Abstract:
A microwave filter includes a strip transmission line and a filtering assembly connected to the strip transmission line. The transmission line has an input terminal and an output terminal. The filtering assembly includes a strip first primary branch connected directly to a first connection point of the transmission line. The first primary branch includes a first body portion and a first bent portion at a first end of the first body portion, where the first bent portion is connected directly to the first connection point, and the first body portion is substantially parallel to a body portion of the transmission line. With the microwave filter incorporated in an electric motor, the impedance curve of the transmission line can be matched with a high-frequency EMI curve, and therefore the filtering effect is enhanced, EMI is suppressed and the EMC level improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201410413316.3 filed in The People&#39;s Republic of China on Aug. 19, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to the technical field of microwave filtering and in particular, to an electric motor having a microwave filter. 
       BACKGROUND OF THE INVENTION 
       [0003]    An idealized LC filtering circuit is shown in  FIG. 1 , and a schematic diagram of an equivalent circuit of the LC filtering circuit in the practical application (particularly in the microwave domain) is shown in  FIG. 2 . In the practical application, an inductor L is equivalent to an idealized inductor LCM connected in parallel with a parasitic resistor (an equivalent parallel resistor EPR) and a parasitic capacitor (an equivalent parallel capacitor EPC), and a capacitor C is equivalent to an idealized capacitor Cy connected in serial with a parasitic inductor (an equivalent series inductor ESL/ 2 ) and a parasitic resistor (an equivalent series resistor EPR/ 2 ). 
         [0004]    Further, as shown in  FIG. 3 , a conductor has similar characteristics to the above description in a high-frequency domain. In  FIG. 3 , an actual wire is shown on the left side, and two equivalent models of the wire in the high-frequency domain are shown on the right side. 
         [0005]    A parasitic inductance and a parasitic capacitance of the conductor such as the inductor and the capacitor may influence the insertion loss and EMI (electromagnetic interference) of the circuit.  FIG. 4  shows a graph of a simulation result of the circuit shown in  FIG. 1 . Referring to  FIG. 4 , a curve  1  represents the insertion loss of a filter constructed with ideal elements, a curve  2  represents the insertion loss in the case where only the series parasitic inductance is taken into account, a curve  3  represents the insertion loss in the case where the parallel parasitic resistance and the series parasitic resistance are taken into account, a curve  4  represents the insertion loss in the case where the parallel parasitic capacitance is taken into account, and a curve  5  represents the insertion loss in the case where the above four parasitic parameters are taken into account. 
       SUMMARY OF THE INVENTION 
       [0006]    Since the electromagnetic interference (EMI) is significantly influenced, there is a need for a microwave filter in which the filtering effect is improved by adjusting an impedance curve of a transmission line. 
         [0007]    Accordingly, in one aspect thereof, the present invention provides a microwave filter, comprising: a strip transmission line having an input terminal and an output terminal, and a filtering assembly connected to the transmission line, wherein the filtering assembly comprises a strip first primary branch connected directly to a first connection point of the transmission line and a strip first secondary branch connected directly to the first primary branch, and wherein a width of the first secondary branch is different from a width of the first primary branch. 
         [0008]    Preferably, the first primary branch extends in a direction perpendicular to the transmission line. 
         [0009]    Preferably, the filtering assembly further comprises a strip second primary branch connected directly to a second connection point of the transmission line, and a second secondary branch connected directly to the second primary branch, wherein a width of the second secondary branch is different from a width of the second primary branch. 
         [0010]    Preferably, a width of the second primary branch is different from a width of the first primary branch. 
         [0011]    Preferably, the first primary branch and the second primary branch are arranged at two opposite sides of the transmission line respectively. 
         [0012]    Preferably, the transmission line is bent in a direction of the first primary branch on both sides of the first connection point to form a first “         ”-shaped portion; and the transmission line is bent in a direction of the second primary branch on both sides of the second connection point to form a second “         ”-shaped portion, wherein an opening of the first “         ”-shaped portion is oriented in an opposite direction to an opening of the second “         ”-shaped portion. 
         [0013]    Preferably, the transmission line further comprises a third “         ”-shaped portion for connecting the first “         ”-shaped portion to the second “         ”-shaped portion in series. 
         [0014]    According to a second aspect, the present invention provides a microwave filter, comprising: a strip transmission line having an input terminal and an output terminal, and a filtering assembly connected to the transmission line, wherein the filtering assembly comprises a strip first primary branch connected directly to a first connection point of the transmission line, wherein the first primary branch comprises a first body portion and a first bent portion at a first end of the first body portion, wherein the first bent portion is connected directly to the first connection point, and the first body portion is substantially parallel to a body portion of the transmission line. 
