JP3058425B2 - Temperature compensated oscillator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature-compensated oscillator that compensates for a change in oscillation frequency due to a temperature change, and more particularly to an improvement in a bias circuit that finely adjusts an oscillation frequency according to a bias voltage value. About.

(Technical Background of the Invention and Problems Thereof) In recent years, a crystal oscillator has been widely used as a reference for frequency, time, and the like. By the way, a crystal oscillator used for a crystal oscillator generally has a temperature coefficient, and the frequency changes with a change in temperature. For example, a general AT-cut crystal unit used at a frequency of about several MHz to several tens of MHz has a substantially cubic curve-like temperature coefficient as shown in FIG. 3, and its characteristics depend on the cutting angle. The inflection point is around 25 ° C.

On the other hand, as the accuracy of electronic devices increases, the oscillation frequency tends to be required to be more stable even in a crystal oscillator.

As a crystal oscillator that satisfies such demands, there is one in which an oscillation circuit is housed in a thermostat. However, in the case of using a thermostat, the shape becomes large, the power consumption is large, and it takes time until the frequency is stabilized when the power is turned on.
In addition, since parts are exposed to a relatively high temperature of about 70 ° C., there is a problem in reliability.

For this purpose, a voltage-capacitance conversion element such as a varicap is connected to the crystal oscillator, and a compensation voltage is generated by a compensation voltage generation circuit in accordance with the temperature detected by a temperature detection element such as a thermistor. In some cases, temperature compensation is performed by applying a voltage to the cap.

FIG. 2 is a block diagram showing an example of this type of conventional temperature-compensated oscillator. A variable voltage capacity element 3, for example, a varicap, is connected in series to a crystal oscillator 2 of a so-called Colpitts-type crystal oscillator 1. Then, a temperature signal detected by a temperature sensor 4 such as a thermistor is supplied to a compensation voltage generating circuit 5 to obtain a temperature compensation voltage and apply it to the varicap 3. By the way, in such a temperature-compensated oscillator, when the frequency slightly changes due to the aging of the crystal oscillator or the like, it is necessary to perform fine adjustment so that the oscillation frequency exactly matches the target frequency. Therefore, a bias voltage Vb is applied to the varicap 3 from the bias input terminal 7 via the high resistance 6.

Generally, as shown in the graph of FIG. 3, in order to use the variable width of the bias voltage Vb most efficiently, the target frequency f is set at approximately the midpoint potential Vb0 of the variable width of the bias voltage Vb.
Designed to be zero.

In this way, the oscillation frequency can be finely adjusted according to the voltage applied to the bias input terminal 7, and can be accurately adjusted to a predetermined target frequency.

However, in such a case, strictly speaking, if the internal resistance of the voltage source connected to the bias input terminal 7 changes, the temperature compensation characteristic of the compensation voltage generating circuit 5 due to the compensation voltage changes, and a desired compensation characteristic cannot be obtained. Therefore, it is necessary to connect a variable voltage source having a predetermined characteristic to the bias input terminal 7, and the actual handling is extremely troublesome.

When the bias input terminal 7 is left open, the oscillation frequency becomes substantially the lower end of the variable width of the frequency by the bias voltage Vb, and a predetermined temperature compensation characteristic cannot be obtained.

In addition, a relatively high-grade temperature-compensated oscillator having such a bias input terminal and a relatively simple-specification oscillator without a bias input terminal have different temperature compensation circuit configurations even at the same oscillation frequency. Need to be done. For this reason, there is also a problem that different temperature compensating circuits are required depending on the presence or absence of the bias input terminal even for the same oscillator.

For this reason, there is a device in which a voltage-capacitance conversion element for temperature compensation and a voltage-capacity conversion element for bias are separately provided. However, such a configuration has a problem in that the electrical configuration becomes complicated, the number of elements that affect the oscillation frequency increases, and it becomes difficult to perform high-precision temperature compensation.

(Object of the Invention) The present invention has been made in view of the above circumstances, and does not impair the temperature compensation characteristics due to the DC characteristics of the bias source. If a bias voltage is applied, the frequency can be finely adjusted according to the value. A temperature-compensated oscillator that can be adjusted, and if it is not necessary to finely adjust the oscillation frequency, the terminal can be opened and the frequency can be automatically set to the approximate center frequency of the variable range by the bias voltage to obtain a predetermined temperature compensation characteristic. The purpose is to provide.

(Summary of the Invention) The present invention provides an oscillator that performs temperature compensation by applying a temperature compensation voltage to a voltage-capacitance conversion element that controls an oscillation frequency. In the oscillator, a bias circuit that finely adjusts the oscillation frequency has an input connected to a bias input terminal. A high input impedance amplifier for providing an output to the voltage-capacitance conversion element, and a pull-up circuit for pulling up the input of the high input impedance amplifier to a substantially middle point potential in the variable range of the bias voltage. Things.

