Superconducting microstrip filter having current density reduction parts

ABSTRACT

A superconducting microstrip filter capable of achieving an improvement of power resistance without enlarging the overall size and while maintaining steep cut characteristics. This filter has a resonator section including at least one resonator. This resonator forms a current density reduction part in one part of its line pattern. Also, the filter has an input line section arranged adjoining the resonator of an initial stage. Current density reduction parts are formed in one part of this input line section. Alternatively, the input line section is comprised of a normal conductor.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

BEST MODE FOR CARRYING OUT THE INVENTION

In order to further facilitate understanding of the present invention, first, an explanation will be made of the general configuration.

FIG. 13 is a view of a front end section of a base station as an example to which the present invention is applied.

In the figure, a front and section is comprised of a duplex antenna , a receiver apparatus for receiving input power from the antenna , and a transmitter apparatus for transmitting the power from the antenna .

The receiver apparatus is comprised including a band-pass filter (BPF) for extracting only signals of intended frequency bands from among signals received from the antenna and a low noise amplifier .

On the other hand, the transmitter apparatus is comprised including a signal amplifier (AMP) and a distortion compensating circuit (DCC) and generates a signal to be transmitted from the antenna .

In the front end section , it is particularly the band-pass filter (BPF) in the receiver apparatus to which the present invention is applied. This filter is comprised of a superconducting microstrip filter (superconducting filter).

The main function of this superconducting filter is to extract a signal of the intended frequency band from among signals Rx received by a path indicated by a solid arrow from the antenna (filter for reception wave).

On the other hand, this superconducting filter also functions to cut a wraparound signal TX by a path indicated by a dotted arrow among the transmitted signals from the transmitter apparatus side. Similarly, it also functions to cut the penetrated signal tx by the path indicated by the dotted arrow from the antenna among signals transmitted from other antennas (not illustrated) of the base station (filter for transmission waves).

Below, an explanation will be made of a general superconducting filter used for the main function, that is as a filter for reception waves.

FIG. 14 is a view of an example of the general superconducting microstrip filter. The present invention is particularly effectively applied to a superconducting filter having a format shown in the figure.

In the figure, the superconducting filter is comprised of an input conductor to which the signal RX is input, and input line section connected to this, a resonator section for extracting only signals of intended frequency bands from among signals RN applied to this input line section , and an output line section for transmitting the extracted signals to for example a low noise amplifier (LNA) . Here, the resonator section is comprised including at least one resonator . Note, in the figure, as an example, nine stages of resonators -, -, . . . - are shown.

Also, in the figure, as each resonator , a microstrip hair pin type resonator configured of a λ/2 resonator bent in a hair pin shape is shown. Such a hair pin type resonator is obtained by coating superconducting thin films YBCO (Y—Ba—Cu—O) on both surfaces of a substrate made of for example magnesium oxide (MgO) or aluminum lanthanum oxide (LaAlO3) first and then forming a line pattern on the illustrated one surface by photolithography or the like. Note that, the other surface (not illustrated) of the substrate is a ground plane.

The superconducting filter provided with the thus obtained hair pin type resonators -, -, -, -, -, -, -, - and - is advantageous in that design and fabrication are easy and, in addition, is extremely effective for reduction of size and lightening of weight, so will probably be widely employed in the future.

FIGS. () and () provide enlarged views of two examples of the shapes of the bent portions of the resonators in FIG. .

FIG. () shows a shape where corners of the line pattern are cut off and the lines bent at right angles (first example) and FIG. () shows a shape where the line width of the line pattern of the straight line parts is held as it is and an arc state is exhibited (second example).

Note that, the superconducting filter is operated by cooling the filter as a whole to an extremely low temperature such as 70K by an external cooling machine. By this, steep cut characteristics can be obtained without insertion loss.

FIG. 16 is a view for explaining cut characteristics.

In the figure, both characteristics of <1>and <2>represent cut characteristics of the superconducting filter . On the other hand, the characteristics of <3>represent the cut characteristics by the general filter made of a non-superconducting metal. W in the figure indicates the pass-band, and W and W on the two ends thereof indicate cut zones.

A conspicuous difference between the characteristic <3> (filter made of ordinary metal) and the characteristics <1> and <2> (superconducting filter) resides in a difference ΔL of the insertion loss. The insertion loss of the superconducting filter is almost zero.

