Abstract:
The present invention is to provide a PCB based band-pass filter for cutting out harmonic of high frequency wherein in manufacturing a circuit board of a wireless communication product technology of PCB production is utilized to mount an input microstrip line, an output microstrip line, an open circuit microstrip line, and a short circuit microstrip line of the band-pass filter on the circuit board. By utilizing the present invention, it is possible of utilizing an unoccupied area of the circuit board to mount a band-pass filter thereon without using an additional filter of high frequency. Moreover, the band-pass filter is adapted to cut out harmonic of two, three, or four times of a fundamental frequency caused by nonlinear distortion of a power amplifier of the wireless communication product.

Description:
FIELD OF THE INVENTION 
   The present invention relates to band-pass filters and more particularly to a PCB (printed circuit board) based band-pass filter for cutting out harmonic of high frequency (e.g., harmonic of two, three, or four times of a fundamental frequency). 
   BACKGROUND OF THE INVENTION 
   In recent years there has been a significant growth in WLAN (wireless local network) due to the ever increasing demand of wireless communication products. Such growth is particularly obvious after the promulgation of IEEE 802.11 WLAN protocol in 1997. IEEE 802.11 WLAN protocol not only provides many novel features to the current wireless communications but also provides a solution of enabling two wireless communication products manufactured by different companies to communicate each other. As such, the promulgation of IEEE 802.11 WLAN protocol is a milestone of the development of WLAN. Moreover, IEEE 802.11 WLAN protocol ensures that core device is the only solution of implementing a single chip. Thus, it can significantly reduce the cost of adopting wireless technology so as to enable WLAN to be widely employed in various wireless communication products. 
   Conventionally, electromagnetic waves are susceptible of generation when a wireless communication product is transferring data in high power. And in turn, EMI (electromagnetic interference) may be caused by the electromagnetic waves. For solving problems associated therewith, many rules are promulgated by advanced countries. These rules impose a limitation on the import and use of wireless communication products found not complying therewith. In view of the above, the developers and manufacturers of wireless communication products have to take related rules into consideration in developing the control circuitry of the wireless communication product. Typically, a filter is provided after a power amplifier so as to cut out the generated harmonic spuriousness of high frequency. In such a manner, the produced wireless communication products are able to comply with the related rules. Conventionally, filters used in the production of the control circuitry of the wireless communication product are filters of high frequency made of ceramic material. The advantages of such filters of high frequency are that they can cut out harmonic of two or three times of a fundamental frequency and are much compact. The disadvantages thereof are expensive, requiring an additional installation procedure, cumbersome process, and much increased manufacturing cost. As an end, the production is low. 
   For solving the above problems, some manufacturers in the art use waveguide elements to simulate the desired filter circuit which is in turn employed to form a microstrip circuit  10  as illustrated in  FIG. 1 . Also, technology of PCB production is utilized in manufacturing circuit boards of wireless communication products in which the microstrip circuit  10  is formed on the circuit board. It is desired that the above configuration can effectively cut out harmonic spuriousness of at least two times of a fundamental frequency caused by nonlinear distortion of the power amplifier of the wireless communication product by means of the microstrip circuit  10  without using an additional filter of high frequency. However, the miniaturization of microstrip circuit contradicts the cut out range of harmonic spuriousness. As such, for achieving the purpose of cutting out a wider range of harmonic spuriousness (i.e., having a frequency of a larger number of times of a fundamental frequency) the produced microstrip circuit may be too large and complicated. As an end, it undesirably greatly limits applications of microstrip circuit, significantly increases the manufacturing difficulties, and compromises the goal of miniaturizing the circuit board and its product. 
   Harmonic of signals transmitted in high power is the most possible one that will not pass an EMI test conducted on a wireless communication product. This is particularly true for a signal having a harmonic of two, three, or four times of a fundamental frequency. Such is not acceptable. 
   Thus, it is desirable among developers and manufacturers of the art to provide wireless communication products complying with the related rules without greatly increasing the manufacturing cost and the size. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a PCB based band-pass filter for cutting out harmonic of high frequency wherein in manufacturing a circuit board of a wireless communication product technology of PCB production is utilized to mount an input microstrip line, an output microstrip line, an open circuit microstrip line, and a short circuit microstrip line of the band-pass filter on the circuit board. By utilizing the present invention, it is possible of utilizing an unoccupied area of the circuit board to mount a band-pass filter thereon without using an additional filter of high frequency. Moreover, the band-pass filter is adapted to cut out harmonic of two, three, or four times of a fundamental frequency caused by nonlinear distortion of a power amplifier of the wireless communication product. Additionally, the above drawbacks of the prior art associated with cutting out harmonic of at least two times of the fundamental frequency in developing wireless communication products can be eliminated. 
