FFT Analysis and SRL Prediction — Seeing Inside the Cable in Real Time

Most capacitance gauges tell you what the cable measures. The CG-i4 tells you what it means.

The FFT and SRL analysis built into the CG-i4 system goes beyond reporting a capacitance value — it analyses the structure of that measurement, identifies any periodic variation hidden within it, and translates that directly into a prediction of how the finished cable will perform electrically. All of this happens inside the Interface Box, continuously, as the cable is produced.

The FFT display — reading the signature of your process

The upper graph in the PCIS-CG software displays the FFT spectrum of the capacitance measurement: amplitude in pF/m on the vertical axis against spatial frequency in Hz and physical defect pitch in centimetres on the horizontal axis. Every peak in this spectrum represents a repeating periodic variation in the insulation structure — and because the x-axis shows the corresponding defect pitch in centimetres, operators can immediately connect what they see on screen to what is happening on the line.

The software reports four key FFT parameters in real time:

• FFT Sampling Rate — the number of capacitance measurements per second feeding the analysis (e.g. 56 Hz or 213 Hz depending on line speed and gauge configuration)
• FFT Peak Frequency — the dominant periodic defect frequency in the capacitance signal (e.g. 0.1–0.2 Hz), identifying the strongest repeating irregularity
• FFT Peak Length — the spatial period of that dominant defect along the cable (e.g. 761 cm or 1600 cm) — the physical distance between repetitions in the insulation structure
• FFT Peak amplitude — the magnitude of the dominant capacitance variation in pF/m, quantifying the severity of the periodic defect
• FFT Cable Length — the length of cable analysed in the current FFT window, confirming the spatial coverage of the measurement

The SRL graph — predicting electrical performance before the test lab

The lower graph translates the FFT spectrum directly into a predicted Structural Return Loss curve: SRL in dB on the vertical axis against electrical frequency in MHz — the same axes used in cable certification testing to IEC 61156 and TIA/EIA 568.

The blue curve shows the predicted SRL across the full frequency range (up to 2000 MHz as configured). The red markers highlight frequency points where SRL exceeds the configured tolerance threshold — immediately flagging the frequency bands where the cable is at risk of failing specification. The software reports:

• SRL Peak — the worst-case predicted SRL value across the measured frequency range (e.g. −25.56 dB or −38.26 dB)
• SRL Peak Frequency — the electrical frequency at which that worst-case reflection occurs

Configurable for every cable type

The FFT/SRL Settings panel allows engineers to configure the analysis for any cable construction and target standard:

• FFT Length — controls the number of samples in each analysis window (e.g. 1024 points), setting the frequency resolution of the spectrum
• FFT Average Number — applies averaging across multiple FFT frames to reduce noise on stable production runs
• SRL Max Frequency — sets the upper frequency limit of the SRL prediction (e.g. 2000 MHz for CAT 6A or higher-category cables)
• Relative Propagation Velocity — enters the cable’s velocity of propagation as a percentage of the speed of light (e.g. 80%), ensuring the physical defect pitch is correctly mapped to electrical frequency for that specific cable construction

What this means on the production line

When a peak appears in the FFT spectrum, the operator knows immediately: there is a repeating structural variation in the insulation at a specific pitch. The SRL graph then shows the electrical consequence of that variation. And because the defect pitch is displayed in centimetres alongside the frequency axis, it is straightforward to match the dominant peak to a known machine cycle — extruder screw period, haul-off drive frequency, or cooling variation — and take targeted corrective action before non-compliant cable is produced.

This is the difference between measuring cable and understanding it.