Crosstalk - Crosstalk - XAPP1392

Crosstalk is unwanted coupling between aggressor signals and victim signals, which can happen anywhere throughout the link. Common sources of crosstalk are connectors, BGA vias, PCB/package traces, cables, AC coupling capacitors and accompanying vias. In all cases, crosstalk is a function of the relative signal strength of the aggressor versus the victim at the region of coupling. For instance, if the victim is a receive channel that has incurred 30 dB of attenuation, and the aggressor is a nearby transmitter that has yet to encounter any attenuation, the crosstalk isolation must account for the strength of the aggressor. In this example, if the aggressor is 30 dB stronger than the victim, the crosstalk isolation must include at least 30 dB of isolation to counteract the aggressor signal strength. Because crosstalk is dependent on the relative strength of the victim and the aggressor, crosstalk isolation is especially important when adjacent channels have very different insertion loss profiles.

Figure 1. Crosstalk Examples

As illustrated in the following figure, within the BGA pin field there are three types of crosstalk:

TX-TX Coupling

This type of coupling is not influenced by the channel loss because the signal has yet to encounter the channel. Therefore, the crosstalk isolation required is independent of the channel loss for either the victim or aggressor.

RX-RX Coupling

An important factor in calculating the required crosstalk isolation is the relative insertion loss of the victim and aggressor channels. If the victim channel is an LR channel with 30 dB of insertion loss and the aggressor is a short reach channel with only 10 dB of insertion loss, the crosstalk isolation must include attenuation of 20 dB to account for the difference between the aggressor and victim signal strength. Because of this, it is important to identify cases in the application where there is a disparity between the insertion loss of the victim and the aggressor.

TX-RX Coupling

The TX incurs little attenuation while the attenuation of the RX can vary significantly. This type of crosstalk coupling is very dependent on insertion loss in the RX channel and is independent of the channel loss in the TX channel. The greater the loss in the RX channel, the more crosstalk isolation is required between the TX and RX. This coupling does not depend on the type of channel like LR and very short reach (VSR) for TX as is the case for TX-TX coupling. Instead, it is completely dependent on the RX channel insertion loss. More channel insertion loss requires more crosstalk isolation.

Figure 2. Crosstalk Components

The crosstalk isolation requirement is a function of the assigned crosstalk allowed in the channel link impairment budget, the victim, and the relative strength of the victim and the aggressor. For NRZ signaling, the type of coupling is used to define the relative strength of the victim and the aggressor. For PAM4 signaling, an additional 10 dB of isolation is required to cover the case when the aggressor signal swing is full amplitude and the victim swing is only 1/3 of that amplitude. So, for PAM4 signaling, if the budget for the crosstalk component is 10%, the crosstalk isolation for all types of coupling would be 30 dB. The isolation due to the type of coupling, TX-TX, RX-RX, or TX-RX, is added. See the following table for examples of crosstalk targets for various interface boundary conditions.

The reason that the BGA pin field is usually a high crosstalk region is the via-via and trace-via coupling. The breakout layer should be assigned in a way that minimizes the via-via and trace-via coupling. Some good practices for minimizing the crosstalk in a BGA pin field are as follows:

• A shallow breakout layer with a short via barrel and thus small vertical parallelism is preferred to minimize via-via coupling.
• If a long via barrel must be used, staggering the routing layers of adjacent pairs is a good practice to reduce via vertical parallelism.
• Route inner pairs on lower layers rather than outer pairs to avoid trace-via coupling.
• Place GND vias along the BGA field edge to improve isolation for pairs at the edge of the ball array.