JPH0563438A - Voltage control oscillator - Google Patents

Abstract

PURPOSE:To provide the wide-band voltage control oscillator with superior modulation sensitivity characteristic by reducing the deviation of the modulation sensitivity for oscillation frequency in the voltage control oscillator with FM modulation function. CONSTITUTION:The voltage control oscillator is provided with a resonance circuit 3, a first variable capacity circuit 1 supplying the capacity decided by a control signal linked and impressed to the resonance circuit 3, a second variable capacity circuit 2 supplying the capacity decided by a modulation input signal to be linked and impressed to the resonance circuit 3, and an oscillation circuit. The second variable capacity circuit 2 is composed of a variable capacity diode 10, capacitor 12 parallely connected to the variable capacity diode 10, and a serial circuit of an inductor 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement in modulation sensitivity of a voltage controlled oscillator having an FM modulation function.

[0002]

2. Description of the Related Art FIG. 2 shows a circuit diagram of a conventional example of a voltage controlled oscillator. This voltage controlled oscillator includes a resonance circuit 3, a first variable capacitance circuit 1, a second variable capacitance circuit 2 and a negative resistance generation circuit 4.
It is composed of.

The resonance circuit 3 is composed of an LC circuit including an inductor 13 and a capacitor 14, and determines the resonance frequency of the entire circuit together with the capacitances determined by the first variable capacitance circuit 1 and the second variable capacitance circuit 2 described below. There is. In addition, the negative resistance generation circuit 4 includes active elements such as the transistor 22,
It oscillates at a frequency equal to the above resonance frequency and outputs the oscillation signal from the output terminal 7.

The first variable capacitance circuit 1 comprises a variable capacitance diode 8 and a capacitor 9 having a relatively wide capacitance change range,
The capacitance is largely changed according to the control signal applied to the control voltage input terminal 5, so that the output frequency of the voltage controlled oscillator is largely changed and the output center frequency is set. Further, the second variable capacitance circuit 2 is composed of a variable capacitance diode 10 and capacitors 11 and 12 having a relatively narrow capacitance change range, and minutely changes the capacitance in accordance with the modulation input applied to the modulation input terminal 6, thus performing voltage control. The oscillation frequency of the oscillator is changed within a narrow range near the output center frequency. That is, the oscillation frequency of the output can be minutely shifted according to the change of the modulation input signal, and it can be used as a frequency modulator.

Next, let us consider the relationship of the frequency shift amount of the oscillation frequency with respect to the modulation sensitivity, that is, the change of the modulation input signal. 2, the equivalent capacitance of the entire circuit is C, the inductance of the inductor 13 of the resonant circuit 3 to L _O, the oscillation frequency f of the output is given by the following equation (1).

[0006]

[Equation 1]

When the equivalent capacitance C of the entire circuit is set to C _H when the oscillation frequency f is high and C _L when the oscillation frequency f is low, and L _O is constant in the equation (1), the following equation (2) is established.

C _L > C _H ‥‥‥‥‥‥‥‥‥‥ (2)

On the other hand, considering the capacitance change of the variable capacitance diode 10 due to the modulation signal input from the modulation input terminal 6, generally, the DC bias voltage value of the modulation input signal is constant, and the change in voltage of the modulation input signal is caused. The capacitance change of the variable capacitance diode _{ΔC VD2} is the oscillation frequency f
Does not depend on Consider the change in capacitance with respect to the change in the modulation input signal in the entire second variable capacitance circuit 2. The equivalent capacitance C _V2 of the second variable capacitance circuit 2 is given by the following equation (3).

C _V2 = C ₂ (C _VD2 + C ₃ ) / [C ₂ + (C _VD2 + C ₃ )] (3) Here, C ₂ and C ₃ are capacitors 11 and 12, respectively.
Is the capacity value of.

