JPS641967B2 - - Google Patents

Description

[Detailed description of the invention]

FIG. 1 shows the simplest equivalent circuit of a crystal oscillator (the crystal oscillator is shown by a symbol). Y is a crystal resonator, γ is the capacitance ratio of the crystal resonator Y, and C _O is the interelectrode capacitance. C _L is the capacitance of the entire circuit excluding the crystal resonator, and is called the load capacitance. Note that information related to loop gain that is unrelated to the oscillation frequency is omitted. The oscillation frequency in this circuit is determined by the deviation amount Δ(=- _S )/from the series resonant frequency _S of the crystal resonator.
As is well known, the equation for _S is expressed as equation (1). Δ/ _S = C _O /2γ (C _O + C _L ) ...(1) To perform temperature compensation, the load capacitance C _L in this equation should be changed according to the temperature. Figure 2 shows a thermistor instead of C _L in Figure 1.
This circuit is known for low-temperature compensation and includes a parallel connection of _{RP and} capacitor _CP . The resulting change in the equivalent load capacity C _L is used for high temperature compensation. Thermistor R _P and capacitor C _P in Figure 2
If we equivalently convert the parallel circuit into a series circuit as shown in Figure 3 to obtain a resistor R _S and a capacitor C _S , then R _S = R _P /1 + R _P ² ω ² C _P ² …(2) C _S =1+R _P ² ω ² C _P ² /R _P ² ω ² C _P (3). In this case, it is not directly related to the oscillation frequency.
We ignore R _S and focus only on C _S. The value of the resistance R _P of the thermistor at a certain arbitrary temperature T is R _P(T) ,
Let the value of R _P at the reference temperature T _O be R _P(O) , and this temperature T
_The value C _{S (T) of} ^the _{series capacitance} C _{S at )} ) ² +1}C _P ...
(4) becomes. TawasiX _C = 1/ωC _P. On the other hand, by modifying equation (1), and assuming that the deviation amount at temperature T is Δ _(T) / _S and the load capacity is C _{L (T)} , C _{L (T)} = (1/2γ・Δ _(T) / _S - 1) C _O …(5) becomes. Furthermore, the deviation amount of the reference temperature T _O is calculated as Δ _(O) / _S
= C _O /2γ (C _O +C _L(O) , load capacity is C _L(O) , temperature T
Letting the compensation frequency amount M _(T) be the difference between the deviation amount at and the deviation amount at the reference temperature T _O , then M _(T) = Δ _(T) / _S - Δ _(O) / _S Δ _(T) / _S = M _(T) +Δ _(O) / _S = M _(T) +C _O /2γ
(C _O +C _L(O) ) ...(6) The right side of equation (6) does not include _S. Next, substituting equation (6) into equation (5), we get C _L(T) = {C _O +C _L(O) /2γM _(T) (C _O +C _L(O) +C _O −1}C _O
...(7), but this equation (7) is the load capacitance (compensation capacitance) C _L(T) required to obtain the compensation frequency amount M _(T) at the temperature T.
This is the formula to find. If the compensation capacitances of the crystal resonator required at temperatures T ₁ and T ₂ are determined as C _L(T1) and C _L(T2) , respectively, using equation (7), then
Since C _L(T) = C _S(T) , the following equation holds true from equation (4). C _L(T1) = (b ² / β ₁ ² + 1) C _P … (8) C _{L (T2)} = b ² / β ₂ ² + 1) C _P … (9) (Here, b = X _C / R _P(O) , β _(T) = R _P(O) ) By solving equations (8) and (9) for C _P and b, the required element value R _P(O)・C _P is as follows. It is obtained as follows. ∴C _P =C _L(T2) β ₂ ² -C _L(T1) β ₁ ² /β ₂ ² -β ₁ ² …(11) ∴R _P(O) =X _C /b=1/ωC _P b …(12) Note that the load capacity C _L(O) at the reference temperature T ₀ is R _P(O)
And from C _P , paying attention to (R _P(O) /R _P(O) ) ² = 1, we get C _S(O) = (X _C ² /R _P(O) ² +1) C _P … (13 ) becomes. Now, when verifying the possibility of compensation over each temperature using the above calculation formula, the temperature compensation circuit shown in Figure 2 will be used in the high temperature region (lower than T _{0 )} of the temperature characteristics of the crystal resonator shown in Figure 4. It is known that it is particularly suitable for compensating for high-temperature areas. Furthermore, during third-order overtone oscillation, the compensation effect is greater if the load capacitance C _L(O) at the reference temperature T ₀ is selected to be 100 pF or less. The reason for this is that in equation (1)
