JP2000115740A - Television signal transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To save-power and to make the size of the transmitter small by simplifying a configuration of a band pass filter section. SOLUTION: The transmitter is provided with a band pass filter section that consists of a high pass filter 13 whose cut-off frequency is a frequency less than a minimum frequency of a broadcast band of a television signal and of a low pass filter section 14 having a plurality of low pass filters 21c, 22c, 23c,... whose cut-off frequencies differ from each other. A band pass filter is configured by selecting a specific low pass filter among the low pass filters 21c, 22c, 23c,... with the high pass filter 13 so as to attenuate harmonics whose frequency is twice or over of the frequency of the television signal passing through the band pass filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal transmitter used in a head end (transmission side) of a CATV (cable television) system.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing a configuration of a conventional television signal transmitter. In FIG. 4, a video signal V and an audio signal A are output from a baseband processing circuit 4.
After being appropriately processed in step 1, the modulator 42 converts the signal into an intermediate frequency television signal (intermediate frequency signal). The intermediate frequency signal is input to the frequency converter 43, and is converted into a television signal of a specific channel to be transmitted within a broadcast band (about 50 MHz to 1000 MHz).

A television signal of a specific channel is amplified by a preamplifier 44, passes through a bandpass filter section 45 having a plurality of bandpass filters, and is then amplified by a main amplifier 46 to a predetermined level. Sent to The band-pass filter section 45 is for removing harmonics of the television signal generated by the preamplifier 44.

FIG. 5 shows a detailed configuration of the band-pass filter section 45 in FIG. 4, and a plurality of filter switching circuits 53, 54, 55,.
Are provided in parallel, and one of the filter switching circuits is selected. Each of the filter switching circuits 53, 54, 55,... Has the same configuration. First, the first filter switching circuit 53 includes an input-side switch diode 53a having an anode connected to the input terminal 51, an output-side switch diode 53b having an anode connected to the output terminal 52, and two switch diodes 53a, 53b. A first bandpass filter 53c connected to the input side, a collector connected to a connection point between the input side switch diode 53a and the first bandpass filter 53c and an emitter grounded; A collector is connected to a connection point between the switch diode 53b and the first bandpass filter 53c, and an output-side switch transistor 53e whose emitter is grounded is provided. The base of the input side switch transistor 53d and the base of the output side switch transistor 53e are connected to each other.

Similarly, a second filter switching circuit 54 includes an input-side switch diode 54a, an output-side switch diode 54b, a second band-pass filter 54c,
It has an input side switch transistor 54d and an output side switch transistor 54e, and has a base of the input side switch transistor 54d and an output side switch transistor 54e.
e are connected to each other. The third filter switching circuit 55 also includes an input-side switch diode 55a, an output-side switch diode 55b, a third band-pass filter 55c, and an input-side switch transistor 55d.
And the output-side switch transistor 55e. The base of the input-side switch transistor 55d and the base of the output-side switch transistor 55e are connected to each other.

Then, voltages are applied to the collectors of the switch transistors 53d and 53e of the first filter switching circuit 53 via feed resistors 53f and 53g, respectively.
Similarly, the second filter switching circuit 54 and the third filter switching circuit 55 also have power supply resistors 54f, 54g, 55f, 5
5g, and the switching transistors 54d, 54
The voltage E of e, 55d, and 55e is applied. The same applies to other filter switching circuits not shown. Further, each input-side switch diode 53a, 54a, 55a,.
Are connected to each other, their cathodes are connected to ground via a resistor 56, and the cathodes of the output side switching diodes 53b, 54b, 55b,... Are also connected to each other. Connected to the ground via

[0007] The filter switching circuit 45 is provided with a switching control circuit 60. The switching control circuit 60 has the same number of output terminals 60a, 60b, 60c,... As the number of filter switching circuits 53, 54, 55,. The first output terminal 60a is connected to the first filter switching circuit 5
3 is connected to the base of the input-side switch transistor 53d and the base of the output-side switch transistor 53e, and the second output terminal 60b is connected to the base of the input-side switch transistor 54d and the output-side switch transistor 54e in the second filter switching circuit 54. The third output terminal 60c is connected to the base of the input side switch transistor 55d and the base of the output side switch transistor 55e in the third filter switching circuit 55. The same applies hereinafter.

