JP2899002B2 - CATV amplifier - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CATV amplifying device used to compensate for transmission loss in a CATV system.

[Prior Art] Conventionally, as shown in FIG. 3, this type of amplifying apparatus used in a CATV system includes a main amplifying circuit 1, an automatic gain control circuit (hereinafter abbreviated as an AGC circuit) 2, and a manual gain control circuit. (Hereinafter M
3), a changeover control circuit 4 and a changeover switch 5.

The main amplifier circuit 1 amplifies and outputs the input television signal and pilot signal. The AGC circuit 2 extracts only the pilot signal from the output of the main amplifier circuit, and controls the gain of the main amplifier circuit 1 by adjusting the attenuation of the variable attenuator 12 of the main amplifier circuit 1 according to the level of the pilot signal. . MGC
The circuit 3 determines the degree of attenuation of the variable attenuator 12 according to a preset value, and controls the gain of the main amplifying circuit 1. The switching control circuit 4 switches the gain control of the main amplifying circuit 1 from automatic gain control to manual gain control when the pilot signal stops or attenuates below a predetermined value, and when the pilot signal returns to a predetermined value or higher. And outputs a switching signal for switching the gain control of the main amplifier circuit 1 from manual gain control to automatic gain control. The changeover switch 5 performs a switching operation between the AGC circuit 2 and the MGC circuit 3 according to a changeover signal from the changeover control circuit 4.

An example of the input / output characteristics of the above-mentioned conventional CATV amplifier is shown in FIG.
Shown in the figure. In the figure, the broken line indicates the input / output characteristics under the control of the AGC circuit 2, and the solid line indicates the input / output characteristics under the control of the MGC circuit 3.

As shown in the figure, the AGC circuit 2 is adjusted so that the output becomes 92 dBμ when the input of the pilot signal is, for example, 67 dBμ to 77 dBμ. The MGC circuit 3 is adjusted so that the output becomes 92 dBμ when the input of the pilot signal is 72 dBμ. When the output of the pilot signal is 92 dBμ, the switching operation point of the switching control circuit 4 is set. When the input of the pilot signal attenuates to 67 dBμ or less, the switching from the AGC circuit 2 to the MGC circuit 3 is performed. Is switched from the MGC circuit 3 to the AGC circuit 2 when the input has returned to 72 dBμ or more.

[Problems to be Solved by the Invention] The conventional CATV amplifying device described above uses the AGC circuit 2
The input level of the pilot signal when switching to the circuit 3 is different from the input level of the pilot signal when switching from the MGC circuit 3 to the AGC circuit 2. For this reason, when switching from the MGC circuit 3 to the AGC circuit 2, there is a disadvantage that the switching operation may not be performed even in the range of the input level of the pilot signal from which the AGC circuit 2 is to operate, 67 to 72 dBμ.

Specifically, in FIG. 4, when the MGC circuit 3 is operating, the switching to the AGC circuit 2 is not performed unless the input of the pilot signal becomes 72 dBμ or more, so that the AGC circuit 2 should operate from 67 dBμ to 72 dBμ. However, there is a disadvantage that the MGC circuit 3 still works.

[Means for Solving the Problems] The present invention is to solve the above-mentioned conventional problems and to make the AGC circuit of the CATV amplifier more effectively function.

