JPH1070413A - Resonance circuit for high frequency oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resonance circuit in which a relation between a control voltage and an oscillated frequency is adjusted for both low and high frequencies and fluctuation in an oscillated frequency is hardly caused even when a very small capacitance change is in existence at the oscillation circuit. SOLUTION: A varactor diode CV1 and a capacitor C2 are connected in parallel and a coil L and a capacitor C1 are connected in series with the parallel connection circuit and a capacitor C0 is connected in parallel with the series connection circuit to configure the resonance circuit, a resonance frequency is changed by varying a control voltage applied to the varactor diode and the capacitance of the circuits C1, C2 is adjusted to adjust a frequency characteristic toward low and high frequencies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonance circuit used for a high-frequency oscillation circuit, and more particularly to a resonance circuit for a local oscillation circuit that needs to perform tracking with an RF filter in a wide band in a television tuner or the like.

[0002]

2. Description of the Related Art A high-frequency oscillation circuit used in a television tuner is used to output a selected channel frequency as an intermediate frequency, and is used at a position as shown in FIG. As shown in FIG. 5, the television tuner first selectively amplifies a television wave input from an antenna by a single tuning filter 10, an AGC amplifier 12, and a detuning filter 14. The single tuned filter 10 and the detuned filter 14 are called an RF filter. Usually, by selecting a channel, a control voltage (tuning voltage) applied to a variable capacitance diode used for the channel changes and is selected. It works as a bandpass filter that passes only the frequency band of the channel.

On the other hand, the local oscillation circuit 16
a and a resonance circuit 16b for generating a frequency higher by an intermediate frequency (IF) than the video carrier frequency of the selected channel. The generated local oscillation frequency is input to a mixer 18, mixed with a video carrier frequency of a selected channel, converted into an intermediate frequency, and output as an intermediate frequency via an intermediate frequency amplifier 20.

In this case, similarly to the RF filter, the oscillation frequency of the local oscillation circuit 16 changes the control voltage applied to the variable capacitance diode included in the resonance circuit 16b in accordance with the selected channel and changes the oscillation frequency of the selected channel. A frequency higher than the video carrier frequency by an intermediate frequency is generated.

In order to select a desired channel (frequency) from a large number of wideband signals and convert it to an IF like a TV tuner, as shown in FIG. It is necessary that the change and the frequency change due to the change of the control voltage of the local oscillation circuit draw the same curve, and that the difference is always the intermediate frequency.

However, the difference between the frequency change due to the change in the control voltage of the RF filter and the frequency change due to the change in the control voltage of the local oscillation circuit generally increases as the frequency increases, as shown in FIG. There is a tendency.

Therefore, conventionally, a circuit has been used that takes into account tracking for making the difference an intermediate frequency at any frequency. In this circuit, the resonance circuit of the local oscillation circuit can be adjusted, and tracking is attempted by the adjustment. 8 and 9 show specific examples.

The resonance circuit shown in FIG. 8 is configured by connecting a capacitor C3 in series to a capacitor C4 and a variable capacitance diode CV2 connected in parallel, and connecting a coil L2 in parallel to the capacitor C3. When the control voltage is low,
The capacitance of CV2 becomes the maximum, and if CV2≫C4, the resonance frequency in the low region is determined by L, C3, and CV2. Also, when the control voltage is high, the capacity of CV2 becomes minimum,
If CV2 ≒ C4 and C3≫CV2, the resonance frequency in the high region is determined by L, C4 + CV2.

In this case, since the capacitance of C4 + CV2 is small in a high frequency range, the oscillation frequency is apt to fluctuate due to a subtle capacitance change on the oscillation circuit side to which the resonance circuit is connected.

On the other hand, in the resonance circuit shown in FIG. 9, a coil L3 is connected in series to a capacitor C5 and a variable capacitance diode CV3 connected in parallel, and a capacitor C6 is connected to the coil L3.
Are connected in parallel. In the case of such a configuration, setting C6 to a few tens of pF has the effect that the oscillation frequency hardly fluctuates even if there is a slight capacitance change on the oscillation circuit side.

