
5 DUT Board Performance Considerations Correctly Terminating Signal Lines
120 System Reference, January 2001
Correctly Terminating Signal
Lines
The guidelines below give you a simplified view for
handling transmission lines.
Why use Impedance Matching Techniques?
At the high edge-transition speeds of the tester, traces and
coaxial cables start to behave like transmission lines. If
not properly terminated, every pulse transition that you
send is reflected back down the line. This can cause severe
distortion in the signal you are transmitting.
The way to prevent the reflections is to terminate at least
one end of the line with a load which matches the charac-
teristic impedance of the transmission line.
How Transmission Lines are Terminated
To prevent reflections in the connection between the IO
Channel and the DUT, the tester uses an impedance-
matched environment. This means that the coaxial cables
have a characteristic impedance of 50 Ω. At the tester end,
the driver in each IO channel also has an impedance of
50 Ω, used to terminate the transmission line.
Thus, all the signal-input lines to the DUT are terminated
already so that if the signal frequency causes transmission
line effects, they are impedance-matched to prevent any
reflections. Traces on the DUT board also have to be
impedance-matched.
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