VOLTAGE CONTROLLED OSCILLATOR

Abstract: In a voltage controlled oscillator, an input terminal of a first inverter (7) is connected to an output terminal of a second inverter (8) and an output terminal of the first inver­ ter is connected to an oscillation frequency control voltage terminal (Vc) through a series circuit of a first resistor (7) and a second resistor (8). A series circuit of a capacitor (6) and a third resistor (4) forming a time constant circuit is connected between a junction point of the first resistor and the second resistor, and an input terminal of the second inverter. Voltage clamp means (3) is connected between the junction point of the first resistor and the second resistor, and the output terminal of the first inverter or the output terminal of the second inverter. Thus, an oscillation output of an exact square waveform is produced at the output terminal of the first inverter or the second inverter in accor­ dance with the oscillation frequency control voltage (Vc).

Description: VOLTAGE CONTROLLED OSCILLATOR The present invention relates to a voltage controlled oscillator, and more particularly to a voltage controlled oscillator which is highly reliable in operation, consumes a low power and produces an oscillation output of an exact square wave. Fig. 1 shows a circuit diagram of a prior art voltage controlled oscillator and Fig. 2 shows an oscillation output waveform of the oscillator of Fig. 1 at a point A. The circuit of Fig. 1 constitutes a well-known astable multivibrator in which a first transistor Trl and a second transistor Tr2 are alternately turned on and off and an oscillation output frequency of is determined by a time constant of a capacitor C1 and a resistor R1, and a time constant of a capacitor C2 and a resistor R2. Such a voltage controlled oscillator is shown in "Pulse Circuit" by K. Gotoh on pages 149-157, published by Sangyo Tosho Co., Ltd. in 1978, October. Referring to the waveform of Fig. 2, a period X is determined by the time constant of R2 and C2 and a period Y is determined by the time constant of R1 and C1. Thus, two time constant circuits (R1, C1) and (R21 C2) are necessary to determine the oscillation output frequency f. Assuming that K1 and K2 are constants, the oscillation output frequency f is given by f=1 (1) X+Y X+Y = K1-R1-C1 + K2- R2- 2 (2) for a given control voltage Vc. From the equations (1) and (2), 1 = 1 (3) - 1 1 2 2 X+Y K1 Xl-RI-Cl R K2 R C2 Usually, since K1 = K2 = K f X+Y K(R1.C11+ R2.C2) (4) By varying the magnitude of the control voltage Vc, the charge target voltages of the capacitors change and hence the periods X and Y change so that a variable frequency output is produced. Such a prior art voltage controlled oscillator has the following disadvantages. First, when a power supply +B is turned on, an astable condition may not be established by some reason and both the transistors Trl and Tr2 are turned on in a stable state. In such a case, the oscillation starts if one of the transistor circuits is shorted to artificially create an unbalanced condition. Thus, the prior art voltage controlled oscillator has a possibility of stop of oscillation and lacks a reliability in the oscillation operation. Secondly, as a problem inherent to the circuit of the prior art voltage controlled oscillator, a shoulder S of a leading edge of the output waveform is dull as seen from Fig. 2 and an exact square wave is difficult to achieve. Such a dull edge is produced because the capacitor C1 is charged through the resistor R3 when the first transistor Trl is turned off from its on state, that is, the dull edge is produced when the output changes from a low level (L) to a high level (H). In order to supress the dull edge, the resistance values of the 'col- lector resistors R3 and R4 may be reduced but it creates another problem of increase of a power consumption in the circuit. Furthermore, the prior art circuit also requires two sets of time constant circuits (R1, C1 and R2, C2). It is a primary object of the present invention to provide a voltage controlled oscillator which is highly reliable in operation, consumes a low power and produces an exact square wave output. It is a secondary object of the present invention to provide an economic voltage controlled oscillator which has a reduced number of time constant circuit, and is simple in circuit configuration. In order to achieve the above objects, the voltage controlled oscillator of the present invention is characterized in that an input terminal of a first inverting amplifier and an output terminal of a second inverting amplifier are connected (the junction point thereof being referred to as a first junction point), an output terminal of the first inverting amplifier is connected to a voltage control input terminal through a series circuit of a first resistor and a second resistor (the junction point thereof being referred to as a second junction