Fluke DSP-2000 - Manual (Page 131)

Basic Cable Testing
ACR
7
7-19
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.