JP4494951B2 - Band stop filter - Google Patents

Description

  The present invention relates to a band rejection filter used in a microwave band and a millimeter wave band.

  FIG. 20 is a circuit diagram of a conventional band-stop filter disclosed in the literature BMS Schiffman and GLMatthaei, “Exact Design of Band-stop Microwave Filters”, IEEE Trans. On MTT, vol. MTT-12, pp6-15, etc. FIG. 21 shows reflection characteristics and transmission characteristics of the filter of FIG. In FIG. 20, 3a is an input terminal, 3b is an output terminal, 4a is a power supply circuit which is an external circuit, 4b is a load circuit which is an external circuit, 14 is an open stub, and 15 is a part of the main line of the filter. A 1/4 wavelength transmission line is shown.

  In this filter, n shunt open stubs having a quarter wavelength at the center frequency f0 of the stop band and n transmission lines having a quarter wavelength at f0 are alternately connected. Is. The impedance on the power supply circuit (input side) 4a side is ZA, and the impedance on the load circuit (output side) 4b side is ZB. The characteristic impedance of the transmission line composing the open stub is Zi (i = 1, 2,..., N), the characteristic impedance Zii + 1 (i = 1, 2,. n). Zii + 1 is larger than the impedance ZA on the power source side.

Next, the operation will be described.
At the center frequency f0, the signal input from the power supply side is short-circuited at the root of the open stub, so that most of it is reflected or dissipated as heat in the stub circuit. Almost no output is given to the load side. On the other hand, at a frequency lower than the center frequency f0, the electrical length (passing phase) of the open stub is 90 degrees or less, the open stub is equivalent to the capacitance of the shunt, and the electrical length of the transmission line between the open stubs is 90. And the characteristic impedance Zii + 1 (i = 1, 2,..., N) is selected to be higher than the impedance ZA on the power supply side. The line is equivalent to a series inductance. Therefore, since low-pass impedance matching is performed, the signal input from the power supply side is gradually output to the load side as the distance from the center frequency f0 increases.

  Furthermore, at a frequency higher than the center frequency f0, the electrical length of the open stub is 90 degrees or more, which is equivalent to the shunt inductance, and the transmission line between the open stubs is selected to have a characteristic impedance higher than ZA. In addition, since the electrical length is 90 degrees or more, the circuit is equivalent to a series capacitance, so that high-pass impedance matching is performed. Therefore, the signal input from the power supply side is gradually output to the load side as the distance from the center frequency f0 increases. In general, based on the above principle, this filter operates as a band rejection filter that does not transmit a signal to the output side only in a frequency band near the center frequency f0, and exhibits frequency characteristics as shown in FIG.

  In addition to a simple circuit configuration, this filter has the characteristics that it is easy to implement because it does not have elements that complicate the structure, such as short-circuit means. As can be seen from the fact that the open stub that operates as a resonant element is directly connected to the transmission line connecting the input and output, the coupling between the resonant element and the power circuit (and load circuit) is large, so The filter circuit is suitable for a wide band-stop filter, so that a large attenuation can be easily obtained in the stop band.

  However, depending on the line structure for realizing the circuit, unnecessary coupling may occur between the open stubs, and the required pass characteristics may not be obtained. In particular, when a filter is realized with a line that is structurally simple, such as a microstrip line, and that is likely to leak high frequency, unnecessary coupling becomes large and the characteristics are likely to deteriorate. FIG. 22 shows a conceptual diagram for explaining this problem.

  In this figure, open stubs are represented as shunt series resonant circuits (resonant frequencies f0) 16a and 16b. When unnecessary coupling occurs between the resonance circuits 16a and 16b, the signal is transmitted between the resonance circuits 16a and 16b, so that the signal is not sufficiently suppressed in the vicinity of the center frequency f0 of the stop band. As shown in the graph, the pass characteristic as originally shown by the dotted line is the pass characteristic shown by the solid line. In order to avoid such a problem of characteristic deterioration, it is easy to increase the distance between the resonance circuits to keep them away and to reduce unnecessary coupling.

  The filter shown in FIG. 20 can obtain the same frequency characteristics as the 1/4 wavelength even if the electrical length of the transmission line between the open stubs is selected to be an odd multiple of the 1/4 wavelength. A method of setting the length of the transmission line between the stubs to 3/4 wavelength or 5/4 wavelength is conceivable. A conceptual diagram for explaining this method is shown in FIG. The distance between the two resonance circuits 16a and 16b is widened to reduce the coupling between the two, and the pass characteristic as shown in FIG. 23 is obtained to improve the attenuation near the center frequency f0 (the dotted line without coupling). Close to the passing characteristics of However, there is a drawback that the longitudinal dimension of the filter becomes large.

