JPH0440112A - Voltage controlled oscillator - Google Patents

Abstract

PURPOSE:To start oscillation without fail by initializing a potential V lower than an Lth or higher than an Hth by a switch element for initialization when starting oscillation. CONSTITUTION:When starting oscillation, the output of an initial reset circuit 10 is turned to 'H' and applied to the gate of an N type MOSFET 9, and this FET is turned to an 'ON' state. Then, the electric charge of a capacitor 6 is discharged through this switch element 9 for initialization to a ground. Until the potential V of a terminal not grounding the capacitor 6 by discharging becomes lower than the threshold level Lth of a schmitt trigger inverter 7, the output of the initial reset circuit 10 is kept 'H', afterwards, this output is turned to 'L' and the switch element 9 for initialization is turned 'OFF'.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oscillation circuit, and more particularly to a voltage-controlled oscillator that oscillates by charging and discharging a capacitor.

[Prior Art] Conventionally, there is a voltage controlled oscillator (VCO) as shown in FIG. 4, whose oscillation frequency can be controlled by an input voltage. In the figure, 20 is a VCO, and one end of the capacitor is grounded, and the other end of the capacitor is connected to a P-type MO8FET 4 that performs a switching operation for charging, an N-type MO8FET 5 that performs a switching operation for discharging, and a Schmitt trigger inverter. 7 inputs are connected. The output of the Schmitt trigger inverter 7 is connected to an inverter 8, which constitutes the output of this oscillation circuit, and a part of this output is fed back to the input of the transistor 4 for charging and the transistor 5 for discharging. has been done. Transistor 4 and transistor 5 are connected to power supply line Vcc and ground line via current mirror constant current sources 2 and 3, respectively. The magnitude of the current of these current mirror constant current sources is controlled by the input voltage of the voltage-current conversion circuit (V/I) 1.

Here, the output of the Schmitt trigger inverter 7 is Lot
level (hereinafter referred to as “L”) to High level (hereinafter referred to as “H”)
) when the input threshold level Lth changes to
The potential of the ungrounded terminal of capacitor 6 is
When the signal becomes low, this output becomes "H". At this time, the output of the inverter 8 becomes "L", so the transistor 4 for charging is "ON" and the transistor 5 for discharging is "OFF". A current is supplied to the capacitor 6 via the capacitor 4. Therefore, the potential V increases with the passage of time as shown in FIG. 5(a). When this is higher than the threshold level Hth when the output of Schmitt trigger inverter 7 changes from "H" to "L", the output becomes "L" and the output of inverter 8 changes to "H". As a result, the transistor 4 for charging becomes "OFF" and the transistor 5 for discharging becomes "ON", so that the charge in the capacitor 6 flows through the transistor 5 for discharging and the current mirror constant current source 3. The potential V drops over time, but when V < L th, the output of the Schmitt trigger inverter 7 changes to "H" and the potential V begins to rise again. Similarly, the capacitor 6 is repeatedly charged and discharged, and this circuit operates as an oscillator.The magnitude of the current of the current mirror constant current sources 2 and 3, which determines the time required for charging or discharging the capacitor 6, is determined by the voltage-current conversion circuit 1. Because it changes depending on the input voltage of
The oscillation frequency can be controlled by this input voltage.

Similarly, when the potential V becomes higher than 8th, oscillation occurs as shown in the timing chart of FIG. 5(b).

[Problem to be solved] However, in such conventional oscillation circuits, when the power is turned on,
In the initial state of “, the potential V is at the threshold level H.
There are cases where the value does not change to the upper and lower thresholds Hth and Lth when it is near the middle between th and Lth. At this time, the inverted output levels of Schmitt trigger inverter 7 and inverter 8 do not sufficiently turn on or turn off transistors 4 and 5, and oscillation does not occur. At this time, the transistor 4 for charging and the transistor 5 for discharging are both in a transition state between "ON" and "OFF", and current flows through the current mirror constant current source 2 and the transistor 4 for charging. The system current flows through a transistor 5 for discharging and a current mirror constant current source 3.
The potential V hardly changes from the intermediate state because it flows out through the capacitor. As a result, oscillation does not occur. Further, at this time, a current corresponding to the input voltage of the voltage-current conversion circuit 1 is supplied to the current mirror constant current source 2, the transistor 4 for charging, the transistor 5 for discharging,
In addition to flowing from the power source to the ground via the current mirror constant current source 3, there is also the disadvantage that the through current of the inverter 8 increases, resulting in an increase in power consumption.

The present invention solves the problems of the prior art, and aims to provide a VCO that can reliably start oscillation and that oscillates quickly.

[Means for Solving the Problem] In order to achieve such an object, the means in the oscillation circuit of the present invention is such that the initial value of the potential V of the capacitor 6 is set to Lt of the Schmitt trigger inverter 7 at the start of oscillation.
By adding a circuit that makes the voltage less than h or more than 8th, the transistor 4 for charging and the transistor 5 for discharging are prevented from being in a transition state between "ON" and "OFF", This ensures that oscillation starts.

