Abstract:
Low pass equivalent processing is a method for use with a network analyzer, that gethers a sequence of NDATA samples, each sample including real and imaginary components of a signal, over a frequency range from F1 to f2 in frequency steps of (f2-f1)/(NDATA-1), where f1 is greater than zero. The sequence of samples is processed to estimate a characteristic of the signal with respect to a selected discontinuity in the signal. The sequence of samples is converted to time domain, to generate time domain data over a range from zero to t. The selected discontinuity is identified as lying within a range of t1 to t2. Next, the phase of the time domain data is unwound to correct a portion of the phase error resulting from the one sided spectrum that is not dependent on the distance to the discontinuity through the network. Next, the magnitude peak JMAX of the data samples between time range t1 to t2, is determined. The phase angles of the data at two points removed from the magnitude JMAX by a number of data points dependent on the type of biasing window used in measurement of the data are used to determine a slope through JMAX. From this information, an estimate is made of the location of the discontinuity in the network. The phase of the time domain data is unwrapped in response to that estimate of location according to an empirically derived equation.

Description:
LIMITED COPYRIGHT WAIVER 
     A portion of this patent document, contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document, or the patent disclosure as it appears in the U.S. Patent and Trademark Office patent file records, but reserves all other rights whatsoever. 
     CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention is related to copending U.S. patent application Ser. No. 07/176,202 entitled MICROWAVE MEASUREMENT SYSTEM AND ASSOCIATED METHOD, invented by Martin I. Grace et al., filed on the same date as the present application, and owned currently and at the time of invention by a common assignee, and is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic network analysis, and more particularly, to methods and apparatus for processing data samples indicating the transmission or reflection characteristics of an electronic network. 
     2. Description of Related Art 
     Network analyzers, such as the WILTRON 360, available through WILTRON, 490 Jarvis Drive, Morgan Hill, Calif. 95037, measure the real and imaginary components of the reflection and transmission coefficients of a network over a range of equally-spaced frequencies between operator-delivered frequencies f1 and f2. The analyzer will display the results of the measurements in a real and imaginary format, or they can be converted to magnitude and phase and displayed in a variety of graph types for the user&#39;s convenience. One technique, which has proven useful in practice, for analyzing the measurements, involves taking the data measured in a frequency domain and converting it to the time domain by means of an inverse Fourier transform. The Fourier transform will give the response of the network in the time domain. With time domain data, the results of the measurements can be used to determine distances in the network to discontinuities reflected in the data, because time is directly related to distance through the network by the velocity of propagation of the signals through the network. Therefore, this technique is useful for analyzing complex electronic networks in order to isolate discontinuities. 
     If only the magnitudes and locations of various discontinuities are to be determined, it is then a simple matter to transform the frequency domain data exactly as it is measured. However, it is possible to increase the utility of the time domain data by performing additional processing steps to identify correct phase measurements for the data. 
     In the prior art, a true low-pass algorithm has been utilized to construct the additional time domain data reflecting accurate phase information. However, for the true low-pass algorithm, f1 must be zero. The algorithm works by constructing a complete double-sided spectrum from -f2 to f2 by taking the complex conjugate of the data at each positive frequency and placing it at the corresponding negative frequency as shown below. 
     
         ______________________________________DATA FROM MEASUREMENT                  0               f2  FREQUENCY                                      DOMAIN                  x1  x2  x3  . . .                                  xn  (real part of data)                  0   y2  y3  . . .                                  yn  (imaginary part)CONSTRUCTED COMPLEX CONJUGATE SPECTRUM-f2                    0               f2  FREQUENCY                                      DOMAIN xn  . . .  x3     x2  x1  x2  x3  . . .                                  xn  (real part)-yn  . . . -y3    -y2  0   y2  y3  . . .                                  yn  (imaginary part)______________________________________ 
    
     The inverse Fourier transform of the constructed complex conjugate spectrum is real, that is, the imaginary part is zero for all data samples as shown below. 
     