Note that when the number of stages of resonators is decreased, as shown by the characteristic <1>, the steep cut characteristic is lost. This is the same also for the characteristic <3>.

As explained above, when realized a superconducting filter giving steep cut characteristics while keeping the insertion loss extremely low, in comparison with a general filter comprised of ordinary metal having exactly the same shape as this, the former has the defect of an inferior power resistance. It is important to overcome this defect. This will be explained in further detail.

In general, in a microstrip line, the “edge effect” of the current flowing through there being concentrated at an edge portion of that line is seen. This edge effect does not become such a large obstacle in a microstrip line made of non-superconducting metal. In a microstrip line made of a superconducting material, however, that edge effect exerts a serious influence. If the current density on the line approaches the critical current density (Jc), at even only one position, the superconducting characteristic thereof is lost, and the superconducting state of the entire microstrip line ends up being broken. That is, the superconducting state is broken at particularly the edge portion of the line of a line pattern comprised of a superconducting microstrip line.

A superconducting filter attempting to deal with this problem is the superconducting filter disclosed in the above document. This is shown in FIG. .

FIG. 17 is a view of an example of a conventional superconducting filter suppressed in the edge effect. Note that the same reference numerals or symbols are attached to similar components throughout all of the figures.

In the superconducting filter according to the conventional example shown in this figure, the input line , the resonator section comprised of for example five stages of -, -, -, - and -, and the output line section are formed on the substrate by the microstrip line. In this superconducting filter, as already explained, by reducing the characteristic impedances of the resonators -, -, -, - and -, to be small, i.e., 10 Ω, although the characteristic impedances of the input line section and the output line section are set at 50 Ω), the line width of the line pattern is expanded and a suppression of the current concentration is achieved.

For this reason, in the superconducting filter, the line width of each line pattern is formed wide over the entire length thereof (for example 3 mm). Also the pitch p between adjacent resonators has become wide. Accordingly, the superconducting filter becomes necessarily large in size, and only a few stages of resonators can be formed on an inexpensive leading substrate having a diameter of about 5 cm.

In addition, when it is desired to configure the microstrip hair pin type resonator as shown in FIG. 14 by such a resonator having a wide line width, a large arc must be formed at each corner of the line pattern . A substrate of about 5 cm just cannot accommodate nine stages of the resonators (-, -, -, -, -, -, -, - and -).

Therefore, the present invention provides the superconducting filters of the first to fifth aspects explained above.

FIG. 1 is a view of the basic configuration of a superconducting filter based on the first aspect according to the present invention. The basic form is similar to the form of FIG. . The superconducting microstrip filter is comprised on an input conductor to which the signal RX is input, and input line section connected to this, a resonator section for extracting only signals of intended frequency bands from among signals RX applied to this input line section and an output line section for transmitting the extracted signals.

This fundamental configuration is as follows: the superconducting microstrip filter having a resonator section including at least one resonator -(k=1, 2, 3, . .), wherein the resonator -forms a current density reduction part -in one part of the line pattern thereof. Note that, in the figure, the k-th current density reduction part -is illustrated as the current density reduction part .

The major difference from the configuration of FIG. 17 shown as the conventional example resides in that the current density reduction part is formed by broadening the line width of only one part of the line pattern of each resonator in the configuration of FIG. 1 in contrast to the conventional example wherein the line width of the line pattern of each resonator is broadened over the entire length thereof.

In the present invention, since the line width of only the part where the current density becomes the maximum is selectively broadened (selective formation of the current density reduction part ), the size does not become so large when seen from the filter as a whole and rather the size can be reduced.

Accordingly, a larger number of resonators having the improved power resistance can be accommodated on the substrate having a limited area, and it becomes possible to keep the current density to not more than the critical current density (Jc) while sufficiently satisfying the steep cut characteristics explained above.

Incidentally, the idea of the present invention of forming the current density reduction part for reducing the current density of only part of the resonator by paying attention to the part where the current density becomes the maximum may seen a natural idea at first glance. However, a superconducting filter achieving both an improvement of the power resistance and a reduction of size based on such a natural idea is not yet known.