   In one aspect of the present invention, the input microstrip line and the output microstrip line are coupled together to form a first straight line, the open circuit microstrip line and the short circuit microstrip line are coupled together to form a second straight line cross and perpendicular to the first straight line, and the short circuit microstrip line is coupled to ground as a short circuit. As a result, a required band-pass filter can be formed on the circuit board by a simple microstrip circuit for greatly reducing the manufacturing cost of the wireless communication products. 
   In another aspect of the present invention, length of the short circuit microstrip line is about one fourth of wavelength of a fundamental frequency, length of the open circuit microstrip line is about one fourth of the wavelength of three times of the fundamental frequency, the short circuit microstrip lines are adapted to modify to be bent or curve slightly so as to reduce the area of the microstrip circuit without adversely affecting the function of the band-pass filter, and the smaller microstrip circuit is adapted to mount on a limited unoccupied area on the circuit board for greatly decreasing the area of the circuit board. 
   The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a conventional microstrip circuit mounted on a band-pass filter; 
       FIG. 2  schematically depicts a first preferred embodiment of band-pass filter according to the invention simulated by waveguide elements; 
       FIG. 3  is a plan view of the band-pass filter shown in  FIG. 2 ; 
       FIG. 4  is a frequency response graph by plotting dB versus frequency for the band-pass filter shown in  FIG. 3  mounted in a wireless communication product being measured after filtering frequency; 
       FIG. 5  schematically depicts a second preferred embodiment of band-pass filter according to the invention simulated by waveguide elements; 
       FIG. 6  is a frequency response graph by plotting dB versus frequency for a microstrip line formed of the band-pass filter shown in  FIG. 5  mounted in a wireless communication product being measured after filtering frequency; 
       FIG. 7  is a plan view of the microstrip circuit of a band-pass filter according to a third preferred embodiment of the invention; and 
       FIG. 8  is a plan view of the microstrip circuit of a band-pass filter according to a fourth preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Principles of a first preferred embodiment of the invention will be described in details below with respect to a wireless communication product complying with IEEE 802.11b WLAN protocol. 
   The fundamental frequency of a signal transmitted by the wireless communication product is at the range of 2.4 GHz to 2.5 GHz. Referring to  FIG. 2 , waveguide elements are used to simulate a band-pass filter in a design phase according to the invention. In the configuration of the band-pass filter, there are provided a first microstrip line  21 , a second microstrip line  23 , an open circuit third microstrip line  25 , and a short circuit fourth microstrip line  26 . One end of the first microstrip line  21  is coupled to an input port  20  to form an input of the band-pass filter and the other end thereof is coupled to a microstrip cross component  22 . The microstrip cross component  22  is in turn coupled to one end of the second microstrip line  23 . Note that the first microstrip line  21  and the second microstrip line  23  are coupled to opposite ends of the microstrip cross component  22  to form a straight line. The other end of the second microstrip line  23  is coupled to an output port  24  to form an output of the band-pass filter. The other two ends of the microstrip cross component  22  are coupled to the third microstrip line  25  and the fourth microstrip line  26  respectively so as to form another straight line. The other end of the third microstrip line  25  is open so as to form an open circuit microstrip line of the invention. The other end of the fourth microstrip line  26  is coupled to ground (i.e., short circuit) so as to from a short circuit microstrip line of the invention. 
   In the simulated configuration, the line impedance of each of the microstrip lines is 50 ohm and the line width is 18 mil. Length of each of the first microstrip line  21  and the second microstrip line  23  is 100 mil. Length of the open circuit third microstrip line  25  is 223.94 mil. Length of the short circuit fourth microstrip line  26  is 652.1 mil. The simulation signal transmitted by the wireless communication product in high power is fed to the input port  20  of the band-pass filter. Next, a frequency response graph is plotted by measuring a frequency of the signal at the output port  24 . It is obvious that the invention can cut out harmonic of two, three, or four times of a fundamental frequency. 