         ______________________________________0                              t   TIME DOMAINx1  x2    x3    . . .              (real part)0   0     0     . . .              (imaginary part)______________________________________ 
    
     From the real part of the transform, it is possible to deduce not only the magnitude of the network discontinuities, but their nature. If the discontinuity consists of a change in impedance, that fact can be recognized and an increase of impedance can be distinguished from a decrease. If a discontinuity is reactive, it can be identified as inductive or capacitive. The location of each discontinuity can be determined within the resolution limits of the measurement. 
     This true low pass method has been used for a number of years. However, it is subject to the constraint that the measured data must be available with the entire spectrum down to zero frequency (f1 must be zero). If the reflection of interest is from a narrow band circuit, such as a filter or wave guide, the true low pass method cannot be used. Further, for equipment that is incapable of providing equal frequency steps all the way down to zero frequency, the true low pass method is unavailable. 
     Thus, in the more general case, f1 is not zero. The missing information from the unmeasured part of the spectrum between zero frequency and fl, causes a distortion in the time domain response. 
     Another source of distortion is the phase rotation in the time domain which is an unavoidable consequence of transforming a single sided spectrum. The arbitrary phase rotation which appears in this case makes it impossible, using prior art techniques, to determine the nature of the discontinuity at a given distance. 
     Accordingly, there is a need for apparatus and methods for processing data measured over a range of frequencies from f1 to f2 where f1 is greater than zero. In particular, it is desirable to identify accurate phase information for data samples of interest in the time domain where the complete complex conjugate spectrum is unavailable. 
     SUMMARY OF THE INVENTION 
     The present invention, providing an apparatus and method for low pass equivalent processing, reduces the difficulty of interpreting time domain information constructed from an inverse Discrete Fourier Transform over a single sided spectrum. 
     According to one aspect, the present invention is a method for use with a network analyzer that gathers a sequence of NDATA samples, each sample including real and imaginary components of a signal, over a frequency range from f1 to f2 in equal frequency steps, where f1 is greater than zero. The method processes the sequence of samples to estimate a characteristic of the signal with respect to a selected discontinuity in the signal. The sequence of samples is converted to time domain, to generate a sequence of time domain samples over a range from zero to t. The selected discontinuity is identified as lying within a range of t1 to t2. Then the phase of the time domain data is unwound to correct a portion of the phase error resulting from the one sided spectrum that is not dependent on the distance to the discontinuity through the network. Next, an estimate of the location of the discontinuity in the first corrected time domain data is generated. The phase of the time domain data is unwrapped in response to that estimate of location. Finally the resulting data is displayed. 
     The data resulting after the above processing have consistent real and imaginary parts which can be interpreted in terms of the nature of the discontinuity. The interpretation will be somewhat different from that of true low pass data. The real part will be positive or negative if there is an increase or decrease in impedance, respectively. The imaginary part will be positive or negative if there is an inductive or capacitive reactance, respectively. 
     According to another aspect, the present invention is an apparatus for performing the low pass equivalent processing outlined above and for displaying the results to a user. 
     Other aspects and advantages of the present invention can be determined from a study of the following description of the preferred embodiment, the figures and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the apparatus according to the present invention. 
     FIGS. 2A-2D are charts illustrating processing of the measured data samples with a rectangular window according to the present invention. 
     FIGS. 3A-3D are charts illustrating processing of the measured data samples with a Hamming window according to the present invention. 
     FIGS. 4A-4I are samples of the data samples for various discontinuity types after processing according to the present invention. 
     FIG. 5 is a flowchart illustrating the process of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description of a preferred embodiment of the present invention is described with reference to the figures. FIG. 1 provides a system overview. FIGS. 2A-2D, 3A-3D and 4A-4I are graphs illustrating characteristics of the data samples before and after processing according to the present invention. FIG. 5 is a flowchart illustrating the processing according to the present invention. 
     The present invention has been incorporated into a network analyzer called the WILTRON 360, manufactured by WILTRON, 490 Jarvis Drive, Morgan Hill, Calif. 95037. This system is described in detail in the co-pending U.S. patent application entitled MICROWAVE MEASUREMENT SYSTEM AND ASSOCIATED METHOD referred to above. 
     FIG. 1 schematically illustrates a system of the present invention set up for a typical reflection characteristic measurement. The system includes a network analyzer 10 and a network 11 coupled to the analyzer across input lines 12. The network analyzer 10 includes measurement apparatus 13, a storage facility 14, a display 15, a user input interface 16, a central processing unit 17 and a transform engine 18, all coupled to bus 19 for communication. 
     The measurement apparatus 13 samples the reflection or transmission characteristics under control of the CPU 17 across a frequency spectrum from f1 to f2 and stores the measured data samples into the storage 14. The CPU processes the stored data for display according to a variety of formats as desired by the user. The user communicates with the CPU 17 through a user input interface 16 consisting of a plurality of control switches and a keyboard. Associated with the CPU is a discrete Fourier Transform (DFT) engine 18 which is utilized for transforming the information taken in the frequency domain to the time domain. In the preferred system, the transform engine is a software utility which performs the chirp Z-transform as described below with respect to the preferred embodiment. The chirp Z-transform is described in detail in L. R. Rabiner, &#34;Chirp Z-Transform Algorithm Program&#34;, Programs for Digital Signal Processing, IEEE Press, New York, N.Y., 1979, pp. 1.6-1 to 1.6-13. 
