JP2786969B2 - Voltage controlled oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage controlled oscillation circuit. In particular, the present invention relates to a voltage controlled oscillation circuit that outputs a signal of a scanning frequency such as a CRT used for a television receiver or a display device of a computer.

[0002]

2. Description of the Related Art Voltage-controlled oscillator circuits (hereinafter referred to as VCOs) have been widely used in the electric industry because they have various application fields. In particular, this is an essential circuit when configuring a PLL (phase locked loop).

A typical example of a circuit using a PLL is an AFC (automatic frequency control) circuit used in a horizontal synchronization circuit of a television receiver.

[0004] AF used in a television receiver
FIG. 4 is a block diagram showing an example of a conventional VCO in the C circuit.

[0005] As shown in FIG.
Connects the input side and the output side of the phase amplification circuit 2 and the voltage control phase shift circuit 4 to each other to form a signal loop.

The phase amplifier circuit 2 includes a first phase shift amplifier circuit 6 using a resonator such as a crystal or a ceramic. The resonator of the first phase amplifier circuit 6 such as crystal or ceramic
Since the impedance changes abruptly before and after the resonance frequency, this circuit is a phase shift circuit in which the amount of phase shift of a signal changes abruptly depending on the frequency. The phase shift amount is 0 degree at the resonance frequency, and asymptotically approaches +90 degrees at higher frequencies, and approaches -90 degrees at lower frequencies. Further, the phase amplifier circuit 2 includes a phase shift output circuit 8 that further shifts the phase of the output signal of the first phase shift amplifier circuit 6 and outputs the resultant signal. This phase shift output circuit 8 performs a phase shift as an offset for securing a predetermined or more phase shift amount in the entire signal loop of the present VCO, and its output signal is a differential signal.
Generally, in an oscillator using feedback, the phase shift amount of the entire loop of the signal must be an integral multiple of 360 degrees. In other words, oscillation is performed at a frequency such that the phase shift amount is an integral multiple of 360 degrees. Since the configuration of the subsequent voltage control phase shift circuit 4 is based on a differential amplifier as described later, the output signal of the phase shift output circuit 8 is also a differential output signal in accordance with the configuration.

The voltage control phase shift circuit 4 shifts the output signal of the phase shift output circuit 8 by a maximum phase shift amount (which can be set by the control voltage) of the voltage control phase shift circuit 4. It includes a two-phase amplifying circuit 10. Furthermore, the voltage control phase shift circuit 4 includes a mixing circuit 12 that mixes the output signal of the phase shift output circuit 8 and the output signal of the second phase shift circuit at a ratio according to an external control voltage. In. The output signal of the mixing circuit 12 is fed back to the first phase shift amplification circuit 6. As described above, a desired phase shift amount can be set by mixing a signal that has not been phase-shifted and a signal that has been phase-shifted at a ratio set by the control voltage.

FIG. 5 shows a specific circuit diagram of a conventional VCO. FIGS. 6 and 7 show V in the circuit diagram of FIG.
A circle diagram illustrating the operation of CO is shown. Hereinafter, the operation of the VCO will be described with reference to the drawings.

The first phase shift amplifier circuit 6 includes a transistor Q
1 and Q2, and a signal is input to the base of the transistor Q2 via the resistor R1, but the resistor R2 and the base of the transistor Q1 A signal is input through a phase shift network including the resonator CX. Therefore,
The phase shift amount of the first phase shift amplifier circuit 6 changes around the resonance frequency. The transistors Q1 and Q1 of the first differential amplifier 20
A circle diagram based on the output signal of No. 2 is shown in FIG. In the figure, the Y axis is the real axis and the X axis is the imaginary axis. now,
A resistor R4 connected to the collector of the transistor Q2
Is represented by a vector indicated by a point f (hereinafter, referred to as a vector f), a voltage signal appearing at a resistor R3 connected to the collector of the transistor Q1 having a phase opposite to that of the vector f is represented by a vector It is represented by g.

The output of the first phase shift amplifier circuit 6 is taken out as a differential signal from the two transistors Q 1 and Q 2 and input to the phase shift output circuit 8. The phase shift output circuit 8 includes a first phase shift network 2 including resistors R3 to R8 and a capacitor C1.
1 and transistors Q3 and Q4 connected to the emitter follower. The vector representing the output signal at the emitter of the transistor Q3 moves on the semicircle A in the circle diagram of FIG. 6 according to the frequency of the signal. The movement of the tip of the vector on the semicircle A is described in, for example, JP-A-60-41806. This semicircle A is due to the characteristics of the first phase shift network 21. That is,
If the signal is direct current, it is equal to the vector f, and if the frequency is infinity, it is equal to the vector e. The magnitude ratio between the vector e and the vector f is represented by (R4 + R5 + R6) / R6 (1). In the circuit shown in FIG.
Since 3 = R4, R5 = R8, and R6 = R7,
The above equation (1) is equal to (R3 + R8 + R7) / R7 (2).

