JP2876709B2 - Electronic tuner tuning circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tuner selection circuit of an electronic tuner used for a television receiver, a video tape recorder and the like.

2. Description of the Related Art In recent years, television receivers and video tape recorders using conventional tuners have become widespread.

Hereinafter, a channel selection circuit of a conventional electronic tuner will be described with reference to the drawings. FIG. 4 is a block diagram showing a configuration of a channel selection circuit of a conventional electronic tuner, and FIG. 3 is a graph showing a change over time of a search voltage when a VHF channel is searched and selected.

In FIG. 4, reference numeral 1 denotes an electronic tuner, 9 denotes a VIF, and 14 denotes a search channel selection circuit which controls the electronic tuner 1 to receive a television signal. The music selection operation section 19 has an up search button and a down search button, and the tuning voltage of the electronic tuner 1 can be sequentially increased or decreased by operating each button. A band switching circuit 15 is connected to the VIF low band power terminal 3, VHF high band power terminal 4, and UHF band power terminal 5, and selects a reception band.

When a search channel is selected, a tuning voltage that is sequentially increased or decreased and output from the tuning voltage sweep circuit 18 in an open loop operation is applied to the tuning voltage terminal 7 of the electronic tuner 1. VHF during up search (tuning voltage increases gradually)
After the tuning voltage has risen in the low band, the tuning voltage starts from the low tuning voltage in the VHF high band. After the tuning voltage has dropped in the VHF high band during the down search (the tuning voltage decreases gradually), the VHF low band Is controlled by the input control circuit 16 so as to start from a high tuning voltage.

Next, the operation of actually tuning the television signal will be described. When the electronic tuner 1 receives the television wave during the search, the synchronization signal from the synchronization signal output terminal 13 of the VIF 9 and the change in the AFT voltage from the AFT voltage terminal 12 are transmitted to the tuning control circuit unit 17 of the tuning control unit 14. The AFT operation is performed by applying feedback to the tuning voltage output from the tuning voltage sweep circuit unit 18 to pull in a television signal and enter a tuning state.

At this time, the output from the RF-AGC output terminal 10 of the VIF 9 is
It is connected to the RF-AGC terminal 6 of the electronic tuner 1 and works to keep the output level from the IF output terminal 8 of the electronic tuner 1 below a certain level. During the search channel selection, many spurious signals are output from the IF output terminal 8, and a normal search channel selection operation may not be performed.

FIG. 3 shows the movement of the tuning voltage in the domestic channel during the search channel selection. FIG. 3 shows the behavior of the search voltage applied to the tuning voltage terminal 7 during the search selection from the VHF high band to the VHF low band, and the upper limit voltage of the VHF low band when the VHF high band is switched to the VHF low band. To the lower voltage, and the tuning voltage corresponding to ch3 and ch1 exists in the middle. The upper limit voltage of the VHF low band varies from device to device, and the factors are the lower limit voltage of the VHF high band, the sensitivity of the tuning voltage sweep circuit section 18, the responsiveness of the electronic tuner 1, VIF 9, and the search tuning circuit section 14, and the like. is there. Due to this voltage variation, it is not possible to reliably avoid spurs generated in the vicinity. That is, when a television signal of a strong input is applied to the electronic tuner 1, many spurious signals are output from the IF output terminal 8. In particular, when a VHF low channel television signal is input, a voltage that can tune the spurious signal is VHF. It exists at a voltage slightly lower or slightly higher than the upper limit voltage of the low band.

Problems to be Solved by the Invention In such a tuning circuit of a conventional electronic tuner, VHF
During the channel search, for example, when switching from high band to low band, try to pull in many spurious signals and tune in with fake signals,
In the worst case, switching from the high band to the low band may not be performed, and the low channel may not be selected. The same phenomenon occurs when switching from the low band to the high band.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide an electronic tuner tuning circuit capable of reliably performing a search tuning when switching between a low band and a high band of a VHF channel.

Means for Solving the Problems In order to achieve the above object, the present invention provides a VIF circuit that amplifies and detects an intermediate frequency output from an electronic tuner that receives a television wave, and selects a channel by controlling the electronic tuner. A tuning control unit for performing, the tuning control unit controls the electronic tuner to perform a switching operation between a low band and a high band in a VHF band, a tuning search operation, and a tuning operation, and during the tuning search operation. The RF-AGC operation by the VIF circuit is stopped for a predetermined time from the time of the band switching operation to perform the forced gain reduction operation of the RF amplification.

