JPH0332118Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial application field This invention uses a VHF mixing circuit when receiving UHF.
UHF- used as amplifier circuit of UHF section IF circuit
This relates to a VHF combination type electronic tuner.

Conventional technology When receiving UHF in a TV tuner, the VHF mixing circuit is used as an amplifier circuit for the UHF IF circuit.
There is a UV combination type electronic tuner, and a specific example thereof is shown below with reference to FIG. The diagram is
The circuit diagram of the UHF part IF circuit is shown, where L ₁ and C ₁ are the coil and capacitor connected in parallel in the primary side tuning circuit, and L ₂ and C ₂ are the coil and capacitor connected in parallel in the secondary side tuning circuit. The capacitor, S is a switching diode that connects or disconnects the UHF part IF circuit and the VHF part mixing circuit which is its amplification circuit, and _C3 is the UHF part.
Coupling capacitor between part IF circuit and VHF part mixing circuit,
_C4 is a coupling capacitor between the VHF local oscillation circuit and the mixing circuit, _C5 is a coupling capacitor between the VHF section input circuit and the mixing circuit, and UB is the B power supply terminal of the UHF section tuner.

In the above configuration, when receiving VHF, the B power terminal
Since UB is open, the switching diode S becomes reverse biased and becomes open, and the VHF section mixing circuit and the UHF section IF circuit are separated. Next, when receiving UHF, a DC 12V voltage is applied to the B power supply terminal UB, so the switching diode S becomes forward biased and connects to the VHF mixing circuit.
It is connected to the UHF section IF circuit, and the former is used as an amplifier circuit for the latter.

Here, the equivalent circuit of the secondary side tuning circuit (the part surrounded by the dotted line in the figure) of the UHF section IF circuit during UHF reception (switching diode S is in a conductive state) is shown in FIG. 3. As shown in the figure, the above equivalent circuit connects the coupling capacitors C ₄ and C ₅ and the input capacitor C ₆ (not shown in Fig. 2) of the VHF section mixing circuit in parallel, and connects the coupling capacitor C ₃ in parallel.
It is connected in series with , and further connected in parallel with a parallel circuit of tuning coil L ₂ and capacitor C ₂ .
FIG. 4 shows an equivalent circuit of the secondary side tuning circuit of the UHF section IF circuit during VHF reception (switching diode S is open).
At this time, the switching diode S has an AC capacity even if it is in an open state, and as shown in the figure, the above equivalent circuit is a switching diode S connected to a parallel circuit of the tuning coil L ₂ and the capacitor C ₂ . The apparent capacitor _C7 is connected in series with the reverse capacitance of the switching diode and the capacitor _C3 .

Problems that the invention aims to solve By the way, as mentioned above, the switching diode S has an AC capacity even when it is in an open state, and when receiving VHF, it is not connected to the UHF IF circuit.
A VHF tuner is coupled to high frequencies.
Therefore, if the input radio wave when receiving VHF matches the resonant frequency of the circuit (inside the dotted line) when looking at the coil _L2 side from point A shown in Figure 4, this input radio wave will be UHF.
The signal is tuned and trapped by the external IF circuit.

Here, in the equivalent circuit shown in FIGS. 3 and 4, when the switching diode S is conductive, C ₂ =
1PF, C ₃ = 6PF, C ₄ +C ₅ +C ₆ = 6PF, the capacitance connected to coil L ₂ is 4PF, and when switching diode S is open, C ₂ = 1PF, C ₇ = 1PF.
Then, the tuning frequency ₁ during UHF reception is ₁ = 1/2π [L ₂ {C ₂ + C ₃ (C ₄ + C ₅ + C ₆ ) / (C ₃ + C ₄ + C ₅ + C ₆ )}] ^1/2. Since ₁ = 3MHz, 34 = 1/2π (4L ₂ ) ^1/2 , therefore (L ₂ ) ^1/2 = 1/34×4π Next, the resonance frequency ₂ when receiving VHF is ₂ = It is expressed as 1/2π {(C ₂ + C ₇ ) L ₇ } ^1/2 , ₂ = 1/2π (2L ₂ ) ^1/2 = 34×4π/2π2 ^1/2 = 34×2/2 ^1/2 =48MHz.

