JPH0374966B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention is applied to a satellite broadcasting television receiver by using a microwave signal existing in the vicinity of the second intermediate frequency (70MHz) to perform an AFT (automatic frequency control circuit). This relates to a mute circuit in an AFT that prevents the local oscillation frequency of the tuner circuit from changing due to the output being out of order.

"Prior Art" Generally, satellite television broadcasts are broadcast from communication satellites.
After receiving a 3.7~4.2GHz signal with an antenna and amplifying it, a block down converter converts it to 430~4GHz.
A first intermediate frequency signal of 930MHz is obtained. This 430～
The 930MHz signal becomes a 70MHz second frequency signal in the tuner circuit. The output waveform at that time has a center frequency of 70MHz, as shown in solid line characteristic A in Figure 2.
, with a frequency bandwidth of at least ±10 MHz on both sides.

However, there were areas where microwave signals of 60MHz and 80MHz existed just ±10MHz away from 70MHz, and there was a problem that these microwave signals became noise and mixed into the TV signal. Therefore, conventionally 60MHz and 80M
A Hz filter circuit was installed to form the characteristic line B in Figure 2, and the interference waves were removed.

``The problem that the invention seeks to solve'' However, if such a filter circuit is constantly inserted, in areas where there are no interference waves, not only will the bandwidth become narrower and the image and sound quality will deteriorate, but the AFT
Originally, the output should be tuned to have a center frequency of 70MHz, as shown in characteristic A in Figure 2, but for example, as shown in dotted line characteristic C in Figure 2, it should be tuned to have a center frequency of 60MHz. I had a problem with putting it away. Therefore, in this state, the filter circuit is 60M
When trapping Hz, the characteristics shown in Figure 2 D occur, causing problems such as the local oscillation frequency of the tuner circuit being different from the desired channel, and normal image and sound quality not being obtained.

"Means for Solving the Problems" The present invention has been made to solve the above problems. In other words, the intermediate frequency signal output from the tuner circuit is amplified by the intermediate frequency amplifier circuit, the video detection circuit performs video detection, and part of the output is returned to the tuner circuit via the AFT circuit to control the local oscillation frequency. In the receiver, a filter circuit for removing interference waves within a certain band from the intermediate frequency signal is inserted after the intermediate frequency amplifier circuit together with a switch circuit for turning on and off the filter circuit, and the AFT During the return path from the circuit to the tuner circuit, in conjunction with the on/off switch circuit of the filter circuit,
It is equipped with an AFT switch circuit that temporarily turns off the AFT circuit.

``Operation'' When a microwave signal exists as an interference wave, the switch circuit is operated to activate the filter circuit. A switch circuit is activated in synchronization with this switch circuit to temporarily turn off the AFT circuit.
Then, the local oscillation frequency of the tuner circuit is returned to the normal frequency side and no longer synchronizes with the interference wave. By turning on the AFT circuit in this state, the local oscillation frequency is controlled normally.

“Example” An example of the present invention will be described below based on the drawings.

In Fig. 1, 1 is an input terminal for a first intermediate frequency signal of 430 to 730 MHz, and this input terminal 1 is coupled to a tuner circuit 2 and an intermediate frequency amplification circuit 3. It is coupled to a trapping filter circuit 4, a video detection circuit 5, and an output terminal 6.

The filter circuit 4 consists of two trap circuits that remove 60 MHz microwave and 80 MHz microwave, and also includes a switching diode 7 inserted in parallel and an anode of this diode 7. A filter circuit on/off switch circuit 9 is connected, which is constituted by a switch 8 inserted between the power source (+B) and the power source (+B).

A part of the video detection circuit 5 is coupled to a comparator circuit 10 as an AFT circuit that compares it with a reference voltage E ₁ and outputs an AFT signal, and its output is further coupled to a channel selection circuit 11 . The input side of the tuning circuit 11 is connected to a tuning signal input terminal 12 and an AFT switch circuit 13, and the output side is connected to the tuner circuit 2 via a prescaler 14. The AFT switch circuit 13 includes a power source E ₂ , a switch 15 interlocking with the switch 8,
It is comprised of a differentiation circuit 16 which obtains a differential waveform output when the switch 15 is turned on and off, and a rectification circuit 17 which performs full-wave rectification of the differential waveform.