The reason for this is that the belief that provision of an additional part changing the shape of the line, that is, the current density reduction part , in one line pattern in a general device handling super high frequency bands like microwaves would probably change the impedance of the resonator per se and the impedance between resonators, seems to be the general thinking of persons skilled in the art.

However, the present applicant found that this type of additional part does not always greatly change the impedance of the resonator per se and that between resonators. The idea of the present invention resides in this point. The present applicant found this fact by verification using electromagnetic field simulation. The results of the verification will be explained later.

FIG. 2 is a plan view of an embodiment based on the first aspect. The basic form is similar to the form of FIG. . The superconducting microstrip filter is comprised on an input conductor to which the signal RX is input, and input line section connected to this, a resonator section for extracting only signals of intended frequency bands from among signals RX applied to this input line section and an output line section for transmitting the extracted signals.

In the embodiment based on the first aspect, each of the resonators -, -, -, -, -, -, -, - and - is a λ/2 resonator. Current density reduction parts -, -, -, -, -, -, -, - and - are formed at the center portion and the vicinity thereof along the length direction of the line pattern thereof.

Each λ/2 resonator (each of -, -, -, -, -, -, -, - and -) is similar to the form shown in FIG. . It is bent in half at the center portion thereof and the length of each side is λ/4. The current is concentrated at this bent portion where the maximum current density is exhibited. On the other hand, each end portion of each λ/2 resonator is open, and the current becomes almost zero.

Therefore, each of the current density reduction parts (-, -, -, -, -, -, -, - and -) is formed at the bent portion, that is, the center portion and the vicinity thereof of the λ/2 resonator.

Various methods of reducing the current density can be considered. In the embodiment shown in FIG. 2, the line width of the line pattern at the center portion and the vicinity thereof is made broader than the line width of the portions other than this to form the current density reduction part (indicated as as representative of -, -, -, -, -, -, -, - and -).

At the broadening of the line width, it is possible to form a triangular shape or square shape or heart shape at the current density reduction part . In the embodiment shown in FIG. 2, however, the current density reduction part is formed to exhibit a circular shape as a whole. By imparting the circular shape, the corners which are always formed in the case of a triangular shape etc. can be eliminated. This is because, if there is a corner in the microstrip line, the already explained edge effect appears there, and the superconducting characteristic is apt to be lost.

Note that, a concrete example of the superconducting filter shown in FIG. 2 will be explained in further detail as follows.

First, a high-temperature superconducting thin film made of YBCO (Y-Ba-Cu-O) is coated over a substrate having a thickness of 0.5 mm, made of magnesium oxide (MgO) and having a dielectric constant ∈=9.7. Next, microstrip line patterns having the line patterns shown in FIG. 2 are formed by photolithography. At this time, when the characteristic impedance is set to 50 Ω, the line width w of each resonator (indicated by as representative of -, -, -, -, -, -, -, - and -) is 0.5 mm. Also, the radius of the circular density reduction part is set to 2.0 mm. Note that, in FIG. 2 (same also in FIG. ), the adjoining resonators are alternatively rotated by 180°, but it is not always necessary to do this in principle. For example, all resonators -, -, -, -, -, -, -, - and - may be oriented in the same direction.

In the case of the present invention, however, the adjoining resonators are preferably alternately rotated by 180°. This is because if all resonators -, -, -, -, -, -, -, - and - are oriented in the same direction, the adjoining current density reduction parts become considerably close to each other, so a deleterious interference occurs.

Thus, according to the superconducting filter of FIG. 2, in each resonator , the current density at the so-called antinode part where the current becomes the maximum is greatly reduced, and the edge effect is suppressed. Accordingly, the power resistance is improved. In this case, there is no enlargement of size of the superconducting filter due to the introduction of the current density reduction parts , where nine stages of resonators -, -, -, -, -, -, -, - and -) can be accommodated on a substrate of about 5 cm length (left and right direction of FIG. 2) easily, like FIG. .

As already explained, in a filter for the super-high frequency bands, the provision of an additional part like the current density reduction part changes the impedance of the resonator per se and the impedance between resonators. Therefore, usually, a person skilled in the art would expect that a superconducting filter having intended characteristics could no longer be obtained. However, the present applicant confirmed by using electromagnetic simulation that such a change or deterioration of characteristics was small. This will be explained.