Each band-pass filter 53c shown in FIG.
Are designed to be different from each other. For example, the first band-pass filter 53c is 47 MHz to 66 MHz, the second band-pass filter 54c is 64 MHz to 96 MHz, and the third band is The pass filter 55c has a frequency range of 95 MHz to 139 MHz. The circuit is composed of a series-type induction M-type high-pass filter 71 and a parallel-type induction M-type low-pass filter 72, as shown in FIG.

In the above configuration, if the frequency of a specific television signal input to the input terminal 51 is, for example, within the pass band of the first band-pass filter 53c, the first output of the switching control circuit 60 Only the terminal 60a outputs a low-level voltage (for example, 0 volt), and the other output terminals 60b, 60c,... Output high-level voltages. As a result, the input side switch transistor 53d and the output side switch transistor 53e in the first filter switching circuit 53 are turned off. Then, the input-side switch diode 53a and the output-side switch diode 53b are turned ON, and the input television signal of the specific channel is output to the output terminal 52 through only the first band-pass filter 53c.

On the other hand, if the frequency of the specific television signal input to the input terminal 51 is within the pass band of the second band-pass filter 54c, for example, the switching control circuit 60
Output the low level voltage, and the other output terminals 60a, 60c,... Output the high level voltage. Then, the input side switch transistor 54d and the output side switch transistor 54e in the second filter switching circuit 54 are turned off,
The input-side switch diode 54a and the output-side switch diode 54b are turned on, and the input television signal of the specific channel is supplied to the second band-pass filter 5.
4c and is output to the output terminal 52. Similarly, the input specific television signal is output to the output terminal 52 after passing only through a band-pass filter whose frequency is within the pass band.

[0011]

In the conventional television signal transmitter described above, each of the filter switching circuits 53, 54, 5 in the band-pass filter section 45 is used.
Control each input-side switch diode 53a, 54a, 55a,... To be turned ON or OFF by each input-side switch transistor 53d, 54d, 55d,. The output-side switch diodes 53b, 54b, 55b,...
······························ Turn ON or OFF Therefore, each of the filter switching circuits 53, 54, 5
5, two switch transistors were used, and the number of components (switch transistors) increased. As a result, the cost of parts has increased, the complexity of the assembling work has increased, and the size of the printed circuit board for forming the band-pass filter unit 45 has been increased. Further, there is a problem that power consumption is increased.

In the conventional television signal transmitter, each filter switching circuit 53, 54, 55,...
Provided with a band-pass filter including a low-pass filter 71 and a low-pass filter 72, the number of components of each filter switching circuit 53, 54, 55,... Therefore, also in this regard, increase in parts cost,
As a result, the assembly work becomes complicated, the size of the printed circuit board for forming the bandpass filter section 45 becomes large, and the manufacturing cost increases.

Therefore, in the television signal transmitter of the present invention, the configuration of the band-pass filter is simplified.
An object is to achieve power saving and downsizing.

[0014]

In order to solve the above-mentioned problems, a television signal transmitter according to the present invention comprises: a high-pass filter having a cut-off frequency equal to or lower than a lowest frequency of a broadcast band of a television signal; A low-pass filter having a plurality of low-pass filters having different off-frequency components, and a band-pass filter together with the high-pass filter by selecting a specific low-pass filter among the low-pass filters; A harmonic that is twice or more of the television signal passing through the band-pass filter is attenuated.

Further, in the television signal transmitter according to the present invention, the low-pass filter, an input-side switch diode connected between the input terminal of the low-pass filter and the high-pass filter, and an output terminal of the low-pass filter The low-pass filter section is configured by connecting a plurality of filter switching circuits each including the connected output-side switch diode in parallel, and only the input-side switch diode and the output-side switch diode connected to the specific low-pass filter are made conductive. Thus, the band-pass filter is constituted by the high-pass filter and the specific low-pass filter.

Also, in the television signal transmitter according to the present invention, the input terminal and the output terminal of the low-pass filter are connected to each other in a DC manner and are DC-insulated from the ground, and the low-pass filter and the input side are connected to each other. A collector is connected to a connection point with the switch diode, and a switch transistor having an emitter connected to the ground is provided.A voltage is applied to the collector via a power supply resistor, and a voltage is applied to the specific low-pass filter among the switch transistors. Only switch transistor connected
I made it FF.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a television signal transmitter according to the present invention will be described with reference to FIGS. Here, FIG.
FIG. 2 is a circuit diagram illustrating a configuration of a television signal transmitter according to the present invention. FIG. 2 is a circuit diagram illustrating a configuration of a band-pass filter unit in the television signal transmitter according to the present invention. FIG. 3 is a circuit diagram of a low-pass filter in the illustrated band-pass filter unit.

