Abstract:
In a circuit for connection of at least two signal sources with at least one signal output, the signal sources are connected with a control unit. The control unit activates or deactivates the signal sources by transmission of control signals. Each signal source is connected with a blocking unit. Each blocking unit is connected with the control unit. The blocking unit can block or enable a connection between the signal source and the signal output, with the connection being automatically enabled when the signal source is activated.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns a circuit for connection of at least two signal sources with at least one signal output, the signal sources being connected to a control unit that activates or deactivates the signal sources via transmission of control signals.  
         [0003]     2. Description of the Prior Art  
         [0004]     In the transmission of electrical signals from signal sources to a signal output that, for example, is connected to a signal evaluation unit, it is frequently necessary to selectively connect a number of signal sources with only one signal evaluation unit. This is in particular the case in magnetic resonance apparatuses where it is necessary to connect a large number of surface coils with a lesser number of preamplifiers. It is additionally necessary that only a few or only one single surface coil are/is active at the same time in order to prevent crosstalk from the other surface coils. The large number of surface coils is, for example, advantageous when a larger region of a patient is to be examined without intrusion into the examination space. The use of larger coils is disadvantageous because, in this case, the signal-to-noise ratio is significantly smaller in comparison to smaller coils. In most magnetic resonance apparatuses, the number of the preamplifiers that are connected with the surface coils is the limiting factor for the number of usable surface coils. Should a large number of smaller coils be used, these must be selectively connected with the preamplifier inputs.  
         [0005]     A circuit is known from U.S. Pat. No. 6,677,755 by means of which a number of coils can be connected with preamplifiers of a magnetic resonance apparatus. For this purpose, a switching matrix composed of radio-frequency switches is controlled via a control line by a switch controller, so the coils can be selectively connected with the preamplifiers. The coils can be activated or deactivated by a coil control unit. A disadvantage of this known circuit is its high circuit complexity.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the present invention is to provide a simple circuit for connection of at least two signal sources with at least one signal output.  
         [0007]     The above object is achieved in accordance with the present invention by a circuit for connecting at least two signal sources with at least one signal output having a control unit connected to the signal sources that activates or deactivates the signal sources by the transmission of control signals, wherein each signal source has a blocking unit connected thereto, each blocking unit also being connected with said control unit, each blocking unit being switchable to block or to enable a connection between the signal source to which it is connected and the signal output, with the blocking unit automatically enabling such a connection with the signal source to which it is connected is activated.  
         [0008]     The blocking units block or enable the respective connection between the signal source and the signal output. The appropriate connection of the signal source with the signal output is also simultaneously enabled by the control signals of the control unit that serve for activation and deactivation of the signal sources. No separate switch controller is necessary, which allows a lower circuit complexity.  
         [0009]     In an advantageous embodiment of the invention, the circuit has at least one primary line that is connected with the signal output and with which each signal source is connected via a secondary (branch) line. This design is particularly simple to execute since here a plurality of signal sources can be connected with the signal output with low circuit complexity.  
         [0010]     In a further embodiment, the blocking unit has at least one blocking element (impedance). This can be, for example, a controllable switch. Respective lines between the signal source and the signal output can be blocked or enabled in a simple manner via the switch.  
         [0011]     In another embodiment, at least one blocking element is arranged in the stub between the signal source and the primary line. Given a deactivated signal source, the stub can be disconnected from the primary line by means of this blocking element, such that no signals can flow therein.  
         [0012]     In a further embodiment, at least one blocking element is connected in the primary line between a connection point of the primary line with the stub for the first signal source and a connection point of the primary line with the stub for the second signal source. Parts of the primary line can therewith also be partially disconnected from the signal path. For example, if the signal source lying closer to the signal output (with regard to the signal path) is activated, the blocking element allows the unused part of the primary line, for the connection point of the primary line with the stub, for the deactivated signal source to be cut off. Particularly for radio-frequency electrical signals, it is important to have no unused stub since, for example, reflections can occur that lead to unwanted signal alterations.  
         [0013]     Preferably the blocking elements are PIN diodes. High resistances in the conductors, which effectively disconnect the lines, can be achieved in a simple manner by control signals by means of such PIN diodes. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  schematically illustrates a system for reception of magnetic resonance signals with a preferred embodiment of the invention.  
