JP3129255B2 - 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 crystal oscillation circuit using a crystal oscillator, and more particularly to a crystal oscillation circuit suitable for generating a local oscillation frequency signal of a radio selective calling receiver.

[0002]

2. Description of the Related Art Generally, a radio selective call receiver is configured to take out a baseband signal by converting a received VHF or UHF band line frequency into a one-stage or two-stage intermediate frequency. Frequency conversion is performed by mixing a local oscillation frequency signal generated by a local oscillator with a received line frequency. For example, in the radio selective calling receiver of the double superheterodyne system shown in FIG. 10, a signal received by the antenna 11 is amplified at a high frequency by the RF amplifier 12, passed through the BPF 13, and then combined with the first local oscillation signal by the first mixer 14. Mix and convert to a first intermediate frequency signal. This first intermediate frequency signal passes through the BPF 15,
After being mixed with the second local signal in the second mixer 16, converted into a second intermediate frequency signal and passed through the BPF 17, the demodulator 1
8 demodulates to a baseband signal. The first local oscillation signal and the second local oscillation signal are generated by using a crystal oscillator. In particular, the first local oscillation signal is constituted by a PLL circuit here. This PLL circuit uses a crystal oscillation circuit 27 including a crystal oscillator XTL as a reference frequency signal, and a value obtained by counting the reference frequency signal by a counter 26 and a VCO 22
Is compared with the value counted by the counter 21 by the phase comparator 23, and the phase difference signal is compared with the charge pump 2
4. By obtaining a control voltage of the VCO 22 via the LPF 25, a predetermined frequency signal is output. And this VC
The output signal of O22 is multiplied by the multiplication circuit 19 to obtain the first local oscillation signal. Here, the second local oscillation signal is obtained by multiplying the output of the crystal oscillation circuit by the multiplication circuit 20.

In the receiver having this configuration, the first and second receivers are used.
Is offset in the frequency of the local oscillation signal, the offset also occurs in the intermediate frequency of the converted intermediate frequency signal. For example, in the case of frequency conversion by the lower local, if the local oscillation frequency shifts to a higher one, the intermediate frequency shifts to a lower one, and if the local oscillation frequency shifts to a lower one, the intermediate frequency shifts to a higher one. That is, the intermediate frequency has a deviation whose polarity is opposite to that of the local oscillation frequency and whose absolute value is substantially equal. This frequency deviation affects the characteristics of the receiver, mainly the reception sensitivity. When the line frequency is relatively high and the transmission speed of the signal is high, the deterioration of the receiving sensitivity due to the local offset appears remarkably. For example, if the local oscillation frequency is ± 5
It is assumed that this line frequency has a deviation of ppm and is converted to a first intermediate frequency (1STIF). Line frequency is 15
At 0 MHz, the offset of the intermediate frequency is ± 600 Hz
However, when the line frequency is 900 MHz, the offset of the intermediate frequency is about ± 4.4 kHz. The 3 dB bandwidth of the channel filter (FIG. 12, BPF 63) is usually ± 7.5 to obtain a certain C / N.
It is designed for 10 kHz. Since the modulation degree is ± 4.5 or ± 4.8 kHz, in the 900 MHz band where the local offset is large, the signal band is attenuated by the filter, and the deterioration of the reception sensitivity band characteristic becomes remarkable.

[0004] The largest cause of such a deviation in the local oscillation frequency is a change in the ambient temperature in the crystal oscillation circuit. In the case of a crystal oscillation circuit, the frequency-temperature characteristics of the oscillation circuit and the frequency-temperature characteristics of the crystal unit alone are affected. Therefore, some countermeasures have conventionally been considered for eliminating this deviation. One is a method using a temperature compensated crystal oscillator (TCXO). TCXO is a crystal oscillator whose purpose is to obtain a characteristic that is flatter than the theoretical frequency temperature characteristic of a crystal unit by using a built-in temperature compensation circuit. By using this TCXO, high frequency stability can be obtained. It is used in car phones, mobile phones, navigation systems, etc. However, TCXO is very expensive compared to a crystal oscillator,
It is difficult to apply to a device requiring low cost such as a radio selective calling receiver from the viewpoint of cost. Also,
Automatic frequency control (AFC) has also been proposed. AFC
Is a method of detecting a frequency offset in an analog or digital manner and feeding back the oscillation circuit. However, this method inevitably complicates the configuration of the receiver, and it is also difficult to apply the method to a device that requires a low price.