FIG. 3 is a view showing that the filter characteristics do not deteriorate even if a current density reduction part according to the present invention is introduced.

In FIG. 3, the abscissa represents the frequency, and the left and right ordinates represent pass characteristics S and correspond to the graph of FIG. 16 explained above. W in the figure indicates the pass band, and W and W on the two ends thereof indicate cut zones.

The characteristic curve <2> shown in FIG. 3 is the characteristic curve obtained by the superconducting filter according to the present invention shown in FIG. . On the other hand, the characteristic curve <4> of FIG. 3 is the characteristic curve showing the enlarged ordinate of the characteristic curve <2>. Accordingly, the ordinate of the characteristic curve <2> is indicated on left side of FIG. and the ordinate of the characteristic curve <4> is indicated on the right side of the figure.

At the time of design of the superconducting filter described above, the ripple value, set as the initial value, is 0.01 dB. When performing the simulation under this design condition, the ripple exhibited a value of 0.2 dB at the maximum as shown in FIG. .

In this way, a ripple value of 0.2 dB or less is the practical value. This shows that steep attenuation characteristics were ensured. Incidentally, a value of ripple up to about 2 to 3 dB is thought to be a practical value (a value more than 2 to 3 dB means a defective filter), so the value (0.2 dB or less) is kept smaller than this (2 to 3 dB) by one order. In this way, the value of the ripple slightly deteriorates to an extent where no problem occurs in practical use, but the effect that the power resistance can be greatly improved is much greater than the deterioration.

Additionally explaining this ripple, when designing a small number of stages of resonators , the smaller the ripple, the gentler the attenuation characteristics in the pass bands (refer to the characteristic curve <1> of FIG. ). In FIG. 2, the number of stages of resonators is set to as large as nine in the design, so there is no large influence exerted upon the attenuation characteristics even if the ripple is made small.

FIG. 4 is a view of the basic configuration of a superconducting filter based on the second aspect according to the present invention.

According to this basic configuration, there is provided a superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals RX to be filtered, wherein at least resonators (-(1), -, -(1)) cascaded at the center portion and in the vicinity thereof of the propagation path form current density reduction parts (-(1), -, -(1)) at parts of the line patterns thereof and the resonators nearer the center portion form current density reduction parts becomes larger. Note that, when the number of stages of the resonators forming the resonator section is set to nine stages as explained above, k of -at the center thereof is equal to 5.

In the above first aspect, easing of the current concentration at the center portion was explained for each individual resonator . This time, however, when viewing the entire resonator section as one resonator, in the pass band, the current becomes more easily concentrated at the resonators cascaded nearer the center portion. The second aspect (FIG. 4) pays attention to this point. The shape of the current density reduction part is made larger in the resonators cascaded nearer the center portion (-(1)→-←-(1)). When the section is comprised of nine stages of resonators, the current density reduction part -(k=5) given to the resonator -(k−5) becomes the largest.

FIG. 5 is a plan view of an embodiment based on the second aspect. The basic form is similar to the form of FIG. . In the string of resonators -→-→-→-, the current density reduction parts become larger in the sequence of -→-→-→-. Similarly, in the string of resonators -→-→-→-, the current density reduction parts become larger in the sequence of -→-→-→-. The current density reduction part - given to the resonator - at the center portion becomes the largest. In this case, the pitch p between adjacent resonators is made larger toward the center portion, while the pitch between adjacent resonators, at the input side and output side, maintains the pitch of the resonator section in the configuration shown in FIG. . By this, the size of the overall superconducting filter is made as small as possible. Note that, in FIG. 5, the configuration is the same as the case of the already explained first aspect in the following items:

(i) The resonators are λ/2 resonators. The current density reduction parts are formed along the length direction of the line patterns thereof at the center portions and in the vicinities thereof,

(ii) The current density reduction parts are formed by mating the line width of the line patterns at the center portions and in the vicinities thereof broader than the line width of the other portions, and

(iii) The current density reduction parts exhibit circular shapes as a whole.

FIG. 6 is a plan view of an embodiment based on a third aspect of the present invention.

The basic form of the third aspect is similar to the form of FIG. 17, but the thinking of the above second form is further introduced into this form of FIG. .

