HP 54753A User Manual page 190

TDR Fundamentals
Step Reflection Testing
The part of E reflected from the junction of Z
rL
again reflected off the load and heads back to the monitoring point only to be
partially reflected at the junction of Z
after some time the magnitude of the reflections approaches zero.
Practical Handling of Multiple Discontinuities.
It is now seen that although TDR is useful when observing multiple
discontinuities, one must be aware of the slight complication they introduce
when analyzing the display. It is fortunate that most practical measuring
situations involve only small mismatches (e.g., Z
multiple reflections is almost nil. Even in this situation, however, it is advisable
to analyze and clean up a system from the generator end. The reflection from
the first of any number of discontinuities is unaffected by the presence of others.
Therefore if it is remedied first and one then moves on to the second
discontinuity, the complications introduced by re-reflections will not exist.
Matching Source Impedance to Transmission Line Impedance
Until now nothing has been said concerning reflections that may have occurred
at the generator end of the transmission line. In general, the source impedance
of the step generator may not be equal to the characteristic impedance of the
transmission line it drives. When this is the case, voltage waves returning from
a mismatch or discontinuity in the system under test will be re- reflected at the
generator end and will complicate the analysis of the display. Referring to Figure
9-14 and Figure 9-15, it is almost essential that the source impedance of the
step generator match the cable it drives. When this is the case, all re-reflections
returning from the system under test pass the oscilloscope's monitoring point
only once and are then absorbed in the source impedance of the step generator.
Figure 9-14 is the oscilloscope display of a TDR system investigating a
transmission line terminated into an open circuit. The source impedance of the
step generator matches the characteristic impedance of the line under test
= 50 Ω).
(Z = Z
s o
9-16
' and Z , such as ρ
o o
'‚ and Z
. This continues indefinitely, but
o o
≅ Z ') and the effect of
o o
'E
, is
1 rL