HP 54753A User Manual page 189

The oscilloscope's display for this situation would be similar to the diagram in
Figure 9-13 (drawn for the case where Z
Figure 9-13
It is seen that the two mismatches produce reflections that can be analyzed
separately. The mismatch at the junction of the two transmission lines
generates a reflected wave, E
Similarly, the mismatch at the load also creates a reflection due to its reflection
coefficient
Two things must be considered before the apparent reflection from Z
on the oscilloscope, is used to determine ρ
Z
but this is not equal to E
junction of the two transmission lines. The wave that returns to the monitoring
point is
Since ρ
E =
r1
is (1 + ρ
) E , not merely E
L 1 i
[ ρ (
2
'
( ρ )E
E = 1 +
r2 1
' = -ρ
, E may be re-written as:
1 1 r2
E
r2
> Z
L o
, where
r1
'
 
Z – Z
ρ o o
 
E = ------------------ - E
1 i ' i
 
Z + Z
o o
'
Z – Z
ρ
L o
= -------------------
'
2
Z + Z
L o
. First, the voltage step incident on
2
: Second, the reflection from the load is
i
ρ )E ]
1 + = E
1 i rL
since a re-reflection occurs at the mismatched
r2
'
( ρ ) ρ [ ( ρ
= 1 + 1 +
rL 1 2 1
2
[ ρ ( 1 ρ ) ]E
= –
2 1 i
TDR Fundamentals
Step Reflection Testing
> Z' ):
o
, as shown
L
)E ]
i
9-15