JP2005136516A - Frequency variable band stop filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a conventional band variable filter increases a reflection amount at a pass band particularly when a frequency variable range is increased and increases a reflection amount in general at an attenuation band in the case of a band stop filter because the reflection characteristic of the pass band depends on a circuit comprising a fixed inductor and the inductor circuit is fixed while the resonance frequency of a resonance circuit varies when the center frequency is changed by switching of switching diodes. <P>SOLUTION: A pair of identical circuits is configured to be a balanced type, each circuit comprising: two 3dB hybrids wherein a termination resistor is connected to an isolation terminal; two systems of transmission lines for interconnecting distribution/combination terminals of the two 3dB hybrids; and one series resonance circuit or more connected between each of the two systems of the transmission lines and a ground conductor and provided with a variable reactive element having a variable function of the resonance frequency. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a frequency variable band rejection filter capable of changing a rejection frequency in a band rejection filter.

  In a conventional variable band filter, a variable reactance element is applied to a resonance circuit that constitutes the filter in order to change a center frequency to form a variable resonance circuit, and the variable resonance via a reactance element or a distributed constant line. The circuit and the input / output terminal and the resonant circuit were connected.

  FIG. 6 is a diagram illustrating the configuration of a frequency band variable filter disclosed in Japanese Patent Laid-Open No. 7-321586. In the figure, a capacitor 103 and an inductor 104 constitute a series resonance circuit, and the filter exhibits an attenuation pole at the resonance frequency. In the figure, fixed inductors 105 and 106 determine the amount of filter reflection in the pass band by determining the degree of coupling between the resonators. Further, the switching diode 107 is switched by appropriately connecting a bias circuit (not shown) for controlling the switching diode 107 using a DC blocking capacitor 108 to change the resonance frequency of the series resonance circuit described above. Thus, a frequency band variable filter is obtained.

Japanese Patent Laid-Open No. 7-321586 (FIG. 2)

  In general, in designing a filter, an element value of a circuit connecting resonance circuits is determined so as to obtain a desired passband reflection characteristic. As described above, in the band-variable filter shown in FIG. 6, the reflection characteristic of the pass band is determined by the circuit composed of the fixed inductor. However, when the center frequency of the same-band variable filter is changed by switching the switching diode, the resonance frequency of the resonance circuit changes, but the inductor circuit is fixed, so it passes especially when the frequency variable range is enlarged. There has been a problem that the amount of reflection in the region becomes large.

  In addition, the band rejection filter generally has a very large amount of reflection in the attenuation band, and in this case, there is a problem in that it sometimes has an adverse effect such as unnecessary oscillation on active circuits such as amplifiers and mixers connected before and after the filter. there were.

  The present invention has been made to solve the above-described problems, and obtains a frequency variable band rejection filter in which the reflection amount of the filter is reduced in both the pass band and the attenuation band when the attenuation frequency is greatly changed. For the purpose.

  The frequency variable band rejection filter according to the present invention includes two 3 dB hybrids having a termination resistor connected to an isolation terminal, two transmission lines connecting between the distribution and combining terminals of the two 3 dB hybrids, and the two systems. A pair of identical circuits each including one or more series resonant circuits each having a variable reactance element having a variable resonance frequency function, connected between each of the transmission lines and the ground conductor, The 3 dB hybrid and the pair of identical circuits are configured in a balanced manner.

  According to the present invention, even when the attenuation frequency is changed in a wide range by configuring the band rejection filter in which the resonance frequency variable resonance circuit forming the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, the attenuation band and the pass Thus, it is possible to obtain a frequency variable band rejection filter having a small amount of reflection at the input / output terminals in both regions.

Embodiment 1 FIG.
FIG. 1 is an explanatory diagram of a configuration of a frequency variable band rejection filter according to Embodiment 1 for carrying out the present invention. In FIG. 1, the distribution and combination branches of the 3 dB hybrids 1 a and 1 b are constituted by a balanced circuit in which two identical circuits each consisting of a transmission line 2, variable capacitors 3 a and 3 b, and short-circuited short stubs 4 a and 4 b are connected. Is done. Termination resistors 5 are connected to the isolation terminals of the 3 dB hybrids 1a and 1b, respectively, for absorbing reflected waves from the circuit.

