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Operating Instructions
1502C MTDR User Manual
1–17
The reflection coefficient is a measure of the impedance change at a point in the
cable. It is the ratio of the signal reflected back from a point, divided by the
signal going into that point. It is designated by the Greek letter r and is written
in this manual as rho. The 1502C measures the reflection coefficient in millirho
(thousandths of a rho).
To measure a reflection, adjust VERT SCALE to make the reflection one
division high. Read the reflection coefficient directly off the display above the
VERT SCALE control. For reflections that are greater than 500 mr/div, adjust
VERT SCALE for a reflection that is two divisions high and multiply the VERT
SCALE reading by two.
O
F
F
O
F
F
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F
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ac 0.000 ft
Figure 1–13: Reflection Adjusted to One Division in Height
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 mr. 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 mr for the open
and –1000 mr 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
Reflection Coefficient
Measurements