Means for solving the problem The present invention connects the VHF part mixing circuit and the UHF part IF circuit through a switching diode,
In a UHF-VHF combination type electronic tuner in which the switching diode is closed during UHF reception and the VHF section mixing circuit is used as an amplifier circuit for the UHF section IF circuit, the UHF section IF
A feature is that a variable capacitance diode is provided as part of the tuning capacitance of the circuit, and the tuning capacitance of the UHF section IF circuit is changed between when receiving UHF and when receiving VHF.

Effect In a UHF-VHF combination electronic tuner, if a variable capacitance diode is provided as part of the tuning capacitance of the UHF section IF circuit, and the tuning capacitance of the IF circuit is changed between UHF reception and VHF reception,
When receiving VHF, it is possible to prevent the trap frequency by the IF circuit from falling within the reception band.

Embodiment An embodiment of the electronic tuner according to the present invention will be described below with reference to FIG. The figure shows a circuit diagram of the UHF section IF circuit, and the same reference numerals as in FIG. 2 indicate the same parts. In the figure, L ₁ and C ₁ are the coil and capacitor of the primary side tuning circuit, L ₂ is the coil of the secondary side tuning circuit, VD is the variable capacitance diode connected in parallel to coil L ₂ , and S is the UHF section IF. A switching diode connects or disconnects the circuit and the VHF mixing circuit serving as its amplifier circuit, C ₃ , C ₄ , and C ₅ are coupling capacitors, and UB is the B power supply terminal of the UHF tuner.

The feature of this invention is that a variable capacitance diode VD is connected in parallel to the coil L ₂ to form a secondary side tuning circuit, and when receiving UHF, a DC 12V voltage is applied to the B power terminal UB, so the switching diode S becomes a forward bias and connects to the VHF mixing circuit.
It is connected to the UHF section IF circuit, and the variable capacitance diode VD has a small capacitance. Next, the B power terminal
Since UB is open, the switching diode S becomes reverse biased and becomes open, and the variable capacitance diode VD changes to a large capacity. Therefore, when receiving UHF and receiving VHF,
Since the tuning capacity of the secondary side tuning circuit of the UHF section IF circuit changes, it is possible to prevent the trap frequency by the UHF section IF circuit from falling within the reception band during VHF reception.

In the above embodiment, a diode designed for a switching diode may be used as the variable capacitance diode VD for changing the tuning capacitance.

Effects of the invention According to the invention, in a UV combination type electronic tuner, there is a difference between UHF reception and VHF reception.
Because I changed the secondary side tuning capacitance of the UHF part IF circuit,
When receiving VHF, the trap frequency by the UHF IF circuit can be set outside the reception band.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a UHF section IF circuit showing an embodiment of an electronic tuner according to the present invention, Fig. 2 is a circuit diagram of a UHF section IF circuit showing a specific example of a conventional electronic tuner, and Fig. 3 is a circuit diagram of a UHF section IF circuit showing a specific example of a conventional electronic tuner. The equivalent circuit diagram of the secondary side tuning circuit during UHF reception of the circuit in Figure 2, and Figure 4 is the equivalent circuit diagram of the secondary side tuning circuit during UHF reception of the circuit in Figure 2.
FIG. 3 is an equivalent circuit diagram of the secondary side tuning circuit of the circuit shown in the figure during VHF reception. S... Switching diode, VD... Variable capacitance diode.

Claims (1)

[Scope of utility model registration request] When receiving UHF, connect the VHF part mixing circuit and the UHF part IF circuit via a switching diode.
The UHF-VHF combination tuner is characterized in that a variable capacitance diode is attached to a part of the tuning capacitance of the UHF section IF circuit, and the tuning state of the UHF section IF circuit is changed between when receiving UHF and when receiving VHF. Electronic tuner.