Abstract:
A distance measuring, device including a lookup table storing values to calibrate a Class AB amplifier to produce a pulse pair with the desired characteristics. The distance measuring device analyzes the characteristics of the output signal and recursively adapts the values stored in a lookup table to force the output the power amplifier to meet the required performance characteristics.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward distance measuring equipment, and specifically toward distance measuring equipment with software defined radio architecture. 
     BACKGROUND OF THE INVENTION 
     Distance measuring equipment (DME) is a transponder based radio technology that measures distance by timing the propagation delay of radio signals. Aircraft use DME to determine their distance from a land-based transponder by sending and receiving two pulses of fixed duration and separation (pulse pairs). 
     An aircraft communicates with a ground transponder using a series of pulse pairs (interrogations) and, after a precise time delay (50 microseconds for Mode X or 56 microseconds for Mode Y), the ground station relies with a pulse pair with the correct spacing. The DME receiver in the aircraft searches for pulse-pairs with the correct time spacing (12 microseconds for Mode X and 30 microseconds for Mode Y). 
     Aircraft have several independent radios. Integrating the various L-Band radios on an aircraft would reduce the size, weight, power and cost of the radios. However, DME transmitters are typically implemented using a saturated Class C amplifier with a drain modulator; integrating DME transmitters with other L-Band radios would be difficult to achieve using a saturated Class C amplifier with drain modulation transmitter architecture. Furthermore, some implementations of integrated L-Band radios may require extensive calibration under various operating conditions. extensive calibration may be prohibitive. 
     Consequently, it would be advantageous if a method and apparatus existed that are suitable for integrating various L-band radios into a single self-calibrating radio architecture. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for integrating various L-band radios into a single self-calibrating radio architecture. 
     One embodiment of the present invention is a software defined radio architecture with a Class AB amplifier. This embodiment utilizes an adaption loop that updates the values within a lookup table in order to maintain a defined pulse width, rise time, fall time and amplifier compression point. A low-pass filter then interpolates between the points of the lookup table. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of one implementation of distance measuring equipment; 
         FIG. 2  shows a block diagram of one implementation of distance measuring equipment with an adaption loop; 
         FIG. 3  shows a graphic representation of a lookup table; 
         FIG. 4  shows a graphic representation of power amplifier input versus output in DME without an adaption loop; 
         FIG. 5  shows a graphic representation of power amplifier input versus output in DME with an adaption loop; and 
         FIG. 6  shows a flowchart of a method for updating a lookup table of values. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a DME that generates a pulse pair  100  having desired characteristics may include a signal generator  104 , one or more driver amplifiers  106 , a Class C power amplifier  108 , and a drain modulator  110 . The DME may also include digital-to-analog (D/A) converters  102 ,  112  to control the function of the signal generator  104  and drain modulator  110 . The signal generator  104  may produce an input signal; the input signal may be amplified by the one or more driver amplifiers  106  for use by the Class-C power amplifier  108 . The Class-C power amplifier  108  may operate at saturation to produce the pulse pair  100 . The drain modulator  110  may alter the supply voltage of the Class-C power amplifier  108  to create the desired pulse pair  100  shape. 
     DME generally operates in the L-Band, as do other radios commonly found in aircraft. It is desirable to integrate the various L-Band radios to reduce the size, weight, cost and power consumption of an aircraft&#39;s radio equipment. However, it would be difficult to integrate multiple radios using Class-C power amplifiers with drain modulation because the drain modulation must be precisely tuned to achieve the desired pulse pair and the circuit is therefore unsuitable for any other purpose. 
     A software defined radio architecture having a Class AB amplifier may be suitable for integrating various L-Band radios in an aircraft. One problem with using a Class AB amplifier is that the Class AB amplifier does not behave linearly, especially when driven close to saturation. Furthermore, the amplifier may behave differently under different thermal conditions; a Class AB amplifier would therefore require extensive calibration related to the anticipated operating conditions. 
     Referring to  FIG. 2 , a self calibrating software defined radio for producing a pulse pair  100  is shown. The self calibrating software defined radio may include a lookup table  204 , an adaption processor  222  and an interpolation filter  206 . The self calibrating software defined radio may also include a sequencer  202  connected to the lookup table  204  and the adaption processor  222  to control the execution of an adaption loop. An adaption loop is a recursive process executed by the adaption processor  222  to modify values stored in the lookup table  204  according to an output. 
