Patent Publication Number: US-6700462-B2

Title: Microstrip line filter combining a low pass filter with a half wave bandpass filter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microstrip line filter and a high-frequency transmitter using the microstrip line filter. In particular, the present invention relates to a microstrip line filter constituting a low-pass filter which eliminates any unwanted radiation and relates to a high-frequency transmitter using the microstrip line filter. 
     2. Description of the Background Art 
     In recent years, the radio (high-frequency) communication has undergone remarkable developments in numerous systems like the broadcast and communication satellites for example. On the other hand, the widespread use of the Internet has caused increasing demands for the two-way communication. 
     FIG. 11 schematically shows a system for two-way communication by means of a communication satellite. Referring to FIG. 11, an IDU (indoor unit)  1  is contained within a television receiver or housed in a board in a personal computer, and processes a signal for two-way communication with a broadcast station via a communication satellite  2 . IDU  1  is connected to a high-frequency transmitter  4  via a transmission-adapted coaxial cable  3  and IDU  1  is also connected to an LNB (low noise block down converter)  6  via a reception-adapted coaxial cable  5 . 
     High-frequency transmitter  4  and LNB  6  are coupled to a feed horn  8  via an orthogonal polarization isolator  7 . A transmission signal from high-frequency transmitter  4  is radiated as the microwave from feed horn  8 , reflected by a parabolic antenna  9  and transmitted toward communication satellite  2 . The microwave from communication satellite  2  is reflected by parabolic antenna  9  and then received by LNB  6  via feed horn  8 . 
     FIG. 12 is a block diagram of the high-frequency transmitter employed in the system shown in FIG.  11 . Referring to FIG. 12, high-frequency transmitter  4  receives, from IDU  1  shown in FIG. 11, a transmission signal of an intermediate frequency ranging from 950 to 1450 MHz superimposed on a direct-current voltage. The intermediate-frequency signal is supplied via a high-pass filter (HPF)  401  to an IF amplifier  402  to obtain a gain, adjusted to a proper level by an attenuator  403 , further amplified by an IF amplifier  404 , and then supplied to a mixer  406  via a bandpass filter (BPF)  405 . 
     A local oscillator  407  generates a local oscillator signal of 13.05 GHz which is provided via a buffer amplifier  408  to mixer  406 . Mixer  406  combines the local oscillator signal of 13.05 GHz with the intermediate-frequency signal of 950-1450 MHz in order to convert the intermediate-frequency signal into a high-frequency signal of 14.0-14.5 GHz. The high-frequency signal supplied from mixer  406  is input to a half-wave bandpass filter  409  where any unwanted radiation component (spurious radiation component) of the high-frequency signal that is generated in mixer  406  is attenuated, and then amplified by two high-frequency amplifiers  410  and  411  to obtain a great gain. 
     The output from high-frequency amplifier  411  is supplied to a bandpass filter  412  where the amplified spurious component is attenuated, and then supplied to a driver amplifier  413  to obtain a further gain. The output from driver amplifier  413  is supplied to a reception-bandwidth noise filter  414  where any noise level in a reception frequency range is substantially reduced to a thermal noise level. Then, the high-frequency signal is converted by a power amplifier  415  to a signal of high power required for transmission to the satellite. The high-frequency signal from power amplifier  415  is provided to a reception-bandwidth noise filter  416  where the noise level in the reception frequency range that is increased from the thermal noise level due to the gain of power amplifier  415  is attenuated, and then the signal supplied via noise filter  416  from high-frequency transmitter  4  is radiated as the microwave from feed horn  8 , reflected by parabola antenna  9  and transmitted toward communication satellite  2  that are shown in FIG.  11 . 