This is because when C _L exceeds 100 PF, Δ/ _S hardly changes as is clear from FIG. 5, and the compensation capacity necessary to obtain the compensation frequency amount cannot be obtained. FIG. 5 is a graph calculated by inserting typical values γ=2500 and C _O =5PF for a third-order overtone oscillator into equation (1). Next, specific examples of the present invention will be described. In general, when the γ and C _O of a crystal resonator change due to manufacturing variations, or when the temperature characteristics vary depending on the cut angle of the diaphragm as shown in Figure 6, R _P , C A compensation circuit must be designed each time _P is selected. Therefore, in order to solve this problem, the present invention may be modified as follows. That is, FIG. 7 shows a temperature compensation circuit showing a modified embodiment of the present invention. The feature of this circuit is that instead of designing and selecting R _P and C _P each time, the thermistor R _P
In order to _{adjust the}
It is located where you inserted the . It goes without saying that this resistance R _C can be replaced by the equivalent resistance of a semiconductor device such as a variable resistor or a PIN diode. The series capacitance value C _{S (T)} of the compensation circuit at this time is obtained by the following equation (in the same way as formula (4) was derived) by setting R _C /R _{P (O)} = a. C _S(T) = {b ² / (a + β _(T) ) ² + 1} C _P … (14) Tadashi (b = X _C /R _P(O) , X _C = 1/ωC _P , β _{( T)} =
R _P(T) /R _P(O) ) Therefore, b and which are appropriately designed and selected in advance
Any desired C _S(T) by using C _P and adding R _C to it
The value of a obtained from equation (14) is found to be β _(T)
>0, b>0, so Therefore, the value of R _C for sensitivity adjustment is determined by this equation (15) within a range that satisfies the conditions of C _{S (T)} C _P and a>0. The practical way to use this R _C is to substitute a variable resistor for this R _C and adjust the variable resistor so that the oscillation frequency at a temperature T ₂ matches the oscillation frequency at a reference temperature T ₀ . It's both reliable and easy. Next, adjust the sensitivity of this thermistor R _P by using a resistor R _C
If this is done, the deviation in the oscillation frequency of the reference temperature before and after that is calculated as follows. First, the series capacitance value at the reference temperature in equation (14)
C _S(O) is β _(T) =
The calculation is performed paying attention to R _P(O) /R _P(O) = 1, and it is found that C _S(O) = {b ² /(a+1) ² +1}C _P (16). Using this equation (16),
Practical value C _P = 18.55PF, b = 0.28
When calculating the value of C _S(O) , if a=0.1, C _S(O) =
It becomes 19.75PF. Also, when a=0 (when R _C is short-circuited), C _S(O) = (calculated using equation (13))
It will be 20PF. Furthermore, with these C _S(O) values, γ is a typical constant example of a third-order overtone crystal resonator.
= 2500, C _O = 5PF, calculate equation (1) with this, and compare the respective values.
It is known that the fluctuation rate of the oscillation frequency when changing from a=0.1 is +0.4×10 ^-6 , which is very small and can be almost ignored. This is one of the effects of the present invention, and the effect is particularly remarkable under conditions where C _S(O) is 100 pF or less. FIG. 8 shows a graph of temperature compensation performed by the method of the present invention. The frequency is 50MHz, and the B constant of the thermistor is approximately
3000〓, the resistance value at 25℃ is 613Ω. Table 1 shows the crystal oscillator's γ, C _O , and the seventh
The values of each element of the compensation circuit shown in the figure are listed. Table 2 shows the amount of compensation frequency M _(T) at each temperature obtained with the configuration of the elements shown in Table 1.