Embodiment An embodiment of the present invention will be described below in detail with reference to the block diagram shown in FIG.

In the figure, reference numeral 11 denotes a temperature sensor, for example, a thermistor whose resistance value changes according to a temperature change. Reference numeral 12 denotes a compensation voltage generation circuit that receives a temperature signal according to the detected temperature from the temperature sensor 11 and generates a temperature compensation voltage Vt. The temperature compensation voltage Vt output from the compensation voltage generation circuit 12 is a resistor
The voltage is applied to the voltage-capacitance conversion element 14 via 13 to control the capacitance.

This voltage capacity conversion element 14 is connected in series to a crystal oscillator 16 of a Colpitts-type crystal oscillation circuit 15, for example, and finely varies its oscillation frequency to compensate for a change in the oscillation frequency due to a temperature change.

Reference numeral 17 denotes a high input impedance amplifier, for example, a so-called voltage follower circuit in which the inverting input of the operational amplifier is connected to the output, that is, the amplification factor is 1. The input of this high input impedance amplifier 17 is connected to a bias input terminal 18 for applying an external control voltage,
The voltage at approximately the midpoint of the variable range of the control voltage via 20
Pulled up to Vn. Then, the output of the high input impedance amplifier 17, that is, the bias voltage Vb is applied to the voltage-capacitance conversion element 14 via the resistor 19.

With such a configuration, the compensation voltage Vt corresponding to the temperature detected by the temperature sensor 11 can be obtained by the compensation voltage generation circuit 12 and applied to the voltage-capacitance conversion element 14 to perform temperature compensation. The output Vt of the compensation voltage generating circuit 12 and the output Vb of the high input impedance amplifier 17 are applied to the voltage-capacitance conversion element 14 via the resistors 13 and 19, respectively, so that these voltages Vt and Vb are The values obtained by multiplying the respective constant ratios are independently added without being influenced by each other and applied to the voltage-capacitance conversion element. Therefore, when the bias input terminal 18 is open and the control voltage is not applied, the input of the high input impedance amplifier 17 becomes the above-mentioned midpoint potential, and automatically becomes the approximate center frequency of the frequency variable width by the control voltage. When a control voltage is applied to the bias input terminal 18, the oscillation frequency can be finely adjusted according to the value.

However, since the compensation voltage generation circuit 12 and the bias input terminal 18 are cut off in a DC manner by the high input impedance amplifier 17, the temperature compensation characteristic may be impaired by the internal impedance of the circuit connected to the bias input terminal 18. Absent.

Furthermore, a relatively high-grade temperature-compensated oscillator with a bias input terminal and a relatively simple specification oscillator without a bias input terminal can be manufactured with the same configuration and a common crystal unit can be used. There are also benefits that you can do.

The present invention is not limited to the above embodiment. For example, the high input impedance amplifier may be configured using not only an operational amplifier but also a field effect transistor.

Also, the compensation voltage generation circuit may be of a type that generates a compensation voltage in an analog manner, or may be of a type that digitally reads out compensation data from a storage element and performs compensation.

(Effects of the Invention) As described above in detail, according to the present invention, the oscillation frequency can be finely adjusted by the bias voltage, and the characteristics of the bias source do not affect the temperature compensation characteristics, and the oscillation frequency is adjusted. If it is not necessary, by opening the bias input terminal, it is possible to provide a temperature-compensated oscillator that can automatically adjust to the target frequency without impairing the temperature compensation characteristics.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing an example of a conventional temperature-compensated oscillator, and FIG. 3 is a graph for explaining frequency adjustment by a bias voltage. 11… Temperature sensor 12… Compensation voltage generating circuit 14… Voltage capacity conversion element 15… Oscillation circuit 16… Crystal oscillator 17… High input impedance amplifier 18… Bias input terminal

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-108801 (JP, A) JP-A 1-22291 (JP, A) JP-A-60-129758 (JP, U) (58) Field (Int. Cl. ⁷ , DB name) H03B 5/00-5/42

Claims (1)

(57) [Claims] A temperature sensor for detecting a temperature, a compensation voltage generation circuit for outputting a temperature compensation voltage corresponding to a detection output of the temperature sensor, and a capacitance controlled according to an output of the compensation voltage generation circuit. A voltage-capacitance conversion element, a piezoelectric vibrator whose frequency is controlled by the voltage-capacity conversion element, an oscillating circuit that forms an oscillator together with the piezoelectric vibrator, A bias circuit for applying a bias voltage set to a frequency, wherein the bias circuit connects an input to a bias input terminal and provides an output to the voltage-capacitance conversion element, and the compensation voltage generation circuit And a resistor inserted between each output of the high input impedance amplifier and the voltage-capacitance conversion element, and Temperature compensated oscillator input of vessels characterized by comprising a pull-up circuit for pulling up a substantially middle point potential of the variable range of the bias voltage.