Abstract:
The invention discloses a dual frequency multiplexer by which a first and second coaxial harmonic oscillator type band pass filters are disposed in a box. The box includes a base body, a cover plate and a cover body. The two coaxial harmonic oscillator type hand pass filters are located on the base body and spaced each other by a metal plate; the multiplexer port, first and second ports are positioned on lateral side of the base body. The blocking capacitors are contained in the coaxial chamber of the two coaxial harmonic oscillator type band pass filters. The cover plate is secured on the base body; the first and second direct current circuits are placed on the cover plate; the low pass filters of the first and second direct current circuits are fixed on an edge of a top surface of the coaxial chamber by means of a support member; the cover body and the base body are fastened with each other. The blocking capacitors each are of distributed parameter capacitor. Utilization of distributed blocking capacitors makes the product of the invention small. Moreover, improvement of the structure of the invention brings effect such as less differential loss, large power capacity, as well as high isolation degree between circuits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national phase entry under 35 U.S.C. §371 of International Application No. 2007/CN2007/001161 filed Apr. 11, 2007, published in Chinese, which claims the benefit of Chinese Patent Application No. 200710027116.4, filed Mar. 12, 2007. The disclosures of said applications are incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a processing device for multiplexing between the second generation and third generation communication systems, and more particularly relates to dual frequency multiplexer. 
     BACKGROUND OF THE INVENTION 
     With rapid development of mobile communications, a scheme in which multiple systems share a common station as well as a common antenna feeder resource has gotten its population by more and more operators. By this way, advantage of sharing resource and reducing system device cost can be obtained. Within a system in which 2G/3G sharing a common antenna feeder, dual frequency multiplexer is a necessary microwave component that mainly serves to multiplex/de-multiplex signals of different systems so as to save the length of feed cable, simplify system construction and reduce cost. In addition, power is supplied to the devices at base station tower via a radio frequency cable and accordingly, the multiplexer connected with the feed cable must have the ability of passing direct current there through. 
     With reference to the schematic diagram of  FIG. 1 , a multiplexer is a microwave component having three ports, i.e., two direct current feed circuits and two RF signal circuits, wherein each direct current feed circuit is constructed by a lump parameter low pass filter, a switch and a lightning protection device. The low pass filter is used to suppress RF signal at high frequency such that control signal at certain frequency (such as 3 MHz) can pass the filter with ease. The switch serves to selectively conduct direct current there through. The RF signal circuit consists of a blocking capacitor and a band pass filter. The band pass filters of respective two RF signal circuits have their band pass range be set so as to be suited to frequency range of two signals to be multiplexed. During operation, the signal input from a common port Port 1  is shunted to Port 2  or Port 3  according to the frequency range. Alternatively, the signal input from ports Port 2  or Port 3  may also be combined and output via the port Port 1 . 
     The RF signal frequency range of the system in which 2G/30 sharing a common antenna feeder is 806 MHz-960 MHz and 1710 MHz-2170 MHz respectively. By now, to obtain such wide a work frequency band and capability of passing direct current, most multiplex products employ dielectric substrates and realize it by micro-strip circuits. The disadvantages of product of this type include large bulkage and low power capacity. Moreover, inactive inter-modulation is greatly depended on property of dielectric substrate material and therefore, it is difficult to control the dielectric substrate material during batch production. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a dual frequency multiplexer which can be minimized in size, reduce differential loss, obtain large power capacity, and have high isolation degree between direct current circuit and RF signal circuit. 
     To this end, the invention utilizes the following technical scheme. 
     The dual frequency multiplexer of the invention includes a multiplexer port, a first port used to receive corresponding a first frequency band, a second port used to receive corresponding a second frequency band, two coaxial harmonic oscillator type band pass filters and two direct current circuits. 
     The first direct current circuit is connected between the first port and the multiplexer port, while the second direct current circuit is connected between the second port and the multiplexer port. 
     The first coaxial harmonic oscillator type band pass filter is connected electrically with the first port through a first blocking capacitor, while the second coaxial harmonic oscillator type band pass filter is connected electrically with the second port through a second blocking capacitor. The other ends of both first and second coaxial harmonic oscillator type band pass filters are electrically coupled to the multiplexer port by a third blocking capacitor. 
     The first direct current circuit includes a first low pass filter coupled electrically with the first blocking capacitor, whilst the second direct current circuit includes a second low pass filter coupled electrically with the second blocking capacitor. The first and second direct current circuits share a common third low pass filter which is connected in electrical manner with the third blocking capacitor. 
