSE:To provide a voltage controlled crystal oscillator capable of inhibiting voltage control characteristics from being influenced by the change of a trimmer capacitor CT, making a load capacity sufficiently large and maintaining the drive level of a crystal resonator above a fixed level. CONSTITUTION:In this voltage controlled crystal oscillator composed of the crystal resonator X, a crystal resonator intrinsic oscillation frequency adjustment part Z including a trimmer capacitor CT, an oscillator circuit part Y including a transistor TR for oscillation and a control voltage varying part W including a voltage variable capacitive element CV, the crystal resonator intrinsic oscillation frequency adjustment part Z is connected to one end side of the crystal resonator X, the oscillator circuit part Y is connected to the other end side and the control voltage varying part W is respectively parallelly connected between the crystal resonator X and the oscillation circuit part Y.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage-controlled crystal oscillator in which the capacitance of a voltage variable capacitance element (varicap diode or the like) is controlled by a control voltage to change the oscillation frequency of a crystal oscillator.

[0002]

2. Description of the Related Art A crystal oscillator for generating a reference frequency used in a mobile radio device such as a cellular phone or a PHP (personal handheld horn) is subject to severe operating conditions (for example, surroundings in which it is used) as the device becomes smaller. The temperature is changing drastically) and downsizing of frequency and stabilization of frequency are required.

In general, a crystal resonator used in a crystal oscillator has an inherent frequency-temperature characteristic represented by a cubic curve. Therefore, in stabilizing the frequency, the ambient temperature is measured and the ambient temperature is measured. It is necessary to compensate for frequency fluctuations. One of the specific circuit configurations has a structure in which a temperature compensation circuit including a capacitor, a thermistor, and a resistor is directly connected to a crystal oscillator. Further, there is a structure in which a voltage controlled crystal oscillation circuit including a crystal oscillator is supplied with a control voltage corresponding to the ambient temperature to obtain a stable oscillation frequency independent of the ambient temperature.

In the latter case, in the voltage controlled crystal oscillator,
By connecting a control voltage variable unit including a voltage variable capacitance element such as a varicap diode in an equivalent loop circuit for determining the load capacitance component CL included in the crystal unit, and applying a predetermined control voltage based on the ambient temperature. By changing the capacitance of the varicap diode, the load capacitance component CL is substantially set to a predetermined value and a stable oscillation frequency is output.

Conventionally, a voltage controlled crystal oscillator as shown in FIG. 4 has been used. In the figure, X is a crystal unit, Y is an oscillation circuit unit, Z is a crystal unit-specific oscillation frequency adjustment unit, and W is a control voltage variable unit.

A crystal unit oscillation frequency adjusting section Z composed of a capacitor C ₂ and a trimmer capacitor CT is connected to one end of the crystal unit X, and at the same time, a control voltage variable unit composed of a variable capacitance diode Cv, a capacitor C _{1 and the} like. Part W is connected. Also, at the other end of the crystal unit X,
Capacitor C ₃ -C _7, an oscillation circuit portion Y composed of such oscillating transistor Tr is connected.

Specifically, the varicap diode Cv
A DC voltage determined by the voltage dividing resistors R ₁ and R ₂ is applied to the varicap diode Cv by combining the DC voltage with the control voltage supplied from the terminal VT.
A controlled voltage is applied to the crystal oscillator X to change its capacitance by a predetermined amount to change the load capacitance component CL for the crystal unit X.

The crystal oscillator natural oscillation frequency adjusting section Z
Is for adjusting different natural oscillation frequencies by different crystal oscillators X, and the adjustment amount differs depending on the crystal oscillator X used.

In general, the voltage-capacitance characteristic of the varicap diode Cv exhibits non-linearity, so that in a voltage controlled crystal oscillator, a frequency deviation (Δf / Δf /
f) also becomes a non-linear change. For example, if the control voltage is high, the frequency deviation is small, and if the control voltage is low, the frequency deviation is large.

