JP2004165793A - Channel converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a channel converter capable of satisfactorily suppressing spurious components with a comparatively simple circuit configuration. <P>SOLUTION: Impedance elements 8, 9 are inserted between an input side transmission circuit 3 and a mixer circuit 6, and between the circuit 6 and an output side transmission circuit 5 so that each element is parallel to each circuit, respectively. The impedances from the viewpoints of the input and output sides of the circuit 6 are put into a mismatching state to a frequency 2L by the elements 8, 9, where a local frequency outputted from a local oscillator circuit 7 used for the circuit 6 is L. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a channel converter used in a CATV system, which converts a signal frequency of a reception channel into a frequency of another channel.
[0002]
[Prior art]
At the head end of the CATV system, a channel converter used for retransmission of a television broadcast converts, for example, a UHF band reception channel received by an antenna into a frequency of a VHF band free channel and transmits the converted signal to a trunk line. It works to output. A mixer circuit used in this channel converter mixes a UHF television signal (received television signal) with a signal of a local frequency output from a local oscillation circuit, and mixes the UHF television signal with a frequency corresponding to the sum or difference of the two. The signal is taken out. In the case of single conversion, a signal having a frequency corresponding to the difference is extracted.
[0003]
A problem in performing this frequency conversion is spurious. When the local frequency is L, in addition to the output signal of the frequency of (received television signal frequency-L), (2L-received television signal frequency) and (3L-2 × received television signal frequency-L) Frequency spurs occur and interfere with television signals output to the mains.
[0004]
Most of such spurious can be removed by a band filter or a trap. However, the spurious of (2L-received television signal frequency) has a high level and is generated within the band of the channel after conversion. There is a problem that removal is difficult.
[0005]
That is, for example, when converting the UHF 21 channel to the VHF 4 channel, a spurious of (2L-frequency of the video carrier of the 21 channel UHF) is generated in the portion of the VHF 4 channel video carrier frequency + 1 MHz. Attempts to remove this with a bandpass filter or trap will degrade the signal on the conversion channel. Therefore, it is technically impossible to perform such a combination of channel conversions with a channel converter having a normal specification, and it is specified in a catalog of a channel converter manufacturer.
[0006]
[Problems to be solved by the invention]
By the way, the conventional techniques as described above have the following problems to be solved.
In a VHF band in a city with few available channels, there are cases where it is necessary to cope with the above-described channel conversion. Therefore, a channel converter of a double conversion system using two local oscillators and two mixers is specially designed to cope with the problem. In this channel converter, the UHF 21 channel is once converted to another intermediate frequency, and then converted to the target VHF 4 channel. This can prevent spurious components from occurring in the band of the conversion channel. However, this method has a problem that the circuit is complicated, the power consumption is large, and the cost is greatly increased.
The present invention has been made in view of the above points, and an object of the present invention is to provide a channel converter capable of sufficiently suppressing spurious signals with a relatively simple circuit configuration.
[0007]
[Means for Solving the Problems]
The present invention solves the above problem by the following configuration.
<Configuration 1>
A transmission circuit on the input side for transmitting a reception television signal to be subjected to frequency conversion, a reception television signal input via the transmission circuit on the input side, and a local frequency signal output from the local oscillation circuit are mixed. A mixer circuit for extracting the frequency-converted television signal; and an output-side transmission circuit for transmitting the frequency-converted television signal. An impedance element is inserted between the transmission circuit on the input side of the mixer circuit and the local frequency output from the local oscillation circuit used in the mixer circuit is set to L between the transmission circuit on the input side and the transmission circuit on the input side of the mixer circuit. Between the impedance seen from above and the impedance seen from the output side of the mixer circuit with respect to the frequency 2L Channel converter, characterized in that a state.
[0008]
<Configuration 2>
An input-side transmission circuit that transmits a reception television signal to be subjected to frequency conversion, a reception television signal input through the input-side transmission circuit, and a local frequency L signal output from a local oscillation circuit. A mixer circuit for mixing and extracting the frequency-converted television signal, and an output-side transmission circuit for transmitting the frequency-converted television signal, wherein the input-side transmission circuit and the mixer circuit, When an impedance element is inserted between the output side transmission circuit and the transmission circuit in parallel with each other to change the signal level of the local oscillation circuit output within a range where the mixer circuit can maintain normal operation, (2L- The impedance element is set so that the spurious signal level of the received television signal frequency shows a substantially V-shaped characteristic having a downward convex minimum value. Along with selecting a value, channel converter signal level spurious is characterized by being selected to a value indicating a minimum value of the signal level of the local oscillation circuit output (2L- received television signal frequency).
