JPH0563442A - Oscillation circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] An AGC loop is configured without inserting a feedback resistor on the crystal unit side, and this makes it possible to increase the load impedance of the crystal unit.
(EN) Provided is a crystal oscillating circuit capable of obtaining a stable oscillation output without lowering the output. [Configuration] An FE is provided on the emitter side of the oscillation transistor 12.
The T element 17 is inserted as a variable resistance element. The gate potential of the FET element before the start of oscillation is zero, and when the oscillation starts, the output of the tuning transformer 13 is taken into the AGC loop, converted into a direct current by the rectifier circuit 18, and a reverse bias voltage is applied to the gate G of the FET element. The resistance value between the drain and the source of the FET element is increased. At the same time that the resistance value on the emitter side of the transistor 12 increases, the potential on the emitter side rises, relatively limiting the base current of the transistor 12 and lowering the gain.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a crystal oscillator circuit using a crystal oscillator as a frequency control element, and particularly to an excellent C / N and stable oscillation output without lowering the Q of the crystal oscillator. And a crystal oscillation circuit capable of obtaining

[0002]

2. Description of the Related Art A crystal oscillation circuit in which a crystal oscillator and a transistor are combined is well known by utilizing the fact that the natural frequency of the crystal oscillator is extremely accurate and the stability is high. For example, in a radio frequency oscillator, a crystal oscillation circuit that uses the piezoelectric effect of crystal is almost universally used as a device that can satisfy the frequency stability requirements stipulated in the International Telecommunications Convention.

In a highly stable crystal oscillator, the oscillation output C
In order to increase / N (signal-to-noise ratio), it is necessary to suppress the current (crystal current) flowing through the crystal resonator during oscillation (crystal current) as much as possible within a range in which oscillation is possible and keep it constant. Therefore, a circuit for automatically controlling the oscillation level of the crystal unit, that is, an automatic gain control loop (AGC loop) is often provided.

Conventionally, as shown in FIG. 4, in gain control, an oscillation output is converted into a direct current by a rectifier circuit 2, and a negative resistance is applied to a base of an oscillating transistor 3 by a feedback resistor 6 to lower the base potential. , Oscillation transistor 3
The AGC loop is configured to reduce the apparent gain.

However, the above-mentioned method necessarily requires the bypass capacitor 7 for eliminating the ripple of the rectifier output voltage, and the resistance value of the feedback resistor 6 cannot be increased.

Since the feedback resistance 6 is dominant in the load resistance from the crystal unit 1, the Q of the crystal unit 1 is lowered,
There was a problem of lowering the stable value of the oscillation frequency.

In the figure, in order to feed back the base bias of the oscillating transistor 3, the resistance value of the feedback resistor 6 must be made relatively small.
The base of the oscillation transistor 3 has a low impedance. Further, even if the crystal oscillator 1 having an originally high Q is used due to the influence of the capacity of the bypass capacitor 7, the apparent Q of the entire oscillation circuit becomes low, and there is a limit in improving the C / N of the oscillation output. Further, since the base bias of the transistor is directly controlled, noise may be mixed through the AGC loop, and it has been difficult to cope with the noise.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to construct an AGC loop without inserting a feedback resistor on the crystal unit side, and thereby to realize the crystal unit of the crystal unit. The load impedance can be increased and the C / N can be increased without lowering the Q of the crystal unit.
It is to provide a crystal oscillation circuit capable of obtaining a stable oscillation output.

[0009]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a crystal oscillator, an oscillating transistor having a base connected to one end of the crystal oscillator, and a bias resistor for setting a base bias of the oscillating transistor. And a tuning circuit connected to the emitter of the oscillating transistor for selecting an oscillating frequency, in a crystal oscillating circuit, an FET element inserted as a variable resistance element on the emitter side of the oscillating transistor, and an oscillating circuit It is characterized in that an AGC loop including a rectifying circuit for rectifying the oscillation output of the transistor to generate a negative potential and a feedback resistor for setting the gate potential of the FET element is added.

[0010]

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of a crystal oscillator circuit according to the present invention.

The circuit shown in FIG. 1 is a Colpitts type crystal oscillating circuit which has been widely used in the past except the AGC circuit, and a detailed description thereof is unnecessary. The oscillator is a capacitor inserted between the base of the oscillation transistor 12 and the ground, and the capacitor C connected in series with the crystal oscillator is for fine adjustment of the oscillation frequency.

