JPS6276801A - Digital temperature compensation crystal oscillator - Google Patents

Abstract

PURPOSE:To improve the C/N and to make the titled oscillator suitable for large scale circuit integration by allowing each charge/discharge circuit to output an analog signal while the impedance of each transistor (TR) switch element is changed timewise consecutively. CONSTITUTION:Charge/discharge circuits 28-31 each consist of a capacitor 40 and a resistor 41, a terminal 38 is connected to a serial/parallel converter 19 and a terminal 39 is connected to a gate of switch elements 24-27. Since only a gate of a MOS TR switch element is connected to the terminal 39 and the impedance of the gate is very high, it is regarded that a nearly infinite impedance is connected. That is, the charge/discharge circuits 28-31 constituting LPFs are inserted between the serial/parallel converter 19 and the switch elements 24-27 to suppress the sudden change in the oscillated frequency. Thus, the deterioration of the C/N is prevented, the oscillator is suitable for large scale circuit integration and an oscillation output with high purity is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a temperature compensated crystal oscillator having a digital temperature compensation circuit, and more particularly to a digital temperature compensated crystal oscillator used for communication devices.

[Conventional technology]

As a digital temperature-compensated crystal oscillator, one shown in FIG. 2 has conventionally been used. In FIG. 2, the temperature detection circuit 10 functions as a temperature sensor that outputs a voltage according to the temperature, and the detected output is sent to the A-D converter 11.
is configured to output to. A-D converter 1
1 converts the output voltage of the temperature detection circuit 10 into digital data. This data is then stored in the storage circuit 12. Memory data for adjusting the oscillation frequency is written in advance in the memory circuit 12, and when the output of the A-D converter 11 is supplied to the memory circuit 12, an address according to this output is specified, and the memory circuit 12 is Specific data from among the stored data is read out and output to the DA converter 13. Then, in the DA converter 13, it is converted into an analog voltage according to the read data. The output of the DA converter 13 is supplied directly to the variable capacitance diode 17 via an LPF (low pass filter) 14 or without passing through the LPF 14 . Capacitor 1 is connected to variable capacitance diode 17.
8 are connected in series, and a negative resistance generating circuit 15 is also connected via a crystal resonator 16, and these elements form a crystal oscillation circuit.

When the output of LPF+4 is supplied to variable capacitance diode 17, the capacitance of variable capacitance diode 17 changes according to the output voltage of LPF 14, and the frequency of the oscillation circuit is adjusted.

That is, by outputting a signal corresponding to the temperature to the variable capacitance diode 17, the frequency of the crystal oscillation circuit can be adjusted according to the temperature, and the oscillation frequency can be stabilized.

Here, the reason why the LPF 14 is inserted into the output of the DA converter 13 will be explained.

That is, since the output voltage of the DA converter 13 is output as a step waveform, if this signal is directly applied to the variable capacitance diode 17, the oscillation frequency f repeats steep changes as shown in FIG. Therefore, the oscillator exhibits a state equivalent to being modulated by pulsed noise, and as a result, the purity (C/N ratio) of the oscillator output deteriorates. Therefore, an LPF is applied to the output of the D-Δ converter 13.
]4 is inserted to suppress steep frequency fluctuations. The LPF 14 has a resistor 4 as shown in FIG.
7.48, has capacitor 49, LPF14 is D-A
When inserted into the output of the converter 13, the oscillation frequency control characteristic becomes as shown by the solid line in FIG.

Note that the dotted line in FIG. 6 shows the control characteristics (same as in FIG. 5) without the LPF 14. Like this LP
By inserting F14, steep frequency fluctuations can be suppressed. This represents a state in which the high frequency component of the modulated wave in frequency modulation is reduced, resulting in a decrease in the C/N ratio. Digital temperature compensated crystal oscillators for communications equipment are often required to have not only stability but also a good C/N ratio.
F is used.

It is clear that the configuration shown in FIG. 2 forms a kind of open-loop control system, and the variable capacitance diode 17 is controlled one by one based on stored data that has been set in advance so that it can be controlled within a certain control error. Frequency stabilization is achieved by changing the capacitance. The means for generating data to be managed with respect to temperature is the temperature detection circuit 10. shown in FIG. 8-D
There are many methods other than using converter 11 and storage circuit 12. For example, analog systems are more commonly used than before. Despite this, the reason why digital temperature-compensated crystal oscillators are now in demand is thought to be due to their improved mass production and economic efficiency. That is, this is because a means for producing with high precision and with little individual variation can be easily and economically realized using a digital circuit configuration.

Although the digital temperature-compensated crystal oscillator shown in FIG. 2 can improve cost performance over the conventional analog type, a digital-type temperature-compensated crystal oscillator has been proposed to further increase the effect. [[Digital TC
``Position configuration method of X○'' (1981 IEICE General Conference, Document No.

