JPS61103308A - Dual tuning circuit - Google Patents

Abstract

PURPOSE:To obtain a desired frequency characteristic by changing a control voltage source so as to change simultaneously the capacitance of a varactor diode thereby changing the tuning frequency of two turning circuits whose sharpness is set differently respectively in opposite directions at the same time. CONSTITUTION:An IF signal incoming to an input terminal 15 is amplified by a transistor (TR) TR1 and fed to tuning circuits 13, 14, where dual tuning is taken with tuning frequencies f1, f2, the impedance is converted by a TR2 and the result is extracted from an output terminal 16. Since the tuning circuits 13, 14 are provided respectively with varactor diodes C1, C2 in this case, when the voltage of a control voltage source V3 is changed, the tuning frequencies f1, f2 are changed in opposite directions with the same ratio at the same time. An F characteristic (frequency characteristic) shown in (B) is obtained at f1=f2=f0 for example, the F characteristic shown in (A) is obtained with f1<f2, an the F characteristic shown in (C) is obtained at f1>f2.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a double-tuned circuit, and is used for color television reception f&
The present invention relates to double-tuned circuits for video high-frequency signals such as TVs and home-use small VTRs.

BACKGROUND OF THE INVENTION FIG. 3 shows a block diagram of an example of a general video signal receiving circuit. The video high-frequency signal occupies a bandwidth allocated to each channel, and is transmitted through the antenna 1 to the high-frequency (hereinafter referred to as RF) tuning circuit 3 of the television tuner 2. RF amplifier4. The signal is supplied to the RF tuning circuit 5.

In the tuner 2, when receiving the selected channel frequency, a predetermined tuning voltage is applied to the variable capacitance diode in a reverse bias state, and the RF tuning circuit 3.5 and the local oscillator 7 are respectively selected to have a predetermined resonance frequency. A mixer 6 that mixes the output of the RF tuning circuit 5 and the output of the local oscillator 7
From this, a signal representing the difference between the RF signal frequency and the local oscillation frequency is obtained as an intermediate frequency (hereinafter referred to as IF) signal. The video IF signal taken out from the tuner 2 has unnecessary frequency components removed by an IF filter 8, and is detected by a video detector 9.
It is taken out from the output terminal 10.

By the way, it is necessary that the difference between the RFF signal frequency and the local oscillation frequency always correctly matches the intermediate frequency.
Further, it is necessary that the tuning frequencies of the RFF tuning circuits 3.5 match. An important issue is the quality of so-called tracking, which indicates that these match at all channel frequencies.

However, in the above circuit, if the tuning frequencies of the RFF tuning circuits 3 and 5 and the local excavation circuit 7 change due to variations in the characteristics of the variable capacitance diodes or the influence of temperature changes, and the tracking becomes defective, each Since the bandwidth of each channel is as large as 4.2 MHz, the frequency versus amplitude characteristic (hereinafter referred to as F characteristic) of the video IF signal output from the tuner 2 differs greatly depending on the receiving channel. As a result, the F characteristic of the video signal obtained from the terminal 10 differs for each channel, causing a so-called inter-station difference.
image quality.

There was a problem in that the S/N ratio was different for each channel.

Therefore, in order to solve the above problems, the applicant proposed
- Utility model registration order of No. 52428 “Video signal receiving circuit”
, a circuit detects the level of the audio IF signal or the level of the color burst signal included in the demodulated video signal and extracts a control wit pressure proportional to the difference between this level and a preset reference level; We propose a circuit that is provided with an IFF variable bandpass filter that is installed in the transmission line and whose frequency transmission characteristics are varied according to the 1lJtll voltage,
The frequency transmission characteristics of the IFF bandpass filter are varied according to the level of the audio IF signal or the level of the color burst signal, thereby keeping the ratio between the video carrier level and the audio carrier level constant regardless of the receiving channel. This makes it possible to obtain high-quality video signals with no differences between stations.

