5. TDR Measurement Theory
Figure 5.6.
Relationship of impedance (ohms) to reflection coefficient (rho) for 50 ohm
source impedance (marked by red circle).
When impedance measurements are very important, accuracy can be improved using Vert. Ref.
(page 47) and normalized TDR traces (page 56).
5.7. Return Loss
Return loss is another way of measuring impedance change in a cable. Return loss is given in
decibels (db) and is always calculated using the relative reflection coefficient. Return loss is
related to the reflection coefficient Γ by the formula:
Return Loss =
−
20
·
log
10
|
Γ
|
db
The larger the fraction of energy in the reflected signal, the lower the numerical return loss value,
so that an open or a short that returns 100% of the signal has a return loss of 0 dB. A very low
loss cable will have a large return loss in dB. Likewise a 50 ohm cable terminated with a perfect
50 ohm load will have very large return loss in dB. This relationship is shown in Figure 5.7.
Return loss measured directly from a CT100B TDR trace represents the average return loss over
all frequencies in the step-rise test signal. The CT100B can measure TDR return loss at cursor
and relative return loss between cursors and optionally displays these values at the right hand of
the screen in units of dB, with ∆dB used for the relative return loss measurement.
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CT100B TDR Cable Analyzers Operator’s Manual