In this way, the variable capacitance diode 10
Since the other elements have a constant capacitance value regardless of the oscillation frequency, the capacitance change ΔC _{V2 of the} entire second variable capacitance circuit 2 with respect to the voltage change of the modulation input signal is also independent of the oscillation frequency. From the above, when considering the modulation sensitivity, that is, the amount of deviation of the oscillation frequency with respect to a constant change of the modulation input signal, by considering the relationship between the change capacitance ΔC _V2 in the second variable capacitance circuit 2 and the equivalent capacitance of the entire circuit, The following expression (4) is established from the expression (2).

ΔC _V2 / C _L <ΔC _V2 / C _H (4)

In equation (4), the higher the oscillation frequency,
It shows that the modulation sensitivity becomes high. Where the second
As a supplementary explanation of the variable capacitance circuit 2, the capacitor 12 connected in parallel with the variable capacitance diode 10 has a capacitance C
_Although it is possible to correct the magnitude of the modulation sensitivity by changing the capacitance change ΔC _V2 by changing ₃ , it is not related to the deviation of the modulation sensitivity due to the change of the oscillation frequency because it is irrelevant to the oscillation frequency. .. A solid line 24 in FIG. 3 shows an example of the characteristic of the modulation sensitivity in the conventional technique.

[0014]

As described above, the above-mentioned conventional technique has a drawback that the modulation sensitivity changes to a higher value as the oscillation frequency increases. Under these conditions, if the modulation sensitivity at the highest frequency in the oscillation frequency range is adjusted within the specified value, the modulation will be insufficient at low frequencies, while conversely, if the modulation sensitivity is set to a predetermined level at the lowest frequency, the high frequency will be high. There may be illegal modulation sensitivity outside the specifications. For this reason, there is a drawback that frequency modulation must be performed only within a narrow range of the oscillation frequency range.
It is an object of the present invention to provide a voltage controlled oscillator which has a small deviation of modulation sensitivity with respect to an oscillation frequency and has a wide range of excellent modulation sensitivity characteristics.

[0015]

In order to achieve the above-mentioned object, the present invention provides an inductor in series with a capacitor 12 for modulation sensitivity correction which is connected in parallel to a variable capacitance diode 10 in a second variable capacitance circuit 2. Is added, and the equivalent capacitance of the series circuit of the capacitor and inductor increases as the oscillation frequency increases.

[0016]

As a result, as the oscillation frequency increases, the capacitance change ΔC _V2 of the second variable capacitance circuit 2 with respect to the voltage change of the modulation input signal decreases as described later, and thus the modulation sensitivity decreases. .. Therefore, it is possible to reduce the deviation of the modulation sensitivity with respect to the change of the oscillation frequency in the circuit in which the modulation sensitivity changes with the increase of the oscillation frequency.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIG. Each part 1 to 22 is the same as the conventional voltage controlled oscillator shown in FIG. 2 and operates in the same way. In the present invention, the inductor 23 is provided in series with the modulation sensitivity correction capacitor 12 connected in parallel with the modulation variable capacitance diode 10. When the equivalent capacitance C _X of the series circuit of the modulation sensitivity correcting capacitor 12 and the inductor 23 is set, C _X is expressed by the following equation (5).

C _X = C ₃ / [1- (2πf) ² L ₁ C ₃ ] ... (5)
Here, L ₁ is the inductance of the inductor 23.

As can be seen from the equation (5), the equivalent capacitance C _X of the series circuit of the modulation sensitivity correcting capacitor 12 and the inductor 23 increases as the oscillation frequency f increases. On the other hand, the equivalent capacitance C _{V2 of the} entire second variable capacitance circuit 2
Is

C _V2 = C ₂ (C _VD2 + C _X ) / [C ₂ + (C _VD2 + C _X )] = C ₂ C _y / (C ₂ + C _y ) ... (6) where _Let C _y = C _VD2 + C _X.

Therefore, the capacitance change ΔC _V2 of the second variable capacitance circuit 2 with respect to the voltage change of the modulation input signal is given by the following (7)
It is shown by the formula.

