Abstract:
A device and method for splitting received signal energy among plural receivers in which received signal energy is provided to a first receiver which uses a primary portion of the received signal energy and reflects a reflected portion of the received signal energy, and in which the reflected portion is directed to an additional receiver(s) for use. Low loss directional circulators Or couplers may be used to direct the signal. The loss of signal energy to the first and subsequent receiver(s) is significantly reduced.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to radio receivers, and more particularly to a method and device for splitting received radio signals among plural receivers. 
     A radio system may operate with redundant receivers to increase operational reliability of the system. Often it is beneficial to provide the same signal to several receivers. Typically a primary (or first) receiver and one or more standby (or second) receivers use the same antenna and the received signal is provided to all of the receivers through a splitter or switch. Splitters are generally preferred because switches require time to switch the signal path and are generally less reliable and more expensive. 
     With reference now to FIG. 1, a conventional radio system with a primary receiver  10  and secondary receiver  12  typically uses a power splitter  14  to direct a portion of a signal received at antenna  16  to secondary receiver  12 . However, the splitter and the redirection of the received signal cause power losses which degrade system performance and are desirably avoided. Losses in one path may be reduced by intentionally splitting the signal unequally so that a primary receiver(s) performance is degraded less than that of a secondary receiver(s). If the signal is split  50 — 50  between the primary and secondary receivers, the splitter loss for each receiver (denoted often the noise figure) would be 3 dB, and if the split were  90 - 10  the primary receiver noise figure would be 0.5 dB and the secondary receiver noise figure would be 10 dB. The use of high quality factor splitters which do not waste signal energy may further reduce losses. 
     In the prior art the signal is typically provided to a low noise amplifier in the receiver. A characteristic of low noise amplifiers is that they exhibit poor impedance matches when they are optimized for a low noise figure. The prior art inserts isolators or directional couplers before the low noise amplifier to minimize the undesirable effects of the poor impedance matches. However, these components introduce further losses which degrade system performance. 
     Accordingly, it is an object of the present invention to provide a novel method and device which splits a received signal while avoiding most of the losses of the prior art, thereby obviating the problems of the prior art. 
     It is another object of the present invention to provide a novel method and low noise figure power splitter for redundant receivers which eliminates most of the loss prior to the low noise amplifier in which the entire received signal is directed to a low noise amplifier in the primary receiver which is intentionally optimized for low noise (thereby aggravating the impedance mismatch) to cause a usable signal to be reflected, and in which the reflected signal is routed to the remaining receivers for use. 
     It is yet another object of the present invention to provide a novel method and device in which the normally undesirable mismatch of the noise optimized amplifier in the primary receiver is used to cause a reflection of power to standby receivers. 
     It is still another object of the present invention to provide a novel method of splitting received signal energy among plural receivers in which received signal energy is provided to a directional circulator (or coupler) which is coupled to a first receiver which reflects a reflected portion of the received signal energy, and in which the reflected portion is directed by the directional circulator (or coupler) to a second receiver for use. 
     It is a further object of the present invention to provide a novel device for providing a received signal to primary and standby receivers in which a first circulator (or coupler) is coupled to the primary receiver for directing a received signal thereto, the primary receiver having a low noise amplifier having a low return loss so that usable signals are reflected from its input, and in which a second circulator (or coupler) is coupled to the first circulator (or coupler) and to a standby receiver for directing the signal reflected from the low noise amplifier to the standby receiver. 
     It is yet a further object of the present invention to provide a novel method of splitting received signal energy between a primary and a standby receiver so that when the primary receiver is inoperable otherwise removed, signal energy to the standby receiver is increased. 
    
