Abstract:
RF switches are known from the art for selecting an RF signal from a multiple of inputs. For instance a TV-tuner and TV-Front-end can have two or more antenna inputs but are implemented with a relative bad cost performance. The present invention relates to an RF selection switch circuit ( 200 ). The switch comprises a first switch ( 202 ), a second switch ( 204 ) with both an RF input port and a common RF output port a control signal circuitry ( 206 ). In a preferred embodiment the switches comprise pin-diodes. The RF output port comprises two transistors (T 1,  T 2 ), one of which at a time can put the non-selected switch to a low-impedance level in order to minimize signal leakage the common output port.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an RF (Radio Frequency) input circuit provided with an RF switch with multiple inputs and at least one output, having a good isolation between the inputs, a good sensitivity and provided at low cost. 
         [0002]    The present invention is particularly relevant for a TV-tuner and TV-Front-end that have two or more antenna inputs such as one for cable TV and one for terrestrial TV. 
       BACKGROUND OF THE INVENTION 
       [0003]    RF switches are known from the art for selecting an RF signal from a multiple of inputs. For instance a TV-tuner and TV-Front-end that have two or more antenna inputs such as one for cable TV and one for terrestrial TV or one with provided with two cable TV inputs (A/B). 
         [0004]    Two main solutions are known today for an RF switch, an electro-mechanical switch and an electronical switch. The electromechanical switch is bulky, relatively expensive (BOM), high voltage and current compared to electronics means. In the electronics solution, if good isolation is required (e.g., FCC requirement) more than one RF switch in cascade may be required in order to meet an isolation requirement. 
         [0005]    In both the cases of RF switch, a control circuit is needed.  FIG. 1  shows an example of a prior art electronical switch with a 2× diode RF switch cascaded with a MOSFET SPDT (Single Pole Double Throw) switch. An IC can implement a switching control circuit. 
         [0006]    Examples of RF switches can be found in U.S. Pat. No. 4,400,735 (named “Multi-component video system controller”, described by Stramello), U.S. Pat. No. 3,047,741 (named “Multiple channel electronic switching circuit” and described by Snow), U.S. Pat. No. 4,742,249 (named “RF switch with diode network and control latch sharing common element” and described by Alpaiwalla) and U.S. Pat. No. 6,650,199 (named “RF A/B switch with substantial isolation” described by Dobrovolny). 
         [0007]    In an application where the RF module is connected to the two antenna inputs, for example cable and terrestrial inputs, certain isolation is needed to ensure that there is no undesired signal leaking through or cross interference between two inputs signal either when the device is power on or off. Beside the fitness for use, it is a regulatory requirement such as an FCC recommendation. The FCC require an isolation between two inputs as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  54 MHz to 216 MHz 
                 80 dBmin 
               
               
                   
                 216 MHz to 550 MHz 
                 60 dBmin 
               
               
                   
                 550 MHz to 806 MHz 
                 55 dBmin 
               
               
                   
                   
               
             
          
         
       
     
         [0008]    To meet the above requirement, relatively expensive and complicated RF switches have been deployed in the prior art. Beside the isolation requirement, a good RF switch should have as minimum insertion loss as possible to ensure the good sensitivity still visible. 
         [0009]    The inventor proposes an alternative for the relative expensive and complicated switch, such as an existing RF switch solution used in the Philips&#39; TUV1236D, namely the TUV1236D RF switch. 
       SUMMARY OF THE INVENTION 
       [0010]    It is, accordingly, an object of the present invention to provide an RF switch with sufficient to superior isolation between the inputs and with little insertion loss. 
         [0011]    It is another object of the invention to teach and practice a low cost RF switch that requires little printed circuit board space. 
         [0012]    It is yet another object of the invention to provide a tuner or tuning system comprising an RF switch with sufficient to superior isolation between the inputs and with little insertion loss. 
         [0013]    It is yet another object of the invention to provide an apparatus for receiving RF signals from a multiple of sources comprising an RF switch with sufficient to superior isolation between the inputs and with little insertion loss. 
         [0014]    In one embodiment, dual diode switches have been applied for switching between two signal inputs. The switch in off state, and thereby the non-selected input signal, is made low-impedance. This is achieved using a transistor that has an output switched to low-impedance and connected to the switch in off state, in order to provide a minimum non-selected input signal leakage to the output of the switch. In a related embodiment pin-diode switches have been applied to provide a superior isolation between the inputs. 
         [0015]    These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: 
           [0017]      FIG. 1  shows a circuit diagram of a prior art RF switch circuit; 
           [0018]      FIG. 2  shows a circuit diagram of an RF switch circuit in accordance with the invention; and 
           [0019]      FIG. 3  shows a tuning system comprising an RF switch circuit in accordance with the invention; 
           [0020]      FIGS. 4   a  and  4   b  show examples of extended switches in accordance with the invention comprising a multiple of the RF switch circuits; 
           [0021]      FIG. 5  shows a circuit diagram of an RF switch in accordance with the invention that comprises an extended number of diodes; 
           [0022]      FIG. 6  shows a circuit diagram of an RF switch in accordance with the invention that comprises an extended the number of transistors; and 
           [0023]      FIG. 7  shows a circuit diagram of an RF switch that comprises and extended number of transistors and diodes in accordance with the invention. 
       