630]. This digital temperature compensated crystal oscillator is shown in FIG. This oscillator is not shown in FIG. 23 respectively MOS) transistor (FET) switch element 2
4 to 27 are arranged.

In FIG. 3, the ratio of capacitance values of capacitor arrays 2o to 23 is determined to be, for example, 1:2:4:8. Further, the output of the storage circuit 12 is supplied to a serial/parallel converter 19, and the output of the serial/parallel converter 19 is configured to turn on/off the MOS transistor switching elements 24 to 27. Note that when the output of the memory circuit 12 is a parallel output, the serial-to-parallel converter 19 is not necessary.

In the case of the oscillator shown in FIG.
The oscillation frequency can be adjusted by connecting the capacitor arrays 2o to 23 in parallel with the capacitor 18 by turning on and off the capacitors 7, and since the variable capacitance diode 17 is not required, the capacitor arrays 20 to 23 are suitable for LSI integration.
Since the switching elements 24 to 27 can be realized with a chip area comparable to that of the D-A converter 13 shown in FIG. 21, it is possible to achieve miniaturization and cost reduction.

[Problem that the invention seeks to solve]

However, in the case of the oscillator shown in FIG. 3, the oscillation frequency is adjusted by turning on and off the switch elements 24 to 27, so the oscillation frequency becomes as shown in FIG. 5, and the C/N ratio is reduced. The problem was that it got worse. That is, due to the influence of the transient changes when the switching elements 24 to 27 are turned on and off, the amount of instantaneous capacitance fluctuation when viewed from the crystal resonator 16 side is magnified compared to the value of a normal stepwise change.

In particular, the effect is large when one of the capacitor arrays 20 to 23 having a large capacity is intermittent. As a result, sharp pulse-like changes as shown in the frequency control characteristics of FIG. 5 also appear.

This kind of impulse-like frequency change occurs because
This is equivalent to frequency modulation by an impulsive noise voltage, and indicates that frequency fluctuation components exist widely from low frequencies to high frequencies.

This kind of phenomenon also occurs to a greater or lesser extent in the model shown in Figure 2, but in the model shown in Figure 2, the combined fluctuation components of both the stepwise frequency change and the impulse frequency change are processed by the LPF 14. LPF14
The C/N ratio is controlled to prevent changes in the C/N ratio. However, in the case of the oscillator shown in FIG. 3, since the oscillation frequency is adjusted by directly switching the capacitor arrays 20 to 23, there is a problem that the purity of the oscillation frequency deteriorates.

Further, in the oscillator shown in FIG. 3, the switch elements 24 to 27
As shown in FIG. 7, a MOS transistor 32 as shown in FIG. 7 is used, or a MOS transistor 37 as shown in FIG.
Even if these switching elements are turned on and off, only the oscillation frequency control characteristics shown in FIG. 5 can be obtained.

Specifically, as shown in FIG. 9, when a control voltage is applied to the gate 33 of the switching element 24 to turn on and off the switching element 24, an equivalent circuit consisting of a series circuit of the capacitor 20 and the switch 24 is shown in FIG. You will be able to do it. Then, assuming that the control voltage of the gate 33 is Vg, Vg and C
FIG. 11 shows the changes in the values of x and Rx.

In normal cases, it is easy to set Rx so large that it can be ignored compared to the impedance of Cx at the oscillation frequency, so even under the condition of Vg2, where Rx takes the minimum value at the time of design, the influence on the impedance of Cx can be ignored. Also, if the input voltage of the gate 33 changes from Vg1 to Vg3 by turning on and off, Cx changes from CX1 to Cx2. Therefore, the frequency also changes accordingly. However, the control voltage is from Vgl to V
Since it instantaneously changes to g2, Cx also instantaneously changes to CX2. Similar changes occur in other circuits, and as a result, the frequency characteristics of the oscillation circuit become as shown in FIG.

An object of the present invention is to provide a digital temperature-compensated crystal oscillator that is suitable for LSI implementation and can improve the C/N ratio.

[Failure to solve the problem]

The present invention includes a group of oscillation frequency adjustment capacitors connected in series with a crystal resonator and connected in parallel to capacitors in a crystal oscillation circuit, and a group of oscillation frequency adjustment capacitors connected in series with each oscillation frequency adjustment capacitor, and a crystal oscillator of each oscillation frequency adjustment capacitor. A group of transistor switch elements that control the input to the oscillation circuit, a temperature sensor that outputs a voltage according to the temperature, a conversion means that converts the output of the temperature sensor into digital data, and control according to the digital data output from the conversion means. It is equipped with a voltage generation means for outputting a voltage to a specific transistor switch element, and a charge/discharge circuit group that supplies a control voltage output from the voltage generation means to each transistor switch element. It is characterized by outputting an analog signal that changes continuously over time.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a preferred embodiment of the present invention. In this embodiment, charging/discharging circuits 28 to 31 are inserted between the serial/parallel converter 19 and the switching elements 24 to 27, and the other configurations are the same as those in FIG. Components are given the same reference numerals and their descriptions will be omitted.