  By the way, as one of the circuits advantageous to the problem of unnecessary coupling as described above, there is a spur line (also referred to as a coupled resonance circuit in the present specification). Sparline is also introduced in the above-mentioned literature. FIG. 24 shows a circuit diagram of the spar line. In this figure, 5 indicates a coupled line constituting a spur line, 17 indicates a direction that operates as a resonance element in one line 1 constituting the coupled line 5, and 18 similarly constitutes a coupled line 5. The other line 2 is the main line. 6 is an open end of the coupled line 5, 19 is a first connection terminal, and 20 is a second connection terminal. Ze is the even mode impedance of the coupled line, Zo is the odd mode impedance, C is the voltage coupling degree, and Zc is the coupled line impedance. The spur line is a circuit that is also introduced in the above-mentioned document, and is a circuit that is a constituent element of a band rejection filter.

  In this circuit, as shown in FIG. 24, the two terminals on one side of the coupled line having a quarter wavelength at the center frequency f0 of the stop band are combined into a first connection terminal 19, and Of the two terminals on the other side, one is the second connection terminal 20 and the other is the open end 6. In other words, the 1/4 wavelength open stub is coupled with a predetermined degree of coupling along the main line to which the stub is attached. As described above, the portion of the line 17 operates as a resonant element. However, since the electromagnetic field generated by the portion of the line 17 is guided by the line 18 which is the main line, the spar line has unnecessary coupling with the surrounding circuits. Basically it has the feature of being small.

  For this reason, as shown in FIG. 25, a band rejection filter configured by cascading a plurality of (n) spur lines easily obtains a large attenuation near the center frequency f0 of the stop band. It has an advantage over the filter of FIG. In addition, as shown in FIG. 26, the spar line has a one-to-one relationship with the combination of the open stub and the transmission line. From this relationship, it can be understood that the band rejection filter of FIG. 20 can be immediately converted into a filter as shown in FIG. 25 configured using spur lines.

  However, one problem arises when an attempt is made to convert the multistage filter of FIG. 20 having more than three stages or four stages into the filter of FIG. In a multistage filter, the coupling degree required for the coupled line is loosely coupled in the spur line at the center of the filter circuit. For this reason, the line spacing between the line 17 and the line 18 in FIG. 24 becomes large, and the feature that the unnecessary coupling with the adjacent circuit is small cannot be obtained.

  27 to 30 show, as calculation examples, the relationship between the stop bandwidth and the degree of coupling obtained for a filter in which spur lines (coupled resonance circuits) are cascaded from one to four stages. 27 shows the characteristics when n = 1, FIG. 28 shows the characteristics when n = 2, FIG. 29 shows the characteristics when n = 3, and FIG. 30 shows the characteristics when n = 4. 27 to 30, when a Chebyshev response band rejection filter having a ripple value of 0.01 dB is configured by a spar line, the voltage coupling degree Ci (i = 1, 2,...) Required for each spar line. n) is obtained and plotted. According to FIG. 29, in the case of three stages, the coupling degree of the second stage in the center of the filter circuit is smaller than that of the first and third stages, and in the case of four stages, as shown in FIG. It can be seen that the second and third level coupling degrees are smaller than the first and fourth level coupling degrees, respectively.

Next, assuming a case where a filter is constituted by a structurally simple microstrip line 11, a coupling line Ci and a strip conductor interval are obtained with respect to a coupling line in which two strip conductors 11 are arranged on the same main surface of a dielectric substrate. The relationship of S was calculated.
FIG. 31 shows a cross-sectional structure diagram of the coupled line, and FIG. 32 shows the relationship between the degree of coupling Ci and the strip conductor spacing S. As the dielectric substrate 8, assuming a standard alumina substrate, a dielectric constant and a thickness h are selected, and calculation is performed at 35 GHz. Reference numeral 10 denotes a cut-off block.
Note that the vertical axis of FIG. 32 represents a value obtained by standardizing the strip conductor interval S by the thickness h of the substrate. It can be seen that when the degree of coupling is about -20 dB, the strip conductor spacing S exceeds the thickness h of the substrate.

  Thus, when an attempt is made to construct a multistage band rejection filter that exceeds three stages with a wide rejection bandwidth, the resonant circuit close to the input end and the output end can be a spur line. The resonance circuit in the center of the filter circuit is difficult in principle to be a spur line, and all the resonance circuits cannot be a spur line with small unnecessary coupling. If some resonant circuits become open stubs, unnecessary coupling must be reduced by using a transmission line with a large electrical length for the main line near the open stub and increasing the distance between adjacent circuits. The filter must be enlarged. Alternatively, a method of providing a structure that shields the space in order to suppress the coupling can be considered, but it is inevitable that the structure becomes complicated.

B.M.Schiffman and G.L.Matthaei, “Exact Design of Band-stop Microwave Filters”, IEEE Trans. On MTT, vol.MTT-12, pp6-15

  As described above, in the conventional band-stop filter, when a multi-stage band-stop filter having a relatively wide bandwidth is configured with a simple line structure such as a microstrip line, the band-pass filter is located near the center frequency of the stop band. In order to obtain a large amount of attenuation, there is a problem that the size of the filter increases, loss increases, or the structure of the filter becomes complicated. This problem becomes obvious as the frequency increases and the thickness of the dielectric substrate increases with wavelength.