     The first and second coaxial harmonic oscillator type band pass filters are disposed in a box including a base body, a cover plate and a cover body. The two coaxial harmonic oscillator type band pass filters are located on the base body and spaced each other by a metal plate. The multiplexer port, first and second ports arc positioned on lateral side of the base body. The blocking capacitors are contained in the coaxial chamber of the two coaxial harmonic oscillator type band pass filters. The cover plate is secured on the base body. The first and second direct current circuits are placed on the cover plate. The low pass filters of the first and second direct current circuits are fixed on an edge of a top surface of the coaxial chamber by means of a support member. The cover body and the base body are fastened with each other. 
     Preferably, a gap with a width not less than 0.2 mm is defined between the top surface of the support member and a bottom surface of the cover plate in order to maintain good electrical. performance of the RF signal. 
     Preferably, the blocking capacitors each are of distributed parameter capacitor. 
     More particularly, each blocking capacitor includes an inner conductor, an insulator and a sleeve. The insulator surrounds the inner conductor at outer perimeter thereof, while the sleeve surrounds the outer perimeter of the insulator. The sleeve serves to electrically connect the first and/or second coaxial harmonic oscillator type band pass filter. The inner conductor is used to connect electrically with the first and/or second direct current circuits so as to be connected with an adjacent port. 
     The first and second coaxial harmonic oscillator type band pass filters each a coaxial chamber and a plurality of harmonic posts arranged in the chamber sequentially. For the two coaxial harmonic oscillator type band pass filters, a ridge is disposed between two adjacent harmonic posts for purpose of enhancing coupling effect. Preferably, the first coaxial harmonic oscillator type band pass filter has 5 harmonic posts, while the second coaxial harmonic oscillator type band pass filter has 6 harmonic posts. 
     Furthermore, corresponding to the two coaxial harmonic oscillator type band pass filters, the cover plate has several turning screws which pass through the cover plate and extend into the two coaxial chambers with the aim of adjustment of tuning frequency and coupling degree of the coaxial harmonic oscillators. 
     The cover plate further has a through hole defined therein with which a Gore permeable film is covered. 
     Compared with prior art, the invention can obtain the following advantages. The 2G/3G dual frequency multiplexer of the invention is implemented by a plurality of coaxial harmonic oscillator type band pass filters. By this way, the direct current circuit and the RF circuit are isolated from each other. Utilization of distributed blocking capacitors makes the product of the invention small. Moreover, improvement of the structure of the invention brings effect such as less differential loss, large power capacity, as well as high isolation degree between circuits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of the invention; 
         FIG. 2  illustrates a perspective view of the product of the invention; 
         FIG. 3  illustrates an enlarged view of portion A of  FIG. 2 ; 
         FIG. 4  shows a cross-sectional view of a first coaxial harmonic oscillator type band pass filter of  FIG. 2 ; 
         FIG. 5  shows a cross-sectional view of a second coaxial harmonic oscillator type band pass filter of  FIG. 2 ; 
         FIG. 6  shows a view of a direct current circuit board of a cover plate of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 2 , a dual frequency multiplexer of the invention is shown, which is used mainly to multiplex 2G and 3g signals. 
     As shown in  FIG. 2 , the multiplexer is of a box type, and is constructed by a base body  6 , a cover plate  2  and a cover body  4  collectively. 
     A first port Port 2  and a second port Port 3  are provided at left side of the base body  6 , both of which are adapted to receive radio frequency (RF) signals with frequency of 806-960 MHz and 1710-2170 MHz respectively. A multiplexer port Port 1  is located at the right side of the base body  6 . The multiplexer port Port 1  is able to output RF signal multiplexed by the first and second ports (Port 2  and Port 3 ). Alternatively, an input signal can be de-multiplexed through the first and second ports (Port 2  and Port 3 ). 
     Two RF circuits, that is, a first RF circuit and a second RF circuit are integrated in the base body  6 . The first RF circuit is consisted of the first port Port 2 , a blocking capacitors (not shown, see a third blocking capacitor  68 ), two coaxial harmonic oscillator type band pass filters ( 610  and  611 ), a third blocking capacitor  68  and a multiplexer port, all of these components being connected with each other electrically. The second RF circuit is consisted of the second port Port 3 , a second blocking capacitors (not shown, see a third blocking capacitor  68 ), two coaxial harmonic oscillator type band pass filters ( 620  and  621 ), a third blocking capacitor  68  and a multiplexer port, all of these components being connected with each other electrically. 
     It is clear that each RF circuit includes not only coaxial harmonic oscillator type band pass filters ( 610  and  611 ;  620  and  621 ), but also the third blocking capacitor  68  in common. 
