Reference
80E00 ElectricalSampling Modules User Manual 27
TDR Measurements BackgroundTDR is based on a simple concept:Whenever energy transmitted through any
medium encountersa change in impedance, some of the energy is reflected back
toward the source.The amount of energy reflected is a function of the trans-
mitted energyand the magnitude of the impedance change. The time lapse
between energytransmission and the reflection returning is a function of the
distance fromthe source to the impedance discontinuity, and the propagation
velocity.
The effectsof this phenomenon are evidenced through echoes that occur when
sound encountersa wall. In electrical systems, a similar phenomenon occurs
when electricalenergy traveling in a transmission line encounters a changein
impedance. Any change in the imped ance of the transmission line, such as a
variation in the width of a circuit board trace, causes a reflecti on with an
amplitude relatedto the magnitude of the impedance change.
A TimeDomain R eflectometersends out a step on the cable, circuit board, or
integratedcircuit under test. The reflection (or echo) received by the TDR is
measured to find events along the path of the step.
Reflections arecaused both by events that are expected, such as width changes
and components, and by thosethat shouldn’t be there, such as bridges, shorts,
and opens. The strengthof a TDR measurement is that it not only tells you there
is a fault, but it also tells you the magnitude and the distance to that fault.
TDR can note any change in the characteristicimpedance of the device-under-
test (DUT). Any changein the impedance is shown on the TDR display as an
upward bump or downwarddip in the waveform, depending on the type of event
(see Figure 12 for examplediscontinuities in a microstrip).
Connector
Roundtrip time
Open
circuit
Volts or ρ
Incidentstep
Capacitive
discontinuity
Inductive
discontinuity
Conductor
Figure12: Microstripdiscontinuities