Insertion Loss vs. Return Loss: Signal Transmission and Reflection

The term loss is almost unilaterally associated with a negative in nearly every conceivable scenario. Take, for example, the loss administered to a certain NFL team (New England) by the magnificence of a certain quarterback (Lamar Jackson) from Baltimore. In this case, if you are a fan of TB 12’s team, then this is most definitely a negative. However, if you are not a fan, then this is one of those rare occasions in which a loss is not a negative.

In the field of electronics, the term loss can also hold a dual relative meaning. So, depending on the circumstances, the term loss can be either a good thing or a bad thing. However, as it is in most cases, the term loss tends to lend itself to the side of negative in the field of electronics as well.

All losses are not equal, and a loss can come in many forms, such as a power loss, connectivity loss, insertion loss, and even a return loss. Furthermore, in the field of electronics, losses such as insertion loss are an essential performance parameter measurement in designs consisting of fiber-optic links.  So, over the next few paragraphs, I will discuss the significance of both insertion loss and return loss as well as their effects on performance and functionality.

What is Insertion Loss?

The loss of signal, which occurs along the length of a fiber optic link, is called insertion loss. This particular measurement parameter is expressed in decibels and should always be a positive number. However, should, does not mean always, and if by chance, it is negative, that is not a favorable measurement parameter.

Insertion loss is, however, a natural occurrence that occurs with all types of transmissions, whether it is data or electrical. Furthermore, as it is with basically all physical transmission lines or conductive paths, the longer the path, the higher the loss. Moreover, these losses also occur at each connection point along the line, including splices and connectors.

As stated initially, we express insertion loss in dBs (decibels), and ordinarily, it is a positive number since it indicates how much signal loss by comparing the input power to the output power. In summary, a signal will always come out lesser than their input level. Furthermore, the lower the number equals a better insertion loss performance; for example, an insertion loss of 0.3dB is better than 0.5dB.

In some instances, an insertion loss may appear as a negative parameter measurement. However, if this is the case, a negative insertion loss means that there is an issue, one of which usually indicates an improper reference setting. For instance, if a reference cable requires cleaning when setting the zero benchmark, and you clean it prior to testing, the insertion loss may show a gain and possibly indicate a negative measurement parameter.

Balancing signals can be tremendously difficult without the right tools.

What is Return Loss?

The measurement of the amount of light that is reflected back toward the source is called Return loss, and its unit of expression is also in decibels (dBs). Furthermore, this measurement parameter is always a positive number, and a high return loss is a favorable measurement parameter, and it typically correlates to a low insertion loss. Similarly, reflectance, which is also a measurement parameter that expresses reflection in decibels, is a negative number, and if it is excessive, it is not a favorable measurement parameter.

In summary, return loss is the loss of signal power due to signal reflection or return by a discontinuity in a fiber-optic link or transmission line. This impedance mismatch can be with a device inserted in the line or with the terminating load. Moreover, return loss is the relationship between both the reflection coefficient (Γ) and the standing wave ratio (SWR). Incidentally, if you increase the return loss, it will correlate to a lower SWR.

Overall, return loss is a measurement parameter that expresses how well a device or line matches. The rule of thumb here is, it is favorable if the device or line match, providing that the return loss is high. Furthermore, a high return loss is advantageous as it will result in a lower insertion loss.

In today’s electronics practices, in terms of use, return loss is preferable to SWR since it affords better resolution for smaller values of reflected waves.

Insertion Loss vs. Return Loss

As previously stated, regardless of type, when a signal travels through a system or a component, power (signal) loss is unavoidable. This loss I am referring to happens while the signal is traversing through a system or a component; it, of course, is called Insertion Loss. 

Furthermore, there a myriad of reasons for this loss of signal power or insertion loss, but the main three are as follows:

1. Dielectric Losses: Loss can occur due to power dissipation in the dielectric materials.

2. Reflected Losses: Loss can occur due to the Voltage Standing Wave Ratio (VSWR). VSWR is a measure of the efficiency of the transmission of radio frequency power from its source through a transmission line and into a load, such as from a power amplifier, through a transmission line, and to an antenna.

3. Copper Losses: Loss can occur due to the power dissipation of conducting surfaces.

So now, let us examine the above diagram in detail so that we may gain a better understanding of how insertion loss and return loss interact. As you can see, incident power travels down a transmission line from the left until it reaches the component. Once it reaches the component, a portion of the signal is reflected back down the transmission line towards the source from which it came. Also, keep in mind that this portion of the signal does not enter the component.

The remainder of the signal does indeed enter the component. There some of it gets absorbed, and the rest passes through the component into the transmission line on the other side. The power that comes out of the component is called the transmitted power, and it is less than the incident power for two reasons:

1.   A portion of the signal gets reflected.

2.   The component absorbs a portion of the signal.

So, in summary, we express insertion loss in decibels, and it is the ratio of incident power to transmitted power. Furthermore, we can summarize that return loss, which we also express in decibels is the ratio of incident power to reflected power. Therefore, we can see how the two types of loss measurement parameters help to accurately gauge the overall efficiency of a measurable signal and component within a system or in a through path.

Determining insertion loss and return loss in high speed circuits requires understandings of component relationships. 

In conclusion, when we combine the measurement parameters of both insertion loss and return loss, we can more accurately assess efficiency and performance. Furthermore, it can determine if there are impedance mismatches at the pins of the receiver and transmitter as well as the vias, connectors, and various other discontinuities. In short, the overview that these two parameter measurements provide is an essential assessment tool in understanding signal performance. 

Insertion loss vs return loss parameters can be measured easily within the suite of design and analysis tools from Cadence. If you're looking for easy layout solutions and comprehensive analysis or simulation options, look no further than Allegro PCB Designer. 

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