First, in FIG. 1, a video signal V and an audio signal A are appropriately processed by a baseband processing circuit 1 and then output by a modulator 2 to a television signal of an intermediate frequency (a video intermediate frequency of 45.75 MHz in the US specification). , An intermediate frequency signal whose audio intermediate frequency is 41.25 MHz)
Is converted to The intermediate frequency signal is input to the frequency converter 3 and is converted into a television signal of a specific channel in a broadcast band (about 50 MHz to 1000 MHz).

The frequency converter 3 comprises a first mixer 3a, a first local oscillator 3b, a first intermediate frequency band-pass filter 3c,
It has a first intermediate frequency amplifier 3d, a second mixer 3e, and a second local oscillator 3f.
A local oscillation signal of 1.25425 GHz is input from b. Therefore, the first mixer 3a outputs a first intermediate frequency signal of 1.3 GHz (video carrier). The first intermediate frequency signal is input to the second mixer 3e via the first intermediate frequency band pass filter 3c and the first intermediate frequency amplifier 3d. The output from the second local oscillator 3f is output to the second mixer 3e.
A local oscillation signal whose oscillation frequency changes from about 300 MHz to 1.25 GHz is input. As a result, the second mixer 3e outputs a second intermediate frequency signal, which has been frequency-converted to any frequency of 50 to 1000 MHz, that is, a television signal to be transmitted.

This television signal is amplified by the preamplifier 4, is input to the main amplifier 6 through the band-pass filter section 5, is amplified to a predetermined level by the main amplifier 6, and is transmitted to a cable (not shown). The band-pass filter section 5 is for removing harmonics of the television signal generated by the preamplifier 4, and for preventing an interference signal having a relatively low frequency below the broadcast band of the television signal from entering. .

FIG. 2 shows a detailed configuration of the band-pass filter section 5 in FIG. 1. A high-pass filter 13 and a low-pass filter section 14 are provided between an input terminal 11 and an output terminal 12.
Are connected in series. The low-pass filter unit 14
A plurality of filter switching circuits 21, 22, 23,... Are connected in parallel with each other, and one of the filter switching circuits is selected. First, the first filter switching circuit 21 includes an input-side switching diode 21a having an anode connected to the high-pass filter 13 side.
An output-side switch diode 21 b having an anode connected to the output terminal 12, and two switch diodes 21 b
a, a first low-pass filter 21 connected between 21a and 21b
c, and a switch transistor 21d whose collector is connected to the connection point between the input side switch diode 21a and the first low-pass filter 21c and whose emitter is grounded.

Similarly, the second filter switching circuit 22
An input-side switch diode 22a having an anode connected to the high-pass filter 13, an output-side switch diode 22b having an anode connected to the output terminal 12, and a second low-pass filter connected between the two switch diodes 22a and 22b. 22c, and a switch transistor 22d having a collector connected to a connection point between the input side switch diode 22a and the second low-pass filter 22c and having an emitter grounded.

Similarly, the third filter unit 23 has an input-side switch diode 23a, an output-side switch diode 23b, a third low-pass filter 23c, and a switch transistor 23d.

Then, each switch transistor 21d,
A power supply resistor 21 is connected to each of the collectors 22d and 23d.
The voltage E is applied via e, 22e, 23e,. Also, the input-side switch diodes 21a, 22a,
Are connected to each other, and the cathodes are grounded by the resistor 15. Similarly, the output side switching diodes 21b, 22b, 23
The cathodes b,... are connected to each other, and the cathodes are grounded by a resistor 16.

The filter switching circuit 5 includes a switching control circuit 1
7 are provided. The switching control circuit 17 has the same number of output terminals 17a, 17b, 17c,... As the number of the filter switching circuits 21, 22, 23,. The first output terminal 17a is connected to the first filter switching circuit 2
1, the second output terminal 17b is connected to the base of the switch transistor 22d in the second filter switching circuit 22, and the third output terminal 17c is connected to the third filter switching circuit 23. Connected to the base of switch transistor 23d. Hereinafter, the same applies to a filter switching circuit (not shown).

The high-pass filter 13 shown in FIG. 2 is composed of, for example, a well-known constant-K high-pass filter or a series or parallel induction M-type high-pass filter.
The cutoff frequency is set to be equal to or lower than the lowest frequency of the broadcast band of the television signal, for example, about 47 MHz in the U.S. specification.