FIG. 4 shows a circuit diagram of an example of a conventional IFF variable band filter used in the above-mentioned IFF variable band filter.

In the figure, reference numeral 11.12 is a tuning circuit, and the tuning frequency f of the tuning circuit 11 is 1/2πf1−℃−1, and its sharpness Q1 is 2πf+
C+ R+, and the tuning frequency of the tuning circuit 12 [2 is 1/2πf1-7]5;-9, its sharpness Q2 is 2π r
The tuning circuits 11 and 12 are coupled in a critical coupling state or a state close to it by a coupling capacitor C3, and the tuning circuits 11 and 12 form a double tuning circuit. Also, the control m+voltage V to is the voltage application resistor R3
Furthermore, the cathode side of the variable capacitance diode C+ is coupled to the inductance L+ via the bypass capacitor C4, and when the control voltage V to is varied, the variable capacitance diode C1 is connected to the cathode side of the variable capacitance diode C1. The capacitance value of C1 is varied, and the tuning frequency f1 and sharpness Q+ are respectively varied.

As shown in Fig. 5, if the center frequency in the IF signal band A shown by the broken line is 0, then in the state where r+ = rz = [0, the F characteristic will be as shown in Fig. 5 (B). , Ql <02 is maintained, and the variable capacitance diode C
When the + capacitance value is varied, the F characteristic is varied. For example, when fl < fl, the F characteristic shown in FIG. 5(A) is set,
When r+ > fl, the F characteristic shown in FIG. 3(C) is established.

Problems to be Solved by the Invention In the above configuration circuit, under the condition fl < fl, fI
As rl becomes smaller, the F characteristic changes from the characteristic shown in Fig. 5 (A) to the characteristic ■, while under the condition rl > fl, as rl becomes larger, the F characteristic changes to that shown in Fig. 5 (C). The characteristic shown changes from ■ to characteristic ■.

As is clear from Figures (A) and (C), the tuning frequency f1
When the difference between and fl becomes large, the center frequency f in the IF signal band and the center of the slope of F characteristic will be extremely shifted, especially,
There was a problem in that the F-characteristic became steep at the inner end of the IF signal band and the F-characteristic was disturbed. For this reason, with conventional circuits, the range in which the F characteristic can be corrected is limited to a narrow range, and it is only possible to correct deviations of about 2cB to 3d3. In other words, it is not possible to sufficiently correct IF signals with deviations larger than this. There was a problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide a double-tuned circuit in which the tuning frequencies of each tuning circuit can be varied simultaneously, and which can obtain a desired F-characteristic over a wide frequency range.

Means for Solving the Problems In FIG. 1(A), tuning circuits 13 and 14 each have an inductance element and a capacitance element for resonance, and each has a different sharpness. Circuit 13.
14, and a control circuit (3) that simultaneously changes the tuning frequencies of the tuning circuits 13 and 14 in opposite directions.

By changing the pressure source ■3, the capacitances of the variable capacitance diodes C+ and C2 change simultaneously, and the tuning frequencies of the tuning circuits 13 and 14 simultaneously move in the opposite direction'! 11
Changes to

Embodiment FIGS. 1(A) and 1(B) respectively show a circuit diagram of an embodiment of the circuit of the present invention and a specific circuit diagram applied to an IFF variable band filter. are given the same symbol. In the figure, the cathode of a variable capacitance diode C1 of the tuning circuit 13 is connected to a reference voltage source (for example, 12V) via a voltage applying resistor R3, and the anode thereof is connected to a control voltage source via a voltage applying resistor R4. ■Connected to 3. The cathode of the tuning circuit 14σ variable length diode C2 is connected to the control voltage source 3 via the voltage applying resistor R5, and the anode thereof is connected to the reference voltage source (for example, ground) V2. Here, V2 <V3
<Vl is set.

