TECHNICAL FIELD  
       [0001]     The present invention generally relates to compensation of cable loss, and more particularly relates to automatic compensation of radio frequency cable losses for aircraft systems and other applications.  
       BACKGROUND  
       [0002]     There are many applications today where communication system receivers, transmitters, power amplifiers and other elements are interconnected by electrical cables carrying radio frequency (RF) signals. For convenience of description, the combination of a receiver and transmitter is referred to herein as a “transceiver”, abbreviated as “T/R”. For proper operation the signal losses occurring in these cables must be taken into account in designing and constructing the systems. If the size and/or configuration of the installations vary from application to application, then the cable losses will likely also vary and must therefore be adjusted or compensated for each system installation. In the aviation industry for example, various standards have been adopted to attempt to limit the variability encountered in such system installations. A non limiting example is in the installation in aircraft of satellite communication systems for use with the Inmarsat® satellites.  
         [0003]      FIG. 1  is a simplified electrical schematic block diagram of airborne satellite communication system  20  according to the prior art, suitable for use with the Inmarsat satellites, which operate for example at frequencies in the range of 1,626.5 to 1,660.5 mega-Hertz, but such frequencies are not critical to the present invention. System  20  comprises transceiver (T/R)  22  coupled by RF pathway  23  to high power amplifier (HPA)  24 . HPA  24  is coupled by RF pathway  25  to diplexer  26 . Diplexer  26  is coupled by RF pathway  27  to antenna  30  and by RF pathway  29 - 1  to low noise amplifier (LNA)  28 . LNA  28  is coupled by RF pathway  29 - 2  to T/R  22 . LNA  28  may be combined with diplexer  26  so that only a single pathway (hereafter RF pathway or link  29 ) is needed. Either arrangement is useful. Diplexer  26  is conventional and separates the incoming and outgoing RF signals. Incoming RF signals received from antenna  30  are directed by diplexer  26  to LNA  28  where they are amplified and sent over RF link  29  to T/R  22  where they are demodulated and/or decoded and the results presented to the user in audio or other form via communication link  32 . Similarly, outgoing communications received from the user via link  32  are modulated and/or encoded by T/R  22  to form a modulated and/or encoded RF signal that is sent via RF link  23  to HPA  24  where it is amplified and sent via RF link  25  to diplexer  26 , which in turn directs it to antenna  30  over RF link  27 . Elements  22 ,  24 ,  26 ,  28  and  30  of RF communication system  20  are conventional and well known in the art.  
         [0004]     HPA  24  is typically physically located close to diplexer  26  and antenna  30  to minimize loss of signal power over link  25 . However, T/R unit  22  may be near or far from HPA  24  depending upon the size and configuration of the aircraft Thus signal losses in, for example, link  23  can be a serious concern. To accommodate this installation variability, a standard has been adopted in the aviation industry requiring that transceiver (T/R)  22  deliver a power level sufficient to overcome up to 25 dB of cable loss in link  23  and still provide adequate drive at input  24 - 1  of HPA  24 . A lower limit of 19 dB of cable loss is also specified to minimize the dynamic range that is required at input  24 - 1  to HPA  24 . If the actual loss along RF cable or link  23  for a particular installation is less than the 19 dB minimum, then additional loss must be inserted in the cabling to force the signal arriving at HPA  24  to conform to the 19-25 dB loss range specified in the standard. One or more fixed or manually settable attenuators  34  are provided at input  24 - 1  of HPA  24  or in RF cable or link  23  between T/R  22  and HPA  24  to adjust the RF signal loss along link  23  to meet the desired specification, for example, 19-25 dB total loss in the case of Inmarsat communication systems. Attenuator(s)  34  are set to the necessary attenuation during system design and installation and generally depend upon the aircraft size and configuration. Attenuator(s)  34  will often vary from installation to installation and aircraft to aircraft because of differences in aircraft size and wiring configuration.  