Each low-pass filter 21c, 22
The cutoff frequencies of c, 23c,... are designed to be different from each other, and each cutoff frequency is set so that the second harmonic of the television signal passing through each low-pass filter does not pass. For example, the cutoff frequency of the first low-pass filter 21c is 66 MHz, which is not more than twice the cutoff frequency (47 MHz) of the high-pass filter 13, and the cutoff frequency of the second low-pass filter 22c is the cut-off frequency of the first low-pass filter 21c. 96 MH which is less than twice the off frequency (66 MHz)
z, the cutoff frequency of the third low-pass filter 23c is set to 139 MHz, which is not more than twice the cutoff frequency (96 MHz) of the second low-pass filter 22c. Then, each low-pass filter 21c, 22
, 23c,... have the parallel induction M shown in FIG.
As in the case of the type low-pass filter 31, a low-pass filter is used in which the input terminal 31a and the output terminal 31b are DC-connected, and the input terminal 31a and the output terminal 31b are DC-insulated from the ground.

In the above configuration, if the frequency of the specific television signal input to the input terminal 11 is, for example, lower than the cutoff frequency of the first low-pass filter 21c, the first output of the switching control circuit 17 Only the terminal 17a outputs a low-level voltage (for example, 0 volt), and the other output terminals 17b, 17c,... Output high-level voltages. As a result, the switch transistor 21d in the first filter switching circuit 21 turns off. Then, the feed resistor 21e, the input side switch diode 21
a, a current flows through the resistor 15, and the input-side switch diode 21a is turned on. Also, a feed resistor 21e, a first low-pass filter 21c, an output-side switch diode 2
1b, a current flows through the resistor 16, and the output-side switch diode 21b is turned on. On the other hand, the input-side switch diodes 22a, 23a,.
And output-side switch diodes 22b, 23b,.
Is turned off. For example, in the second filter section 22, when the switch transistor 22d is turned on, the input side switch diode 22a and the output side switch diode 22b are turned off. Accordingly, a band-pass filter is formed by the high-pass filter 13 and the first low-pass filter 21c, and the specific television signal input is output to the output terminal 12 through only the high-pass filter and the first low-pass filter 21c. You.

The frequency of the specific television signal input to the input terminal 11 is equal to the frequency of the first low-pass filter 21c.
Is higher than the cut-off frequency of the second low-pass filter 22c, only the second output terminal 17b of the switching control circuit 17 outputs a low-level voltage, and the other output terminals 17a , 17c, ...
・ Outputs high level voltage. Then, the switch transistor 22d in the second filter switching circuit 22
Is turned off, the input side switch diode 22a and the output side switch diode 22b are turned on, and the input side switch diodes 2 of all the other filter switching circuits are turned on.
, And the output side switching diodes 21b, 23b,... Are turned off, and the specific television signal input is passed through only the high-pass filter 13 and the second low-pass filter 22c and output to the output terminal. 12 is output.

Similarly, the frequency of the specific television signal input to the input terminal 11 is higher than the cut-off frequency of the second low-pass filter 22c and lower than the cut-off frequency of the third low-pass filter 23c. In this case, only the third output terminal 17c of the switching control circuit 17 outputs a low-level voltage, and the other output terminals 17a,
.. Output high-level voltages. Then, the input side switch diode 23a and the output side switch diode 2 in the third filter switching circuit 23
3b is turned ON, and the input side switch diode and the output side switch diode of all the other filter units are turned off.
The specific television signal input as F is output to the output terminal 12 through only the high-pass filter 13 and the third low-pass filter 23c. Similarly, the input specific television signal passes through the low-pass filter having the lowest cut-off frequency among the low-pass filters that can pass the frequency, and is output from the output terminal 12.

As described above, the bandpass filter unit 5
Connects a high-pass filter 13 and a low-pass filter unit 14 in series between an input terminal 11 and an output terminal 12, and the low-pass filter unit 14 includes a plurality of filter switching circuits 21,
Are connected in parallel with each other, and one of the filter switching circuits is selected. Therefore, each filter switching circuit 21, 2
The configuration of 2, 23,... Can be simplified. Also, the input terminals and the output terminals of the low-pass filters 21c, 22c, 23c,... In the filter switching circuits 21, 22, 23,. Therefore, each low-pass filter 21c,
The input side switch diodes and the output side switch diodes 22c, 23c,... Can be controlled to be turned ON or OFF by one switch transistor.

