SARA-R4 series - System integration manual
UBX-16029218 - R20 Design-in Page 74 of 128
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• selecting a cellular antenna providing the worst possible return loss / VSWR / efficiency figure in
the GNSS frequency band: the lower is the cellular antenna efficiency between 1575 MHz and
1610 MHz, the higher is the isolation between the cellular and the GNSS systems
• ensuring at least 15 – 20 dB isolation between antennas in the GNSS band by implementing the
most suitable placement for the antennas, considering in particular the related radiation diagrams
of the antennas: better isolation results from antenna patterns with radiation lobes in different
directions considering the GNSS frequency band.
• adding a GNSS pass-band SAW filter along the GNSS RF line, providing very large attenuation in
the cellular frequency bands (see Table 26 for possible suitable examples). It has to be noted that,
as shown in Figure 4, a SAW filter and an LNA are already integrated in the GNSS RF path of the
SARA-R422M8S: the addition of an external filter along the GNSS RF line has to be considered
only if the conditions above cannot be met.
Additional countermeasures
In case all the aforementioned countermeasures cannot be implemented, adding a GNSS stop-band
SAW filter along the cellular RF line may be considered. The filter shall provide very low attenuation in
the cellular frequency bands (see Table 33 for possible suitable examples). It has to be noted that the
addition of an external filter along the cellular RF line has to be carefully evaluated, considering that
the additional insertion loss of such filter may affect the cellular TRP and/or TIS RF figures.
Table 33 lists examples of GNSS band-stop SAW filters that may be considered for the cellular RF
input/output in case enough isolation between the cellular and the GNSS RF systems cannot be
provided by proper selection and placement of the antennas beside other proper RF design solutions.
Table 33: Examples of GNSS band-stop SAW filters
Additional considerations
As far as the RF Tx power is involved in the cellular / GNSS RF coexistence, it has to be noted that
high-power transmission occurs very infrequently: typical values are in the range of -3 to 0 dBm (see
Figure 1 in the GSMA official document TS.09 [12]). Therefore, depending on the application, careful
PCB layout, antenna selection and placement should be sufficient to ensure accurate GNSS reception.
For an example of vehicle tracking application in a small form factor featuring cellular and short-range
connectivity alongside a multi-constellation GNSS receiver, with successful RF coexistence between
the systems, refer to the u-blox B36 vehicle tracking blueprint [23]. The distance between the cellular
and GNSS antennas for the u-blox B36 blueprint is annotated in Figure 45.
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