Return loss

One professor commented that return loss is usually specified in receiver input, for example LNA. Is is a specification in power amplifier?

Goolged the answer, it is a general definition. Actually it can be used for any device in power transmission. In power amplifier, input return loss and output return loss are also used in specification, but some people prefer to use VSWR instead of input/output return loss.

 

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In telecommunications, Return loss or Reflection loss is the reflection of signal power resulting from the insertion of a device in a transmission line or optical fiber. It is usually expressed as a ratio in dB relative to the transmitted signal power.
If the power supplied by the source is P_I (incident power) and the power reflected is P_R, then the return loss in dB is given by

Optical Return Loss is a positive number, historically ORL has also been referred to as a negative number. Within the industry expect to see ORL referred to variably as a positive or negative number.
This ORL sign ambiguity can lead to confusion when referring to a circuit as having high or low return loss; so remember:- High Return Loss = lower reflected power = large ORL number = generally good. Low Return Loss = higher reflected power = small ORL number = generally bad.

Electrical

In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio of the amplitude of the reflected wave V_r to the amplitude of the incident wave V_i is known as the reflection coefficient Γ.

When the source and load impedances are known values, the reflection coefficient is given by

where Z_S is the impedance toward the source and Z_L is the impedance toward the load.
Return loss is simply the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, then return loss is given by

where the vertical bars indicate magnitude. Thus, a large positive return loss indicates the reflected power is small relative to the incident power, which indicates good impedance match from source to load.
When the actual transmitted (incident) power and the reflected power are known (i.e. through measurements and/or calculations), then the return loss in dB can be calculated as the difference between the incident power P_i (in dBm) and the reflected power P_r (in dBm).

RL(dB) = P_i(dBm) − P_r(dBm)