Microwave filter

Abstract: A microwave filter is disclosed in which open-ended lines (14, 15, 16; 25, 25'; 34, 35, 35', 36; 44, 45, 46, 47; 54, 55, 56, 57; 64, 65, 66, 67) are connected at three or four points of a main line (13; 33; 43; 52; 63) having input and output terminals (11, 12; 31, 32; 41, 42; 51, 52; 61, 62), the open-ended lines have a length about one fourth the wavelength of an image signal frequency thereby to produce a band-stop filter characteristic with an image frequency, all or parts of the open-ended lines constitute open-ended parallel-coupled lines arranged in parallel to the main line, and the intervals of connection of the open-ended lines with the main line are properly selected. The insertion loss of the filter within the pass band of a radio frequency signal is reduced and the filter attenuation within a stop band of an image signal is increased, thus providing a compact band-stop filter which has a steep rise characteristic.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a microwave filter using a strip line or a micro-strip line, or more in particular to a microwave filter configuration with a pass-band frequency higher than a stop-band frequency and both the pass-band and stop-band frequencies limited in bandwidth. DESCRIPTION OF THE PRIOR ART In a mixer supplied with a radio frequency signal (f s in frequency) and a local oscillation signal (f ℓ in frequency) different in frequency from the radio frquency signal for producing an intermediate frequency signal (f s - f ℓ in frequency; f s > f ℓ ) making up a frequency component representing the difference between the former two signals, a filter (hereinafter called the "signal-pass image-rejection filter) for passing the radio frequency signal without loss but stopping an image signal (with frequency f m = 2f ℓ - f s ) having a frequency (2f ℓ - f s ) twice the local oscillation signal (2f ℓ ) less the frequency (f s ) of the radio frequency signal, is inserted in a main line for transmitting the radio frequency signal to a mixer diode. Further, a local band-pass filter (hereinafter called the "local BPF") for selectively passing a local oscillation signal alone is interposed between an input terminal for the local oscilaltion signal and the mixer diode. Upon application of a radio frequency signal and a local oscillation signal to a mixer diode making up a nonlinear element, a side band or a high harmonic of mf s ± nf ℓ (m, n: Integers) in frequency are generated. The waves of the image signal frequency f m and the sum frequency f s + f ℓ in these spectra contain a radio frequency component. By returning the image signal, in particular, out of these signals to the mixer diode through a signal-pass image-rejection filter and mixing it with the local oscillation signal again, therefore, it is possible to produce a reconverted intermediate frequency signal and thereby to reduce the conversion loss of the mixer. further, the signal-pass image-rejection filter is capable of preventing an interference wave signal having the same frequency as the image signal frequency from entering the frequency band of the intermediate frequency signal by way of the radio frequency signal input terminal. Especially, a single-ended mixer using only one mixer diode has the performance thereof greatly affected by the manner in which the image signal generated in the mixer diode is processed. The impedance as viewed from a diode terminal is normally set to be reactive against the image signal frequency. A signal-pass image-rejection filter and a local BPF for rejecting an image signal thus constitute indispensable elements for configuring a single-end mixer. The signal-pass image-rejection filter is provided on or in coupling with a main line for transmitting a radio frequency signal to the mixer diode, and therefore the characteristics of the signal-pass image-rejection filter have a direct effect on the mixer performance. In other words, it is not too much to say that the mixer performance is determined by the characteristics of the signal-pass