  The present invention has been made to solve the above-described problems, and a multi-stage band stop filter having a relatively wide stop band and a large attenuation near the center frequency of the stop band. The objective is to achieve a small and simple structure. In particular, an object of the present invention is to overcome the above-described problem when a filter is configured by a planar circuit having a simple structure such as a microstrip line.

The band rejection filter according to the present invention is:
A plurality of transmission lines having a length of 1/4 wavelength of the center frequency in the stop band are arranged in parallel, and all the terminals on one side of the coupled line are electrically connected to each other. A single coupled resonance circuit configured as a first connection terminal, one of the terminals on the opposite side of the coupled line as a second connection terminal and the other as an open end, or Two unit cells are connected in cascade to form a unit cell, and a plurality of unit cells are connected via a connection means having a transmission line having an electrical length of 180 degrees or an integral multiple thereof at the center frequency of the stop band. It is a cascade connection.
Further, another band rejection filter according to the present invention is:
A plurality of transmission lines having a length of 1/4 wavelength of the center frequency in the stop band are arranged in parallel, and all the terminals on one side of the coupled line are electrically connected to each other. A single coupled resonance circuit configured as a first connection terminal, one of the terminals on the opposite side of the coupled line as a second connection terminal and the other as an open end, or Two unit cells are connected in cascade to form a unit cell, and a plurality of the unit cells are connected in cascade through a connection means formed of a transmission line having an electrical length of 90 degrees or less at the center frequency of the stop band. It consists of.

  According to the present invention, the unit cell is a one-stage or two-stage band-stop filter configured using a coupled resonance circuit that has less unnecessary coupling with surrounding circuits, and the interval between the end portions of adjacent unit cells is determined. A plurality of unit cells are connected in cascade through connection means for physically expanding and adjusting the frequency characteristics of the filter, and a multi-stage band-stop filter in which all resonance circuits are coupled resonance circuits is configured. There is little unnecessary coupling between the resonant circuits in the circuit, and there is an effect that a large attenuation can be obtained in the vicinity of the center frequency of the stop band. In particular, when the filter is configured with a line structure such as a microstrip line, the effect is greater as the filter is configured with a higher frequency at which the thickness of the dielectric substrate increases with respect to the wavelength. In addition, the use of connection means having a small physical length has an effect that a band-stop filter having a large stop-band attenuation can be obtained in a compact manner.

Embodiment 1 FIG.
FIG. 1 is a circuit diagram of a band rejection filter showing a first embodiment of the present invention. In this figure, 1-1 to 1-n are unit cells, 2-1 to 2-n-1 are connection means, 3a is an input terminal, 3b is an output terminal, 4a is a power circuit that is an external circuit, 4b Indicates a load circuit which is an external circuit.
FIG. 2 shows a specific circuit used as a unit cell of the filter of FIG. 1, wherein (a) is the same as the spar line shown in FIG. 24, 5 indicates a coupled line constituting the spar line, 6 Is an open end thereof, 19 is a first connection terminal, and 20 is a second connection terminal.
(B) and (c) are modified two-stage filters in which two stages of spar lines are connected, and 7 is an interstage line of a modified two-stage filter in which the end interval between two spar lines is widened. It has a length of 1/4 wavelength at the center frequency of the stop band.
(B) is configured by connecting the first connection terminals of the two spur lines to the interstage line 7 side.
(C) is configured by connecting the second connection terminals of the two spur lines to the interstage line 7 side.

  As described above, the unit cell according to the present invention is composed of a one-stage or two-stage band-rejection filter element configured using one or two spur lines. The modified two-stage filters (b) and (c) are modified from the case of n = 2 of the filter shown in FIG. Both are obtained based on the two-stage filter of FIG. 20, and (b) can be easily obtained by devising a combination of an open stub and a quarter wavelength transmission line, and (c) This is obtained by using Kuroda's theorem and dual transformation (the specific derivation is omitted here because it is described in the literature). In the modified two-stage filter, since the end spacing of the spar line is 1/4 wavelength due to the interstage line 7, the unnecessary coupling is small due to the nature of the spar line that the unnecessary coupling is originally small. It is a circuit in which good frequency characteristics can be easily obtained.

  As described above, the band rejection filter of this embodiment uses a single-stage or two-stage filter for a unit cell, and a plurality of unit cells are cascade-connected via the connection means 2 to form a multistage filter. Is. The connection means 2 has a role of improving the reflection characteristics in the pass band and a role of reducing unnecessary coupling between unit cells by widening the end interval between adjacent unit cells. In order to have the latter role, for example, a connection means 2 having a physically necessary and sufficient size is applied. Needless to say, the filter according to the present embodiment uses a spur line as a resonance circuit for suppressing the passage of high frequency in a specific band.

  In addition, in a filter used at a high frequency such as a microwave or a millimeter wave, a plurality of filters (unit cells) designed individually are cascaded to form a multistage filter as in the present embodiment. In particular, it is not often performed to cascade a plurality of unit cells to form a multistage filter with a space between unit cells. This is because multiple reflection occurs between unit cells and the frequency characteristics deteriorate. In particular, when unit cells having band stop characteristics are combined, each unit cell generates a large reflected wave in the stop band. Therefore, in a multistage band stop filter configured by combining unit cells, the stop band passes. Large deterioration in characteristics occurs.