         [0005]     These additional attenuators and/or other custom components add weight, increase installation time and reduce overall system reliability due to the extra cable connectors and fittings that may loosen or degrade over time. They also make system maintenance more complex and expensive since different aircraft in the same fleet may have different attenuator configurations and/or settings so that different parts and documentation are needed for the various planes being serviced by the same installation and/or maintenance organizations. Accordingly, it is desirable to provide a cable loss compensation system that avoids the need for different attenuation and compensation devices. In addition, it is desirable that cable loss compensation and/or industry standard loss specifications be achievable with a common system for different aircraft. It is further desirable that the cable loss compensation means and method be capable of automatic operation so that loss compensation is achieved without human intervention. It is additionally desirable that the system be able to compensate in whole or part for changes in cable loss that occur over time due to system aging or other factors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY  
       [0006]     An apparatus is provided for compensation of RF coupling between a transmitter and amplifier of a communication system. The apparatus comprises a signal source coupled to the transmitter for providing an RF test signal of a first magnitude to the RF coupling, a test signal measuring apparatus at the RF input of the amplifier for measuring a second magnitude of the test signal reaching the RF input of the amplifier through the RF coupling, and an electronically adjustable attenuator serially coupled between the transmitter and the RF input of the amplifier and responsive to differences between the first and second magnitudes so as to provide attenuation in an RF communication signal passing into the amplifier from the RF coupling such that the sum of RF signal loss in the coupling and the attenuator has a predetermined value  
         [0007]     A method is provided for compensation of an RF communication link between a transmitter and amplifier of a communication system. The method comprises sending a test signal of known initial strength through the RF communication link to an input port of the amplifier, comparing a received strength of the test signal at the input port to the initial strength, determining the path loss of signal strength through the RF communication link, and automatically setting a variable attenuator in the communication link so that the combination of the path loss and attenuation loss has a predetermined value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]      FIG. 1  is a simplified electrical schematic block diagram of an airborne satellite communication system according to the prior art;  
         [0010]      FIG. 2  is a simplified electrical schematic block diagram of a loss compensation system of the present invention, applied to a communication system of the type illustrated in  FIG. 1 , according to a first embodiment;  
         [0011]      FIG. 3  is a simplified electrical schematic block diagram of a loss compensation system of the present invention, according to a further embodiment;  
         [0012]      FIG. 4  is a simplified flow chart illustrating a method of the present invention according to a first embodiment; and  
         [0013]      FIG. 5  is a simplified flow chart illustrating a method of the present invention according to a further embodiment and showing further details. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. As used herein the term “radio frequency” and the abbreviation “RF” are intended to be interpreted broadly and include all portions of the electromagnetic spectrum suitable for the transmission of modulated or coded signals. For convenience of explanation, the present invention is described for a communication system useful with the Inmarsat satellite system, but this is not intended to be limiting and the present invention is application to any type of RF systems where automatic compensation of a connecting cable or other transmission medium is useful. For convenience of illustration in  FIGS. 1-3 , RF signal pathways are shown as heavy lines and control or input signals (whether analog or digital) are shown as light lines.  
         [0015]      FIG. 2  is a simplified electrical schematic block diagram of loss compensation system  40  of the present invention, applied to a communication system of the type illustrated in  FIG. 1 , according to a first embodiment. System  40  comprises diplexer  26  coupled to antenna  30  and low noise amplifier (LNA)  28  coupled to diplexer  26 , similar to those employed in prior art system  20 . System  40  further comprises transceiver (F/R)  42  that receives signals from LNA  28  over RF link  29  and during ordinary communications provides the same receive and transmit functions as T/R  22  of system  20 . The internal components for providing these normal communication functions are well known in the art and are generally omitted in  FIGS. 2-3  in order not to obscure the elements of the present invention needed for providing cable loss compensation. T/R  42  also comprises modem  42 - 1  and T/R processor  42 - 2  coupled by signal link  42 - 3 . T/R  42  also comprises memory  42 - 4  coupled to processor  42 - 2  by signal link  42 - 5 . Modem  42 - 1 , T/R processor  42 - 2  and/or memory  42 - 4  are useful for providing cable loss compensation but may also provide functions associated with the normal communication mode of operation of system  40 . User communication I/O is conveniently provided via signal link  52  to modem  42 - 1  during normal communication operation of system  40 . System  40  is switched into the cable loss compensation mode via an initiate signal received over signal link  53 . This initiate signal may be provided automatically upon system power-up or by an operator input (e.g., a user activated “compensate” switch or other means) or by a command issued by the aircraft flight control or maintenance management system according to instructions stored therein. Either arrangement is useful. T/R  42  is coupled by RF cable or link  43  (analogous to pathway or link  23  of system  20 ) to high power amplifier (HPA)  44 .  
