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Operating Instructions
1–18
1502C MTDR User Manual
similar to the Vertical Compensation for Higher Impedance Cable procedure (see
the VERT SET REF section).
Return loss is another was of measuring impedance changes in a cable. Mathe-
matically, return loss is related to rho by the formula:
Return Loss (in dB) = –20 * log (base ten) of Absolute Value of Rho (Vref/Vinc)
The 1502C can be made to display in dB instead of mr/div through the menu:
1. Press MENU.
2. Select Setup Menu.
3. Press MENU again.
4. Select Vertical Scale is: Millirho.
5. Press MENU again. This should change is to Vertical Scale is: Decibels.
6. Press MENU twice to return to normal operation.
To measure return loss with the 1502C, adjust the height of the reflected pulse to
be two divisions high and read the dB return loss directly off the LCD. The
incident pulse is set to be two divisions high at zero dB automatically when the
instrument is turned on.
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.
Return Loss
Measurements