  However, depending on how to select the connection means, it is possible to reduce the deterioration of the pass characteristics in the stop band. Therefore, if the deterioration can be reduced to a local frequency band only, or if the deterioration can be reduced to a level below the required performance, the center frequency of the stop band can be reduced. A large attenuation that cannot be obtained in the normal configuration in the vicinity of f0 can be obtained with a simple structure and in a compact manner, and a band-rejecting filter having a sufficient utility value can be obtained. This is the point of the present invention.

  By the way, although the basic operation of the filter of the present embodiment is the same as that of the conventional filter, the reflection characteristic in the pass band is adjusted by the characteristic of the connection means 2. Further, since the unit cell 1 has a large reflection in the frequency band that is a stop band, the pass characteristics are deteriorated due to multiple reflections between the unit cells 1. This deterioration of the pass characteristic is mainly adjusted by the pass phase of the connecting means 2.

  The filter of the present embodiment is significantly different from the conventional filter in that all the resonance circuits are spur lines with less unnecessary coupling, and the end portions of adjacent unit cells (= the end portions of the spur lines). Is maintained at a constant distance by the interposition of the connecting means 2, and these are combined, and unnecessary coupling between the resonance circuits becomes very small. This feature can be obtained regardless of the form of the line, but it is particularly effective when the filter is configured with a line structure that is easy to radiate and cause unnecessary coupling, such as a microstrip line, although the structure is simple. It becomes.

As described above, the filter according to the present embodiment has a circuit configuration in which unnecessary coupling between the resonance circuits in the filter circuit is reduced, so that a large attenuation is obtained in the vicinity of the center frequency of the stop band. There is an effect that it is. In particular, when the filter is configured with a line structure such as a microstrip line, the effect is greater as the filter is configured with a higher frequency at which the thickness of the dielectric substrate increases with respect to the wavelength. Further, by suppressing the physical dimension of the connecting means 2 to 3/4 wavelength or less, there is an effect that a band-stop filter having a large stop-band attenuation can be obtained in a compact manner.
In addition, when the unit cell is configured by cascading two coupled resonance circuits, an interstage line having a length of 1/4 wavelength at the center frequency of the stop band is interposed between the coupled resonance circuits. Therefore, there is little unnecessary coupling between the resonance circuits even in the unit cell, and there is an effect that a larger attenuation can be obtained in the vicinity of the center frequency of the stop band.

Embodiment 2. FIG.
FIG. 3 is a circuit diagram showing the band rejection filter of the second embodiment. Reference numerals 1a, 1b2, 3a, 3b, 4a, 4b and 7a, 7b are the same as those in the first embodiment, and will not be described. Reference numerals 13a to 13d denote spur lines (coupled resonance circuits) in the unit cell 1, which are the same as 5a and 5b shown in FIG. The filter according to the present embodiment is a four-stage filter in which a modified two-stage filter is used as a unit cell and two unit cells 1a and 1b are connected via a connecting means 2. The two unit cells 1a and 1b are both configured by connecting the second connection terminals of the coupled resonance circuit to the interstage line 7 side.
The connecting means 2 is a transmission line having a characteristic impedance Za, and its electrical length (passing phase) θa is 180 degrees at the center frequency f0 of the stop band.

FIG. 4 is an exploded perspective view showing the structure of the filter. In FIG. 4, 10 is a cut-off block in which a groove having a bowl-shaped cross section is formed in order to form a cavity that is cut off in the use frequency band of the filter, 8 is a filter substrate, 9 is a filter substrate 8 A carrier that is mounted so as to be positioned in the groove portion of the cut-off block 10 and forms a blocking cavity by combination with the groove-side surface of the cut-off block 10, 3a is an input terminal of the filter, and 3b is an output terminal of the filter It is.
The width A of the groove of the cut-off block 10 is selected so as to cut off the cavity in the used frequency band. By disposing the filter substrate 8 in the blocking cavity, unnecessary coupling between the resonance circuits and between the input and output is reduced.

FIG. 5 shows a view of the filter substrate 8 as viewed from above. 12a is an input line, 12b is an output line, and the input / output terminals 3a and 3b of the filter, the interstage lines 7a and 7b, and the spar lines 13a to 13d are formed by the strip conductor 11.
FIG. 6 shows reflection characteristics and transmission characteristics. The modified two-stage filter, which is a unit cell of this filter, is designed with a center frequency f0 of 30 GHz and a bandwidth of 80% and a ripple value of 0.03 dB.

Next, the operation of the filter of this embodiment will be described.
The basic operation principle is the same as that of the conventional filter and the filter described in the first embodiment. However, in the band rejection filter of the present embodiment, the connection means 2 is a transmission line having an electrical length of 180 degrees. This is a key point in determining the frequency characteristics. In order to explain this point, FIGS. 7 to 11 show the relationship between the electrical length of the transmission line used as the connecting means 2 and the frequency characteristics of the filter.