The voltage control phase shift circuit 4 includes a buffer amplifier 2
2, a second phase-shift amplifier 10, and a mixing circuit 12 that mixes these output signals based on a ratio determined by an external control signal.

The buffer amplifier 22 includes a transistor Q5
And Q6, and amplifies the differential output signal from the phase amplifier circuit 2 at a predetermined amplification factor. This is a circuit for matching the level of the signal which is supplied to the mixing circuit 12 and which is not phase-shifted with the level of the signal which is phase-shifted by the second phase-shift amplification circuit 10. In this conventional example, the amplification factor is set to 1 (0 db). Therefore, the vector representing the output signal of the buffer amplifier 22 is
The vector a in FIG. 5 is the same as the output signal of the transistor Q3. The output signal of the buffer amplifier 22 is taken out from the collector on the transistor Q5 side and supplied to the subsequent mixing circuit 12.

The second phase shift amplifier 10 includes a transistor Q7
And Q8, and a differential output signal from the phase amplifier circuit 2 is connected to resistors R12 to R1.
4 and a capacitor C2 are supplied to the bases of transistors Q7 and Q8 via a second phase shift network 28. Therefore, the vector representing the output signal from the transistor Q7 of the second phase shift amplifier circuit 10 has the tip of the vector at any point on the semicircle C in the circle diagram of FIG. 6 according to the frequency of the signal. To position. This semicircle C is due to the characteristics of the second phase shift network 28. In this conventional example, the vector representing the output signal from the transistor Q7 of the second phase shift amplifier circuit 10 is represented by c in the figure. By the way, in this conventional example, in order to further shift the phase shift, a signal is extracted from a transistor on the opposite side of the buffer amplifier 22 and supplied to the subsequent mixing circuit 12. That is, the output signal is extracted from the transistor Q8. The vector represented by the extracted signal is a vector d having a phase shift opposite to that of the vector c.

The mixing circuit 12 includes transistors Q9, Q1
0, a fourth differential amplifier 30 and a transistor Q1.
1, a fifth differential amplifier 32 composed of Q12. An output resistor R15 is connected to the collectors of the transistors Q10 and Q12, and a signal appearing at the output resistor R15 is fed back to the phase amplifier circuit 2.

A control voltage V _AFC is applied to the bases of the transistors Q9 and Q10 of the fourth differential amplifier 30. The control voltage V _AFC is applied to the transistors Q11 and Q12 of the fifth differential amplifier 32 in a direction opposite to that of the fourth differential amplifier 30. Therefore, when the control voltage _VAFC is the positive maximum voltage, the transistor Q9
When the control voltage _VAFC is the negative maximum voltage, the transistors Q10 and Q11 are turned on. By adjusting the control voltage V _AFC between the positive maximum voltage and the negative maximum voltage, the output signals of the buffer amplifier 22 and the second phase shift amplifier circuit 10 can be mixed at a desired ratio. is there. Therefore, since the vector representing the signal of the mixing result is current addition, it is located on the line connecting the vectors a and d in FIG.

Next, the difference in operation when the control voltage V _AFC is changed will be described.

First, the case where the control voltage _VAFC is the negative maximum voltage will be described. The oscillation frequency of the circuit in this case is the lowest frequency that can be set. In this case, the transistor Q10 is ON, and the transistor Q12 is
It is in the F state. That is, only the output signal of the buffer amplifier 22 appears on the output resistor R15. Buffer amplifier 2
Since the amplification factor of 2 is 1 (0 dB) as described above, the vector representing the output signal appearing at the output resistor R15 is equal to the vector a.

The output signal represented by the vector a is fed back to the phase amplifier circuit 2, and the first differential amplifier 20
Becomes a signal represented by a vector parallel to the Y axis (real axis). Note that FIG. 6 depicts the output signal of the first differential amplifier 20 on a (phase) basis. In order for such amplification to be performed by the first differential amplifier 20, the ratio between the impedance of the resistor R2 connected to the base of the transistor Q1 of the first differential amplifier 20 and the resonator CX such as quartz or ceramic is required. Must be equal to the ratio between the real and imaginary components of vector a. In other words, oscillation occurs at such a frequency. In this case, that is, when the control voltage V _AFC is the maximum negative voltage, the oscillation frequency is slightly lower than the series resonance frequency of the resonator CX such as a crystal or a ceramic, and is the lowest oscillating frequency of this VCO. .