The charging/discharging circuits 28 to 31 each include a content 40 and a resistor 41, as shown in FIG. connected to the gate. Therefore, Vg for the on/off output of the serial/parallel converter 19
changes between Vgl and Vg2 over time, so
It becomes possible to set the frequency characteristics of the oscillation circuit to those shown in FIG. Note that it is also possible to use a constant current source 42 as shown in FIG. 13 as the charging/discharging circuits 28 to 31. However, in order to charge and discharge the constant current source 42, the direction of current must change in accordance with the voltage at the terminal 38. Since the terminal 39 is connected only to the gate of the MOS transistor switching element, and the impedance of the gate is very high, it can be seen as connected to an almost infinite impedance. FIG. 14 shows the input voltage waveform 43 of terminal 38 and the input voltage waveform 43 of terminal 38.
9 output voltage waveforms 44 are shown.

Here, a capacitance of 1 pF is used as the content 40, the current of the constant current source 42 is 1O-8A, and the content 40 is
When the maximum voltage across 0 is 5■, τ in Figure 14
-0.5X1 is 8 seconds, and the cutoff frequency is several k for equal deflection.
The L P F of H2 can be realized.

That is, in this embodiment, charging/discharging circuits 28 to 31 constituting the LPF are inserted between the serial/parallel converter 19 and the switching elements 24 to 27 to suppress sudden changes in the oscillation frequency. Therefore, it is possible to prevent the C/N ratio from deteriorating.

Furthermore, when an LPF of τ-0.5 x 10-3 seconds is constructed, this means that the changes in capacitance Cxl and CX2 in Fig. 11 occur continuously and relatively smoothly in about 0.5 mm seconds. .

〔Effect of the invention〕

As explained above, according to the present invention, a charging/discharging circuit group is inserted between the voltage generating means and the transistor switching element group, thereby preventing the oscillation frequency from changing rapidly due to turning on and off of the transistor switching element. Since it is controlled, it is possible to obtain an excellent effect of being able to obtain a high-purity oscillation output that is suitable for LSI implementation without sacrificing miniaturization or economic efficiency.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram of a conventional oscillator using a variable capacitance diode, FIG. 3 is a block diagram of a conventional oscillator using a capacitor array, and FIG. 5 is a diagram showing the relationship between temperature and oscillation frequency, FIG. 6 is a diagram showing oscillation frequency characteristics according to the present invention, and FIG. 7 is a diagram showing the oscillation frequency characteristic of the MOS transistor 32. 8 is an explanatory diagram of the configuration of a switching element using a MOS transistor 37 and an inverting amplifier 50. FIG. 9 is a diagram of the content 20 and transistor switches. FIG. 12 is a specific diagram of the charging/discharging circuits 28 to 31. configuration diagram, 1st
3 is a block diagram showing another embodiment of FIG. 2, and FIG. 14 is a diagram showing the relationship between the input voltage and output voltage of FIG. 13. 10 ・・・・・・・・・・・・・・・・・・・・・
Temperature detection circuit 11 ・・・・・・・・・・・・・・・
...... Δ-D converter 12 ...
・・・・・・・・・・・・・・・ Memory circuit-13・
・・・・・・・・・・・・・・・・・・・・・ D-A Combark 14 ・・・・・・・・・・・・・・・・・・
・・・・LPF15 ・・・・・・・・・・・・・・・
...... Negative resistance generation circuit 16 ...
・・・・・・・・・・・・・・・River crystal oscillator 19
・・・・・・・・・・・・・・・・・・ Serial-to-parallel converter 20, 21.22.23 ... Capacitor array 24, 25.26.27 ... FET switch element 28, 29.30.31... Charge/discharge circuit 47
．． 48 ・・・・・・・・・・・・ Resistance 49
・・・・・・・・・・・・・・・・・・ Condenser agent Patent attorney Yoshiyuki Iwasa (t! alO Figure 5 VDD Figure 7 Figure 8 Figure 9 Figure 10 N force Voltage Figure 11 Figure 14

Claims (1)

[Claims] (1) A group of oscillation frequency adjustment capacitors connected in series with the crystal resonator and connected in parallel with the capacitors in the crystal oscillation circuit, and a group of oscillation frequency adjustment capacitors connected in series with each oscillation frequency adjustment capacitor,
A group of transistor switching elements that control input of each oscillation frequency adjustment capacitor to the crystal oscillation circuit, a temperature sensor that outputs a voltage according to temperature, a conversion means that converts the output of the temperature sensor into digital data, and a conversion means Each charging/discharging circuit includes a voltage generating means for outputting a control voltage according to output digital data to a specific transistor switching element, and a charging/discharging circuit group for supplying the control voltage output from the voltage generating means to each transistor switching element. is a digital temperature-compensated crystal oscillator characterized by outputting an analog signal in which the impedance of each transistor switch element changes continuously over time.