Patent Abstract:
A method for cancelling magnetic coupling in an amplifier is disclosed. The amplifier includes a first path and a second path for outputting a first signal and a second signal, respectively, and the first signal and the second signal have a specific phase difference. The method includes forming a first LC tank and a second LC tank in the first path, and forming a third LC tank and a forth LC tank in the second path.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and amplifier for cancelling magnetic coupling, and more particularly, to a method and amplifier capable of reducing in-phase/quadrature-phase (IQ) phase imbalance and gain imbalance, and minimizing layout area. 
     2. Description of the Prior Art 
     In communication systems, a carrier is frequently utilized for carrying baseband signals that contain data. Generally, a carrier is a high frequency signal. However, due to bandwidth limitation, a transmitter adopts a modulation scheme with high bandwidth efficiency. A quadrature amplitude modulation (QAM) is one of frequently utilized modulation schemes. 
     Generally, in a QAM system, signals are processed in two different paths. Ideally, the incoming signals are multiplied by an in-phase carrier (in-phase carrier) g sin wt and a quadrature-phase carrier g cos wt two mixers for modulation in the two paths, respectively, wherein g is the gain, and w is the angular frequency. In practice, factors such as magnetic coupling between inductors, temperature, process and supply voltage offset may result in gain imbalance and phase imbalance between the in-phase carrier g sin wt and the quadrature-phase carrier g cos wt, i.e. in-phase/quadrature-phase (IQ) imbalance. In other words, the oscillating signals utilized by the mixers may become (g+α)sin wt and g cos(wt+θ), where α is the gain imbalance and θ is the phase imbalance. In such a condition, there will be gain imbalance and phase imbalance between the mixed in-phase signal and the mixed quadrature-phase signal. 
     For example, please refer to  FIG. 1A , which is a schematic diagram of a conventional quadrature amplifier  100 . For clearly illustrating, the quadrature amplifier  100  shown in  FIG. 1A  only includes differential amplifiers  102 ,  104  and inductors  106 ,  108 , while other components of the quadrature amplifier  100  are not shown. The inductors  106 ,  108  form inductors of two inductor capacitor (LC) tanks, respectively. The amplifier  102  and the inductor  106  are in an in-phase path (I path), and the amplifier  104  and the inductor  108  are in a quadrature-phase path (Q path), wherein a distance between the inductors  106  and  108  is H. In the quadrature amplifier  100 , terminals IN and IP in the in-phase path output an in-phase negative signal S 1  and an in-phase positive signal S 2 , respectively, and terminals QN and QP in the quadrature-phase path output a quadrature-phase negative signal S 3  and a quadrature-phase positive signal S 4 , respectively. Ideally, a phase difference between the in-phase positive signal S 2  and the quadrature-phase positive signal S 4  is 90 degree. However, in order to keep carrier frequency within a range, inductors are utilized to achieve band-pass effect, and magnetic coupling between inductors may induce magnetic fields and generate corresponding induced currents, causing IQ imbalance. 
     For example, as shown in  FIG. 1B  to  FIG. 1D , which are schematic diagrams of IQ phase imbalance between the in-phase positive signal S 2  and the quadrature-phase positive signal S 4  when the inductors  106  and  108  shown in  FIG. 1A  have different coil winding directions. In  FIG. 1B , coil winding directions of the inductors  106  and  108  are clockwise, and thus magnetic field directions are downwards through paper. In such a situation, magnetic coupling between inductors having magnetic fields with a same direction generates induced currents, which shifts the in-phase positive signal S 2  and the quadrature-phase positive signal S 4  from original solid lines to dotted lines. Take a point A shown in  FIG. 1B  as an example, ideally, a phase of the in-phase positive signal S 2  is 0 degree and a phase of the quadrature-phase positive signal S 4  is 90 degree, such that a phase difference in between is 90 degree. However, after being affected by the induced currents generated by magnetic coupling between inductors having magnetic fields with the same direction, the phase of the in-phase positive signal S 2  is greater than 0 degree and the phase of the quadrature-phase positive signal S 4  is less than 90 degree, such that the phase difference in between is less than 90 degree, causing IQ phase imbalance. 
