Effects of TP cable capacity

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Voltage vs. TP cable capacity

Assuming a significantly simplified model of the cable and its drivers, the following equivalent circuit can be used: Equivalent RC network of a TP cable
Resistors of half of characteristic impedance represent serial driving/matching resistors between the output of the integrated driver and the hybrid filter/transformer. Effects of equivalent impedance of the filter and transformer is included into the driving signal and is not considered here. Equivalent impedance of the cable is significantly simplified, further calculations take into account cable capacity only. The load at the end of the cable is equal to one half of characteristic impedance. This load is incorporated into receiver input circuitry.

Characteristics of real TP cables:
Belden is one of the leading world manufacturers. Let's take a look at two typical samples from their catalog -- a low-end cable and a high-end cable:

Type Resistance Capacity Belden cable
1700A (DataTwist 350) 78,7 ohm/km 45,9 pF/m extended Cat.5 up to 350 MHz
1247A (DataTwist Three) 88,6 ohm/km 72,2 pF/m Cat.3

Both cables have nominal impedance of 100 Ohm.

In the following figures, colors are used to distinguish signals:
input signal 10 MHz (data 1111 or 0000) / 5 MHz (data 0101), output signal 10MHz / 5 MHz

Voltage at the cable for 1 nF capacity (corresponds to 21 m of Belden type 1700 or 13 m of Belden type 1247 cable)
Voltage at the cable, capacity 1 nF
This waveform explains why the given shape is used to drive the cable at 5 MHz. The waveform ensures 10 Mhz-like build-up in the first half, while respecting the RC element (driver-cable) and its effects on amplitudes of different frequencies in the second half.

More figures show the same signal vs. different cable capacities.

Voltage at the cable for 7 nF (corresponds to 150 m of Belden type 1700 cable or 90 m of Belden type 1247 cable
Voltage at the cable, capacity 7 nF
Detail of the output signal
Voltage, capacity 7 nF, detail

Voltage at the cable for 300 pF (corresponds to 6 m of Belden type 1700 or 4 m of type 1247 cable
Voltage, capacity 300 pF

References

xlsExcel sheet for experiments with cable capacity




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