In order to make up for such a drawback, the present applicant has previously shown that, as shown in FIG. 5, a trimmer is provided at one end of a crystal resonator X as a voltage controlled crystal oscillator which is not affected by fluctuations in frequency deviation. There is provided a voltage-controlled crystal oscillator having a structure in which a crystal oscillator natural oscillation frequency adjusting section Y including a capacitor CT and a control voltage varying section W including a varicap diode Cv are connected in series.

As a result, since the trimmer capacitor CT for correcting the frequency variation of each crystal unit X is connected in series to the varicap diode Cv, when the frequency is controlled by changing the control voltage, the load is reduced. Capacity CL
Therefore, stable frequency control becomes possible. In particular, since the load capacitance CL is not significantly affected, the characteristic of the nonlinear portion of the variable capacitance diode is not promoted when the variable range of the control voltage changes at a relatively low voltage. .

[0012]

However, in the voltage-controlled crystal oscillator shown in FIG. 5, the crystal oscillator intrinsic oscillation frequency adjusting section Z including the varicap diode Cv and the capacitor C ₁ is provided for the crystal oscillator X. , And is connected in series to the trimmer CT, it acts so as to reduce the entire load capacitance component CL as compared with FIG. 4, so that the drive level of the crystal unit X is lowered and oscillation occurs. There was a problem that it was difficult to do so.

The present invention has been devised to solve the above-mentioned problems, and an object thereof is to reduce the influence of the trimmer capacitor CT for adjusting the natural oscillation frequency of the crystal unit. Further, the present invention provides a voltage controlled crystal oscillator capable of ensuring a sufficient load capacitance component and performing stable oscillation.

[0014]

According to the present invention, a crystal unit, a crystal unit-specific oscillation frequency adjustment unit including a trimmer capacitor, an oscillation circuit unit including an oscillation transistor, and a voltage variable capacitor. In a voltage controlled crystal oscillator comprising a control voltage variable unit including an element, the crystal oscillator natural oscillation frequency adjusting unit is connected to one end side of the crystal oscillator, and the oscillation circuit unit is connected to the other end side, In the voltage-controlled crystal oscillator, the control voltage varying unit is connected in parallel between the crystal unit and the oscillation circuit unit.

[0015]

According to the present invention, the control voltage variable section including the voltage variable capacitance element and the crystal oscillator natural oscillation frequency adjusting section including the trimmer capacitor CT are connected to both ends with the crystal oscillator interposed therebetween. Since the change in the capacitance of the voltage variable capacitance element and the trimmer capacitor do not affect each other, the variation of the modulation impression due to the variable capacitance of the trimmer capacitor is alleviated, and the linearity is sufficient in the relationship between the control voltage and the oscillation frequency. Can be obtained.

Further, an element that generates a load capacitance for the crystal unit, that is, an oscillation circuit unit, a control voltage variable unit, and a crystal unit specific oscillation frequency adjusting unit are connected in parallel to the crystal unit. Therefore, a sufficient load capacity can be secured, the drive level of the crystal unit can be sufficiently maintained, and stable oscillation can be performed.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A voltage controlled crystal oscillator according to the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a voltage controlled oscillator circuit used in the voltage controlled crystal oscillator of the present invention. The voltage controlled oscillator circuit is mainly composed of a crystal unit X, an oscillator circuit unit Y, a crystal unit specific oscillation frequency adjustment unit Z, and a control voltage variable unit W.

A crystal oscillator natural oscillation frequency adjusting section Z including a capacitor C ₂ and a trimmer capacitor CT is connected to one end of the crystal oscillator X. The crystal oscillator natural oscillation frequency adjustment unit Z is for adjusting the variation in frequency due to the variation in the thickness of the crystal piece that is different for each crystal oscillator X, and the crystal oscillator X is controlled under certain conditions. Of the trimmer capacitor CT
To obtain a desired oscillation frequency by changing the capacity of the.

Further, the oscillation circuit section Y including capacitors C _{3 to} C ₇ , transistors Tr, resistors R _{3 to} R _{6 and the} like is connected to the other end of the crystal resonator X. The oscillation circuit unit Y creates an oscillation signal of the crystal unit X into an oscillation signal of a predetermined frequency by the oscillation transistor Tr and supplies it from the output terminal OUT to, for example, an amplification stage. Although not denoted including a capacitor C ₃ to the oscillator circuit part Y, the capacitor C ₃ is a crystal resonator X and oscillating transistor Tr
Is a coupling capacitor for connecting to.

