Tektronix Findingt he Velocityof Propagation and Locating Mismatches, TDR Measurement Units

Reference
30 80E00 ElectricalSampling Modules User Manual
The time between the incidentedge and the reflected edge is valuable in
determining the length of the transmissionline from the TDR to a mismatch, or
between two mismatches. The formulais:
D=v䴍×T
2=v䴍T
2
where:
v䴍=velocity of propagation
T=the time from the TDR to the mismatch and back again,
as measuredon t he instrument
D=distance to the fault
Velocityof Propagation (vρ) is a measure of how fast a signal travels in that
transmission line.
NOTE. The factor of 2 in the denominator is present because TDR systems
display round-triptime (incident and reflected edges), whereas with distance it is
usually desirable to display one-way distance. It is important to note that the
distance scale does not inject this factor of two and, therefore,the distance
displayed is round-trip.See the main instrument user documentation and online
help for moreinformation about distance scale operation.
All TDR impedance measurementsare based on the ratio of transmitted voltage
to reflectedvoltage. As a result, measurements are not generally taken in
absolute units, such as volts. Instead, TDR measurements are madeon a relative
scale, calledreflection coefficient, and abbreviated as ρ.The definition of ρis the
reflectedsignal amplitude divided by the incident signal amplitude. For example,
if a 100-millivolt reflectionresults from a 1-volt incident step, the reflection is
called a100 millirho reflection: ρ=E
reflected/Eincident = 100 mρ= 100 mV/1 V.
Given a known impedanceand a measured reflection coefficient, the unknown
impedance that caused the reflection can be calculated from the fol lowing
equation:
=Ereflected
Eincident
=ZL–Zo
ZL+Zo
where Zois the known impedance,ρis the measured reflection coefficient, and
ZLis the unknown impedance. An alternateform of the equation is:
ZL=ZOᏋ1+
1−Ꮠ
Findingt he Velocityof
Propagation and Locating
Mismatches
TDR Measurement Units