   Hence, the inventor develops a microstrip circuit by incorporating the simulated configuration of  FIG. 2  in manufacturing the circuit boards of wireless communication products. Also, technology of PCB production is utilized so as to mount the microstrip circuit on a surface of the circuit board of the wireless communication product as illustrated in  FIG. 3 . As shown, the microstrip circuit comprises an input microstrip line  30 , an output microstrip line  31 , an open circuit microstrip line  32 , and a short circuit microstrip line  33 . The input microstrip line  30  and the output microstrip line  31  are coupled together to form a first straight line. The open circuit microstrip line  32  and the short circuit microstrip line  33  are coupled together to form a second straight line cross and perpendicular to the first straight line. The short circuit microstrip line  33  is coupled to ground (i.e., short circuit). In the embodiment, length L 1  of the short circuit microstrip line  33  is about one fourth of the wavelength of a fundamental frequency (i.e., about 16 mm). This can implement a simple band-pass filter having a frequency range of 2.4 GHz to 2.5 GHz. Also, harmonic of two or four times of the fundamental frequency (e.g., 50 GHz or 10 GHz) can be prevented from passing (i.e., band rejection filtering). That is, at least harmonic of two or four times of the fundamental frequency can be cut out. Further, length L 2  of the open circuit microstrip line  32  is about one fourth of the wavelength of three times of the fundamental frequency (i.e., about 6 mm). This can implement a simple band rejection filter. Also, harmonic of three times of the fundamental frequency (e.g., 7.5 GHz) can be prevented from passing (i.e., band rejection filtering). Moreover, the frequency range cut out by the short circuit microstrip line  33  will not affect adversely the open circuit microstrip line  32  since the short circuit microstrip line  33  is open circuit with respect to the frequency range. Hence, at least harmonic of three times of the fundamental frequency can be cut out. As an end, a band-pass filter capable of cutting out harmonic of two, three, or four times of the fundamental frequency can be produced on the circuit board of the wireless communication product. 
   The signal transmitted by the wireless communication product in high power is fed to the input of the input microstrip line  30 . Next, a frequency response graph is plotted as shown in  FIG. 4 . It is seen from the frequency response graph that an insertion loss is 0.268 dB at a frequency of 2.4 GHz, an insertion loss is 0.357 dB at a frequency of 2.5 GHz, a cut out amount of harmonic (e.g., 5.03 GHz or 5.2 GHz) of two times of the fundamental frequency is 20 dB, a cut out amount of harmonic (e.g., 7.12 GHz or 7.58 GHz) of three times of the fundamental frequency is 20 dB, and a cut out amount of harmonic (e.g., 10.1 GHz or 10.3 GHz) of four times of the fundamental frequency is 20 dB. In view of the above, it is found that the band-pass filter of the invention can not only cut out harmonic of two times of the fundamental frequency but also cut out harmonic of three or four times of the fundamental frequency. 
   Referring to  FIG. 5 , there is shown a second preferred embodiment of the invention in which waveguide elements are used to simulate a band-pass filter having a wider band rejection width in a design phase. The widths and lengths of a third microstrip line  55  and a fourth microstrip line  56  are changed in order to simulate various band-pass filters. It is found from an experiment that if the fourth microstrip line  56  having a width of 652.1 mil (see  FIG. 2 ) is decreased to 605 mil (see  FIG. 5 ) the produced band-pass filter has a wider band rejection width under the same conditions. Also, technology of PCB production is utilized so as to mount the microstrip circuit on a surface of the circuit board of the wireless communication product as illustrated in  FIG. 5 . The signal transmitted by the wireless communication product in high power is then fed to the input of the band-pass filter. Next, a frequency response graph is plotted at  FIG. 6  by measuring a frequency of the signal at the output. It is seen from the frequency response graph that an insertion loss is 0.155 dB at a frequency of 2.4 GHz, an insertion loss is 0.324 dB at a frequency of 2.5 GHz, a cut out amount of harmonic (e.g., 4.78 GHz or 5.28 GHz) of two times of the fundamental frequency is 20 dB, a cut out amount of harmonic (e.g., 7.15 GHz or 7.62 GHz) of three times of the fundamental frequency is 20 dB, and a cut out amount of harmonic (e.g., 9.69 GHz or 10.3 GHz) of four times of the fundamental frequency is 20 dB. In view of the above, it is found that the band rejection width at a frequency of about 5 GHz or about 10 GHz of the second embodiment is wider than that of the first embodiment. 
   Moreover, Referring to  FIG. 3  again, length of the short circuit microstrip line  33  of the band-pass filter is much longer than that of the open circuit microstrip line  32  in the invention. Referring to  FIGS. 7 and 8 , for preventing the produced circuit board from being too large due to large microstrip circuit, short circuit microstrip lines  73  and  83  have to be bent or curve slightly so as to reduce the area of the microstrip circuit without adversely affecting the function of the band-pass filter. The smaller microstrip circuit is then adapted to mount on a limited unoccupied area on a circuit board of a wireless communication product. As an end, the area of the circuit board is much reduced. 
   While the invention has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.