[0011]

In the circuit shown in FIG. 8, however, the only element that contributes to tracking is C4. On the high frequency side, adjustment is possible as shown in FIG. Is difficult to adjust. Also, in the circuit shown in FIG. 9, the element that contributes to tracking is still only C5, and similarly, it is difficult to adjust on the low frequency side.

An object of the present invention is to adjust the relationship between the control voltage and the oscillation frequency on both the low-frequency side and the high-frequency side, and to suppress the fluctuation of the oscillation frequency even if there is a slight capacitance change on the oscillation circuit side. An object of the present invention is to provide a resonance circuit that hardly occurs.

[0013]

In order to solve the above-mentioned problems, the present invention employs the following means. That is,
A resonance circuit for a high-frequency oscillation circuit according to the present invention connects a variable capacitance diode CV1 and a capacitor C2 in parallel,
A capacitor C0 is connected in parallel to a connection formed by connecting the coil L and the capacitor C1 in series, and the resonance frequency is changed by changing a control voltage applied to the variable capacitance diode. It is characterized by.

With this configuration, when the applied control voltage is low, the capacitance of the variable capacitance diode CV1 becomes the maximum, and when CV1≫C2, the combined capacitance of CV1 and C2 connected in parallel is about CV1. Thereby, C1,
The combined capacitance C ′ of CV1 and C2 is C ′ ≒ C1 · CV1 /
(C1 + CV1).

When C 'and L1 are used as the inductive reactance, the resonance frequency on the low frequency side of this circuit becomes C
1, determined by CV1, L1, CO. By adjusting the value of C1, it is possible to change the frequency characteristics on the low frequency side.

On the other hand, when the control voltage to be applied is high, the capacitance of the variable capacitance diode CV1 becomes minimum, and the combined capacitance of CV1 and C2 connected in parallel becomes C2 + CV1. C
When 1≫C2 + CV1, the combined capacitance C ″ 容量 C2 + CV1 of C1, C2, and CV1. When C ″ and L1 are used as the inductive reactance, the resonance frequency on the high frequency side of this circuit becomes C2, CV1, L1, CO Depends on By adjusting the value of C2, it becomes possible to change the frequency characteristics on the high frequency side.

By setting the value of C0 in parallel with C1, CV1, and L1 to be sufficiently large with respect to the capacitance of the oscillation circuit, even if there is a delicate change in capacitance of the oscillation circuit, Oscillation frequency does not easily fluctuate.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of a resonance circuit for a high-frequency oscillation circuit according to the present invention. As shown in the figure, this resonance circuit has a variable capacitance diode CV1 and a capacitor C2 connected in parallel, a coil L and a capacitor C1 connected in series, and thus formed CV1, C2. , C1, and L1 and a capacitor C0 is connected in parallel. Then, by changing the control voltage applied to the variable capacitance diode CV1, the capacitance of the variable capacitance diode CV1 is changed to change the resonance frequency.

When the applied control voltage is the lowest, the capacitance of the variable capacitance diode CV1 becomes the maximum, and CV1≫C2
Then, the combined capacitance of CV1 and C2 connected in parallel is about CV1. Thus, the combined capacitance C 'of C1, CV1, and C2 is C' 、 C1 · CV1 / (C1 + CV1).
Becomes

When C 'and L1 are used as the inductive reactance X'L, the resonance frequency on the low frequency side of this circuit is determined by C1, CV1, L1, and CO. By adjusting the value of C1, it is possible to change the frequency characteristics on the low frequency side. That is, when C1 is increased,
As the inductive reactance X'L decreases, the frequency
When the value C1 is decreased, the inductive reactance X'L increases, and the frequency characteristic moves in the direction a shown in FIG.

On the other hand, when the applied control voltage is the highest, the capacitance of the variable capacitance diode CV1 becomes minimum, and the combined capacitance of CV1 and C2 connected in parallel becomes C2 + CV1. If C1≫C2 + CV1, C1, C2, CV1
Is the combined capacitance C ″ ≒ C2 + CV1.