[0009]
<Configuration 3>
The channel converter according to Configuration 2, further comprising a mixer circuit having a configuration in which a diode bridge is disposed between the input-side balanced transformer and the output-side balanced transformer, and the mixer circuit includes an input-side balanced and an output-side balanced transformer. An impedance circuit is connected to the output side of the transformer, and at the input terminal portion of the diode bridge, the value of the impedance is selected so that the impedance becomes close to zero for a frequency of 2L. Channel converter to do.
[0010]
<Configuration 4>
The channel converter according to Configuration 2, further comprising a mixer circuit having a configuration in which a diode bridge is disposed between the input-side balanced transformer and the output-side balanced transformer, and a mixer circuit provided on an input side of the input-side balanced transformer and an output side of the output-side balanced transformer. , A impedance converter is connected in series, and the value of the impedance is selected so that the impedance becomes near infinity with respect to a frequency of 2 L at the input terminal of the diode bridge.
[0011]
<Configuration 5>
An input-side transmission circuit that transmits a reception television signal to be subjected to frequency conversion, a reception television signal input through the input-side transmission circuit, and a local frequency L signal output from a local oscillation circuit. A mixer circuit for mixing and extracting a frequency-converted television signal, and an output-side transmission circuit for transmitting the frequency-converted television signal, wherein the input-side transmission circuit, the mixer circuit, and the output-side transmission circuit are provided. In addition to inserting an impedance element having a value such that the transmission efficiency of the circuit including the signal of frequency 2L becomes sufficiently lower than the transmission efficiency of other signals, the frequency of the mixer circuit using the signal of frequency 2L is used. The frequency output from the local oscillation circuit is set so that the conversion efficiency is sufficiently lower than the conversion efficiency using the signal of frequency L. Channel converter being characterized in that selects the L signal level.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described using specific examples.
FIG. 1 is a block diagram showing a specific example of the channel converter of the present invention.
This circuit shows the main part of the channel converter at the head end of the CATV system. A television signal received by the antenna 1 shown in FIG. 1 is input to a mixer circuit 6 via a transmission circuit 3 such as a band-pass filter or an amplifier. This signal is mixed with the output signal of the local oscillation circuit 7, frequency-converted, and output from the mixer circuit 6. The television signal after the frequency conversion is sent to the transmission circuit 5 on the output side.
[0013]
In this embodiment, the impedance elements 8 and 9 are connected in parallel to the input terminal and the output terminal of the mixer circuit 6, respectively, and the signal level of the local frequency L output from the local oscillation circuit 7 is optimized to reduce spurious noise. We are trying to decrease. The transmission circuit 3 on the input side is a circuit for transmitting, for example, a UHF received television signal to be subjected to frequency conversion. The mixer circuit 6 is a circuit that mixes a received television signal and a signal of a local frequency output from the local oscillation circuit 7 and extracts a television signal converted into, for example, a VHF band frequency. The transmission circuit 5 on the output side is a subsequent circuit for transmitting the television signal whose frequency has been converted. This television signal is transmitted to the viewer via a main coaxial cable at the head end of the CATV system.
[0014]
The impedance element 8 is inserted between the input-side transmission circuit 3 and the mixer circuit 6 in parallel. The impedance element 9 is inserted in parallel between the mixer circuit 6 and the transmission circuit 5 on the output side. At this time, when the local frequency output from the frequency oscillation circuit 7 connected to the mixer circuit 6 is L, the impedance seen from the input side of the mixer circuit and the impedance from the output side of the mixer circuit are determined by the impedance elements 8 and 9. The observed impedance is set to a mismatch state with respect to the frequency 2L.
[0015]
That is, the transmission efficiency of the transmission circuit including the input-side transmission circuit 3, the mixer circuit 6, and the output-side transmission circuit 5 with respect to the signal of the frequency 2L is sufficiently, preferably the most, as compared with the transmission efficiency of the other signals. The impedance elements 8 and 9 having such a value as to become low are inserted. The impedance element may be inserted at any position, and may be inserted in series or in parallel. Since the impedance element does not trap (2L-received television signal frequency), the output signal does not deteriorate. As will be described later, the local conversion is performed so that the frequency conversion efficiency using the signal of frequency 2L in the mixer circuit 6 is sufficiently, preferably the lowest, as compared with the conversion efficiency using the signal of frequency L. By selecting the signal level of the frequency L output from the oscillation circuit, a very practical level channel converter was completed.