         [0016]     During normal communications, HPA  44  amplifies the modulated RF signal received from T/R  42  and sends it over RF link  25  to diplexer  26  and thence to antenna  30 . System  40  differs from system  20  in that system  40  according to the present invention includes means and methods for providing automatic cable loss compensation, which otherwise with system  20  must be performed by physical modification of the system hardware. HPA  44  of system  40  comprises input filter  46 , input attenuator (ATTN)  48 , input detector  50 , back-off attenuator  52 , power amplifier  54 , and RF output detector  56 , wherein these elements are coupled respectively by RF links  47 ,  49 ,  51 ,  53 , and  55 . Input filter  46 , back-off attenuator  52 , power amplifier  54  and output detector  56  are conventional and provide the same function as similar elements (not shown) in system  20 . HPA  44  of system  40  further comprises HPA processor  58  and associated memory  59 . Input filter  46  receives an RF signal from RF pathway or link  43  and removes unwanted sidebands or noise signals. RF output detector  56  provides feedback to HPA processor  58  on the output power being developed by amplifier  54 . Detector  56  is coupled to RF output path  25 . In the present invention, processor  58  is coupled to input attenuator  48  by signal path or bus  58 - 1 , to input detector by signal path or bus  58 - 2 , to back-off attenuator  52  by signal path or bus  58 - 3 , to RF output detector  56  by signal path or bus  58 - 4  and to memory  59  by signal path or bus  58 - 5 . HPA processor  58  is also coupled to T/R processor  42 - 2  by digital communication bus or signal link  58 - 6 , and to amplifier  54  by optional bus or control link  58 - 7 .  
         [0017]      FIG. 3  is a simplified electrical schematic block diagram of loss compensation system  60  according to a further embodiment of the present invention. System  60  comprises elements  42 ,  42 - 1 ,  42 - 2  and  42 - 3  that are analogous in function to similarly identified elements of system  40 , which description thereof is incorporated herein by reference. Similarly, system  60  comprises elements  44  to  58 - 6  that are analogous in function to similarly identified elements of system  40 , which description thereof is also incorporated herein by reference. Systems  40  and  60  differ in that in system  40  of  FIG. 2 , input attenuator  48  is located in the RF signal pathway ahead of input detector  50 , whereas in system  60 , input detector  50 ′ is located in the RF signal pathway ahead of input attenuator  48 ′. In system  60 , the RF signal arriving over RF cable or link  43  passes through input filter  46  and is sent to input detector  50 ′ via RF link  47 . From input detector  50 ′, the RF signal passes via RF link  51 ′ to input attenuator  48 ′. From input attenuator  48 ′, the RF signal passes to back-off attenuator  52  via RF link  49 ′. The rest of the elements and signal links of system  60  are otherwise arranged in substantially the same manner as for the elements of system  40 . As will be explained the operation of the systems  40  and  60  is slightly different, but either arrangement is useful. Accordingly, the operation of systems  40  and  60  will be described together. While input attenuator  48 ′ and back-off attenuator  52  are shown as separate elements in  FIG. 3 , this is not essential and they may be combined as a single attenuator providing a combined function.  
         [0018]     The operation of systems  40  and  60  in a cable loss compensation mode is now described. Acting under the control of T/R processor  42 - 2 , modem  42 - 1  generates an RF test signal of known strength that is sent via link  43  to HPA  44  where it is received via input filter  46 . Element  42 - 1  is identified in  FIGS. 2-3  as a “modem” and a modem is useful for generating the cable compensation test signal. However, a modem per se is not essential for the cable compensation mode of operation and any type of test signal generator may be used for element  42 - 1 . Accordingly, element  42 - 1  is also more generally referred to in the cable compensation mode of operation as a “test signal source” and the label “modem” is intended to include this broader description of element  42 - 1 , that is, comprising any suitable form of signal generator for providing the RF test signal used to determine the cable loss. In the case of system  40 , after transiting RF link  43  this RF test signal is then passed through input attenuator  48  to input detector  50  and thence to back-off attenuator  52 . In the case of system  60 , after transiting RF link  43  this RF test signal is then passed via input detector  50 ′ to input attenuator  48 ′ and thence to back-off attenuator  52 . Back-off attenuator  52  prevents the RF test signal from being coupled to power amplifier  54  and diplexer  26  via output detector  56  when systems  40 ,  60  are operating in the cable loss compensation mode. After cable loss compensation is complete and during normal communications, back-off attenuator  52  is reset to zero attenuation (or other predetermined value) and the conventional RF communication signals pass through to power amplifier  54  and via RF output detector  56  to diplexer  26  and antenna  30 . Output detector  56  is used to monitor the power output of HPA  44 . Back-off attenuator  52  is also useful for adjusting the power output from amplifier  54 . When used in conjunction with HPA processor  58  and back-off attenuator  52 , output detector  56  can facilitate maintaining a predetermined power output from HPA  44 .  