     The DC voltage with the intermediate-frequency signal superimposed thereon is supplied via an inductor L to a power supply circuit  421 . Inductor L prevents the intermediate-frequency signal from being input to power supply circuit  421 . Power supply circuit  421  converts the supplied DC voltage into a predetermined voltage which is provided to a power supply sequence circuit  422 . Then, the converted DC voltage is supplied to IF amplifiers  402  and  404 , mixer  406 , local oscillator  407 , buffer amplifier  408 , high-frequency amplifiers  410  and  411 , driver amplifier  413  and power amplifier  415 . 
     In high-frequency transmitter  4  shown in FIG. 12, the gain of IF amplifiers  402  and  404  and the degree or amount of attenuation by attenuator  403  are adjusted to prevent the output level from varying when the level of the input intermediate-frequency signal varies in the range from −5 dBm to −25 dBm. Even if a high-level signal of approximately −5 dBm is input, IF amplifiers  402  and  404  operate in a saturation region to distort the signal component in order to output the signal at a predetermined level. However, the distorted signal component generates harmonic components resulting in increase of spurious components. 
     Any spurious of 14.95-15.95 GHz generated in mixer  406  resultant from mixing of the input signal of twice the frequency of 950 MHz-1450 MHz and the local oscillator signal of 13.05 GHz differs from the output frequency range 14 GHz-14.5 GHz of high-frequency transmitter  4  merely by 450 MHz. Then, in order to reduce such a spurious, a microstrip filter as shown in FIG. 13 is used as the half-wave bandpass filter  409  shown in FIG.  12 . 
     The microstrip filter shown in FIG. 13 includes a plurality of (e.g.  8 ) rectangular elements shifted so that respective halves of the longitudinal sides of respective elements are opposite to and in parallel with each other. This bandpass filter  409  has a passband of 14 GHz-14.5 GHz so as to attenuate an image-frequency signal of 11.6-12.1 GHz and a signal above 14.5 GHz. However, proper attenuation of the spurious of 14.95 GHz which is close to 14.5 GHz could be impossible. 
     FIG. 14 shows cutoff characteristics of a combination of half-wave bandpass filter  409  and high-frequency amplifiers  410  and  411 . It is seen from FIG. 14 that the attenuation achieved by the cutoff characteristics is merely 11.9 dB, which means that an attenuation of 20 dB or more by half-wave bandpass filter  409  with its elements arranged as shown in FIG. 13 is extremely difficult. Even if attenuation of at least 20 dB is possible, it is impossible to make the cutoff characteristics more steeper. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a microstrip line filter constituting a low-pass filter with a large out-of-band attenuation and a small in-band deviation, and to provide a high-frequency transmitter employing the microstrip line filter. 
     In summary, according to one aspect of the present invention, a microstrip line filter formed on a substrate includes a plurality of composite elements arranged in parallel with each other. The composite elements each include a rectangular microstrip line element, an input microstrip line and an output microstrip line that are formed on the substrate. The composite elements are connected to constitute a low-pass filter. 
     The rectangular microstrip line element has one longer side, the other longer side, one end and the other end. The input microstrip line is connected at the one end to the one longer side, and the output microstrip line is connected at the other end to the other longer side. 
     The composite elements adjacent to each other have respective input microstrip line and output microstrip line connected to each other and, the adjacent composite elements are symmetrical with respect to a center line between the input microstrip line and the output microstrip line connected to each other of the adjacent composite elements respectively. 
     Rectangular microstrip line elements of the composite elements differ in the length of longer side. 
     The rectangular microstrip line elements include outer microstrip line elements and inner microstrip line elements. The inner microstrip line elements have longer sides shorter than those of the outer microstrip line elements to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics. 
     Microstrip line elements of the composite elements are arranged symmetrically with respect to a center line of the arrangement of the composite elements, and the microstrip line filter includes a metal casing having a partition on the center line and covering microstrip line elements of the composite elements. 
     Microstrip line elements of the composite elements have respective input microstrip lines and respective output microstrip lines that connect the microstrip line elements and that have respective widths selected to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics. 
     A half-wave bandpass filter connected in series to the low-pass filter is further formed on the substrate. 