In order to achieve the above object, the present invention provides a main amplifier circuit for compensating for a transmission loss of a television signal in a CATV system, and a gain of the main amplifier circuit based on a pilot signal transmitted together with the television signal from a head-end device. An automatic gain control circuit for adjusting the gain, a manual gain control circuit for adjusting the gain of the main amplifier circuit according to a preset value, and a pilot signal output from the automatic gain control circuit has dropped below a predetermined switching operation point. Sometimes, the gain control of the main amplifier circuit is switched from automatic gain control to manual gain control, and when the pilot signal output from the automatic gain control circuit returns to the switching operation point, the gain control of the main amplifier circuit is changed to manual gain control. And a switching control circuit for switching from control to automatic gain control. An input of a pilot signal for obtaining an output, for the CATV in which the automatic gain control circuit sets the input of the pilot signal for obtaining the output of the switching operation point higher than a lower limit of the input of the pilot signal by a predetermined difference value. In the amplifying device, the automatic gain control circuit may further include a level adjuster that interlocks with the switching control circuit and increases the input level of the pilot signal by the difference value when the manual gain control circuit is operating. Features.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the CATV amplifier of the present invention, and FIG. 2 is a table showing an example of input / output characteristics of the CATV amplifier of the present invention.

As shown in FIG. 1, the CATV amplifier of the present embodiment
It comprises a main amplifier circuit 1, an AGC circuit 2, an MGC circuit 3, a changeover control circuit 4, and a changeover switch 5.

The main amplifying circuit 1 includes amplifiers 11 and 13 for amplifying an input television signal and a variable attenuator 12 for adjusting a gain.
Consists of

The AGC circuit 2 is a surface acoustic wave filter (SAW filter) 2 that extracts only a pilot signal from the output of the main amplification circuit 1.
1 and amplifiers 22, 24 and a detector 23 for adjusting the attenuation of the variable attenuator 12 based on the level of the extracted pilot signal.
And a level adjuster 6 for adjusting the level of the pilot signal in conjunction with the switching control circuit 4.

The MGC circuit 3 includes a power supply 31 having a negative electrode grounded, and a variable attenuator 32. The MGC circuit 3 is variable regardless of the level of the pilot signal in accordance with a value set by adjusting the output of the power supply 31 with the variable attenuator 32. The degree of attenuation of the attenuator 12 is determined.

The switching control circuit 4 receives the pilot signal output from the AGC circuit 2 and controls the gain of the main amplifier circuit 1 when the pilot signal attenuates below a predetermined switching operating point.
A switching signal for switching from the AGC circuit 2 to the MGC circuit 3 is output. When the input of the pilot signal returns to the switching operation point or higher, the gain control of the main amplifier circuit 1 is performed by MG.
A switching signal for switching from the C circuit 3 to the AGC circuit 2 is output. When the MGC circuit 3 operates, the level adjuster 6
To raise the level of the pilot signal input to the switching control circuit 4 by a predetermined difference value described later.

The changeover switch 5 performs a switching operation between the AGC circuit 2 and the MGC circuit 3 according to a changeover signal from the changeover control circuit 4.

Next, an example of the input / output characteristics of the present embodiment will be shown, and the operation of the present embodiment will also be described.

2, a broken line 7 indicates input / output characteristics under the control of the AGC circuit 2, and a solid line 8 indicates input / output characteristics under the control of the MGC circuit 3.

As shown in the figure, the AGC circuit 2 is adjusted so that the output becomes 92 dBμ when the input of the pilot signal is 67 dBμ to 77 dBμ. The MGC circuit 3 is adjusted so that the output becomes 92 dBμ when the input of the pilot signal is 72 dBμ. The reason for setting the AGC circuit 2 and the MGC circuit 3 in this way is that the standard input level of the pilot signal is 72
It is related to what is determined as dBμ. That is, the purpose of the AGC circuit 2 is to adjust the gain so that the output level becomes a specified level (92 dBμ) even if the input level of the pilot signal fluctuates up and down within a certain range from the standard input level 72 dBμ. Therefore, the setting is performed as described above. Further, even when the input of the pilot signal is lowered or the wave is stopped, the MGC circuit 3 needs to operate the MGC circuit 3 assuming that the standard 72 dBμ pilot signal is input, and thus is set as described above. ing.

Here, the switching operation point at which the switching control circuit 4 switches between the AGC circuit 2 and the MGC circuit 3 is a specified level at which the AGC circuit 2 can adjust the input within the above-mentioned fixed range, that is,
92dBμ is set. That is, the switching control circuit 4 monitors the output of the pilot signal, and switches to the MGC circuit 3 when the output becomes 92 dBμ or less, and switches to the AGC circuit 2 when the output returns to 92 dBμ or more.