Further, a control voltage variable section W is provided between the ground and a connection point V between the other end of the crystal unit X and the input side of the oscillation circuit section Y (the crystal unit X side of the capacitor C ₃ ). It is connected in parallel to the crystal unit X. The control voltage variable unit W includes, for example, a voltage variable capacitance element such as a varicap diode (hereinafter, a varicap diode will be described as an example) Cv, a capacitor C ₁ , resistors R ₁ and R _{2, and the} like. Specifically, a control voltage corresponding to ambient temperature information is supplied from the control voltage terminal VT, the control voltage is divided by the voltage dividing resistors R ₁ and R ₂ , and the varicap diode Cv is divided. The voltage will be applied as the applied voltage. As a result, the varicap diode C
v will generate a capacitive component corresponding to the ambient temperature.

Therefore, the load capacitance component CL of the entire circuit viewed from the crystal unit X changes, and the fluctuation characteristic of the frequency of the crystal unit due to the ambient temperature can be compensated.

In addition to the ambient temperature information as described above, the control voltage is supplied with the control voltage in order to switch the communication channel of the communication device, that is, to change the frequency. As a result, a desired oscillation frequency can be obtained with the predetermined control voltage.

According to the configuration of the above circuit diagram, considering the load capacitance CL for the crystal unit X, the load capacitance component CL ₁ of the crystal unit natural oscillation frequency adjusting unit Z on one end side of the crystal unit X, Control voltage variable unit W on the other end side of the crystal unit X
Combined load capacitance component CL ₂ of the oscillator circuit section Y, the combined load capacitance component CL of the coupling capacitor C ₃ of the oscillation circuit unit Y and other capacitance components C _{W
It is} the sum of each load capacity component of ₃ . The equivalent circuit diagram of the load capacitance component is as shown in FIG.

That is, the crystal oscillator natural oscillation frequency adjusting section Z
The load capacitance component CL _{1 at} is the trimmer capacitor C
Since T and the capacitor C ₂ are connected in parallel,
It is represented by CL ₁ = CT + C ₂ .

A load capacitance component C in the control voltage variable section W
L ₂ is a varicap diode Cv and a capacitor C ₁
Since and are connected in series, CL ₂ = C ₁ · Cv
It is represented by / (C ₁ + Cv).

The combined load capacitance component CL ₃ in the oscillation circuit section Y is CL ₃ = C ₃ · C _W / since the coupling capacitor C ₃ and the combined capacitance C _W are connected in series.
It is represented by (C ₃ + C _W ).

Further, the load capacitance component CL ₂ of the control voltage variable portion W and the combined load capacitance component CL _{3 of the} oscillation circuit portion Y are summarized as shown in FIG. 2 (b). In FIG. 2B, C _O is the above-mentioned crystal unit natural oscillation frequency adjustment unit Y
Is the load capacitance CL ₁ , and C _O 'is the load capacitance component CL
It is the sum of ₂ and the combined load capacitance component CL ₃ .

That is, the load capacitance of the entire circuit with respect to the crystal unit X is 1 / CL = 1 / C _O + 1 / C _O '.

Therefore, the varicap diode Cv is
It is in the term of 1 / C _O 'and the voltage variable sensitivity by Cv is
C ₁ · Cv / (C ₁ + Cv) and C ₃ · C ₆ / (C ₃ + C
Will be determined by the ratio of the _6), the trimmer capacitor CT is a term of 1 / C _O, would never influence each other.

The load capacitance CL for the crystal unit X
Since the constituent elements (the crystal oscillator natural oscillation frequency adjusting unit Z, the oscillation circuit unit Y, and the control voltage varying unit W) are in a parallel relationship, a sufficient load capacitance CL can be obtained, and the crystal oscillator X A stable oscillation operation can be performed without lowering the drive level.

