Diameter measurement during primary wire extrusion using a LASER 2010 XY

Photos: Sikora

FFT analysis and SRL prediction inside the measuring head

Combined diameter and eccentricity measurement with CENTERVIEW 8010

Visualization of SRL prediction results at SIKORA’s ECOCONTROL 2000 display unit

Online Prediction of Structural Return Loss (SRL) for Twisted-pair Data Cables

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SIKORA offers compact and easy-to-use measuring equipment specifically designed for in-process quality control of twisted-pair data cables. Concerning measuring accuracy, resolution, and measuring rate, these devices meet all conditions to successfully conduct online SRL predictions at early stages during the cable production process.
Increasing quality requirements are common to all types of electrical communication cables. This is best reflected by permanently new data cable standards known as “categories”.
Variations in the structure of such cables (e.g. dimensional variations or fluctuations in the electric properties of the insulation material) will cause reflections of the high-frequency data signals transmitted via these cables. The relationship between signal parts reflected back (returned) to the source and the forward traveling signal is generally referred to as structural return loss (SRL) of a cable.
Unlike randomly distributed structural problems, minor diameter, eccentricity and cable capacitance variations become critical if they occur with regular spacing. Currently, compliance with specifications is ensured by a post-production SRL measurement at the finished cable.
In order to minimize the number of rejects and optimize productivity, a need has developed for more advanced measuring technology. With respect to minor periodical variations, data of such parameters like capacitance, diameter, and/or eccentricity can be continuously monitored at early stages during cable production. An appropriate method for analyzing this data is online SRL prediction. This prediction delivers an estimate of the impact of periodical problems on the SRL performance of the later cable.

The SRL performance of a twisted-pair cable mainly depends on the geometry of the pair. Monitoring the outer diameter during the primary wire extrusion process is shown in Fig. 1. This measurement does not directly give the later inter-axial distance between the wires of a pair. However, an inter-axial distance change of DD/2 can be estimated from a primary wire diameter change DD when the second wire is assumed to be perfect. Accordingly, a capacitance measuring tube would not measure the capacitance between both wires of a pair during the primary wire extrusion process, but instead the coaxial capacitance of the primary wire. For converting this coaxial capacitance into the two-wire capacitance, the second wire has to be assumed as a perfect one, too.
For estimating the required measuring sensitivities, the maximum allowable amplitudes of periodic fluctuations in outer diameter, conductor eccentricity, and coaxial capacitance of CAT-6 and CAT-7 primary wires are compiled in the Table. These calculated values are based on a SRL limit of -23 dB which is equal to 7% of reflected signal strength.
The data in the upper part of the table indicates, that diameter gauges should be able to detect outer diameter variations well below 1 µm. Because quite fast fluctuations must be detected, the gauge should achieve this sensitivity not just for mean values but for single diameter readings.
As apparent from the data in the center part of the table, the sensitivity of an eccentricity gauge should be well below 1 µm, too. Again, this sensitivity has to be achieved for single eccentricity readings.
According to the values in the bottom part of the table, a capacitance measuring tube should be able to detect fast capacitance variations of primary wires well below 1 pF per meter.

The LASER Series 2000 XY diameter gauges of SIKORA operate after the patented laser shadow projection principle. By evaluating diffraction information for the diameter determination, a single reading precision as high as 0.2 µm is achieved. This precision is maintained for years because no moving parts are included in these gauges.
The compact CENTERVIEW 8000 gauges of SIKORA as shown in Fig. 3 combine diameter and eccentricity measurement. Using optical and inductive measuring technologies high-precision single eccentricity readings are supplied be these gauges.
In SIKORA’s capacitance gauges named CAPACITANCE 2000, a short and a long measuring electrode are integrated in one tube. The short sensing electrode of 10 mm length allows for a reliable measurement of fast periodic capacitance fluctuations. The measuring accuracy is 0.1 pF per meter. SRL predictions up to 2 GHz are feasible at line speeds up to 3,750 m/min.
Powerful digital signal processors are by standard included in all SIKORA measuring heads. Because of this computational power, a complete SRL prediction is realized just inside the measuring heads (Fig. 2). Thus, the on-line prediction of the SRL performance is entirely executed were the input data (i.e. diameter, eccentricity, or capacitance readings) is available in high resolution and high precision. Due to the all-digital signal processing inside the SIKORA gauges noise is kept at a minimum allowing to detect even minor periodic cable parameter fluctuations most reliable.
By integrating the online SRL prediction into the measuring heads, SIKORA was able to realize a very compact analysis tool. Moreover, the cable manufacturer is relieved from the hassles caused by setting-up and wiring different devices (measuring gauge, spectrum analyzer, SRL computer etc.). Because no analog transmission of high-bandwidth data is necessary (e.g. no communication of single readings to an external spectrum analyzer), pseudo SRL peaks by electromagnetic interferences are avoided.
SIKORA’s measuring devices can be equipped with a variety of digital interfaces (e. g. Profibus, DeviceNet, CANOpen, Ethernet). Using one of these interfaces, the low-bandwidth SRL prediction data can be communicated, e.g. to the central control and display unit of the production line or to one of SIKORA’s ECOCONTROL display units.
The very intuitive to operate graphical user interface for SRL predictions at the ECOCONTROL 2000 display unit of SIKORA is shown in Fig. 4.

f [MHz] 20 100 200 300 600
outer diameter DD [µm]
UTP CAT-6 11.0 (0.43 mil) 4.8 3.6 3.1 -
UTP CAT-7 10.4 (0.41 mil) 4.3 3.0 2.5 1.9 (0.07 mil)
conductor eccentricity De [µm]
UTP CAT-6 5.5 (0.22 mil) 2.4 1.8 1.5 -
UTP CAT-7 5.2 (0.20 mil) 2.1 1.5 1.2 0.9 (0.03 mil)
coaxial capacitance DC� [pF/m]
UTP CAT-6 4.9 (0.19 mil) 2.2 1.6 1.4 -
UTP CAT-7 4.6 (0.18 mil) 1.9 1.3 1.1 0.8 (0,2 pF/ft)