FIG. 7 shows the reflection and transmission characteristics when the electrical length of the transmission line of the connecting means 2 is 0 degrees at the center frequency f0 of the stop band, that is, when two unit cells are directly connected, and FIG. The reflection and transmission characteristics are shown. 9 shows the characteristics when the electrical length is 90 degrees, FIG. 10 shows the characteristics when the electrical length is 135 degrees, and FIG. 11 shows the characteristics when the electrical length is 180 degrees. In FIGS. 7 to 11, the calculation is performed assuming that the circuit is a lossless circuit in order to make it easy to understand the deterioration of the pass characteristic in the stop band. The characteristic impedance Za of the transmission line of the connecting means 2 is the same as the impedance ZA on the power source side.
As can be seen from these figures, in the stop band, the pass characteristics are locally degraded due to unnecessary resonance caused by multiple reflection occurring between unit cells. And it turns out that the resonance frequency of an unnecessary resonance changes with the change of the electrical length of the connection means 2. FIG.

  Next, FIG. 12 shows the relationship between the electrical length of the connecting means 2 and the resonance frequency of unnecessary resonance. It can be seen that the resonance frequencies fsh (> f0) and fsl (<f0) of unnecessary resonance decrease as the electrical length increases. Furthermore, FIG. 13 shows the relationship between the resonance frequency of unnecessary resonance and the external Q value at the unnecessary resonance. FIG. 13 also shows the relationship between the non-resonant resonance frequency and the pass level at the resonance frequency using the no-load Q value of unnecessary resonance as a parameter.

  According to FIG. 13, when the resonance frequency of unnecessary resonance approaches f0, the external Q value of unnecessary resonance gradually increases, and becomes infinite at f0. Based on this result and looking at FIG. 12, when the electrical length of the connecting means 2 is 0 degree or 180 degrees, one unnecessary resonance frequency coincides with f0, and furthermore, a high frequency band and a low frequency centering on f0. It can be seen that one unnecessary resonance appears in each band. At f0, as can be seen from the calculated values shown in FIG. 13, the external Q value is infinite, so that no deterioration occurs in the passband pass characteristics. For the remaining two unnecessary resonances, the resonance frequency is away from the center frequency f0 and is in the vicinity of the passband. The pass characteristic is deteriorated in the vicinity of the resonance frequency of the unnecessary resonance. However, since the frequency interval between the two unnecessary resonances is large, a large attenuation can be obtained in a wide range of the frequency band between the two unnecessary resonances. I can do it. Since the resonance frequency of the unnecessary resonance is in the vicinity of the pass band, the amount of attenuation is small even when there is no unnecessary resonance, so deterioration of the amount of attenuation hardly poses a problem.

  However, when the electrical length is 0 degrees, the unit cells are directly connected, so there is no room for adjusting the reflection characteristics in the pass band, and the spar lines at the ends of the unit cells are close to each other, so that the stop band is There is no denying that it is disadvantageous in terms of securing the amount of attenuation. That is, it can be seen that the electrical length (passing phase) of the connecting means 2 is preferably selected at 180 degrees at the center frequency f0. Since the electrical length of the connecting means 2 is 180 degrees, the interval between the end portions of the unit cells is 180 degrees, and since a sufficient distance is maintained, unnecessary coupling is small. On the other hand, the reflection characteristic in the pass band can be adjusted by the characteristic impedance of the transmission line constituting the connection means 2. In the present embodiment, only the characteristic impedance Za is made about 5% lower than the impedance ZA of the power source, and as shown in FIG. 6, good reflection characteristics can be obtained in the bands in the vicinity of 20 GHz and 50 GHz as the cutoff frequencies. It was.

  As described above, the filter according to the present embodiment has a modified two-stage filter configured by using a spur line as a unit cell, and the two unit cells are connected by using a transmission line having an electrical length of 180 degrees as a connection means. This is a band rejection filter equivalent to a stage. For this reason, all the resonance circuits are composed of spur lines, and the end cell spacing is sufficiently secured, so that the unnecessary coupling between the resonance circuits is small while being compact. Unwanted resonance due to reflection also appears only in the vicinity of the passband. Further, all the resonance circuits are configured by spur lines, the lateral width of the filter substrate can be reduced, the filter substrate can be arranged in a cavity having a high cutoff frequency, and this is advantageous in terms of suppressing unnecessary coupling. Combined with these conditions, a large attenuation characteristic of a multistage filter in a wide frequency range can be obtained with a simple line structure such as a microstrip line.

Embodiment 3 FIG.
FIG. 14 is a circuit diagram of the band rejection filter of the third embodiment. A modified two-stage filter is used as the unit cell, and the unit cell 1a, 1b is a filter corresponding to four stages formed by connecting via the connecting means 2, which is almost the same as the circuit of FIG. 3 described in the second embodiment. Are the same. However, in the present embodiment, the electrical length θa of the transmission line constituting the connecting means 2 is selected to be 30 degrees (less than 90 degrees), and the characteristic impedance Za of the transmission line is a value lower than the power supply impedance ZA. The point of selecting 25 ohm is different.

