How to Improve digital cable return loss (RL) Indicators

1. Introduction to return loss indicators

Ripple loss is an important indicator of digital cable products. Ripple loss combines two kinds of reflection effects, including the deviation of nominal impedance such as 100 Ω and the influence of structure, used to characterize the performance of a link or channel. It is caused by the non-uniformity of the characteristic impedance of the cable length. It is caused by the non-uniformity of the cable structure. Because of the reflection caused by signals in different locations in the cable, the signal arriving at the receiving end is equivalent to the multi-path effect in the transmission of wireless channels, resulting in time diffusion and frequency selective fading of signals, time diffusion leads to pulse broadening, so that the signals at the receiving end overlap and cannot be determined. Multiple signal reflections in the cable also lead to signal power attenuation, affecting the signal-to-noise ratio of the receiver, leading to an increase in the bit error rate, and thus limiting the transmission speed. In the process of digital cable production, the Return Loss of the cable is prone to unqualified.

2. Measures to Improve the Return Loss Index

There are three measures to increase the return loss RL:

1) Improve concentricity

In the production process of insulation series, the diameter tolerances of copper conductor must be within ± 0. 002mm, and the deviation of Insulation Outer diameter must be within ± 0. 01mm. The concentricity is above 96%, and the surface is smooth and round. Otherwise, the characteristic impedance of the cable after the twisted pair of a single line will be larger than the required peak value.

2) use a certain proportion of "pre-Twist" or "backtwist" technology and use cross-shaped plastic skeleton

The use of a certain proportion of "pre-Twist" or "back-Twist" technology can eliminate the impact of single insulation eccentric line on the Characteristic Impedance, while reducing the requirements of single insulation concentricity. The cross plastic skeleton can maintain the stability of the cable structure, so that the variation of characteristic impedance caused by uneven single line is smooth, so that the performance of the near-end crosstalk and ripple loss is quite good at high frequencies.

As we all know, the fluctuation of the center distance between the two wires in a line will lead to the fluctuation of the line impedance. Because the eccentric insulation of a single insulated line is inevitable, the line changes the impedance to some extent and remains unchanged in some local lengths, in the total length, the sudden high and low fluctuations show a tiered pattern. The line pairs are composed of several segments with different impedance uneven segments. These uneven segments are long, long, or short, when the usage frequency of the cable is greater than 1/8 of the wavelength corresponding to the half wavelength, the impedance changes will be "noticed" by the traveling electromagnetic wave, resulting in the reflection of the electromagnetic wave, some of the reflections are superimposed due to phase consistency, resulting in Impedance fluctuations, decreased return loss, and additional loss attenuation-peak value on the frequency curve ). As the frequency increases, the wavelength decreases, causing more uneven segments to cause electromagnetic reflection.

Through the "pre-Twist" or "back-Twist" of the line pair, the length corresponding to a periodic change completed by the distance S between the line pairs of conductors contains several twisted pair pitch, but it does not exceed the 1/8 wavelength corresponding to the maximum usage frequency of the cable, the line-to-line impedance also completes a period of rapid change within a pitch, and its size fluctuates in a sine shape, in this way, the impedance changes of the line to the total length become smooth, the reflection does not occur, and the uniformity of the line to the impedance is greatly improved. In addition, the cross plastic skeleton is used together to maintain the stability of the cable structure, so that the variation of characteristic impedance caused by uneven single line becomes smoother.

Through the above measures, the performance of the adjacent crosstalk and ripple loss of the digital cable is quite good at high frequencies.

3) use the sticking wire pair Technology

The sticking line technique refers to the combination of two plastic extrusion machines and one head, and the two insulated core wires of the same line are simultaneously extruded to bond them together. After the adhesion between the twisted pair wires, the stability of the structure of the twisted pair wire can be ensured, and the uniformity of the line impedance can be improved by maintaining the stability of the center distance between the two wires (S; the Return Loss Index of the cable can also be avoided when the conductor of the insulation conductor is dispersed after bending and twisting.

1. When the link is too short, be careful about the crosstalk and ripple loss at the end.
2. DSP-4000 Series Digital cable Tester