Namely, according to the third aspect, there is provided a superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along the propagation path of signals RX to be filtered, wherein at least resonators cascaded at the center portion and in the vicinity thereof of the propagation path form current density reduction parts over the entire length of the line patterns thereof and the resonators nearer the center portion form the current density reduction parts become larger.

More concretely, in the configuration of FIG. 6, the current density reduction parts are formed by gradually making the line width of the line pattern broader in the resonators nearer the center portion.

In the example shown in FIG. 6, in a superconducting filter having seven stages of resonators -, -, -, -, -, - and -, the current density reduction part - given to the center resonator - is the largest. Namely, the line width of the line pattern forming the resonator - is the broadest, while the line width becomes successively thinner as one moves from resonator - to each of resonators -, - and -. Similarly, the line width becomes successively thinner as one moves from resonator - to each of resonators -, - and -. When compared with the configuration of FIG. 17, only the resonator at the center portion becomes a resonator having a thick line width, so the entire superconducting filter does not become so large.

Note that the pitch p between adjoining resonators similarly becomes larger toward the center portion.

Above, a filter for reception waves was explained, so a filter for transmission waves will be explained below. These filter for reception waves and filter for transmission waves are not separate and independent. In actuality, preferably one superconducting filter is formed combining the configuration of the filter for reception waves explained above and the configuration of the filter for transmission waves as will be explained from now on. This is because the filter for reception waves provided in the base station according to the above example is simultaneously strongly affected by its own wraparound transmission power and the transmission power from other adjacent antennas of the base station as well, so must also combine the function of a filter for transmission waves.

Before explaining the embodiment of a filter for transmission waves, a general problem concerning the filter for transmission waves will be explained.

As clear also from FIG. 13 explained above, the transmission power from the transmitter apparatus side usually reaches tens to hundreds of watts. Most of the power is radiated from the antenna to the cell or sector. However, part of the power is wrapped around to the receiver apparatus side. Also, when the transmitter apparatus and receiver apparatus of FIG. 13 are provided in the above base station, a strong transmission power radiated from the antenna other than the illustrated antenna among the antennas provided in the base station flows to the receiver apparatus side through the antenna .

When the base station is used in for example a W-CDMA system, the reception frequency band and transmission frequency band of the base station are for example 1960 to 1980 MHz and 2150 to 2170 MHz. In this case, signals of undesired transmission frequency bands are eliminated without a problem when using a general filter using ordinary metal. When using a superconducting filter, however, the following problem occurs.

Namely, referring to FIG. 14, the transmission frequency bands (2150 to 2170 MHz) are sufficiently separate from the reception frequency bands (1960 to 1980 MHz). Therefore, when the transmission power is wrapped around into the superconducting filter , the current is liable to concentrate at the input line section thereof and be reflected there. However, as it approaches the critical current density (Jc), the superconducting state starts break down, and the filter characteristic of the superconducting filter deteriorates. That is, when high transmission power out of the band flows into the superconducting filter , the problem arises that only the input line section becomes unable to keep the superconducting state.

That problem will be further clarified experimentally.

In a superconductor, a distortion wave is produced due to its own nonlinearity. For example, when assuming that two waves having frequencies slightly different from each other are input to the pass band of the superconducting filter , a so-called third-order inter modulated distortion wave (third-order IMD wave) is produced. FIG. 7 is a graph of the third-order IMD characteristic of a superconducting filter.

In FIG. 7, Pin and Pout are the input power and the output power of the superconducting filter . Note that, if the frequencies of the fundamental waves are ω1 and ω2, the third-order IMD waves are 2ω2−ω1 and 2ω1−ω2.

This graph of FIG. 7 shows the situation of a change of a third-order IMD wave which rises with an inclination of three times the fundamental waves when two waves (ω, ω) separated from each other by 1 MHz are input to the pass band of a YBCO superconducting microstrip hair pin type filter (referred to as specimen ) having the microstrip pattern shape of FIG. and having C-axis oriented YBCO thin films formed on both surfaces of the substrate . It is seen from this graph that an intercept point IF at which the fundamental waves and the third-order IMD wave coincide has a value of 33 dBm.

Also, when the transmission power is input to the superconducting filter of the specimen , the third-order IMD becomes further larger.