FIG. 15 is a view of the filter substrate of the filter according to the present embodiment as viewed from above. This filter is composed of the microstrip line 11 as in the band rejection filter of the second embodiment, and is formed in the cutoff cavity formed by using the cutoff block 10 and the carrier 9 as shown in FIG. A filter substrate 8 is arranged.
FIG. 16 shows the reflection characteristics and transmission characteristics of the filter of this embodiment. In the present embodiment, of the two spar lines in the unit cell, the spar lines 13b and 13c having a high degree of coupling C are connected to the connection means 2 side.

  As can be seen from the fact that the filter of the present embodiment is different from the filter of the second embodiment only in the connection means 2, the basic operation is the same as that of the filter of the second embodiment. Referring to FIG. 12 described in the second embodiment, when the electrical length of the transmission line used as the connection means 2 is selected to be shorter than 90 degrees, unnecessary resonance that appears in the stop band is low near the center frequency f0 of the stop band. It can be seen that it appears on the band side and on the high band side of f0. Further, according to FIG. 13, the external Q value of unnecessary resonance increases in the vicinity of f0. The passage loss at the resonance frequency of unnecessary resonance near f0 depends on the loss (unloaded Q value) of the resonance circuit. The unloaded Q value depends on the structure and frequency of the line.

  For this reason, depending on the required attenuation in the stop band and the balance between the circuit structure and the frequency, the deterioration of the pass characteristics due to unnecessary resonance in the vicinity of f0 may not be a problem. For example, when a circuit is configured in a 30 GHz band with a microstrip line having a cross-sectional dimension as shown in FIG. 31, the unloaded Q value of the half-wave resonator is only about 200 at most. In this case, if the amount of attenuation desired to be secured in the stop band is 30 dB or more, the required amount of attenuation can be secured even if the frequency of the unwanted resonance is 0.942f0 according to FIG. From FIG. 12, it can be seen that when the electrical length of the transmission line is 90 degrees or less, the electrical length at which the unnecessary resonance (fsl_2) in the vicinity of f0 is 0.942f0 is about 45 degrees.

  In this case, the unnecessary resonance appearing at a frequency higher than f0 is lowered to 1.2 f0 or less, which is not suitable for obtaining a large attenuation in a wide range of frequencies higher than f0. Considering securing a large amount of attenuation in a band or a frequency band lower than that, there is no deterioration in pass characteristics due to unnecessary resonance, and good characteristics can be obtained. In addition, since the point with a short electrical length of the transmission line which comprises the connection means 2 is equivalent to that the edge part space | interval of a unit cell becomes small, in this Embodiment, among two spar lines in a unit cell, The unit cell and the connection means are connected with the side having the higher degree of coupling with which the unnecessary coupling with the surrounding circuits becomes smaller on the side of the connection means.

  Further, as can be seen from the characteristics shown in FIG. 16, by adjusting the characteristic impedance Za of the transmission line used as the connection means 2 appropriately, a wide frequency range from the cut-off frequency to DC is obtained in the low-frequency passband. Flat and good reflection characteristics can be obtained over a range. This is because, in the low-pass band, the length of the transmission line constituting the connection means 2 becomes sufficiently short, and when the transmission line characteristic impedance Za is low, the capacity of the shunt is increased. When the characteristic impedance Za of the transmission line is high, it approaches the inductance of the series, so when it is sandwiched between unit cells that behave like a low-pass filter on the lower frequency side than the center frequency f0, This is because the impedance mismatch between cells is well adjusted. In the case of the present embodiment, a flat and good reflection characteristic is obtained by selecting the characteristic impedance Za of the transmission line constituting the connection means 2 lower than the power source impedance ZA.

As described above, in the filter of the present embodiment, the modified two-stage filter configured using the spur line is used as a unit cell, and the two unit cells are connected via the transmission line serving as the connecting means 2. In the case of the band-rejection filter corresponding to the stage, the electrical length of the transmission line constituting the connection means 2 is selected to be less than 90 degrees. For this reason, even if the filter is configured with a simple line structure such as a microstrip line, a large attenuation near the center frequency of the stop band can be obtained, and good performance from the center frequency to the cutoff frequency on the low frequency side can be obtained. Pass characteristics in a wide band.
Further, by adjusting the characteristic impedance Za of the transmission line in the connecting means 2 to an impedance lower than the power source impedance ZA, there is an effect that a flat and good reflection characteristic can be obtained in the pass band on the low frequency side. . In particular, it is effective in a high frequency band where the thickness of the substrate is larger than the wavelength.

Embodiment 4 FIG.
FIG. 17 is a circuit diagram of the band rejection filter of the present embodiment. Similarly to the second and third embodiments, the present embodiment is a band rejection filter configured by a microstrip line, and two modified two-stage filters 1a and 1b serving as unit cells are connected via the connecting means 2. A band rejection filter corresponding to four stages is connected. FIG. 18 is a view of the filter substrate 8 as viewed from above. FIG. 19 shows a comparison between calculated values and measured values of the reflection characteristics and pass characteristics of the band rejection filter of the present embodiment. The measurement values are overwritten with the measurement values of three samples.

