The metric is used because it is useful both for short cables where crosstalk related problems dominate and for longer cables where attenuation is the major concern. It is closer to a single number figure of merit for cables than anything else.
Crosstalk is the transfer of signal from one pair to another. It is highest near the transmitter and falls off further away as the signal in the wires gets weaker. The value actually measured is Near End Crosstalk (NEXT) which is the ratio of transmitted to induced signals at the transmitter end. Large numbers are good.
Attenuation is the signal loss from transmitter to receiver. It is typically specified for a 100 meter length of cable. Large numbers are bad.
ACR is NEXT minus Attenuation. Low values indicate either lots of crosstalk or lots of signal loss or both. ACR needs to meet specified values if a cable is going to work. 4db or 6db are typical specified values. ACR is sometimes specified as "ACR margin" -- the difference between measured ACR and required ACR. Positive values are desirable. Negative values are very bad.
ACR varies with frequency and temperature. For cables with more than two wire pairs, there are multiple ACRs (one ACR for each pairing), but all are assumed to be similar. When only one ACR is specified, it will normally be quoted at the highest frequency at which the cable claims to be usable.
Return To Index Copyright 1994-2008 by Donald Kenney.