DD IEC PAS 62260:2001 pdf download Pulse/Step Return Loss from measurement in the frequency domain using the Inverse Discrete Fourier Transformation (IDFT)
General
The test determines the regularity of impedance of RF-cables by measuring the return loss in thefrequency domain using a vector network analyser equipped with a reflection test set (bridge) andtransfer them into the time domain by using the lnverse Discrete Fourier Transformation (IDFT). ThePulse/Step Return Loss is displayed against time to show the magnitude and the local distribution otirregularities of the characteristic impedance of the cable under test. Depending on the number ofpoints of the network analyser cable length up to and above 5km may be measured.
Principle
2
The test determines the return loss of coaxial RF-cables in the time domain by measuring the returnloss in the frequency domain, and transforming the results into the time domain by using the lnverseDiscrete Fourier Transformation (IDFT).
The network analyser measures the magnitude and phase of the CUT’s return loss (S11) in thefrequency domain at discrete frequencies specified by the harmonic portions contained in anequivalent time domain pulse of predetermined shape. After processing a complete sweep over thesefrequencies the network analyser has collected the spectral representation of the cable’s pulseanswer into memory. The pulse answer in time domain representation can be calculated now byapplying a mathematical algorithm called lnverse Discrete Fourier Transformation (lDFT) to the storeddata. Because from the mathematically point of view the pulse is the derivative of the step, theresponse to a step function can be computed easily by integrating the pulse response data over time.The Discrete Fourier Transformation calculates the complex factors H, of the sine and cosine wavescontained in an arbitrary waveform. The lnverse Discrete Fourier Transformation synthesises acomplex arbitrary waveform by calculating the sum of the sine and cosine functions multiplied with thecomplex Fourier transforms H.. The transformation does not depend on any dimensional parametersuch as a time scale or frequency.
Because the actual measurements are made in the frequency domain, the calibration of the networkanalyser has to be done in the frequency domain as well using a full one port calibration for reflectionmeasurements.
3Equations
The performance of a RF-cable depends on its mechanical homogeneity. Deviations in its mechanicadimensions will cause reflections of the RF-signal which is travelling through the cable. The complexreflection factor rx of such a reflection is given by:
wheret
LxZ,+Z,
is the nominal impedance of the cableis the actual impedance of the cable on a given point x.
61
The transformation of one single reflection to the cables input with attention to the attenuation andpropagation is given by:
Lx=Le-2yl
e
where y is the propagation constant

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