Operating Instructions
1502C MTDR Service Manual
1–17
O
F
F
O
F
F
O
F
F
O
N
ac 0.000 ft
Figure 1–14: Return Loss
A large return loss means that most of the pulse energy was lost instead of being
returned as a reflection. The lost energy might have been sent down the cable or
absorbed by a terminator or load on the cable. A terminator matched to the cable
would absorb most of the pulse, so its return loss would be large. An open or short
would reflect all the energy, so its return loss would be zero.
The 1502C can compute and display what impedance mismatch would cause a
reflection as high (or low) as the point at the cursor. This measurement is useful for
evaluating the first impedance mismatch (first reflection) or small impedance
changes along the cable (e.g., connectors, splices).
This function can be selected in the Setup Menu. Once it is enabled, the impedance
value will be displayed under the distance in the distance window.
O
F
F
O
F
F
O
F
F
O
N
ac
Ohms-at-Cursor
2.800 ft
50.0
W
Readout
Figure 1–15: Ohms-at-Cursor
The accuracy of the difference measurement in impedance between two points near
each other is much better than the absolute accuracy of any single point
measurement. For example, a cable might vary from 51.3
W
to 58.4
W
across a
connector – the 7.1
W
difference is accurate to about 2%. The 51.3
W
measurement
by itself is only specified to be accurate to 10%.
Ohms-at-Cursor
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