Abstract:
A transmission circuit for radio frequency signals has a power mixer and includes a radio frequency input port, a first radio frequency output port, a second radio frequency output port, and a dummy load port. The transmission circuit also has a first switch unit and a second switch unit. The first switch unit is connected to the first radio frequency output port and outputs the radio frequency signals which are emitted from the first radio frequency output port or reflects the radio frequency signals which are emitted from the first radio frequency output port back to the first radio frequency output port. The second switch unit is connected to the second radio frequency output port and outputs the radio frequency signals which are emitted from the second radio frequency output port, or reflects the radio frequency signals which are emitted from the second radio frequency output port back to the second radio frequency output port. The radio frequency signals reflected to the radio frequency input port by the first switch unit and the radio frequency signals reflected to said radio frequency input port by the second switch unit cancel each other. The transmission circuit has reduced complexity and cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the technology field of radio frequency signal transmission and, particularly, to a transmission circuit for radio frequency signals and a method for transmitting radio frequency signals. 
     2. Description of the Prior Art 
     In current magnetic resonance imaging systems, a transmitting link of radio frequency signals for a body coil is shown in  FIG. 1 . Referring to  FIG. 1 , the transmitting link primarily includes a radio frequency amplifier  101 , a transmitting antenna shift switch  102 , a power mixer  103 , a transmitting antenna  104 , and a power load  105 . In which case, the main role of the power amplifier  101  is to amplify the radio frequency signals to be transmitted. The main role of the transmitting antenna shift switch  102  is to switch the output power of the radio frequency amplifier  101  to the power mixer  103  or the power load  105 , namely, to transmit the output power of the radio frequency amplifier  101  to a radio frequency input port  11  of the power mixer  103 , so as to further provide it to the transmitting antenna  104  for transmitting; or to transmit the output power of the radio frequency amplifier  101  to the power load  105 , so as to calibrate the radio frequency amplifier  101  by measuring. The main role of the power mixer  103  is to mix the radio frequency signals that are supplied as inputs from the transmitting antenna shift switch  102 , so as to form the two orthogonal radio frequency signals, and to output them to the transmitting antenna  104  via the radio frequency output ports  12  and  13 . The transmitting antenna  104  is mainly used for transmitting (emitting) the two channels of the radio frequency signals supplied thereto as inputs. 
     When the power mixer  103  transmits the two channels of the orthogonal radio frequency signals to the antenna via the radio frequency output ports  12  and  13 , since the radio frequency device does not match or the reflection coefficients of multiple input ports of the transmitting antenna  104  are not the same, it will produce therefore a certain amount of reflected power, and this part of the reflected power is emitted to the transmitting antenna shift switch  102  from the dummy load port  14  of the power mixer  103 . It is then switched to the power load  105  by the transmitting antenna shift switch  102  and is consumed there to avoid being returned to the radio frequency amplifier  101 , which may damage the radio frequency amplifier  101 . 
     In the transmitting link shown in  FIG. 1 , the transmitting antenna shift switch  102  not only switches the power signals that are emitted from the radio frequency amplifier  101  to the power mixer  103  or the power load  105 , but also outputs the reflected power outputted from the dummy load port  14  of the power mixer  103  to the power load  105 . Such a transmitting antenna shift switch  102  has high operating requirements, so its structure is relatively complicated, thereby leading to a relatively complicated transmitting link as shown in  FIG. 1 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a transmission circuit for radio frequency signals exhibiting reduced complexity for the transmission of such radio frequency signals. The present invention also provides a method for transmitting radio frequency signals utilizing a transmission circuit for radio frequency signals. 
