SARA-R5 series - System integration manual
UBX-19041356 - R04
Design-in
Page 66 of 118
C1-Public
SARA-R510M8S
31
ANT_GNSS
GND
F1
SAW
LNA
U1
Figure 43: Typical circuit for best performance and improved jamming immunity with GNSS passive antenna
The external LNA can be selected to deliver the performance needed by the application in terms of:
•
Noise figure (sensitivity)
•
Selectivity and linearity (robustness against jamming)
•
Robustness against RF power
Depending on the characteristics of the supply source (DC/DC regulator, linear LDO regulator or other)
used to supply the external LNA, make sure some good filtering is in place for the external LNA supply
because of the noise on the external LNA supply line can affect the performance of the LNA itself:
consider adding a proper series ferrite bead (see
for possible suitable examples) and a proper
decoupling capacitor to ground with self-resonant frequency in the GNSS frequency range (as for
example the 27 pF 0402 capacitor Murata GCM1555C1H270JA16) at the input of the external LNA
supply line.
It should be noted anyway that the insertion loss of the filter directly affects the system noise figure
and hence the system performance. The selected SAW filter has to provide very low loss (no more
than 1.5 dB) in the GNSS pass-band, beside providing very large attenuation (more than 40 to 60 dB)
in the out-of-band jammers
’
cellular frequency bands (see
for possible suitable examples).
SARA-R510M8S already provides an integrated SAW filter and LNA (as illustrated in
). The
addition of such external components should be carefully evaluated, especially in case the application
power consumption should be minimized, since the LNA alone requires an additional supply current
of typically 5 to 20 mA.
Moreover, the first LNA of the input chain will dominate the receiver noise performance, therefore its
noise figure should be less than 2 dB. If the antenna is close to the receiver, then a good passive
antenna (see
can be directly connected to the receiver with a short (a few cm) 50
line.
From a noise point of view, this design choice offers comparable performance as an active antenna
with a long (~3 to 5m) cable attached to the application board by means of an SMA connector without
the increased power consumption and BOM cost. If the goal is to protect the GNSS receiver in a noisy
environment then an additional external SAW filter may be required. If a degradation in the C/No of 2
to 3dB (depending on the choice of the filter) is not acceptable for the application, then, to
compensate for the filter losses and restore an adequate C/No level, an external LNA with good gain
and low Noise Figure (see
) is to be considered.
lists examples of SAW filters suitable for the GNSS RF input of SARA-R510M8S modules.
Manufacturer
Part number
Description
Murata
SAFFB1G56AC0F0A
GPS / SBAS / QZSS / GLONASS / Galileo / BeiDou RF band-pass SAW
filter with high attenuation in Cellular frequency ranges
Murata
SAFFB1G56AC0F7F
GPS / SBAS / QZSS / GLONASS / Galileo / BeiDou RF band-pass SAW
filter with high attenuation in Cellular frequency ranges
Table 22: Examples of GNSS band-pass SAW filters
