JPH03226103A - Digital temperature compensated oscillator - Google Patents

Abstract

PURPOSE:To keep a prescribed frequency offset quantity in a prescribed temperature range by expanding a theoretical value of a control voltage into a series and calculating a 1st order term for the initial value of the control voltage and the temperature and a 2nd order term for an offset to control the oscillating frequency. CONSTITUTION:A control voltage Vc is expressed in equation I in a digital temperature compensation oscillator, in which a detected output from a temperature sensor 11 is digitally converted, a corresponding data is read from storage elements 13, 14 storing a temperature compensation data in advance and converted into an analog data as a control voltage, which is applied to a voltage capacity conversion element controlling the frequency of a crystal oscillator 16 for the temperature compensation, where K00, K01, N10 are constants Vco is an initial value of the control voltage, T is a temperature and F is a frequency offset quantity. Thus a prescribed frequency offset quantity is maintained over a wide temperature range regardless of the setting value of the frequency compensation voltage and the temperature compensation characteristic is not affected by the frequency compensation voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a digital temperature-compensated oscillator that digitally compensates for changes in oscillation frequency due to temperature changes, and is particularly concerned with improving analog-to-digital converters.

(Technical background of the invention and its problems) Recently, crystal oscillators have been widely used as standards for frequency, time, etc. Incidentally, a crystal resonator used in a crystal oscillator generally has a temperature coefficient, and its frequency changes with changes in temperature. For example, a typical AT-cut crystal oscillator used at a frequency of several MHz to about 10-odd MHz,
It exhibits a temperature coefficient in the shape of a substantially cubic curve, and its characteristics change minutely depending on the cutting angle, with an inflection point around 25°C.

On the other hand, as electronic equipment becomes more precise, crystal oscillators are also required to have more stable oscillation frequencies.

A crystal oscillator that satisfies these requirements is one that is housed in a thermostatic oven. However, those that use a constant temperature oven are larger in size, consume more power to heat the inside of the oven to a constant temperature, take time to stabilize the frequency when the power is turned on, and have parts that are heated to around 70°C. Reliability is also an issue due to exposure to relatively high temperatures.

In addition, there are devices that connect a temperature detection element such as a thermistor to a crystal resonator and perform temperature compensation by changing the reactance of the element. However, in such a device, the frequency stability is 10 times worse than that in the above-mentioned device using a constant temperature bath.

For this purpose, a digital temperature compensated oscillator having a configuration as shown in FIG. 4, for example, is known.

In this oscillator, the detection output of the temperature sensor is applied to a compensation voltage generation circuit 2 to generate a temperature compensation voltage Vco(T). Then, the temperature compensation voltage Vco (T) is connected to the frequency adjustment terminal 3.
By adding the frequency compensation voltage Vf given to the control voltage Vc
Obtain (T).

The frequency compensation voltage Vf is used to finely adjust and offset the oscillation frequency in order to compensate for deviations in the actual oscillation frequency from the reference frequency due to aging or the like.

Therefore, the temperature compensation voltage ■ co (T) is the resistance 4 (resistance (l
The frequency compensation voltage is applied via the resistor 5 (resistance value R
2) to obtain the control voltage Vc(T).

Then, the control voltage Vc (T) is applied to, for example, a varicap diode 7 inserted in series with the crystal oscillator 6 of a Colpitts-type crystal oscillator to finely vary the oscillation frequency and maintain a constant oscillation frequency. I have to.

Therefore, the control voltage Vc(T) is given by the following equation 0.

Vc (T)=AXVco (T)+BXVf...■
However, A=R2/ (R1+R2) B=R1/ (R1+R2) However, in general, in such an oscillation circuit, the amount of change in frequency with respect to the applied voltage of the varicap is non-linear, as shown in the graph shown in Figure 5. Become. For example, the frequency offset amount ΔFl when the control voltage Vcl is changed by a minute voltage ΔVc and the control voltage Vc2
The frequency offset amount ΔF2 is different from the frequency offset amount ΔF2 when only the minute voltage ΔVc is changed. Also, the control voltage
The amount of change in frequency when C is changed by a certain value is also affected by temperature.

For this reason, even if the control voltage Vc is finely adjusted so that the oscillation frequency has a constant offset ΔF at room temperature, the frequency offset ΔF cannot be maintained at a constant value over a wide temperature range.

Figure 6 shows a crystal oscillator as shown in Figure 4 at 70°C.
OPPM, +2PPM, -2PPM and +4 at temperatures of
This is a graph showing the results of actually measuring the ratio ΔF/F of the frequency offset ΔF from the reference frequency F when changing the temperature from -20℃ to +70℃ after setting the frequency offset ΔF of PPM and -1PPM, respectively. be.

