JPH0314845Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field This invention relates to an IF signal coupling circuit in a high frequency tuner, and particularly to a circuit for connecting an output signal of a UHF tuner to a VHF tuner in a UHF/VHF combination.

Conventional technology UHF/VHF combination tuner
The intermediate frequency signal (hereinafter referred to as IF signal) generated by the UHF tuner section is passed through a bandpass selector.
It is known to couple to a VHF tuner section and use the mixer stage of the VHF tuner section as an IF signal amplification stage. For example, the combination tuner shown in FIG. 3 is composed of a UHF tuner section that processes an input signal from a UHF band input terminal 1 and a VHF tuner section that processes an input signal from a VHF band input terminal 2. Each tuner part is RF
Amplification stages 3 and 4, mixer stages 5 and 6, local oscillation stage 7,
8, and interstage tunable bandpass filters 9 and 10 inserted between the RF amplification stage and the mixer stage,
A bandpass filter 11 for IF signal coupling is inserted between the mixer stage 5 of the UHF tuner section and the mixer stage 6 of the VHF tuner section, and a bandpass filter 12 for passing the IF signal from the mixer stage 6 of the VHF tuner section is provided. , is derived from the output terminal 13 as an IF signal.

Here, when receiving the UHF band, the UHF tuner section operates, and its output signal is connected from the coupling bandpass filter 11 to the mixer stage 6 of the VHF tuner section. In the VHF tuner section, this mixer stage 6 is operated as an IF signal amplification stage, and its output signal is led out from the IF signal output terminal 13 via the bandpass filter 12. On the other hand, when receiving the VHF band, the UHF tuner section does not operate and the coupling circuit of the bandpass filter 11 is also kept in the OFF state by the switching element.

The coupling circuit consisting of the bandpass filter 11 is
The high frequency equivalent circuit has the configuration shown in FIG. 4, and is coupled between the UHF tuner 14 and the VHF tuner 15 with a tuning circuit 16 including coils L ₁ , L ₂ , L ₃ and capacitors C ₁ and C ₂ and a switching diode 17. A capacitor 18 is connected. The switching diode 17 performs conduction/cutoff ON/OFF operation depending on the presence or absence of the operating power supply for the UHF tuner 14, and connects the IF signal from the UHF tuner to the VHF tuner side (Utility Model No. 59-57049). (see publication).

Problems to be Solved by the Invention However, when the coupling tuning circuit 16 is added to the primary input coil L ₁ and the secondary output coil L ₂ as well as the coupling air-core coil L ₃ , the arrangement space can be used effectively. However, in the channel system where the frequency of the IF signal is low, the coupling capacitor 18,
A series circuit of switching diode 17 and coupling air core coil _L3 forms a trap at frequencies within the VHF band. This is because when the frequency of the IF signal is low, the primary input side coil L ₁ and the secondary output side coil L ₂ are miniaturized as cored coils, so compared to the Q of the cored coil L ₂ , the coupling air-core coil is used. This is because the Q of _L3 is high, and specifically, the trap frequency f is determined by f=1/2π√LC. For example, the capacitance of the coupling capacitor 18 is 6 pF, the breaking capacitance of the switching diode 17 is 1.3 pF, and the coupling coil
The trap frequency f when the inductance of _L3 is 0.7μH is It forms a trap circuit of approximately 190MHz. This means that the Chinese channel system (P _IF = 37MHz, S _IF = 30.5MHz) and the Australian channel system (P _IF = 36.875MHz,
S _IF =31.375MHz), for example, 190MHz near channel 8 is trapped, causing problems in reception.

Therefore, the present invention has been proposed in view of the above, and an object of the present invention is to provide a new and improved IF signal coupling circuit that prevents trap interference during VHF band reception in a tuning circuit using a coupling air-core coil. This is the purpose.

Means for Solving the Problems According to the present invention, the UHF tuner transmits the intermediate frequency signal generated in the UHF tuner section to the VHF tuner section via a tuning circuit and a switching diode.
In a VHF combination tuner, the tuning circuit includes an input side coil that is a cored coil, an output side coil that is a cored coil, and a coupling coil that is an air-core coil inserted between the connection point of these two coils and the ground. , and a damping resistor is connected in parallel with this coupling coil. The damping resistor lowers the Q by dipping the coupling air-core coil, and performs the coupling function during UHF reception as well as the function of preventing reception channel trap interference during VHF reception. Therefore, a UHF/VHF combination tuner having a coupling circuit advantageous for UHF/IF signals is obtained.