    
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
     FIG. 1 is a partial schematic diagram of a signal receiver system. 
     FIG. 2 is a partial schematic diagram of signal receiver system illustrating operation of the present invention. 
     FIG. 3 is a block diagram illustrating a preferred embodiment of the present invention. 
     FIG. 4 is partial circuit diagram of a low noise amplifier which finds application in the receivers of the present invention. 
     FIG. 5 is a simplified block diagram of another embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference now to FIG. 2, in an embodiment of the present invention in which received signal energy is divided among plural receivers (represented in FIG. 2 by amplifiers  20  and  22  as the remainder of the receiver is not significant to the present invention), the received signal energy is provided in its entirety to a first amplifier  20  in a first receiver which amplifies a primary portion of the received signal energy and reflects a reflected portion of the received signal energy, and the reflected portion is thereafter redirected to a second amplifier  22  in a second receiver. 
     By way of further explanation, and with reference to FIG. 3, in a preferred embodiment the received signal enters antenna  30  and passes through a circulator  32  and filter  34  which typically serves to keep the strong transmitter signal(s) out of the receivers. The received signal is provided from filter  34  to the invention, a first three port directional circulator  36  which routes the entire signal to first low noise amplifier  38  in the primary receiver. Circulator  36  directs signals which are input to port A to be output at port B, and directs signals which are input to port B to be output at port C. Circulator  36  may be low loss microwave circulator which has less than 0.2 dB loss (preferably no loss) in the X-band. As is apparent, the losses in the signal delivered to amplifier  38  are limited to those small losses in circulator  36 , in contrast to the power splitter and attendant losses of the prior art. 
     Low noise amplifier  38  amplifies a primary portion of the received signal and is intentionally optimized for low noise (thereby aggravating the impedance mismatch) to cause a usable signal to be reflected back to port B of circulator  36 . Circulator  36  directs the reflected signal out of its port C to a second three-port directional circulator  40  (whose operation is the same as circulator  36 ) which routes the reflected signal to second low noise amplifier  42  in the standby receiver. Low noise receiver  42  may also reflect a portion of the signal directed to it, and the reflected portion is routed back to circulator  40  which directs it to a terminator  44  where the signal is absorbed or otherwise terminated. 
     Low noise amplifier  38  may be a single ended amplifier such as illustrated in FIG.  4 . The preferred design includes but a single transistor  48  optimized for low noise. Such a transistor desirably exhibits a return loss of less than 6 dB (noise figure of 1.5 dB in the X-band). 
     The losses afforded by the present invention are apparent from the following example of a 10.7-11.7 GHz receiver with circulator losses of 0.2 dB. As is apparent, the losses at the primary receiver in the present invention are significantly less than in the prior art, and the losses at the standby receiver are also reduced. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Present Invention 
               
             
          
           
               
                   
                 Return Loss of 
                   
                   
                   
                   
                   
               
               
                   
                 Low Noise Amp 
                   
                 Primary Loss 
                   
                 Standby Loss 
               
               
                   
                   
               
             
          
           
               
                   
                 2.0 
                 dB 
                 0.2 
                 dB 
                 2.4 
                 dB 
               
               
                   
                 3.0 
                   
                 0.2 
                   
                 3.4 
               
               
                   
                 4.0 
                   
                 0.2 
                   
                 4.4 
               
               
                   
                 5.0 
                   
                 0.2 
                   
                 5.4 
               
               
                   
                 6.0 
                   
                 0.2 
                   
                 6.4 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Prior Art 
               
             
          
           
               
                 Power Split 
                   
                   
                   
                   
               
               
                 to Primary 
                 Primary Loss 
                   
                 Standby Loss 
               
               
                   
               
             
          
           
               
                 50% 
                 3.0 
                 dB 
                 3.0 
                 dB 
               
               
                 60  
                 2.2 
                   
                 4.0 
               
               
                 70  
                 1.5 
                   
                 5.2 
               
               
                 80  
                 1.0 
                   
                 7.0 
               
               
                 90  
                 0.5 
                   
                 10.0 
               
               
                   
               
             
          
         
       
     
     The present invention is not limited to two receivers, as any appropriate number of receivers and associated circulators may be cascaded in the manner suggested above. 
     The present invention affords the further advantage of splitting received signal energy between a primary and a standby receiver so that when the primary receiver is removed or otherwise inoperable creating a mismatch, signal energy to the standby receiver is increased. With reference again to FIG. 3, received signal energy is provided to circulator  36  which is coupled to the primary receiver which, if operable, amplifies a portion of the received signal energy and reflects a reflected portion of the received signal energy. The reflected portion is directed back to circulator  36  and provide to circulator  40  which is coupled to standby receiver. When the primary receiver is decoupled from Circulator  38  the signal energy exiting port B of circulator  36  is reflected back in its entirety to circulator  36  (less any losses encountered) and is directed to circulator  40  and to the standby receiver. Thus, the signal to the standby receiver is almost the entire signal which would have been received by the primary receiver. In contrast, the prior art would continue to direct only the split portion of the signal to the standby receiver if the primary receiver became inoperable. 
     By way of further explanation, and with reference to FIG. 3 (the preferred embodiment) and FIG. 5 (the directional coupler/balanced amplifier approach), the received signal enters antenna  30  and passes through a circulator  32  and filter  34  which typically serves to keep the strong transmitter signal(s) out of the receivers. The received signal is provided from filter  34  to an optional isolator  36  which routes the entire signal to first balanced low noise amplifier  50  in the primary receiver. 
     The received signal is then input to a first directional coupler  51  in the first balanced low noise amplifier  50 . The directional coupler has the property of splitting the input signal at  49  between ports  52  and  53 . The signal is then directed from the directional coupler to two Low noise amplifiers  54  and  55  which amplify a primary portion of the received signal, which is combined in a subsequent directional coupler  58  for the primary receiver. The amplifiers  54  and  55  are intentionally optimized for low noise (thereby aggravating the impedance mismatch) to cause a usable signal to be reflected back to each port ( 53  &amp;  53 ) of the directional coupler. 
     The directional coupler having a directional property combines the reflected signals at the reflection port  56 . The combined reflected portions at the reflected port  56  are then. connected to a subsequent low noise amplifier  57  in the secondary receiver. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full Range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.