If C ″ and L1 are used as the inductive reactance X ″ L, the resonance frequency on the high frequency side of this circuit is determined by C2, CV1, L1, and CO. By adjusting the value of C2, it becomes possible to change the frequency characteristics on the high frequency side. That is, when C2 is increased,
As the inductive reactance X "L decreases, the frequency characteristic
When the distance C moves in the direction d shown in FIG. 2 and C2 decreases, the inductive reactance X ″ L increases, and the frequency characteristic moves in the direction c shown in FIG.

If this resonance circuit is used as a resonance circuit for a local oscillation circuit of a television tuner, the tracking with the frequency characteristics of the RF filter is first set so that a difference is generated only by the intermediate frequency in an intermediate frequency band. The low frequency side can be adjusted so as to obtain the same difference by adjusting C1, and the high frequency side can be adjusted by adjusting C2.

Further, by setting the value of the capacitor CO sufficiently large with respect to the capacitance on the oscillation circuit side, it is possible to suppress the influence of the capacitance fluctuation due to the temperature change in the chip on the oscillation circuit side on the oscillation frequency. . Further, as shown in FIG. 4, the impedance characteristic on the resonance circuit side decreases in inverse proportion to the frequency, and becomes a frequency characteristic similar to the input impedance characteristic on the oscillation circuit side. The decline can be canceled.

[0025]

As described above, according to the present invention,
When changing the control voltage to generate various resonance frequencies from the low band to the high band, the capacitor C1
Can be changed to adjust the value of the capacitor C2, and the frequency characteristic of the high range can be changed by adjusting the value of the capacitor C2. Therefore, when used as a resonance circuit of a local oscillation circuit of a television tuner, tracking with an RF filter can be easily performed. Further, by setting the value of the capacitor CO to be sufficiently larger than the capacitance of the oscillation circuit, it is possible to suppress the influence of the capacitance variation due to the temperature change in the chip of the oscillation circuit on the oscillation frequency.

[Brief description of the drawings]

FIG. 1 is an embodiment of a resonance circuit for a high-frequency oscillation circuit according to the present invention.

FIG. 2 shows a frequency characteristic of the resonance circuit shown in FIG. 1, and shows a state where the frequency characteristic changes by adjusting a capacitor.

FIG. 3 is a diagram showing a change in frequency characteristics and is a diagram for reference in FIG. 2;

FIG. 4 is a diagram illustrating a relationship between frequency and impedance of the resonance circuit illustrated in FIG. 1;

FIG. 5 is a schematic block diagram of a television tuner.

FIG. 6 is a diagram for explaining the necessity of tracking the frequency characteristics of an RF filter and a local oscillation circuit of a television tuner.

FIG. 7 is a diagram illustrating that it is difficult to track the frequency characteristics of an RF filter and a local oscillation circuit of a television tuner.

FIG. 8 is a diagram showing a conventional resonance circuit.

FIG. 9 is a diagram showing a conventional resonance circuit.

FIG. 10 is a diagram illustrating frequency characteristics of the resonance circuits illustrated in FIGS. 8 and 9 and illustrating an adjustment range thereof.

[Explanation of symbols]

10: Single tuning filter, 12: AGC amplifier, 14
... detuning filter, 16 ... local oscillation circuit, 16b ...
... Resonant circuit, 18 ... Mixer, 20 ... Intermediate frequency amplifier, C0, C1, C2, C3, C4, C5, C6 ... Capacitor, CV1, CV2, CV3 ... Variable capacitance diode, L1, L2, L3 ... …coil.

Claims (1)

[Claims] 1. A variable capacitance diode CV1 and a capacitor C2 are connected in parallel, and a coil L and a capacitor C
1 and a capacitor C
0 is connected in parallel, and the resonance frequency is changed by changing the control voltage applied to the variable capacitance diode.