JPH0257755B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention simplifies the circuit configuration of a CATV converter, particularly a CATV converter that selectively converts a cable television signal to, for example, a second, third, or fourth channel. CATV that is less susceptible to the effects of local frequency drift
It concerns converters.

The problems of the prior art will be outlined below. FIG. 1 shows a conceptual diagram when a cable television signal and an airwave television signal are respectively received. A wired television signal is guided from a wired cable 1 via a tap 2 to a CATV converter 3, which performs frequency conversion to a selected one of a plurality of predetermined channels, and converts the frequency of the wired television signal to a selected one of a plurality of predetermined channels. The signal is switch 5
is supplied to the TV receiver 6 via. When receiving airborne television, the high frequency signal captured by the antenna 4 is guided to the TV receiver via the switch 5, a channel is selected, and the signal is received. The airwave television signal is used in one channel jump to avoid interference from adjacent channels, and the output channel of the CATV converter 3 is used in the airwave to avoid interference from the airwave during CATV reception. Not set to an empty channel. Needless to say, the number of empty channels differs depending on the region, and the above CATV converter is designed to be able to selectively switch the output channels, and in half the range it switches between the second, third, and fourth channels. They are often given a choice.

FIG. 2 shows a conventional CATV converter with selectable output channels. The input signal supplied to the received signal input terminal 7 passes through an input filter 8 and is guided to a first mixer 9. Then, it is mixed with the local oscillator signal from the first oscillator 10. The local oscillator coil 11, the local oscillator varactor 12, the bypass capacitor 13, and the connecting resistor 14 are used to make the frequency of the first oscillator 10 variable.
In the synthesizer receiver, the desired output voltage is set by applying the charge pump output voltage (not shown).
CATV reception channel is selected.

The intermediate frequency signal produced by the first mixer 9 is guided to the second mixer 19 through an intermediate frequency input filter 16, an intermediate frequency amplifier 17, and an intermediate frequency output filter 18. Then, it is mixed with the local oscillator signal from the second oscillator 20. Local coil 21, local varactor 22, bypass capacitor 23, connection resistor 24
is for switching the frequency of the second oscillator 20, and by applying the voltage from the switching voltage supply device 25 to the local varactor 22 through the switching switch 33, a desired output channel is selected.

The output channel and signal created by the second mixer 19 are passed through the output filter 26 and the output trap 2.
9. The signal is selectively supplied to any one of the output filter 27 and the output trap 30, the output filter 28 and the output trap 31 by the changeover switches 34 and 35, and is outputted to the output terminal 36. In addition,
The switches 33, 34, and 35 have an interlocking mechanism 32 to switch the output channels in one motion.

FIG. 3 shows the output filters 26, 27, 28
This is an example of the passage characteristics between the output channel and the output traps 29, 30, and 31. In the figure, P and S indicate the image carrier signal and the audio carrier signal of the output channel, and S-6 and P+6 indicate the adjacent channels. It represents the audio and video carrier signals of the channel.

A conventional CATV converter is configured as described above, and includes, for example, an output filter 26 and an output trap 29 to obtain an output channel corresponding to the second channel, and an output filter to obtain an output channel corresponding to the third channel. 27 and an output trap 30, and an output filter 28 and an output trap 31 for obtaining an output channel corresponding to the fourth channel, each of which must be provided with independent circuit sections, and must be switched in conjunction with the frequency switching of the second oscillator. The structure is relatively complicated. This is expensive and has the potential disadvantage of being susceptible to interference between output filters or output traps. Furthermore, since the output traps 29, 30, and 31 are inserted after the second mixer 19, if the frequency of the second oscillator 20 drifts, a discrepancy will occur between the trap frequency and the interference signal, causing beat interference. There was a drawback that it was easy for this to occur. Furthermore, there is a problem that interference with adjacent signals is likely to occur in the second mixer. Furthermore, the changeover switches 34 and 35 are for switching high-frequency signals, and have the problem of deterioration of signal transmission characteristics and easy leakage of interference signals.

FIG. 4 shows the configuration of an embodiment of the present invention.
Reference numerals 7 to 25, 33, and 36 in the figure correspond to those in FIG. 2, 37 represents an intermediate frequency trap provided in the present invention, and 38 represents an output low-pass filter also provided in the present invention. .

Until the input signal is applied to the received signal input terminal 7 and the desired CATV reception channel is selected and routed to the intermediate amplifier 17, it may be considered that the case is exactly the same as in FIG. In the case shown in Figure 4,
An intermediate frequency trap 37 is provided following the intermediate frequency output filter 18, and the output from the amplifier 17 is guided to the second mixer 19 after sufficiently attenuating interfering signals such as adjacent signals. The frequency of the second oscillator 20 is switched to select the output channel as in the case shown in FIG.
The output of mixer 19 is directed to output terminal 36 through output low pass filter 38 in the form of one of the desired second, third, or fourth channels. The output low-pass filter 38 has a pass characteristic as shown in FIG. 5, for example, and passes the output from the second mixer 19. However, the characteristics of the filter 38 are not limited to those shown in FIG. 5, and may be any characteristics that allow the output from the second mixer 19 to pass and prevent undesired interference signals from passing as much as possible. , may be a band pass filter.

In the case shown in FIG. 4, when selecting, for example, the third channel as the output channel, it is sufficient to set the switch 33 to the corresponding signal. Then, in the second mixer 19, the CATV television signal transformed to the frequency of the third channel is guided to the output terminal 36 through the output low-pass filter 38.

The CATV of the present invention is configured as described above.
According to the converter, the configuration is simplified and the cost is reduced, as can be seen by comparing FIG. 2 and FIG. 4. On the other hand, since a trap circuit is inserted before the second mixer, the second mixer Therefore, adjacent interference is less likely to occur. Furthermore, even if the frequency of the second oscillator drifts, the trapping performance of the preceding trap circuit is not affected, nor is it undesirably attenuated by the output low-pass filter. The influence of such frequency drift is then connected to the output terminal 36.
It will be absorbed by the AFC circuit (not shown) built into the TV receiver. Furthermore, the switch only changes the DC voltage;
The problems described in connection with FIG. 2 do not occur.

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram of the case where a cable television signal and an airwave television signal are respectively received;
The figure shows an example of the conventional configuration of the CATV converter shown in FIG. 1, FIG. 3 shows an example of the frequency pass characteristics of the output filter and output trap shown in FIG. The figure shows an example of the frequency characteristics of the output low pass filter shown in FIG. In the figure, 9 is the first mixer, 10 is the first oscillator, 1
6 is an intermediate frequency input filter, 17 is an intermediate frequency amplifier, 18 is an intermediate frequency output filter, 19 is a second mixer, 20 is a second oscillator, 37 is an intermediate frequency trap, and 38 is an output low pass filter.

Claims (1)

[Scope of Claims] 1. A converter that is provided with a frequency conversion circuit stage that selectively converts the frequency of a cable television signal into one of the television signals of a plurality of adjacent channels and inputs it to a television receiver, comprising: The frequency conversion circuit stage includes a first mixer circuit and a second mixer circuit.
A mixer circuit, and the frequency conversion circuit stage includes a band filter between the first mixer circuit and the second mixer circuit that allows only the band of the specific channel obtained by the first mixer circuit to pass. ,
a trap circuit for removing signals of other channels adjacent to the specific channel, and a filter whose pass band includes all the bandwidths of the plurality of adjacent channels between the frequency conversion circuit stage and the output end. The CATV converter is further characterized in that the frequency conversion circuit stage includes a local oscillator having oscillation frequency switching means for oscillating different frequencies corresponding to each of the plurality of adjacent channels.