[Operation] In this way, the circuit that initializes the potential V of the capacitor 6 sets the initial value of the potential V to below Lth or to Hth.
With the above settings, the initial states of the transistor 4 for charging and the transistor 5 for discharging are set to "ON" and "OFF", or "OFF" and "ON", respectively.

As a result, it is possible to reliably start oscillation from the power "ON", and the reliability of the oscillation circuit can be improved.

Furthermore, when the transistor 4 for charging and the transistor 5 for discharging enter a transition state, current flows from the power supply to the ground, so power is not wasted, so the circuit consumption is reduced. Power can also be reduced.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an embodiment of an oscillation circuit to which the present invention is applied. Here, the same components as in FIG. 4 are indicated by the same reference numerals.

In Fig. 1, the difference from the conventional one is that an N-type MO8FE is connected between the ungrounded terminal of the capacitor 6 and the ground.
An initialization switch element 9 made of T is connected, and an initial reset circuit 10 is added for turning the initialization switch element 9 "ON" at the start of oscillation.

To start oscillation, the output of the initial reset circuit 10 is set to "H" and applied to the gate of the N-type MO8FET 9, turning it "ON", and the charge of the capacitor 6 is passed through this initialization switch element 9. Discharge to earth. The output of the initial reset circuit 10 is kept at "H" until the potential V of the non-grounded terminal of the capacitor 6 falls below the threshold level Lth of the Schmitt trigger inverter 7 due to discharge, and thereafter this output is kept at "L". ”
to turn off the initialization switch element 9. The Schmitt trigger inverter 7 and the inverter 8 invert and amplify the terminal potential 6 of the capacitor 6 at this time with sufficient gain to generate "H" and "L" outputs, respectively.Therefore, the transistor 4 for charging is turned ON. '', the transistor 5 for discharging is turned OFF, and charging of the capacitor 6 begins. FIG. 3(a) shows a timing chart of the potential V and the output of the initial reset circuit 10 at this time. When this charging potential V exceeds the thread 1 shillold level Hth, the Schmitt trigger inverter 7 performs an inversion operation, its output becomes "L", and the output of the inverter 8 changes to "H". From this, oscillation starts. .After this, capacitor 6 is connected as in the conventional oscillation circuit.
Oscillating operation occurs through repeated charging and discharging.

FIG. 2 shows another embodiment of an oscillation circuit to which the present invention is applied. The difference from the embodiment shown in FIG. 1 is that the initialization switch element 9 is connected between the terminal on the non-grounded side of the capacitor 6 and the power supply. As shown in the timing chart of FIG. 3(b), at the start of oscillation, the output of the initial reset circuit 10 is set to "L", the initialization switch element 9 is turned "ON", and the capacitor 6 is charged.

When the potential V becomes Hth or higher, the initial reset terminal 10 is "H" and the initialization switch element 9 is "OFF".
From this point, the capacitor 6 starts discharging, and when the potential V of the capacitor 6 falls below Lth, the Schmitt trigger inverter 7 performs an inversion operation and oscillation starts.

[Effects of the Invention] As can be understood from the above explanation, in this invention, no matter what the potential V of the capacitor 6 is at the start of oscillation, the initialization switch element 9 lowers the potential V to Lth.
Since the initialization is performed below or above Hth, oscillation can be reliably started and the reliability of the oscillation circuit is improved. Furthermore, since there is no wasted current consumption from the power source to the ground when oscillation has not started, which was a problem in conventional circuits, there is an effect of reducing power consumption.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing embodiments of the present invention;
The figure shows a timing chart of an embodiment of the present invention, FIG. 4 shows a circuit diagram of a conventional VCO, and FIG. 5 shows its timing chart. l... Voltage-current conversion circuit, 2.3... Current mirror constant current source, 4... P-type M
O8FET. 5...N type MO8FET. 6... Capacitor, 7... Schmitt trigger inverter, 8... Inverter, 9... Switch element for initialization, 10... Initial reset circuit, 20... VCO.

Claims (1)

[Claims] (1) A Schmitt trigger inverter is connected to the other end of a capacitor whose one end is connected to a reference potential line, a first switch circuit for charging the capacitor from the power supply side, and a first switch circuit for charging the capacitor with charge from the power supply side; A second switch circuit for discharging to the power supply line is connected to the other end of the capacitor, and returns the output of the Schmitt trigger inverter to the first switch circuit and the second switch circuit via the inverter, In an oscillation circuit that changes these switch circuits from either "ON" or "OFF" state to the other state, when the power is turned on, the output of the Schmitt trigger inverter changes the potential at the other end of the capacitor to a low level. Below the threshold level or H when changing from to High level
A voltage controlled oscillator comprising a third switch circuit that is initially set to a threshold level or higher when changing from a high level to a low level.