JP3203664B2 - Crystal oscillation circuit - Google Patents

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 oscillation circuit.

[0002]

2. Description of the Related Art Conventionally, for example, in a case where a transmission signal from a transmitter is received by a receiver and the receiver is operated based on the received signal, the receiver side operates the received signal. It is required to synchronize the clock of the receiver with the operation clock of the receiver. The synchronization on the receiver side is performed, for example, by changing the oscillation frequency of the crystal oscillation circuit so that the clock in the reception signal is synchronized with the oscillation frequency of the operation clock generation crystal oscillation circuit in the receiver. This is done by controlling.

Here, as the above-mentioned crystal oscillation circuit, for example, there is a voltage-controlled crystal oscillation circuit (VCXO).
This voltage controlled crystal oscillation circuit is an element whose reactance changes according to the voltage (for example, a variable capacitance diode).
Is incorporated in a crystal oscillation loop, and the oscillation frequency of the crystal oscillation circuit is controlled by changing the load capacitance on the circuit side viewed from the oscillator (crystal oscillator) side by a control voltage. . In such a voltage-controlled crystal oscillation circuit, it is important how the oscillation frequency can be greatly changed by changing the control voltage, which is the performance of the voltage-controlled crystal oscillation circuit. . This is mainly determined by factors such as the control sensitivity of the crystal unit.
The above-mentioned control sensitivity means, for example, when the load capacitance on the circuit side viewed from the crystal unit side is changed, for example, every 1 pF.
It indicates how much the oscillation frequency of the crystal oscillation circuit changes (the amount of change in the oscillation frequency), and is unique to each crystal resonator.

FIG. 6 shows a general circuit configuration of the above-mentioned voltage controlled crystal oscillation circuit. The voltage-controlled crystal oscillation circuit of FIG. 6 includes a crystal resonator 1 as a resonator, a load resistor R1,
R2, the inverter 2, and the fixed capacitor Ca,
Cc, a trimmer capacitor Ct, and a variable capacitance diode Cva as a variable capacitance element whose reactance changes according to a voltage (control voltage Vcon from the terminal 3). Here, the load resistor R1 and the inverter 2 are connected in parallel to the crystal unit 1, respectively, and the fixed capacitor Ca and the trimmer capacitor Ct are connected to the input side of the inverter 2. The fixed capacitance capacitor Cc is provided on the output side of the inverter 2, and the variable capacitance diode Cva is provided on the input side of the inverter 2. The variable capacitance diode Cva only has to be provided at one of the input / output terminal of the inverter 2, that is, at both ends of the crystal unit 1. Therefore, the variable capacitance diode Cva is disposed on the output side of the inverter 2. It may be done. In this case, the fixed capacitor Cc is arranged on the input side of the inverter 2. However, since the fixed capacitance capacitor Cc is used for pull-down, if the capacitance is too large, the oscillation stops. Usually, the fixed capacitance capacitor Cc is provided on the output side of the inverter 2.
The load resistor R2 is a control voltage limiting resistor. Further, the trimmer capacitor Ct is provided for fine adjustment of the reference frequency in the crystal unit 1.
This is an element for absorbing variations in characteristics of the crystal unit 1 and compensating the reference frequency.

In the above-described voltage-controlled crystal oscillation circuit shown in FIG. 6, by changing the control voltage Vcon at the terminal 3, the capacitance of the variable capacitance diode Cva changes, thereby changing the oscillation frequency of the crystal oscillation circuit. I will be. By providing such a voltage-controlled crystal oscillation circuit on the receiver side, for example, a clock of a received signal and an operation clock based on the oscillation frequency of the crystal oscillation circuit can be synchronized. The receiver operates normally based on the transmission signal.

[0006]

By the way, in recent years, high-density mounting technology has been advanced, and miniaturization of components has been rapidly achieved. Therefore, miniaturized parts have also been developed for the quartz oscillator. For example, the volume is half (or 1)
/ 4) have also been developed. However, downsizing of this crystal unit is convenient for miniaturization of products incorporating it, but on the other hand, the size of the crystal unit encapsulated in the crystal unit is small, so it depends on the size of the crystal unit. However, there is a disadvantage that the control sensitivity of the quartz crystal resonator becomes smaller.

