JPH10242752A - Clap type voltage controlled oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the voltage controlled oscillator with a wide frequency variable range and stable oscillation. SOLUTION: A varactor diode CV1 is connected in series with an inductance L, and the oscillator is oscillated at an oscillating frequency in response to a voltage Vc applied to the varactor diode CV1. In this case, A varactor diode CV2 is connected to any of feedback capacitors and the capacitance of the varactor diode CV2 is controlled with a control voltage Vc depending on the oscillating frequency so as to change the feedback amount in response to the oscillating frequency thereby keeping a loop gain to be a prescribed gain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clap type voltage controlled oscillator suitable for use as, for example, a variable frequency oscillator for a PLL.

[0002]

2. Description of the Related Art For example, a PLL is used as a synthesizer for high-frequency signal conversion processing of a portable telephone terminal. However, as a variable frequency oscillator for the PLL, particularly when the oscillation frequency can be varied over a wide band, Clapp type voltage controlled oscillators are often used. FIG. 4 shows a basic circuit configuration of the clap type voltage controlled oscillator. FIG. 4 shows only basic elements from which the bias circuit is omitted.

That is, in the example of FIG. 4, the amplifier uses a transistor Q1 which is a current control element, and a first feedback capacitor C1 is connected between the base and the emitter of the transistor Q1. , A second feedback capacitor C2 is connected between the collector and the emitter of the transistor Q1. A series circuit of a resonance inductance L and a variable capacitance diode CV is connected between the base and the collector of the transistor Q1.

In the circuit shown in FIG. 4, the capacitance of the variable capacitance diode CV depends on the control voltage Vc applied to the variable capacitance diode CV. Therefore, the oscillation determined by the inductance L and the combined capacitance including the variable capacitance diode CV. The frequency is changed according to the applied control voltage Vc.

In the clap type voltage controlled oscillator shown in FIG. 4, in order to increase the frequency variable ratio, it is necessary to increase the variable ratio of the combined capacitance of the resonance circuit. For this purpose, the capacitance of the variable capacitance diode CV is
1 and C2. For example, a method of using a variable capacitance diode itself having a small capacitance value or connecting a plurality of variable capacitance diodes in series is adopted.

As described above, if the capacitance of the variable capacitance diode CV is made smaller than the capacitances of the capacitors C1 and C2, in the resonance circuit that determines the oscillation frequency, the capacitance of the variable capacitance diode dominates the entire capacitance. Therefore, the frequency variable ratio can be approximated to a value close to the square root of the variable capacitance ratio of the variable capacitance diode CV. This is because the oscillation frequency fo can be expressed by the following equation.

[0007]

(Equation 1)

[0008]

In the oscillator, when the gain of the amplifier is represented by G (real number) and the feedback coefficient (transfer function of the feedback circuit) is represented by H, the loop gain (open loop gain) becomes G · H. The following conditions are required for the loop gain.

The condition for starting oscillation is as follows: GH> 1 (1)

The condition for sustaining the oscillation is GH = 1, and G is a real number.

H = 1 / G (2) H is a real number (the imaginary part is 0). (3) FIG. 5 shows a principle circuit of a three-point connected oscillator which is a basic circuit of the clap type oscillation circuit, and FIG. 6 shows an equivalent circuit thereof. Here, the principle of the amplifier is described using a transistor which is a current control type element.

In FIG. 6, hfe: current amplification factor of the transistor (hfe = G) hie: input impedance of the transistor. The output admittance hoe was omitted because it was considered to be sufficiently small.

According to FIG. 6, the relationship between the collector current ic and the base current ib of the transistor is given by the following equation (4).

[0014]

(Equation 2)

Therefore, the feedback coefficient H is given by: H = ib / ic = ib / hfe · ib (5) Therefore, when the equation (4) is substituted into the ib of the numerator of the equation (5), the following equation ( 6).

[0016]

(Equation 3)

In FIG. 5, since Z1, Z2 and Z3 are constituted by pure reactance elements, they are pure imaginary numbers and h
Since ie is a real number, in equation (6), the numerator is a real number, the first term of the denominator is an imaginary number, and the second term of the denominator is a real number.
Therefore, in the state where the oscillation continues, the feedback coefficient H is a real number according to the above equation (3), so that Z1 + Z2 + Z3 = 0 (frequency condition) (7) is obtained.

