Tektronix 1503C, User Manual, Page: 31 / 128

Operating Instructions
1503C MTDR User Manual
1–15
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ac 299.80 ft
Cursor
Figure 1–12: 455-ft Cable with 20 ft/div, Cursor Off Screen
A “rise” or “fall” is a signature of an impedance mismatch (fault). A dramatic rise
in the pulse indicates and open. A dramatic lowering of the pulse indicates a short.
Variations, such as inductive and capacitive effects on the cable, will appears as
bumps and dips in the waveform. Capacitive faults appear as a lowering of the pulse
(e.g., water in the cable). Inductive faults appear as a rising of the pulse (e.g., kinks
in the cable). Whenever an abnormality is found, set the cursor at the beginning of
the fault and read the distance to the fault on the distance window of the LCD.
Return loss is another was of measuring impedance changes in a cable.
Mathematically, return loss is related to rho by the formula:
Return Loss (in dB) = –20 * log (base ten) of Absolute Value of Rho (V
ref
/V
inc
)
To measure return loss with the 1503C, note the height of the incident pulse, then
adjust the reflected pulse to be the same height that the incident pulse was and read
the dB on the LCD display. The amount of vertical scale change that was needed
is the return loss in dB.
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ac
455.00 ft
Loss
Figure 1–13: 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
Return Loss
Measurements