Next, a case where the control voltage V _AFC is the maximum positive voltage will be described. The oscillation frequency of the circuit in this case is the highest frequency that can be set. In this case, the transistor Q10 is in the OFF state, and the transistor Q12 is
N state. That is, only the output signal of the second phase shift amplifier circuit 10 appears on the output resistor R15. Therefore, the vector representing the output signal appearing at the output resistor R15 is equal to the vector d.

In this case, as in the case where the control voltage V _AFC is the maximum negative voltage, the ratio between the impedance of the resistor R2 and the impedance of the resonator CX such as crystal or ceramic is determined by the real component and the imaginary component of the vector d. Must be equal to the ratio. In other words, oscillation occurs at such a frequency. In this case, that is, when the control voltage V _AFC is the maximum positive voltage, the oscillation frequency is slightly higher than the series resonance frequency of the resonator CX such as crystal or ceramic.
Is the highest frequency at which oscillation is possible.

As described above, the conventional voltage controlled oscillator circuit can oscillate at a frequency corresponding to the vector a to the vector d with the series resonance point of the resonator CX such as crystal or ceramic interposed therebetween.

[0022]

A resonator such as quartz or ceramic has a higher-order parasitic resonance point due to its structure. Since the parasitic resonance point is often higher than the reference resonance point, in order to prevent oscillation at the parasitic resonance point, the closed loop gain at the higher-order parasitic resonance point of the VCO must be reduced. No.

FIG. 7 shows a circle diagram in the case where oscillation occurs at such a higher-order parasitic resonance point. As shown in FIG. 7, the vector a is closer to the vector e because the frequency increases. Therefore, the real component of the vector a is smaller than that in the case of FIG. However, the real component of the vector d is larger than the case shown in FIG. This is because the vector c has moved closer to the point b because the frequency has increased, and the position of the vector d has moved closer to the Y axis (real axis).

As described above, in a state where the closed-loop gain at a high frequency is larger than the gain at a low frequency, oscillation may occur at a higher-order parasitic resonance point of a resonator such as a crystal or a ceramic. is there.

The present invention has been made in view of such problems, and an object of the present invention is to provide a voltage-controlled oscillation circuit which is less likely to oscillate at a higher parasitic resonance frequency of a resonator.

[0026]

In order to solve the above-mentioned problems, the present invention provides an input side of a phase amplifier circuit and an output of a voltage controlled phase shift circuit which shifts a phase by an amount of phase shift according to an external control voltage. A voltage controlled oscillation circuit in which a signal loop is formed by connecting an output side of the phase amplification circuit and an input side of the voltage controlled phase shift circuit.
The phase amplification circuit uses a resonator whose impedance changes according to the oscillation frequency, a first phase shift amplification circuit whose phase shift amount changes according to the oscillation frequency, and an output signal of the first phase shift amplification circuit. An output circuit that generates the voltage control phase shift circuit, wherein the voltage control phase shift circuit shifts an output signal of the phase amplifier circuit by a predetermined amount of phase shift, and the voltage control phase shift circuit responds to the external control voltage. By mixing the output signal of the phase amplifier circuit and the output signal of the second phase shift amplifier circuit at the same ratio, a signal shifted in phase by an amount corresponding to the control voltage from the outside is created. A mixing circuit for supplying this signal to the input side of the phase amplifier circuit, wherein the sum of the phase shift amount of the voltage controlled phase shift circuit and the phase shift amount of the phase amplifier circuit is 3
Oscillates at a frequency that is an integral multiple of 60 degrees, the second phase-shift amplifier circuit includes a differential amplifier circuit including first and second transistors, a differential output signal pair of the phase amplifier circuit, and the differential amplifier. A phase-shifting network connecting the base input terminals of the first and second transistors of the dynamic amplifier circuit, and the phase-shifting network includes a differential output signal of the phase amplifier circuit. A first resistor for connecting one signal line to a base terminal of the first transistor, and connecting a second signal line of a differential output signal of the phase amplifier circuit to a base terminal of the first transistor. A second resistor, a third resistor connecting one signal line of the differential output signal of the phase amplifier circuit and a base terminal of the second transistor,
A series circuit of a fourth resistor and a capacitor connecting the other signal line of the differential output signal of the phase amplifier circuit and the base terminal of the second transistor, and having a higher resonance frequency than the resonance frequency of the resonator. A voltage controlled oscillator circuit characterized in that the gain of the second phase shift amplifier circuit decreases with respect to the frequency, and the closed loop gain of the entire signal feedback path decreases.