Therefore, there has been proposed a method of improving the frequency-temperature characteristics by focusing on the frequency-temperature characteristics of the crystal resonator and the oscillation circuit. That is, FIG.
As shown in the figure, the frequency temperature characteristic of the AT-cut quartz resonator is a cubic curve characteristic. For example, the deviation at −10 ° C. (vs. 25 ° C.) can be either positive or negative. Therefore, if the temperature characteristic of the circuit has a positive or negative slope, the temperature characteristic of the crystal oscillator may be offset, but the characteristic may be strengthened and become out of the standard. Therefore, in order to improve the temperature characteristics of the crystal oscillation circuit at a low cost and with a simple configuration, the frequency temperature characteristics of the circuit may be set to zero. As a result, the temperature characteristic of the oscillation frequency becomes only the temperature characteristic of the single crystal oscillator, and it is not necessary to consider the circuit deviation.
When the oscillation frequency temperature characteristic of the circuit has a negative slope, the frequency temperature characteristic can be generally reduced to zero by using a capacitive element having a negative temperature coefficient as a load capacitance. RH products (−220 ± 60 ppm / ° C.), UJ
(-750 ± 120 ppm / ° C.). However, when the frequency temperature characteristic of the oscillation circuit has a positive slope as in the example of FIG. 2A, a capacitor having a positive temperature coefficient standard is required. The coefficient standard type is difficult to obtain because it is not mass-produced, and it becomes expensive if custom-ordered.

[0006] For these reasons, there has been proposed a device using a variable capacitance diode which is easily available and inexpensive. For example, Hirofumi Kawashima et al., “Voltage Control Using NS-GT Cut Quartz Crystal Resonator: S-TCXO” (Transactions of the Institute of Electronics, Information and Communication Engineers, C
-I Vol. J78-CI No.11pp.533-540 November 1995)
By controlling the control voltage applied to the variable capacitance diode in accordance with the temperature change, the frequency temperature characteristic of the crystal resonator is corrected by the change in the capacitance. There are also other techniques using such a variable capacitance diode, for example, Japanese Patent Application Laid-Open Nos. 1-205607 and 64-49406.

[0007]

However, in each of the techniques described in the above-mentioned documents using the variable capacitance diode, since the voltage applied to the variable capacitance diode is controlled, a circuit independent of the oscillation circuit is used. Is added, the circuit scale of the crystal oscillation circuit becomes large.
In particular, in recent years, an oscillation circuit has been integrated into an IC, but adding a variable capacitance diode and its control circuit to such an integrated oscillation circuit requires space for the control circuit as the circuit scale increases. However, there is a problem in that the above-mentioned radio selective calling receiver becomes an obstacle in miniaturizing the radio selective calling receiver, and it becomes difficult to reduce the cost by the additional control circuit, so that the intended purpose cannot be achieved.

An object of the present invention is to reduce the cost while stabilizing the frequency-temperature characteristic, thereby enabling application to equipment requiring low cost such as a radio selective calling receiver. It is to provide a crystal oscillation circuit.

[0009]

SUMMARY OF THE INVENTION The present invention provides a transistor
And a crystal oscillator connected to the oscillation circuit, the oscillation circuit includes
With a self-biased emitter grounding circuit,
A base-emitter voltage of the transistor is applied;
Variable capacitor is connected to the variable capacitor.
The capacitance characteristic with respect to the temperature change of the sensor
Is set to cancel the oscillation frequency temperature characteristic of the oscillation circuit.
It is characterized by having. In this case, the crystal unit is connected between the collector and the base of the transistor.

In the present invention, utilizing the fact that the capacitance of the variable capacitance diode is given by the VBE of the transistor of the oscillation circuit, and that the temperature characteristic is given by the temperature characteristic of VBE and the temperature characteristic of the variable capacitance diode itself, The combined capacitance of the external circuit elements additionally connected to the oscillation circuit cancels out the temperature characteristics in the oscillation circuit that cause the temperature characteristics of the oscillation circuit, and the temperature characteristics of the oscillation frequency of the crystal oscillation circuit change the temperature characteristics of the crystal resonator. The characteristics depend only on.

