Structural return loss (SRL) and FFT- frequency analysis SIKORA measuring devices provide FFT analysis and SRL predictions

Prof. Dr.-Ing. Werner Blohm, Professor for engineering sciences at the Jade University
The production of state-of-the-art cable products, e.g. data cables, requires a constant monitoring of the cable diameter, concentricity and/or cable capacitance with regard to smallest, periodically occurring cable parameter fluctuations.
These fluctuations may occur from irregularities such as a “pumping” of the extruder, imbalance of rotating parts, periodical fluctuations of the line speed, cyclic fluctuations of the foaming degree and/or periodical fluctuations of the conductor preheating.
The most commonly used method for the quality control of data and high-frequency cables is the measurement of the return loss of a certain cable length as a function of the operation frequencies, which are supposed to be transmitted on the cable. The Structural Return Loss (SRL) represents the relation – expressed in dB – between a fed signal and the re-reflected signal fraction along the complete cable length towards the entry. While reflections, caused by smaller randomly occurring fluctuations, may be neglected, reflections caused by periodical re-occurring cable parameter fluctuations should be detected and eliminated.
It becomes clear with the following estimation: Caused by irregularities of the production process, periodical re-occurring “dents” are pushed into the cable insulation with a gap of 10 cm in between. If a reflection of 0.1‰ of the fed signal is taken as a basis at each imperfection, a maximal total reflection of 1,000 x 0.1‰ = 10% can be estimated (neglecting the cable attenuation at 1,000 impurities at a cable length of 100 m). This complies with a return loss of 20 dB. This value sums up when the signals of all single reflections at the cable entrance are added up in phase.
In order to optimize the cable quality as well as to reduce the scrap rate it is essential to search for the root cause of periodical fluctuations on the return loss of the finished cable at an early stage and to eliminate those by adjusting the production parameter. The online prediction of the return loss from short-term cable capacity data offers necessary information. SIKORA offers a variety of measuring systems for quality control of telephone wires, data cables and coax cables (e.g. CENTER VIEW 8000 for measuring diameter, ovality and concentricity, CAPACITANCE 2000 for capacitance measurement). Regarding the measurement accuracy, the measurement resolution and the measuring rate, all of these devices meet the requirements for providing FFT analysis and SRL predictions. Powerful digital signal processors are by default integrated in the gauge heads. On the basis of the computing power of the gauges, SIKORA is able to realize a digital spectrum analyzer as well as a prediction of the return loss directly in the gauge head. This means, that the online calculation of the spectrum of cable parameter fluctuations as well as the online prediction of the return loss are executed at the same place where the input data (that is diameter, eccentricity or cable capacitance measuring values) is available with high chronological and amplitude resolution as well as high precision with minimal measuring value noise. A problematic (normally analog) transmission of high-rate single measuring values is not necessary. For the cable manufacturer the complex installation, configuration and cabling of several single devices is not required. Furthermore, the risk of a distortion of the analysis results caused by the coupling of electromagnetic disturbances is reduced to a minimum as no broadband data transfer is necessary.
Prof. Dr.-Ing. Werner Blohm, Professor for engineering sciences at the Jade University, Wilhelmshaven and free consultant of SIKORA AG: “The usage of the FFT frequency analysis offers not only advantages for the production of communications cables. The manufacturer of other cables also benefits from this modern control tool if it is used to detect periodical occurring diameter fluctuations and if these fluctuations are eliminated in order to optimize the production process regarding material savings. The consumption of the insulation material can be decreased to a minimum while a constant wall thickness is guaranteed.”