Appendix D: Application Note1502C MTDR User Manual D–5If the reflection is downward from the 50 W or 75 W reference level, set thereference level to the top of the chosen scale. If the reflection is toward a higherimpedance than the reference level, set the reference level to the bottom of thechosen scale. Then count off the right number of divisions and subdivisions tolocate the level corresponding to the peak of the reflection and read the corre-sponding impedance levels (Ohms) on the adjacent sliding scale.
SOURCE
IMPULSE
GENERATOR INCIDENT
VOLTAGE REFLECTED
VOLTAGE
2EPEAK
RS
75 W
ZO
75 W
LINE
–0dB =
–6dB =
–6dB =
–0dB =
+100%
+50%
0%
-50%
-100%
1 W
225 W
75 W
25 W
0 W
LOAD
RL
% / DIV % / DIV % / DIV % / DIV
OHMS OHMS OHMS
20 10 5 2 1 .5
75 W SOURCE 75 W SOURCE 75 W SOURCE
75W
1dB =
EPEAK
100
60
50
40
30
20
15
10
5
0
90
60
50
45
40
35
30
25
20
15
10
5
75W
80
70
65
60
55
50
45
40
35
30
70
65
60
55
76
74
73
72
70
69
68
67
66
65
64
63
71
75W
1
74
73
72
71
70
69
34
40
46
46
40
34
RETURN LOSS (–dB)
(20% / division, downward 5 major divisions)–100% = 0 WFigure D–4: Slide Rule 75 ohm SourceIf the line impedance and the source resistance are known, the expectedamplitude of a reflection can be approximated. First, select the side of the sliderule having the correct source impedance. For cables having a higher impedancethan the selected source resistance, put the sliding reference level even with thebottom stationary scale markings. For cables having a lower impedance than theselected source resistance, move the reference level even with the top of the scalemarkings. For best accuracy, select the scale farthest to your right in which theimpedance level of interest is in view. Read from the adjacent stationary scale thereflection coefficient or percent reflected voltage that corresponds to the Ohmsselected.