TX-to-RX Crosstalk Analysis for 56 Gbps Full-density Quad Usage - TX-to-RX Crosstalk Analysis for 56 Gbps Full-density Quad Usage - XAPP1392

PCB Channel Design Guidelines for 112 Gbps GTM Transceivers (XAPP1392)
Document ID
XAPP1392
Release Date
2023-05-23
Revision
1.0 English

Similarly, for the TX-to-RX crosstalk analysis of a triple-triple BGA pin pattern, the routing layers (via length) for the TX in the middle column are swept to understand the effects of vertical distance between TX lead-out traces and the RX via barrel to the TX-to-RX coupling.

  • The routing layers for RX are constrained to L3, L5, and L7.
  • The routing layer of the TX middle column is swept from L5, L7, L9, and L22.
  • The routing layers of the TX right and left columns are kept to L22 and L24.

The following table lists the simulated power sum crosstalk for various cases. The following can be observed:

  • The TX middle column is routed on a layer at least 18 mil (L9 – L5) below the lowest RX routing layer to fulfill the –65 dB TX-to-RX crosstalk requirement.
  • While routing the TX middle column on L22 minimizes TX-to-RX crosstalk, it violates the –35 dB TX-to-TX crosstalk requirement due to the long via vertical parallelism.
  • The attainable RX-to-RX crosstalk is –44 dB, allowing a 14 dB victim-to-aggressor insertion loss delta.
Table 1. Simulated Power Sum Crosstalk per Via Length
PowerSum TX-TX Xtalk (dB) PowerSum TX-RX Xtalk (dB) PowerSum RX-RX Xtalk (dB) RX Routing Layers TX Routing Layers Comments
–28.63 1 –68.9 3 –43.9 2 L3, L7 L22, L24 TX middle column routed on L22
–41.2 3 –54.1 1 –43.8 2 L3, L7 L5, L22, L24 TX middle column routed on L5
–38.6 3 –55.3 1 –44.4 2 L3, L5 L7, L22, L24 TX middle column routed on L7
–36.7 2 –63.8 2 –44.4 2 L3, L5 L9, L22, L24 TX middle column routed on L9
–36.7 2 –64.7 2 –44.4 2 L3, L5 L9, L22, L24 TX middle column routed on L9 with layer mis registration core L6/L7 – 4 mils
  1. Not allowed.
  2. OK with constraints.
  3. Good.
  4. TX-TX isolation requirement is 35 dB, allowing for 5 dB TX channel insertion loss delta.
  5. TX-RX isolation requirement is 65 dB, allowing for 35 dB maximum RX channel loss.
  6. RX-RX isolation requirement is 45 dB, allowing for 15 dB RX insertion loss delta.

The simulated TX-to-TX and RX-to-RX crosstalk amplitudes from individual aggressors with the TX middle column routed on L9 (the fourth entry of Table 1) are shown in the following figure. It can be seen that the crosstalk from the diagonal aggressors is more severe than that from the non-diagonal aggressors. The calculated RX-to-RX power sum crosstalk is 44.4 dB, and the TX-to-TX power sum crosstalk is 36.7 dB.

Figure 1. Simulated TX-to-TX & RX-to-RX Crosstalk with TX Middle Column Routed on L9