     The half-wave bandpass filter includes a plurality of rectangular microstrip line elements arranged in parallel with each other at predetermined intervals and inclined at a certain angle, and halves of respective longitudinal sides of the microstrip line elements are opposite to halves of respective longitudinal sides of adjacent microstrip line elements. 
     According to another aspect of the present invention, a high-frequency transmitter converts an intermediate-frequency signal into a high-frequency signal and transmits the high-frequency signal. The high-frequency transmitter includes a mixer circuit combining the intermediate-frequency signal with a local oscillator signal, a filter circuit connected to an output of the mixer circuit, and a high-frequency amplifier circuit connected to an output of the filter circuit. The filter circuit is formed on a substrate and includes a half-wave bandpass filter including a plurality of rectangular microstrip line elements that are arranged in parallel with each other at predetermined intervals and inclined at a certain angle, halves of respective longitudinal sides of the microstrip line elements being opposite to halves of respective longitudinal sides of adjacent microstrip line elements. The filter circuit further includes a low-pass filter including a plurality of composite elements arranged in parallel with each other and cascaded, the composite elements including respective rectangular microstrip line elements, respective input microstrip lines and respective output microstrip lines. 
     According to the present invention, the low-pass filter provides a large out-of-band attenuation and a small in-band deviation and accordingly has improved spurious elimination characteristics. Specifically, attenuation of at least 40 dB out of the passband above the higher limit of the passband is achieved all the time without deterioration in deviation within the passband and accordingly elimination of spurious above 14.95 GHz is possible. 
     In addition, the low-pass filter of the present invention has composite elements symmetrically arranged. Specifically, composite elements adjacent to each other are symmetrical with respect to the center line between respective input and output lines connected to each other. Accordingly, the low-pass filter occupies a minimum space as compared with composite elements that are simply cascaded. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a high-frequency transmitter including a microstrip line filter according to one embodiment of the present invention. 
     FIG. 2 shows a shape of an element of the microstrip line filter according to the embodiment of the present invention. 
     FIG. 3 shows a shape of a low-pass filter according to the embodiment of the present invention. 
     FIG. 4 shows a shape of the low-pass filter according to another embodiment of the present invention. 
     FIG. 5 shows respective shapes of the low-pass filter and a half-wave bandpass filter according to the present invention. 
     FIGS. 6A-6C show the low-pass filter housed in a metal casing according to the present invention, FIGS. 6A and 6B showing cross sections of principal parts of the low-pass filter and FIG. 6C showing a plan view thereof. 
     FIG. 7 shows signal pass characteristics of the half-wave bandpass filter and the low-pass filter shown in FIG. 5 connected in series, the characteristics being obtained through simulation. 
     FIG. 8 shows signal pass characteristics of a conventional half-wave bandpass filter obtained through simulation. 
     FIG. 9 shows cutoff characteristics of the low-pass filter of the present invention. 
     FIG. 10 shows cutoff characteristics obtained by connecting the half-wave bandpass filter and low-pass filter shown in FIG. 5 in series. 
     FIG. 11 schematically shows a system for two-way communication via a communication satellite. 
     FIG. 12 is a block diagram of a high-frequency transmitter used in the system shown in FIG.  11 . 
     FIG. 13 shows a shape of a half-wave bandpass filter used in the high-frequency transmitter shown in FIG.  12 . 
     FIG. 14 shows cutoff characteristics of a combination of the conventional half-wave bandpass filter and high-frequency amplifiers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of a high-frequency transmitter including a microstrip line filter according to one embodiment of the present invention. Referring to FIG. 1, high-frequency transmitter receives, as the conventional transmitter shown in FIG. 12, a transmission signal of an intermediate frequency ranging from 950 to 1450 MHz superimposed on a direct-current voltage. The intermediate-frequency signal is supplied via a high-pass filter (HPF)  401  to an IF amplifier  402  to obtain a certain gain, adjusted to a proper level by an attenuator  403 , further amplified by an IF amplifier  404 , and then supplied to a mixer  406  via a bandpass filter (BPF)  405 . 
