JP4713331B2 - Constant attenuation filter - Google Patents

Description

  The present invention relates to a constant attenuation filter that flattens an attenuation band at an arbitrary attenuation level.

  With the start of terrestrial digital TV broadcasting, analog frequency changes (ana-ana change) are performed in many areas in order to secure the frequency (channel) for terrestrial digital TV broadcast while maintaining the current terrestrial analog TV broadcast. In this analog frequency change, the current UHF band analog TV broadcast is moved to, for example, a high band of 40 channels or more in the UHF band, and terrestrial digital TV broadcast is performed on the low band 13 to 30 channels. And in 2011, the terrestrial analog TV broadcast will be finished, and it is scheduled to completely shift to the terrestrial digital TV broadcast.

  Therefore, in the area where the analog frequency change is performed, it is inevitable that the existing receiving equipment is improved during the simulcast period until the transition to the terrestrial digital TV broadcast is completed. In particular, there is a problem in the facility where the amplifier is used, in addition to the strength of the received electric field strength, causing a distortion failure due to gain saturation of the amplifier due to an increase in the wave number of the received radio wave.

  Many countermeasures employ a method of reducing the gain of the amplifier or using a filter. The former method of lowering the gain of the amplifier reduces the level of weak radio waves other than strong radio waves, so that reception of strong radio waves is good, but weak radio waves cause degradation of image quality.

The latter method using the filter attenuates the strong radio wave and allows the weak radio wave to pass therethrough, so that the reception of the weak radio wave is good. However, the strong radio wave is attenuated by the action of the filter and causes deterioration of the image.
As the filter, a high-pass filter 10 configured as shown in FIG. 11 is used. In the high-pass filter 10, a plurality of, for example, four capacitances (capacitors) 13a to 13d are provided in series between the input terminal 11 and the output terminal 12, and the capacitance 14 and reactance (between the connection points of the capacitances 13a to 13d and the ground). Series resonant circuits 16a to 16c each including a high frequency coil) 15 are provided.

  FIG. 12 shows the frequency characteristics of the high-pass filter 10, with the horizontal axis representing frequency and the vertical axis representing attenuation. The high-pass filter 10 is a polar high-pass filter that uses a resonance circuit to sharpen the characteristics, and has a polar characteristic in which the amount of attenuation partially increases in the attenuation band.

  Since the conventional high-pass filter 10 exhibits a polar characteristic in the attenuation band, a channel existing in a frequency band having a large attenuation characteristic is not able to be normally received because the received radio wave is significantly attenuated.

Further, even in a TV reception booster used in an area where radio waves are weak, there is a possibility that cross modulation may occur and disturb the TV receiver with the start of digital terrestrial TV broadcasting. In order to prevent such a reception failure, a UHF band TV reception signal is amplified by an amplifier, then divided into two by a distributor, and input to a band pass filter for an analog TV band and a band pass filter for a digital TV band. Thus, a booster that eliminates interference waves has been considered (see, for example, Patent Document 1).
JP 2003-125231 A

  The conventional polar high-pass filter has a problem in that the attenuation amount in the attenuation band becomes large and the received image of the terrestrial digital TV broadcast is deteriorated, and the reception of a specific channel is impossible due to the polar characteristic generated in the attenuation band. There is a problem of becoming.

  Further, a booster provided with a band-pass filter for analog TV band and a band-pass filter for digital TV band to remove interference waves is a band-pass filter having different characteristics corresponding to the digital TV broadcast channel in each region. Need to be implemented, and there are so many types. Also, the booster is expensive, and it is not preferable in terms of production, product management, etc., to prepare various types according to each region.

  The present invention has been made to solve the above-described problems. The attenuation band can be flattened at an arbitrary attenuation level, the signal output level is leveled, distortion distortion due to the strength of the received electric field strength is reduced, and good reception is achieved. An object of the present invention is to provide a constant attenuation filter capable of obtaining a level.

