JPS6238322Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an LC type variable frequency oscillator suitable for application to a synchronous detection circuit of an AM stereo receiver.

It oscillates at a frequency of several MHz or more, and the ratio Δ/ ₀ between the center oscillation frequency ₀ and the frequency variable width Δ is relatively large, ranging from 1 to several tens of percent, and the noise figure C/N is relatively large.
An LC type variable frequency oscillator is generally used as a variable frequency oscillator with good performance. A variable capacitance element such as a variable capacitor is usually used as the frequency variable means of this oscillator, but this variable capacitance element is expensive and requires a circuit to drive this variable capacitance element. When integrated, the variable capacitance element must be externally attached, resulting in an increase in the number of pins.

On the other hand, it is possible to use a PN junction capacitance (junction capacitance between the base and emitter of a transistor) made by a normal bipolar planar process instead of a variable capacitance element, but in this case, the amount of capacitance change with respect to the control voltage is very small, so in order to widen the range of capacitance change, the control voltage must be increased or the PN junction area must be increased.

Since the control voltage cannot be increased in integrated circuits that operate with a source voltage of 2 to 3 V, the PN junction area must be expanded, but this would make it unsuitable for use as an oscillator in an LSI, and the PN junction surface area would have to be increased. As the size increases, there is a drawback that stability against fluctuations in parameters such as the emitter resistance value and _hFE of the transistor, internal disturbances such as external mechanical vibrations, and temperature changes, and external disturbances decreases.

Therefore, this invention proposes a variable frequency oscillator that is highly stable and suitable for integration with a wide frequency variable range even without using a variable capacitance element.

An example of the LC type variable frequency oscillator according to this invention will be described below with reference to FIG. 1 and subsequent figures.

In Figure 1, 10 is a resonant circuit (tank circuit) in which a coil L _T and a capacitor C _T are connected in parallel.
This resonant output voltage is supplied to the amplifier circuit 20. The amplifier circuit 20 performs oscillation at an oscillation frequency defined based on the impedance of the resonant circuit 10 by feeding back the output signal So to its input side. That is, the resonant circuit 10 and the amplifier circuit 2
0 constitutes an oscillation circuit.

Further, the output signal So of the amplifier circuit 20 is supplied to the output terminal 20a.

Further, the output signal So of the amplifier circuit 20 is supplied to a variable gain amplifier circuit 30, and the output signal is supplied to the resonant circuit 10 and the amplifier circuit 20 via an impedance circuit consisting of a capacitor C. The gain of variable gain amplifier circuit 30 is controlled by control voltage Vc supplied to terminal 30a.

According to this configuration, the impedance circuit for signal feedback consisting of the capacitor C is connected in parallel with the resonant circuit 10 in terms of alternating current as shown in FIG. Furthermore, this capacitor C increases from its actual capacitance due to the mirror effect of the variable gain amplifier circuit 30. Therefore, the equivalent capacitance C ^* seen from the resonant circuit 10 to the variable gain amplifier circuit 30 is the resonant circuit 1.
This means that it is connected in parallel to 0.

If the gain of the variable gain amplifier circuit 30 is A, the equivalent capacitance C ^* is C ^* =(1-A)C (1). Then, from the dynamic range relationship, when the gain A is selected as -3≦A≦0...(2), the amount of change ΔC in the equivalent capacitance C ^*
* becomes ΔC ^* = 3C …(3), and when C = 1 pF, the amount of change ΔC ^* is
It becomes 3pF.

When the variable gain amplifier circuit 30 is configured as a limiter amplifier circuit, this limiter amplifier circuit can be equivalently used as the variable gain amplifier circuit 30 by changing the output level B of the limiter amplifier circuit. .

When the variable gain amplifier circuit 30 is a limiter amplifier circuit configured as a differential amplifier as shown in FIG.
When the current I flowing into the collector resistor R of _Q11 and the current source transistor _Q14 is given, the limiter output vL is vL=B(cosωt+1/3cos3ωt+...) =RI(cosωt+1/3cos3ωt+...)...( 4) Here, ω: Oscillation angular frequency Therefore, if Vo is the level of the oscillation output So input to the base of the differential transistor _Q11 , then A=RI/Vo...(5) Therefore, the equivalent capacitance C ^* is C ^* = (1-R/Vo・I)C...(6) If the current I is varied by controlling the transistor _Q14 with the control voltage Vc, the equivalent capacitance C ^* will be increased over the range of ΔC ^* . can be changed. Therefore, the oscillation frequency is varied.

