Viavi CAA06M, User Manual

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Reflection Test
26
In telecommunications, return loss is a measure of power reflected from imperfections in an electrical or
optical communication link. The ratio (PR / PT), represents the wave power reflected from the
imperfection (PR) to that of the incident, or transmitted, wave, (PT). For maximum transmitted power, the
return loss should be as small as possible, meaning the ratio PR / PT should be as small as possible.
Return loss is usually expressed in dB and the return loss value describes the reduction in the amplitude
of the reflected energy, as compared to the forward energy. It will always be a loss and therefore a
negative dB. However, you can write -3 dB simply as 3 dB of loss, dropping the negative sign and adding
loss. For example, if a device has 15 dB of return loss, the reflected energy from that device (PR), is
always 15 dB lower than the transmitted energy (PT). When expressed in dB, larger (in magnitude)
negative numbers represent larger return losses and thus smaller reflected power (PR).
In electrical systems, return losses often occur at junctions between transmission lines and terminating
impedances. It is a measure of the dissimilarity between impedances in metallic transmission lines and
loads. For devices that are not perfect transmission lines or purely resistive loads, the return loss value
varies with the frequency of the transmitted signal.
NOTE:
You can also view reflection measurements in
Smith Chart to help you graphically identify
problems with transmission lines.
Reflection measurement screen