Fluke DSP-100, DSP-2000 user manual Acr

Basic Cable Testing 7

ACR

A negative reflection percentage indicates that the polarity of the reflection is the opposite of the original signal. Negative reflections are caused by abrupt decreases in the cable’s impedance, such as those caused by mismatches in cable types or shorts on the cable.

The results plotted in Figure 7-10 are the results from a TDR test on pair 4,5 in a good twisted pair cable. The results show a positive anomaly caused by the open end of the cable at about 219 feet. Note that the left edge of a TDR reflection represents the location of the anomaly on the cable, while the peak of the reflection represents the size of the anomaly.

The test tool’s TDR plot includes a movable cursor with a readout that displays the cursor’s position and the reflection percentage of the anomaly at the cursor’s position. You can move the cursor left and right by pressing the L R keys on the test tool. Figure 7-10 shows the cursor positioned near the beginning of an anomaly.

ACR

ACR (attenuation to crosstalk ratio) is the difference between NEXT in dB and attenuation in dB. The ACR value indicates how the amplitude of signals received from a far-end transmitter compares to the amplitude of crosstalk produced by near-end transmissions. A high ACR value means that the received signals are much larger than the crosstalk. In terms of NEXT and attenuation values, a high ACR value corresponds to high NEXT and low attenuation.

7-19