[0016]
FIG. 2 is a connection diagram illustrating a specific example of the mixer circuit.
In this mixer circuit, an input-side balanced transformer 11, a diode bridge 15, and an output-side balanced transformer 12 are sequentially connected. The diode bridge 15 includes four diodes D1, D2, D3, and D4, as shown in FIG. These diodes D1, D2, D3 and D4 are used in a bridge connection as shown in FIG. The input terminals of this diode bridge are No. 1 and No. 3, and the output terminals are No. 6, No. 7, No. 5 and No. 8.
[0017]
An impedance element 8 is connected in parallel to an input terminal of the input-side balanced transformer 11. This impedance element 8 is composed of a trimmer capacitor. The output side of the output side balance transformer 12 is connected to the impedance element 9. This impedance element 9 includes a trimmer capacitor CT2 and a coil L1. In this example, each of the trimmer capacitors CT1 and CT2 used had a capacity of 10 picofarats (pF).
[0018]
Each of the impedance elements forms a series resonance circuit by the inductance of a capacitor and its connection lead wire. In the embodiment, the value of the impedance element 8 and the value of the impedance element 9 are set so that the spurious signal level of (2L-received television signal frequency) has a downwardly convex minimum value within a range where the mixer circuit maintains a normal operation. It is adjusted so as to show a V-shaped characteristic. The resonance frequency of the series resonance circuit fluctuates relatively greatly depending on the configuration and arrangement of the transmission circuit 3 on the input side, the transmission circuit 5 on the output side, and the mixer circuit 6, so that the capacitor used for the impedance element is a variable capacitor. It is preferable to adopt a configuration in which optimum conditions can be found. Here, in order to make the spurious signal level V-shaped, for example, at the input terminal portion of the diode bridge 15, the impedance element 8 and the impedance element 9 are connected so that the impedance becomes almost zero with respect to the frequency 2L. adjust.
[0019]
FIG. 4 is an explanatory diagram showing a combination of channels that cannot be conventionally converted due to 2L spurious.
"Conversion channels" shown on the left side of the figure list combinations that cannot be converted. That is, conversions such as 20 to 4 channels, 21 to 4 channels, and 23 to 5 channels use special expensive circuits because the level of spurs in the band and spurs to adjacent channels are too high. Otherwise, it was not available. According to the present invention, the following problems can be solved by the following configuration, and the spurious level can be sufficiently reduced with a simple circuit.
[0020]
FIG. 5 is an explanatory diagram showing a relationship between a local frequency signal output from the local oscillation circuit and spurious.
Using the circuit shown in FIG. 2, the output level of the local oscillation circuit 7 (FIG. 1) is gradually changed, and the 2L spurious signal level is measured. The horizontal axis in FIG. 7 is a local frequency signal output from the local oscillation circuit, and the vertical axis is a spurious level. The spurious level indicates a ratio to the main signal. The comparative example is an example in which no impedance element is inserted. As shown in this figure, in the case of the comparative example, the spurious level is in the range of −20 dB to −30 dB. However, when an impedance element is connected, a downwardly convex V-shaped characteristic is obtained in the range of −30 dB to −60 dB as shown in FIG. Conventionally, the output level of the local oscillation circuit 7 has been set to the maximum possible level in order to obtain a required conversion gain.
[0021]
On the other hand, in the present invention, by connecting the impedance element as described above, a V-shaped characteristic as shown in the figure is obtained, and the output level from the local oscillation circuit is adjusted. That is, in this example, the output level of the local frequency signal output from the local oscillation circuit is 115 dBμ, and the spurious level indicates a value of −60 dB. With this value, the spurious of (2L-received television signal frequency) is at a level that can be ignored sufficiently. That is, according to this method, channel conversion which has been impossible in the past can be performed. In addition, since it can be realized only by connecting a simple impedance element to a conventionally used mixer circuit having a simple configuration and selecting optimum conditions, a very low-cost channel converter can be provided.
