HP 54753A - Page 193

TDR Fundamentals
Step Reflection Testing
9-19
Shunt resistance,
for Z
o
< 50 Ω:
Resistance in series with source,
Shunt resistance,
The incident step and the reflections will be attenuated considerably. Refer to
Figure 9-17. The sacrifice made to achieve the reflectionless connection is
sensitivity, and a loss of calibration. It is a good rule of thumb to use the L-pad
technique when major discontinuities are to be encountered and a tapered
section when small discontinuities are present (such as in cable testing).
The ±100% reflection points may be determined with the voltage markers and
by using a short and open at the transmission line's end.
Figure 9-17
Α 50 Ω
TDR System with a Matching L-Pad to the 75
Ω
cable. The Amplitude Corresponding to Rho
=
±
1 is Reduced Using the Matching L-Pad
R
2
50
Z
o
R
1
------------=
R
2
50 50
Z
o
–
()
=
R
2
50
Z
o
R
1
------------=