FIG. 8 is a graph of the third-order IMD deterioration characteristic of the superconducting filter. TWO waves (the input powers are three types of Pin=12.75 dBm, 8.74 dBm, and 5.75 dBm) separate from each other by 1 MHz are input to the pass band of the superconducting filter , and the third-order IMD is produced. Further, it is shown in this FIG. 8 how the third-order IMD become large in a case where the transmission wave of a band separate from the center frequency by 190 MHz is assumed, and the power of this band is input to the superconducting filter of the specimen while gradually enlarging the power of this band.

In this way, it is understood that the third-order IMD abruptly increases as the transmission power is raised.

FIG. 9 is a graph of the insertion loss characteristic of the superconducting filter.

This is a graph showing how the insertion loss in the pass band of the superconducting filter of FIG. 14 (near the center, low frequency band end, high frequency band end) deteriorates due to the increase of the transmission power.

It is seen also from this FIG. 9 that the insertion loss abruptly increases along with an increase of the transmission power.

With the background explained above, an explanation will be made of a fourth aspect and fifth aspect of the present invention (filter for transmission waves).

FIG. 10 is a view of an example of the configuration of a superconducting filter based on the fourth aspect according to the present invention.

In this fourth aspect, there is provided a superconducting microstrip filter having an input line section to which signals RX to be filtered are input and a resonator section arranged adjoining this input line section and including at least one resonator , wherein that input line section forms a current density reduction part (′) in one part of its line pattern .

The current caused by the transmission power flowing into the filter as the signal RX concentrates at the input line section . Then, that current concentrates at the portion of λ′/4 (λ′ is the wavelength of the related transmission wave) from the open end (upper end portion of the line pattern in the figure) of the input line section , whereupon the current density becomes the maximum. Accordingly, the current density reduction part is formed in this portion of λ′/4 to keep the density to not more than Jc and prevent breakdown of the superconducting state due to the transmission power.

In this case, the line width of the line pattern of the portion (λ′/4) where the current concentration becomes the maximum in the line pattern of the input line section is made broader than the line width of the portions other than this to form the current density reduction part .

In this fourth aspect, another current density reduction part ′ can be included.

Namely, when the line pattern of the input line section and the line pattern ′ of the input conductor to which the signal RX is input are coupled in almost an L-shape, the line width of these line patterns in the coupling portion is made broader than the line width of the portions other than this to form the current density reduction part ′.

The superconducting filter is usually accommodated in a housing (not illustrated) accommodating this and connected to an external conductor (not illustrated) via a connector (not illustrated). This connector is usually arranged on the left side (on the side of the left side of the substrate ) in FIG. . For this reason, the end portion opposite to the open end of the input line section is bent to the side of the left side of the substrate at substantially a right angle. In actuality, for the input line section , the input conductor is coupled from a direction perpendicular to this.

This being so, the already explained edge effect may appear at this coupling portion. Another current density reduction part ′ eases the current density at that portion so that this edge effect does not conspicuously appear.

Both of the current density reduction parts and ′ desirably exhibit circular shapes as a whole similar to the current density reduction part explained above. Note that, in FIG. 10, the example where another current density reduction part ′ is projects out to the exterior angle side of the coupling portion is shown, but it is also possible, contrary to this, to project this to the interior angle side circularly (indicated by the dotted line in the figure).

Note that at least one of the above explained two current density reduction parts and ′ is formed. In practical use, desirably both of these two reduction parts and ′ are formed.

Finally, an explanation will be made of a fifth aspect of the present invention.

FIG. 11 is a view of an example of the configuration of a superconducting filter based on the fifth aspect according to the present invention.

In this fifth aspect, there is provided a superconducting microstrip filter having an input line section to which signals RX to be filtered are input and a resonator section arranged adjoining this input line section and including at least one resonator , wherein only that input line section is formed by a line pattern made of a material other than a superconducting material.

Here, the above material other than a superconducting material is preferably a normal conducting material.

The power of the transmission power flowing into the filter from the outside concentrates at the input line section as explained above. Therefore, in the fourth aspect, the current density reduction part and/or ′ was provided in part of the input line section to ease the current density. On the other hand, in the fifth aspect, as described above, an effect of reduction of the current density was obtained relatively not by directly reducing the current density, but by increasing the permissible current density at the input line section .