     Similarly, in  FIG. 1D , coil winding directions of the inductors  106  and  108  are counterclockwise, and thus magnetic field directions are upwards through paper. Magnetic coupling between inductors having magnetic fields with a same direction generates induced currents, which results a phase difference between the in-phase positive signal S 2  and the quadrature-phase positive signal S 4  less than 90 degree, causing IQ phase imbalance. Similarly, in  FIG. 1C , coil winding directions of the inductors  106  and  108  are clockwise and counterclockwise, respectively, and thus magnetic field directions are downwards through paper and upwards through paper, respectively. Take a point C shown in  FIG. 1C  as an example, after being affected by induced currents generated by magnetic coupling between inductors having magnetic fields with opposite directions, a phase of the in-phase positive signal S 2  less than 0 degree and a phase of the quadrature-phase positive signal S 4  is greater than 90 degree, such that a phase difference in between is greater than 90 degree, causing IQ phase imbalance. 
     In the prior art, in order to reduce IQ imbalance caused by magnetic coupling between inductors, the distance H between the inductors  106  and  108  is required to be very long, so as to reduce induced currents. As a result, large layout area is required, and magnetic coupling effect can not be totally eliminated, wherein the issue of IQ imbalance can not be effectively solved. Thus, there is a need for improvement of the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a method and amplifier for cancelling magnetic coupling. 
     The present invention discloses a method for cancelling magnetic coupling in an amplifier. The amplifier includes a first path and a second path for outputting a first signal and a second signal, respectively. The second signal and the first signal have a specific phase difference. The method includes steps of forming a first inductor capacitor (LC) tank and a second LC tank in the first path; and forming a third LC tank and a fourth LC tank in the second path. 
     The present invention further discloses an amplifier capable of cancelling internal magnetic coupling between inductors on different paths. The amplifier includes a first path, for outputting a first signal, comprising a first inductor capacitor (LC) tank and a second LC tank; and a second path, for outputting a second signal, comprising a third LC tank and a fourth LC tank, the second signal and the first signal having a specific phase difference. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram of a conventional quadrature amplifier. 
         FIG. 1B  to  FIG. 1D  are schematic diagrams of IQ phase imbalance between an in-phase positive signal and a quadrature-phase positive signal when the inductors shown in  FIG. 1A  have different coil winding directions. 
         FIG. 2A  is a schematic diagram of an amplifier according to an embodiment of the present invention. 
         FIG. 2B  is a schematic diagram of magnetic coupling between inductors shown in  FIG. 2A . 
         FIG. 3A  and  FIG. 3B  are schematic diagrams of inductors shown in  FIG. 2A  having different coil winding directions. 
         FIG. 4A  is a schematic diagram of IQ imbalance magnitude of the quadrature amplifier shown in  FIG. 1A . 
         FIG. 4B  is a schematic diagram of IQ imbalance magnitude of the amplifier shown in  FIG. 2A . 
         FIG. 5A  is a schematic diagram of an amplifier according to another embodiment of the present invention. 
         FIG. 5B  is a schematic diagram of magnetic coupling between inductors shown in  FIG. 5A . 
         FIG. 6A  and  FIG. 6B  are schematic diagrams of inductors shown in  FIG. 5A  having different coil winding directions. 
         FIG. 7  is a schematic diagram of a process according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2A , which is a schematic diagram of an amplifier  200  according to an embodiment of the present invention. For simplicity, the amplifier  200  shown in  FIG. 2A  only includes an in-phase path (I path), for outputting an in-phase negative signal S 1 ′ and an in-phase positive signal S 2 ′ at a terminal IN and a terminal IP, respectively, and a quadrature-phase path, for outputting a quadrature-phase negative signal S 3 ′ and a quadrature-phase positive signal S 4 ′ at a terminal QN and a terminal QP, respectively. In detail, the in-phase path includes differential amplifiers  202 ,  204  and inductors  206 ,  208 . The quadrature-phase path includes differential amplifiers  210 ,  212  and inductors  214 ,  216 . The inductors  206 ,  208 ,  214 ,  216  form inductors of inductor capacitor (LC) tanks, respectively. The in-phase path and the quadrature-phase path are cross, i.e. the differential amplifiers  202 ,  212  and the inductors  206 ,  216  are in parallel with the differential amplifiers  210 ,  204  and the inductors  214 ,  208 . Noticeably, the amplifier  200  can further include components such as feedback circuit, which is not limited to this, and amplifiers  202 ,  204 ,  210 ,  212  are not limited to differential amplifiers. 