image-rejection filter. The performance described below is required of such a signal-pass image-rejection filter. (1) A minimum insertion loss against a radio frequency signal. (2) Characteristics to reject an image signal sufficiently. (3) A pass bandwidth and a rejection bandwidth required for a radio frequency signal and an image signal respectively. (4) The more steep the out-of-band characteristics, the closer the frequencies of the radio frequency signal and the image signal to each other. A conventional signal-pass image-rejection filter used with a mixer is disclosed in JP-A-63-10601. This signal-pass image-rejection filter is shown in Fig. 5. In Fig. 5, an input terminal 1 and an output terminal 2 for a radio frequency signal are connected by a main line 3 configured of a strip line. Open-ended stubs 4, 5, 6 having lengths of ℓ 1 , ℓ 2 , ℓ 3 respectively at equal intervals of ℓ 0 sequentially are connected in shunt with the main line 3. The lengths ℓ 1 , ℓ 2 , ℓ 3 of the open-ended stubs 4, 5, 6 are selected as equal or near to one fourth of the wavelength of the image signal so that poles of attenuation are placed within or in the vicinity of the image signal band. The length, ℓ 1 , ℓ 2 , ℓ 3 and the intervals ℓ 0 of the open-ended stubs 4, 5, 6 are also determined in such a manner as to hold the relations of both ℓ 2 < ℓ 1 < ℓ 0 < 2ℓ 2 and ℓ 2 < ℓ 3 < ℓ 0 < 2ℓ 2 at the same time or the relations ℓ 2 < ℓ 1 = ℓ 3 < ℓ 0 < 2ℓ 2 , while the length ℓ 0 is selected at a value about 1.5 times one fourth of the wavelength of the radio frequency signal. Numerals 7, 8 designate input and output lines connected to the input and output terminals 1 and 2 respectively. The forementioned signal-pass image-rejection filter with the open-ended stubs 4, 5, 6 projected in the directions perpendicular to the main line 3 has disadvantages in that: (1) The fact that the open-ended stubs 4, 5, 6 are mounted in the form projected in the directions perpendicular to the main line 3 easily causes radiation, thereby increasing an insertion loss within the pass band of a radio frequency signal. (2) The open-ended stub 5 has poles of attenuation on high-frequency side as compared with the stubs 4, 6. If the characteristic impedance of the open-ended stub 5 is increased, a filter having a comparatively steep rise characteristic would be obtained. Since there is only one open-ended stub with poles of attenuation on high frequency side, however, it is impossible to produce a filter having a steep rise characteristic. (3) In view of the fact that the open-ended stubs 4, 5, 6 are projected in the directions perpendicular to the main line, the filter is widened for an increased filter size. SUMMARY OF THE INVENTION The present invention has been developed in order to obviate these disadvantages, and the object thereof is to provide a compact microwave filter small in the insertion loss caused by radiation within the pass band of a radio frequency signal and having a steep rise characteristic. The invention is defined in claim 1, whereas the other claims specify advantageous embodiments thereof. A filter section similar to the filter depicted in figure 4 is known from figure 4 of IEEE Transactions on Microwave Theory and Techniques, Vol. 24, no 5, May 1976, New York US, pages 242-248: J.A.G. Malherbe "TEM Pseudoelliptic-function bandstop filters using noncommensurate lines". BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a pattern diagram showing a microwave filter configured of a strip line according to a first embodiment of the present invention. Fig. 2 is a pattern diagram showing a microwave filter configured of a strip line according to a second embodiment of the present invention. Fig. 3 is a characteristic diagram showing a specific example of the frequency characteristic of insertion loss of the filter circuit shown in Fig. 2. Fig. 4 is a pattern diagram showing a microwave filter configured of a strip line according to a sixth embodiment of the present invention. Fig. 5 is a pattern diagram showing a microwave filter circuit