         [0015]    Preferably, the filtering assembly further comprises a strip second primary branch connected directly to a second connection point of the transmission line, the second primary branch comprises a second body portion and a second bent portion at a first end of the second body portion, wherein the second bent portion is connected directly to the second connection point, and the second body portion is substantially parallel to the body portion of the transmission line. 
         [0016]    Preferably, the body portion of the transmission line is arranged between the first body portion and the second body portion. 
         [0017]    Preferably, the body portion of the transmission line is arc-shaped. 
         [0018]    Preferably, a width of the body portion of the transmission line, a width of the first body portion and a width of the second body portion are different. 
         [0019]    Preferably, the filtering assembly further comprises a first secondary branch, the first secondary branch comprises a third body portion and a third bent portion at a first end of the third body portion, wherein the third bent portion is connected directly to a second end of the first body portion, and the third body portion is substantially parallel to the first body portion. 
         [0020]    Preferably, a pad is formed between a terminal and the first connection point of the transmission line. 
         [0021]    Preferably, a width of the transmission line and a width of the first primary branch are in a range between 0.15 mm and 1.1 mm. 
         [0022]    According to a third aspect, the present invention provides an electric motor, comprising a power supply circuit for the motor, wherein at least one microwave filter as defined above, is connected in series in the power supply circuit. 
         [0023]    Preferably, there are two microwave filters arranged in a same plane in the motor. 
         [0024]    Preferably, there are two microwave filters arranged in a ring shape in the motor. 
         [0025]    By implementing the invention, the impedance curve of the transmission line can be matched with a high-frequency EMI curve, and therefore the filtering effect is enhanced, EMI may be suppressed and the EMC level improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0027]      FIG. 1  is a schematic diagram of an ideal LC filtering circuit, and 
           [0028]      FIG. 2  is a schematic diagram of a practical LC equivalent filtering circuit; 
           [0029]      FIG. 3  shows an equivalent circuit of a conductor in a microwave domain; 
           [0030]      FIG. 4  shows different insertion losses of an LC circuit with different parasitic parameters being considered; 
           [0031]      FIG. 5  is a schematic diagram of a microwave filter according to a first embodiment of the disclosure; 
           [0032]      FIG. 6  is a schematic diagram of the microwave filter shown in  FIG. 5  integrated into a printed circuit board; 
           [0033]      FIG. 7  is a diagram of a simulation result of the microwave filter shown in  FIG. 5 ; 
           [0034]      FIG. 8  is a schematic diagram of a microwave filter according to a second embodiment of the disclosure; 
           [0035]      FIG. 9  is a schematic diagram of the microwave filter shown in  FIG. 8  integrated into a printed circuit board; 
           [0036]      FIG. 10  is a diagram of a simulation result of the microwave filter shown in  FIG. 8 ; 
           [0037]      FIG. 11  is a schematic diagram of a microwave filter according to a third embodiment of the disclosure; 
           [0038]      FIG. 12  is a schematic diagram of the microwave filter shown in  FIG. 11  integrated into a printed circuit board; 
           [0039]      FIG. 13  is a schematic diagram of a microwave filter according to a fourth embodiment of the disclosure; 
           [0040]      FIG. 14  is a schematic diagram of the microwave filter shown in  FIG. 13  integrated into a printed circuit board; and 
           [0041]      FIG. 15  is a schematic diagram of a motor incorporating the microwave filter according to the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]      FIG. 5  illustrates a microwave filter  30  according to a first embodiment of the present invention, having a strip transmission line  31  and a filtering assembly connected to the transmission line  31 . Two terminals  33 ,  34  of the transmission line  31  are configured as an input terminal and an output terminal respectively. The filtering assembly includes a strip first primary branch  36  connected directly to a first connection point of the transmission line  31  and a strip second primary branch  38  connected directly to a second connection point of the transmission line  31 . The filtering assembly further includes a strip first secondary branch  37  connected directly to the first primary branch  36  and a second secondary branch  39  connected directly to the second primary branch  38 . According to the first embodiment, the transmission line  31  and the second primary branch have a width of 0.2 mm, the first primary branch  36  has a width of 0.5 mm, and the first secondary branch  37  and the second secondary branch  39  have a width of 1.0 mm. 