     The network 11 in FIG. 1 consists of a transmission line 20, having a characteristic impedance of 50 ohms. Along the transmission line 20, a discontinuity 21 exists. The nature of the discontinuity can be a step-up in impedance, a step-down in impedance, capacitive, inductive, or a combination of these characteristics. 
     The user of the network analyzer 10 desires to determine the location of the discontinuity 21 along the transmission line 20 as well as its nature. Thus, the user sets up the measurement apparatus 13 through the CPU 17 to take measurements over a range of equally spaced frequencies from frequency f1 to frequency f2. The analyzer samples the real and imaginary components of the reflection coefficient at each frequency. Results can be displayed in a real and imaginary format, or they can be converted to magnitude and phase for user convenience. 
     FIGS. 2A-2D and 3A-3D illustrate the stages of data processing according to the present invention for a simple short circuit discontinuity with a rectangular window (FIGS. 2A-2D), or a two-term Hamming window (FIGS. 3A-3D). Each of FIGS. 2A-2D and 3A-3D includes plots of the real and imaginary components of the data and a polar plot with the same data. 
     FIG. 2A illustrates the windowed frequency domain data with a rectangular window. The real component is illustrated in chart 300, the imaginary component in chart 301 and the polar graph is shown in chart 302. As can be seen, it is simply a constant magnitude signal with a phase rotating around 360 degrees. After transforming the single-sided spectrum, the data from point t1 to point t2 takes the form shown in FIG. 2B, with the real component shown in chart 303, the imaginary component shown in chart 304 and the polar magnitude plot shown in chart 305. The portion of the time domain data shown in FIG. 2B is those data points taken around the main lobe of the discontinuity of interest as selected by a user through the keyboard interface. This does not illustrate the entire range from 0 to t which could be generated by the transform. As is illustrated in the chart 305, the phase information is quite difficult to decipher. 
     FIG. 2C illustrates the partially processed time domain data from point t1 to point t2 after removing the portion of the phase error that is not dependent on the distance to the discontinuity through the network. The real component is shown in chart 306, the imaginary component in chart 307 and the polar plot in chart 308. As can be seen, the phase information in the data is now normalized to a simple line in the polar plot. However, this includes an error that is dependent on the particular location of the discontinuity in the network. In the final unwinding step, that error is removed and the resulting data is shown in FIG. 2D. The real component of the resulting data is shown in chart 309, the imaginary component in chart 310 and the polar plot in chart 311. This takes the form that would be expected for a simple short circuit discontinuity in the network. The imaginary part is zero everywhere as shown at chart 310. The polar plot takes the form of a vector with a phase of minus 180 degrees illustrating a step-down in impedance with no reactive component. 
     FIGS. 3A-3D illustrate the same data using a two-term Hamming window in order to improve the displayed characteristics for the purposes of analysis. Accordingly, FIG. 3A shows the real and imaginary components of the frequency domain data in charts 312 and 313, respectively. The polar plot is shown in chart 314. The raw time domain data real and imaginary components between points t1 and t2 which illustrate the main lobe of the discontinuity are shown in charts 315 and 316 of FIG. 3B. The polar plot of the raw time domain data is shown in chart 317 of FIG. 3B. After removing a portion of the phase error that is independent of the distance to the discontinuity, the time domain data as partially processed, is shown in FIG. 3C. The real component of the partially processed data is shown in chart 318, the imaginary component in chart 319 and the polar plot in chart 320. After the final unwinding, removing the portion of the phase error that is dependent upon the location of the discontinuity in the network, the low pass equivalent data according to the present invention, is shown in FIG. 3D, with the real component shown in chart 321, the imaginary component in chart 322 and the polar plot shown in chart 323. As can be seen, this is an improved version of the processed data over that shown in FIG. 2D because of the Hamming window. The real component is below zero across the entire discontinuity, the imaginary component is zero everywhere. Thus, the polar plot is the form of a vector from zero to minus one as illustrated in chart 323. Again, this is the form of a polar plot that would be expected for a simple short circuit discontinuity in the network. The expected characteristics of time domain data for various discontinuities are illustrated in FIGS. 4A-4I. 
     FIGS. 4A-4I are plots illustrating time domain data after correction according to the present invention. Graphs 4A, 4B and 4C are plots in the time domain of corrected data for a discontinuity which is a step-down in impedance (R=49.0099 ohms), no discontinuity (R=50.00 ohms), and for a discontinuity which is a step-up in impedance (R=50.0101 ohms) with no reactive components (L=0, C=0). 
     FIGS. 4D, 4E and 4F are plots of the real and imaginary components of the corrected time domain data for a capacitive discontinuity (C=0.0032 pF) across a frequency range of 18 to 26.5 GHz with step-down in impedance, no impedance change, and a step-up in impedance, respectively. 
     FIGS. 4G-4I illustrate the real and imaginary components of the corrected time domain data for an inductive discontinuity (L=0.0080 mH) across a frequency range of 18 to 26.5 GHz with step-down, no change, and a step-up in impedance, respectively. 
     Using the location of the main lobe of the discontinuity as well as the plots as illustrated in FIGS. 4A-4I, a great deal of information about the nature of the discontinuity and its location can be determined. Of course for complex discontinuities that involve combinations of reactive and non-reactive components, the shapes of these plots may vary widely. Also, the plots shown in FIGS. 4A-4I were generated using a Hamming window with two terms as a bias to the time domain data. The Hamming window is used to enhance the characteristics of the discontinuity so that plots as illustrated in FIGS. 4A-4I are more readily readable. The present invention will work with or without biasing windows. 
     Let us diagram the frequency domain measurement, the transformed time domain data and their relationships. The frequency domain is shown below: 
     