     Each of coaxial harmonic oscillator type band pass filters ( 610  and  611 ;  620  and  621 ) includes corresponding coaxial chambers  610  and  620  and plural harmonic posts  611  and  621 . As illustrated in  FIG. 2 , a compartment defined in middle portion of the base body  6  is divided into two coaxial chambers  610  and  620  by a metal plate  63 . A first coaxial chamber  610  is corresponding to the first RF circuit, while a second coaxial chamber  620  is corresponding to the second RF circuit. The separation caused by the metal plate  63  can result in higher isolation between the first and second RF circuits. Five harmonic posts  611  are sequentially arranged in the first coaxial chamber  610 . A harmonic post close to the first port Port 2  is electrically coupled to the first blocking capacitor (see  68 ) via a wire, thereby further being coupled with the first port Port 2 . Another harmonic post located far away from the first port Port 2  is electrically coupled to the third blocking capacitor  68  via a wire  671 . Similarly, six harmonic posts  621  are sequentially arranged in the second coaxial chamber  620 . A harmonic post close to the second port Port 3  is electrically coupled to the second blocking capacitor (see  68 ) via a wire, thereby further being coupled with the second port Port 3 . Another harmonic post located far away from the second port Port 3  is electrically coupled to the third blocking capacitor  68  via a wire  672 . 
     The coaxial chambers  610  and  620  are not completely isolated by the metal plate  63  located between the two coaxial harmonic oscillator type band pass filters ( 610  and  611 ;  620  and  621 ). 
     The first and second blocking capacitors have the same construction as that of the third capacitor  68 . The third capacitor  68  includes an inner conductor  683 , an insulator  682  and a sleeve  681 . The insulator  682  surrounds the inner conductor  683 , while the sleeve  681  surrounds the insulator  682 . The insulator  682  is implemented by medium film. The sleeve  681  is electrically coupled with the final posts of respective first and second coaxial harmonic oscillator type band pass filters ( 610  and  611 ;  620  and  621 ). The inner conductor  683  is coupled with the multiplexer port Port 1  directly. As such, the sleeve  681  and inner conductor  683  can be isolated from each other by the insulator  682  so as to construct distributed parameter capacitor. For two RF circuits, transmission of RF signal is carried out by coupling between the inner conductor  683  and sleeve  681  with direct current being blocked to pass the sleeve  681 , thereby making it possible for the RF circuit to block direct current. 
     As described above, the first and second blocking capacitors employ the same construction as the third blocking capacitor  68 . However, the sleeve of the first blocking capacitor (not shown) is connected only to a harmonic post adjacent the second blocking capacitor, of the first coaxial harmonic oscillator type band pass filter, whilst the sleeve of the second blocking capacitor (not shown) is connected only to a harmonic post adjacent the second blocking capacitor, of the second coaxial harmonic oscillator type band pass filter. 
     The inner conductors of corresponding blocking capacitors are extended out of and electrically connected with the ports Port 1 , Port 2  and Port 3  respectively. 
     The cover plate  2  has a printed circuit board supported thereon. A circuit shown in  FIG. 6  is printed on said printed circuit board. The cover plate  2  is just covered on the top surfaces of the two coaxial chambers  610  and  620 . Direct circuits of the dual multiplexer are techniques of well known and description thereof is set forth below in brief. 
     Referring to  FIG. 1  and  FIG. 6 , two direct current circuits, namely a first direct current circuit and a second direct current circuit are incorporated into the cover  2 . Each direct current circuit is mainly constructed of low pass filters  201 ,  202  and  203 , switches, lightning protection components  205 . Signals coming from the first/second ports Port 2 /Port 3  of the first/second direct current circuits are filtered by a first/second low pass filter  202 / 203 , then combined together and outputted to the third low pass filter  201 , and finally outputted to the multiplexer port Port 1 . The low pass filters  201 ,  202  and  203  serve to suppress high frequency signal while permitting transmission of control signal with a frequency less than 3 MHz. Moreover, a switch may be provided between the two direct current circuits depending upon need to determine whether transmission of direct current is allowed. Lightning protection component  205  made of discharging tube may be further provided. 
     The printed circuit diagram of  FIG. 6  shows three connection nodes  281 ,  282  and  283  of the low pass filters  201 ,  202  and  203 . The low pass filters  201 ,  202  and  203  are supported independently on three support members. As shown in  FIG. 1 , the three support members are disposed on top edges of the openings of the two coaxial chambers  610  and  620 , and are adjacent to respective ports Port 1 , Port 2  and Port 3 . Each support member has respective low pass filters  201 ,  202  and  203  supported thereon. 
     One ends of respective low pass filters  201 ,  202  and  203  are connected to blocking capacitors of adjacent ports Port 1 , Port 2 , Port 3 . More specifically, one end of the third low pass fitter  201  is electrically connected with the inner conductor  683  of the third blocking capacitor  68 ; one end of the first low pass filter  202  is electrically connected with the inner conductor of the first blocking capacitor (not shown); while one end of the second low pass filter  203  is electrically connected with the inner conductor of the second blocking capacitor (not shown). The other ends of the low pass filters  201 ,  202  and  203  have corresponding contact pads  26  which contact with the connection nodes  281 ,  282  and  283  of the printed circuit board shown in  FIG. 6 . Corresponding to three contact pads  26 , three holes are defined in the cover plate  2 . Engagement of three holes defined in the cover plate  2  with three contact pads  26  ensures fixation between the cover plate  2  and base body  6 . At this time, three connection nodes  281 ,  282  and  283  of the printed circuit board shown in  FIG. 6  contact with the contact pads  26  of three low pass filters  201 ,  202  and  203  of three support members respectively, thus resulting connection of the low pass fillers  201 ,  202  and  203  with the direct current circuit. A gap with a width not less than 0.2 mm is defined between the top surface of the support member and the cover plate in order to maintain good electrical performance of the RF signal. 