Here, the linearity characteristic of the relationship between the control voltage (applied voltage) of the varicap diode Cv and the capacitance will be described. Particularly, in the conventional voltage controlled oscillator circuit of FIG. 4, a comparatively high value of the applied voltage is obtained. In, it becomes non-linear. However, in the present invention, by selecting the capacitor C ₁ connected in series with the varicap diode Cv, the linearity characteristic does not become a problem in a practical range.

For example, as a circuit constant, CT = 100p
F, C ₂ = 15 pF, C ₃ = 47 pF, C _W = 75 pF
(C ₄ = 150 pF, C ₅ = 150 pF), varicap diode Cv (Toshiba 1SV217), crystal oscillator X constants C _O = 2.6 pF, Cx = 16 mpF,
As a strip-shaped crystal oscillator having a frequency of 19.2 MHz, a fluctuation rate (Δf / f) of the oscillation frequency from the entire circuit when the capacitor C ₁ connected in series with the varicap diode Cv is changed to 10 pF, 15 pF, and 20 pF. ) Was investigated. The result is shown in FIG. The voltage applied to the varicap diode Cv was centered on 2.5V.

In FIG. 3, the characteristic when the capacitor C ₁ is 10 pF is a line A, and the characteristic when the capacitor C ₁ is 15 pF is a line B.
The line C shows the characteristic at 20 pF. As can be understood from the figure, if the capacitor C ₁ is set to ₁₀ to 20 pF, the normally used varicap diode Cv
It is possible to obtain linearity in which the fluctuation rate (Δf / f) of the oscillation frequency is very stable in the range of the voltage applied to.

[0035]

As described above, according to the present invention, the control voltage variable section including the varicap diode Cv and the like is connected between the crystal oscillator and the oscillation circuit section. Since it is connected to one end on the side facing the crystal oscillator natural oscillation frequency adjustment unit, the capacitance of the varicap diode Cv is changed with respect to the variation of the capacitance variable amount of the trimmer capacitor CT of the crystal oscillator natural oscillation frequency adjustment unit. Stable operation is possible without being affected.

Further, the load capacitance for the crystal unit, which is composed of the load capacitance component of the control voltage variable unit including the varicap diode Cv, the capacitance component of the oscillation circuit unit, and the load capacitance component of the crystal oscillator specific oscillation frequency adjusting unit, Because of the parallel relationship, the total load capacitance component becomes extremely large,
It is possible to oscillate with the stable drive level of the crystal unit.

That is, a voltage-controlled crystal oscillator capable of achieving extremely stable oscillation is achieved.

[Brief description of drawings]

FIG. 1 is an oscillation circuit diagram of a voltage controlled crystal oscillator according to the present invention.

2A and 2B are equivalent circuit diagrams showing a load capacitance component of the oscillation circuit diagram shown in FIG.

FIG. 3 is a characteristic diagram showing a variation characteristic of a relationship between a control voltage and a frequency variation rate due to a change in a capacitor C ₁ connected to a varicap diode Cv.

FIG. 4 is an oscillation circuit diagram of a conventional voltage controlled crystal oscillator.

FIG. 5 is an oscillation circuit diagram of a conventional voltage controlled crystal oscillator.

[Explanation of symbols]

X ... Crystal oscillator Y ... Oscillation circuit unit Z ... Crystal oscillator specific oscillation frequency adjustment unit W ... Control voltage variable unit C _{1 to} C ₇ ... Capacitor Cv .... Voltage variable capacitance element (varicap diode) CT ... Trimmer capacitor Vt ... Control terminal O ... Output terminal Vcc ... Power supply terminal

Claims (1)

[Claims] 1. A voltage control type comprising a crystal unit, a crystal unit specific oscillation frequency adjusting unit including a trimmer capacitor, an oscillation circuit unit including an oscillating transistor, and a control voltage changing unit including a voltage variable capacitance element. In the crystal oscillator, the crystal oscillator natural oscillation frequency adjusting unit is connected to one end side of the crystal oscillator, the oscillation circuit unit is connected to the other end side, and the control voltage varying unit is connected to the crystal oscillator and the oscillation unit. A voltage-controlled crystal oscillator characterized in that it is connected in parallel with the circuit section.