JPS6195604A - Voltage controlled oscillator - Google Patents

Abstract

PURPOSE:To eliminate the temperature dependancy by adopting the constitution that a discharge circuit absorbs a prescribed current from a charging circuit. CONSTITUTION:In opening a switch 5, a capacitor C of the charging circuit 1 is charged via a resistor R20 and the charging voltage is subjected to comparison by a Schmitt trigger comparator circuit 3. When the charging voltage rises up to a high threshold value of the comparator circuit 3, the output of the comparator circuit 3 is inverted to close the switch 5. Then the charged electric charge of the capacitor C is discharged via the discharge circuit 2. The discharge circuit 2 consists of transistors Q7-Q12 and resistors R5-R9 and designed to absorb a prescribed current from the charging circuit 1. When the voltage by the electric charge in the capacitor C exceeds the low threshold value of the comparator circuit 3, the output is inverted to open the switch 5 thereby restarting the charging. Thus, the output of the comparator circuit 3 is used as an oscillation output. Then the temperature dependancy of the oscillated output is avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a voltage controlled oscillator, and particularly to improvements in the temperature characteristics of its free-running oscillation frequency.

[Conventional technology]

Conventionally, there have been devices of this type as shown in FIGS. 2 to 4. Figure 2 shows a conventional voltage controlled oscillator (hereinafter referred to as
Vco) is shown, and in the figure, 1
2 is a charging circuit consisting of a capacitor C2 and a resistor R20, 2 is a discharging circuit, 3 is a Schmitt trigger voltage comparison circuit, 4 is a voltage-current conversion circuit, 5 is a switch, 6 is an output circuit, and Vcc is a power supply.

FIG. 3 shows the waveform at the connection point between capacitor C and resistor R20 in FIG. The falling portion is a waveform when the charge of the capacitor C is discharged via the discharge circuit 2.

FIG. 4 shows the internal configuration of each block in FIG. 2, and in the figure, the Schmitt trigger circuit 3 is a transistor Q1.
4-Q17. The voltage-current conversion circuit 4 uses the resistors RIO to R16 to connect the transistors Q1 to Q6 . The output circuit 6 is connected to the transistors Q19 to Q23 . by the resistors R1 to R4. Resistance R
17 to R19, switch 5 is connected to transistor Q13.
．． Due to Q18, the discharge circuit 2 is configured by an external resistor R22.

Next, the operation will be explained. The reason why the waveform at the capacitor C becomes a triangular waveform as shown in FIG. A voltage that rises to the value is discharged - capacitor - "NPy
The charging voltage of C drops to the low threshold of the voltage comparator circuit 3. The output of the voltage comparator circuit 3 is inverted. The switch 5
This is because the operation of opening the capacitor C and charging the capacitor C is repeated. Therefore, the voltage of Vcc in this form is
The running oscillation frequency fo becomes fo=□TC+TDC, where TC is the charging period and TDC is the discharging period.

The free-running oscillation frequency f in the conventional example shown in FIG.

I'll try to find it. Here, the low threshold of voltage comparator circuit 3,
Letting the high threshold values be Vl and V2, respectively, V 1 =
(Vcc-1)・R1))(R1)+R14+R15)
/R15+(R1)/R16). Here ■1 is v
This is a current value obtained by converting the control voltage input to the CO into a current, and is constant during free-running oscillation. When the control voltage is VIN, the temperature characteristics of the resistors cancel each other out, so the temperature characteristics of the low threshold value 1 and the high threshold value v2 of the voltage comparison circuit 3 become zero.

Next, the charging period TC and the discharging period TDC are as described in (1) above, V
It is expressed by the following equation using 2. That is, Vcc-V2 Here, VBB is the forward voltage between the base and emitter of the transistor Q13.

[Problem that the invention seeks to solve]

In this way, TC has no temperature characteristics, but TDC has VB
Since it includes the term H, it has a temperature characteristic, and the conventional VCO has the disadvantage that the free-running oscillation frequency fo has a temperature characteristic.

The present invention was made in order to eliminate the drawbacks of the conventional ones as described above, and its object is to provide a voltage controlled oscillator without temperature characteristics.

[Means for solving problems]

The voltage controlled oscillator according to the present invention has a discharge circuit of a constant current sink type.

[Effect]

In this invention, since the discharging circuit draws a constant current from the charging circuit, the discharging period of the charging circuit becomes constant and there is no temperature characteristic.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a voltage controlled oscillator according to an embodiment of the present invention, and in the figure, the same reference numerals as in FIGS. 2 and 4 indicate the same parts. 2 is a constant current sink type discharge circuit in this embodiment, and the circuit 2 includes transistors Q7 to Q12. It is composed of resistors R5 to R9. Note that R21 is an external resistor for determining the binary value of the discharge current by the discharge circuit 2.

Next, the operation will be explained.

The oscillation mechanism is the same as that of the conventional example, and the waveform at the connection point between capacitor C and resistor R20 is also a triangular waveform as shown in FIG. The low threshold value (1) and the low threshold value (2) of the voltage comparator circuit 3 in this embodiment are also the same as in the conventional example.

On the other hand, the charging period TC and the discharging period TDC are expressed by the following equations.

Vcc-V2 TC=-R2(l C-1n Vcc-VI V1+R20・I2-Vcc TDC=-R2(l C-1n V 2 +R20-12-Vcc Here, 12 is a constant current sink type discharge circuit In I2, the temperature characteristics of resistors R5 and R6 cancel each other out, and the resistor R21 has no temperature characteristics because it is an external resistance.Therefore, I2 has no temperature characteristics. Since there is no term with temperature characteristics in TDC, the free-running oscillation frequency is Although the loop gain of the PLL must be set so that a range can be obtained, by using the device of this embodiment, the pull-in range may be narrow, and the degree of freedom in design can be increased.

〔Effect of the invention〕

As described above, according to the present invention, since the discharging circuit is of the constant current sink type, the discharging period of the charging circuit becomes constant, and there is an effect that the temperature characteristics of the voltage controlled oscillator can be eliminated.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a voltage controlled oscillator according to an embodiment of the present invention, FIG. 2 is a block diagram of a conventional voltage controlled oscillator, FIG. 3 is a waveform diagram showing the charging voltage of the charging circuit, and FIG. FIG. 4 is a diagram showing the internal configuration of each block in FIG. 2. 1...Charging circuit, C...Capacitor, R20...
Resistor, 2...Discharge circuit, 3...Schmitt trigger voltage comparison circuit, 4...Voltage-current conversion circuit, 5...Switch. In the drawings, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] (1) A charging circuit that charges with a predetermined voltage, a discharging circuit that discharges the charge accumulated in this charging circuit, and
A switch inserted between the discharge circuit compares the charging voltage with two high and low threshold voltages determined by the control current, and when the charging voltage reaches the high and low thresholds, the switch is turned on. a voltage comparison circuit that turns on and off respectively and outputs the comparison result as an oscillator output;
A voltage-controlled oscillator comprising a voltage-current conversion circuit that converts an external control voltage for frequency control of the oscillator output into the control current, characterized in that a constant current sink circuit is used as the discharge circuit. voltage controlled oscillator.