     A local oscillator  407  generates a local oscillator signal of 13.05 GHz which is provided via a buffer amplifier  408  to mixer  406 . Mixer  406  combines the local oscillator signal of 13.05 GHz with the intermediate-frequency signal of 950-1450 MHz in order to convert the intermediate-frequency signal into a high-frequency signal of 14.0-14.5 GHz. The high-frequency signal supplied from mixer  406  is input to a half-wave bandpass filter  409  and a low-pass filter  417  characterizing the invention where an unwanted radiation component (spurious radiation component) of the high-frequency signal that is generated in mixer  406  is attenuated. 
     According to this embodiment, half-wave bandpass filter  409  and low-pass filter  417  are combined to achieve attenuation of frequencies higher than 14.95 GHz by at least 40 dB all the time. The high-frequency signal with its spurious component thus attenuated is then amplified by two high-frequency amplifiers  410  and  411  to obtain a great gain. 
     The output from high-frequency amplifier  411  is supplied to a bandpass filter  412  where the amplified spurious component is attenuated, and then supplied to a driver amplifier  413  to obtain a further gain. The output from driver amplifier  413  is supplied to a reception-bandwidth noise filter  414  where any noise level in a reception frequency range is substantially reduced to a thermal noise level. Then, the high-frequency signal is converted by a power amplifier  415  to a signal of high power required for transmission to the satellite. The high-frequency signal from power amplifier  415  is provided to a reception-bandwidth noise filter  416  where the noise level in the reception frequency range that is increased from the thermal noise level due to the gain of power amplifier  415  is attenuated, and then the signal supplied via noise filter  416  from high-frequency transmitter  4  is radiated as the microwave from a feed horn  8 , reflected by parabola antenna  9  and transmitted toward communication satellite  2  that are shown in FIG.  11 . 
     FIG. 2 shows a shape of an element of the microstrip line filter, as one component of low-pass filter  417  shown in FIG. 1, according to the embodiment of the present invention. 
     Referring to FIG. 2, the microstrip line filter uses, as a substrate material, a double-sided substrate (dielectric constant: 2.65, copper foil thickness: 20 μm, thickness: 0.61 mm). The line element  40  is rectangular in shape. An earth electrode of copper foil is formed on the entire rear surface of line element  40 . One of the longer sides of line element  40  has an end where an input microstrip line  41  is formed, and the other side of line element  40  has an end where an output microstrip line  42  is formed. The composite element is accordingly formed. 
     FIG. 3 shows a shape of the low-pass filter according to the embodiment of the present invention. Referring to FIG. 3, low-pass filter  417  shown in FIG. 1 includes line elements  40   a - 40   d  as shown in FIG.  2 . At least four line elements are cascaded each having input microstrip line  41  connected to output microstrip line  42  of an adjacent line element, and the line elements adjacent to each other are symmetrical with respect to a center line between the connected input microstrip line  41  and output microstrip line  42 . Preferably, line elements  40   a - 40   d  are symmetrical with respect to a center line which evenly divides the arrangement of the line elements. 
     The low-pass filter shown in FIG. 3 can be represented by a distributed constant circuit of LCR. 
     FIG. 4 shows a shape of the low-pass filter according to another embodiment of the present invention. According to this embodiment, in order to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics, central line elements  40   b  and  40   c  have longer sides that are shorter than those of outer line elements  40   a  and  40   d . Moreover, any width of the microstrip line connecting line elements  40   b  and  40   c  to each other is selected so as to obtain desired input/output impedance characteristics, in-band pass characteristics and out-of-band attenuation characteristics. 
     FIG. 5 shows the low-pass filter and the half-wave bandpass filter of the present invention. Low-pass filter  417  and half-wave bandpass filter  409  connected in series shown in FIG. 2 are formed on a substrate. Half-wave bandpass filter  409  includes a plurality of rectangular microstrip line elements  40   h  inclined at a certain angle and arranged in parallel with each other at predetermined intervals. The microstrip line elements  40   h  have respective halves of the longitudinal sides opposite to those of adjacent microstrip line elements  40   h.    