configured of a conventional strip line. DESCRIPTION OF THE PREFERRED EMBODIMENTS A signal-pass image-rejection filter according to a first embodiment of the invention is shown in Fig. 1. An input terminal 41 and an output terminal 42 of a radio frequency signal are connected by a main line 43 configured of a strip line. Open-ended parallel-coupled lines 44, 45, 46, 47 having lengths of ℓ 2 , ℓ 3 , ℓ 3 ', ℓ 2 ' (here, ℓ 2 ≅ ℓ 2 ', ℓ 3 ≅ ℓ 3 ') respectively, are coupled in parallel to the main line 43. The main line 43 is connected to the parallel-coupled lines 44, 45, 46, 47 with the distances ℓ 0 , ℓ 1 , ℓ 0 ' respectively. The parallel-coupled lines 44, 45, 46, 47 are selected at lengths ℓ 2 , ℓ 3 ' ℓ 3 ', ℓ 2 ' which are one fourth or approximately one fourth the wavelength of the image signal so that poles of attenuation may be positioned within or in the vicinity of the image signal band. The lengths ℓ 2 , ℓ 3 , ℓ 3 ', ℓ 2 ' and the intervals ℓ 0 , ℓ 1 , ℓ 0 ' of the parallel-coupled lines 44, 45, 46, 47 are selected to satisfy the conditions ℓ 1 < ℓ 3 ≅ ℓ 3 ' < ℓ 0 ≅ ℓ 0 ' < 2ℓ 3 ≅ 2ℓ 3 '. At the same time, the length ℓ 0 , ℓ 0 , is determined to be about 1.5 times one fourth the wavelength of the radio frequency signal, and the length ℓ 1 about 0.5 to 1.0 times one fourth the wave-length of the radio frequency signal. In addition, the characteristic impedance of the parallel-coupled lines 45, 46 with poels of attenuation thereof located on the high-frequency side of the image signal, that is, on the side nearer to the radio frequency signal, is selected to be higher than the characteristic impedance (normally 50 Ω) of the input and output lines 48, 49 connected to the input and output terminals 41, 42. According to this embodiment, the arrangement of the parallel-coupled lines 44, 45, 46, 47 coupled in parallel to the main line 43 reduces the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines 44, 45, 46, 47, with the result that the insertion loss of the filter within the pass band of the radio frequency signal is reduced, thereby increasing the attenuation capacity of the filter within the stop band of the image signal. Also, in view of the fact that the characteristic impedance of the parallel-coupled lines 45, 46 with poles of attenuation located on the side nearer to the pass band of the radio frequency signal is set to a value higher than that of the input and output lines 48, 49, the quality factor within the stop band of the parallel-coupled lines 45, 46 is improved. Further, since the poles of attenuation are composed of the two parallel-coupled lines 45, 46, a filter with a steep rise characteristic is obtained. Furthermore, the arrangement of the parallel-coupled lines 44, 45, 46, 47 in parallel to the main line 43 reduces the whole width of the filter for a smaller filter size. Fig. 2 shows a signal-pass image-rejection filter according to a second embodiment of the present invention. An input terminal 51 and an output terminal 52 for a radio frequency signal are connected by a main line 53. Open-ended parallel-coupled lines 54, 55, 56, 57 having the lengths of ℓ 2 , ℓ 3 , ℓ 3 ', ℓ 2 ' (here, ℓ 2 ≅ ℓ 2 ', ℓ 3 ≅ ℓ 3 ') respectively are coupled in parallel to the main line 53. The main line 53 is connected to the parallel-coupled lines 54, 55, 56, 57 with the intervals ℓ 0 , ℓ 1 , ℓ 0 ' respectively. The parallel-coupled lines 54, 55 are arranged in opposed relations to each other on the opposite sides of a main line portion 53, and the parallel-coupled lines 56, 57 in opposed relations to each other on the opposite sides of the other main line portion 53. The parallel-coupled lines 54, 55, 56, 57 are selected at lengths ℓ 2 , ℓ 3 , ℓ 3 ', ℓ 2 ' which are one fourth or almost one fourth the wavelength of the image signal so that the poles of attenuation thereof are included within or in the vicinity of the image signal band. The lengths ℓ 2 , ℓ 3 , ℓ 3 ', ℓ 2 ' and the intervals ℓ 0 , ℓ 1 , ℓ 0 of the parallel-coupled lines 54, 55, 56, 57 are selected to satisfy the conditions ℓ 1 < ℓ 3 ≅ ℓ 3 ' < ℓ 2 ≅ ℓ 2 ' < ℓ 0 ≅ ℓ 0 ' < 2ℓ 3 = 2ℓ 3 ', while determining the length ℓ 0 , ℓ 0 ' at a value about 1.5 times one fourth the wavelength of the radio frequency signal, and the length ℓ 1 about 0.5 to 1.0 time one fourth the wavelength of the radio frequency signal. In addition, the characteristic impedance of the parallel-coupled lines 55, 56 with poles of attenuation located on the high frequency ,. side of the image signal, that is, on the side nearer to the radio frequency signal, is selected to be higher than the characteristic impedance (normally 50 Ω) of the input and output liens 58, 59 connected to the input and output terminals 51, 52 respectively. Fig. 3 shows a filter characteristic assuming that the relative dielectric constant of the dielectric substrate for the strip line is 2.6, the thickness thereof 0.6 mm, the characteristic impedance of the parallel-coupled lines 54, 55, 56, 57 is 120 ohm, the characteristic impedance of the input and output lines 58, 59 is 50 ohm, ℓ 0 = ℓ 0 ' = 6.5 mm, ℓ 1 = 2.8 mm, ℓ 2 = ℓ 2 ' = 5.5 mm and ℓ 3 = ℓ 3 ' = 5.2 mm in Fig. 2, while at the same time taking into consideration the fringing capacitance due to the open end effect at the open ends of the parallel-coupled lines 54, 55, 56, 57. In the filter shown in Fig. 3, VSWR is less than 1.4 in the frequency range from 12.1 to 14.0 GHz, so that a characteristic with an attenuation of more than 30 dB is obtained in the frequency range from 9.5 to 10.6 GHz. As a result, the filter having the characteristic as shown in Fig. 3 sufficiently satisfies the performance required of a signal-pass image-rejection filter for a mixer having a radio frequency range from 12.1 to 14.0 GHz and an image signal frequency range from 9.5 to 10.6 GHz. In addition, since the component parts of the filter are limited to parallel-coupled lines, a compact signal-pass image-rejection filter for a mixer is provided, which, very small in insertion loss for a radio frequency signal, is used effectively for rejecting an image signal and passing a radio frequency signal without loss. According to the embodiment under consideration, the parallel-coupled lines 54, 55, 56, 57 are coupled in parallel to the main line 53, and therefore the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines 54, 55, 56, 57 is reduced, with the result that the insertion loss of the filter within the pass band of the radio frequency signal is decreased, thus increasing the filter attenuation capacity within the stop band of the image signal. Also, the characteristic impednace of the parallel-coupled lines 55, 56 with poles of attenuation thereof located on the side nearer to the pass band of the radio frequency signal is selected higher than that of the input and output lines of the filter, so that the quality factor (Q) within the stop band of the parallel-coupled lines 55, 56 is higher. In addition, the fact that the poles of attenuation are comprised of two parallel-coupled lines 55, 56 assures a steep rise characteristic for the filter. Further, the parallel-coupled lines 54, 55, 56, 57 are coupled in parallel to the main line 53 and in opposed relations on the opposite sides thereto, thereby shortening the width and length of the whole filter for a greatly reduced filter size. Fig. 4 shows a signal-pass image-rejection filter according to a third embodiment of the present invention. An input terminal 61 and an output terminal 62 of a radio frequency signal are connectec by a main line 63 made up of a strip line. Open-ended stubs 64, 67 having the length ℓ 2 , ℓ 2 ' (ℓ 2 ≅ ℓ 2 ') are connected in shunt with the main line 63, while parallel-coupled lines 65, 66 having the length ℓ 3 , ℓ 3 ' (ℓ 3 ≅ ℓ 3 ') are coupled in parallel to the main line 63. The main line 63, the open-ended stub 64, the parallel-coupled lines 65, 66 and the open-ended stub 67 are connected with intervals of ℓ 0 , ℓ 1 , ℓ 0 ' (ℓ 0 ≅ ℓ 0 ') respectively. The length ℓ 2 , ℓ 2 ' of the open-ended stubs 64, 67 and the length ℓ 3 , ℓ 3 ' of the parallel-coupled lines 65, 66 are selected to