Equation (6) is as follows: H = -Z2 / (Z2 + Z3) (8) = Z2 / Z1 (9)

Incidentally, according to the gain condition (Equation (2)), H = Z2 / Z1 = 1 / hfe (gain condition) (10) holds in a state where oscillation continues.

From equation (10), it becomes clear that Z1 and Z2 must have the same sign. Then, it is further apparent from Expression (7) that Z3 has the opposite sign to Z1 and Z2.

Next, the clap type voltage controlled oscillator shown in FIG. 4 can be rewritten as shown in FIG. Therefore, in the clap type voltage controlled oscillator, each of the impedances Z1, Z2, Z3 in FIG. 6 can be changed as follows.

Z1 → ZC1 (capacitance) Z2 → ZC2 (capacitance) Z3 → ZL + ZCV1 (ZL is inductance, ZCV
1 is a variable capacitance).

Then, the frequency condition of the above equation (7) is as follows. Z1 + Z2 + Z3 = 0 → ZC1 + ZC2 + ZCV1 + ZL = 0 (11) Z1 = 1 / jωC1, Z2 = 1 / jωC2, Z3 = 1 /
Substituting jωCV1, ZL = jωL into equation (11),
When the oscillation angular frequency ω is obtained, the following equation (12) is obtained.

[0024]

(Equation 4)

The feedback coefficient H is as follows: H = Z2 / Z1 → H = ZC2 / ZC1 = C1 / C2 (13)

As described above, in order to obtain stable oscillation, it is necessary to maintain the loop gain G · H,
Generally, as the oscillation frequency increases, the gain G of the amplifier decreases. On the other hand, in the circuit of FIG. 4, the feedback coefficient H is constant irrespective of the oscillation frequency, as shown in Expression (13).

For this reason, as described above, even if the variable capacitance diode CV is reduced in capacitance and the variable frequency range is widened, the gain G of the amplifier is reduced in a high frequency range. Is small, and stable oscillation cannot be performed.

In view of the above, an object of the present invention is to provide a clap type voltage controlled oscillator which has a wide variable frequency range and oscillates stably.

[0029]

In order to solve the above problems, a clap type voltage controlled oscillator according to the present invention has a variable capacitance diode connected in series with a resonance inductance, and a control voltage applied to the variable capacitance diode. In a clap type voltage controlled oscillator that oscillates at a corresponding oscillation frequency, a variable capacitance diode different from the variable capacitance diode is connected to one of the feedback capacitances, and the capacitance of the other variable capacitance diode is controlled by the control voltage. It is characterized in that control is performed in accordance with the oscillation frequency, and the feedback amount is changed in accordance with the oscillation frequency.

In the present invention having the above configuration, for example, when the oscillation frequency increases and the gain of the amplifier decreases, the feedback amount is increased according to the oscillation frequency. Therefore, the frequency variable range can be increased, and the loop gain can be ensured within the frequency variable range, so that stable oscillation can always be performed.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a clap type voltage controlled oscillator according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a basic circuit configuration diagram of the clap type voltage controlled oscillator of this embodiment, in which a bias circuit is omitted. That is, in FIG.
The variable capacitance diode CV of the conventional clap type voltage controlled oscillator shown in FIG.
And the feedback capacitor C2 is replaced with a series circuit of a variable capacitance diode CV2 and a fixed capacitance capacitor C4.

The control voltage Vc applied to the variable capacitance diode CV1 is also applied to the variable capacitance diode CV2.
Apply it. In this case, the variable capacitance diode C
The voltage-capacity characteristics of V1 and the variable capacitance diode CV2 are as follows:
They need not be equal and may be set separately. The other components in FIG. 1 are the same as those in FIG.

In the case of the circuit shown in FIG. 1, when the oscillation frequency is low, the gain of the amplifier is large, but the control voltage Vc is low, so that the capacitance of the variable capacitance diode CV2 is large. Is large, the feedback amount is small. When the oscillation frequency is high, the control voltage Vc is high, but the capacitance of the variable capacitance diode CV2 is small. Therefore, the combined capacitance C20 of the capacitor C4 and the variable capacitance diode CV2 is also small. For this reason, the gain of the amplifier decreases, but the amount of feedback increases. When the feedback amount is large and the gain G is small, the distortion becomes low.