     FIGS. 6A-6C each show a principal part of the low-pass filter of the present invention housed in a metal casing. FIG. 6A shows a cross section along line VIA—VIA in FIG. 6B, FIG. 6B shows a cross section along line VIB—VIB in FIG. 6C, and FIG. 6C is a plan view of the metal casing. 
     Referring to FIG. 6B, a substrate  60  with a pattern  61  for the microstrip line filter formed thereon is mounted on a chassis  52 . A frame  50  has a rib  51  on pattern  61  on substrate  60  for reinforcing and shielding purposes. 
     In this way, patterns  61  of the microstrip line filter are covered with frame  50  and shielded from each other by rib  51  so as to reduce leakage of the spurious component to the outside. 
     FIG. 7 shows signal pass characteristics of the half-wave bandpass filter and the low-pass filter shown in FIG. 5 connected in series, the characteristics being obtained through simulation. Referring to FIG. 7, the passband of transmission frequencies is 14-14.5 GHz, and optimization is achieved by minimizing the loss within the passband (in-band loss) and maximizing the attenuation range out of the passband above 14.95 GHz (out-of-band attenuation). Specifically, the loss of the transmission frequency is 4 dB or less and the attenuation out of the passband above 14.95 GHz is at least 52 dB. 
     FIG. 8 shows signal pass characteristics of the conventional half-wave bandpass filter obtained through simulation. It is seen from FIG. 8 that the characteristics shown in FIG. 7 exhibit improvements in the amount of attenuation of 32.9 dB, i.e., from 19.1 dB to 52 dB, of the receiving frequency. Moreover, the steeper cutoff characteristics shown in FIG. 7 as compared with FIG. 8 show that the ability of reducing the spurious component is improved. 
     FIG. 9 shows cutoff characteristics of the low-pass filter of the present invention, and FIG. 10 shows cutoff characteristics of the combination of the half-wave bandpass filter and low-pass filter shown in FIG.  5  and high-frequency amplifiers  410  and  411 . 
     Low-pass filter  417  has cutoff characteristics as shown in FIG. 9 and, as shown in FIG. 10, overall characteristics of bandpass filter  409 , low-pass filter  417  and two-stage high-frequency amplifiers  410  and  411  exhibit the amount of attenuation of 47.3 dB at 14.95 GHz relative to the level in the passband. Here, this combination achieves the attenuation of 47.3 dB while the attenuation by the conventional bandpass filter  409  shown in FIG. 14 is merely 11.9 dB. It is thus seen that an improvement of 35.4 dB from 11.9 dB to 47.3 dB is obtained. In this way, this embodiment provides a greater amount of attenuation out of the passband and a smaller in-band deviation as compared with use of only the conventional half-wave bandpass filter  409  shown in FIG.  13 . Consequently, the spurious elimination feature is enhanced. 
     As heretofore discussed, according to the embodiment of the present invention, a plurality of composite elements each are constituted of a rectangular microstrip line element, an input microstrip line and an output microstrip line, and the composite elements are arranged in parallel and cascaded on a substrate to constitute a low-pass filter providing a large amount of attenuation out of the passband and a small deviation within the passband to be improved in the spurious elimination characteristics. Specifically, out-of-band attenuation of at least 40 dB is achieved all the time above the higher limit of the passband, without deterioration in in-band deviation characteristics, and accordingly, spurious elimination characteristics above 14.95 GHz is accomplished. 
     Moreover, the low-pass filter of the present invention includes the composite elements arranged so that the composite elements adjacent to each other are symmetrical with respect to the center line between connected input line and output line of respective composite elements adjacent to each other. The composite elements thus arranged occupy a minimum area as compared with the simply cascaded composite elements. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.