Constant attenuation filter according to the first invention has a high-pass filter polar provided between an input terminal and an output terminal, provided in parallel with the high-pass filter, the characteristic of passing the stop band of the high pass filter apolar A general low-pass filter, and by combining the high-pass filter and the low-pass filter, the attenuation range of the overall characteristic is made to be a substantially flat frequency characteristic, and a constant attenuation value in the attenuation range is set by the slope characteristic of the low-pass filter. It is characterized by that.

  According to a second invention, in the constant attenuation filter according to the first invention, an attenuator is provided in series with the low-pass filter, and the value of the constant attenuation in the attenuation region is adjusted by the attenuator.

  According to the present invention, the attenuation band can be flattened at an arbitrary attenuation level with a very simple configuration, the signal output level is leveled, distortion distortion due to the strength of the received electric field strength is reduced, and a good reception level is obtained. Can do.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Basic configuration diagram)
FIG. 1 is a block diagram showing a basic configuration of a constant attenuation filter 20 according to the present invention.
The constant attenuation filter 20 according to the present invention is configured by connecting a polar high-pass filter (HPF) 23 and a non-polar low-pass filter (LPF) 24 in parallel between an input terminal 21 and an output terminal 22. . For example, a signal received by an antenna is input to the input terminal 21.

  The polar high-pass filter 23 is a polar filter similar to the conventional high-pass filter, and has an attenuation amount of β in the attenuation region. Further, the nonpolar low-pass filter 24 has a characteristic of passing the stop band of the polar high-pass filter 23.

  As described above, the polar high-pass filter 23 and the non-polar low-pass filter 24 having the characteristic of passing the stop band of the polar high-pass filter 23 are connected in parallel, thereby reducing the attenuation range in the overall characteristic. It can be made flat.

  The value of the constant attenuation amount α in the attenuation region is set using the slope characteristic of the nonpolar low-pass filter 24. Therefore, by changing the attenuation amount of the inclined portion of the nonpolar low-pass filter 24, the constant attenuation amount α can be arbitrarily variably set. The constant attenuation amount α in the attenuation region is smaller than the attenuation amount β in the polar high-pass filter 23.

  By flattening the attenuation band as described above and setting the constant attenuation amount α to a predetermined level, the input level of the amplifier is leveled, distortion distortion due to the strength of the received electric field strength is reduced, and a good reception level is obtained. Obtainable.

(First embodiment)
Next, a first embodiment of the present invention will be described.
FIG. 2 is a circuit configuration diagram of the constant attenuation filter 20A according to the first embodiment of the present invention.
The polar high-pass filter 23 has the same configuration as the conventional high-pass filter shown in FIG. 11, and a plurality of, for example, four capacitances 31 a to 31 d are provided in series between the input terminal 21 and the output terminal 22, and each capacitance ( Series resonance circuits 34 a to 34 c each including a capacitance 32 and a reactance (high frequency coil) 33 are provided between the connection points of the capacitors 31 a to 31 d and the ground.

  In the nonpolar low-pass filter 24, a plurality of, for example, four reactances 41a to 41d are provided in series between the input terminal 21 and the output terminal 22, and capacitances 42a to 42c are provided between the connection points of the reactances 41a to 41d and the ground. Provided.

  3A shows the frequency characteristic of the polar high-pass filter 23, and FIG. 3B shows the frequency characteristic of the non-polar low-pass filter 24. The horizontal axis represents frequency (MHz) and the vertical axis represents attenuation ( dB). FIG. 3 shows an example in the case where the HF band (13 to 62 channels) of 470 to 770 MHz is set to 535 to 770 MHz as the pass band and 535 MHz or less as the attenuation band.

  FIG. 4 shows the overall characteristics of the constant attenuation filter 20A. The horizontal axis represents frequency (MHz) and the vertical axis represents attenuation (dB).

  As described above, the polar high-pass filter 23 and the non-polar low-pass filter 24 having the characteristic of passing the stop band of the polar high-pass filter 23 are connected in parallel, thereby reducing the attenuation range in the overall characteristics, As shown in FIG. 4, for example, the attenuation range A of 470 to 535 MHz can be flattened.