     The transmission circuit for radio frequency signals in accordance with the present invention has a power mixer, this power mixer including a radio frequency input port, a first radio frequency output port, a second radio frequency output port and a dummy load port. Radio frequency signals reflected back to the power mixer from the first radio frequency output port and radio frequency signals reflected back to the power mixer from the second radio frequency output port cancel each other at the radio frequency input port, and are emitted as an output from the dummy load port. The transmission circuit for radio frequency signals further has a first switch unit and a second switch unit. The first switch unit is connected to the first radio frequency output port for emitting the radio frequency signals supplied from the first radio frequency output port, or for reflecting the radio frequency signals supplied from the first radio frequency output port back to the first radio frequency output port. The second switch unit is connected to the second radio frequency output port for emitting the radio frequency signals supplied from the second radio frequency output port, or for reflecting the radio frequency signals outputted from the second radio frequency output port back to the second radio frequency output port. 
     In the above technical solution, the first switch unit reflects the radio frequency signals emitted from the first radio frequency output port back to the first radio frequency output port by forming a short circuit in the first radio frequency output port, and the second switch unit reflects the radio frequency signals emitted from the second radio frequency output port back to the second radio frequency output port by forming a short circuit in the second radio frequency output port. 
     Preferably, the short circuit in the first switch unit and the second switch unit is realized by a connection to ground. 
     The first switch unit has a diode, with its anode connected to the first radio frequency output port and its cathode to ground, and when the diode is non-conducting, the radio frequency signals supplied from the first radio frequency output port are emitted as an output, and when it is conducting, the radio frequency signals supplied from the first radio frequency output port are reflected back to the first radio frequency output port. The second switch unit also has a diode, with its anode connected to the first radio frequency output port and its cathode to ground. When the diode is non-conducting, the radio frequency signals from the second radio frequency output port are emitted as an output. When it is conducting, the radio frequency signals supplied from the second radio frequency output port are reflected back to the second radio frequency output port. 
     Preferably, the first switch unit reflects the radio frequency signals from the first radio frequency output port back to the first radio frequency output port by making the first radio frequency output port an open circuit, and the second switch unit reflects the radio frequency signals from the second radio frequency output port back to the second radio frequency output port by making the second radio frequency output port an open circuit. 
     The first switch unit has a radio frequency switch, with one side thereof connected to the first radio frequency output port. When this radio frequency switch is closed, it emits, from the other side thereof, the radio frequency signals supplied from the first radio frequency output port, and when it is open, it reflects the radio frequency signals supplied from the first radio frequency output port back to the first radio frequency output port. The second switch unit can also have a radio frequency switch, with one side thereof connected to the second radio frequency output port. When this radio frequency switch is closed, it emits, from the other side thereof, the radio frequency signals supplied from the second radio frequency output port. When it is open, it reflects the radio frequency signals supplied from the second radio frequency output port back to the second radio frequency output port. 
     In the above technical solution, the power supplied as an output from the first radio frequency output port to the first switch unit and the power supplied as an output from the second radio frequency output port to the second switch unit are equal, and the reflectance of the first switch unit and that of the second switch unit are equal. Alternatively; the power supplied from the first radio frequency output port to the first switch unit and the power supplied from the second radio frequency output port to the second switch unit are not equal, and the reflectance of the first switch unit and that of the second switch unit are not equal. 
     In the above technical solution, the power mixer is either an active power mixer or a passive power mixer. 
     The above transmission circuit for radio frequency signals further has a radio frequency amplifier, a transmitting antenna and a power load, wherein, the output of the radio frequency amplifier is connected to the radio frequency input of said power mixer; the two inputs of the transmitting antenna are respectively connected to the first radio frequency output and the second radio frequency output of the power mixer, and the power load is connected to the port of the dummy load of the power mixer. 
     The present invention also provides a method for transmitting radio frequency signals which utilizes the abovementioned transmission circuit for radio frequency signals, including the steps of:
         emitting from the first switch unit and the second switch unit, respectively, the radio frequency signals supplied from the first radio frequency output port and the radio frequency signals supplied from the second radio frequency output port; and   reflecting, by the first switch unit and the second switch unit respectively, the radio frequency signals supplied from the first radio frequency output port and the radio frequency signals supplied from the second radio frequency output port back to the first radio frequency output port and the second radio frequency output port.       

     In the above technical solution, the first switch unit is used to make the first radio frequency output port short-circuited or open, so as to produce the reflection; and the second switch unit is used to make the second radio frequency output port short-circuited or open, so as to produce that reflection. 