From these facts, the switching operation point is 92 dBμ in the MGC circuit 3.
72dBμ of pilot signal input necessary to obtain the output of
Is different from the lower limit value 67 dBμ of the input of the pilot signal necessary for obtaining the output of the switching operation point 92 dBμ in the AGC circuit 2, and a difference value (72−67 =
5 dBμ) is clearly present. The existence of such a difference value is the same as in the above-described conventional case.

Therefore, in the present embodiment, the level adjuster 6 is operated during the operation of the MGC circuit 3 to increase the input level of the pilot signal by the above difference value 5 dBμ. As a result, the pilot signal input to the switching control circuit 4 during the operation of the MGC circuit 3 is raised by 5 dBμ by the level adjuster 6. For example, the input before adjustment by the level adjuster 6 is 65 dB
If μ, the input after adjustment is 70 dBμ, the input before adjustment is 67 dB
If μ, the input after adjustment is 72 dBμ, so the switching control circuit 4 sets the input / output characteristic of the MGC circuit 3 to a characteristic that is always 5 dBμ higher than the characteristic shown by the solid line 8 (the solid line 8 a Characteristic). Therefore, when the level of the pilot signal returns to 67 dBμ or more, the main amplification circuit 1
Is switched from the MGC circuit 3 to the AGC circuit 2.

[Effects of the Invention] As described above, the CATV amplifying apparatus of the present invention is provided by the MGC
During circuit operation, the level of the pilot signal input to the switching control circuit is increased by the difference value, so the input level of the pilot signal to the AGC circuit and the input level of the pilot signal to the MGC circuit to obtain the switching operation point match. I do. Thus, the AGC circuit always operates normally at the input level of the pilot signal that should be operated by the AGC circuit, and the function can be more effectively exhibited.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the CATV amplifying apparatus of the present invention, FIG. 2 is a table showing input / output characteristics of the embodiment of FIG. 1,
FIG. 3 is a circuit diagram showing a conventional CATV amplifying device, and FIG. 4 is a table showing input / output characteristics of the device of FIG. 1: Main amplifier circuit 2: Automatic gain control circuit (AGC circuit) 3: Manual gain control circuit (MGC circuit) 4: Switch control circuit 5: Switch 6: Level adjuster 7: Input of main amplifier by automatic gain control Output characteristics 8: Input / output characteristics of main amplifier by manual gain control 8a: Input / output characteristics of main amplifier recognized by switching control circuit when MGC circuit is operating

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03G 3/00 H03G 3/20 H04N 7/16

Claims (1)

(57) [Claims] 1. A main amplifier circuit for compensating a transmission loss of a television signal in a CATV system, and an automatic gain control for adjusting a gain of the main amplifier circuit based on a pilot signal transmitted together with the television signal from a head end device. Circuit, a manual gain control circuit that adjusts the gain of the main amplifier circuit according to a preset value, and the main amplifier circuit when the pilot signal output from the automatic gain control circuit falls below a predetermined switching operating point. The gain control is switched from automatic gain control to manual gain control, and when the pilot signal output from the automatic gain control circuit returns to the switching operation point, the gain control of the main amplifier circuit is changed from manual gain control to automatic gain control. A switching control circuit for switching, and a pilot control circuit for obtaining an output at the switching operation point by the manual gain control circuit. In the CATV amplifying apparatus, the input of the input signal is set higher by a predetermined difference value than the lower limit of the input of the pilot signal for obtaining the output of the switching operation point in the automatic gain control circuit, In conjunction with the switching control circuit, a level adjuster for raising the input level of the pilot signal by the difference value when the manual gain control circuit is operating is provided in the automatic gain control circuit for CATV. Amplifying device.