[0011]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a first embodiment of the present invention, and particularly shows only a main part of an oscillation circuit. This crystal oscillation circuit is configured as a self-biased emitter grounded circuit, and the active circuit is stabilized by the negative feedback effect of the feedback resistance between the base and collector, and the oscillation frequency fluctuation with respect to the ambient temperature can be reduced. I am taking. This circuit is described in "Stable Collector Negative Feedback Crystal Oscillator" by Sakutoro Chiba (Transactions of the Institute of Electronics, Information and Communication Engineers, C-
II Vol.J73-C-II No.3 pp.143-153 March 1990), but here, low voltage operation (1V system etc.) is realized. Therefore, the configuration is different in that the emitter follower type circuit is a grounded emitter circuit.

Referring to FIG. 1, an oscillation circuit formed inside an IC comprises a base-collector having a load resistor R1 connected between the collector of an NPN transistor Q1 and a stabilized power supply VSTB, and a feedback resistor R2 also connected between the base and collector. It is an inter-resistance negative feedback type configuration. The load resistor R1 provides the collector current of the transistor Q1 and the gain of the oscillation circuit. The power supply VSTB is connected to the base of the transistor Q1 so that the collector current is constant with respect to the IC manufacturing conditions and temperature. A stabilizer configured to have the same temperature characteristics as the emitter-to-emitter voltage VBE is used. Further, the feedback resistor R2 performs a negative feedback between the base and the collector of the transistor Q1 to stabilize the transistor circuit.

On the other hand, a quartz oscillator XT is provided outside the IC.
L is connected between the base and the collector of the transistor Q1, and capacitors C1 and C2 for determining the oscillation frequency are connected to the collector and the base of the transistor Q1, respectively. Further, a variable capacitance diode CV is added to the base side of the transistor Q1. Therefore, the reverse voltage that determines the capacitance of the variable capacitance diode CV corresponds to VBE of the transistor Q1. As a result, the capacitance of the variable capacitance diode CV is automatically given by the VBE of the transistor Q1, and there is no need to add a circuit for performing an external control voltage or a feedback system.

The oscillation frequency of this crystal oscillation circuit is
An external combined capacitance including the capacitors C1 and C2 having fixed capacitances and the variable capacitance capacitor CV;
It is given by the internal capacitance of the oscillation circuit inside the IC. Since the capacity of the variable capacitor CV is controlled by the VBE of the transistor Q1, the constant of the capacitors C1 and C2 is set to an appropriate value in accordance with the change in the capacity. It can be set to 0. In this case, the capacitors C1 and C2 include a CG product (0 ± 30 ppm / ° C.) and a CH product (0 ± 60 ppm).
It is desirable to use a capacitance element having a temperature characteristic standard of 0 (ppm / ° C.).

Next, the operation of the crystal oscillation circuit of FIG. 1, particularly the operation for setting the frequency temperature characteristic of the oscillation circuit using the variable capacitance diode CV to zero will be described. First, the operation of the circuit without the variable capacitance diode CV will be considered. The oscillation frequency of the crystal oscillation circuit is determined by the load capacitance of the crystal oscillator XTL. The load capacitance CL is an effective capacitance as viewed from both ends of the crystal unit. (1)
As shown in the equation, the internal capacitance C of the oscillation circuit formed in the IC
I and an external combined capacitance C composed of elements connected outside the IC
This is the combined capacity with O. CL = CO + CI (1) Here, in the case of the circuit of FIG.
Is given by equation (2) from the capacitors C1 and C2. In the following expressions, C1 and C2 indicate capacitance values. CO = C1 × C2 / (C1 + C2) (2)

Incidentally, the crystal oscillation circuit can be equivalently regarded as an LC resonance circuit, and the resonance frequency f of the LC resonance circuit is generally expressed by the following equation (3). f = 1 / {2π · {(LC)} (3) Then, considering that the crystal unit XTL operates with L (inductance) property, the frequency of the crystal unit alone can be obtained from the overall frequency-temperature characteristics. The frequency temperature characteristics of the circuit excluding the temperature characteristics can be considered to be caused by the fact that CL changes with temperature. Temperature T1 → T2 (T1 <T
The change in capacitance of each of the capacitors in 2) is represented by ΔCL, ΔCO,
Assuming that ΔCI, there is a relationship of equation (4). ΔCL = ΔCO + ΔCI (4)