In the above configuration, the satellite broadcast TV signal of 3.7 to 4.2 GHz received by the antenna is converted into a first intermediate frequency signal of 430 to 930 MHz by the DUN converter, which is input to the tuner circuit 2 via the input terminal 1. , a second intermediate frequency signal with a center frequency of 70 MHz is obtained. This second intermediate frequency signal is amplified by the intermediate frequency amplification circuit 3 and input to the filter circuit 4.

Here, if 60MHz and 80MHz microwaves are not present, if the switch 8 is turned on and the filter circuit 4 is short-circuited, the output of the intermediate frequency amplification circuit 3 will remain the same as the dotted line characteristic A in FIG. The signal passes through a switching diode 7 in line with the circuit 4 and is sent to the video detection circuit 5. At this time,
The switch 15 of the AFT switch circuit 13 remains off, and the signal subjected to AFT is sent to the channel selection circuit 11 as usual.

When 60MHz and 80MHz microwaves are present, when switch 8 is turned off, switch 15 is turned on in conjunction. The differential waveform when the switch 15 is turned on is converted into a pulse signal with a constant time width by the rectifier circuit 17 and sent to the channel selection circuit 11, and the AFT signal from the comparison circuit 10 is cut off by the time width of this pulse signal. Then, the local oscillation frequency in the tuner circuit 2 is controlled to a regular frequency without synchronizing with the interference wave. When the certain period of time has elapsed, the channel selection circuit 1
1 disappears, and the AFT signal is now sent to the tuner circuit 2.

When the switching diode 7 is turned off, a reverse bias is applied to the switching diode 7, and the switching diode 7 is turned off. Therefore, the output of the intermediate frequency amplification circuit 3 is transmitted to the filter circuit 4,
The 60MHz and 80MHz parts are trapped, and the 60MHz and
The 80 MHz microwave is removed, resulting in a characteristic as shown by the solid line B in FIG. 2, and this output is sent to the video detection circuit 6 at the next stage.

"Effects of the Invention" Since the present invention is configured as described above, the AFT circuit is temporarily turned off when the filter circuit removes microwaves existing within the band of the second intermediate frequency signal in satellite broadcast television. This prevents the local oscillation frequency of the tuner circuit from being drawn into the microwave signal. Therefore, it is possible to prevent the channel from not matching the image or from being unable to obtain the image or sound due to interference waves.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of a trap circuit for a satellite broadcast television receiver according to the present invention, and FIG.
The figure is a frequency characteristic diagram. 1...Input terminal, 2...Tuner circuit, 3...
Intermediate frequency amplification circuit, 4... Filter circuit, 5...
Video detection circuit, 6... Output terminal, 7... Switching diode, 8... Switch, 9... Filter circuit on/off switch circuit, 10... Comparison circuit (AFT circuit), 11... Tuning circuit , 13...
AFT switch circuit, 15...Switch, 16
... Differential circuit, 17 ... Rectifier circuit.

Claims (1)

[Claims] 1. The intermediate frequency signal output from the tuner circuit is amplified by an intermediate frequency amplification circuit, and further video detection is performed by a video detection circuit, and a part of the output is returned to the tuner circuit via an AFT circuit. In a receiver configured to control a local oscillation frequency, a filter circuit for removing interfering waves within a certain band from the intermediate frequency signal is provided at a subsequent stage of the intermediate frequency amplifier circuit, together with a switch circuit for turning on and off the filter circuit. In the AFT, an AFT switch circuit is inserted, and is provided in the middle of the return path from the AFT circuit to the tuner circuit, for temporarily cutting off the AFT signal in conjunction with when the filter circuit is turned on. Miyuto circuit. 2 The AFT switch circuit turns on the filter circuit,
The AFT according to claim 1, which comprises a switch that interlocks with the off switch, a differentiating circuit that differentiates the voltage when the switch is on, and a rectifier circuit that rectifies the output of the differentiating circuit to produce a pulse signal.
Mute circuit in.