  In the present embodiment, the connection means 2 is constituted by a cascade connection of three short transmission lines 2a, 2b and 2c having different characteristic impedances, that is, constituted by three short transmission lines 2a, 2b and 2c. The point of using the low pass filter (pseudo lumped constant type low pass filter) as the connection means 2 is different from the band rejection filter of the other embodiments. The cut-off frequency of the low-pass filter serving as the connection means 2 is selected to be higher than the cut-off frequency on the low-pass side of the band stop filter, and the pass phase at the stop band center frequency f0 is less than 90 degrees.

  The operation of the band rejection filter in the present embodiment is the same as that in the third embodiment. Since the pass phase of the connecting means 2 is less than 90 degrees at the stopband center frequency f0, unnecessary resonance caused by multiple reflection occurring between unit cells is the same as that of the filter of the third embodiment. Therefore, the band rejection filter emphasizes the characteristics in the frequency band lower than the stop band.

The filter of the present embodiment is effective when it is desired to ensure better reflection characteristics in a local frequency band in the pass band. Since the three short transmission lines 2a, 2b, and 2c are combined, the degree of freedom in adjusting the reflection characteristics is large, and it is possible to ensure good reflection characteristics for various unit cells. As shown in FIG. 19, when the stop band is in a certain frequency range of 35 GHz band, while the pass band is in a certain frequency range of 18 GHz band, the band stop filter of the present embodiment has good reflection characteristics. It can be seen that both large attenuations in the stopband can be obtained.
From the relationship of the required attenuation, the pass phase of the connecting means 2 was selected so that the unnecessary resonance that occurs in the vicinity of the stopband center frequency f0 is on the low frequency side outside the stopband of the 35 GHz band. Further, it can be seen from FIG. 19 that in the actual structure, the attenuation in the stopband is deteriorated compared to the calculated value due to unnecessary coupling, and the attenuation obtained in the actual structure is limited.

  As described above, the band rejection filter according to the present embodiment uses the modified two-stage filter configured using two spur lines as a unit cell, and connects the two unit cells via the connection means 2 to form four stages. Although the corresponding band rejection filter is configured, the connection means is formed of a quasi-lumped constant type low-pass filter formed by cascading short transmission lines having different characteristic impedances, and the connection phase of the connection means is It was made to be less than 90 degrees at the stopband center frequency f0. For this reason, since the degree of freedom of impedance matching is high, in addition to obtaining the same effect as the band stop filter of the second embodiment, in the local frequency range of the pass band on the lower side of the stop band. Thus, there is an effect that better reflection characteristics can be obtained.

  The present invention realizes a multistage bandstop filter having a relatively wide stopband and a large attenuation near the center frequency of the stopband with a small and simple structure. Widely applicable to wave antenna circuits and the like.

1 is a circuit diagram of a band rejection filter according to a first embodiment of the present invention. It is a figure which shows the unit cell of the band-stop filter used as Embodiment 1 of this invention. It is a circuit diagram of the bandstop filter used as Embodiment 2 of this invention. It is a figure which shows the structure of the band-stop filter used as Embodiment 2 of this invention. It is a top view of the filter board | substrate of the band-stop filter used as Embodiment 2 of this invention. It is a figure which shows the reflection characteristic and the passage characteristic of the bandstop filter which becomes Embodiment 2 of this invention. It is a figure which shows the relationship between the electrical length 0 degree | times of a connection means and frequency characteristics in the band elimination filter which becomes Embodiment 2 of this invention. It is a figure which shows the relationship between the electrical length of 45 degree | times of the connection means in the band-stop filter used as Embodiment 2 of this invention, and a frequency characteristic. It is a figure which shows the relationship between the electrical length 90 degree | times of a connection means and frequency characteristics in the band elimination filter which becomes Embodiment 2 of this invention. It is a figure which shows the relationship between the electrical length 135 degree | times of a connection means in the band elimination filter used as Embodiment 2 of this invention, and a frequency characteristic. It is a figure which shows the relationship between the electrical length 180 degree | times of a connection means in the band elimination filter used as Embodiment 2 of this invention, and a frequency characteristic. It is a figure which shows the relationship between the electrical length of the connection means in the zone | band elimination filter used as Embodiment 2 of this invention, and an unnecessary resonance frequency. It is a figure which shows the relationship of the unnecessary resonant frequency and external Q value of Embodiment 2 comprised with the microstrip line, and the unnecessary resonant frequency and the pass level of the frequency. It is a circuit diagram of the band rejection filter used as Embodiment 3 of this invention. It is a top view of the filter board | substrate of the band-stop filter used as Embodiment 3 of this invention. It is a figure which shows the reflection characteristic and pass characteristic of the band-stop filter which becomes Embodiment 3 of this invention. It is a circuit diagram of the band rejection filter used as Embodiment 4 of this invention. It is a top view of the filter board | substrate of the band-stop filter used as Embodiment 4 of this invention. It is a figure which shows the reflection characteristic and the passage characteristic of the band stop filter which becomes Embodiment 4 of this invention. It is a circuit diagram of the conventional band stop filter. It is a figure which shows the reflection characteristic and the passage characteristic of the conventional band stop filter. It is a conceptual diagram explaining the relationship between the unnecessary coupling between the resonance circuits and the deterioration of the pass characteristics when the resonance circuit interval is 1/4 wavelength in the conventional band rejection filter. It is a conceptual diagram explaining the relationship between the unnecessary coupling between the resonance circuits and the deterioration of the pass characteristics when the resonance circuit interval is 3/4 wavelength in the conventional band rejection filter. It is a circuit diagram for demonstrating a spur line. FIG. 5 is a circuit diagram of a band rejection filter configured by connecting spur lines in cascade. It is a figure explaining the relationship between the combination of a 1/4 wavelength shunt open stub and a 1/4 wavelength transmission line, and a spar line. It is a figure which shows the relationship between the coupling | bonding degree of a band-stop filter comprised by 1 step | paragraph of a spur line, and a stop bandwidth. It is a figure which shows the relationship between the coupling | bonding degree of a band-stop filter comprised by the super stage 2 steps | paragraph, and stop bandwidth. It is a figure which shows the relationship between the coupling | bonding degree of a zone | band stop filter comprised by 3 steps | paragraphs of a super line, and stop bandwidth. It is a figure which shows the relationship between the coupling | bonding degree of a band-stop filter comprised by four stages of spur lines, and a stop bandwidth. It is a figure which shows the cross-sectional dimension example of a microstrip line type | mold coupling line. It is a figure which shows the relationship between the coupling degree of the microstrip line type | mold coupling line of FIG. 31, and strip conductor space | interval.