As is clear from this result, it is not possible to maintain a constant frequency offset ΔF because the frequency offset ΔF that has been set changes in response to changes in temperature, and the offset amount does not vary particularly in the low temperature region. growing.

Furthermore, since varying the frequency compensation voltage Vf affects the compensation characteristic itself by the temperature compensation voltage Vco(T), there is a problem that accurate temperature compensation cannot be performed.

(Purpose of the Invention) The present invention has been made in view of the above circumstances, and is capable of maintaining a constant frequency offset amount in a wide temperature range regardless of the set value of the frequency compensation voltage. It is an object of the present invention to provide a digital temperature compensated oscillator whose temperature compensation characteristics are not affected.

(Summary of the Invention) The present invention digitally converts the detected value of a temperature sensor that detects temperature, selects the address of a storage element, reads out pre-stored temperature compensation data, and converts this data into an analog control voltage. In a crystal oscillator circuit whose frequency is controlled by applying it to a voltage-capacitance conversion element, the theoretical value of the control voltage described above is expanded into a series, and the initial value and temperature of the control voltage are expressed as a first-order term, and the offset amount is a second-order term. This feature is characterized in that the oscillation frequency is controlled by calculating the above.

(Embodiment) Hereinafter, an embodiment of the present invention will be described in detail with reference to a block diagram of a digital temperature compensated oscillator shown in FIG.

In the figure, 11 is a temperature sensor, for example, a thermistor whose resistance value changes depending on the temperature. Then, this change in the resistance value of the temperature sensor 11 is given to the arithmetic circuit 12 as a detection signal. The arithmetic circuit 12 is further provided with compensation data regarding the offset ΔF and the initial value VCO of the control voltage actually measured by changing the temperature at every predetermined temperature step from the first and second digital storage elements 13.14. .

The arithmetic circuit 12 then performs a predetermined digital arithmetic operation, which will be described later, and provides the arithmetic result to the digital-to-analog converter 15.

Then, the analog conversion output of the digital-to-analog converter 15 is applied as a control voltage Vc to a voltage capacitance conversion element, such as a varicap diode, which controls the oscillation frequency of the crystal oscillator, thereby varying its capacitance. This voltage-capacitance conversion element is connected in series with the crystal resonator of a Colpitts-type crystal oscillator, for example, as shown in FIG. 4, and finely changes the oscillation frequency by varying its capacitance.

The arithmetic circuit 12 is, for example, a central processing unit (CPU), and its arithmetic operations are performed as follows.

That is, in such a temperature compensated oscillator, if the amount of minute change in control voltage Vc required to change the oscillation frequency by minute frequency df is dVc, then dV c /
df is not constant but is given as a function of control voltage Vc and temperature T by the following equation 0.

dVc/df=f(■c, T)...If the 00 formula is transformed, the following 0 formula is obtained.

dVc/f (Vc, T) = df...■Convert this formula 0 to Δ from the state before frequency adjustment (Vc=Vco, △F=0)
Integrating to the state where the frequency is offset by F results in the following 0 formula.

The left side of this equation 0 is the number between the control voltage Vc and the temperature T, and when solved for Vc, Vc becomes the number between Vco, T, and ΔF, which is generally given by the following equation 0.

Vc=Vco+ΣΣakjiV co'T'ΔF'-
-■ However, if the second term on the right side of equation 0 is added to the control voltage, a constant frequency offset ΔF can be maintained regardless of temperature changes even if the frequency offset amount is changed.

However, performing temperature compensation strictly based on Equation 0 requires a huge amount of calculation, which is unrealistic.

Therefore, in order to determine the interval number f (Vc, T) of equation 0, the control voltage V for controlling the oscillation frequency fO to be constant is determined.
c is measured over the entire temperature range to be temperature compensated, and then a control voltage V c for maintaining the oscillation frequency fO at a frequency fO + df offset by a certain minute frequency df.
+ d V c was similarly measured over the entire temperature range.

FIG. 2 is a graph showing an example of the results of this measurement.

From this result, ±0. When the frequency stability of IPPM is targeted, it has been experimentally confirmed that the above equation 0 can obtain necessary and sufficient accuracy with a first-order approximation for the control voltage Vc and temperature T. In this case, dVc/df is expressed as the following 0. It is given by Eq.

dVc/df=10Vc+KO1-T to K00+
・■ However, K001KIO5KOI is a constant, so the control voltage Vc is given by the following equation from equation 0.

