JPS6226908A - Voltage controlled oscillation circuit - Google Patents

Abstract

PURPOSE:To prevent the oscillation of a crystal resonator except at fundamental wave and to obtain an oscillation frequency with high signal purity by providing an amplifier, a low pass filter varactor element as a positive feedback circuit and a fundamental wave crystal resonator and adding a low resistor across a serial resonance circuit composing of the varactor element and the crystal resonator. CONSTITUTION:Low resistors 5, 6 are connected between common and input/ output terminals of a series resonance circuit comprising a variable capacitance element 3 and a crystal resonator 4 to increase the attenuation including the low resistors 5, 6 at a frequency apart from a series resonance frequency, that is, with high effective impedance of the series resonance circuit thereby improving the frequency selectivity. That is, the Q of the series resonance circuit is increased to improve the signal purity. Further, a low pass filter circuit 2 is used for the output side of the amplifier 1 to attenuate the positive feedback quantity at the sub resonance point or the higher order resonance point thereby obtaining the preventing effect of oscillation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a voltage-controlled crystal oscillator circuit that can be directly modulated and serves as a reference frequency source for a transmission signal, particularly in an audio interrupt transmission circuit such as a microwave repeater. .

[Conventional technology]

Crystal oscillator circuits are widely used as reference frequency sources for frequency control (AFc) circuits and phase control (APC) circuits because they can easily obtain high frequency stability and high purity signals.

Most conventionally used crystal oscillation circuits are of the Colpitts type, as shown in FIG. 3, in which a single transistor 21 is provided with a capacitor and a Tycho capacitor. When applying frequency modulation to this circuit, the most sensitive and commonly used method is to connect the modulating variable capacitor 3 in series with the crystal resonator 4 connected to the pace of the transistor 21.

As another conventional example, as shown in FIG. 2, there is a circuit in which a crystal resonator 4 and a modulating variable capacitance element 3 are connected in series to a feedback circuit of one amplifier 1.

[Problem that the invention seeks to solve]

However, in the case of the one shown in FIG. 3, a sufficient frequency variable range cannot be obtained due to the capacitance between the terminals of the transistor 21 and the capacitance connected between the terminals, and there is also a problem in the linearity of the frequency modulation sensitivity. Following this, on the other hand, in the second figure,
Since the capacitance between the input and output terminals of the amplifier 10 is small, the drawbacks of the Colpitts type described above are eliminated. Therefore, the frequency variable range becomes wide, and it is advantageous to obtain linearity of frequency modulation sensitivity. However, due to the widening of the frequency variable range, the oscillation frequency may shift to a higher-order resonance frequency due to the influence of the sub-resonance of the crystal resonator 4. Furthermore, by connecting the modulation variable capacitance element 3 in series with the crystal resonator 4, the external Q of the oscillator decreases, and the output signal purity (C/
The disadvantage is that the N ratio) deteriorates.

[Means for solving problems]

The voltage-controlled crystal oscillator circuit of the present invention that can be directly modulated has a low-pass filter, a variable capacitance element, and a crystal resonator connected in series to the feedback circuit of an amplifier, and a series resonance consisting of the crystal resonator and the variable capacitance element. The feature is that a low resistance resistor is connected to the ground at both ends of the circuit.

As a result, the reference frequency of the transmission signal and [2 and high purity 1
A directly modulated voltage controlled crystal oscillator circuit is provided.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. The positive feedback circuit of the amplifier 1 is composed of a series connection circuit of a low-pass filter 2, a modulation variable capacitance element 3, and a crystal resonator 4.

The oscillation frequency is determined by the resonant frequency of the series resonant circuit consisting of the modulating variable capacitance element 3 and the crystal resonator 4. Therefore, the maximum frequency modulation sensitivity can be obtained by the modulation preparation variable 1 element 3. At the resonant frequency, the effective impedance of the series resonant circuit is only the resistance, and the amount of positive feedback becomes maximum, causing oscillation. At frequencies far from the resonant frequency, the effective impedance of the series resonant circuit is high and the amount of positive feedback is attenuated, so oscillation does not occur.

As shown in Figure 1, by connecting the variable resistor to the input and output of the series resonant circuit consisting of the element 3 and the crystal resonator 4, and the low resistors 5 and 6 between the ground, the series resonant frequency can be reduced. At distant frequencies, that is, when the effective impedance of the series resonant circuit is high, the amount of attenuation including the low resistors 5 and 6 increases, increasing frequency selectivity. That is, as the Q of the series resonant circuit increases, signal purity can be improved. Furthermore, when a general-purpose broadband amplifier is used as an amplifier, oscillation 'fc may occur at the sub-resonance point or higher-order resonance point of the crystal resonator 4. This often occurs because there is no difference in atb between the resistance component at these resonance points and the resistance component at the fundamental resonance point.

As shown in Figure 1, a low-pass filter circuit 2 is connected to the output shaft of the amplifier 1.
), it is possible to attenuate the positive feedback i' at the sub-resonance point or higher-order resonance point, thereby obtaining the effect of preventing oscillation.

〔Effect of the invention〕

As explained above, the present 8A has an amplifier, a low-pass filter variable capacitance element, and a fundamental wave crystal resonator in the positive feedback circuit, and a low By adding a resistor, it is possible to prevent oscillations caused by waves other than the fundamental wave of the crystal resonator and to obtain an oscillation frequency with high signal purity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a crystal oscillation circuit according to an embodiment of the present invention, and FIGS. 2 and 3 are a block diagram and a circuit diagram of a conventional oscillation circuit, respectively. 1... Amplifier, 2... Curved low-pass filter, 3... Curved capacitive element, 4... Crystal oscillator, 5, 6... Curved low resistor, 11 ...... Output terminal, 12... Input terminal for music modulation. 21...Transistor, 22...Song power supply terminal. Anchor Chihaya 1 to figure 22 Densu #I child figure 3

Claims (1)

[Claims] A modulation variable capacitance element and a crystal oscillator are connected in series between the input and output of an amplifier, a low-pass filter is inserted on the output side of the amplifier, and the modulation variable capacitance element and the crystal oscillator are connected in series. A voltage controlled oscillator circuit characterized in that a resistor is connected between both ends of a series resonant circuit and a reference potential point.