Further, bias resistors 15 and 16 for applying a potential to the base are added to the base of the oscillation transistor 12 so that the transistor functions as an amplifier, and further, between the base and the emitter of the transistor 11 and between the emitter and the ground. The dividing capacitors C1 and C2 are connected.
In the circuit shown in this example, the primary side of the transformer 13 is connected in parallel to the split capacitor C2, and the oscillation output is obtained from the secondary coil thereof. In this case, the capacitor C2 outputs the oscillation output together with the coil of the transformer. The value is tuned to the frequency and has a capacitance value necessary as a dividing capacitor of the Colpitts oscillator circuit. The capacitor C inserted between the capacitor C2 and the ground is for high frequency bypass.

In this embodiment, the emitter of the transistor of such Colpitts oscillator has the following A
It has a GC loop.

That is, a capacitor C3 having a relatively small capacitance and a resistor R1 connected to the output terminal of the oscillation circuit, diodes D1 and D2 for rectifying the branched high frequency signal, a capacitor C4, and a rectified DC current. Resistor R2
, R3 and an FET 17 to which the voltage is applied to the gate, and the drain and source of this FET are inserted between the emitter resistor R4 of the oscillation transistor 12 and the ground. The FET element 17 connected to the emitter side of the oscillation transistor 12 is used as a variable resistance element, but a transistor may be used. The rectifier circuit 18 connected to the gate side of the FET element 17 serves to rectify the oscillation output to generate a negative potential, and the feedback resistor 19 sets the gate potential of the FET element 17.

The operation of this structure will be described. Before the start of oscillation, the gate potential of the FET element 17 is zero, and the resistance between the drain and source of the FET element 17 is small and apparently short-circuited. Oscillation starts, and as the oscillation level rises,
The output from the tuning transformer 13 is taken into the AGC loop, converted into a direct current by the rectifier circuit 18, and a reverse bias voltage is applied to the gate G of the FET element 17. As a result, the resistance value between the drain D and the source S of the FET element 17 is increased. Therefore, transistor 1
At the same time that the resistance value on the emitter side of No. 2 increases, the potential on the emitter side rises, the base current is relatively limited, and the gain can be reduced.

In this way, the AGC loop can be constructed without inserting a feedback resistor on the crystal oscillator side. As a result, in the initial stage of the amplitude, the gain of the amplifier is large and the oscillation start-up is stable, and the load impedance of the crystal unit becomes high at the time when the output level increases and the Q of the crystal unit does not decrease. It is possible to improve C / N and obtain a stable oscillation output.

The oscillation circuit to which the present invention is applied is not limited to the Colpitts oscillation circuit described above, and the gain of the oscillation amplifier is
The present invention can be applied to anything that can indirectly control the signal input end bias without directly controlling it.

Since the Colpitts oscillator circuit has various modified circuits, it can be applied to each of them.

For example, in FIG. 2, the oscillation signal is directly derived from the emitter without the output transformer in the emitter of the transistor 12, or as shown in FIG. 3, a small resistance is inserted in the collector of the oscillation transistor to form the transistor. The same can be applied to the case where the oscillation signal is derived from the collector of the above.

[0021]

As described above, according to the present invention,
Since the FET element is inserted on the emitter side of the oscillation transistor to form the AGC loop, the load impedance of the crystal oscillator in the crystal oscillation circuit can be increased, and in the crystal oscillator circuit with high stability requiring AGC. In addition, a remarkable effect can be obtained in further increasing the stability.

[Brief description of drawings]

[0022]

FIG. 1 is a configuration diagram of a crystal oscillation circuit according to the present invention.

[0023]

FIG. 2 shows a modified embodiment of the present invention.

[0024]

FIG. 3 is a modified embodiment of the present invention.

[0025]

FIG. 4 is a configuration diagram of a conventional crystal oscillation circuit.

[0026]

[Explanation of symbols]

 11 crystal oscillator, 12 oscillation transistor, 13 tuning transformer, 14 tuning circuit, 15 and 16 bias resistor, 17 FET element, 18 rectifier circuit, 19 feedback resistor,

Claims (1)

[Claims] 1. In a Colpitts oscillation circuit including a piezoelectric vibrator and a transistor amplifier, an FET or a transistor inserted as a variable resistance element on the emitter side of the transistor amplifier and a part of the oscillation output are rectified to obtain the FE.
An oscillating circuit including an AGC loop including a rectifying circuit for generating a gate or base bias voltage of T or a transistor.