HP 54753A User Manual page 202

TDR Fundamentals
Instrument Configuration
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 is the inductance of the wire bond. Refer to Figure 9-
wb
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
in the 50 Ω line has a risetime given by:
by L
wb
t = 2.2T
r
/100 Ω = 2.2 × 3 nH/100 Ω = 66 ps
where T = time constant = L
wb
9-28