TDR Fundamentals
Instrument Configuration
9-28
Using TDR to Test Interconnects
One of the largest applications of TDR measurements is optimizing and testing
transmission line systems. An example of this involves the interface from a PC
board 50 Ω line to a thickfilm hybrid 50 Ω line. If the connection was made with
a 3 mm wire bond, then this would introduce a series inductive discontinuity
into the line. Where L
wb
is the inductance of the wire bond. Refer to Figure 9-
24. A wire bond in free space would have an inductance of about 1.26 nH/mm
but since it is located near the ground planes of the transmission lines the
inductance is somewhat lower. A measured inductance for typical wire bonds
on hybrids is about 1 nH/mm. If we assume this number, then the inductance
of the 3 mm wire bond is 3 nH. This then says that the low-pass filter created
by L
wb
in the 50 Ω line has a risetime given by:
where T = time constant = L
wb
/100 Ω = 2.2 × 3 nH/100 Ω = 66 ps
t
r
2.2T=
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