TDR Fundamentals
Instrument Configuration
9-29
Therefore the risetime of the signal that is to pass through this discontinuity
should be greater than 66 ps if it is not to be significantly degraded. If the signal
to be transmitted through the discontinuity was a 350 ps risetime logic signal,
then the risetime degradation would be small. Even though the risetime
degradation is small there will be a significant reflection off the wire bond.
Assuming the reflection is less than 10%, then an equation predicts a maximum
reflection of:
or
if the edge was an ideal Gaussian step.
Figure 9-24
If the step is not ideal, this gives an approximate answer. This reflection may
or may not be a problem. If the circuit driving the 50 Ω line is source-terminated
in 50 Ω then this will not be a problem, but if it is driven from a current source
such as an open collector of a transistor, then it could. If it is desired to minimize
reflections of this discontinuity, then there are methods to do this. Refer to
Figure 9-24. If a TDR system is used to measure the transmission line, a
response would be seen as shown in Figure 9-26 which is an inductive response
with a max reflection of about 8.7% as predicted before. If the capacitance along
the wire bond could be increased, this would reduce the maximum reflection
since the wire bond section is moving towards a 50 Ω line. While it may not be
possible to do this, it is possible to increase the capacitance at the two ends of
the wire bond by widening the 50 Ω lines there. The circuit would now resemble
the circuit shown in Figure 9-25. When the value of C1 and C2 are chosen
properly, the TDR response of the system would now be as shown in
Figure 9-27. The value of C1 and C2 which minimizes the maximum reflection
is 0.6 pF which can be calculated from the equation.
L 98.4
ρ
t
r
system
=
ρ
L
98.4t
r
system
-----------------------------
3 nH
98.4
()
350 ps
()
------------------------------------ 8 . 7== =
L
wb
Z
o
50
Ω
L
C
----==
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