JPS6012372Y2 - Fault detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a failure detection device in a cable television system.

A cable television system (hereinafter referred to as a CATV system) has the configuration shown in Fig. 1, in which a television signal received by an antenna 1 is amplified to a predetermined level at a head end 2, and then sent to a main line 3 consisting of a coaxial cable. Sent out.

This television signal is transmitted while compensating for transmission loss through a plurality of main line amplifiers 4 inserted into the main line 3, and then taken out from the main line 3 to a branch system at a brancher 5. The signal is amplified and inputted via a distributor 6 to a television receiver 7 at each subscriber's home.

In such a CATV system, the level setting of the entire system is set so that the transmission loss of the TV signal in the coaxial cable is compensated for by each amplifier, and the loss is inputted to the TV receiver 7 at the correct signal level. It is done.

This level setting was performed by connecting a measuring device to one terminal of a monitor provided in the system, measuring the signal level value at that point, and adjusting the output level of the amplifier based on the measured value.

However, with this method, if there is impedance mismatching due to poor contact of the connector that connects the coaxial cable to various devices in the system such as an amplifier, or because a dummy resistor has been forgotten, the monitor terminal will not function properly. Since no signal level is output, an erroneous level value will be measured, and even if the amplifier output level is adjusted according to this erroneous level value, the system will not be adjusted correctly.

In this case, the various devices in the CATV system will be adjusted to a state where they cannot demonstrate their proper capabilities, and there is even a risk that the system will be seriously damaged.

This invention attempts to improve the above problem by providing a changeover switch and a pseudo impedance at one terminal of the monitor, and when the changeover switch is switched to the core side of the coaxial cable and to the pseudo impedance side. The present invention provides a failure detection device that can measure the signal level in the relevant part of the system by comparing the signal level of one monitor terminal of the system, and can easily confirm the presence or absence of a mismatching state.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which 11 is an amplifier inserted into a CATV system, 12 and 13 are input terminals and output terminals, the center conductors of which are the inner conductors of the coaxial cables on the input and output sides, respectively. and the outer conductor is grounded.

14 and 15 are resistors connected in series with each other and to the output terminal of the amplifier 11, and the series connection point thereof is connected to the center conductor of the monitor terminal 16.

The outer conductor of the terminal 16 is grounded.

17 is a pseudo-impedance element having a value of 75Ω, which is equal to the characteristic impedance of the coaxial cable.

This pseudo-impedance element 17 may be a pure resistance.

18 is a changeover switch whose movable contact is connected to the coaxial cable on the output side of the amplifier 11, and its two fixed contacts are connected to the terminal 13 side,
That is, the coaxial cable on the output side and the pseudo impedance 1
Connected to one end of 7.

The other end of this element 17 is grounded. Changeover switch 18
Normally, it is better to use a non-locking type switch, connect its movable contact to the fixed contact on the output terminal 13 side as shown in the figure, and connect the output terminal of the amplifier 11 to the output side coaxial cable. This is preferable because it prevents forgetting.

Level measurement and fault detection are performed by connecting a field strength measuring device to the monitor terminal 16.

The output of the amplifier 11 is supplied by resistors 14 and 15 by a predetermined amount, for example, 20
Since the signal is attenuated by dB, the correct output level of the amplifier 11 is the signal level measured at the monitor terminal 16 plus this attenuation.

Detection of faults in the CATV system, such as impedance mismatching, is performed by comparing the level values of the monitor terminal 16 when the changeover switch 18 is switched to the output terminal 13 side and the pseudo-impedance element 17 side.

When the CATV system is normal, the signal level at the terminal 16 is the same when the pseudo-impedance element 17 is connected and when the coaxial cable is connected, so that it can be detected that the system is normal.

If mismatching occurs in the coaxial cable due to a break or the like, these measured level values will be different, making it easy to detect the fault.

Of course, if a failure is detected, necessary measures will be taken to eliminate the cause.

When both level values of the monitor terminal 16 match, the amplifier 11
Adjust the output level.

This is done while monitoring the measurement value of the electric field strength measuring device.
Adjust the gain of 1 so that the output level of the monitor 1 terminal becomes a predetermined value.

As explained in detail above, in this invention, when adjusting the signal level of the CATV system, the measurement level of the CATV system at one monitor terminal is compared with the measurement level when the changeover switch is connected to the pseudo-impedance element. Impedance mismatching failures such as cable breaks, poor contact between coaxial cables and connectors for various devices, and forgetting to attach dummy resistors can be easily detected, and the signal level of the CATv system can be prevented from incorrect level values. It is never set to .

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a CATV system, and FIG. 2 is a circuit diagram showing the configuration of a fault detection circuit according to the present invention. In the figure, 7 is a television receiver, 14.15 is a resistor, and 16
is a monitor terminal, 17 is a pseudo impedance element, 1B
is a changeover switch.

Claims (1)

[Scope of utility model registration request] In a wired television system that transmits at least a television signal to a television receiver in a general subscriber's home via a coaxial cable, two series-connected resistors connected between an internal conductor system and an external conductor system of the coaxial cable; A monitor terminal connected to the series connection point of the resistor and the external conductor system, a pseudo impedance element whose one end is connected to the external conductor system, and the internal conductor system to which the resistor is connected to the other internal conductor system or A fault detection device comprising: a changeover switch for selectively connecting to the other end of the pseudo-impedance element.