That is, for example, when considering synchronization between a plurality of video cameras as a specific example of the transmitter and the receiver, the change amount of the oscillation frequency in the conventional crystal oscillation circuit used for these video cameras is For example, it is about ± 36 ppm from the reference frequency of the crystal resonator.
However, as devices such as the video camera are further miniaturized in the future, it is expected that the size of the crystal unit will be reduced, and accordingly, the control sensitivity of the crystal unit will also decrease. For example, in the case of a crystal resonator having a volume half that of the conventional crystal resonator, the variation of the oscillation frequency in the crystal oscillation circuit using the crystal resonator is, for example, about ± 26 ppm from the reference frequency. . As described above, when the size of the crystal unit is reduced, the amount of change in the oscillation frequency required for synchronization between devices (for example, the amount of change in frequency required for synchronization between video cameras) cannot be obtained. There is a possibility that synchronization may be lost between the devices.

Accordingly, the present invention has been proposed in view of the above situation, and provides a crystal oscillation circuit which can obtain a sufficient variation in oscillation frequency even if the crystal oscillator is downsized. It is intended to do so.

[0009]

SUMMARY OF THE INVENTION A crystal oscillation circuit according to the present invention has been proposed to achieve the above-mentioned object, and includes a crystal oscillator and a first crystal oscillator connected in parallel to the crystal oscillator. A load resistance, an inverter, and a first variable capacitance element provided on the input side of the inverter;
A second variable capacitance element provided on the output side of the inverter, a second and a third load resistances having one end commonly connected and the other end connected to the input side or the output side of the crystal unit; And a capacitance element connected in parallel between the input and output of the third load resistor, and controlling the voltage applied to the common connection terminal of the second and third load resistors to change the oscillation frequency. It is intended to be. Further, a crystal oscillation circuit of the present invention includes an adjusting element provided on an input side of the inverter, for adjusting a deviation of a reference frequency of the crystal resonator.

[0010]

According to the crystal oscillation circuit of the present invention, the first and second terminals are connected to both terminals of the crystal oscillator (input side and output side of the inverter).
The variable capacitance element is provided so that the amount of change in the load capacitance can be increased, so that the amount of change in frequency can be increased, and a capacitance element is provided between the input and output of the third load resistor. The response speed of the frequency change can be improved. Further, an adjustment element for adjusting the deviation of the reference frequency of the crystal unit is provided on the input side of the inverter.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the crystal oscillation circuit according to the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic circuit diagram of the crystal oscillation circuit of the first embodiment. In FIG. 1, a crystal oscillation circuit according to a first embodiment includes a crystal unit 1 and the crystal unit 1.
A load resistor R1 and an inverter 2 connected in parallel to each other, and a first variable capacitance diode Cva as a first variable capacitance element provided on the input side of the inverter 2;
A variable capacitance diode Cvb as a second variable capacitance element provided on the output side of the inverter 2 to control a voltage applied between the input and output of the first and second variable capacitance diodes Cva _and Cvb. Thus, the oscillation frequency of the crystal oscillation circuit is changed.

That is, the crystal oscillation circuit of the first embodiment shown in FIG. 1 has the above-described crystal oscillator 1, load resistance R1,
Inverter 2, first and second variable capacitance diodes Cv
a, Cvb, etc., other than the main components of the first embodiment of the present invention, load resistors R3, R4, and a fixed capacitor C
a, Cb and a trimmer capacitor Ct. The fixed capacitor Ca and the trimmer capacitor Ct are connected to the input side of the inverter 2, and the fixed capacitor Cb is provided on the output side of the inverter 2. Further, the fixed capacitance capacitors Ca, Cb
Is a DC cut capacitor for removing a DC voltage component, and Cva≪ is smaller than the capacitance of the variable capacitance diode.
Ca, Cvb≪Cb. The oscillation frequency of the crystal oscillation circuit mainly depends on the first and second variable capacitance diodes Cv
a, and Cvb. Further, the trimmer capacitor Ct is provided for fine adjustment of the reference frequency of the crystal unit 1, and is an element for absorbing variations in the characteristics of the crystal unit 1 and compensating for the reference frequency. is there. The load resistors R3 and R4 are control voltage limiting resistors.

When the crystal unit 1 having a certain control sensitivity is applied to a voltage-controlled crystal oscillation circuit, the amount of change in load capacitance on the circuit side as viewed from the crystal unit 1 is increased. It is possible to obtain a larger change amount of the oscillation frequency than the conventional crystal oscillation circuit described above.

