JPH0134442Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an input tuning circuit for a VHF tuner, and its purpose is to improve the image interference ratio.

Using variable capacitance diodes for channel selection
A VHF tuner generally uses an input tuning circuit as shown in FIG. In Figure 1, 1 is a coil connected to the hot side for low band, 2 is a coil that is used for high band and low band and connected to hot side, 3 is a coil connected to cold side for low band, and 4 is a coil for high band. A coil that serves both the band and low band and is connected to the cold side, 5 and 8 are DC blocking capacitors, 6 is a variable capacitance diode for selecting a desired channel, and 7 is for switching between the high band and low band. 9 is a coupling capacitor, 10 is a transistor for high frequency amplification, and 11 is a terminal for supplying a switching voltage.

In the input tuning circuit configured as described above, during high band reception, a positive voltage is applied to the terminal 11, and the switching diode 7 receives a forward bias and becomes conductive. Therefore, the equivalent circuit at this time is shown in FIG. 2a. That is, an input tuning circuit is constituted by a parallel circuit in which one end of the coil 1 and the coil 3 are substantially connected, the coils 2 and 4, and the variable capacitance diode 6, and covers the high band frequency. The ratio with the coil 4 matches the antenna side and the input tuning circuit side, and determines the bandwidth of the high band band.

On the other hand, during low band reception, a negative voltage is applied to the terminal 11, and the switching diode 7 is biased in the opposite direction and becomes non-conductive, and the equivalent circuit at this time is as shown in FIG. 2b. That is, a tuning circuit is constructed of a series circuit of coil 1 and coil 2, a series circuit of coil 3 and coil 4, and a variable capacitance diode 6, and covers the low band frequency. In addition to matching the antenna side and the input tuning circuit side, the low band bandwidth is determined.

However, in the input tuning circuit configured as above, for example, the 10th channel (center frequency 207MHz)
When receiving, the first channel (center frequency 93M
Hz), an image signal of 210.5 MHz is generated, which interferes with the received signal of the 10th channel in the mixing circuit (not shown), resulting in the deterioration of the received image.

The present invention provides a solution to the above-mentioned drawbacks, and will be explained in detail below with reference to an embodiment shown in the drawings. Fig. 3 is a circuit diagram of an input tuning circuit showing an embodiment of the present invention, where 1 is a coil connected to the hot side for low band, and 2 is a coil used for both high band and low band and connected to hot side. The coils 1 and 2 are connected in series to form a fourth series body. 3 is a coil connected to the cold side for the low band, 4 is a coil used for both the high band and the low band and connected to the cold side, and the coils 3 and 4 are connected in series to form a third series body. ing. 5, 8, and 14 are DC blocking capacitors whose impedance is negligible for low-band and high-band signals; 6 is a variable capacitance diode for selecting a desired channel; and 7, 15 are high-band capacitors. 9 is a coupling capacitor, 10 is a transistor for high frequency amplification, 11 is a terminal for supplying a switching voltage, 12 is a trap capacitor 13 which is used for both the low band and the high band. It is a trap coil for low band,
Capacitor 12 and coil 13 are connected in series to form a first series body. Reference numeral 16 denotes a high band trap coil, and a series circuit consisting of the DC blocking capacitor 14, the switching diode 15, and the trap coil 16 forms a second series body, one end of which is connected to the trap capacitor 1.
2 and the trap coil 13, and the other end is grounded. 17 is a signal input terminal.

In the input tuning circuit configured as described above, during high band reception, since a positive voltage is applied to the terminal 11, the switching diodes 7 and 15 are forward biased and turned on. In this case, if the equivalent circuit is shown by omitting the capacitor 9 and the transistor 10, it becomes the circuit shown in FIG. 4a. That is, a series resonant circuit consisting of a parallel circuit of trap coils 13 and 16 and a trap capacitor 12 forms an image trap circuit for a high band. As described above, the image trap circuit formed in this way removes the 210.5 MHz image signal that causes interference when receiving the 10th channel, so that the received image does not deteriorate.

On the other hand, during low band reception, a negative voltage is applied to terminal 11, so switching diodes 7 and 1
5 receives a bias in the reverse direction and becomes non-conductive. If the capacitor 9 and transistor 10 are omitted and an equivalent circuit is shown, the circuit shown in FIG. 4b is obtained.
That is, a series resonant circuit consisting of a trap capacitor 12 and a trap coil 13 forms an image trap circuit for the low band. This circuit removes image signals that cause interference during low-band reception.

It goes without saying that during high and low band reception, the respective tuning circuits and rectifying circuits operate in the same manner as in the conventional example.

As mentioned above, according to the input tuning circuit of the VHF tuner of the present invention, at the same time as the tuning coil is switched between low band reception and high band reception, the trap circuit for low band and the trap circuit for high band are switched. Since the trap circuit can attenuate both the low band and high band image signals, the image interference ratio characteristics can be significantly improved to 5 to 10 dB in the high band and 15 to 20 dB in the low band. The practical effects of this are great.

In addition, the image trap circuit is provided independently from the tuning circuit for channel selection, and has a large impedance at the frequency of the selected channel, so the effect on the tuning circuit and matching circuit can be ignored in practical terms. It is.

In the embodiment described above, switching between the low and high bands was carried out by turning on and off the switching diodes 7 and 15, but it goes without saying that the switching can also be carried out simply by a mechanical switch.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional input tuning circuit, Figures 2a and b are equivalent circuit diagrams during high band reception and low band reception in the input tuning circuit of Figure 1, and Figure 3 is an implementation of the present invention. FIGS. 4a and 4b are equivalent circuit diagrams of essential parts of the input tuning circuit shown in FIG. 3 during high-band reception and low-band reception. 1, 2, 3, 4...Coil, 5, 8, 14...
DC blocking capacitor, 6... Variable capacitance diode, 7, 15... Switching diode, 9
...Coupling capacitor, 10...Transistor,
11... terminal, 12... trap capacitor,
13, 16...Trap coil, 17...Input terminal.

Claims (1)

[Scope of utility model registration request] A capacitor and a first coil are connected in series, a first series body is connected to a signal input terminal and ground, and a first diode and a second coil are connected in series. a second series body connected in parallel to the coil; a third series body consisting of a third and fourth coil connected in series and connected to the signal input terminal and ground; a fourth series body consisting of coils connected in series and connected to the signal input terminal and the signal output terminal; a connection point between the third and fourth coils and the fifth and sixth coils; and a variable capacitance diode connected to the signal output terminal and ground, and the first and second diodes are both switched to a non-conducting state during low band reception. , an input tuning circuit for a VHF tuner, characterized in that both are switched to a conductive state during high band reception.