  Next, the operation of the frequency variable band rejection filter configured as described above will be described. The variable capacitor 3a and the tip short-circuit stub 4a, and the variable capacitor 3b and the tip short-circuit stub 4b constitute a series resonance circuit, and form an attenuation pole at each resonance frequency. By arranging the circuit in a balanced manner by the 3 dB hybrid circuit, the reflected waves from the above-described resonant circuit cancel each other out of phase with each other at the input / output terminals. Only the remaining reflection component has.

  In this way, by configuring the band rejection filter in which the resonance frequency variable resonance circuit that forms the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, even when the attenuation frequency is changed in a wide range, the attenuation band and the pass band In both cases, it is possible to obtain a frequency variable band rejection filter with a small amount of reflection at the input / output terminals. In FIG. 2 of the present embodiment, an example in which two types of resonance circuits are used has been described. Further, another resonance circuit having a different resonance frequency is connected between the transmission line 12 and the ground conductor to be attenuated. It is also possible to increase the number of poles.

Embodiment 2. FIG.
FIG. 2 is a diagram illustrating the configuration of the frequency variable band rejection filter according to the second embodiment for carrying out the present invention. In FIG. 2, a balanced circuit in which two sets of the same circuit including the transmission line 12, the variable capacitor 13, and the inductor 14 are connected to the distribution and combination branches of the 3 dB hybrids 11a and 11b. The terminating resistor 15 is connected to the isolation terminal of the 3 dB hybrid and absorbs the reflected wave from the circuit.

  Next, the operation of the frequency variable band rejection filter configured as described above will be described. In the present embodiment, the resonance circuit is constituted by a parallel resonance circuit including a variable capacitor 13 and an inductor 14. Since the circuit forms an attenuation pole at the resonance frequency, it can exhibit the same operation as in the first embodiment. In the present embodiment, an example in which only one type of resonance circuit is used has been described. However, by connecting another resonance circuit having a different resonance frequency to the transmission line 12 in series, a plurality of resonance circuits may be used. Attenuating poles can also be provided.

  In this way, by configuring the band rejection filter in which the resonance frequency variable resonance circuit that forms the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, even when the attenuation frequency is changed in a wide range, the attenuation band and the pass band In both cases, it is possible to obtain a frequency variable band rejection filter with a small amount of reflection at the input / output terminals.

Embodiment 3 FIG.
FIG. 3 is an explanatory diagram of the configuration of the frequency variable band rejection filter according to the third embodiment for carrying out the present invention. In the figure, a balanced circuit is formed by connecting two sets of the same circuit including the transmission line 22, the variable capacitor 23, the tip short-circuited stub 24, and the fixed capacitor 26 to the distribution and combination branches of the 3 dB hybrids 21a and 21b. The The terminating resistor 25 is connected to the isolation terminal of the 3 dB hybrid and absorbs the reflected wave from the previous circuit.

  Next, the operation of the frequency variable band rejection filter configured as described above will be described. In the present embodiment, the variable capacitor 23 and the short-circuited short stub 24 constitute a parallel resonance circuit, and the inverter operation of the fixed capacitor 26 selected so as to be substantially equivalent to a quarter wavelength line at the resonance frequency. By forming an electrical open point at the resonator side end of the fixed capacitor 26 by resonance of the parallel resonant circuit composed of the variable capacitor 23 and the tip short-circuited stub 24, the transmission line side end of the fixed capacitor 26 is electrically connected. By forming an electrical short-circuit point on the transmission line 22 by making a short circuit state, the signal is prevented from passing and an attenuation pole is formed. However, since the resonance circuit is coupled to the transmission line 22 via the fixed capacitor 26, the transmission line and the resonance line are compared with the case where the resonance circuit is directly connected to the transmission line as in the first or second embodiment. The degree of coupling of the resonance circuit can be reduced. As a result, the attenuation bandwidth is smaller than that of the previous embodiment, and this embodiment is more effective when an attenuation characteristic with higher selectivity is desired.