         [0043]    According to the first embodiment, all of the first primary branch  36 , the first secondary branch  37 , the second primary branch  38  and the second secondary branch  39  extend in a direction perpendicular to the transmission line  31 . The first primary branch  36  and the second primary branch  38  are arranged on the same side of the transmission line  31 . 
         [0044]    Referring to  FIG. 6 , the microwave filter may be integrated into a printed circuit board or a flexible circuit board  51 . A via hole  53  may be pre-formed in the circuit board  51  for passing through a related wire or pin. 
         [0045]      FIG. 7  is a graph of simulation results for the microwave filter of  FIG. 5 . The horizontal axis represents frequency and the vertical axis represents bandwidths. The curve in  FIG. 7  shows that the microwave filter of  FIG. 5  effectively attenuates signals in the frequency band between 1.0 MHz and 2.0 MHz (the curve in  FIG. 7  is in compliance with the international standard CISPR25(2008)PK). The frequency band of the filter can be adjusted or tuned, primarily because the impedance of the transmission line  31  is changed due to the branches  36 ,  37 ,  38 ,  39 , since the impedance of the transmission line is typically varied depending on different element dielectric constants. 
         [0046]    Referring to  FIG. 8 , a microwave filter according to a second embodiment of the present invention is a variant of the microwave filter shown in  FIG. 5 . One of the variations is that the first primary branch  36  and the second primary branch  38  are connected on opposite sides of the transmission line  31 . Moreover, the transmission line  31  is bent in the direction of the first primary branch  36  on both sides of the first connection point to form a first “         ”-shaped portion (see the “         ”-shaped portion formed by sections  31   a ,  31   b  and  31   c  of the transmission line in  FIG. 8 ). The transmission line  31  is bent in the direction of the second primary branch  38  on both sides of the second connection point to form a second “         ”-shaped portion, and the opening of the first “         ”-shaped portion is oriented in the opposite direction to the opening of the second “         ”-shaped portion. The space occupied by the microwave filter is significantly reduced with this arrangement. 
         [0047]    Furthermore, the transmission line  31  further includes a third “         ”-shaped portion for connecting the first “         ”-shaped portion and the second “         ”-shaped portion in series. A similar “         ”-shaped portion may also be arranged between the first connection point and the terminal  33 , and the similar “         ”-shaped portion may also be arranged between the second connection point and the terminal  34 . The above first “         ”-shaped portion, second “         ”-shaped portion and third “         ”-shaped portion may be used to improve the filtering effect of the microwave filter. 
         [0048]    Similarly, as shown in  FIG. 9 , the microwave filter shown in  FIG. 8  may be integrated into a printed circuit board or a flexible circuit board  51 . A via hole  53  may be pre-formed in the circuit board  51  for passing through a related wire or pin. 
         [0049]      FIG. 10  is similar to  FIG. 7 , providing a graph of simulated results of the filter of  FIG. 8 . The horizontal axis represents frequency and the vertical axis represents bandwidths. The curve in  FIG. 10  shows that the microwave filter of  FIG. 8  may attenuate signals in a high frequency band. 
         [0050]    Referring to  FIG. 11 , according to a third embodiment of the disclosure, an independent microwave filter  30  and an independent microwave filter  40  are arranged facing each other to form a ring shape. Preferably, the microwave filter  30  and the microwave filter  40  are symmetrical about a center of the ring. The microwave filter  30  includes a strip transmission line  31 , two terminals  33 ,  34  of which are configured as an input terminal and an output terminal respectively. The filter assembly includes a strip first primary branch  36  connected directly to a first connection point of the transmission line  31 , the first primary branch  36  includes a first body portion  36   h  and a first bent portion  36   f  at a first end of the first body portion  36   h , where the first bent portion  36   f  is connected directly to the first connection point, and the first body portion  36   h  is substantially parallel to a body portion  31   h  of the transmission line  31 , that is, the first body portion  36   h  and the body portion  31   h  of the transmission line  31  are approximately evenly spaced or extend in the same direction. 
         [0051]    The filtering assembly further includes a strip second primary branch  38  connected directly to a second connection point of the transmission line  31 , the second primary branch  38  includes a second body portion  38   h  and a second bent portion  38   f  at a first end of the second body portion  38   h , where the second bent portion  38   f  is connected directly to the second connection point, and the second body portion  38   h  is substantially parallel to a body portion  31   h  of the transmission line  31 . 