That is, in the conventional crystal oscillation circuit shown in FIG. 6, a variable capacitance element (variable capacitance diode Cva in FIG. 6) is connected to one of both terminals of the crystal unit 1.
To control the oscillation frequency. On the other hand, in the circuit of the first embodiment shown in FIG.
By disposing variable capacitance elements (first and second variable capacitance diodes Cva, Cvb) to both of the terminals, a change in load capacitance on the circuit side as viewed from the crystal unit 1 side is increased. .

In the circuit of the first embodiment shown in FIG. 1, by changing the control voltage Vcon at the terminal 3,
The capacitances of the first and second variable capacitance diodes Cva and Cvb change, thereby changing the oscillation frequency of the crystal oscillation circuit.

FIG. 2 shows a configuration of an equivalent circuit of the circuit of FIG. That is, in the circuit of FIG. 2, the first and second variable capacitance diodes Cva, Cva for increasing the load capacitance change amount are provided on the circuit side as viewed from the crystal unit 1 side.
vb is provided. It should be noted that the second variable capacitance diode Cvb and fixed capacitance capacitor Cb surrounded by a dotted line in FIG. 2 are replaced with the fixed capacitance capacitor Cc in FIG. Become.

Using FIG. 2 as an equivalent circuit of FIG. 1 (or FIG. 6), the amount of change in load capacitance on the circuit side as viewed from the quartz oscillator 1 between the first embodiment circuit and the conventional circuit is shown. Compare. The capacitances Cout and Cin in FIG. 2 are the input capacitance (Cin) and the output capacitance (Cou) of the inverter 2 in FIG.
t).

Based on the equivalent circuit of FIG. 2, the load capacitance on the circuit side as viewed from the quartz oscillator 1 is determined. First, the load capacitance C _{1 on the} circuit side in the circuit of the first embodiment is represented by Cva≪C
a _, Cvb≪Cb, which can be obtained from Equation 1.

(Equation 1)

In the equation (1), Cva represents the capacitance of the first variable capacitance diode, Cvb represents the capacitance of the first variable capacitance diode, and Ct represents the capacitance of the trimmer capacitor.

Further, the load capacitance C4 on the circuit side in the conventional circuit can be obtained from Equation 2 from Cva≪Ca.

(Equation 2)

In the equation (2), Cva indicates the capacitance of the variable capacitance diode shown in FIG. 6, Cc indicates the capacitance of the fixed capacitance capacitor, and Ct indicates the capacitance of the trimmer capacitor.

As specific numerical examples of the capacitances of these equations 1 and 2, in the crystal oscillation circuit of the first embodiment, for example, Cout = 9 pF, Cin = 7 pF, Cva = Cv
b = 10 pF to 20 pF, and Ct = 2 pF to 6 pF. However, since Ct of the trimmer capacitor is used for compensating the reference frequency of the crystal unit 1, Ct is used here.
= 4 pF. By substituting these numerical values into Equation 1 in the circuit of the first embodiment, Cva = Cvb = 1
At 0 pF, C ₁ becomes the minimum value C ₁ min, and Cva =
C ₁ when Cvb = 20 pF is the maximum value C ₁ max. That is, from equation 1, C ₁ min = 9.98 pF C ₁ max = 14.98 pF Therefore, the load capacitance change amount ΔC on the circuit side as viewed from the crystal unit 1 side in the crystal oscillator circuit of the first embodiment.
₁ results in ΔC ₁ = C ₁ max−C ₁ min = 5.00 pF.

Further, when the same numerical values of the respective capacitances as those of the circuit of the first embodiment are substituted into the above equation 2 of the conventional circuit, Cva = 1
When 0 pF, C ₄ is the minimum value C ₄ min, Cva = Cvb
When C = 20 pF, C ₄ becomes the maximum value C ₄ max. That is, from equation 2, C ₄ min = 11.20 pF C ₄ max = 13.53 pF Therefore, the load capacitance change ΔC ₄ on the circuit side as viewed from the crystal unit 1 side in the conventional crystal oscillation circuit is ΔC ₄ = C ₄ max−C ₄ min = 2.33 pF.

As shown by the calculation results of Equations 1 and 2, the circuit of the first embodiment can obtain a load capacitance change amount twice or more as compared with the conventional circuit. It can be seen that this is advantageous for frequency control of the voltage controlled crystal oscillation circuit.

