Symmetrical cable's electrical characteristics: asymmetry

 In the previous article, we reviewed the basic parameters NEXT and FEXT. The Attenuation to Crosstalk Ratio (ACR) is used when evaluating structured cabling. This parameter is equivalent to the signal/noise parameter concerning the transient effects at the near end of the NEXT, i.e., it serves as an estimate at the receiver input for the undergone line attenuation of the signal and for noise from the transient effects at the near end. Quantitatively, ACR is expressed as a logarithmic measure of the difference between NEXT and cable attenuation. If, for example, the ACR value is 10 dB, this means that the NEXT power of the interference at the receiver input will be 10 times less than the power of the useful signal, i.e., the signal-to-noise ratio will be 10.

Suppose the communication system operates in single-cable mode, and the signal levels at the outputs of the transmitters at points A and B are the same and equal to 0 dB. If the line attenuation at frequency F denotes by Ak, then at transient attenuation NEXT at the same frequency signal levels, Pc and transient noise Pp at the input of receiver A will be Ak and NEXT.

Then ACR = Pc - Pp = NEXT - Ak.

The practical meaning of the ACR parameter becomes more precise if the frequency characteristics of the attenuation of a symmetrical pair (A), transient interference (NEXT), and the parameter (ACR) are presented on the same graph. The frequency at which the values of attenuation and NEXT are the same (in this case, it is 100 MHz) determines the upper limit of the operating frequency range. At frequencies above the boundary value, the power of the interference NEXT exceeds the signal strength.

Another cable system indicator, the Equal Level Far End Crosstalk (ELFEXT), has the same physical meaning as ACR. The only difference is that ACR is associated with NEXT, while ELFEXT is associated with FEXT. The ELFEXT parameter becomes critical when several transmitters of the same system are transmitting in the same direction on pairs located in the same cable.

In this case, ELFEXT = FEXT - Ak.

In addition to the ACR and FEXT parameters, two additional parameters are used - PS-ACR (Power Sum ACR) and PS-ELFEXT (Power Sum ELFEXT), which consider the real influence on this pair of all other cable pairs.

ASYMMETRY (IMBALANCE) LINE

Asymmetry is a transmission parameter, as it is determined by the pair parameters and affects its bandwidth, and an influencing parameter, as it affects the transitions between other pairs.

Each symmetrical line must be balanced concerning the ground in a certain way. Two types of asymmetry are distinguished depending on the current, DC, or AC.

DC asymmetry is estimated by the relative value of the difference of resistance of the cores of a symmetrical line and should not exceed 1%. The presence of the resistive unbalance of the line, equal to the difference of its core resistances measured at AC current, can be interpreted as the inclusion of an additional low-pass filter with the resistance of the longitudinal arm dR. In addition to the resistive component, the longitudinal unbalance of the line in the general case contains a capacitive component; it can arise, for example, due to accidental crossing of the cores of different pairs in the connection points of cables. This component can be interpreted as the transverse capacitance of the additional low-pass filter mentioned above.

The AC Longitudinal asymmetry can be caused by loose contact at cable core joints (twist or splice points, switch cabinets, etc.). The longitudinal unbalance problem cannot be solved, even if the longitudinal asymmetry of the pair in question is reduced to normal. This fact is a necessary but insufficient condition for solving the problem of longitudinal asymmetry in a particular cable. The state of sufficiency requires that all pairs of the bundle or the loop must be checked for compliance with the asymmetry norms. The fact is that any unbalance of even a non-working pair is a source of interference to all working pairs, the consequence of which is a decrease in their carrying capacity.