V c = (V c o +(K10+KO1-T)
/KIO) Xexp(KlO゛△F)−(K10+
KO1-T)/KIO...■ If the frequency offset ΔF is not too large, +
i<io to 8+<<1 and exp(KIO
For ΔF), sufficient accuracy for practical use can be obtained using the following 0 formula, which is expanded to the second-order term by the tiller.

exp (KIOThF)=1+KIO△F+1/2
(KIOΔF)2·■ Therefore, by substituting the 0 formula into the above 0 formula, the following 0 formula is obtained.

V c =V co + (K00+V co・K
lO+KO1-T) X(△F+△F2×K10/2)
...■Then, the control voltage Vc obtained by calculating this equation 0 is applied to a digital-to-analog converter for analog conversion, and then applied to the voltage capacitance conversion element to perform temperature compensation.

(Concrete example) A specific example will be explained below.

As a result of actually measuring the control voltage Vc for maintaining a constant frequency offset value in a predetermined temperature range in an oscillator with an oscillation frequency of 12.8 MHz, the constants in Equation 0 were as follows.

K00=3.481X10-3 (V/Hz)KIO=
2.627X10-3 (1/Hz)KO1=8.76
6X10-5 (V/Hz/℃) and set this constant to 0
The standard error between the value calculated by substituting it into the formula and the actual value is 7.4
8×10 −5 (V/Hz), which was a sufficient approximation for practical use. Figure 3 shows a temperature compensated oscillator that has been temperature compensated by such calculations, and is operated OP at room temperature.
PM, +2PPM, -2PPM and +4PPM, -4
After setting the PPM frequency offset ΔF,
It is a graph showing the results of actually measuring the oscillation frequency when the temperature was changed from -20°C to +70°C. As is clear from these results, a constant amount of offset could be maintained in a predetermined temperature range, and the temperature compensation characteristics were not impaired.

(Effects of the Invention) As detailed above, according to the present invention, a constant frequency offset amount can be maintained in a predetermined temperature range,
Furthermore, it is possible to provide a digital temperature compensated oscillator whose temperature compensation characteristics are not affected by the value of the frequency offset.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of the digital temperature compensated oscillator of the present invention, Fig. 2 is a graph showing actual measurement results with respect to temperature changes in the control voltage required to obtain a constant frequency offset, and Fig. 3 is the invention of the present invention. Figure 4 is a block diagram showing an example of a conventional temperature compensated oscillator, and Figure 5 is a graph showing changes in oscillation frequency with respect to temperature changes in a conventional temperature compensated oscillator. Graph showing changes. FIG. 6 is a graph showing changes in oscillation frequency with respect to temperature changes when a constant offset voltage is applied in the temperature compensated oscillator shown in FIG. 11...Temperature sensor 12...Engraving of calculation circuit V side (no change in content) Fig. 2 over LT→ Fig. 3 Fig. 4 Fig. 5 Fig. 6;! ITjiC] Procedural amendment (method) % formula %) 1. Indication of the case Japanese Patent Application No. 2-20970 2. Title of the invention Digital temperature compensated oscillator 3. Relationship with the person making the amendment Patent applicant address Tokyo Nippon Den, 1-21-2 Nishihara, Shibuya-ku, Miyako
Nami Kogyo Co., Ltd. Procedural Amendment January 10, 1991 1゜2゜3゜Indication of the case Japanese Patent Application No. 2-20970 Title of the invention Relationship with the digital temperature compensation oscillator correction case Address of the patent applicant Nippon Den, 1-21-2 Nishihara, Shibuya-ku, Tokyo
Nami Kogyo Jihatsu Co., Ltd. 5, Specification subject to amendment・3・S≧＼6, Vc= (Vco+ (K10+KO14 ) /KI
O) Xexp (K10・ΔF) = (K10+K
O1-T) /KIO...■ A certain thing V c = (V co + (K00+KO14)
/ KIO) XeX1) (KIO−ΔF) −(K
00+KO1-T) /KIO...■ Correct.

Claims (1)

[Claims] A voltage-capacitance conversion element that converts the detection output of a temperature sensor into digital, reads out the corresponding data from a storage element that stores temperature compensation data in advance, and controls the frequency of a crystal oscillator using a control voltage that is converted into analog. In the case where temperature compensation is performed by applying the above control voltage (Vc) to Vc=Vco+(K00+Vco・K10+K01・T
)×(ΔF+ΔF^2×K10/2) where K00, K01, and K10 are constants Vco is an initial value T of a control voltage, and temperature ΔF is a frequency offset amount.