Further, in order to compare the change amount of the oscillation frequency between the conventional circuit and the circuit of the first embodiment, each circuit is actually constructed, and for example, the frequency change is performed by using ten quartz resonators as samples. The amount was measured. FIG. 3 shows the measurement results. As shown in FIG. 3, in the conventional circuit, the frequency change amount was about ± 26 ppm on the average as shown by the dotted line in the figure, whereas in the circuit of the first embodiment, the characteristic shown by the solid line in the figure was obtained. As described above, it was possible to obtain an average frequency variation of about ± 48 ppm. The measurement result in the circuit of the first embodiment is, for example, a value sufficient to establish synchronization between the video cameras described above.

The oscillation waveform of the circuit of the first embodiment also has a practically acceptable shape, and does not cause any malfunction. Further, for example, even when the volume of the crystal unit is reduced to 1/4 of the conventional volume, if it is applied to the circuit of the first embodiment of the present invention, it is necessary and sufficient for synchronization between various devices. It is possible to obtain a large change amount of the oscillation frequency.

It should be noted that, in the circuit of the first embodiment, the frequency variation is substantially symmetrical at the upper and lower limits with respect to the reference frequency of the crystal unit 1 as described above. This is because the value is configured to be adjustable (for example, adjusted using a volume) so as to absorb variations in the characteristics of the first and second variable capacitance diodes Cva and Cvb.

As described above, according to the circuit of the first embodiment, the first and second variable capacitance diodes Cva and Cva for increasing the amount of load capacitance change on the circuit side as viewed from the crystal unit 1 side. By connecting Cvb to both terminals of the crystal unit 1 (input / output terminals of the inverter 2), a large variation in oscillation frequency can be obtained.

That is, when a voltage controlled crystal oscillation circuit is formed using the crystal resonator 1 having a certain control sensitivity, the circuit viewed from the crystal resonator 1 side is obtained by applying the configuration of the first embodiment. The amount of change in the load capacitance on the side can be twice or more as compared with the conventional circuit. As a result, the circuit of the first embodiment can operate under the same conditions (the same crystal sensitivity as the control sensitivity) of the conventional circuit. ), It is possible to improve the frequency variation about twice. Therefore, the first
If the embodiment circuit is applied to a video camera, for example,
In the future, it will have enough performance to cope with the decrease in control sensitivity due to the downsizing of the crystal unit expected due to the downsizing of the product body, and as a new method of a voltage controlled crystal oscillation circuit using a variable capacitance element Becomes effective.

In the crystal oscillation circuit of the first embodiment, since the output impedance of the inverter 2 is low, the control voltage Vco
It is conceivable that a surge current is generated in response to the change in n, and the response speed of the change in the oscillation frequency is reduced.

That is, while the input impedance Ri of the inverter 2 is, for example, about 100 kΩ to 1 MΩ, the output impedance Ro is as low as about 10 Ω to 50 Ω. In this case, the first of FIG.
In embodiments circuits (such as for example, a voltage value suddenly changes from v _L to v _H) rectangular specific, as shown in FIG. 5 when applying a control voltage Vcon, the input side voltage vi of the inverter 2 Vi / v between output side voltage vo ₁
DC value of o ₁ is to converge to the control voltage Vcon (converging to v _H in the example of FIG. 5), it is considered that it is necessary t ₁ some time. Therefore, there is a possibility that the response speed of the change of the oscillation frequency is reduced by the time t ₁ .

In the present invention, the second type shown in FIG.
With the configuration of the crystal oscillation circuit of the embodiment, it is possible to improve the response speed of the frequency change. That is, the crystal oscillation circuit of the second embodiment shown in FIG.
And the first connected to the crystal unit 1 in parallel.
And a first variable capacitance element provided on the input side of the inverter 2 and a first variable capacitance element provided on the input side of the inverter 2.
And a second variable capacitance diode Cvb, which is a second variable capacitance element provided on the output side of the inverter 2, has one end commonly connected and the other end connected to the crystal resonator 1. It comprises second and third load resistors R4 and R3 connected to the input side or output side, and a fixed capacitance capacitor Cs which is a capacitance element connected in parallel between the input and output of the third load resistor R3. It becomes. Here, the voltage (the control voltage Vcon) applied to the common connection terminal of the second and third load resistors R4 and R3.
By controlling the oscillation frequency of the crystal oscillation circuit. Note that, in FIG. 4, components denoted by the same reference symbols as those in FIG.
Since the operation is the same as that of each component in FIG. 1, the description of these details will be omitted. That is, the input impedance Ri of the inverter 2 is, for example, about 100 kΩ to 1 MΩ, as in FIG. 1, whereas the output impedance Ro is as low as about 10Ω to 50Ω.