    
    
       [0024]    Throughout the drawings, the same reference numeral refers to the same element, or an element that performs substantially the same function. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  shows a circuit diagram of a prior art RF switch circuit. 
         [0026]      FIG. 1  shows a TUV1236D RF switch circuit  100  comprising 2× diode  102  and  104 , switching control circuit  106  and Peregrine MOSFET Switch  108 . RF input signals are Off-Air and Cable and the RF output signal is RF_SW_Out. 
         [0027]    In general, signal isolation is realized by cascading a 2× diode and a Peregrine MOSFET Switch. Peregrine MOSFET switch  108  is a single pole double throw (SPDT) switch or 2 input ports with one output port. Depending on the status of control signal RFSW, the output port/pin is connected at the either one of the input ports/pins at one time. An MSOP-8-pins IC can realize the Peregrine MOSFET switch  108 . 
         [0028]    From  FIG. 1 , the operation switch is relative simple. When a user wants to receive an off-air signal Off-Air, the corresponding 2× diode  102  at the off-air signal path will be switched-on and the Peregrine MOSFET switch  108  will be switched to the desired input port/pin (in  FIG. 1  to the upper input position) to allow the Off-Air signal path to appear at the output (RF_SW_out) with minimum insertion loss. At the same time Cable signal path should be maximum attenuated/isolated when the user is in the off-air signal-receiving mode. In this case the 2× diodes  104  at the cable signal path will be in the off-state and the Peregrine MOSFET switch will have the cable input port/pin (in  FIG. 1  the lower input position) disconnected. A MOPLL IC or circuitry can supply the control signal, RFSW. 
         [0029]    Vice versa, when a user wants to receive the cable antenna input Cable, 2× diode  102  will be switched off, 2× diode  104  will be switched on, and Peregrine MOSFET Switch  108  will be passing the lower input signal from the Cable signal path while the Off-Air signal path will be given a maximum attenuation. 
         [0030]      FIG. 2  shows circuit diagram of an RF switch circuit  200  in accordance with the invention. RF switch circuit  200  comprises 2× diode  202  (comprising diodes D 1  and D 2 ), 2× diode  204  (comprising diodes D 3  and D 4 ) and control signal circuitry  206 .  FIG. 2  shows RF input signal ports Off-Air and Cable and output signal port RF_SW_OUT. Control signal circuitry  206  performs part of an insulation function between the non-selected input and the output RF_SW_OUT. The proposed circuitry in accordance of the invention exhibits an, insulation and insertion loss performance that is similar or even better compared to cascading two prior art RF switches. 
         [0031]    When in operation, point  210  (cathode of D 1 ) is set at a DC level of about half the supply voltage (in  FIG. 2  around 2.5 V) using resistors R 1  and R 2 . Points  212  (cathodes of D 2  and D 3 ) and  214  (cathode of D 4 ) are set at a DC level of about half the supply voltage in a similar way. 
         [0032]    In one state of operation, when control input port RFSW is set to high state (typically supply voltage level) transistor T 1  starts conducting and will bring its collector output to a low state (close to ground) and thus point  216  (anodes of D 1  and D 2 ) becomes low. That means that the diodes in 2× diode  202  become non-conductive and will effectively block a signal coming from port Off-Air to pass 2× diode  202  to point  212 . Moreover point  216  becomes a low impedance point through transistor T 1  that is in a conductive state. The low impedance will provide a better insulation from signal port Off-Air to output port RF_SW_OUT. As the collector of transistor T 1  is low, also the basis of transistor T 2  becomes low and transistor T 2  will be in a non-conductive state. As a consequence, the collector of transistor T 2  will float and will become in a high state through resistor R 8 . As a consequence, the diodes in 2× diode  204  will get in a conductive state and will pass through an RF signal coming from signal port Cable to point  212 . The signal coming from signal port Cable will be passed to signal output port RF_SW_OUT. Capacitors C 1 , C 2 , and C 3  block any DC to pass but will be big enough to pass an RF-signal. 
         [0033]    In another state of operation, when RFSW is set to low state (typically ground level) transistor T 1  becomes non-conducting and will bring its collector output to a floating state and thus point  216  becomes high through resistor R 7 . That means that the diodes in 2× diode  202  become conductive and will effectively pass a signal coming from port Off-Air to pass 2× diode  202  to point  212 . As the collector of transistor T 1  is high, also the basis of transistor T 2  becomes high and transistor T 2  will get in a conductive state. As a consequence, the collector of transistor T 2  will get in a low state. As a consequence, the diodes in 2× diode  204  will get in a non-conductive state and will not pass through an RF signal coming from signal port Cable to point  212 . Moreover point  218  (anodes of D 3  and D 4 ) becomes a low impedance point through transistor T 2  that is now in a conductive state. The low impedance will provide a better insulation from signal port Cable to output port RF_SW_OUT. Capacitors C 1 , C 2 , and C 3  block any DC to pass but will be big enough to pass an RF-signal. 