As described above, according to the configuration of FIG. 1, when the gain G of the amplifier changes due to the frequency change, the feedback amount H changes with the characteristic opposite to the change, so that the open loop gain is widened. An appropriate value can be maintained over the frequency range, and stable oscillation can be performed.

Therefore, according to the clap type voltage controlled oscillator of this embodiment, even when the capacitance of the main variable capacitance diode CV1 is reduced and the capacitance change ratio of the resonance circuit is increased, the feedback is performed as described above. By varying the capacitance according to the oscillation frequency, open loop gain can be maintained over a wide band, and stable oscillation can be performed with a large frequency variable ratio.

When the oscillation frequency is high, the combined capacitance C
20, the variable capacitance diode CV1
Is dominant in the fixed circuit of the capacitor C2 shown in FIG. 4, but this does not cause a reduction in the frequency variable ratio. This is because the decrease in the combined capacitance C20 means that the equivalent capacitance value of the resonance circuit that determines the oscillation frequency becomes smaller than the case of the fixed capacitance of the capacitor C2. Is higher.

FIG. 2 shows an equivalent circuit using the h parameter of the clap type voltage controlled oscillator of FIG. This is equivalent to the above-described equivalent circuit of FIG. 7 which is modified as ZC2 → ZC4 + ZCV2 (ZC4 is a capacitance and ZCV2 is a variable capacitance). Therefore, the oscillation angular frequency ω and the feedback coefficient H in this case are as shown in the following equations (14) and (15).

[0039]

(Equation 5)

Here, the oscillation frequency fo and the feedback coefficient H in the conventional circuit shown in FIG. 4 and the circuit shown in FIG.
Obtained by (15) is as follows. In this case, the following values were used as circuit constants.

That is, C1 = 10 pF C2 = 10 pF CV1 = 10 pF to 2 pF L = 20 mH C4 = 20 pF CV2 = 20 pF to 4 pF In the case of the conventional circuit, fo (minimum) = 616 MHz and fo (maximum) = 943 M
Hz, H = 1.00 In the case of the circuit of FIG. 1, fo (minimum) = 616 MHz, H = 1.00 fo (maximum) = 1068 MHz, and H = 3.00.

From the above, it can be seen that according to the clap type voltage controlled oscillator of this example, the variable frequency range becomes large and a stable oscillator can be obtained.

Next, a specific circuit example of the clap type voltage controlled oscillator of this embodiment, including a bias circuit, is shown in FIG.

In FIG. 3, resistors R1 and R2 are resistors for applying a control voltage Vc. The resistance R
3, R4 and R5 are bias resistors of the transistor amplifier. C5 is a coupling capacitance, and C6 is a bypass capacitance.

As can be seen from this specific circuit example, according to the circuit of the present invention, the number of components added to the conventional circuit is extremely small, so that it can be easily applied to a module having a limited size. is there.

[0046]

As described above, according to the present invention, it is possible to provide a wrap type voltage controlled oscillator having a wide frequency variable range and excellent oscillation stability.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a basic configuration of an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the circuit of FIG.

FIG. 3 is a circuit diagram showing a specific circuit example of the embodiment of the present invention.

FIG. 4 is a circuit diagram showing a basic configuration of a conventional clap type voltage controlled oscillator.

FIG. 5 is a circuit diagram illustrating a basic configuration example of an oscillator.

FIG. 6 is an equivalent circuit diagram of the circuit of FIG. 1;

FIG. 7 is an equivalent circuit diagram of the circuit of FIG. 6 using h parameters.

[Explanation of symbols]

L: inductance of resonance circuit, CV1: main variable capacitance diode, CV2: variable capacitance diode for variable feedback amount, C1, C2, C4: feedback capacitance, C5: coupling capacitance, C6: bypass capacitance, R1, R2: control Voltage application resistors, R3, R4, R5 ... bias resistors

Claims (1)

[Claims] A variable capacitance diode is connected in series with a resonance inductance, and oscillates at an oscillation frequency according to a control voltage applied to the variable capacitance diode. A variable capacitance diode different from the variable capacitance diode is connected, the capacitance of the other variable capacitance diode is controlled according to the oscillation frequency by the control voltage, and the feedback amount is changed according to the oscillation frequency. A clap-type voltage controlled oscillator.