Quectel RG520N-AT Hardware Design page 65

The following principles of PCIe interface design should be complied with to meet PCIe specifications.
⚫
Route the PCIe signal traces as differential pairs with ground surrounded. The differential impedance
is 72.5–97.5 Ω and 85 Ω is recommended.
⚫
PCIe signals must be protected from noise signals (clocks, DC-DC, RF and so forth). All other
sensitive/high-speed signals and circuits must be routed far away from PCIe traces.
⚫
For each differential pair, intra-lane length match should be less than 0.7 mm. The total bus length
should be less than 300 mm for PCIe 3.0.
⚫
Inter-lane length match, i.e., the trace length matching between the reference clock (Tx and Rx pairs)
is not required.
⚫
The space between Tx and Rx, and the space between PCIe lanes and all other signals, should be
larger than 4 times of the trace width.
⚫
PCIe Tx AC coupling capacitors can be placed anywhere along the trace, but better to be placed
close to source or connector side to keep good signal integrity of main route on PCB. PCIe Tx AC
coupling capacitors should be 220 nF for Gen 3 and 100 nF for Gen 2/Gen 1.
⚫
Ensure not to stagger the capacitors since this will affect the differential integrity of the design and
create EMI.
⚫
In the case of trace serpentines, one line of a differential pair must be routed to make up a length
delta, then it must be routed at the source (breakout) – this ensures that traces stay differential
thereafter.
⚫
To reduce the probability for layer-to-layer manufacturing variation, minimize layer transitions on the
main route (in other words, apply layer transitions only at module breakouts and connectors to
ensure minimum layer transitions on the main route).
Table 27: PCIe Trace Length in the Module
Pin No. Pin Name
40 PCIE_REFCLK_P
38 PCIE_REFCLK_M
46 PCIE_TX0_P
44 PCIE_TX0_M
43 PCIE_TX1_P
41 PCIE_TX1_M
34 PCIE_RX0_P
32 PCIE_RX0_M
37 PCIE_RX1_P
RG520N-AT_Hardware_Design
Length (mm)
7.52
7.58
12.87
12.90
10.36
10.37
3.92
4.09
4.88
5G Module Series
Length Difference (P-M) (mm)
-0.06
-0.03
-0.01
-0.17
0.03
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