     Compared to the transmission circuit for radio frequency signals in the prior art, the transmission circuit for radio frequency signals provided by the present invention omits the complicated transmitting antenna shift switch and the related links in the prior art, and adds respectively in the circuit a switch unit to each of the two radio frequency output ports of the power mixer. Such a switch unit only transmits the radio frequency signals outputted from the corresponding radio frequency output port to the subsequent devices, or reflects the radio frequency signals back to the corresponding radio frequency output port so that the power mixer outputs from its dummy load port the reflected power. Since the two switch units and their connection relationship in the transmission circuit for radio frequency signals according to the present invention are much simpler than the transmitting antenna shift switch in the prior art, the present invention reduces the complexity of the circuit. Moreover, by utilizing the simple switch unit in place of the complicated transmitting antenna shift switch, the present invention also reduces the costs. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmitting link for the radio frequency signals in the prior art. 
         FIG. 2  is a schematic diagram of the power mixer of an embodiment according to the present invention. 
         FIG. 3  is a schematic diagram of a transmitting link for the radio frequency signals in an embodiment according to the present invention. 
         FIG. 4  is a schematic diagram of an embodiment of signal transmission circuit according to the present invention. 
         FIG. 5  is a schematic diagram of another embodiment of signal transmission circuit according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to make the object, technical solution and advantages of the present invention more apparent, the present invention will be further described in detail below using exemplary embodiments. 
     First, the working principles of a power mixer in an embodiment according to the present invention will be described.  FIG. 2  is a schematic diagram of the power mixer. As shown in  FIG. 2 , the power mixer comprises four ports, which are respectively a radio frequency input port (RF in), a first radio frequency output port (RF output 1 ), a second radio frequency output port (RF output 2 ) and a dummy load port (Dummy load). For the sake of simplicity,  FIG. 2  does not provide particular internal structures of the power mixer; it shows the signal flows by lines and arrows. 
     Between the radio frequency input port and the first radio frequency output port, and between the second radio frequency output port and the dummy load port there are respectively odd number times of 90° phase shifts. Between the radio frequency input port and the second radio frequency output port, and between the first radio frequency output port and the dummy load port there are respectively even number times of 90° phase shifts. For the sake of simplicity, in the following description, a phase shift of 90° will be taken as an example the above odd number times of 90° phase shifts, and a phase shift of 0° will be taken as an example of the above even number times of 90° phase shifts. 
     It is well known that, power mixers can be divided into active power mixers or passive power mixers, the power mixer in the present invention can be either a passive power mixer or an active power mixer. In the case of using a passive power mixer, for the radio frequency signals which are supplied from the radio frequency input port, one half of the power each (approximately −3 dB) is transmitted to the first radio frequency output port or the second radio frequency output port. In the case of using an active power mixer, the radio frequency signals transmitted from the radio frequency input port to the first radio frequency output port and the second radio frequency output port will have the some gain, such as 2 dB gain, 5 dB gain, and so on. For the sake of simplicity, in the following description, the implementation of the embodiments according to the present invention are illustrated by means of passive power mixers as examples, the embodiments of the active power mixers are similar to that of the passive power mixers, so they need not be described herein redundantly. 
     As shown in  FIG. 2 , the radio frequency signals are inputted from the radio frequency input port, and one half the power each (approximately −3 dB) is transmitted to the first radio frequency output port or the second radio frequency output port, wherein, the radio frequency signals transmitted to the first radio frequency output port generate a phase shift of 90°, and the radio frequency signals transmitted to the second radio frequency output port generate a phase shift of 0°. 
     If there is a reflection coefficient k 1  at the first radio frequency output port, then after the reflection, the power of the radio frequency signals which is equal to the product of k 1  and the output power supplied from the first radio frequency output port will be reflected into the first radio frequency output port. For the radio frequency signals reflected from the first radio frequency output port, one half the power each (approximately −3 dB) is transmitted to the radio frequency input port or the dummy load port. The radio frequency signals transmitted to the radio frequency input port generate a phase shift of 90°, and the radio frequency signals transmitted to the dummy load port generate a phase shift of 0°. 