[0022]
In the above example, a mixer circuit using a diode bridge was used. However, similar effects were obtained by experiments with other general mixer circuits. In either case, an impedance element may be connected in parallel to the input terminal and the output terminal of the mixer circuit in advance, and the value thereof may be set so that the impedance of the transmission circuit is close to 0 with respect to the frequency 2L. The impedance may be such that the minimum value of the V-shaped characteristic shown in FIG. 5 is such that spurious can be ignored, and strict accuracy is not required. Note that a similar result is obtained by inserting an impedance element in series with this transmission circuit such that the impedance becomes near infinity with respect to the frequency 2L. In short, the circuit constant of the impedance element may be selected so that the transmission efficiency becomes the worst for the signal of the frequency 2L, not for the signal to be converted. Further, circuit conditions may be selected so that conversion efficiency is the lowest for a signal having a frequency of 2L.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a specific example of a channel converter according to the present invention.
FIG. 2 is a connection diagram illustrating a specific example of a mixer circuit.
FIG. 3 is a connection diagram of a diode bridge.
FIG. 4 is an explanatory diagram showing a combination of channels that cannot be conventionally converted due to 2L spurious.
FIG. 5 is an explanatory diagram showing a relationship between a local frequency signal output from a local oscillation circuit after connection of impedance elements 8 and 9, and spurious.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Antenna 3 Input-side transmission circuit 5 Output-side transmission circuit 6 Mixer circuit 7 Local oscillation circuit 8, 9 Impedance element

Claims (5)

A transmission circuit on the input side for transmitting a received television signal to be frequency-converted,
A mixer circuit that mixes a received television signal input through the input-side transmission circuit and a signal of a local frequency output from a local oscillation circuit to extract a frequency-converted television signal,
An output-side transmission circuit that transmits the frequency-converted television signal,
Between the input-side transmission circuit and the mixer circuit, and between the mixer circuit and the output-side transmission circuit, insert an impedance element in parallel with each circuit,
Assuming that the local frequency output from the local oscillator circuit used in the mixer circuit is L, the impedance element causes the impedance viewed from the input side of the mixer circuit and the impedance viewed from the output side of the mixer circuit to have a frequency of 2L. A channel converter characterized by making a mismatch state with respect to a channel. A transmission circuit on the input side for transmitting a received television signal to be frequency-converted,
A mixer circuit that mixes a received television signal input through the input-side transmission circuit with a signal of a local frequency L output from a local oscillation circuit and extracts a frequency-converted television signal;
An output-side transmission circuit that transmits the frequency-converted television signal,
Between the input-side transmission circuit and the mixer circuit, and between the mixer circuit and the output-side transmission circuit, insert an impedance element in parallel with each circuit,
When the signal level of the output of the local oscillation circuit is changed within a range where the mixer circuit can maintain normal operation, the spurious signal level of (2L-received television signal frequency) has a downward convex minimum value. While selecting a value of the impedance element so as to exhibit a substantially V-shaped characteristic,
A channel converter characterized in that a signal level of an output of a local oscillation circuit is selected to a value at which a spurious signal level of (2L-received television signal frequency) shows a minimum value. The channel converter according to claim 2,
A mixer circuit having a configuration in which a diode bridge is arranged between the input-side balanced transformer and the output-side balanced transformer,
This mixer circuit
Connect an impedance circuit to the input side of the input side balanced transformer and the output side of the output side balanced transformer,
A channel converter characterized in that the value of the impedance is selected so that the impedance is near zero for a frequency of 2 L at the input terminal portion of the diode bridge. The channel converter according to claim 2,
A mixer circuit having a configuration in which a diode bridge is arranged between the input-side balanced transformer and the output-side balanced transformer,
An impedance circuit is connected in series to the input side of the input side balanced transformer and the output side of the output side balanced transformer,
A channel converter characterized in that, at the input terminal of the diode bridge, the value of the impedance is selected so that the impedance is near infinity with respect to a frequency of 2L. A transmission circuit on the input side for transmitting a received television signal to be frequency-converted,
A mixer circuit that mixes a received television signal input through the input-side transmission circuit with a signal of a local frequency L output from a local oscillation circuit and extracts a frequency-converted television signal;
An output-side transmission circuit that transmits the frequency-converted television signal,
An impedance element having a value such that the transmission efficiency of the circuit including the input-side transmission circuit, the mixer circuit, and the output-side transmission circuit with respect to a signal having a frequency of 2L is sufficiently lower than the transmission efficiency of other signals. And insert
The signal level of the frequency L output from the local oscillation circuit was selected such that the frequency conversion efficiency using the signal of frequency 2L in the mixer circuit was sufficiently lower than the conversion efficiency using the signal of frequency L. A channel converter characterized by that:

Images