         ______________________________________0   f1                      f2       FREQUENCY                                DOMAIN    x1    x2    x3  . . .   xn       (real part of data)    y1    y2    y3  . . .   yn       (imaginary part)______________________________________ 
    
     The inverse Fourier transform can be used to obtain n complex points of independent data in the time domain, as shown below: 
     
         ______________________________________0   t1      t2        t          TIME DOMAINx1  x2    x2    . . .   xn         (real part of data)y1  y2    y3    . . .   yn         (imaginary part)______________________________________ 
    
     The time domain covers a range from 0 to t, after which it repeats indefinitely. This behavior is called aliasing. The range t is the inverse of the frequency domain step size: 
     
         t=(NDATA-1)/(f2-f1). 
    
     The time domain step size t/(NDATA-1) is the inverse of the frequency domain range: 
     
         t/(NDATA-1)=1/(f2-f1). 
    
     In the preferred embodiment, the chirp Z-transform is used to display the arbitrary time range from t1 (starttime) to t2 (endtime) indicated in the last figure. Other DFT algorithms can be employed if desired. The number of equally spaced points calculated for this range t1 to t2 is equal to the number of points in the frequency domain, although the chirp Z-transform does allow the number of input and output points to differ. Note that the chirp Z-transform performs a band limited interpolation because t1 to t2 is less than the unaliased time range 0 to t. 
     The chirp Z-transform as published by the IEEE uses certain conventions which will be adopted here, for the parameters used in calling the transform. The published algorithm is nominally a forward transform. Converting it to an inverse transform can be accomplished by reversing the rotation of the output data, which can be done by using negative values for the parameters DLTOMG and OME0. In addition, NDATA and NOPTS are set to a common integer value and FS is set to the same value, using a floating point variable to satisfy the variable type requirements of FORTRAN, the language in which the chirp Z-transform is written. 
     Phase rotation of complex data is accomplished by doing a complex multiplication of the data by a unit vector with an angle equal to the desired change in radians. Where array index values are specified, they fall in the range of 1 to NDATA. This is the FORTRAN indexing convention. 
     When the foregoing conventions are followed, the low pass equivalent algorithm can be expressed as shown in FIG. 5. First, the system generates the NDATA data samples in a frequency range from f1 to f2 (block 500). The routine processes the samples as follows: 
     1. Apply a suitable window to the frequency domain data with NTERMS terms (block 501). A Hamming raised cosine window with two terms (NTERM=2) is found to provide useful results for many measurements. Also, 3 or 4 term raised cosine windows, known as Blackman windows, can be used. For information concerning selection of windows, see F. J. Harris, &#34;On the Use of Windows for Harmonic Analysis with the Discrete Fourier Transform&#34;, Proc. IEEE, Vol 66, no. 1, pp. 51-83, January 1978. Biasing with Hamming windows is not necessary for all measurements, but can aid interpretation of the data. A &#34;rectangular&#34; window (NTERMS=1) is suitable for many measurements. 
     2. Convert data to the time domain with the chirp Z-transform, selecting DLTOMG and OME0 as shown, so that the main lobe of the discontinuity from t1 to t2 (block 502), fills the display of time domain data. 
     