         [0034]    The control signal circuitry  206  is arranged to select one of the 2× diode switch  202  and the 2× diode switch  204  for passing respective selected RF-input signal to the RF signal output port. The control signal circuitry  206  is also arranged to establish a low-impedance mode on a non-selected of one of the 2× diode switch  202  and the 2× diode switch  204  as to minimize cross-talk from a non-selected RF-input signal to the RF signal output port. 
         [0035]    Summarized, a high on control input port RFSW will enable a signal from input port Cable to pass to output port RF_SW_OUT and a low on control input port RFSW will enable a signal from input port Off-Air to pass to output port RF_SW_OUT. The switching is executed with a minimum insertion loss from input to output and a high insulation between the two input ports. 
         [0036]    RF switch circuit  200  is clearly simpler compared to the prior art RF switch circuit  100  as, e.g., the SPDT MOSFETs Switch  108  and its related components have been removed and the Control signal circuitry  106  has been simplified. This results in a significant lower Bill Of Materials (BOM), higher reliability, and also in a PCB area saving that enables in turn a more compact design, relevant for miniaturization. 
         [0037]    The inventor found that when applying pin-diodes for in 2× diode switch  202  and 2× diode switch  204 , the switches provide, e.g., superior isolation behavior, e.g., between inputs. The inventor however envisions that other types of diodes with superior switching behavior will emerge as technology improvement proceeds and can be applied in the spirit of the invention. 
         [0038]      FIG. 3  shows a tuning system  300  comprising an RF switch circuit  310  in accordance with the invention. Furthermore, tuning system  300  comprises a terrestrial input port  302 , a cable input port  304 , LNA  306 , Tuner part  320  that comprises a hybrid tuner  322 , analog part  330  that comprises analog IF  332  and analog IF demodulation  334 , digital part  340  that comprises digital IF  342  and digital IF demodulation (Channel decoder)  344 , OOB tuner  350 , output port CVBS/SIF  360  and output port TS/POD  370 . RF-switch  310  comprises RF switch circuit  200  of  FIG. 2 . 
         [0039]    Tuning system  300  can be used in an apparatus for receiving RF signals from a multiple of sources. Such sources can include a cable-TV source, an off-air TV source, an in house TV signal distribution source, an RF data source, an RF audio source, etc. Examples of such apparatus include a television receiver, a multi-media receiver, a personal computer, etc. 
         [0040]      FIGS. 4   a  and  4   b  show examples of extended switches comprising a multiple of the RF switch circuit  200  of  FIG. 2  (and/or RF switch circuit  500 ,  600  and  700  of  FIGS. 5 ,  6  and  7  respectively or derivations thereof as a person skilled in the art could think of).  FIG. 4   a  shows a three RF input to two RF-output switch.  FIG. 4   b  shows a three RF input to one RF-output switch. One skilled in the art understands that other configurations can easily be configured as well. 
         [0041]      FIG. 5  shows a circuit diagram of an RF switch that comprises an extended number of (preferably pin) diodes (D 5 , D 6 , D 7  and D 8 ) with periphery circuitry that will improve the input signal isolation performance. 
         [0042]      FIG. 6  shows a circuit diagram of an RF switch that comprises an extended the number of transistors (T 3  and T 3 ) with periphery circuitry that also will improve the input signal isolation performance. 
         [0043]    Finally,  FIG. 7  shows a circuit diagram of an RF switch that comprises and extended number of transistors (T 3  and T 4 ) and (preferably pin) diodes (D 5 , D 6 , D 7  and D 8 ) that will even more improve the input signal isolation. 
         [0044]    One of ordinary skill in the art will recognize that alternative schemes can be devised of designing the RF switch. For instance, instead of a (pin) diode a (pin) transistor may be used or another device that in performs in a similar way. Alternatively, it is also possible to use a power supply voltage other than 5V. The invention is also useful for other than RF switching, e.g., TV-IF-signal switching, audio- and/or video-base-band, other modulated signals (analogue, digital for audio, video and or digital signals). 
         [0045]    The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. Also one skilled in the art understands that certain components, such as coupling or decoupling capacitors can be added or deleted, depending on a design beyond the switching circuitry or certain design and performance criteria.