     The interpolation filter  206  may be connected to a signal generator  210  to produce an input for a Class AB amplifier  212 . A digital-to-analog converter  208  may also functionally interpose between the interpolating filter  206  and the signal generator  210 . The output from the Class AB amplifier  212  may be a pulse pair  100 . The output from the Class AB amplifier  212  may be applied to a down-converting mixer  214  connected to an analog-to-digital converter  216  to produce a digital signal for processing in an adaption loop. 
     The self calibrating software defined radio may be implemented in a field-programmable gate array (FPGA)  200 . Where the self calibrating software defined radio is implemented in a FPGA  200 , the digital signal from the analog-to-digital converter  216  may be connected to a digital down converter (DDC)  218 . The DDC  218  may convert the signal from the analog-to-digital converter  216  to a lower sampling rate complex baseband signal for processing in the adaption loop. The DDC  218  may be connected to a magnitude function (MAG)  220 , converting the complex baseband signal into an amplitude signal useable in the adaption loop. The MAG processed down sampled digital signal may comprise a representation of the pulse pair amplitude at the output of the Class AB amplifier  212 . The adaption processor  222  may analyze the received digital amplitude signal in order to determine if the performance parameters of the pulse pair output signal produced by the Class AB amplifier  212  accurately match the predefined characteristic values stored in the adaption processor  222 . If the adaption processor  222  determines that the pulse pair output signal does not conform to the predefined required characteristic values stored in the adaption processor  222 , the adaption processor  222  may modify the values in the lookup table  204  in order to force the output of the Class AB amplifier  212  to more closely match the predefined required performance parameters. 
     A software defined radio according to  FIG. 2  may recursively self-calibrate as operating conditions alter the performance of the Class AB amplifier  212 , thereby obviating the need to manually calibrate the system. 
     Referring to  FIG. 3 , a representation of output from the lookup table  204  is shown. An interpolating filter  206  may receive the output from the lookup table  204  and produce an interpolated signal. The interpolating filter  206  may comprise a low pass filter. 
     Referring to  FIG. 4  and  FIG. 5 ,  FIG. 4  shows a graphical representation of an input signal  400  and the corresponding output signal  402  in a DME utilizing a Class AB amplifier  212  without utilizing an adaption loop. Without an adaption loop, an output signal  402  may deform during processing.  FIG. 5  shows a graphical representation of an input signal  500  and the corresponding output signal  502  in a DME utilizing a Class AB amplifier  212  with an adaption loop. The adaption loop may continuously update the input values in the lookup table  204  to accommodate varying performance parameters of the Class AB amplifier  212  according to varying thermal conditions. 
     Referring to  FIG. 6 , a flowchart of a method for modifying values in a lookup table  204  to produce a pulse pair signal  100  utilizing a Class AB amplifier  212 . The method may include generating  600  a signal based on values stored in a lookup table  204 . The values in the lookup table  204  may control the pulse power, pulse width and rise and fall time requirements of a DME. An interpolating filter  206  may then receive the signal and interpolate  602  a complete pulse or waveform. A signal generator  210  and Class AB amplifier  212  may then generate  604  a pulse pair  100  based on the interpolated signal. Under certain thermal conditions, the Class AB amplifier  212  may not produce the desired pulse pair  100  from the interpolated signal. In that case, the pulse pair  100  may be processed  606  to produce an adaption comparison signal. Processing  606  may include converting the pulse pair to a digital signal, down-sampling the digital signal to a sampling rate more easily analyzed by an adaption processor, and converting the complex down sampled signal into an amplitude signal. An adaption processor  222  may then analyze  608  the processed pulse pair to determine whether the pulse pair  100  conforms to the predefined performance parameters store within the adaption processor  222 . If the pulse pair  100  does not conform to the parameters defined in the lookup table  204 , the adaption processor  222  may modify  610  the values in the lookup table  204  to produce a pulse pair more closely matching the parameters as originally defined in the adaption processor  222 . 
     By this method, a DME utilizing a Class AB amplifier  212  may self-calibrate to produce pulse pairs  100  with consistent characteristics even as operating conditions change. Various L-Band radios may thereby utilize the same software defined radio by altering the lookup table of values to produce a desired self-calibrating signal. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.