         [0052]    According to the present embodiment, the body portion  31   h  of the transmission line  31  is arc-shaped. The body portion  31   h  of the transmission line  31  is arranged between the first body portion  36   h  and the second body portion  38   h . A width of the body portion  31   h  of the transmission line, a width of the first body portion  36   h  and a width of the second body portion  38   h  are different. Specifically, the body portion  31   h  has a width of 0.5 mm, and the first body portion  36   h  and the second body portion  38   h  have a width of 1.0 mm. 
         [0053]    Preferably, the transmission line  31  further includes a bent portion  31   f  formed between the first connection point and the terminal  33  and a bent portion  31   g  formed between the second connection point and the terminal  34 . The bent portion  31   f  and the bent portion  31   g  are used to improve the filtering effect of the microwave filter. 
         [0054]    The microwave filter  40  has a structure symmetrical to the structure of the microwave filter  30 , and thus the specific description is omitted. 
         [0055]    As shown in  FIG. 12 , the microwave filter shown in  FIG. 11  may be integrated into a printed circuit board or a flexible circuit board  51 . 
         [0056]    Referring to  FIG. 13 , a microwave filter according to a fourth embodiment of the disclosure is a variant of the microwave filter shown in  FIG. 11 . One of the variations is that the filtering assembly further includes a first secondary branch  37 , the first secondary branch  37  includes a third body portion  37   h  and a third bent portion  37   f  at a first end of the third body portion  37   h , where the third bent portion  37   f  is connected directly to a second end of the first body portion  36   h , and the third body portion  37   h  is substantially parallel to the first body portion  36   h . Another variation, as shown in  FIG. 14 , is that a bent portion is formed between the first connection point and the terminal  33  of the transmission line  31 , a first pad  31   k  is arranged at the bent portion, a second pad  31   m  is arranged adjacent to the first pad  31   k , and a capacitor may be connected to the first pad  31   k  and the second pad  31   m  by welding or soldering. The second pad  31   m  is connected to a circuit on the back side of the circuit board through a via hole  53 . 
         [0057]    According to the present embodiment, all of the body portion  31   h  of the microwave filter  30 , the first body portion  36   h , the second body portion  38   h  and the third body portion  37   h  have a width of 0.5 mm. 
         [0058]    According to the present embodiment, all of the body portion  41   h  of the transmission line  41 , the first body portion  46   h  of the first primary branch  46 , the second body portion  48   h  of the second primary branch  48  of the microwave filter  40  have a width of 0.5 mm, and the third body portion  47  of the first secondary branch  47  have a width of 1.0 mm. 
         [0059]    It should be noted that, in the above embodiments, the widths of the transmission lines  31  and  41 , the first primary branches  36  and  46 , the second primary branches  38  and  48  and the first secondary branches  37  and  47  may be varied as required, for example, may be increased or decreased by a range from 0.05 mm to 0.1 mm. A bent portion is formed between the first connection point and the terminal  33  of the transmission line  41 , a first pad  41   k  is arranged at the bent portion, a second pad  41   m  is arranged adjacent to the first pad  41   k , and a capacitor may be connected to the first pad  41   k  and the second pad  41   m  by welding or soldering. The second pad  41   m  is connected to a circuit on the back side of the circuit board through the via hole  53 . 
         [0060]    Referring to  FIG. 15 , a motor M according to an embodiment of the disclosure includes the microwave filters  30  and  40  according to the disclosure which are connected in series with the power supply circuit of the motor. Preferably, the motor is a brushed motor, and the microwave filters  30  and  40  are connected between terminals of the motor and the brushes, respectively. For example, the terminal  33  of the microwave filter  30  is connected between one of the terminals of the motor and one of the brushes, the terminal  43  of the microwave filter  40  is connected between the other terminal of the motor and the other brush. The terminals of the motor are connected to an external power supply, and the brushes are in sliding contact with a commutator to supply power to motor windings wound on the rotor. The applied microwave filter may be fitted within a housing of the motor, for example at the inner side of an end cap, or may be arranged at the outer side of the end cap. Preferably, the two microwave filters may be arranged in the same plane; more preferably, the two microwave filters may be printed on a common circuit board with the two microwave filters being arranged into a ring shape, as shown in  FIGS. 11 to 14 . The circuit board may be mounted outside of the end cap of the motor housing or inside the motor. It should be noted that the microwave filters shown in  FIG. 5  and  FIG. 8  may be also applied to the motor M. 
         [0061]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item or feature but do not preclude the presence of additional items or features. 
         [0062]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0063]    The embodiments described above are provided by way of example only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined by the appended claims.