a value one fourth or approximately one fourth the wavelength of the image signal so that the poles of attenuation thereof are placed within or in the vicinity of the image signal band. The length ℓ 2 , ℓ 2 ' of the open-ended stubs 64, 67, the length ℓ 3 , ℓ 3 ' of the parallel-coupled lines 65, 66 and the intervals thereof ℓ 0 , ℓ 1 , ℓ 0 ' are selected to satisfy the conditions ℓ 1 < ℓ 3 ≅ ℓ 3 ' < ℓ 2 ≅ ℓ 2 ' < ℓ 0 ≅ ℓ 0 ' < 2ℓ 3 ≅ 2ℓ 3 ' while at the same time selecting the length ℓ 0 , ℓ 0 ' at a value about 1.5 times one fourth the wavelength of the radio frequency signal and the length ℓ 1 about 0.5 to 1.0 time one fourth the wavelength of the radio frequency signal. Further, the characteristic impedance of the parallel-coupled lines 65, 66 with poles of attenuation thereof located on the high frequency side of the image signal, that is, on the side nearer to the radio frequency signal is selected higher than the characteristic impedance (normally 50 ohm) of the input and output lines 68, 69 connected to the input and output terminals 61, 62. According to this embodiment, the parallel-coupled lines 65, 66 with poles of attenuation thereof located on the side nearer to the radio frequency signal are coupled in parallel to the main line 63. It is thus possible to reduce the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines 65, 66, with the result that the insertion loss of the filter in the pass band of the radio frequency signal, in particular, can be reduced. Also, in view of the fact that the characteristic impedance of the parallel-coupled lines 65, 66 with poles of attenuation thereof located on the side nearer to the pass band of the radio frequency signal is set higher than that of the input and output lines 68, 69 of the filter, the quality factor within the stop band of the parallel-coupled lines 65, 66 is high. In addition, the poles of attenuation are comprised of two parallel-coupled lines 65, 66, and therefore a filter is obtained with a steep rise characteristic. Further, the use of the parallel-coupled lines 65, 66 reduces the size of the filter as a whole. Further, a filter with an especially superior characteristic is configured, if the interval ℓ 0 , ℓ 0 ' of open-ended lines or subs is selected longer than 5/16 and shorter than 7/16 the wavelength of the pass-band frequency (or radio frequency signal), and the interval ℓ 1 is determined longer than 1/8 and shorter than 2/8 the wavelength of the pass-band frequency. The filter having the characteristic shown in Fig. 7 is selected to have a filter size satisfying these conditions. As will be understood from the foregoing description, the present invention has the advantages described below. (1) Parallel-coupled lines having a high characteristic impedance are coupled in parallel to a main line, and therefore the radiation loss of the filter due to the radiation from the open ends of the parallel-coupled lines is reduced, with the result that the insertion loss of the filter within the pass band of a radio frequency signal is decreased, thereby increasing the amount of attenuation of the filter within the stop band of an image signal. (2) In view of the fact that the characteristic impedance of at least two parallel-coupled lines is set high as compared with the characteristic impedance (normally 50 ohm) of input and output lines, the quality factor within the stop band of the parallel-coupled lines is high, thus producing a steep rise characteristic of the filter. This is especially effective with a filter for image rejection used with a mixer having a radio frequency signal comparatively close to an image signal, thus realizing a superior mixer performance. (3) In the case where open-ended lines are connected at four points of a main line, the length ℓ 2 , ℓ 2 ', ℓ 3 , ℓ 3 ' of the parallel-coupled lines or open-ended stubs are selected equal to or substantially equal to one fourth the wavelength of an image signal so that poles of attenuation thereof are placed within or in the vicinity of the image signal band. And the lengths ℓ 