[0032]

As described above, the television signal transmitter according to the present invention comprises a high-pass filter having a cut-off frequency equal to or lower than the lowest frequency of the broadcast band of a television signal, and a plurality of filters having different cut-off frequencies. A low-pass filter section having a low-pass filter, a specific low-pass filter among the low-pass filters is selected to form a band-pass filter together with the high-pass filter, and a television signal passing through the band-pass filter Since a harmonic that is twice or more of the above is attenuated, the band-pass filter can be realized with a simple configuration.

Further, the television signal transmitter of the present invention comprises a low-pass filter, an input-side switch diode connected between the input end of the low-pass filter and the high-pass filter, and an output connected to the output end of the low-pass filter. A plurality of filter switching circuits consisting of a side switch diode are connected in parallel to form a low-pass filter section, and only an input side switch diode and an output side switch diode connected to a specific low-pass filter are made conductive to specify a high-pass filter. , The pass band of the band-pass filter can be easily changed corresponding to the frequency of the television signal.

Further, in the television signal transmitter of the present invention, the input terminal and the output terminal of the low-pass filter are connected to each other in a DC manner and are DC-insulated from the ground, and the connection between the low-pass filter and the input-side switch diode. The switch transistor has a collector connected to a point and an emitter connected to the ground, a voltage is applied to the collector via a feed resistor, and only a switch transistor connected to a specific low-pass filter is turned off. Therefore, the input-side switch diode and the output-side switch diode can be controlled to be turned on or off by one switch transistor, and the bandpass filter can be further simplified.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a television signal transmitter according to the present invention.

FIG. 2 is a circuit diagram illustrating a configuration of a band-pass filter unit in the television signal transmitter according to the present invention.

FIG. 3 is a circuit diagram of a low-pass filter in the low-pass filter section shown in FIG.

FIG. 4 is a circuit diagram showing a configuration of a conventional television signal transmitter.

FIG. 5 is a circuit diagram showing a configuration of a band-pass filter unit in a conventional television signal transmitter.

FIG. 6 is a circuit diagram of the bandpass filter in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Baseband processing circuit 2 Modulator 3 Frequency converter 3a 1st mixer 3b 1st local oscillator 3c 1st intermediate frequency bandpass filter 3d 1st intermediate frequency amplifier 3e 2nd mixer 3f 2nd local oscillator 4 Preamplifier Reference Signs List 5 band-pass filter unit 6 main amplifier 11 input terminal 12 output terminal 13 high-pass filter 14 low-pass filter unit 15, 16 resistor 17 switching control circuit 17a first output terminal 17b second output terminal 17c third output terminal 21 first 21a Input-side switch diode 21b Output-side switch diode 21c First low-pass filter 21d Switch transistor 21e Power supply resistor 22 Second filter-switching circuit 22a Input-side switch diode 22b Output-side switch diode 22c Second low-pass filter 22d switch transistor 22e power supply resistor 23 third filter switching circuit 23a input side switch diode 23b output side switch diode 23c third low pass filter 23d switch transistor 23e power supply resistance 31 low pass filter 31a input terminal 31b output terminal

Claims (3)

[Claims] 1. A band-pass filter comprising a high-pass filter having a cut-off frequency equal to or lower than a lowest frequency of a broadcast band of a television signal and a low-pass filter having a plurality of low-pass filters having different cut-off frequencies. A specific low-pass filter among the low-pass filters to form a band-pass filter together with the high-pass filter, so as to attenuate a harmonic of at least twice the television signal passing through the band-pass filter. A television signal transmitter, comprising: 2. The low-pass filter, comprising: an input-side switch diode connected between an input terminal of the low-pass filter and the high-pass filter; and an output-side switch diode connected to an output terminal of the low-pass filter. The low-pass filter section is configured by connecting a plurality of filter switching circuits in parallel, and only the input-side switch diode and the output-side switch diode connected to the specific low-pass filter are made conductive, so that the high-pass filter and the specific low-pass filter are connected. 2. The filter according to claim 1, wherein said band pass filter is constituted by a filter.
A television signal transmitter as described. 3. The input terminal and the output terminal of the low-pass filter are DC-connected to each other and are DC-insulated from ground, and a collector is connected to a connection point between the low-pass filter and the input-side switch diode. A switch transistor whose emitter is connected to ground, a voltage is applied to the collector via a power supply resistor, and only the switch transistor connected to the specific low-pass filter among the switch transistors is turned off. The television signal transmitter according to claim 2, wherein: