Fast Fourier Transformation (FFT) – Reliably Detect Periodic Fluctuations of Product Parameters

Especially in the production of data cables, periodic variations of diameter, eccentricity, and capacitance pose a risk to the data transmission properties. The online Fast Fourier Transform analysis of these measurement data is a powerful control tool for detecting periodic irregularities in cable extrusion.

Sources of error

Periodic fluctuations of cable parameters can be caused, for example, by irregularities such as extruder “pumping”, imbalance of rotating parts, periodic fluctuations of line speed, cyclic change of foaming degree, vibrating (oscillating) cores, and/or periodic irregularities in conductor preheating.


Measuring devices with FFT function

While the presence of periodic fluctuations is usually difficult to infer from the time course of the measured cable parameter, periodically recurring fluctuations can be detected at an early stage by a Fast Fourier Transformation analysis of the measured values. In addition, the calculated frequency spectrum provides helpful information for identifying the causes of cyclic irregularities in the cable manufacturing process, especially when production parameters such as line speed are changed in a controlled manner.

SIKORA measuring devices with FFT analysis

As standard, all SIKORA measuring devices, such as the CENTERVIEW 8000, the LASER Series 2000, the LASER Series 6000 and the CAPACITANCE 2000, are equipped with powerful digital signal processors. Due to the computing power thus provided in the measuring head, a digital FFT spectrum analyzer and return loss prediction is directly integrated in the measuring head. The analysis is performed where the input data (i.e. the diameter, eccentricity or cable capacitance measurement values) are available with high temporal and amplitude resolution as well as high precision with minimal measurement noise. A mostly analog and therefore problematic transmission of the sensitive single measured values to external analysis systems is therefore not necessary for FFT and SRL.