point), the second junction point is connected to one end of a capacitor directly or through a buffer for impedance conversion, the other end of the capacitor is connected to an input terminal of the second inverting amplifier and to the first junction point through a third resistor which forms a time constant circuit together with the capacitor, and a voltage clamp means is connected between the second junction point and the output terminal of the first inverting amplifier or between the second junction point and the output terminal of the second inverting amplifier so that an oscillation output of a variable frequency is produced at the output terminal of the first inverting amplifier or at the output terminal of the second inverting amplifier in accordance with a variable voltage applied to the voltage control input terminal. The present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a circuit diagram of a prior art voltage controlled oscillator; Fig. 2 shows an oscillation output waveform of the oscillator of Fig. 1; Fig. 3 shows a circuit diagram of one embodiment of the present invention; Fig. 4 shows signal waveforms in the circuit of Fig. 3; Fig. 5 shows potential values in the circuit of Fig. 3; Fig. 6 shows an oscillation frequency characteristic of the circuit of Fig. 3; and Figs. 7, 8A, 8B and 8C show circuit diagrams of other embodiments of the present invention. Fig. 3 shows a circuit diagram of one embodiment of the voltage controlled oscillator of the present invention. Vc denotes an oscillation frequency control voltage, Vcc denotes a power supply voltage, numerals 1 and 2 denote a first resistor R11 and a second resistor R12, respectively, for determining a control sensitivity, numeral 3 denotes a clamping diode D1 for fixing the control voltage Vc, numeral 4 denotes a third resistor R13 for determining an oscillation frequency, numeral 5 denotes a protecting resistor R14, numeral 6 denotes a capacitor C11 for determining the oscillation frequency, numeral 7 denotes a first inverter, numeral 8 denotes a second inverter, and numeral 9 denotes a buffer which functions as an impedance converter for converting an impedance as looked from the capacitor 6 into the resistors 1 and 2, which impedance would be high if the buffer 9 were not present, to a low impedance in order to prevent the affect by the resistors 1 and 2 to the oscillation frequency determined by the time constant of the capacitor 6 and the resistor 4. The buffer 9 and the protecting resistor 5 may be omitted in the embodiment of the present invention. Fig. 4 shows signal waveforms at points B, C and D in the circuit of Fig. 3. VTH denotes a threshold level in the input circuit at which the output level of the second inverter 8 changes from a high level (H) to a low level (L) or vice versa. Referring to Figs. 3 and 4, the operation of the voltage controlled oscillator of the present embodiment will be explained. Assuming that the output level of the first inverter 7 is L (OV), the potential at the point B is OV by the function of the diode 3. Since the output level of the second inverter 8 is now H, a charging current flows into the capacitor C11 through the third resistor 4 and a charge voltage rises toward the power supply voltage Vcc of the inverters 7 and 8. As a result, when the potential at the point C reaches the threshold voltage VTH, the output level of the second inverter 8 changes to L and hence the output level of the first inverter 7 changes to H. The potential Vc' at the point B is given by R11 Vc' = (Vcc - Vc) x R11 + Vc (5) R11+R12 where R11 is a resistance value of the resistor 1 and R12 is a resistance value of the resistor 2. Thus, the potential on one electrode of the capacitor 6 (connected to the buffer 9) rapidly increases from OV to Vc' and the potential at the point C reaches (VTH + Vac'). The input level of the second inverter 8 further increases. Since the output level thereof is L, the charge in the capacitor 6 is discharged through the third resistor 4 and the potential at the point C falls toward OV. When the potential at the point C reaches the threshold voltage VTH, the output level of the second inverter 8 changes to H and the output level of the first inverter 7 changes to L (OV). The potential at the point B is OV by the function of the diode 3. As a result, the output potential of the buffer 9 rapidly falls from Vc' to OV, and the potential at the point C falls from VTH to (VTH - Vc') and the capacitor 6 is charged through the third resistor 5. The above operations are repeated. Namely, the output potential of the buffer 9 (i.e., the potential at the point B) and the potential at the point C change with time as shown in Table of Fig. 5 so that the circuit oscillates. As seen from Fig. 4, the waveform of the maximum potential at the point C is higher than the threshold VTH of the inverter 8 by Vc' and that of the minimum potential is lower