  In addition, by arranging the circuit including the resonance circuit in a balanced manner by the 3 dB hybrid circuit, the reflected waves from the above-described resonance circuit are reversed in phase with each other at the input / output terminals and cancel each other. Only the remaining reflection component of the 3 dB hybrid itself is obtained. In the present embodiment, an example in which only one set of resonance circuits is used has been described. However, a plurality of resonance circuits having different resonance frequencies are connected in parallel to the transmission line 22 so that a plurality of resonance circuits are connected. An attenuation pole can also be provided.

  In this way, by configuring the band rejection filter in which the variable resonance circuit forming the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, both the attenuation band and the pass band are obtained even when the attenuation frequency is changed in a wide range. Thus, it is possible to obtain a frequency variable band rejection filter with a small amount of reflection at the input / output terminals. Further, since the degree of coupling between the transmission line through which the signal propagates and the resonance circuit are coupled via a fixed capacitor can be reduced, it is possible to respond to a request to reduce the attenuation bandwidth.

Embodiment 4 FIG.
FIG. 4 is an explanatory diagram of the configuration of the frequency variable band rejection filter according to Embodiment 3 for carrying out the present invention. In the figure, a distribution line 32, fixed capacitors 33a, 33b, 33c, short-circuited short stubs 34a, 34b, 34c, and an SPnT (Single Pole n-Throw) switch 37 are provided for distribution and combination branches of 3 dB hybrids 31a, 31b. It is constituted by a balanced circuit in which two identical circuits are connected. The terminating resistor 35 is connected to the isolation terminal of the 3 dB hybrid and absorbs the reflected wave from the previous circuit.

  Next, the operation of the frequency variable band rejection filter configured as described above will be described. In this embodiment, instead of the variable capacitor constituting the resonance circuit in the first embodiment, a fixed capacitor having a different capacitance value is changed by switching with an SPnT switch. Therefore, the fourth embodiment exhibits the same operation as that of the first embodiment. 4 shows an example in which the tip short-circuit stubs are divided and connected to the tip short-circuit stubs 34a, 34b, and 34c, respectively. .

  The SPnT switch is generally composed of a PIN diode or FET, while the above variable capacitor is composed of a varactor diode or the like. In many cases, the former is superior in power durability characteristics compared to the latter. Therefore, the power durability characteristic of the circuit can be improved by this embodiment.

  In this way, by configuring the band rejection filter in which the resonance frequency variable resonance circuit that forms the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, even when the attenuation frequency is changed in a wide range, the attenuation band and the pass band In both cases, it is possible to obtain a frequency variable band rejection filter with a small amount of reflection at the input / output terminals. Further, by obtaining a pseudo variable capacitor using an SPnT switch that is generally superior in power resistance characteristics compared to a variable capacitor, the power resistance characteristics of the circuit can be improved.

Embodiment 5 FIG.
FIG. 5 is an explanatory diagram of the configuration of the frequency variable band rejection filter according to the second embodiment for carrying out the present invention. In FIG. 5, two sets of the same circuit including the transmission line 42, fixed capacitors 43a, 43b, 43c, the tip short-circuit stub 44, the fixed capacitor 46, and the SPnT switch 47 are provided for the distribution and combination branches of the 3 dB hybrids 41a, 41b. Consists of connected balanced circuits. The terminating resistor 45 is connected to the isolation terminal of the 3 dB hybrid and absorbs the reflected wave from the previous circuit.

  Next, the operation of the frequency variable band rejection filter configured as described above will be described. In this embodiment, instead of the variable capacitor constituting the resonance circuit in the third embodiment, a fixed capacitor having a different capacitance value is changed by switching with an SPnT switch. Therefore, the fifth embodiment can reduce the attenuation bandwidth as in the third embodiment, and can improve the power handling characteristics by adopting the circuit configuration using the SPnT switch as in the fourth embodiment. it can.

  In this way, by configuring the band rejection filter in which the resonance frequency variable resonance circuit that forms the attenuation pole is arranged in a balanced manner by the 3 dB hybrid, even when the attenuation frequency is changed in a wide range, the attenuation band and the pass band In both cases, it is possible to obtain a frequency variable band rejection filter with a small amount of reflection at the input / output terminals. In addition, since the degree of coupling between the transmission line through which the signal propagates and the resonance circuit are coupled via a fixed capacitor can be reduced, it is possible to respond to a request to reduce the attenuation bandwidth. In addition, by obtaining a pseudo variable capacitor using an SPnT switch that is generally superior in power handling characteristics compared to a variable capacitor, the power handling characteristics of the circuit can be improved.