Here, a capacitor C as an external capacitance element
Since 1, 1 and C2 have a temperature coefficient of 0, ΔCO = 0, and therefore, the relationship of equation (5) is obtained. ΔCL = ΔCI (5) From this, it can be seen that the temperature characteristics of the load capacitance CL in the crystal oscillation circuit are caused by the temperature characteristics of the internal capacitance CI of the oscillation circuit. Therefore, by setting ΔCL = 0, the frequency temperature characteristic of the crystal oscillation circuit can be set to 0.
Equation (6) is obtained. ΔCO = −ΔCI (6) That is, in the case shown in FIG. 2A, since the internal capacitance of the oscillation circuit has a negative temperature coefficient, ΔCI <0.
It is understood that ΔCO> 0 should be satisfied in order to satisfy the expression (6).

Next, consider the case of the configuration shown in FIG. 1 in which a variable capacitance diode CV is connected between the base and the emitter of the transistor Q1. The variable capacitance diode C
The reverse voltage applied to V is VBE of transistor Q1.
It is. If the external combined capacitance at this time is CO ',
Equation (7) is obtained. CO ′ = C1 × (C2 + CV) / (C1 + C2 + CV) (7) Characteristics of the variable capacitor CV FIG.
(A) and (b) show. FIG. 3A shows the relationship between the capacitance with respect to the reverse voltage, and FIG. 3B shows the relationship between the capacitance with respect to the ambient temperature. FIG. 4 shows the temperature characteristics of VSTB and VBE. The temperature characteristic of VBE of the transistor Q1 corresponding to the reverse voltage applied to the variable capacitance diode CV is about −2 mV / ° C. As shown in FIG. 3A, the characteristics of the variable capacitance diode are such that the smaller the reverse voltage is, the larger the capacitance is, and as shown in FIG. 3B, the variable capacitance diode itself has a positive temperature coefficient. Therefore, in the crystal oscillation circuit of FIG. 1, the capacitance of the variable capacitance diode CV has a positive slope temperature characteristic due to a synergistic effect of the VBE of the transistor Q1 and its own temperature coefficient.

As an example, FIG. 5 shows capacitance-temperature characteristics of a variable capacitance diode at -10.degree. C., 25.degree. C., and 50.degree. The capacitance change ΔCV of the variable capacitance diode CV from T1 to T2 (T1 <T2) is as follows: ΔCV> 0 (8) The change amount ΔC of the external combined capacitance CO ′ expressed by the equation (7)
O 'is represented by the following equation if the capacitors C1 and C2 are elements having no temperature characteristic. ΔCO ′ = ΔCV × C1 ² / {(C1 + C2 + CV) (C1 + C2 + CV + ΔCV)} (9) From equation (8), ΔCO ′> 0 (10) The equation is: ΔCO ′ = − ΔCI (11) If the values of the capacitors C1 and C2 as external capacitance elements are set to constants satisfying the above equation, ΔCO ′ and ΔCI cancel each other, and the amount of change in the load capacitance CL ΔCL = 0.

As described above, the capacitance of the variable capacitance diode CV varies with the VBE of the transistor Q1 and its own temperature characteristic, and thereby the variation ΔCI of the internal capacitance of the oscillation circuit.
And the frequency temperature characteristic of the oscillation circuit is set to zero. FIG. 2B shows frequency temperature characteristics of the configuration of FIG.
From this, it can be seen that there is no influence of the frequency temperature characteristic in the oscillation circuit, and the frequency temperature characteristic is only the crystal resonator. Therefore, in this configuration, since the VBE of the transistor Q1 of the oscillation circuit itself operates to control the capacitance of the variable capacitance diode CV, no external control voltage or feedback system is required. As a result, it is not necessary to provide a circuit for controlling the variable capacitance diode CV separately from the oscillation circuit, and the circuit configuration can be simplified, and the temperature characteristics of the control circuit are designed and set in accordance with the frequency temperature characteristics of the oscillation circuit. This eliminates the need for an operation for performing the operation, and as a result, the crystal oscillation circuit can be configured at low cost.

Next, another embodiment of the present invention will be described. In each embodiment, the same reference numerals are given to portions equivalent to the components of the circuit shown in FIG. FIG.
Are the crystal oscillator XTL and the collector of the transistor Q1,
The buffer resistors R3 and R4 are interposed between the bases, and a trimmer capacitor CT is connected in parallel with the capacitor C1. In particular, the oscillation frequency can be adjusted by the trimmer capacitor CT.