【table】

[Table] Now, when looking at the graph in Figure 8 and M _(T) in Table 2, the "sensitivity" of the thermistor R _P is expressed as the resistance inserted in series.
It can be seen from R _C that even if the value of a is changed from 0 to 0.1, the effect hardly appears at low temperatures below 25° C. in the temperature-frequency characteristic. (This is also one of the effects of the present invention.) Furthermore, another effect of the present invention is that, as described above, it is easy to correct variations in the characteristics of the crystal resonator. That is _, when performing high temperature compensation, each constant of _the compensation circuit shown in FIG.
Considering the case where the element values R _p and C _p are designed and selected according to equations (10) to (12), the range of the cut angle of the crystal resonator that can be compensated by these R _p and C _p is the temperature that is required for the time being. For example, when the permissible value of frequency characteristics is ±2×10 ^-6 at 10°C to 70°C, the cutting angle deviation of the crystal resonator of the AT plate must be within ±15 seconds. This is an extremely severe production condition, and is not economically viable. Furthermore, since variations inevitably occur in the γ and _CO of the crystal resonator during the manufacturing process, it is necessary to take into account the effects of these variations. The variation between these two values is ±10% from the specified value (described later).
Calculating the variation in the amount of compensation frequency M _(T) using equations (7) and (1) when using a material that has been carefully selected to fall within the range, the amount of compensation frequency M _(T) required at 70°C is When we set M ₍₇₀₎ = −17×10 ^-6 ,
Assuming that the values and variations of γ and C _O of the crystal oscillator are γ = 2500 ± 10% and C _O = 5PF ± 10%, respectively, and if C _{L (O)} = 20PF, then γ = 2250 ,
At the worst value of C _O = 5.5PF, M ₍₇₀₎ = -20 x 10 ^-6 , which is difficult to keep within the allowable value. The remarkable feature of the compensation circuit of the present invention, especially the compensation circuit in which an adjustment resistor is further inserted into the thermistor, is that these drawbacks can be solved all at once.
By using the resistor R _C , variations in γ and C _O of the crystal resonator and variations in the cut angle can be easily corrected. Also, regarding the range of the correction, the above-mentioned a is a≒∞, that is, from the uncompensated part to a≒
It has the feature that it can be changed continuously up to 0 (that is, the compensation circuit part in FIG. 2). Figure 9 shows this correction range with diagonal lines.As long as the temperature characteristics of the crystal resonator are within this diagonal line, the frequency at any one temperature point within the diagonal line can be adjusted by changing the value of resistor R _C. The amount of deviation can be made zero. Next, stabilization of the output level of the device of the present invention will be explained. FIG. 11 is a specific circuit diagram of the present invention. By shorting resistor _R3 , this circuit becomes a typical collector-grounded Kolbitz circuit. When the oscillation output is taken out from the output terminal OUTB via the transformer T inserted in the emitter circuit of the transistor TR, as is conventionally done in this circuit, the change in the series resistance value R _S of the compensation circuit described above is This is not preferable because it directly affects the oscillation level.
That is, when calculating the R _S value using equation (2), at around 70°C, taking b≒0.28 shown in Figure 8 as an example, R _P(O)
This means that a resistance value approximately twice that of 1 is inserted, and as a result, the output level fluctuates greatly as shown by curve B in FIG. However, when a resistor _R3 is inserted between the collector of the transistor TR and the ground as shown in Fig. 11, and the oscillation output is taken out from the collector side via the capacitor C5 from the output terminal OUTA,
The fluctuation in the output level due to the change in R _S described above is almost eliminated, and the output level becomes extremely stable as shown by curve A in FIG. 10. This is because this output utilizes the saturation effect of the collector output when the base of the transistor is over-input. It is considered that the base input in the oscillation circuit of FIG. 11 during steady oscillation is an excessive input. As mentioned above, this invention has the feature of strongly improving the temperature-frequency characteristics of a crystal oscillator, especially in the high-temperature part, by using an extremely simple circuit configuration, and its effects are remarkable, making it an extremely useful invention industrially. It should be said. Note that the variable resistor R _C of the present invention can be configured with a semiconductor resistance element such as a diode or a transistor. The resistance values of these resistance elements at this time can be changed by the value of the DC current (flowing between the base and emitter of the diode or transistor) or the value of the DC voltage (applied to the gate of the FET). (The diode is
Achieve good results using the PIN type. ) The control of compensation for the frequency-temperature characteristics of a crystal oscillator using such DC current and voltage is of interest because it suggests the possibility of adding active temperature compensation.

[Brief explanation of drawings]

Figure 1 is an equivalent circuit of a crystal oscillator, Figure 2 is an equivalent circuit of a conventional low temperature compensation oscillator, Figure 3 is a circuit equivalently converted into a series version of Figure 2, and Figure 5 is a crystal oscillator load capacitance and oscillation. Graph showing the relationship between frequencies, 4th
The figure is a graph showing an example of the temperature characteristics of a crystal resonator.
Fig. 6 is a graph showing variations in temperature characteristics of a crystal resonator, Fig. 7 is a diagram of a temperature compensation circuit showing a modified embodiment of the present invention, Fig. 8 is a graph of actual measurements in which the present invention is implemented, Fig. 9 10 is a graph showing the correction range of the compensation circuit of the present invention, FIG. 10 is an actual graph showing the temperature characteristics of the oscillation output, and FIG. 11 is a circuit diagram of a specific embodiment of the present invention. A specific circuit diagram is shown below. Y...Crystal resonator, R _P ...Thermistor (and its resistance value), C _P ...Capacitor, R _C ...Variable resistor,
C _L ...Load capacity.

Claims (1)

[Claims] 1. A crystal oscillator including a crystal resonator and an amplifier, in which a temperature compensation circuit configured by connecting a thermistor with negative temperature characteristics and a capacitor in parallel is inserted in series with the crystal resonator. In the temperature-compensated crystal oscillator, the capacitor and the capacitor are arranged so that the amount of change in the ratio between the capacitor value and the equivalent series capacitance value of the compensation circuit with respect to temperature changes is small at room temperature or below, but becomes large at high temperatures. A temperature compensated crystal oscillator characterized by setting the value of a thermistor. 2. Claim 1, characterized in that a resistor is inserted in series with the thermistor of the temperature compensation circuit.
Temperature-compensated crystal oscillator as described in section. 3. The temperature compensated crystal oscillator according to claim 2, wherein the resistor inserted in series with the thermistor is a variable resistor. 4. The temperature compensated crystal oscillator according to claim 2, wherein the resistor is a semiconductor resistance element whose equivalent resistance value changes depending on direct current or voltage. 5. The temperature compensated crystal oscillator according to claim 4, wherein the semiconductor resistance element is a diode. 6. The temperature compensated crystal oscillator according to claim 4, wherein the semiconductor resistance element is a transistor. 7. In the crystal oscillator which uses a transistor as the amplifier and whose collector is equivalently grounded, a resistor is inserted between the collector and the ground point, and the oscillation output is derived from the collector. A temperature compensated crystal oscillator according to any one of claims 1 to 6.