Therefore, this is a voltage controlled oscillation circuit in which the closed loop gain decreases as the oscillation frequency increases.

[0028]

According to the present invention, the phase shift network including the first, second, third, and fourth resistors and capacitors not only shifts the phase of the differential signal from the phase amplifier circuit but also increases the frequency. The amplitude is attenuated accordingly. Therefore, the closed loop gain of the voltage controlled oscillation circuit according to the present invention decreases as the oscillation frequency increases.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

[0030] 1. Configuration FIG. 1 shows a circuit diagram of a VCO of this embodiment. FIG.
As shown in FIG. 2, the VCO of this embodiment is a conventional VC
This is almost the same circuit as O. Also, the phase amplification circuit 1
02, the operation of the mixing circuit 112 is exactly the same.

However, as shown, a second resistor R 116 is newly added to the second phase shift network 128. As described above, conventionally, three resistors and one capacitor were used as the second phase shift network, but in this embodiment, four resistors and one capacitor are used. Further, the amplification factor of the buffer amplifier 122 is made smaller than that of the related art as described later.

A characteristic of this embodiment is that the second phase shift network 12 used by the second phase shift amplifier circuit 110 that shifts the phase of the differential output signal output from the phase amplifier circuit 102 is used.
8 includes a series circuit of a capacitor and a resistor. That is, while the conventional second phase shift network 128 changes only the phase shift and does not change the amplitude of the signal at all, the second phase shift network 128 in this embodiment attenuates the amplitude as the frequency increases. There is a characteristic in having done it. Further, since the amplification factor of the second phase shift amplifier circuit according to the present embodiment is smaller than that of the related art, the amplification factor of the buffer amplifier 122 is set to be smaller in accordance with the amplification factor.

2. Normal Operation First, the normal operation of the present embodiment will be described.

The transistor Q1 of the first differential amplifier 120
A circle diagram based on the output signals of 01 and Q102 is shown in FIG. Similar to the circle diagram described in the description of the conventional VCO, the Y axis is the real axis and the X axis is the imaginary axis. Both the semicircles A and B are exactly the same as the conventional one, and are defined by the characteristics of the first phase shift circuit 106. In the semicircle A, if the signal is a direct current, the signal is represented by a vector f. When the frequency increases, the signal moves on the semicircle A, and when the frequency becomes infinite, the signal is represented by a vector e.

If R112 = R113, the base of Q7 becomes the center point, and the signal of the base of Q8 with respect to the base of Q7 is indicated by vector c.

The feature of this embodiment is the position of the semicircle C. It can be seen that the magnitude of the vector c decreases as the frequency of the signal increases, as shown in FIG.

Next, the difference in operation when the control voltage V _AFC is changed will be described.

First, the case where the control voltage _VAFC is the negative maximum voltage will be described. The oscillation frequency of the circuit in this case is the lowest frequency that can be set. In this case, the transistor Q110 is on, and the transistor Q112 is off. That is, only the output signal of the buffer amplifier 122 appears in the output resistor R15. As described above, the amplification factor of the buffer amplifier 122 is set smaller than in the related art. The amplification factor of the buffer amplifier 122 is represented by the following equation. R110 / (R109 + R110 + R111) (5) Since the matching with the second phase shift amplifier circuit 110 is achieved in this way, the vector representing the output signal appearing at the output resistor R115 is, for example, equal to the vector k in the figure. Become.

The output signal represented by the vector k is fed back to the phase amplifier 102. As in the prior art, the ratio of the impedance of the resistor R102 connected to the base of the transistor Q101 of the first differential amplifier 120 to the resonator CX such as quartz or ceramic is equal to the ratio between the real component and the imaginary component of the vector k. Oscillation is performed at such a frequency. In this case, the oscillation frequency is slightly lower than the series resonance frequency of the resonator CX such as crystal or ceramic, and is the lowest oscillating frequency of this VCO.

Next, a case where the control voltage V _AFC is the maximum voltage will be described. In this case, the oscillation frequency is the maximum frequency that can be set. Instead of the signal represented by the vector k being fed back, the signal represented by the vector d is fed back. Others are exactly the same as the conventional one.