         [0019]     In the case of system  40  of  FIG. 2 , input attenuator  48  is desirably set to zero attenuation when the test RF signal is sent to HPA  44 . In that circumstance, the RF signal strength measured at input detector  50 , can be compared directly to the transmitted signal strength to obtain the cable loss as the difference between the transmitted RF signal power “TP” sent by T/R  42  and the received RF signal power “RP” at input detector  50 . Accordingly, the cable loss CL equals TP-RP. Alternatively, if the loss of input attenuator  48  is not set to zero but has an initial value of ALI, then the cable loss CL equals TP−(RP+ALI) where ALI is the initial attenuator loss. Attenuator  48  is desirably an electrically variable attenuator whose loss AL is determinable. The measured cable loss CL is then used to set the final value of ALF in attenuator  48  so that the desired total loss TL=CL+ALF is obtained and the correct signal strength is provided to power amplifier  54 . Electrically controllable RF attenuators are well known in the art.  
         [0020]     In the case of system  60  of  FIG. 3 , input attenuator  48 ′ is located electrically after input detector  50 ′. Therefore, input detector  50 ′ measures the received RF signal power RP independent of the setting of attenuator  48 ′, and the cable loss CL=TP-RP. This cable loss information is then used to set the value of ALF in attenuator  48 ′ so that the desired total loss TL=CL+ALF is obtained and the correct signal strength is provided to power amplifier  54 . Electrically controllable RF attenuators are well known in the art. Following the cable loss auto-compensation function, back-off attenuator  52  is reset to its ordinary communication function value and system  40  or  60  returned to the normal communication (COM) mode. It will be noted that with the arrangement of  FIG. 3 , system  60 , input attenuator  48 ′ and back-off attenuator  52  may be combined and only a single attenuator used for both functions.  
         [0021]      FIG. 4  is a simplified flow chart illustrating method  200  of the present invention according to a first embodiment. Method  200  begins with START  202  and INITIATE SIGNAL RECEIVED ? query  204 , which determines whether or not an initiation signal has been received by system  40  or  60  to start the cable compensation function. The initiate signal may be generated automatically on system power-up, so that cable compensation is automatically triggered whenever satellite communication system  40  or  60  is turned on. Alternatively, the initiate signal may be provided by an operator action, such as depressing a suitable cable compensation control switch. Still further the initiate signal may be provided at predetermined intervals by the flight control system or other aircraft management system. A yet further alternative is to have the cable compensation initiate signal provided by an maintenance control system according to a predetermined schedule, or any combination of the forgoing approaches or other arrangement selected by the system designer, aircraft operator and/or maintenance organization. Any of these arrangements is useful.  
         [0022]     If the outcome of query  204  is NO (FALSE), then method  200  loops back as shown by path  205  to await the occurrence of an “initiate” event. If the outcome of query  204  is YES (TRUE) indicating that an initiate event has occurred, the method  200  proceeds to step  206  wherein the normal communication mode of system  40  or  60  is turned off and system  40 ,  60  is placed in the cable compensation mode of operation. The normal communication mode of operation may be turned OFF in any number of ways, for example and not intended to be limiting, by having processor  58  bock transmission of the RF signal through back-off attenuator to power amplifier  54  or by disabling power amplifier  54  (e.g., by optional link  58 - 7 ) or by other means. In subsequent step  208 , system  40  or  60  sends an RF test signal through the cable being compensated. As used herein, the word “cable” is intended to include any form of RF signal path coupling T/R  42  and HPA  44  and not be limited merely to conventional coaxial cables. In following step  210  the transmitted and received signals are compared, for example and not intended to be limiting, by comparing TP and RP. In step  212 , the cable loss (e.g., CL=TP−RP) is determined, the exact method or calculation depending upon whether the arrangement of system  40  or system  60  or other configuration is being used, as has been previously explained. In step  214  the attenuator in the RF signal path (e.g., attenuator  48 ,  48 ′) is set by processor  58  or  42 - 2  or a combination of  42 - 2  and  58  or an equivalent system processor, to have a value ALF so that the total loss TL=CL+ALF has the desired value for proper system operation. The correct value of attenuator loss ALF to be set can be determined by any number of means, for example, calculated from the CL value determined in step  212  by processors  42 - 2 ,  58  or a combination thereof using an algorithm relating the attenuator control input to its attenuation, or by use of a look-up table relating control input to desired attenuation value or other appropriate means. Any suitable arrangement may be used. Then in step  216 , system  40  or  60  is returned to its normal communication mode of operation and method  200  returns to START  202  as shown by path  217 .  
