JPH0119500Y2 - - Google Patents

Description

[Detailed explanation of the invention] (1) Technical field of the invention This invention is aimed at preventing high-frequency output leakage from the ground side when the high-frequency signal output is blocked by switching in the converter output circuit of a subscriber's TV. The present invention therefore relates to a converter output circuit for a subscriber TV, which improves the isolation of the RF output circuit.

(2) Background of the technology Generally, in subscriber TVs, when the video and audio on the TV receiver are to be stopped (for example, in the case of non-payment of charges by the subscriber), the switch circuit is switched to stop the output terminal. Typically 70dB at side
The above isolation is required. However, in conventional converters of this type, 55~
I was only able to achieve isolation of about 60dB. The possible cause is a small amount of VHF leaking from the converter circuit through the ground.
The output signal was picked up by the high pass filter coil, which had one end connected to ground, worsening the effective isolation at its output terminal.

(3) Problems with the prior art For example, FIG. 1 shows a converter output circuit for a conventional subscriber TV. In the figure, 1 is the chassis of the entire device, which is at ground potential. 2 is an input terminal, 3 is a converter, 4 is a control circuit, 5 is a switch circuit, and 6 is a high-pass filter circuit, and the filter circuit 6 is connected to two capacitors 8 and 9, and one end is connected to ground. a coil 1 whose other end is connected to the connection point of the capacitor;
Consists of 0.

In such a configuration, an input signal from a transmitting side (head end), not shown, enters the input terminal 2 of the converter 3, but is converted into a constant VHF output signal by a tuning circuit, not shown, and is sent to the switch circuit 5. The switch circuit is the control circuit 4
The VHF output can be turned on and off using control signals from the As mentioned above, the purpose of this is to stop the output from the converter 3 in the case of non-payment of charges by the subscriber. And to stop the video and audio on the TV receiver,
This means that the output side must be able to provide isolation of typically 70dB or more.

As described above, via the switch circuit 5
The VHF output signal is given to a high pass filter consisting of capacitors 8 and 9 and a coil 10,
The output is outputted to an output terminal 7 leading to a TV receiver. Further, when the switch circuit 5 is turned off, the VHF output signal is blocked from passing through the switch circuit 5. The purpose of the high-pass filter 6 is to prevent damage to the semiconductors in the converter 3 from abnormally high voltages induced from the flyback trunk of the TV receiver, and It has the function of protecting the original VHF signal from being distorted by the diode in the switch circuit 5 when a frequency component (several 10 KHz to several MHz) enters the output terminal 7. The filter 6
The cutoff frequency of is usually selected to be approximately 10 to 30 MHz.

Now, in such a conventional converter output circuit, sufficient isolation of about 70 dB cannot be achieved as mentioned above. The reason for this will be explained using the switching equivalent circuits shown in FIGS. 2 and 3.

That is, assuming that each circuit has a common ground point 1, when the switch circuit 5 is off, the VHF output signal leaks from the ground of the converter 3 to the load L via the coil 10 of the filter 6. There isn't. However, in reality, the third
As shown in the figure, a ground loop is formed and the ground itself has impedance to high frequencies, so a small amount of high frequency current flows through the ground point of the coil 10 as shown by the arrow. Therefore, it is necessary to cut off such leakage of high frequency current.

(4) Purpose of the invention The purpose of this invention is to solve the above problem by creating a parallel resonant circuit to eliminate the leakage of high-frequency output from the ground in a high-pass filter that cannot be completely switched off by a switch circuit. The objective is to provide an effectively blocking subscriber TV converter output circuit.

(5) Embodiment of the invention Next, an embodiment of the subscriber converter output circuit of the invention will be described with reference to the attached FIG. 4.

In the figure, 1 is a chassis at ground potential, 2 is an input terminal, 3 is a converter, 4 is a control circuit, 5 is a switch circuit, 6 is a high-pass filter, and the high-pass filter includes capacitors 8 and 9 and a coil. 10, 7 is an output terminal, and 11 is a capacitor added in the present invention. 1 and 4 are the same except for a capacitor 11 loaded in parallel with the coil 10. And the fourth
In the circuit of the illustrated embodiment, the resonant frequency of the resonant circuit constituted by the coil 10 and the capacitor 11 is selected to be the same as the frequency of the VHF output signal of the converter 3.

The operation of the circuit of the present invention constructed as described above will be described next. Since the operation of the circuit of FIG. 4 is the same as that of FIG. 1 except for a part, only the differences will be explained.

If the subscriber is paying, the input signal is given to converter 3 where it is converted to a predetermined value.
When the signal is converted into a VHF output signal and applied to the switch circuit 5, the control circuit 4 turns on the switch circuit 5 and a normal VHF output flows. However, in case of non-payment, the switch circuit 5 is turned off. However, in spite of this, the VHF output signal as described above leaks, but in the case of the present invention, the parallel resonant circuit of the coil 10 and the capacitor 11 allows
That signal can be blocked. Therefore,
The isolation at the output terminal 7 can be set to approximately the same level as the isolation level of the switch circuit 5 alone, that is, about 70 dB.

(6) Effects of the invention As stated above, the subscriber
In the TV converter output circuit, by blocking unnecessary leakage of the VHF output signal, the isolation of the frequency output circuit can be improved, and the function against spurious operation can be improved.

[Brief explanation of the drawing]

Figure 1 shows the converter output circuit side of a conventional subscriber TV, Figures 2 and 3 show the actual switching equivalent circuit of Figure 1, and Figure 4 shows the converter output circuit side of a conventional subscriber TV.
An example of a converter output circuit for a TV is shown. In the figure, 1 is a chassis, 2 is an input terminal, 3 is a converter, 4 is a control circuit, 5 is a switch circuit, 6 is a high-pass filter, and 7 is an output terminal.

Claims (1)

[Scope of utility model registration request] A converter output circuit for a subscriber TV comprising a converter for converting an input signal into a constant VHF output signal, a switch circuit for turning on/off the output of the converter, and a control circuit for controlling the on/off operation of the switch circuit. A high pass filter is provided on the output side of the circuit, and a resonant circuit that resonates with the VHF output signal to be passed from the converter is provided in the ground circuit of the filter, and when the switch circuit is off, A converter output circuit for a subscriber TV, characterized in that the converter output circuit for a subscriber TV is configured to block leakage of the VHF output signal from the ground side of the filter toward the output of the converter.