Explanation of symbols

  1-1 to 1-n unit cell, 2-1 to 2-n-1 connection means, 3a input terminal, 3b output terminal, 4a power supply circuit, 4b load circuit, 5, 5a, 5b coupling line, 6, 6a, 6b open end, 7, 7a, 7b interstage line, 8 dielectric substrate, 9 carrier, 10 cut-off block, 11 strip conductor, 12a input line, 12b output line, 13a-13b spur line (coupled resonance circuit), 14 open stub, 15, 15a to 15c 1/4 wavelength transmission line, 16a, 16b series resonant circuit, 17 coupled line 1, 18 coupled line 2, 19, 19-1, 19-2 first Connection terminal 20, 20-1, 20-2 Second connection terminal.

Claims (6)

A plurality of transmission lines having a length of 1/4 wavelength of the center frequency in the stop band are arranged in parallel, and all the terminals on one side of the coupled line are electrically connected to each other. A single coupled resonance circuit configured as a first connection terminal, one of the terminals on the opposite side of the coupled line as a second connection terminal and the other as an open end, or Two unit cells are connected in cascade to form a unit cell, and a plurality of unit cells are connected via a connection means having a transmission line having an electrical length of 180 degrees or an integral multiple thereof at the center frequency of the stop band. Band stop filter that is connected in cascade. A plurality of transmission lines having a length of 1/4 wavelength of the center frequency in the stop band are arranged in parallel, and all the terminals on one side of the coupled line are electrically connected to each other. A single coupled resonance circuit configured as a first connection terminal, one of the terminals on the opposite side of the coupled line as a second connection terminal and the other as an open end, or Two unit cells are connected in cascade to form a unit cell, and a plurality of the unit cells are connected in cascade through a connection means formed of a transmission line having an electrical length of 90 degrees or less at the center frequency of the stop band. A band rejection filter consisting of: 3. The band rejection filter according to claim 2, wherein the characteristic impedance of the transmission line constituting the connection means is made lower than the power supply impedance. Connecting means, a transmission line having a low characteristic impedance than the source impedance constituted by cascade connecting the transmission line having different characteristic impedances from said transmission line, and, passing phase of the circuit and the cascaded stopband 3. The band rejection filter according to claim 2 , wherein the band rejection filter is a circuit having a central frequency of less than 90 degrees. The unit cell includes two coupled resonant circuits connected in cascade, and an interstage line connected between the coupled resonant circuits and having a length of 1/4 wavelength at the center frequency of the stop band. The first connection terminals of the two coupled resonance circuits are both connected to the interstage line side, or the second connection terminals of the two coupled resonance circuits are both connected to the interstage line side. band rejection filter according to any one of claims 1 to 4 configured by. A filter substrate in which a transmission line of a coupled resonant circuit and an interstage line connecting between the coupled resonant circuit and a connection means are formed of a microstrip line on a dielectric substrate;
A cut-off block having a rectangular groove with a horizontal width that is cut off in the use frequency band;
The filter substrate is mounted so that the filter substrate is positioned in the groove portion of the cut-off block, and includes a carrier that is combined with the cut-off block to form a cutoff cavity in a use frequency band. The band rejection filter according to any one of 5 .

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