     Similarly, if there exists a reflection coefficient k 2  at the second radio frequency output port, after the reflection, the power of the radio frequency signals which is equal to the product of k 2  and the output power supplied from the second radio frequency output port will be reflected into the second radio frequency output port. For the radio frequency signals reflected from the second radio frequency output port, one half the power each (approximately −3 dB) is transmitted to the radio frequency input port or the dummy load port. The radio frequency signals transmitted to the radio frequency input port generate a phase shift of 0°, and the radio frequency signals transmitted to the dummy load port generate a phase shift of 90°. 
     At the dummy load port, the radio frequency signals reflected from the first radio frequency output port and the radio frequency signals reflected from the second radio frequency output port, compared to those supplied from the radio frequency input port, both have the phase shift of 90°, in other word, both are at the same phase; in addition. The powers of the two channels of the radio frequency signals are also the same. Therefore, the radio frequency signals reflected from the first radio frequency output port and the radio frequency signals reflected from the second radio frequency output port are superimposed and emitted as an output at the dummy load port. It is also possible to have reflection at the dummy load port, a part of the power will be reflected to the first radio frequency output port and the second radio frequency output port, but this part of the power has already been very small, and in the embodiments of the present invention, it will not be considered. 
     In the radio frequency input port, the radio frequency signals reflected from the first radio frequency output port and the radio frequency signals reflected from the second radio frequency output port, compared to those supplied from the radio frequency input port, have phase shifts of respectively 180° and 0°, in other word, these signals have a phase difference of 180°. The power of the two channels radio frequency signals is the same. Therefore, the radio frequency signals which are reflected from the first radio frequency output port and the radio frequency signals which are reflected from the second radio frequency output port cancel each other out at the radio frequency input port. Similarly, it is also possible to have reflection at the radio frequency input port, a part of the power will be reflected to the first radio frequency output port and the second radio frequency output port, but this part of the power has already been very small, and in the embodiments of the present invention, it will not be considered. 
     In summary, for the power mixer shown in  FIG. 2 , the power reflected from the first radio frequency output port and the power reflected from the second radio frequency output port cancel each other out at the radio frequency input port, and it is applied to the load connected at the dummy load port. In view of the overall effects, the power reflected from the first radio frequency output port and the power reflected from the second radio frequency output port are all outputted from the dummy load port, and there is no output coming from the radio frequency input port. 
     In summary, the power mixers in the embodiments of the present invention have the following features:
         1) The radio frequency signals reflected from the first radio frequency output port and the second radio frequency output port to the radio frequency input port cancel each other out, namely, their power is the same, and their phase difference is an odd number times of 180°; and   2) the radio frequency signals reflected from the first radio frequency output port and the second radio frequency output port to the dummy load port are superimposed and then outputted, namely, and their phase difference is an even number times of 180°.       

       FIG. 3  is a transmitting link of radio frequency signals of a body coil in the embodiment according to the present invention, it comprises: a radio frequency amplifier  201 , a power mixer  203 , a transmitting antenna  204 , a power load  205 , a first switch unit  206  and a second switch unit  207 . Compared to the prior art in  FIG. 1 , this embodiment of the present invention removes the transmitting antenna shift switch  102 , and adds the first switch unit  206  and the second switch unit  207 . 
     Referring to  FIG. 3 , the first switch unit  206  and the second switch unit  207  are respectively connected to the first radio frequency output port  22  and the second radio frequency output port  23  of the power mixer  203 . The first switch unit  206  and the second switch unit  207  can transmit directly the radio frequency signals supplied from the first radio frequency output port  22  and the second radio frequency output port  23  of the power mixer  203  to the transmitting antenna  204  behind them, or reflect respectively the radio frequency signals outputted from the first radio frequency output port  22  and the second radio frequency output port  23  of the power mixer  203  back to the first radio frequency output port  22  and the second radio frequency output port  23 . 