2 , ℓ 2 ', ℓ 3 , ℓ 3 ' and the intervals ℓ 0 , ℓ 1 , ℓ 0 ' of the parallel-coupled lines or the open-ended stubs are selected to satisfy the conditions ℓ 1 < ℓ 3 ≅ ℓ 3 ' < ℓ 2 ≅ ℓ 2 ' < ℓ 0 ≅ ℓ 0 ' < 2ℓ 3 ≅ 2ℓ 3 ' thereby providing a filter having a more steep rise characteristic and a wide pass bandwidth. (4) In the case where open-ended lines are connected at three points of a main line, the lengths ℓ 1 , ℓ 2 , ℓ 2 ', ℓ 3 of the parallel-coupled lines and the open-ended stub are selected at a value one fourth or substantially one fourth the wavelength of the image signal so that poles of attenuation come within or in the vicinity of the image signal band. The length ℓ 1 , ℓ 2 , ℓ 2 ', ℓ 3 and the interval ℓ 0 , ℓ 0 ' of the parallel-coupled lines and the open-ended stub, on the other hand, are determined to satisfy the conditions ℓ 1 ≅ ℓ 2 (≅ ℓ 2 ') < ℓ 0 ≅ ℓ 0 ' < 2ℓ 1 and ℓ 3 ≅ ℓ 2 (≅ ℓ 2 ') < ℓ 0 ≅ ℓ 0 ' < 2ℓ 3 at the same time, or ℓ 1 = ℓ 3 ≅ ℓ 2 (≅ ℓ 2 ') < ℓ 0 ≅ ℓ 0 ' < 2ℓ 1 , whereby a filter is provided which has a more steep rise characteristic and a wider pass band. If the interval ℓ 0 , ℓ 0 ' is selected to be longer than 5/16 and shorter than 7/16 the wavelength of a radio frequency signal, it is possible to configure a filter especially superior in rise characteristic and pass band width (5) Parallel-coupled lines coupled distributively in parallel to a main line are used as open-ended lines connected to the main line, and therefore the size of the whole filter is reduced.

Claims: A microwave filter comprising: a main line (43; 53; 63) having an input terminal (41; 51; 61) and an output terminal (42; 52; 62), and first, second, third and fourth open-ended lines (44, 45, 46, 47; 54, 55, 56, 57; 64, 65, 66, 67), each connected to said main line at intervals l 0 , l 1 , l 0 ' whereby l 0 ≈l 0 ', respectively having lengths of l 2 , l 3 , l 3 ' and l 2 ' respectively, characterised in that at least said second and third open-ended lines (44, 45, 56, 47; 54, 55, 56, 57; 65, 66) are arranged and coupled in parallel to the main line, and said lengths l 2 , l 3 , l 3 ' and l 2 ' are selected to be the 1/4 wavelength of a stop-band frequency so that the poles of attenuation thereof are positioned within a stop band, said lengths l 0 , l 0 ', l 1 , l 2 , l 2 ', l 3 and l 3 ' being selected to satisfy a condition l 1 < (l 3 and l 3 ') < (l 2 and l 2 ') < (l 0 and l 0 ') < (2l 3 and 2l 3 ') or to satisfy a condition l 1 < l 3 ≈l 3 ' < l 2 ≈l 2 ' < l 0 ≈l 0 ' < (2l 3 and 2l 3 '). A microwave filter according to claim 1, wherein the first, second, third and fourth open-ended lines (44, 45, 46, 47; 54, 55, 56, 57) are coupled in parallel to the main line. A microwave filter according to claim 2, wherein the first and second parallel-coupled lines (54, 55) are at corresponding positions in opposed relationship to each other on the opposite sides of said main line, and the third and fourth parallel-coupled lines (56, 57) are at corresponding positions in opposed relationship to each other on the opposite sides of the main line. A microwave filter according to claim 2, wherein the characteristic impedance of said first second, third and fourth open-ended parallel-coupled lines (44, 45, 46, 47; 54, 55, 56, 57) is selected to be higher than that of the input and output lines (48, 49; 58, 59) connected to be input and output terminals respectively. A microwave filter according to claim 1, wherein the first and fourth open-ended lines (64, 67) comprise open-ended stubs arranged in shunt with the main line, the second and third open-ended lines (65, 66) comprise open-ended parallel-coupled lines arranged in parallel to said main line. A microwave filter according to claim 5, wherein the characteristic impedance of the first, second, third and fourth open-ended lines (64, 65, 66, 67) is selected to be higher than that of the input and output lines (68, 69) connected to the input and output lines respectively.

Inventors: SAKA HIROSHI, SAKA, HIROSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H01P1
IPC codes: H01P1