Tektronix 1503C, User Manual, Page: 114 / 128

Operator Troubleshooting
D–6
1503C MTDR User Manual
Characteristic Line Impedance or Load Resistance vs. Reflection Amplitude
(as seen on your TDR)
Line Impedance (Z
O
) or Load Resistance (R
L
) can be derived directly from the
amplitude of a reflection displayed on a TDR display. The displayed reflection
should be positioned vertically to a known 50
W
reference level. For a reference
level, use either a section of line of known impedance ahead of the line under
test/load, or use a termination of known resistance at the end of the line. The slide
rule can then be used by selecting the side with the same source resistance and the
same scale as the TDR.
RG8/U
51.5
W
RG213/U
49.5
W
.01
r
/div
2 ns / div
Precision Load Resistor used for 50
W
Reference Lev-
el
.01 .005
50W
SOURCE
50W
55
54
53
52
51
51
52
49
48
49
48
47
46
r
/ DIV
OHMS
Figure D–5: Calculating Resistance/Impedance from Waveform Amplitude
Position the 50
W
or 75
W
reference level on the sliding scale to correspond with the
one selected as the reference level on the TDR display. The impedance (Ohms)
causing the reflection can then be read from the sliding scale by noting the position
on the fixed scale that corresponds to the position of the reflection on the TDR
display.
You should note that the peak level of any reflection that does not have a discernable
plateau might be an erroneous indication of the impedance discontinuity that caused
it. There might be several reasons for the error. 1) The discontinuity might occupy
such a short segment of the line compared to the V
P
of the line and the risetime of
the test pulse wavefront that part of the wavefront starts to emerge from the segment
while the remainder is still entering. This causes a spike-shaped reflection, the
amplitude of which might vary depending on the risetime of the test pulse, how
badly the risetime has been degraded by the cable before it arrives, and how much
attenuation the cable might impose on the reflection before it arrives back at the
source. 2) If the risetime of the TDR system is too long, a reflection with a plateau
will appear as a spike.