  The constant attenuation filter 20A according to the first embodiment equalizes the input level of the amplifier by flattening the attenuation band and setting the constant attenuation α to a predetermined level as described in the basic configuration of FIG. Thus, it is possible to reduce the distortion disturbance due to the strength of the received electric field intensity and obtain a good reception level. The constant attenuation filter 20A according to the first embodiment is effective when the guard band (interference countermeasure band) is relatively small in digital TV broadcasting.

(Second Embodiment)
Next, a second embodiment of the present invention will be described.
FIG. 5 is a circuit configuration diagram of a constant attenuation filter 20B according to the second embodiment of the present invention.
The constant attenuation filter 20B according to the second embodiment is the same as the constant attenuation filter 20A according to the first embodiment, but instead of the nonpolar lowpass filter 24, a nonpolar lowpass filter 24a, a variable attenuator (ATT) 43, A series circuit including a nonpolar low-pass filter 24b is provided. The variable attenuator 43 can arbitrarily set the attenuation amount γ. The nonpolar low-pass filters 24a and 24b are configured in the same manner as the nonpolar low-pass filter 24 according to the first embodiment.

  In the second embodiment, the same effects as in the first embodiment can be obtained, and the attenuation amount γ can be adjusted by the variable attenuator 43, so that the constant attenuation amount α in the attenuation region in the overall filter characteristics can be changed to γ. It can be easily variably set, and is effective when a relatively large number of guard bands can be obtained.

In the second embodiment, the variable attenuator 43 is provided between the two nonpolar low-pass filters 24a and 24b on the low-pass filter side. However, as a simple type, as shown in FIG. Two nonpolar low-pass filters 24 may be combined. In FIG. 6, specific circuit configurations of the polar high-pass filter 23 and the non-polar low-pass filter 24 are shown. However, since the configuration is the same as that of the first embodiment, the same portions are denoted by the same reference numerals. Detailed description is omitted.
Even in the case of a simple configuration as shown in FIG. 6, substantially the same effect as that of the constant attenuation filter shown in FIG. 5 can be obtained.

  In the second embodiment, the variable attenuator 43 is used. However, a fixed attenuator may be used.

(Third embodiment)
Next, a third embodiment of the present invention will be described.
FIG. 7 is a circuit configuration diagram showing a basic configuration of a constant attenuation filter 20C according to the third embodiment of the present invention. FIG. 7 shows a configuration example in the case where the high-pass filter characteristic is used.

  As shown in FIG. 7, a 3 dB directional coupler 53 is provided between the input terminal 51 and the output terminal 52, and a low pass filter (LPF 1) 54 and a low pass filter ( LPF2) 55 is connected. The low-pass filters 54 and 55 are grounded via terminators 56 and 57, respectively. The low-pass filters 54 and 55 have the same characteristics.

  As the 3 dB directional coupler 53, for example, a double wound spiral type or a stripline type is used. The double spiral type can be reduced in size, and the strip line type can be made high quality with little variation using a printed circuit board.

  As the low-pass filters 54 and 55, for example, LC filters that are commonly used may be used. As the terminators 56 and 57, for example, a 50Ω (or 75Ω) resistor or a π-type or T-type attenuator is used.

Next, the operation of the constant attenuation filter 20C will be described.
a. The signal inputted from the input terminal 51 is divided into two by the 3 dB directional coupler 53 and outputted to the low-pass filter 54 and the low-pass filter 55 side.
b. The frequency component corresponding to the attenuation region of the low-pass filters 54 and 55 is reflected and output to the input terminal 51 and the output terminal 52 side.
c. The frequency component corresponding to the pass band of the low-pass filters 54 and 55 passes through the low-pass filters 54 and 55 and is absorbed by the terminators 56 and 57. As a result, the frequency component corresponding to the pass band of the low-pass filters 54 and 55 is not output to the output terminal 52 side.

  d. The signals reflected by the low-pass filters 54 and 55 and returned to the input terminal 51 and the output terminal 52 have the phase relationship shown in FIG. 8, and as a result, they are not output to the input terminal 51 but all to the output terminal 52. Is output.