Function The coupling air-core coil that couples the input and output side tuning coils of the coupling circuit is used for the coupling function between the input and output for the IF signal when receiving UHF, but when receiving VHF, the damping resistor in parallel with the coupling coil By lowering the Q, the L component involved in the trap frequency is connected in series with an output coil having an equivalent Q, and as a result, the trap frequency is shifted in a lower direction. That is, trap interference that occurs when receiving a VHF band channel can be prevented by using a resistor.

Embodiments Hereinafter, embodiments of the invention will be described in detail with reference to the drawings.

FIG. 1 shows an IF signal coupling circuit of a UHF/VHF combination tuner according to the present invention, and is shown as a high frequency equivalent circuit diagram. This combination tuner has UHF tuner part 21 and VHF tuner part 21.
The tuner section 22 is connected by an IF signal coupling circuit consisting of an IF tuning circuit 23, a switching diode 24, and a coupling capacitor 25,
The IF signal generated by the UHF tuner section 21 is transmitted to the VHF tuner section 22. Here, the feature of the present invention is that the tuned circuit 23 includes an input coil 26 and a capacitor 27, an output coil 28 and a capacitor 29, a coupling coil 30 inserted between the connection point between the input and output coils and the ground, and this coupling. It is constructed by including a resistor 31 connected in parallel to a coil 30. Specifically, the input/output side coils 26 and 28 are coils with ferrite cores, the coupling coil 30 is an air-core coil, and a damping resistor 31 is connected in parallel with the coupling coil 30, and this resistance value is 10. Selected in the range of ~100KΩ.

The purpose of inserting the resistor 31 is to lower the trap frequency that occurs when the switching diode 24 is turned off during VHF reception so that it does not affect the reception channel. That is, the L component that determines the trap frequency in terms of high frequency is damped by the resistor 31 additionally connected to the coupling coil 30 to lower the Q, and as a result, the L component of the output coil
This component contributes to the determination of the trap frequency and shifts it to the lower frequency side. This can be realized at a lower cost than the means of lowering the Q by using the coupled coil 30 as a cored coil, and also enables inductance adjustment by changing the winding spacing. By the way,
With a cored coil, variable adjustment of the inductance is not possible by changing the winding pitch.

FIG. 2 shows the high-frequency equivalent circuit of FIG. 1 as a concrete circuit, and the same parts are designated by the same reference numerals and detailed explanation thereof will be omitted.
In the figure, a coupling circuit is interposed between the mixer transistor 32 of the UHF tuner section 21 and the mixer FET 23 of the VHF tuner section 22, and the switching diode 24 is activated by the operating voltage from the power supply terminal 34 that drives the UHF tuner section. Continuity
It is turned on. Note that 35 and 36 are diode 2
It is a bias resistor that allows forward current to flow through 4, and 37
is a DC blocking capacitor.

Effects of the invention The IF signal coupling circuit according to the invention has UHF-VHF
In a device equipped with a switching diode and a tuning circuit, an air-core coupling coil and a damping resistor connected in parallel are used in the tuning circuit to eliminate reception channel trap interference during VHF reception. Furthermore, the use of a damping resistor is less expensive than using a cored coupling coil, and also enables inductance adjustment by changing the winding pitch as an air-core coil. Therefore, it has great practical value in combination tuners.

[Brief explanation of drawings]

Fig. 1 is an equivalent circuit diagram of the main part of the IF signal coupling circuit of the combination tuner according to the present invention, Fig. 2 is a specific circuit diagram of Fig. 1, and Fig. 3 explains a general combination tuner. The block diagram and FIG. 4 are main part circuit diagrams of an IF signal coupling circuit of a conventional tuner. 21... UHF tuner section, 22... VHF tuner section, 23... Tuning circuit, 24... Switching diode, 25... Coupling capacitor, 26...
...Input side coil, 27, 29...Capacitor, 2
8... Output side coil, 30... Coupling coil, 31
……resistance.

Claims (1)

[Claims for Utility Model Registration] In an IF signal coupling circuit of a UHF/VHF combination tuner that transmits an intermediate frequency signal generated in a UHF tuner section to a VHF tuner section, the IF signal coupling circuit is connected to a tuning circuit and an ON- OFF
The tuning circuit includes an input side coil containing a core, an output side coil containing a core, and a coupling coil consisting of an air core coil inserted between the connection point of these two coils and the ground. An IF signal coupling circuit characterized by connecting a dipping resistor in parallel with a coupling coil.