For this reason, concretely, the input line section is comprised of a material other than a superconducting material. In practice, the input line section is comprised of a normal conducting material. In this case, the introduction of the normal conducting material must not cause a remarkable increase of insertion loss at the superconducting filter . This will be explained later.

Below, a further detailed explanation will be given of the fifth aspect.

Referring to FIG. 11, when a transmission wave sufficiently apart from the reception frequency band flows into the superconducting filter , the transmission wave is apt to be reflected at the input line section . At this time, the current by that transmission wave concentrates at the input line section , but the input line section is a line pattern made of a metal of a normal conducting material, and something like superconduction breakdown will not occur. Accordingly, the characteristics of the superconducting filter do not deteriorate.

Also, by forming the input line section by a metal of a non-superconducting material, in comparison with the case where all of the superconducting filter is fabricated by a superconductor, increase of the insertion loss cannot be avoided. However, when a good electrical conductor such as gold, silver, copper, or aluminum is used as the pattern , the insertion loss thereof increases by only several tenths of a dB, and the original performance of the superconducting filter is sufficiently maintained.

Further, by forming the line pattern by a normal conducting material, the type of the normal conductor can be selected from a wide range. For this reason, the degree of freedom increases in the selection of solder materials and electrode materials for electrically connecting it to the connector for input explained above. If for example copper is used as the normal conductor, it becomes possible to use Pb—Sn-based ordinary solder.

In the embodiment of the fifth aspect based on the present invention, a substrate having a thickness of 0.5 mm and made of magnesium oxide (MgO) (dielectric constant ∈r=9.7) is formed over it with resonators and an output line section by a high-temperature superconducting thin film and is formed over it with an input line section by a copper thin film as the normal conductor.

For the frequency band, in for example the W-CDMA system, the reception frequency band and the transmission frequency band are for example 1960 to 1980 MHz and 2150 to 2170 MHz. Therefore, when the transmission wave flows into the superconducting filter , components of this transmission wave concentrate at the input line section of the copper thin film and are sufficiently reflected there. Therefore something like superconduction breakdown can not occur.

FIG. 12 is a graph showing that a large insertion loss is not caused even if a normal conductor according to the present invention is introduced into the input line section.

In the figure, the abscissa indicates the frequency, and the ordinate indicates the pass characteristic.

The results of frequency characteristic simulation by a hair pin type superconducting filter having the pattern shape shown in FIG. and having a center frequency of 1.962 GHz, a band width of 23 MHz, and five stages of resonators , designed using electromagnetic field simulation, and in a case where the input line section was formed by a superconductor (Q value by film was 20000) and in a case where the input line section was formed by a normal conductor (Q value by film was 500) are shown in FIG. 12 as characteristics <5> and <6> respectively. At this time, the resonator section and the output line section were formed by superconductors (Q value by film was 20000).

When the input line section was formed by a superconductor, the insertion loss was 0.12 dB, but even if the input line section is formed by a normal conductor, the insertion loss becomes 0.18 dB and the increase of the insertion loss is very small. Accordingly, it is understood that the performance as the superconducting filter is sufficiently maintained irrespective of the introduction of the normal conductor ().

Note that, in FIG. and FIG. 11 used for the explanation of the fourth and fifth aspects, as the resonator section , a resonator section comprised of resonators having patterns similar to that shown in FIG. 14 but having a decreased number of stages was shown for simplification, but in practice, either of the first, second, and third aspects (FIG. 2, FIG. 5, FIG. 6) is desirably employed as this resonator section .

As explained above, according to the present invention, a superconducting filter capable of greatly improving the power resistance while maintaining the steep cut characteristics without enlarging the overall size is realized. Also, the superconducting filter based on the present invention can be used as a filter for reception waves, as a filter for transmission waves, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the basic configuration of a superconducting filter based on a first aspect according to the present invention,

FIG. 2 is a plan view of an embodiment based on the first aspect,

FIG. 3 is a view showing that filter characteristics do not deteriorate even if a current density reduction part according to the present invention is introduced,

FIG. 4 is a view of the basic configuration of a superconducting filter based on a second aspect according to the present invention,