     The switches are implemented by some magnetic beads  208  welded onto the circuit board to suppress high frequency signal. Disconnection can be established by removing the magnetic beads  28 , and resumption of connection can be obtained by placing the same thereon. 
     As illustrated in  FIG. 2 , the third low pass filter  201  is electrically coupled with the multiplexer port Port 1  by means of connection of the wire  272  with the inner conductor  683  of the third blocking capacitor  68 . The same principle applies to the second and third low pass filters  202  and  203 . By this way, it is realized that both the direct current circuit and RF circuit are coupled with the multiplexer port Port 1 . 
     As illustrated in  FIG. 1 , several tuning screws  69  are located at two lateral sides corresponding to two coaxial harmonic oscillator type band pass filters ( 610  and  611 ;  620  and  621 ) of the base body  6 , of the cover plate  2 . Nine tuning screws  69  are disposed at one side corresponding the first coaxial harmonic oscillator type band pass filters ( 610  and  611 ); while the number of the tuning screws  69  located at the other side is  11 . The tuning screws  69  pass the cover plate  2 . When the cover plate  2  and the base body  6  are secured each other, the screws  69  can extend into the interior of the two coaxial chambers  610  and  620 . The screws  69  are used to adjust tuning frequency and coupling degree of the coaxial harmonic oscillators ( 610  and  611 ;  620  and  621 ). 
     With reference to  FIG. 4  which is a cross sectional view of the first coaxial harmonic oscillators ( 610  and  611 ), and belongs to a first RF circuit between the first port Port 2  and the second port Port 3 . To achieve strong coupling among the harmonic posts  611 , a ridge  616  is formed between two adjacent harmonic posts  611 . The height of various ridges  616  may vary and be regulated according to certain situation. Further, a disk is provided on top portion of each harmonic post  611 . A gap with a width preferably not less than 1.5 mm is defined between the harmonic posts and the cover plate  2 . This is for making the size of the coaxial chambers as small as possible during operation of the harmonic posts  611  at a frequency of 806-960 MHz. 
     With reference to  FIG. 5 , which is a cross sectional view of the second coaxial harmonic oscillators ( 620  and  621 ), and belongs to a second RF circuit between the second port Port 2  and the multiplexer port Port 1 . Similarly, ridges  626  with various heights can be placed between respective harmonic posts  621  so as to obtain strong coupling between the harmonic posts  621 . Pass band range is within 1710-2170 MHz. 
     With reference to  FIG. 2  again, which shows the cover body  4  used to cover the base body  6 , thereby protecting components inside the base body  6 . A rubber ring is provided at the perimeter of the cover body  4  so as to enhance waterproof performance and protect the inner circuit. A Through hole may be defined in the surface of the cover plate  2 . A Gore permeable film  40  may be covered on the through hole to keep pressure balance between inside and outside of the base body  6 . 
     In addition, the inner surfaces of the two coaxial chambers  610  and  620  are coated with silver, thereby greatly reducing attenuation of RF signal transmission and keeping insertion loss of the signal within the pass band less than 0.2 dB. 
     In summary, the invention overcomes drawbacks of prior art and brings advantages as follows. 
     The size of the dual frequency multiplexer of the invention can be reduced even to 174 mm*105 mm*61 mm. The sleeve typed coupling construction sufficiently utilizes a room through which the inner conductor of the multiplexer port Port 1  passes. Therefore, coupling of RF signal is obtained, and no additional space is necessary. Addition of the lump parameter low pass filters between the direct current circuit and RF circuit ensures isolation between the direct current circuit and RF signal circuit, and reduces largely the size of the printed circuit board on the cover plate. 
     High isolation is realized. Because each RF circuit is of completely sealed waveguide chamber construction, isolation between the circuits is improved largely. The isolation provided by the first port Port 1  to RF signal at frequency of 1710-2170 MHz is larger than 85 dB, and the isolation provided by the second port Port 3  to RF signal at frequency of 806-960 MHz is larger than 65 dB. 
     Higher capacity is realized. As a sufficiently wide of gap is defined between each harmonic post within the coaxial chamber and walls of the coaxial chamber, RF signal power support ability of the components is enhanced. Mean power supported by each port is up to 250 watts.