     By increasing an amount of LC tanks and adjusting relative positions and coil winding directions, i.e. magnetic field directions, of the inductors  206 ,  208 ,  214  and  216  in the amplifier  200 , the magnetic coupling between inductors can effective cancelled, so as to reduce IQ imbalance. Thus, a phase difference between the in-phase positive signal S 2 ′ and the quadrature-phase positive signal S 4 ′ is substantially 90 degree, and layout area is minimized. Please refer to  FIG. 2B , which is a schematic diagram of the magnetic coupling between the inductors  206 ,  208 ,  214  and  216  shown in  FIG. 2A . K 1  is a magnetic coupling coefficient between the inductors  206  and  214  or the inductors  208  and  216 , K 2  is a magnetic coupling coefficient between the inductors  206  and  216  or the inductors  214  and  208 , and K 3  is a magnetic coupling coefficient between the inductors  206  and  208  or the inductors  214  and  216 . Since IQ imbalance between signals with a same phase is small, an effect the magnetic coupling coefficient K 3  can be ignored. Therefore, as long as relative distances and the coil winding directions of the inductors  206 ,  208 ,  214  and  216  are properly adjusted, induced currents generated by magnetic coupling can be cancelled with each other, so as to reduce IQ imbalance and minimize layout area. For example, by properly adjusting the relative distances and the coil winding directions of the inductors  206 ,  214  and  216 , magnetic coupling the inductor  206  by the magnetic coupling coefficients K 1  and K 2  has a same magnitude with opposite directions, i.e. induced currents can be cancelled with each other, so as to reduce IQ imbalance, and minimize layout area. 
     In detail, please refer to  FIG. 3A  and  FIG. 3B , which are schematic diagrams of the inductors  206 ,  208 ,  214  and  216  shown in  FIG. 2A  having different coil winding directions. In  FIG. 3A , coil winding directions of the inductors  206  and  214  are clockwise, and thus magnetic field directions thereof are downwards through paper, while coil winding directions of the inductor  208  and  216  are counterclockwise, and thus magnetic field directions thereof are upwards through paper. In such a condition, since IQ imbalance between signals with a same phase is small, the effect the magnetic coupling coefficient K 3  can be ignored. Therefore, for the inductor  206 , magnetic coupling generated by the inductor  208  can be ignored, and the inductor  214  and inductor  216  can be adjusted to have a same distance with the inductor  206 . Therefore, the inductor  214  and inductor  216  can induce magnetic fields on the inductor  206  with a same magnitude but a upwards through paper direction and a downwards through paper direction, respectively, such that corresponding induced currents on the inductor  206  are cancelled with each other. By the same token, induced currents on the inductor  208 ,  214  and  216  can also be cancelled. As a result, the present invention can adjust induced currents to be cancelled with each other, so as to reduce IQ imbalance, and minimize layout area. Similarly, as shown in  FIG. 3B , coil winding directions of the inductor  206  and  216  are clockwise, and thus magnetic field directions are downwards through paper, while coil winding directions of the inductor  208  and  214  are counterclockwise, and thus magnetic field directions are upwards through paper. Therefore, the above effect can also be achieved, and is not narrated hereinafter. 
     Please refer to  FIG. 4A  and  FIG. 4B .  FIG. 4A  is a schematic diagram of IQ imbalance magnitude of the quadrature amplifier  100  shown in  FIG. 1A .  FIG. 4B  is a schematic diagram of IQ imbalance magnitude of the amplifier  200  shown in  FIG. 2A .  FIG. 4A  and  FIG. 4B  are obtained by measuring a difference between in-phase positive signal and quadrature-phase positive signal at different frequencies. Ideally, there are only signals at carrier frequency minus baseband frequency, i.e. points D, F. In practice, IQ imbalance magnitude is obtained by measuring a difference between signals at carrier frequency minus baseband frequency and signals at carrier frequency plus baseband frequency, i.e. points E, G, wherein lower difference indicates worse IQ imbalance. As can be seen from  FIG. 4A  and  FIG. 4B , IQ imbalance magnitude of  FIG. 4A  is 37.7 dB, and IQ imbalance magnitude of  FIG. 4B  is 52.3 dB. Therefore, the present invention gain extra 14.6 dB of IQ imbalance magnitude than the prior art. Noticeably, in normal communication systems, IQ imbalance magnitude is required to be higher than 40 dB. As a result, other than adjusting induced currents to be cancelled with each other, so as to reduce IQ imbalance and minimize layout area, the present invention can more comply with requirements of communication systems than the prior art. 
     Noticeably, the spirit of the present invention is to adjust relative positions and coil winding directions of inductors inside an amplifier, to cancel magnetic coupling between different internal paths, so as to reduce IQ imbalance. Those skilled in the art should make modifications or alterations accordingly which belong to the scope of the present invention. For example, the amplifier  200  is not limited to generate in-phase signals and quadrature-phase signals with 90 degree phase difference, and can to generate signals with other fixed phase differences, such as 45 degree or 135 degree. The amplifier  200  is also not limited to only include the in-phase path and the quadrature-phase path, and can include more than two paths with a specific phase difference. An amount of LC tanks is not limited to four, and can be any amount. All modifications or alterations belong to the scope of the present invention, as long as magnetic coupling between different internal paths can be cancelled by adjusting relative positions and coil winding directions of internal inductors of LC tanks of the amplifier. 
