SEPTENTRIO mosaic Series, Руководство по оборудованию

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mosaic Integration
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5.5.4 Avoiding Self-Interference
Antenna input connections are sensitive to interference from higher harmonics of other
signals on the board. Even clock signals of just a few MHz can produce harmful harmonics
at GNSS frequencies (1155-1300 MHz and 1540-1610 MHz).
It is best to keep antenna input traces short, to reduce the area in which signals can be
picked up. Stitching vias at the input trace could be arranged as a via fence to shield it
from interference.
Furthermore, it is important to avoid digital signals in the MHz-
range (SDIO, RMII, MDIO,…)
from running close to antenna inputs.
If an external frequency reference is used, it will get close to the antenna input because
of the proximity of the REF_I and ANT_1 pad.
This is not a problem if it doesn’t have many
harmonics. It can however cause issues if the reference signal is originating from a high-
speed buffer or comparator. This can be avoided at circuit level, by filtering the signal with
an RC-filter near the source (see section 4.7.2).
Most self-interference issues relate to radiated interference into a collocated GNSS-
antenna. The following applies when the GNSS antenna is closer than 1 meter from
electronics which are not in a shielded box:
•
The SDIO, RMII and MDIO signals of mosaic can cause harmful radiated
interference if not properly routed. In designs with a collocated antenna, these
signals shall preferably be routed in an inner layer of the board, shielded by ground
planes or a ground copper pour at top and bottom layers, connected with stitching
vias. This approach puts them in a Faraday cage. Also avoid passing these signals
though an unshielded board-to-board connector if there is no shielding at system
level, like a metal housing or shielding can.
•
The same holds for other high-speed digital signals in other electronics on the
motherboard, like memory busses and clock signals. They should also be routed
in an inner layer, flanked with cupper pours connected to ground.
• Large processor and memory chips sometimes already radiate via the bondwires
inside their package. Connectors like SD card sockets and radio-module sockets
also tend to radiate.
It’s best to put these components at the side of the board
facing away from the collocated antenna. In this way the ground-layer will shield
them. Alternatively, they could be placed underneath an EMI shielding can. There
is less of a concern if the associated clock frequencies have no harmonics in the
GNSS bands.
See also Appendix D.