         [0015]      FIG. 2  is a preferred embodiment of the circuit of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The system schematically shown in  FIG. 1  has m coil groups  102 , of which three are shown. Each coil group  102  has n surface coils  104  for the reception of magnetic resonance signals. Again only three surface coils  104  are shown. The surface coils  104  of the coil group  102  are connected with n signal outputs  108  (of which three are likewise shown) by a multiplex circuit  106 . The multiplex circuit  106  is a preferred embodiment of the invention and is further explained in detail below using  FIG. 2 . The signal outputs  108  are connected with n preamplifiers of a magnetic resonance apparatus  110 , with the preamplifiers of the magnetic resonance apparatus  110  being shown only as a single block. Since the number of surface coils  104  of a coil group  102  is equal to the number of preamplifiers, magnetic resonance signals can be acquired with any complete coil group  102 . For this purpose, the surface coils  104  thereof are tuned to the corresponding frequency of the magnetic resonance signals. During the measurement with one coil group  102 , the surface coils  104  of all other coil groups  102  are detuned so that they receive no magnetic resonance signals. The tuning and detuning of the coil groups  102  occurs by means of m control lines  112  that proceed from the magnetic resonance apparatus  110  to the coil groups  102 . The coil groups  102  can be tuned or detuned by defined voltage signals on the control lines  112 . For transfer of the acquired magnetic resonance signals, each coil group  102  is connected via a coaxial cable  114  with the multiplex circuit  106  that, during the signal acquisition, connects the respective surface coils  104  of the tuned coil group  102  with the signal outputs  108 . At the same time, all untuned coil groups  102  are disconnected (isolated) from the signal outputs  108  so that no interfering signals can emanate from them. The voltage signals of the control lines  112  are used for connection and disconnection of the coil groups  102 , which is why the multiplex circuit  106  is connected with the control lines  112 .  
         [0017]     The design of the multiplex circuit is schematically shown in  FIG. 2 . All lines are executed as coaxial cables with a characteristic impedance of 50 Ω. Of the n signal outputs  2 , only three are shown for clarity. The signal outputs  2  are respectively connected with a primary line  4 . Branching from each primary line  4  are m stubs  6 , of which again only three are shown. The ends of the stubs  6  are connected with the surface coils of the coil groups. A surface coil of each coil group is thereby connected with each primary line  4 . All stubs  6  in the same horizontal row in  FIG. 2  consequently form the connections to a coil group. Sheath wave barriers  7  that prevent the propagation of interfering sheath waves are connected in the primary lines  4  between the stubs  6 .  
         [0018]     The design of one primary line  4  with the corresponding stubs  6  is described in the following; all further primary lines  4  are designed in an analogous manner. A PIN diode  10  of the type BA592 with a resistor  12  of 20 kΩ connected in parallel therewith is located between the connection points  8  of the stubs  6 . Two PIN diodes  10  switched in series are located in each stub  6  with a respective resistor  12  connected in parallel with each PIN diode  10 . The voltage signals of the control lines are coupled into the circuit via an auxiliary connection  14 . A pre-resistor  16  of 5 kΩ is located between the stub  6  and the connection to the control unit. The PIN diodes  10  in the stubs  6  can be switched into the blocking state upon application of a positive voltage on the auxiliary line  14  and are switched a conducting state upon application of a negative voltage. All stubs  6  in the same horizontal row are activated with the same control signal. A capacitor  18  of 1 nF, which brings the control signals to a defined ground potential, is additionally located in the stub  6 . A capacitor  18  that separates the radio-frequency signal and control signal is likewise located in each primary line  4 . A connection to a reference potential  20  branching from the primary line  4  is provided via an inductor  22  of 5 μH. A group of surface coils of the control unit is now activated by the application of a negative voltage, thus the PIN diodes  10  located in the respective stub  6  are simultaneously switched to conduct by the same control signal so that the measured signals of the surface coils arrive at the primary line  4  from the stub  6 . The PIN diodes  10  located in the primary line  4  between the respective connection point  8  and the signal output  2  are switched to a conducting state by the same voltage signal. All further coil groups are simultaneously deactivated by a control signal that serves as a deactivation signal for the surface coils. The PIN diodes  10  are thereby switched into the high-ohmic state in the respective stubs  6  to the coils, such that the surface coils are disconnected from the primary line  4 . At the same time all PIN diodes  10  in the primary line  4  that are located (viewed from the signal output  2 ) behind the connection point  8  to the activated coil are switched into the high-ohmic state so that the subsequent parts of the primary line  4  are disconnected from the signal path. This prevents reflections in the signal and thus reduces the possibility of measurement errors.  
         [0019]     A connection of many surface coils with one signal output can be achieved with low circuit complexity with the specified circuit. The control signals serve to tune selected surface coils to the frequency of the magnetic resonance signals to be received and at the same time they are used to open the corresponding signal path. The surface coils which should not be receiving are detuned by corresponding control signals so that no reception occurs at the reception frequency. At the same time, the signal paths to these surface coils are blocked by the control signals. In contrast to circuits known in the prior art, no auxiliary signals for control of the switches are consequently necessary.  
         [0020]     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.