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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 PI (incident power) and the power reflected is PR, then the return loss in dB is given by

RL(dB) = 10 \log_{10} {P_R \over P_I}

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.

Considering negative values, the lesser the return loss, better it is; e.g. 0dB implies 100% reflection of the signal power getting reflected (PR = PI), -10dB implies 10% of the signal power getting reflected (PR = 0.1 * PI) , -20dB implying 1% of the signal power being reflected (PR = 0.01 * PI), -30dB implying 0.1% (PR = 0.001 * PI) & so on. This practically makes it more clear as lesser loss is better & more loss is bad, even more prominent & clear while looking at the graphical values e.g. Return loss in dB Vs Frequency in MHz wherein peaks would show higher negative values (e.g. -30dB) & Troughs would indicate lower negative values (e.g. -40dB).

Considering positive values, the higher the return loss (in dB), better it is; e.g. 0dB implies entire signal power (100%) got reflected and nothing (0%) got transferred to output load, 10dB return loss implies 10% of the signal power got returned (as loss) & remaining was transferred to output load, 20dB return loss implying 1% signal power got returned (as loss) & remaining got transferred. So one should remember, if positive values (in dB) are used for return loss, the more dB value would actually means lesser % value getting lost as reflection.

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 Vr to the amplitude of the incident wave Vi is known as the reflection coefficient Γ.

\Gamma = {V_r \over V_i}

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

\Gamma = { {Z_L - Z_S} \over {Z_L + Z_S} }

where ZS is the impedance toward the source and ZL 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

RL(dB) = -20 \log_{10} \left| \Gamma \right|

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 Pi (in dBm) and the reflected power Pr (in dBm).

RL(dB) = Pi(dBm) − Pr(dBm)

Optical

In an optical fiber, the loss that takes place at any discontinuity of refractive index, especially at an air-glass interface such as a fiber endface, at which a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss," or simply "Fresnel loss."

Fiber optic transmission systems use lasers to transmit signals over optical fiber, and a high optical return loss (ORL) can cause the laser to stop transmitting correctly. The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser.

ORL(dB) = -10 \log_{10} { {P_e} \over {P_i} }

where Pe is the reflected power and Pi is the incident, or input, power. value of return loss must be high to insure loss free transmission.

See also

References

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