         DLTOMG=-(t2-t1)*(f2-f1)/(NDATA-1) 
    
     
         OME0=-(f2-f1)*(t1+(t2+t1)/(2*NDATA-2)) 
    
     3. Unwind the phase of the time domain data by an amount given by 
     
         -pi*(t2-t1)*(f2-f1)*(J-0.5)/NDATA 
    
     where J is the index into the time domain data (block 503). This corrects the portion of the phase error which is not dependent on the distance to the discontinuity. 
     4. Find the magnitude peak JMAX of the displayed time range t1 to t2 (block 504). Find the phase angles of the data at two points removed from the magnitude peak by a number of data points equal to the integer part of 
     
         NTERMS*(NDATA-1)/(2*(t2-t1)*(f2-f1)) 
    
     where NTERMS is the number of terms in the window used (block 505). NTERMS reflects the width of the main lobe relative to its width for a rectangular window. Determine the difference in phase angle at these two data points to identify the phase slope through JMAX. 
     5. Compare this phase difference to a threshold value determined by experiment for the particular instrument configuration (block 506). If the difference, or phase slope, is less than the threshold value, the discontinuity is not reactive, branch to block 507. 
     6. If the phase slope exceeds the threshold value, JMAX is not the best estimate of the location of the discontinuity, branch to block 508. An improved estimate is generated by comparing the phase at the middle of the spectrum of the discontinuity with the phase of the time domain data over which the phase slope was determined in block 505. 
     The processing performed in step 3 (block 503) is a heterodyne operation in the time domain. It is the equivalent of a shift in the frequency domain of the middle of the spectrum of the discontinuity to zero frequency. The spectral component of interest can be computed with a discrete Fourier transform. Since the component is at zero frequency, the transform degenerates into a simple summation of the real and imaginary parts of the data. The inverse tangent operation is then used to find the angle of interest. 
     Once the angle has been found, it is compared point by point with the angles of the time domain data between t1 and t2 until the best match is found. Linear interpolation can be used to find the point of match between the closest two data points, if desired. This matching point is an improved estimate of the location of the discontinuity. 
     Alternative methods for estimating the location may be derived for a particular application. 
     7. The estimate of location determined in block 505 or block 508 is set as variable Z with a value between 1 and NDATA in blocks 507 and 509, respectively. Z is used to do a final unwrapping of the phase of the time domain data (block 510), using a phase value given by 
     
         pi*((Z-1)*(t2*f1+t2*f2-t1*f1-t1*f2)/(NDATA-1)+2*t1*f1) 
    
     8. Finally, the resulting data is displayed (block 511). The data will now have consistent real and imaginary parts which can be interpreted in terms of the nature of the discontinuity as illustrated in FIGS. 4A-4I. The interpretation will be somewhat different from that for true low pass data. The real part will be positive or negative if there is an increase or decrease in impedance. The imaginary part will be positive or negative if there is an inductive or capacitive reactance. Combined discontinuities will exhibit combined effects. 
     Source code in FORTRAN implementing a preferred embodiment of the present invention is provided as an appendix to the present application. 
     CONCLUSION 
     The present invention provides low pass equivalent processing useful in analyzing data sampled from a complex network. Users of the present invention will be able to better identify discontinuities in networks for design and trouble shooting tasks. 
     It has been established that according to the present invention, frequency domain data taking over a range of frequencies from f1 to f2, where f1 is not equal to zero, can be transformed to the time domain and the phase information for a particular point in that time domain data can be reconstructed with an accuracy that is useful for analyzing characteristics of the network from which the frequency domain samples were taken. Results may not be obtainable for all networks. 
     The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.