In the circuit of the second embodiment shown in FIG.
The fixed capacitor Cs is connected to the third load resistor R3.
5, the high-frequency impedance Zc of the capacitor Cs and the resistance value r of the third load resistor R3 when the control voltage Vcon is applied as shown in FIG. Zc≪r. At this time, the current i flowing in the direction of the arrow in the figure is determined by the impedance Zc of the capacitor Cs and the value of the output impedance Ro of the inverter 2. In the transition period, the relationship among the output voltage vo2 of the inverter 2, the output impedance Ro of the inverter 2, the impedance Zc of the capacitor Cs, and the control voltage Vcon is as shown in Equation 3.

(Equation 3)

For this reason, the control voltage Vcon
Immediately after the voltage is applied, the electric charge is immediately stored in the capacitor Cs.
The output-side voltage vo ₂ comes to a short period of time converges to the control voltage Vcon in _{(t 2) (vo 2 =} Vcon). Therefore, the output side voltage vo _{2 of the} inverter ₂
From the beginning (control voltage Vcon) rather than the third load resistor R3 alone (without capacitor Cs).
Immediately after the application), the potential becomes high. In addition, normal application to the variable capacitance diodes Cva and Cvb also becomes faster.

That is, as shown in FIG. 5, the time t ₂ until the output voltage vo ₂ converges to the control voltage Vcon (v _H ) is the time t ₂ in the circuit of FIG.
_It will be faster than ₁ . Therefore, according to the crystal oscillation circuit of the second embodiment, the response speed of the change in the oscillation frequency can be increased (the response time can be shortened).

[0036]

As described above, in the crystal oscillation circuit of the present invention, a variable capacitance element is provided at both ends of the crystal oscillator, and the oscillation frequency is changed by controlling the voltage applied between the input and output of the variable capacitance element. This makes it possible to increase the amount of change in the oscillation frequency of the crystal oscillation circuit,
By connecting a capacitive element in parallel between the input and output of the third load resistor, the response speed of the change in the oscillation frequency can be increased. Further, in the crystal oscillation circuit of the present invention, by providing an adjustment element on the input side of the inverter, it is possible to adjust the deviation of the reference frequency of the crystal resonator. Therefore, if the crystal oscillation circuit of the present invention is applied to, for example, a video camera or the like, the crystal oscillation circuit may have a performance that can sufficiently cope with a reduction in control sensitivity due to the downsizing of the crystal unit expected in the future downsizing of the product body. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram of a crystal oscillation circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing an equivalent circuit of the crystal oscillation circuit according to the first embodiment.

FIG. 3 is a characteristic diagram showing an oscillation frequency change amount in the first example circuit and the conventional example circuit.

FIG. 4 is a schematic circuit diagram of a crystal oscillation circuit according to a second embodiment of the present invention.

FIG. 5 is a waveform chart for comparing the response speed of the frequency change of the circuits of the first and second embodiments.

FIG. 6 is a schematic circuit diagram of a conventional crystal oscillation circuit.

[Explanation of symbols]

 1 ····· Crystal oscillator 2 ····· Inverter R1, R3, R4 ··· Load resistance Ca, Cb, Cs ··· Fixed capacitor Cva ··· First Of the variable capacitance diode Cvb of the second variable capacitance diode Ct of the trimmer capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-203606 (JP, A) JP-A-62-130410 (JP, A) JP-A-56-15111 (JP, U) JP-A-2-15 64224 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H03B 5/32

Claims (2)

(57) [Claims] And 1. A crystal oscillator, a first load resistor connected in parallel to the crystal oscillator, and an inverter, a first variable capacitance element provided on the input side of the inverter, the A second variable capacitance element provided on the output side of the inverter; and a second and third load resistors, one end of which is commonly connected and the other end connected to the input side or the output side of the crystal unit. A capacitance element connected in parallel between the input and output of the third load resistor, and controlling the voltage applied to the common connection terminal of the second and third load resistors to change the oscillation frequency. A crystal oscillation circuit characterized by the above. 2. An inverter provided on an input side of the inverter,
Adjustment element for adjusting the deviation of the reference frequency of the crystal unit
Characterized in that it comprises the claim 1, wherein the crystal oscillation times
Road.