Percent Reflected Voltage Vs. Characteristic Line Impedance - Tektronix 1502C User Manual

Appendix D: Application Note

Percent Reflected Voltage vs. Characteristic Line Impedance

D–4
To find the characteristic impedance of a line, or section of a line, knowing the
reflection coefficient or the %, you should first know the impedance of the pulse
generator. It should be as close as possible to the nominal impedance of the line
and should be connected to the line through a length of cable having the same
impedance as the source. Select the side of the slide rule that corresponds to the
source resistance (R ) of the generator used, then select the longest scale as
S
follows:
Size of Reflection
100% to 80% (1r to 0.8r)
80% to 40% (0.8r to 0.4r)
40% to 16% (0.4r to 0.16r)
16% to 8% (0.16r to 0.08r)
8% to 4% (0.08r to 0.04r)
4% or less (< 0.04 r)
SOURCE LINE
R Z
S
50 W 50 W
D2E DE
STEP INCIDENT
GENERATOR VOLTAGE
r = REFLECTION COEFFICIENT
r / DIV
OHMS
1000
.20
600
400
300
200
1
1000
150
500
300
100
200
90
80
100 70
70 60
50W
45
40
40
50 W SOURCE
The risetime or amplitude of received reflections may be signi-
ficantly degraded or attenuated by two-way losses of the line .
+1r = ∞ W
Figure D–3: Slide Rule 50 ohm Source
% / division
20
10
5
2
1
0.5
+100%
+1r =
+0.5r= +50%
O
0r =
-50%
-0.5r=
-1r = -100%
REFLECTED
VOLTAGE
r / DIV r / DIV
OHMS OHMS
.10 .05 .02 .01
59
70
120
68
54
58
66
100
57
64
53
90
56
62
55
80
60
52
58 54
70
56 53
54
52
51
60
52
51
50W 50W
49
48
50 W SOURCE 50 W SOURCE
(50 or 75 ohm Source)
r / division
.20
.10
.05
.02
.01
.005
1 W
150 W
50 W
0%
16.7 W
0 W
LOAD
R
L
1r = 100%
r / DIV
.005
+.03r = 53.1W
1502C MTDR User Manual