[0041] 3 . Operation at Parasitic Resonance Point Next, FIG. 3 shows a circle diagram in the case where oscillation occurs at a higher-order parasitic resonance point. As shown in FIG. 3, the vector a is
Since the frequency is increased, the value is closer to the vector e. Therefore, the real component of the vector a representing the lowest oscillation frequency is smaller than that in the case of FIG. The real component of the vector d representing the highest oscillation frequency is also smaller than that in the case shown in FIG. This is because the frequency has increased and the magnitude of the vectors c and d has decreased. This semicircle C is defined by the second phase shift amplifier circuit 110, and its operation is to increase not only the amount of phase shift as the frequency of the signal increases but also the amount of signal attenuation as described above. It is. This effect is apparent from the shape and position of the semicircle C shown in FIGS.

As described above, according to the present embodiment, the second phase shift network 128 inside the second phase shift amplifier circuit 110 is configured using the resistors R112, R113, R114, R116 and the capacitor C102. ing. And the resistance R
116 and a series circuit using a capacitor C102. Therefore, as shown in FIG. 3, as the oscillation frequency increases, the closed loop gain decreases. Therefore, it is possible to configure a voltage controlled oscillation circuit that is very unlikely to oscillate at the parasitic frequency of the resonator CX such as quartz or ceramic.

[0043]

As described above, according to the present invention, since the phase shift network is constituted by four resistors and one capacitor, when the oscillation frequency increases, the oscillation frequency signal can be attenuated. Therefore, the closed loop gain can be reduced at a high-order parasitic oscillation frequency, and an effect is obtained that a voltage-controlled oscillation circuit with extremely low possibility of oscillation at the parasitic oscillation frequency is obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of a voltage controlled oscillation circuit according to the present invention.

FIG. 2 is a circle diagram showing a state of an oscillation signal of the voltage controlled oscillation circuit shown in FIG.

FIG. 3 is a circle diagram showing a state of an oscillation signal when the voltage controlled oscillation circuit shown in FIG. 1 oscillates at a higher-order parasitic resonance point.

FIG. 4 is a block diagram of an embodiment of a conventional voltage controlled oscillation circuit.

FIG. 5 is a circuit diagram of one embodiment of a conventional voltage controlled oscillation circuit.

FIG. 6 is a circle diagram showing a state of an oscillation signal of a conventional voltage controlled oscillation circuit.

FIG. 7 is a circle diagram showing a state of an oscillation signal when a conventional voltage controlled oscillation circuit oscillates at a higher-order parasitic resonance point.

[Explanation of symbols]

 Reference Signs List 102 Phase amplifier circuit 104 Voltage controlled phase shift circuit 106 First phase shift amplifier circuit 108 Phase shift output circuit 110 Second phase shift amplifier circuit 112 Mixing circuit 121 First phase shift network 128 Second phase shift network CX Quartz or ceramic Resonators such as

Claims (1)

(57) [Claims] An input side of a phase amplification circuit is connected to an output side of a voltage controlled phase shift circuit that performs a phase shift by a phase shift amount according to a control voltage from the outside, and an output side of the phase amplification circuit is connected to the input side. A voltage controlled oscillation circuit in which a signal loop is formed by connecting an input side of the voltage controlled phase shift circuit, wherein the phase amplification circuit includes a resonator whose impedance changes according to an oscillation frequency. A first phase-shift amplifier circuit, wherein a phase shift amount changes according to an oscillation frequency; and an output circuit that generates an output signal of the first phase-shift amplifier circuit. A second phase shift amplifier circuit that shifts an output signal of the phase amplifier circuit by a predetermined phase shift amount, and an output signal of the phase amplifier circuit at a ratio according to the control voltage from outside, By mixing with the output signal of the amplifier circuit, A mixing circuit that creates a signal that is phase-shifted with a phase shift amount according to the control voltage and supplies the signal to an input side of the phase amplification circuit. Oscillates at a frequency at which the sum of the phase shift amount and the phase shift amount of the phase amplifier circuit is an integral multiple of 360 degrees. The second phase shift amplifier circuit includes a differential amplifier circuit including first and second transistors. A phase-shifted network that connects a differential output signal pair of the phase amplifier circuit to base input terminals of first and second transistors of the differential amplifier circuit. The phase network includes a first resistor connecting one signal line of the differential output signal of the phase amplifier circuit and a base terminal of the first transistor, and the other of the differential output signal of the phase amplifier circuit. Connect the signal line to the base terminal of the first transistor. A second resistor, a third resistor connecting one signal line of the differential output signal of the phase amplifier circuit to a base terminal of the second transistor, and a second resistor of the differential output signal of the phase amplifier circuit. A series circuit of a fourth resistor and a capacitor connecting the other signal line and the base terminal of the second transistor; and for a frequency higher than the resonance frequency of the resonator, the second phase shift A voltage controlled oscillator circuit, wherein the gain of the amplifier circuit is reduced, and the closed loop gain of the entire signal feedback path is reduced.