1 is a configuration explanatory diagram of a frequency variable band rejection filter according to Embodiment 1 of the present invention. FIG. It is a structure explanatory drawing of the frequency variable band stop filter by Embodiment 2 of this invention. It is a structure explanatory drawing of the frequency variable band stop filter by Embodiment 3 of this invention. It is a structure explanatory drawing of the frequency variable band stop filter by Embodiment 4 of this invention. It is a structure explanatory drawing of the frequency variable band stop filter by Embodiment 5 of this invention. It is a configuration explanatory view of this kind of conventional frequency band variable filter.

Explanation of symbols

  1a, 1b, 11a, 11b, 21a, 21b, 31a, 31b, 41a, 41b 3dB hybrid, 2, 12, 22, 32, 42 Transmission line, 3a, 3b, 13, 23 Variable capacitor, 26, 33a, 33b, 33c, 43a, 43b, 43c, 46, 103, 108 Fixed capacitor, 4a, 4b, 24, 34a, 34b, 34c, 44 Short-circuited short stub, 14, 104, 105, 106 Fixed inductor, 37, 47 SPnT switch, 107 Switching diode.

Claims (6)

Two 3 dB hybrids with termination resistors connected to the isolation terminals;
Two transmission lines connecting between the distribution / combination terminals of the two 3 dB hybrids, and a variable reactance element connected between each of the two transmission lines and the ground conductor and having a function of changing a resonance frequency. A pair of identical circuits comprising one or more series resonant circuits;
And
A frequency variable band-stop filter characterized by being configured in a balanced manner by the two 3 dB hybrids and the pair of identical circuits. Two 3 dB hybrids with termination resistors connected to the isolation terminals;
Two or more transmission lines connecting between the distribution / combination terminals of the two 3 dB hybrids, and one or more equipped with variable reactance elements inserted and connected to the two transmission lines and having a variable resonance frequency function A pair of identical circuits consisting of a parallel resonant circuit of
And
A frequency variable band-stop filter characterized by being configured in a balanced manner by the two 3 dB hybrids and the pair of identical circuits. Two 3 dB hybrids with termination resistors connected to the isolation terminals;
Two transmission lines connecting between the distribution / combination terminals of the two 3 dB hybrids, a fixed capacitor that performs inverter operation with one end connected to the transmission line, and the other end of the fixed capacitor and the ground A pair of identical circuits comprising a parallel resonant circuit having a variable reactance element connected between conductors and having a variable function of a resonant frequency;
And
A frequency variable band-stop filter characterized by being configured in a balanced manner by the two 3 dB hybrids and the pair of identical circuits. Two 3 dB hybrids with termination resistors connected to the isolation terminals;
Two transmission lines connecting between the distribution / combination terminals of the two 3 dB hybrids, a single pole n-throw (SPnT) switch having a common terminal connected to the transmission line, and a plurality of distribution lines of the SPnT switch A pair of identical circuits comprising a plurality of series resonant circuits connected between the branch ends and the ground conductor and provided with reactance elements;
And
A frequency variable band-stop filter characterized by being configured in a balanced manner by the two 3 dB hybrids and the pair of identical circuits.   The reactance element includes a plurality of fixed capacitors each having one end connected to a plurality of branch ends of the SPnT switch, and a short-circuited one end commonly connected to the other end of each of the plurality of fixed capacitors. 5. The frequency variable band rejection filter according to claim 4, wherein the frequency variable band rejection filter is formed of a stub. Two 3 dB hybrids with termination resistors connected to the isolation terminals;
Two transmission lines connecting between the distribution / combination terminals of the two 3 dB hybrids, a fixed capacitor that performs inverter operation with one end connected to the transmission line, and the other end of the fixed capacitor and the ground A reactance element connected between the conductors, an SPnT (Single Pole n-Throw) switch having a common terminal connected to the other end of the fixed capacitor, and a plurality of branch ends of the SPnT switch and a ground conductor A pair of identical circuits comprising a parallel resonant circuit formed by a plurality of reactance elements connected to
And
A frequency variable band-stop filter characterized by being configured in a balanced manner by the two 3 dB hybrids and the pair of identical circuits.

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