Operating Instructions
1–16
1502C MTDR Service Manual
In an ideal transmission system with no changes in impedance, there will be no
reflections, so rho is equal to zero. A good cable that is terminated in its
characteristic impedance is close to ideal and will appear as a flat line on the 1502C
display.
Small impedance changes, like those from a connector, might have reflections from
10 to 100 m
r
. If rho is positive, it indicates an impedance higher than that of the
cable before the reflection. It will show as an upward shift or bump on the waveform.
If rho is negative, it indicates an impedance lower than that of the cable prior to the
reflection. It will show as a downward shift or dip on the waveform.
If the cable has an open or short, all the energy sent out by the 1502C will be
reflected. This is a reflection coefficient of rho = 1, or +1000 m
r
for the open and
–1000 m
r
for the short.
Long cables have enough loss to affect the size of reflections. In the 1502C, this loss
will usually be apparent as an upward ramping of the waveform along the length of
the cable. In some cases, the reflection coefficient measurement can be corrected for
this loss. This correction can be made using a procedure very 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.
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/Vinc)
The 1502C can be made to display in dB instead of m
r
/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.
Return Loss
Measurements
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