than VTH by Vc'. Thus, the capacitor 6 is discharged with the time constant T = Cll.Rl3 when the potential at the point C is between VTH + Vc' and VTH, and is charged with the same time constant T when the potential is between VTH and VTH ¯ Vc', TH TH where C11 is a capacitance of the capacitor 6 and R13 is a resistance value of the resistor 4. Since the time constant T is constant, the higher is the voltage Vc' appearing at the point B depending on the control voltage Vc, the lower is the oscillation frequency f, and the lower the Vc' is, the higher is the oscillation frequency f, as is readily seen from the potential waveform at the point C of Fig. 4. This relation is shown by a solid line 3 in Fig. 6. In Fig. 3, the resistor 5 is the input protecting resistor of a high impedance. It is inserted because the potential at the point C may be higher than the power supply voltage Vcc of the inverter 8 or lower than the ground (GND) potential. Since no capacitor is connected to the output terminal of the first inverter 7, the output waveform of the first inverter 7 does not become dull as is the case of S shown in Fig. 2. Accordingly, all resistors may be of high resistance and hence a power consumption is low. In the description of the circuit of Fig. 3, the potential at the point B is fixed to the L level of the inverter 7 by the diode 3. Alternatively, the potential at the point B may be fixed to the H level by reversing the polarity of the diode 3 and the lower potential side thereof varied by the control voltage Vc. In this case, as the control voltage Vc increases, the oscillation frequency f increases and a characteristic shown by a broken line (b) in Fig. 6 is obtained. Fig. 7 shows another embodiment of the voltage controlled oscillator of the present invention. In the first embodiment shown in Fig. 3, the clamp diode 3 is connected between the junction point of the resistor 1 and the resistor 2, and the output terminal of the first inverter 7. In Fig. 7, the clamp diode 3 is connected between the junction point of the resistors 1 and 2 and the output terminal of the second inverter 8. In this case, the same output waveforms at the points B, C and D as those shown in Fig. 4 are produced and the same function and effect are attained. The clamp means for fixing the voltage may be a transistor Tr3, an analog switch or a mechanical switch RL as shown in Figs. 8A, 8B and 8C, respectively, instead of the diode 3. As described hereinabove, the present invention provides a voltage controlled oscillator which is highly reliable in operation, consumes low power and produces an inherently exact square wave output while using only one set of time constant circuit.

Claims: CLAIMS 1. A voltage controlled oscillator comprising: a series circuit of a first resistor (1) and a second resistor (2) for applying an oscillation frequency control voltage (Vc); a first inverter (7) having an output terminal thereof connected to said second resistor (2); a second inverter (8) having an output terminal thereof connected to an input terminal of said first inverter (7); a capacitor (6) connected to a junction point of said first resistor (1) and said second resistor (2) for determining an oscillation frequency; a third resistor (4) connected to said capacitor (6) and the output terminal of said second amplifier (8), said third resistor (4) forming a time constant circuit together with said capacitor (6); and clamp means (3) connected between the junction point of said first resistor (1) and said second resistor (2) and the output terminal of said first inverter (7) or between the junction point of said first resistor (1) and said second resistor (2) and the input terminal of said first inverter (7) for fixing the oscillation frequency control voltage (vc); whereby an oscillation output of a variable frequency is produced at the output terminal of said first inverter (7) or at the output terminal of said second inverter (8). 2. A voltage controlled oscillator according to Claim 1 further comprising a buffer (9) for impedance conversion connected between the junction point of said first resistor (1) and said second resistor (2), and said capacitor (6). 3. A voltage controlled oscillator according to Claim 1 further comprising a protecting resistor (5) connected between the input terminal of said second inverter (8) and said capacitor (6). 4. A voltage controlled oscillator according to Claim 1 wherein said clamp means (3) is a diode. 5. A voltage controlled oscillator according to Claim 1 wherein said clamp means (3) is a transistor. 6. A voltage controlled oscillator according to Claim 1 wherein said clamp means (3) is an analog switch. 7. A voltage controlled oscillator according to Claim 1 wherein said clamp means (3) is a mechanical switch.

Inventors: IKOMA, JUNICHI, MORI, YONEMITSU
Applicants: HITACHI, LTD.
Application year: 1983
Main classification: H03K3
IPC codes: H03K3