     In addition, relative positions of the inductors inside the amplifier  200  are not limited to the above description that the in-phase path and the quadrature-phase path are cross. In other embodiments, the in-phase path and the quadrature-phase path can be in parallel as well, which can still cancel magnetic coupling between different internal paths by adjusting relative positions and coil winding directions inside inductors of the amplifier. 
     For example, please refer to  FIG. 5A  and  FIG. 5B .  FIG. 5A  is a schematic diagram of an amplifier  500  according to an embodiment of the present invention. A difference between the amplifier  500  and the amplifier  200  is that the amplifier  500  the in-phase path and the quadrature-phase path are in parallel, i.e. differential amplifiers  502 ,  504  and inductors  506 ,  508  are in parallel with differential amplifiers  510 ,  512  and inductors  514 ,  516 . Operating principles of the amplifier  500  are similar to those of the amplifier  200 , and are not narrated hereinafter.  FIG. 5B  is a schematic diagram of magnetic coupling between inductors  506 ,  508 ,  514  and  516  shown in  FIG. 5A . K 1 ′ is a magnetic coupling coefficient between the inductors  506  and  514  or the inductors  508  and  516 , K 2 ′ is a magnetic coupling coefficient between the inductors  506  and  508  or the inductors  514  and  516 , and K 3 ′ is a magnetic coupling coefficient between the inductors  506  and  516  or the inductors  514  and  508 . K 1 ′, K 2 ′ and K 3 ′ are magnetic coupling coefficients between inductors, respectively. In such a condition, since IQ imbalance between signals with a same phase is small, an effect the magnetic coupling coefficient K 2 ′ can be ignored. Therefore, as long as relative distances and the coil winding directions of the inductors  506 ,  508 ,  514  and  516  are properly adjusted, induced currents generated by magnetic coupling can be cancelled with each other, so as to reduce IQ imbalance. 
     Please continue to refer to  FIG. 6A  and  FIG. 6B , which are schematic diagrams of the inductors  506 ,  508 ,  514  and  516  shown in  FIG. 5A  having different coil winding directions. In  FIG. 6A , coil winding directions of the inductors  506  and  514  are clockwise, and thus magnetic field directions are downwards through paper, while coil winding directions of the inductors  508  and  516  are counterclockwise, and thus magnetic field directions are upwards through paper. In such a condition, since IQ imbalance between signals with a same phase is small, the effect the magnetic coupling coefficient K 2 ′ can be ignored. Therefore, in order to make the magnetic coupling coefficients K 1 ′ and K 3 ′ to be cancelled with each other, the inductors  506 ,  508 ,  514  and  516  can be arranged corresponding to four vertices of a rhombus or a distance H′ between the inductors  506 ,  514  can be lengthened, to induce magnetic fields with a same magnitude but opposite directions, such that corresponding induced currents are cancelled with each other. Similarly,  FIG. 6B  is another schematic diagram of coil winding directions, and is similar to the above description. 
     Therefore, a method for the amplifier  200  to cancel magnetic coupling can be summarized into is a process  70 . As shown in  FIG. 7 , the process  70  includes following steps: 
     Step  700 : Start. 
     Step  702 : Form the inductors  206  and  208  in the in-phase path. 
     Step  704 : Form the inductors  214  and  216  in the quadrature-phase path. 
     Step  706 : End. 
     Noticeably, the inductors  206 ,  208 ,  214 ,  216  form inductors of LC tanks, respectively. By adjusting the relative distances and the coil winding directions of the inductors  206 ,  208 ,  214  and  216 , induced currents on the inductors generated by magnetic coupling can be cancelled with each other. Detailed description can be derived by referring the above description, and is not narrated hereinafter. 
     In the prior art, a distance between an in-phase path and a quadrature-phase path has to be lengthened, to reduce induced currents, so as to reduce IQ imbalance. In comparison, the present invention increases an amount of LC tanks on the in-phase path and the quadrature-phase path, respectively, and then adjusts relative distances and coil winding directions of inductors of the LC tanks. Therefore, induced magnetic fields on the inductors can be cancelled with each other, to reduce induced currents, and thus reduce IQ imbalance. As a result, the present invention can reduce IQ imbalance with small layout area. 
     To sum up, the present invention cancels magnetic coupling by increasing and adjusting LC tanks, and thus can effectively reduce IQ imbalance with small layout area. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

Technology Classification (CPC): 7