JPH0230904Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a repeater amplifier for cable television.

Conventionally, a circuit system as shown in Figure 1 has been considered as a means for automatic gain control of the transmitted signal in a coaxial line relay amplifier that is interposed in the transmission system of high-frequency signals transmitted to the receiving section of television etc. . That is, a part of the transmission signal level is extracted from the directional coupler 3 provided at the output section of the main amplifier 1, and the signal detection filter 4 extracts a signal component of the desired frequency, that is, a frequency signal component to be subjected to automatic gain control. (e.g. pilot signal). Furthermore, the high frequency signal selected by this signal detection filter 4 is amplified by a detection signal amplifier 5, detected by a detector 6, and then DC amplified by a DC amplifier 7 to obtain an automatic gain control signal. Automatic gain control is performed by controlling the operation of a voltage-controlled variable attenuator 2 provided at 1.

In such a conventional automatic gain control means, in order to accurately detect only the amount of input level fluctuation of a desired frequency, a signal detection filter 4 is required.
It is necessary to improve the selection characteristics of the transmitter and remove other transmission frequency components. Normally, the amount of attenuation of unnecessary frequency components is required to be -20 dB or more with respect to the desired detection signal, and this value needs to be increased as the number of cascaded repeater amplifiers increases.

In particular, in a relay amplifier that transmits a large number of high-frequency signals with close frequencies, the frequency selection characteristics of the detection circuit (signal detection filter 4) greatly affect the performance of automatic gain control of the main amplifier 1, and the transmission frequency is high. The closer the transmission signal frequency is to the pilot signal frequency, the more the detection circuit must be constructed with filters having a larger number of elements.
For example, in the cable television amplifiers currently in use, the upper limit of the transmitted television signal frequency is 222MHz, but the pilot signal frequency for automatic gain control is set to 246MHz. , the frequency ratio of these signals is 1.11 times. Considering that a cable television amplifier is used in the UHF band at the same ratio, the upper limit of the television signal frequency is
Since it is 770MHz, the frequency of the pilot signal is
855 (≈770×1.11) MHz, and the main amplifier and the detection signal amplifier of the automatic gain control circuit must be wideband. In addition, while a 246MHz pilot signal detection filter can be constructed with a component configuration of about 4 elements, an 855MHz pilot signal detection filter has a component configuration of 8 or more elements due to the higher frequency, and each element has a expensive,
Moreover, advanced circuit technology is required. As described above, in the automatic gain control means in the conventional circuit system shown in FIG. The selection amplifier circuit 1 composed of a signal detection filter 4 and a detection signal amplifier 5 approaches the
0, a large number of expensive parts are required, which leads to a complicated circuit configuration, an increase in the size of the device, etc.
This results in a significant increase in manufacturing costs, making it difficult to realize this type of repeater amplifier that can perform normal automatic gain control even when the frequencies of transmitted signals are close, especially for the UHF band. It was hot.

The present invention was developed in view of the above-mentioned circumstances, and provides stable automatic transmission with good selection characteristics even when the transmission band frequency is high and the frequencies of the transmission signal and pilot signal or other transmission signals are close to each other. The object of the present invention is to provide a relay amplifier for cable television that is gain controllable.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a circuit block diagram showing an embodiment of the present invention. In the figure, the same parts as in FIG. The configuration shown in FIG. 2 is particularly different from the configuration shown in FIG. 1 above in that the selective amplifier circuit 10 shown in FIG.
is composed of a signal detection filter 4 for selecting a high frequency signal to be subjected to automatic gain control, and its detection signal amplifier 5.
In the selective amplification circuit 20 shown in FIG.
2, and a frequency conversion circuit 21 consisting of a local oscillator 13. After converting the detection signal that has passed through the input filter 11 into an intermediate frequency signal, the intermediate frequency filter 14 and the intermediate frequency amplifier 15 perform the necessary selection. The point is that the structure is configured to obtain characteristics and perform signal amplification. In the operation of the selective amplification circuit 20 in FIG. 2, the input filter 11 preliminarily selects a signal to be subjected to automatic gain control and prevents the local oscillation signal from leaking to the outside. A signal that has passed through this input filter 11, such as a pilot signal, is converted into an intermediate frequency signal by a frequency conversion circuit 21 consisting of a mixer 12 and a local oscillator 13, and after obtaining the required selection performance by an intermediate frequency filter 14, The intermediate frequency is amplified by the intermediate frequency amplifier 15. After this intermediate frequency amplified signal is detected by a wave detector 6, it is DC amplified by a DC amplifier 7, and becomes an automatic gain control signal for the main amplifier 1.

