Reference
32 80E00 ElectricalSampling Modules User Manual
A number of issues must be consid ered to make accurate TDR measurements. In
general it is relatively easy to make impedance measuremen ts near the reference
impedance (usually 50 Ω). Higher accuracy or measurements farther from the
referenceimpedance requires more care. The following list covers a few key
considerations in making accurate and repeatable im pedance measurements.
Resolution.Resolution determines the shortest impedance discontinuity that a
TDR instrument can measure. Because of round trip effects, Resolution =
1/2(System ReflectedRise Time). If a discontinuity, such as a variation in the
width of a trace, is small with respect to the system ri setime, the reflectionwill
not accurately represent the impedance of th e discontinuity. In extreme cases, the
discontinuity may effectivelydisappear. System rise time is the combined rise
time of the step generator(TDR), the instrument, and the interconnect between
the TDR and the circuit under test. In general, the most significantlimitation in
impedancetesting is the probe. Close attention to probe geometry and probing
techniques cangreatly enhance resolution.
Reference Impedance. All TDR measurements are relative; they com pare an
unknown impedanceto a known impedance. The accuracy of the resu lts depends
directly on the accuracy of the reference impedance. Any error in the reference
impedancetranslates to error in the measured impedance. It is also a good idea to
use a reference impedance close to the expected meas ured impedance because a
smaller differencebetween the reference and unknown impedance reduces
uncertaintyin the measurement.
CableLosses. Always use the shortest high-quality cable possible to connect to
the test fixture. The cablethat connects the TDR unit to the circuit boardnot
only degrades the system risetime, but can causeother aberrations in the system
response thatadd to measurement error.
TakingDifferential and Common-Mode TDR MeasurementsThis section describeshow to use the 80E04 to take differential and common-
mode time-domainreflectometry (TDR) measurements.
Totake TDR measurements on coupled transmission lines. Using common-mode
and differentialTDR, you can characterize coupled transmission lines.
The Tektronix80E04 sampling module is a true differential sampling module for
more accurate differential TDR measurements.
This featureonly works with an 80E04 sampling module.