High speed signal routing layers

I have tried to attached “General High Speed Signal Routing” a TI document SPRAAR7J – NOVEMBER 2018 – REVISED FEBRUARY 2023

Here is the link

Referring to section 3.2 High Speed Differential Signal Rules.

Point number 4: “When possible, route high-speed differential pair signals on the top or bottom layer of the PCB with an adjacent GND layer. TI does not recommend stripline routing of the high-speed differential signals.”

Given microstrip is not symmetric compared to stripline having reference layers on top and bottom. If differential signals, there are two components, one is common signal or the common voltage level and the other is differential signal or the differential voltage level. I guess in the asymmetric geometry the speed of the common part and the differential part does not remain the same as in case of microstrip and that is the reason we do not route high speed signals on top and bottom layers. Because of symmetric geometry in case stripline, we route the high speed signals in inner layers.

But the TI document is describing something which is opposite.

High-speed signals should be routed on the top or bottom if possible.

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The following text is taken from the book “Signal Integrity and Power Integrity – Simplified” written by Eric Bogatin.

Chapter 11: Differential Pairs and Differential Impedance

Even and Odd Modes

There are two special voltage patterns we can launch into the pair that will propagate down the line undistorted.

The first pattern is when exactly the same signal is applied to either line; for example, the voltage transitions from 0 v to 1 v in each line.

The second special voltage pattern that will propagate unchanged down the differential pair is when the opposite-transitioning signals are applied to each line; for example, one of the signals transitions from 0 v to 1 v and the other goes from 0 v to –1 v.

To distinguish these two states, we call the state where the same voltage drives each line the even mode and the state where the opposite-going voltages drive each line the odd mode.

Velocity of Each Mode and Far-End Cross Talk

The description of the signal in terms of its components propagating in each of the two modes is especially important in edge-coupled microstrip because signals in each mode travel at different speeds.

The velocity of a signal propagating down a transmission line is determined by the effective dielectric constant of the material the fields see. The higher the effective dielectric constant, the slower the speed, and the longer the time delay of a signal propagating in that mode.

In the case of a stripline, the dielectric material is uniform all around the conductors and the fields always see an effective dielectric constant equal to the bulk value, independent of the voltage pattern.

The odd and even-mode velocities in a stripline are the same.

However, in a microstrip, the electric fields see a mixture of dielectric constants, part in the bulk material and part in the air. The precise pattern of the field distribution and how it overlaps the dielectric material will influence the value of the resulting effective dielectric constant and the actual speed of the signal. In the odd mode, more of the field lines are in air; in the even mode, more of the field lines are in the bulk material. For this reason, the odd-mode signals will have a slightly lower effective dielectric constant and will travel at a faster speed than do the even mode signals.

In a stripline, the fields see just the bulk dielectric constant for each mode. There is no difference in speed between the modes for any homogeneous dielectric interconnect.

In an edge-coupled microstrip, a differential signal will drive the odd mode so it will travel faster than a common signal, which drives the even mode.

Should we route high speed differential signals in inner layers or outer layers ?

I prefer to route all controlled impedance traces on outer layers. However, if you need to route controlled impedance traces on internal layers the goals are:

  1. Make the transition onto the internal layer properly (i.e. proper use of vias).

  2. Try to stay on only that layer and don’t jump around from layer to layer.

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I think the main reason for using a differential pair is to control noise from interference. Ideally, the interference will affect both traces the same way, and be seen as common-mode, and therefore be ignored. If it gets there a little before (or after) the real signal, that shouldn’t matter since you’ll still ignore it anyway.

The problem with using an internal layer is getting there and back. The internal trace itself may have slightly more consistent impedance, but the vias will almost certainly present impedance discontinuities, and going to/from an inner layer will also create stubs that smear out the signal.

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