  In FIG. 8, regarding the signal input to the input terminal 51, the frequency component corresponding to the pass band of the low-pass filters 54 and 55 is the fp component, and the frequency component corresponding to the attenuation band of the low-pass filters 54 and 55 is the fr component.

  The fp and fr components of the signal input from the input terminal 51 to the 3 dB directional coupler 53 are divided into two by the 3 dB directional coupler 53 to become fp / 2 and fr / 2. At this time, the phase of the signal component output to the low-pass filter 54 side is 0 °, and the phase of the signal component output to the low-pass filter 55 side is −90 °.

  At this time, the frequency component fp / 2 corresponding to the pass band of the low-pass filters 54 and 55 passes through the low-pass filters 54 and 55 and is absorbed by the terminators 56 and 57, and is not output to the output terminal 52 side.

  Further, the frequency component fr / 2 corresponding to the attenuation region of the low-pass filters 54 and 55 is reflected and again divided into two by the 3 dB directional coupler 53 to become fr / 4 to the input terminal 51 and the output terminal 52 side. Is output. At this time, the phase of fr / 4 reflected from the low-pass filter 54 toward the input terminal 51 is 0 °, and the phase of fr / 4 reflected toward the output terminal 52 is −90 °.

  On the other hand, the frequency component fr / 4 of the signal reflected from the low-pass filter 55 to the input terminal 51 side is delayed in phase by 90 °, and consequently becomes “−90 ° + (− 90 °) = − 180 °”. . The phase of fr / 4 reflected from the low pass filter 55 to the output terminal 52 side is −90 °.

  As a result, the phase of fr / 4 reflected from the low-pass filters 54 and 55 to the input terminal 51 side is canceled out as “0 °” and “−180 °”.

  Further, the phase of fr / 4 reflected from the low-pass filters 54 and 55 to the output terminal 52 side is “−90 °”, and is added to fr / 2 (−90 °).

  Since the signal fr reflected from the low-pass filters 54 and 55 to the input terminal 51 side cancels out with a phase difference of 180 °, all the signals fr are output to the output terminal 52 side.

  e. As described above, the component fp corresponding to the pass band of the low-pass filters 54 and 55 is not output to the output terminal 52 side, but is connected between the terminals of the 3 dB directional coupler 53, that is, between the input terminal 51 and the output terminal 52. Since the amount of attenuation does not exceed the amount of attenuation, an amount of attenuation equivalent to leakage from the input terminal 51 to the output terminal 52 can be obtained. Since the attenuation amount between the input terminal 51 and the output terminal 52 is restricted by the isolation (coupling attenuation amount) of the 3 dB directional coupler 53, the low pass filters 54 and 55 loaded on the 3 dB directional coupler 53 are used. It becomes irrelevant to the amount of attenuation.

  FIG. 9 shows the frequency characteristics of the constant attenuation filter 20C. The solid line a indicates the characteristics between the input terminal 51 and the output terminal 52, and the broken line b indicates the characteristics of the low-pass filters 54 and 55. In FIG. 9, the frequency band of about 400 to 600 MHz is the attenuation band, and the frequency band of 600 MHz to 770 MHz is the pass band.

  According to the above embodiment, the low-pass filters 54 and 55 are connected to the 3 dB directional coupler 53 provided between the input and output terminals, and the signals that have passed through the low-pass filters 54 and 55 are absorbed by the terminators 56 and 57. Thus, a constant attenuation can be obtained over a wide band as shown in FIG. The value of the constant attenuation obtained in the attenuation region can theoretically be any dB, but is practically 20 to 30 dB.

Next, a case where the value of the constant attenuation is changed in the above embodiment will be described.
As described in the above embodiment, the attenuation cannot be greater than the isolation value of the 3 dB directional coupler 53, but can be reduced to a value of 15 dB, 10 dB, or the like.

