JPS6145655Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an input circuit for a UHF tuner in a television receiver.

There are various performances required of a tuner, and among them, prevention of interference caused by interference of signals other than the target frequency is desired. Interference interference includes image frequency interference caused by external signals,
On the other hand, in CATV and the like, there is a problem such as interference due to leakage of local oscillation signals to input signal terminals. Since this type of interference has a major impact on image quality, some countries impose numerical regulations on it.

Here, we will explain the basic configuration of the UHF tuner first.
As shown in the figure. In FIG. 1, a received signal induced in an antenna is supplied to an input circuit 2 via an input terminal 1. In FIG.

Here, a schematic circuit of the input circuit 2 is shown in FIG. 7, and its actual diagram is shown in FIG. In the figure, 9 is a coupling coil (primary coil) into which the received signal is input, 1
0 indicates an input tuning coil (secondary coil) that is mutually inductance coupled (hereinafter referred to as M coupling) with the coupling coil 9, 11 indicates a varactor diode, and 12 indicates a chassis. This input tuning circuit 2 couples the received signal through M coupling between a coupling coil 9 and an input tuning coil 10, and further obtains a signal tuned to the signal frequency of a desired channel together with a varactor diode 11.

The signal output from the input circuit 2 is amplified in the next high frequency amplifier circuit 3 and then input to the mixing circuit 5 via the interstage double tuning circuit 4. Mixing circuit 5
Then, the frequency of the received channel signal is converted into an intermediate frequency signal IF by mixing the local carrier signal from the local oscillation circuit 6 and the amplified signal. This intermediate frequency signal IF is output to a high frequency amplifier circuit or a mixing circuit (not shown) of the VHF tuner.

In the above UHF tuner, narrowing the frequency bandwidth as much as possible up to the stage before the mixing circuit 5, that is, the interstage double-tuned circuit 3, improves the image frequency interference ratio or reduces the input terminal 1 of the local oscillation signal component. Effective in reducing leakage.

However, in the case of a UHF tuner using a varactor diode as a variable capacitance element, the frequency bandwidth up to the interstage double-tuned circuit 4 generally becomes wider as the tuning frequency becomes higher.
FIG. 2 shows an example of its frequency characteristics. this is,
The Q of a varactor diode depends on its applied voltage, and as the applied voltage increases (i.e., as the tuning frequency increases), the Q increases,
This occurs due to the increase in the coupling index of the interstage double-tuned circuit 4.

More details are as follows. Third
The figure shows an example of Q characteristics with respect to the applied voltage V _VD [V]. As can be seen from this figure, the applied voltage V
The higher _{the VD} , the greater the value of Q. On the other hand, FIG. 4 shows an example of the interstage double tuning circuit 4, and the coupling index K of the two tuning circuits 7 and 8 is expressed as K=k√ ₁ , _{2 .} . . (1). Here, k: coupling coefficient, Q ₁ :1
Q of the secondary side tuning circuit 7, Q ₂ : Q of the secondary side tuning circuit 8. What can be seen from equation (1) is that the larger the Q (Q ₁ , Q ₂ ) of each tuned circuit, the larger the coupling index K, and therefore the wider the frequency bandwidth.

In this way, as the receiving channel frequency becomes higher, the frequency bandwidth becomes wider, so the input terminal 1 of the image frequency interference ratio and the local oscillation signal component
The higher the reception channel frequency, the worse the leakage to the signal becomes. Incidentally, FIG. 5 shows the image frequency interference ratio of the American channel tuner having the frequency characteristics shown in FIG. 2, and FIG. 6 shows actual measurement data of the state of leakage of the local oscillation signal component to the input terminal. The above-mentioned problem can be understood from FIGS. 5 and 6 as well.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an input circuit that can improve the image frequency interference ratio and prevent leakage of local oscillation signals, especially in high frequency channels.

