Let’s get on the same page by calling up an accepted industry definition of Return Loss. (IEEE 1988) defines Return Loss as:
(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:
20*log10|(Z1+Z2)/(Z1-Z2)| decibel
where Z1 and Z2 = the two impedances.
(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.
A mathematically equivalent expression is that \(ReturnLoss=\frac{P_{incident}}{P_{reflected}}=\frac{P_{fwd}}{P_{rev}}\).
ReturnLoss is fundamentally a power ratio that can be expressed in dB. This article uses ReturnLoss as simply a power ratio.
Return Loss Bridge (RLB)
A Return Loss Bridge (RLB) is a common implementation of a Directional Coupler, a device that can in a given transmission lines context (meaning wrt some given characteristic impedance Zref), be used to measure forward wave to reverse wave components and calculate their ratio, ie the ReturnLoss.
Let’s look at the common resistive RLB in detail.
Above is an LTSPICE model of an ideal RLB and source with Zref=50+j0Ω. Note:
- ALL the resistors (except for the unknown Zu) are equal to Zref;
- I1 and R3 model an ideal source with Zs=Zref; and
- R13 is the ‘floating’ measurement detector, again it presents a load of Zref.
Let’s explore some interesting properties of this ideal RLB. Continue reading The common resistive Return Loss Bridge
Last update: 11th June, 2024, 2:48 PM