The tuning circuits 13 and 14 are in a critical coupling state or a state close to the critical coupling state with respect to the coupling capacitor C3, and are further set to 01 to Q2. Further, Cs and Ca are bypass capacitors.

11i4(B) 1″= a vs r・xt′ end″F
151 in 76 (The IF signal is the transistor Tr+
The signal is amplified and supplied to the tuning circuits 13 and 14, where it is double tuned at the tuning frequencies f+ and fz, impedance exchanged by the transistor Tr2, and taken out from the output terminal 16.

In this case, since variable capacitance diodes C+ and C2 are provided in the tuning circuits 13 and 14, respectively, the control voltage source V3
When the voltage of changes, the tuning frequency rI.

f2 changes simultaneously and at the same rate in the opposite direction. For example, f
, = f2 = f, it is assumed to be the F characteristic shown in Figure 2 (B), and when f, < f2, it is assumed to be the F characteristic shown in Figure 2 (A),
r) > f2, it is assumed to be F special as shown in Figure (C).

In this way, the respective tuning frequencies f of the tuning circuits 13, 14
+, fz can be changed in the opposite direction at the same rate at the same time, so the center frequency "0" in the IF signal band and the center of the slope of the F characteristic almost match, making it possible to correct the F characteristic compared to conventional circuits. The range is widened, deviations of about 5'' to 6 dB can be sufficiently corrected, and a stable F characteristic can be obtained in each receiving channel.

In addition to coupling the tuning circuits 13 and 14 through a coupling capacitor C3, they may also be coupled through a mutual inductance of inductance L1 and 2.

In addition, a resistor R+ is used as an element that determines the sharpness Q+ and C2.
, R2, the inductance L+, the resistance of L2, etc. may be substituted.

Further, the control voltage source (3) may be a voltage proportional to the difference between the color burst signal level and the reference level, in addition to a voltage corresponding to the audio IF signal level included in the output of the video detection circuit.

Effects of the Invention The circuit of the present invention has two tuned circuits each having an inductance element and a capacitance element for resonance, each having a different sharpness, and a coupling for coupling the two tuned circuits. Since it is composed of a circuit and a control circuit that simultaneously changes the tuning frequencies of the two tuning circuits in opposite directions, it is possible to obtain a desired F characteristic over a wide frequency range. When used as a filter for correcting the F-characteristics of a video 6111F signal, it has the advantage of being able to sufficiently correct deviations of about 5d3 to 6'' and obtaining stable F-characteristics in each receiving channel.

[Brief explanation of the drawing]

1 and 2 are a circuit diagram and a frequency versus amplitude characteristic diagram of an embodiment of the circuit of the present invention, respectively. FIG. 3 is a block system diagram of an example of a general video signal receiving circuit, and FIGS. 4 and 5. 1A and 1B are a circuit diagram and a frequency versus amplitude characteristic diagram of an example of a conventional circuit, respectively. 13.14... Tuning circuit, 15... Input terminal, 16
...output terminal, C+, C2...variable speech diode, V+, V2- reference ffi pressure source, ■3...training
1l'Fi pressure source, L, 1. L2...inductance,
C3...Coupling capacitor, R1-R5...Resistor.

Claims (2)

[Claims] (1) Each has an inductance element and a capacitance element for resonance, and the sharpness of each is set to be different.
double tuning, comprising: a tuning circuit; a coupling circuit that couples the two tuning circuits; and a control circuit that simultaneously changes the respective tuning frequencies of the two tuning circuits in opposite directions. circuit. (2) The capacitive element is a variable capacitance diode, the anode of the one variable capacitance diode is connected to a first reference voltage source, and the cathode of the other variable capacitance diode is connected to the first reference voltage source. the cathode of the one variable capacitance diode and the anode of the other variable capacitance diode are connected to a high second reference voltage source; 2. The double-tuned circuit according to claim 1, wherein the double-tuned circuit is connected to a control voltage source that is variable.