FIG. 5 is a plan view of an embodiment based on the second aspect,

FIG. 6 is a plan view of an embodiment based on a third aspect of the present invention,

FIG. 7 is a graph of a third-order inter-modulation distortion (IMD) characteristic of a superconducting filter,

FIG. 8 is a graph of a third-order IMD deterioration characteristic of the superconducting filter,

FIG. 9 is a graph of insertion loss characteristics of the superconducting filter,

FIG. 10 is a view of an example of the configuration of a superconducting filter based on a fourth aspect according to the present invention,

FIG. 11 is a view of an example of the configuration of a superconducting filter based on a fifth aspect according to the present invention,

FIG. 12 is a graph showing that a large loss is not caused even if a normal conducting material according to the present invention are introduced into an input line section,

FIG. 13 shows a front end section of a base station as an example to which the present invention is applied,

FIG. 14 is a view of an example of a general superconducting microstrip filter,

FIGS. () and () are views of enlarged shapes of bent portions of resonators in FIG. 14 for two examples,

FIG. 16 is a view explaining cut characteristics, and

FIG. 17 is a view of an example of a conventional superconducting filter suppressed in edge effect.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application and is based upon PCT/JP00/00491, filed on Jan. 28, 2000.

CLAIMS

1. A superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, each of the resonators comprising a microstrip hair pin resonator configured as a λ/2 resonator, said plurality of resonators being constructed so that adjoining ones of the plurality of resonators are alternately rotated by 180°, wherein each of said plurality of resonators includes a current density reduction part at a center portion along a length direction of a line pattern thereof, said center portion of the respective line pattern being broader in line width that than the remainder of the line pattern thereof.

2. A superconducting microstrip filter as set forth in claim 1, wherein said current density reduction part exhibits a circular shape.

3. A superconducting microstrip filter having an input line section to which signals to be filtered are input and having a resonator section comprising a superconducting material and arranged adjoining the input line section and including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, each of the resonators comprising a microstrip hair pin resonator configured as a ω/2 resonator, said plurality of resonators being constructed so that adjoining ones of the plurality of resonators are alternately rotated by 180°, wherein only said input line section has a line pattern comprising a non-superconducting material being able to increase the permissible current density relative to that of said superconducting material.

4. A superconducting microstrip filter as set forth in claim 3, wherein said non-superconducting material is a conducting material.

5. A superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, each of the plurality of resonators having respective line patterns and comprising a microstrip hair pin resonator configured as a λ/2 resonator, said plurality of resonators being constructed so that adjoining ones of the plurality of resonators are alternately rotated by 180°, wherein three or more of said plurality of resonators cascaded at a middle cart of said propagation path and in the vicinity thereof include current density reduction parts at center portions of the line patterns thereof; said current density reduction parts being larger in size in said resonators located nearer to said middle part.

6. A superconducting microstrip filter having a resonator section including a plurality of resonators cascaded in a line along a propagation path of signals to be filtered, each of the plurality of resonators having respective line patterns, wherein three or more of said plurality of resonators cascaded at a middle part of said propagation path and in the vicinity thereof include current density reduction parts over the entire length of the line patterns thereof; said current density reduction parts having line widths of said line patterns that are broadest at the center of said middle part and that that become narrower in successive ones of said plurality of resonators, moving away from said middle part.

7. A superconducting microstrip filter as set forth in claim 6, wherein a respective pitch p between adjoining resonators becomes larger moving along said propagation path of signals toward said middle part.

8. A superconducting microstrip filter having an input line section to which signals to be filtered are input and having a resonator section arranged adjoining the input line section and including at least one resonator, wherein said input line section comprises a current density reduction part in one part of a line pattern of the input line section and said current density reduction part has a line width at a portion of the line pattern of the respective line that is broader than the line width at other portions of said line pattern of the respective line, and a current concentration is at a maximum in the broader portion of the line pattern of the respective line.

9. A superconducting microstrip filter as set forth in claim 8, wherein said input line section and an input conductor to which said signal is input are coupled in substantially an L-shape, and a further current density reduction part is disposed at the coupling portion and has a line width broader than the line width of portions of the input line section and the input conductor other than this the coupling portion.

10. A superconducting microstrip filter as set forth in claim 9, wherein said current density reduction part exhibits a circular shape.

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