Insertion Loss Budget Guide

An insertion loss budget is a list of every loss in a high speed channel, added up before the layout begins. The driver, the package, the board, the vias, the connectors and the cable all contribute, and the total has to leave enough signal at the receiver for the interface to work. A budget that is built after the layout is a description of a problem rather than a design tool.

What the Budget Is

The budget is expressed in decibels at a reference frequency, which for a digital link is usually half the data rate. The transmitter has a known output amplitude and the receiver needs a known minimum, so the difference is the loss that the channel may have.

The loss is frequency dependent, so a single number is not enough. The budget is therefore a curve or a set of values at several frequencies, and the shape of the curve matters as much as the value at one point.

A link that just meets the budget at the reference frequency can still fail, because the higher frequency components carry the edges of the eye and they are attenuated more than the fundamental.

The budget should be built with the interface specification in hand, since the standard usually quotes the loss that the channel may present, and that number is the target rather than an opinion.

The Terms in the Budget

The package of the driver and the receiver is the first term. The bond wire and the package trace add a loss and often a significant one, and it is not visible in the board layout.

The board itself contributes a dielectric loss and a conductor loss, both of which grow with the length of the line. The two have different frequency dependence, which is why a short line and a long line can have different limitations.

The vias contribute a loss and a reflection, and a via with a stub is worse than one without. The stub length sets the frequency at which the reflection becomes significant.

The connectors and the cable complete the list. A connector is usually a small loss, but a poorly chosen one is a large reflection, and the two effects appear together in a time domain measurement.

Dielectric and Conductor Loss

The dielectric loss comes from the resin absorbing energy as the field alternates. It is proportional to the frequency and to the loss tangent of the material, and it is the dominant term on a long line at a high frequency.

The conductor loss comes from the resistance of the trace, and it grows with the square root of the frequency because of the skin effect. The roughness of the foil adds to it, as the profile makes the current path longer.

A low loss laminate reduces the dielectric term but does nothing for the conductor term, so a material change only helps if the dielectric loss dominates. That is the case on a long backplane and not on a short card.

The high frequency laminate guide gives the loss tangent figures, and the conductor term should be calculated from the geometry and the foil type rather than assumed.

Vias and Connectors

A signal via has a barrel and a pad, and if the via is not back drilled it also has a stub below the layer where the trace leaves it. The stub is an open transmission line that resonates at a frequency set by its length.

A stub of a few millimetres resonates in the gigahertz range, and the reflection at resonance is large. Back drilling removes the stub and is the standard fix on a thick backplane, while a blind via or a via in pad removes it by construction.

A connector adds a short section of a different impedance, and the discontinuity creates a reflection rather than a loss. The reflection is measured as a return loss, and it should be part of the budget as a separate item.

The via options are discussed in the via guide, and the choice between them is often decided by the stub length rather than by the routing density.

Length and Layer Choices

Length is the term that the layout controls most directly, and a shorter line is better in every term of the budget. A link that is marginal on a long route is often comfortable on a shorter one.

The layer choice changes the loss as well, because a stripline is buried between two dielectric layers and a microstrip has one layer of dielectric and one of air. The field distribution differs, so the same material gives a different loss.

The reference plane spacing matters, since a thinner dielectric gives a tighter field and a lower loss per unit length for the same impedance. The trade is a tighter manufacturing tolerance on the impedance.

The routing in the microstrip and stripline guide describes both structures, and the choice is usually made together with the layer count.

Equalisation and Margin

Most high speed receivers include an equaliser that compensates for the low pass behaviour of the channel. The equaliser restores the high frequency content at the cost of amplifying the noise, so it recovers some of the loss rather than all of it.

The budget should include the equaliser gain as a negative term and the noise penalty as a positive one. A link that relies on the maximum equaliser setting has no margin left for a temperature change or a process variation.

The margin should be stated explicitly, as a number of decibels or as a percentage of the eye height. A budget with no margin is a budget that will fail on the first board that is slightly different from the model.

The high speed design rules give the framework for the routing, and the budget turns the framework into a pass or fail decision before the artwork is released.

Practical Rules

Start the budget before the layout, using the interface specification and the expected length. Update it as the layout changes rather than at the end.

Measure a coupon. A set of traces of different lengths on the same panel gives the loss per unit length for the actual material and process, which is far better than a catalogue figure.

Keep the via stubs short, and back drill the ones that are not. A stub that resonates inside the band of interest is often the difference between a working link and a marginal one.

Record the budget with the board documentation, so that a later change of material or of length can be checked against the same numbers.

FAQ

What is a typical insertion loss budget? It depends on the interface, and the specification for that interface usually quotes the allowed channel loss. A common figure is a few decibels at half the data rate.

Does a low loss laminate always help? It only helps where the dielectric loss dominates. On a short line the conductor loss and the vias are larger terms, and the material change buys very little.

Why is the via stub so important? Because the stub resonates at a frequency set by its length, and the reflection at resonance can exceed the loss of the whole line.