Hereinafter, the present invention will be described in detail based on illustrative embodiments. FIG. 9 shows an embodiment of the input circuit according to the present invention, and FIG. 10 shows its actual diagram. Note that in FIGS. 9 and 10, the same parts as in FIGS. 7 and 8 (prior art) are given the same reference numerals and explained.

As shown in FIGS. 9 and 10, a part of the coupling coil 9 (hereinafter referred to as the proximal part 9A) is close to the chassis 12, and
and are arranged facing each other by a predetermined length (area). Note that the portion facing the proximal portion 9A is the chassis 1.
The ground line is not limited to 2, and may be a ground line of a printed circuit board, etc., and it is sufficient that the ground line is arranged parallel to and opposite to the ground line.

With this arrangement, a predetermined distributed capacitance is formed between the proximal portion 9A and the ground. This distributed capacitance is determined by the length (or area) of the proximal portion 9A. Equivalent circuits of this proximal portion 9A are shown in FIGS. 11a and 11b. In FIG. 11, L indicates the inductance of the adjacent portion, and C indicates the formed distributed capacitance. Note that a indicates a case where the distributed capacitance C is large, and b indicates a case where the distributed capacitance C is small.

As is clear from the equivalent circuit, the proximal portion 9A constitutes a low-pass filter. Therefore,
The proximal portion 9A can attenuate high frequency components, and this effect becomes more pronounced as the frequency increases.

As a result, the frequency characteristics of the input circuit according to the present invention become substantially uniform as shown in FIG. 12. Also, Fig. 13 shows the characteristics of the image frequency interference ratio with respect to the receiving channel number, and the 14th
The figures show the relationship between the leakage level of the local oscillation signal and the frequency. Note that the solid line in the figure shows the characteristics according to the present invention, and the broken line shows the conventional characteristics shown in FIGS. 2 and 3. As can be seen from these Figures 13 and 14, the image frequency interference ratio is high in the high frequency part, and the local oscillation signal input terminal 1
leakage has been significantly improved.

As described above, according to the present invention, it is possible to improve the image frequency interference ratio and prevent leakage of local oscillation signals in the input circuit of a UHF tuner, and it is possible to obtain a playback screen with good image quality.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the general configuration of a UHF tuner, Figure 2 is a frequency characteristic diagram up to the interstage double tuning circuit, Figure 3 is a Q characteristic diagram of the varactor diode, and Figure 4 is the interstage double tuning circuit. A diagram showing an example. Figure 5 is a characteristic diagram of the image frequency interference ratio of a conventional tuner. Figure 6 is a characteristic diagram of the leakage level of a local oscillation signal component of a conventional tuner. Figure 7 is a schematic circuit diagram of a conventional input circuit. , FIG. 8 is its actual diagram, FIG. 9 is a circuit diagram showing one embodiment of the input circuit according to the present invention, and FIG.
Fig. 0 is its actual diagram, Fig. 11 is an equivalent circuit diagram of the adjacent part, Fig. 12 is a frequency characteristic diagram of the input circuit according to the present invention, Fig. 13 is a characteristic diagram of image frequency interference ratio, and Fig. 14 is a local oscillation diagram. FIG. 3 is a characteristic diagram of the leakage level of signal components. 1... Input terminal, 9... Coupling coil (primary coil), 10... Input tuning coil (secondary coil), 9A... Proximity, C... Distributed capacitance.

Claims (1)

[Claims for Utility Model Registration] A receiving signal induced to an input terminal is supplied 1
A UHF device that has a secondary coil, an input-tuned secondary coil electromagnetically coupled to the primary coil, and further includes a varactor diode so that the tuning frequency can be changed by changing the voltage applied to the varactor diode. In the tuner, a part of the primary coil is placed close to and facing the ground line to form a predetermined distributed capacitance between the ground and the primary coil and this distributed capacitance to form a low-pass filter. characterized by comprising
UHF tuner input circuit.