Note that the relationship between ₀ and Δ is given by Δ/ ₀ = 1/2・ΔC ^* /C _T (7), so the oscillation frequency of 1.8MHz _{is 0.}
For example, if a 260pF capacitor C ^T is used to obtain Δ≒10.4kHz (8).

FIG. 3 is a specific example of FIG. 1. The oscillation amplifier circuit 20 is configured as a differential amplifier, and a resonant circuit 10 is connected to the collector side of one differential transistor _Q1 .
is connected to this collector, and the obtained oscillation output is
So is fed back to the base of the other differential transistor Q ₂ via the emitter follower transistor Q ₃ and is also supplied to the variable gain amplifier circuit 30 .

The oscillation output So is further supplied to an AGC circuit 40, and its output is controlled to be constant. In this way, the oscillation output level is controlled to be constant because the output level B of the variable gain amplifier circuit 30 is controlled by the differential transistor Q ₁₁
Since B = RI regardless of ^the input to
This is because it changes.

The AGC circuit 40 has differential transistors Q ₄ and Q ₅ , one transistor Q ₅ is fixed at a reference level, and the oscillation output So supplied to the other transistor Q ₄ is passed through a phase inverting transistor Q ₆ . The current is supplied to the control transistor _Q7 , which controls the base current of the differential transistor _Q1 and keeps the oscillation output So constant.

In this figure, the capacitor C for signal feedback is connected to the tap of the coil L ^T , but this is to achieve impedance matching between the resonant circuit 10 and the amplifier circuit 20 including the variable gain amplifier circuit 30. . By tapping down, ΔC ^* becomes n ²
(n is the input-output turns ratio of the coil ^L The value of should be increased by about 10 times. Karini,
With a capacitor C of 10pF, Δ is 11.4kHz.

Further, in this embodiment, the oscillation output So is taken out via the emitter follower transistor _Q0 . The output of the variable gain amplifier circuit 30 is supplied to the capacitor C via the emitter follower transistor _Q13 . Transistors Q ₉ and Q ₁₀ are constant current source transistors.

As explained above, according to this invention, the equivalent capacitance C ^* can be changed in accordance with the gain of the variable gain amplifier circuit 30. Therefore, by varying the control signal Vc supplied to the variable gain amplifier circuit 30, the oscillation frequency of the signal So output from the oscillation circuit including the resonance circuit 10 and the amplifier circuit 20 can be varied, and the variable capacitance element A variable frequency oscillator can be constructed without using And since the frequency variable range Δ is also determined by the equivalent capacitance C ^* ,
The variable range can be selected relatively freely. According to experiments, it can range from 10 to 40kHz.

In addition, since the value of the feedback capacitor C apparently increases due to the Miller effect based on the variable gain amplifier circuit 30, the value of this capacitor C can be made smaller than the value of the external capacitor ^Cr . The stability of oscillation is improved, and therefore it is suitable for integration. Even when integrated, the number of pins can be reduced because no external variable capacitance element is used.

Although the above-described embodiment uses a tank-configured resonant circuit 10 and a differential amplifier 20, this idea can be applied to any oscillator with an LC configuration.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the principle of this invention, FIG. 2 is a partial AC equivalent circuit diagram thereof, and FIG. 3 is a concrete connection diagram of FIG. 1. 10 is a resonant circuit, 20 is an oscillation amplifier circuit, 30
is a variable gain amplifier circuit, C is a feedback capacitor,
C ^* is an equivalent capacitance, and 40 is an AGC circuit.

Claims (1)

[Scope of utility model registration request] An oscillation circuit having an LC resonant circuit and having an oscillation frequency defined based on the resonance characteristics of the LC resonant circuit, a variable gain amplification circuit to which a signal of the oscillation frequency is supplied from the oscillation circuit, and the variable gain amplification circuit. The output signal of the variable gain amplification circuit, which is supplied to one end of the capacitor, is taken out from the other end and supplied to the LC resonant circuit, so that the output signal of the capacitor is The capacitance is made substantially variable corresponding to the gain of the variable gain amplifier circuit, and the capacitance of the capacitor is added to the capacitance of the capacitor constituting the LC resonant circuit, and the gain of the variable gain amplifier circuit is The oscillation frequency of the oscillation circuit is changed in response to the change.
LC type variable frequency oscillator.