         [0023]      FIG. 5  is a simplified flow chart illustrating method  300  of the present invention according to a further embodiment and showing further details. Method  300  begins with START  302  and INITIATE SIGNAL RECEIVED ? query  304  analogous to query  204  of method  200 , which determines whether or not an initiation signal has been received by system  40  or  60  to start the cable compensation function. The occurrence of the initiate signal is explained in connection with query  204  of method  200  and such discussion is incorporated herein by reference. If the outcome of query  304  is NO (FALSE), then method  300  loops back as shown by path  305  to await the occurrence of an “initiate” event. If the outcome of query  304  is YES (TRUE) indicating that an initiate event has occurred, then method  300  proceeds to step  306  wherein the normal communication mode of system  40  or  60  is turned off and system  40 ,  60  is placed in the cable compensation mode of operation, as previously discussed in connection with step  206  of method  200 , which discussion is incorporated herein by reference. In step  308 , then in either order, sub-steps  308 - 1  and  308 - 2  are executed, wherein in sub-step  308 - 1  input attenuator  48  is set to its minimum value and in sub-step  308 - 2  back-off attenuator  52  is set to its maximum value. In following step  310 , an RF test signal is sent (e.g. by modem  42 - 1  operating under the control of T/R processor  42 - 2 ) from T/R  42  via RF cable link  43  to HPA  44 . In the case of system  40  this RF test signal is sent via input filter  46  and input attenuator  48  to input detector  50 . In the case of system  60 , this RF test signal is sent via input filter  46  to input detector  50 ′. In either case, in step  312  the magnitude of the RF test signal, as for example, the transmitted power value TP is stored in memory (e.g., memory  42 - 4 ), and in step  314  the magnitude of the signal received by HPA  44  (e.g., the received signal power value RP) is measured by input detector  50 ,  50 ′. Any loss occurring in input filter  46  is either negligible or easily taken into account. In step  316 , the transmitted and received signal values, e.g., TP and RP, are reported to whichever processor (e.g., processor  42 - 2 ,  58 , a combination thereof or a separate system processor) is assigned to determine the actual cable loss CL based on these values, as previously explained. In the preferred embodiment, in which T/R processor acts as a primary or supervisory processor and HPA processor  58  acts as a secondary or subsidiary processor, the cable loss determination is carried out by T/R processor  42 - 2  using routines stored in memory  42 - 4 , but this is not essential. Once the cable loss CL is determined, the attenuation loss value ALF needed to provide a total loss TL=CL+ALF of the proper value is determined. In step  320  the corresponding control signal needed to be supplied from HPA processor  58  to attenuator  48  or  48 ′ to provide ALF is determined, for example, in the same way as already described in connection with method  200  by evaluation of an appropriate algorithm or use of a look-up table or by other means based in whole or in part on information stored in memory  42 - 4  or  59  operating in cooperation with processor  42 - 2  or  58  or both. In step  322 , the control signal determined in step  320  is automatically provided by processor  58  over lead  58 - 1  to attenuator  48  or  48 ′ to obtain the desired cable compensation so that TL=CL+ALF. Following cable compensation in step  322 , step  324  is executed wherein back-off attenuator  52  is reset to its normal communication operating value and in step  326  the cable compensation routine is terminated and system  40  or  60  is returned to its normal communication mode of operation. Following step  326 , method  300  returns to START  302  and initial query  304  as indicated by path  327  to await a subsequent cable loss compensation initiation event.  
         [0024]     While system  40  of  FIG. 2  and system  60  of  FIG. 3  are shown as having their signal processing functions (and associated memory) partitioned into T/R processor  42 - 2  with memory  42 - 4  and HPA processor  58  with memory  59 , with processors  42 - 2  and  58  linked by bus  58 - 6 , this is not essential and the functions performed by these separate but linked processors may be combined into a single processor and associate memory. Accordingly, the genera terms “processor” and “system processor” are intended to include either arrangement, that is, either a centralized processing function or a partitioned or distributed processing function. The partitioned arrangement illustrated in  FIGS. 2 and 3  are preferred but not essential.  
         [0025]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. For example, while the present invention has been described for convenience of explanation as applied to an Inmarsat communication system, this is not intended to be limiting and the principles taught herein can be applied to any type of system where RF cable loss compensation is desirable. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.
Method and apparatus are provided for compensation of an RF link between a transmitter and amplifier of a communication system. The apparatus comprises a signal source coupled to the transmitter for providing an RF test signal of a first magnitude to the RF link, a test signal measuring apparatus at the RF input of the amplifier for measuring a second magnitude of the test signal reaching the RF input of the amplifier through the RF link, and an electronically adjustable attenuator serially coupled with the RF link and responsive to differences between the first and second magnitudes so as to provide attenuation in an RF communication signal passing into the amplifier from the RF link such that the sum of RF signal loss in the link and the attenuator has a predetermined value.