  The reason why the constant attenuation is obtained is that the signals that have passed through the low-pass filters 54 and 55 are absorbed by the terminators 56 and 57. Therefore, it is only necessary to create some unabsorbed components (fp components). That is, the amount of attenuation between the input terminal 51 and the output terminal 52 can be changed by the value of VSWR in the pass band of the low-pass filters 54 and 55 loaded on the 3 dB directional coupler 53.

  FIG. 10 shows the relationship between the value of VSWR of the low-pass filters 54 and 55 to be loaded and the amount of attenuation between the input terminal 51 and the output terminal 52. When the VSWR of the pass band is set in the range of 1.10 to 3.00 as shown in FIG. 10, the attenuation amount between the input terminal 51 and the output terminal 52 should be set in the range of about 26 dB to 6 dB. Can do. When the isolation of the 3 dB directional coupler 53 is, for example, 25 dB, the maximum attenuation between input and output is 25 dB.

  The attenuation amount between the input terminal 51 and the output terminal 52 can be set by changing the value of the VSWR of the low-pass filters 54 and 55 loaded as described above. The VSWR of the low-pass filters 54 and 55 can be set to the values shown in FIG. 10 by selecting circuit constants.

  In the above embodiment, the case where the low-pass filters 54 and 55 are connected to the 3 dB directional coupler 53 to obtain the constant attenuation high-pass filter (HPF) characteristic has been described, but the constant attenuation low-pass filter (LPF) characteristic is described. In the case of obtaining, a high pass filter is used instead of the low pass filters 54 and 55.

  When a band pass filter is connected as a load to the 3 dB directional coupler 53, a constant attenuation band rejection filter (BEF) characteristic can be obtained.

Further, when a band rejection filter is connected as a load to the 3 dB directional coupler 53, a constant attenuation bandpass filter characteristic can be obtained.
In any of the above cases, the attenuation amount in the attenuation region is kept substantially constant.

  Further, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage.

It is a block diagram which shows the basic composition of the constant attenuation amount filter which concerns on this invention. It is a circuit block diagram of the constant attenuation amount filter which concerns on 1st Embodiment of this invention. (A) is a frequency characteristic figure of a polar high pass filter in the embodiment, (b) is a frequency characteristic figure of a nonpolar low pass filter. It is a comprehensive characteristic figure of the constant attenuation amount filter in the same embodiment. It is a circuit block diagram of the constant attenuation amount filter which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the structure of the constant attenuation amount filter which concerns on 3rd Embodiment of this invention. It is a circuit block diagram of the simple type constant attenuation amount filter in the same embodiment. It is a figure which shows the phase relationship of the frequency component of the pass region and attenuation region of a low-pass filter in the embodiment. It is a frequency characteristic figure of the constant attenuation amount filter in the embodiment. It is a figure which shows the relationship between the value of VSWR of the low-pass filter in the same embodiment, and the attenuation amount between an input terminal and an output terminal. It is a block diagram of the conventional high pass filter. It is a frequency characteristic figure of the conventional high pass filter.

Explanation of symbols

  20, 20A, 20B, 20C ... constant attenuation filter, 21 ... input terminal, 22 ... output terminal, 23 ... polar high-pass filter, 24 ... nonpolar low-pass filter, 31a-31d ... capacitance, 32 ... capacitance, 33 ... reactance 34a to 34c ... series resonance circuit, 41a to 41d ... reactance, 42a to 42c ... capacitance, 43 ... variable attenuator, 51 ... input terminal, 52 ... output terminal, 53 ... 3dB directional coupler, 54, 55 ... low pass Filters 56, 57 ... terminators.

Claims (2)

A polar high-pass filter provided between the input terminal and the output terminal, and a non-polar low-pass filter provided in parallel with the high-pass filter and having a characteristic of passing the stop band of the high-pass filter,
A constant attenuation filter characterized by combining the high-pass filter and the low-pass filter to make the attenuation range of the overall characteristic a substantially flat frequency characteristic, and setting the constant attenuation value in the attenuation range by the slope characteristic of the low-pass filter. .   2. The constant attenuation filter according to claim 1, wherein an attenuator is provided in series with the low-pass filter, and a constant attenuation value in the attenuation region is adjusted by the attenuator.

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