7. TDR Measurement Theory
7.7. 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 7.7.
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 of the screen
in units of dB, with ∆dB used for the relative return loss measurement.
The CT100B is also able to measure complex frequency-specific S
11
return loss and cable loss
values such as those produced with Vector Network Analyzer (VNA) and Frequency-Domain
Reflectometer (FDR) equipment. This is discussed in further detail in Section 7.11.
Figure 7.7. Relationship of return loss (dB) to reflection coefficient (rho).
CT100B TDR Cable Analyzers Operator’s Manual 101
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