In this way, by converting the detection signal (e.g. pilot signal) into an intermediate frequency signal and performing signal selection (main selection) and amplification, the frequency is much lower than the original signal, so a ferrite core is used in the signal selection filter. It is now possible to use parts with high magnetic permeability such as, making it possible to make the filter part smaller and improve its performance. Further, for the same reason, the required detection circuit input level can be obtained with a small number of amplification stages for the amplifier circuit (intermediate frequency amplifier 15). for example,
Taking as an example a 780MHz UHF signal and a 57MHz signal obtained by intermediate frequency conversion, the selection characteristics of each are compared using a small filter composed of four elements (two stages), as shown in Figure 3. It becomes a characteristic. In other words, in the same figure, curve A showing the attenuation characteristic
is the case when the method of the present invention is adopted with an intermediate frequency of 57MHz, and the frequency is 10MHz away from the center frequency ( ₀ ).
While an attenuation of as much as 30 dB can be obtained, the curve B selected for 780 MHz in the conventional example has an attenuation of 30 dB even at a distance of 50 MHz.
Only 10dB of attenuation can be obtained. As is clear from this figure, if the signal is selected by converting it to an intermediate frequency, the desired signal (pilot signal) can be accurately selected even if the transmission signal frequency interval in the UHF band is only 10 MHz, and automatic gain control operation can be performed. Since the band expansion required for this purpose is only about 10 MHz, there is no need to change the circuit and component configuration of the main amplifier 1. Also, if the amplification gain necessary to obtain the required detection level is 30 dB, the number of amplifier circuit stages at 780 MHz = several GHz
If it is implemented at 57 MHz, the purpose can be achieved with two stages even if an amplifying element of less than 1 GHz is used, and a diode is used for the mixer 12. This is sufficient even if the conversion loss is taken into consideration.

In addition, although the above-mentioned embodiment has been described as an example in which automatic gain control is performed using a pilot signal, the present invention is not limited to this, and automatic gain control can be performed by detecting an arbitrary wave in a transmission signal. Of course, the method of the present invention can also be applied in cases where

As detailed above, according to the cable television relay amplifier shown in the side view, the transmission band frequency is high;
Moreover, even when the frequencies of the transmission signal and the pilot signal or other transmission signals are close to each other, a repeater amplifier that can perform stable automatic gain control with good selection characteristics can be easily created with a simple and inexpensive configuration. realizable.

[Brief explanation of the drawing]

Fig. 1 is a circuit block diagram for explaining automatic gain control means for a relay amplifier for coaxial lines according to the prior art, Fig. 2 is a circuit block diagram showing an embodiment of the present invention, and Fig. 3 is a circuit block diagram of a selective amplifier circuit. The selection characteristics by the filter are shown in the case of the circuit system shown in Fig. 1 above, and
FIG. 2 is a diagram showing a comparison between the circuit system according to the embodiment of the present invention shown in FIG. 2 above; 1... Main amplifier, 2... Voltage controlled variable attenuator,
3... Directional coupler, 6... Detector, 7... DC amplifier, 11... Input filter, 12... Mixer, 13... Local oscillator, 14... Intermediate frequency filter, 15... Intermediate frequency Amplifier, 20... selection amplification circuit, 21... frequency conversion circuit.

Claims (1)

[Scope of utility model registration request] A relay amplifier for cable television that detects transmission level fluctuations using a pilot signal and performs automatic gain control includes a directional coupler, a voltage-controlled variable attenuator provided before the co-directional coupler, and the above-mentioned components. A filter that takes out a part of the transmission signal from the directional coupler and pre-selects the signal to be subjected to automatic gain control, and an intermediate frequency signal that converts the signal that has passed through this filter into an intermediate frequency signal and selects only the pilot signal. A relay amplifier for cable television, comprising a filter, a circuit for amplifying and detecting this signal, and a circuit for feeding back the detected signal to the voltage-controlled variable attenuator.