ΔC _V2 = | C _V2 −C _V2 ′ | = | C ₂ C _y / (C ₂ + C _y ) −C ₂ C _y ′ / (C ₂ + C _y ′) | = | C ₂ ² ΔC _y / (C ₂ + C _y ) (C ₂ + C _y ′) | ... (7)

Here, ΔC _y is ΔC _y = | C _y −C _y ′ | = | (C _VD2 + C _X ) − (C _VD2 ′ + C _X ) | = | C _VD2 −C _VD2 ′ | = ΔC _VD2 ‥‥‥‥‥‥‥‥‥‥‥‥‥ (8)

From the above equations (5), (7) and (8), the following can be concluded. As the oscillation frequency f increases,
As shown in the equation (5), since the equivalent capacitance C _X of the series circuit of the capacitor 12 and the inductor 23 increases, the equivalent capacitance C _{y obtained} by adding the equivalent capacitance C _VD2 of the variable capacitance diode 10 to this also increases. On the other hand, the capacitance change ΔC _y due to the voltage change of the modulation signal of the equivalent capacitance C _y is equal to the capacitance change _{ΔC VD2} of the variable capacitance diode 10 as shown in the equation (8), and does not depend on the oscillation frequency as described above. Therefore, since the denominator C _{y in} Eq. (7) increases and the numerator ΔC _y is constant as the oscillation frequency increases, the capacitance change ΔC _V2 of the second variable capacitance circuit 2 with respect to the voltage change of the modulation signal.
Is decreasing. That is, the capacitance change of the second capacitive circuit 2 with respect to the voltage change of the modulation signal has the following relationship when _{ΔC V2H} is set when the oscillation frequency is high and _{ΔC V2L} is set when the oscillation frequency is low.

△ C _V2L ＞ △ C _V2H ‥‥‥‥‥‥‥‥‥‥ (9)

In the prior art, ΔC is applied to the above equation (9).
_{Since V2L} = _{ΔC V2H} , the modulation sensitivity becomes higher as the oscillation frequency f becomes higher as shown in the equation (4). However, according to the present invention, the values of the added inductor 23 and capacitor 12 are adjusted. As a result, the deviation of the modulation sensitivity due to the oscillation frequency can be reduced, and the modulation sensitivity can be made almost constant in the oscillation frequency range as shown in the following expression (10).

_{ΔC V2L} / C _{L ≈ΔC V2H} / C _H ‥‥‥‥‥ (10)

An example of the characteristic of the improved modulation sensitivity according to the present invention is shown by the solid line 25 in FIG. As shown in this figure, almost constant modulation sensitivity is achieved in a wide oscillation frequency band.

[0029]

According to the present invention, a change in modulation sensitivity with respect to a change in frequency can be made small, and a voltage controlled oscillator in which a change in modulation sensitivity is small in a wide band can be realized, and its effect is remarkable. Further, since the means of the present invention is extremely simple,
Not only does the circuit scale be small, it is also economical, and its application range is wide.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an example of a conventional voltage controlled oscillator.

FIG. 3 is a characteristic diagram showing a change in modulation sensitivity with respect to an oscillation frequency.

[Explanation of symbols]

 1 1st variable capacitance circuit 2 2nd variable capacitance circuit 3 Resonance circuit 4 Negative resistance generation circuit 5 Control voltage input terminal 6 Modulation input terminal 7 Output terminal 8,10 Variable capacitance diode 11 Capacitor 12 Modulation sensitivity correction capacitor 23 Inductor 24 Conventional modulation sensitivity characteristic 25 Modulation sensitivity characteristic according to the present invention

Claims (1)

[Claims] 1. A resonance circuit and a first capacitance coupled to the resonance circuit to provide a capacitance determined by a control signal applied to the resonance circuit.
A variable capacitance circuit; a second variable capacitance circuit coupled to the resonance circuit to provide a capacitance determined by a modulation input signal applied; and a resonance circuit coupled to the input side of the resonance circuit and the first and second variable capacitance circuits. And an oscillation circuit that generates an output signal of a frequency determined by a capacitance circuit, wherein the second variable capacitance circuit includes at least a variable capacitance diode, and a capacitor and an inductor connected in parallel to the variable capacitance diode. And a series circuit of a voltage controlled oscillator.