     When the first switch unit  206  and the second switch unit  207  transmit the radio frequency signals supplied from the power mixer  203  to the transmitting antenna  204 , the circuit is in its normal working conditions, and the radio frequency signals supplied from the radio frequency amplifier  201  are transmitted out by the transmitting antenna  204 . When the first switch unit  206  and the second switch unit  207  reflect the radio frequency signals outputted from the power mixer  203  back to the power mixer  203 , the reflected signals are outputted from the dummy load port of the power mixer to the power load  205 , so that the radio amplifier  201  can calibrated by measurements. 
     The first switch unit  206  and the second switch unit  207  can be implemented in a variety of ways. Taking the first switch unit  206  as an example, when it is needed to transmit the radio frequency signals to the transmitting antenna  204 , the first switch unit  206  transmits directly the radio frequency signals to the transmitting antenna  204 ; when it is needed to transmit to the power load  205 , the first switch unit  206 , by making the first radio frequency output port  22  short-circuited or open, can reflect the radio frequency signals outputted from the first radio frequency output port  22  back to the first radio frequency output port. 
       FIG. 4  provides a structural diagram for implementing the reflection by forming a short circuit. As shown in  FIG. 4 , the first switch unit comprises a diode D 1 , which diode D 1  has its anode connected to the first radio frequency output port, and its cathode grounded. The second switch unit has a diode D 2 , which diode D 2  has its anode connected to the second radio frequency output port, and its cathode grounded. When it is needed to transmit the radio frequency signals to the transmitting antenna, an appropriate negative voltage is applied to the anode of the diode D 1  (node  2  shown in the figure) and the anode of the diode D 2  (node  1  shown in the figure), so that the D 1  and D 2  are in their non-conducting state, the signals emitted from the first radio frequency output port and the second radio frequency output port will be transmitted to the transmitting antenna behind them. When it is needed to output from the dummy load port the radio frequency signals emitted from the power mixer, an appropriate positive voltage is applied to the anode of the diode D 1  (node  2  shown in the figure) and the anode of the diode D 2  (node  1  shown in the figure), so that the D 1  and D 2  are in their conductive state, the signals emitted from the first radio frequency output port and the second radio frequency output port will all be reflected back to the first radio frequency output port and the second radio frequency output port due to the connection to earth, and then be emitted as an output from the dummy load port. 
     The diode shown in  FIG. 4  is only for the purpose of illustration. In this embodiment of the present invention, the way to realize the reflective switch unit is not limited to forming a short circuit. For example, the embodiment of the present invention can also use the way in which one end of the radio frequency switch is connected to the output port of the power mixer, and the other end is grounded, and there are also other ways to achieve the above function. 
       FIG. 5  shows a structural diagram for implementing reflection by forming an open circuit. The first switch unit and the second switch unit can be realized respectively by a radio frequency switch, both ends of the radio frequency switch in the first switch unit are respectively connected to the first radio frequency output port and one input end of the transmitting antenna, both ends of the radio frequency switch in the second switch unit are respectively connected to the second radio frequency output port and another input end of the transmitting antenna. When it is necessary to transmit the radio frequency signals to the transmitting antenna, the above two radio frequency switches will be closed, and the two channels of the radio frequency signals outputted from the power mixer are transmitted respectively to the transmitting antenna by way of the two closed radio frequency switches. When it is needed to transmit the radio frequency signals to the power load, the above two radio frequency switches will be broken and in an open state, then the two channels of the radio frequency signals outputted from the power mixer will be reflected respectively back to the first radio frequency output port and the second radio frequency output port due to the open circuit, and then be outputted from the dummy load port. 
     Similarly, the radio frequency switch shown in  FIG. 5  is only for the purpose of illustration, and the switch unit of the present invention is not limited to this example. The radio frequency switch of the present invention can be an electromechanical switch realized by mechanical contacts, it can also be a radio frequency switch realized by a field-effect transistor (FET), it can further be a radio frequency switch realized by a diode of positive-intrinsic-negative (PIN) type, or any other radio frequency switch which is capable of achieving the above function. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.