Method and apparatus for determining and directly measuring the voltage standing wave ratio of an RF transmission system
FIELD OF THE INVENTION 
     This invention relates to a system for determining the voltage standing wave ratio (VSWR) for an RF transmission system, and more particularly, to a system which allows measurement of the VSWR on a &#34;larger than&#34; or &#34;smaller than&#34; basis, as compared to a predetermined value, as well as allowing for direct VSWR measurement. 
     DESCRIPTION OF THE PRIOR ART 
     The condition of an RF transmission system is indicated by the VSWR as determined by the impedance match between an antenna and a transmission line. As the transmitter power in an RF transmission system is varied, the magnitudes of the forward voltage and reflected voltage also vary. It is therefore desirable to have a system whereby the standing wave ratio of an RF transmission may be monitored, without constant correction for variation. Although a number of systems have been proposed to provide such a monitor, these systems frequently suffer from the need to calculate VSWR on the basis of measured variable inputs, depending on sensors which require constant recalibration. Another common disadvantage of prior art systems is the difficulties introduced in precisely calculating the VSWR at any given time. Frequently, in operation, all that need be determined is the relative position of the VSWR against a known value, not an accurate determination of the actual value. Therefore, if a system providing ease of calibration and insensitivity to variation could be established for determining the relative value of the VSWR of an RF transmission system could be established, monitoring and correction of such transmission systems would be considerably facilitated over prior art practices. 
     Accordingly, it is one object of this invention to provide a system whereby the relative value of the VSWR of an RF transmission system may be determined. 
     It is another object of this invention to provide a system for determining VSWR which is insensitive to voltage variation and may be easily calibrated. 
     It is yet a further object of this invention to provide a system whereby the VSWR of a transmission system may be directly measured. 
     These and other objects of the invention will be apparent from the detailed description below. 
     SUMMARY OF THE INVENTION 
     The VSWR value is determined by selecting a predicted or predetermined VSWR and looking up the ratio of the absolute value of the reflected voltage to the absolute value of forward voltage for the predetermined VSWR. This ratio is then multiplied by the sensed instantaneous forward voltage of the system, the product of that multiplication being compared to the measured instantaneous reflected voltage. Depending on whether the measured reflected voltage is either greater or smaller than the multiplication product of the ratio times the forward voltage, the actual value of the VSWR is greater or less than the predicted value of the VSWR, respectively. 
     To make direct measurement of the VSWR of the transmission system, the results of the above comparisons are fed to a counter. Based on the result of the comparison, the counter is either incremented or decremented and a new predetermined value VSWR is fed to the look-up table containing values of the calculated ratio versus predicted VSWR. The predetermined VSWR is therefore either increased or decreased, and the operation repeated. This repetition is continued until the difference of the measured reflected voltage less the ratio multiplied by the forward voltage is of a sign different than the initial difference. The predetermined VSWR used in the current repetition is the actual VSWR, which may now be displayed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block flow chart diagram illustrating the basic inputs and calculations necessary to the invention. 
     FIG. 2 is a block flow chart diagram illustrating the details of the system of this invention, whereby the relative value of the VSWR may be determined. 
     FIG. 3 is a flow chart illustrating the embodiment of the system whereby the VSWR may be directly measured. 
     FIG. 4 is a partial block flow chart diagram adapted from FIG. 2 and illustrating the details of the system used with the flow chart of FIG. 3 to directly measure the VSWR. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The voltage standing wave ratio of an RF transmission may be defined by the following equation: ##EQU1## where E f  =absolute value of instantaneous forward rf voltage E r  =absolute value of instantaneous reflected rf voltage 
     r=ratio of E r  /E f   
     Therefore, r=(S-1)/(S+1) wherein S=VSWR. 
     From the above equations, it may be seen that, for any given VSWR value of an RF transmission system, each such value will have a single value r, which may be calculated for a range of VSWRs. A table of ratio r versus VSWR for the operating range of any RF transmission system may thereby be generated. This lookup table is stored in a permanent read-only memory. It is apparent that, for any given VSWR, there will be a specific reciprocal for ratio r as well. Tables of reciprocal r versus VSWR may also be generated and stored in the PROM. 
     Based on the above defining equations, it is apparent that, given a predicted value r p  of ratio r, and an actual measured value E fa  of E f , there exists a predicted value E rp  of E r  corresponding to E fa , such that the following equation is valid: 
     
         E.sub.rp =r.sub.p E.sub.fa 
    
     If the actual value E ra  of E r  is equal to the predicted value E rp , then the predicted value r p  is equal to the actual value of r a  in this measurement. Accordingly, the actual value of the VSWR a  equals the predetermined value VSWR pd . Alternatively E ra  must be either larger than, or smaller than, E rp , indicating that the actual value of VSWR a  is larger than, or smaller than, respectively, the predetermined value of VSWR pd . 
     The instant system for determining and/or directly measuring VSWR is based on implementing the above derived relationships. This implementation is broadly illustrated in FIG. 1. The predetermined VSWR pd  addresses a look-up table which contains a complete set of values r p  over the VSWR operating range of the RF transmission system. The VSWR pd  may be selected either as an extreme limit for the transmission system, to determine if the system is actually operating outside those limits, or as an expected value, to determine if the transmission system is operating as anticipated. The output from the look-up table, the ratio of the absolute value of the reflected voltage to the absolute value of the forward voltage, r p , is provided from the look-up table to the digital multiplier of FIG. 1. As noted, this output could as easily be the reciprocal of r p . 