Effect of the Invention With the above configuration, the present invention forcibly reduces the RF amplification to a gain that does not generate a spurious signal for a predetermined time from the time when the band is switched, and therefore, the search tuning without capturing the spurious signal. Will be continued.

Embodiment Hereinafter, a tuning circuit of an electronic tuner according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a tuning circuit of an electronic tuner according to one embodiment of the present invention. In FIG. 1, the tuning circuit of the conventional electronic tuner shown in FIG.
The difference is that an AGC defeat circuit section 20 is provided. The RF-AGC defeat circuit section 20 includes a voltage change detection circuit section 21 and an RF-AGC voltage control circuit section 22. The VHF low band power supply terminal 3 and the VHF high band power supply terminal 4 of the electronic tuner 1 are connected to a voltage. Connected to the change detection circuit section 21. The change in the voltage when the band is switched in the VHF is added to the voltage change detection circuit unit 21, and the time constant predetermined by the RF-AGC voltage control circuit unit 22 is determined as the output voltage, and the forced gain reduction voltage is set. RF-AG of electronic tuner 1
The signal is input to the C terminal 6 and defeet is applied to the RF-AGC for a fixed short time.

The operation of the above configuration will be described. FIG. 2 shows the change in the AGC voltage during the search selection from the high band to the low band with the passage of time when the reverse AGC is applied to the RF amplification stage of the electronic tuner 1. During search tuning, the AGC voltage is at a level sufficient to obtain RF sensitivity, but is forced to a lower voltage, for example, 0 volts as shown in the figure, for a predetermined time after the band is switched. I do. The RF amplifier loses amplification gain due to this voltage,
No spurious signal is generated near the switching between the low band and the high band of the VHF band. This operation is performed similarly when the band is switched from the low band to the high band.

That is, the RF-AGC is defeated for a fixed short time by using the voltage change when the band is switched by the VHF, so that a strong input TV signal is applied to the input terminal 2 of the electronic tuner 1. Also, IF output terminal 8
Has no spurious signals at all. That is, it is possible that there is no spurious signal having a tuning voltage close to the upper limit voltage of the VHF low band in FIG. 3, and it is possible to prevent a phenomenon in which a channel is selected by a spurious spurious signal or the low band high band is not switched by the VHF. It is possible to surely perform a search channel selection.

As is apparent from the above embodiments, the present invention provides a VIF circuit for amplifying and detecting an intermediate frequency output from an electronic tuner for receiving a television wave, and a selection for controlling the electronic tuner to select a station. Station control unit, the tuning control unit controls the electronic tuner to perform switching operation of low band and high band in VHF band, tuning search operation and tuning operation, and the band during the tuning search operation. During the tuning search of the tuner, by using the electronic tuner tuning circuit to stop the RF-AGC operation by the VIF circuit for a predetermined time from the time of the switching operation and perform the forced gain reduction operation of the RF amplification. In the vicinity of the boundary between the low band and the high band of the VHF, the gain of RF amplification is forcibly reduced and spurious signals are not generated. The search operation is continued without performing the tuning operation on the prior signal, and a smooth search operation is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a channel selection circuit of an electronic tuner according to one embodiment of the present invention, and FIG. 2 is a time point of RF-AGC voltage band switching in the channel selection circuit of the electronic tuner according to one embodiment of the present invention. FIG. 3 is a graph showing a time change of a search voltage generated during a search operation by the tuning control device, and FIG. 4 is a block diagram showing a configuration of a tuning circuit of a conventional electronic tuner. is there. 1 ... Electronic tuner, 3 ... Low band power supply terminal (band switching operation voltage), 4 ... High band power supply terminal (band switching operation voltage), 6 ... RF-AGC terminal, 7
…… Tuning voltage terminals (search operation voltage and tuning operation voltage), 9… VIF circuit, 10… VIF RF-AGC voltage terminal, 1
4 ... Tuning control section, 15 ... Band switching circuit section, 21 ...
… Voltage change detection unit (hand switching time detection), 22 ……
AGC voltage control circuit (RF-AGC operation stop control).

Claims (1)

(57) [Claims] A VIF circuit for amplifying and detecting an intermediate frequency output from an electronic tuner for receiving a television wave;
A tuning control unit for controlling the electronic tuner to perform tuning, wherein the tuning control unit controls the electronic tuner to control VH
In the F band, the switching operation of the low band and the high band, the tuning search operation, and the tuning operation are performed, and during the tuning search operation, the RF by the VIF circuit is performed for a predetermined time from the time of the band switching operation.
-An electronic tuner tuning circuit which stops the AGC operation and performs a forced gain reduction operation of RF amplification.