FIG. 7 shows a configuration in which the load of the oscillation circuit is configured as an active load using a current mirror composed of a constant current source I and transistors Q2 and Q2, thereby stabilizing the frequency-temperature characteristics of the oscillation circuit. In addition, it is possible to further enhance the offset effect of the frequency temperature characteristic by the variable capacitance diode CV.

Further, FIG. 8 shows that the resistor R5 is inserted into the emitter of the transistor Q1 of the oscillation circuit, the control voltage of the variable capacitance diode CV is higher than that of FIG. 1, and the variable width of the variable capacitance diode CV is smaller. are doing. Although not shown, the variable width can be increased by connecting a plurality of variable capacitance diodes in parallel.

FIG. 9 shows a transistor Q1 'in which the emitter area of the transistor Q1 shown in FIG. 1 is increased as a transistor of the oscillation circuit to lower VBE, and the control voltage of the variable capacitance diode CV is compared with that of FIG. The temperature characteristics can be stabilized.

[0025]

As described above, the present invention provides a transistor
In a crystal oscillation circuit including an oscillation circuit including a star and a crystal resonator connected to the oscillation circuit, a circuit configuration for canceling the frequency temperature characteristics of the oscillation circuit, for example, by connecting a variable capacitor to the oscillation circuit By applying a base-emitter voltage of a transistor constituting the oscillation circuit to the variable capacitor,
The temperature characteristic of the oscillation frequency of the entire crystal oscillation circuit matches the frequency temperature characteristic of the crystal oscillator itself because the frequency temperature characteristic of the oscillation circuit becomes 0, and thus, the temperature characteristic of the crystal oscillation circuit depends on the frequency temperature characteristic. No characteristic deterioration
Higher frequency stability over temperature can be obtained. Therefore, there is no need to provide a circuit independent of the oscillation circuit for controlling the variable capacitance diode, and the circuit scale can be reduced and the cost can be reduced. In addition, since there is a margin with respect to the temperature characteristic standard of the oscillation frequency, it is possible to reduce the variation accuracy of the crystal unit,
The cost of the crystal oscillator can be reduced, and the cost of the crystal oscillation circuit can be further reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

FIG. 2 is a diagram showing oscillation frequency temperature characteristics of the conventional and the crystal oscillation circuits of the present invention.

FIG. 3 is a diagram showing a VR-CV characteristic of a variable capacitance diode and an ambient temperature-capacity characteristic.

FIG. 4 is a diagram showing VSTB and VBE voltage-temperature characteristics.

FIG. 5 is a diagram showing capacitance characteristics of the variable capacitance diode of the present invention.

FIG. 6 is a circuit diagram according to a second embodiment of the present invention.

FIG. 7 is a circuit diagram of a third embodiment of the present invention.

FIG. 8 is a circuit diagram according to a fourth embodiment of the present invention.

FIG. 9 is a circuit diagram according to a fifth embodiment of the present invention.

FIG. 10 is a circuit diagram showing a part of a receiving circuit for radio selective call reception of a double superheterodyne system using a crystal oscillation circuit.

[Explanation of symbols]

 Q1 to Q3 Transistor XTL Quartz resonator CV Variable capacitance diode C1, C2 Capacitor R1 to R5 Resistance

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03B 5/30-5/42

Claims (3)

(57) [Claims] 1. A crystal oscillation circuit comprising: an oscillation circuit including a transistor; and a crystal resonator connected to the oscillation circuit, wherein the oscillation circuit is a self-biased grounded emitter.
Circuit and the base of the transistor
-A variable capacitor to which the
Key to the temperature change of the variable capacitor.
Capacitance characteristics depend on the oscillation frequency temperature characteristics of the oscillation circuit.
A crystal oscillation circuit characterized in that the crystal oscillation circuit is set so as to cancel the characteristics. 2. A crystal oscillator circuit according to claim 1, wherein the crystal oscillator between the collector and the base of said transistor is connected. 3. The oscillation circuit is configured as an internal circuit of an IC, and the crystal unit and the variable capacitor are connected to the IC.
Claim constructed as an external circuit element to be connected to C 1
Or the crystal oscillation circuit according to 2.