     The sensed actual forward voltage is also inputted to the digital multiplier. The multiplier output, r p  times E fa , is the predicted reflected voltage value E rp , which is then compared, in the comparator, with the actual measured reflected voltage E ra . The comparator determines whether E ra  is larger or smaller than E rp , a decision marked by a &#34;1&#34; level or a &#34;0&#34; level output, respectively. On the basis of that output, the relative value of the actual VSWR a , as compared with the predetermined VSWR pd , is known. Particularly where the predetermined VSWR pd  is a value at the limit of the operating range of the RF transmission system or desired operating range, this information may be satifactory, and the process is then repeated periodically, to determine continued performance by the transmission system. 
     FIG. 2 illustrates, in detail, the apparatus employed in carrying out this system. A set of switches holds the predetermined VSWR pd  value in binary coded decimal (BCD) format. The VSWR pd  value addresses the PROM look-up table, which is the permanent memory part of the microprocessor device of the apparatus of this system. The PROM table output value of r p  is presented in binary format with as many bits as required by the resolution of the measurement and by the magnitude of the corresponding VSWR pd  number. 
     The microprocessor controls the reading of the BCD switches, the addressing of the look-up table, and the loading of the ratio r p  value into the binary latches. The ratio r p  is stored into the binary latches so that the microprocessor can release itself from this measurement. 
     The latches feed the binary input (r) to the digital-to-analog converter. The digital-to-analog converter receives at the reference input the absolute value of the actual forward voltage E fa  in analog form. This value is the output of an analog sensor which detects the envelope voltage of the forward RF signal. The digital-to-analog converter multiplies the two inputs, its output therefore being the absolute value of the predicted reflected voltage E rp . 
     This output is sent to the comparator, where it is compared with the absolute value of the actual reflected voltage E ra , which is, again, the output of an analog sensor which detects the envelope voltage of the reflected RF signal. 
     The comparator constantly compares the two inputs and presents the comparison result to the microprocessor. The microprocessor need only look at the comparator output on a periodic basis, and thereupon determines the outcome of the measurement; failure indication, next measurement preparation, etc. The time-to look by the microprocessor is programmed so that it allows for settling time at the analog voltage channels where E fa  and E ra  originate. 
     As noted, in place of the ratio value r p  for the predetermined VSWR pd , the reciprocal of r p  could be stored in a look-up table. In this system, the absolute value of the actual reflected voltage E ra  would be inputted to the digital-to-analog converter, and the absolute value of the actual forward RF voltage E fa  would be the basis for the comparison made by comparator. In such a system, the actual VSWR will vary inversely with the difference between the actual and predicted forward voltage. However, although the results of the reciprocal system are equal, and it is recognized as an art equivalent, it introduces several difficulties. In particular for a matched system, the reciprocal numbers tend to be quite large, requiring mathematical computations which are subject to error. 
     The above-described system is particularly valuable in determining if actual VSWR a  falls between two sets of predetermined limits, frequently the limits of the operating range of the RF transmission system, as noted above. In such a system, there are two BCD switch settings: an upper and a lower VSWR limit. The microprocessor alternates the predetermined VSWR pd  and corresponding r p  value between the two limits, and the comparator output is processed accordingly. As is apparent, the VSWR determination provided by this system is insensitive to analog voltage drift in E fa  and E ra , provided the variations due to drift track each other. 
     Although, as noted, the above-described system has its own utility, particularly in determining whether a transmission system is operating within desired limits, it may frequently be desirable to directly measure the VSWR of the system at any particular time. An embodiment of this invention suitable for so directly measuring VSWR is illustrated in FIGS. 3 and 4. 
     A synchronous counter, whose count represents VSWR, counts up starting at the minimum value one. Alternatively, the counter may count down starting from a maximum value or have a predetermined value set therein. The counter feeds the look-up table that contains values of the ratio r versus VSWR. For each counter value, a VSWR pd  is selected, the ratio r inputted as above, and the above-described comparison between predicted and actual voltages is made. The comparator output is examined and a decision is made on whether or not to increment the counter. When the comparator indicates the value for the predicted reflected voltage E rp  has just exceeded the actual reflected voltage value E ra , the VSWR counter value has reached the actual VSWR value, and the counter incrementing stops. The display circuit is now loaded with the final VSWR value from the counter, which corresponds to the actual VSWR a  value. 
     As is apparent to those of skill in the art, in terms of the mathematical relationship set forth above, the comparator, on the initial comparison, subtracts from the predicted reflected or forward voltage the actual voltage. If the predetermined VSWR pd  is the actual VSWR, the difference is zero, the counter is stopped, and the predetermined VSWR displayed. If the difference is positive or negative, the incrementing up or down continues until the difference is of a sign opposite that of the initial comparison. The predetermined VSWR pd  employed in making the last comparison then corresponds to the actual VSWR, which is then displayed. 
     The instant invention has been described above with respect to particular embodiments and devices. Variations will occur to those of ordinary skill in the art, particularly with respect to the relationship between the input and output values, and the hardware employed in driving and performing operations on the inputted data, without the exercise of inventive faculty. Such variations remain within the scope of the instant invention.