Integration Manual

SARA-R5 series - System integration manual
UBX-19041356 - R03 System description Page 30 of 123
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Jamming signals come from in-band and out-band frequency sources. In-band jamming is caused by
signals with frequencies within or close to the GNSS constellation frequency used, while out-band
jamming is caused by very strong signals with frequencies different from the GNSS carrier, that is
picked up at the input of the GNSS receiver and that can saturate the receiver front-end. if not
properly taken into consideration those signals cause a reduction in the carrier-to-noise power density
ratio (C/No) of the GNSS satellites.
In-band interference signals are typically caused by harmonics from displays, switching converters,
micro-controllers and bus systems. Moreover, considering for example the LTE Band 13 high channel
transmission frequency (787 MHz) and the GPS operating band (1575.42 MHz ± 1.023 MHz), the
second harmonic of the cellular signal is exactly within the GPS operating band. Therefore, depending
on the board layout and the transmit power, the highest channel of LTE Band 13 is the channel that
has the greatest impact on the C/No reduction.
Countermeasures against in-band interference include:
 maintaining a good grounding concept in the design
 ensuring proper shielding of the different RF paths
 ensuring proper impedance matching of RF traces
 placing the GNSS antenna away from noise sources
 add a notch filter along the GNSS RF path, in front of SAW filter, at the frequency of the jammer
(as for example, a notch filter at ~787 MHz improves the immunity to LTE Band 13 high channel)
Out-band interference is caused by signal frequencies that are different from the GNSS, the main
sources being cellular, Wi-Fi, bluetooth transmitters, etc. For example, the lowest channels in LTE
Band 3, 4 and 66 can compromise the good reception of the GLONASS satellites. Again, the effect can
be explained by comparing the LTE frequencies (low channels transmission frequency is 1710 MHz)
with the GLONASS operating band (1602 MHz ± 8 MHz). In this case the LTE signal is outside the
useful GNSS band, but, provided that the power received by the GNSS subsystem at 1710 MHz is high
enough, blocking effects may appear reducing once again the C/No.
Countermeasures against out-band interference include:
 maintaining a good grounding concept in the design
 keeping the GNSS and cellular antennas more than the quarter-wavelength (of the minimum Tx
frequency) away from each other. If for layout or size reasons the aforementioned requirement
cannot be met, then the antennas should be placed orthogonally to each other and/or on different
side of the PCB.
 ensuring at least 15 – 20 dB isolation between antennas in the GNSS band
 adding a GNSS pass-band SAW filter along the GNSS RF line, providing very large attenuation in
the cellular frequency bands (as for example Murata SAFFB1G56AC0F0A, or SAFFB1G56AC0F7F).
It has to be noted that, as shown in Figure 3, a SAW filter and an LNA are already integrated in the
GNSS RF path of the SARA-R510M8S: the addition of an external filter along the GNSS RF line has
to be considered only if the conditions above cannot be met.
 adding a GNSS stop-band SAW filter along the cellular RF line, providing very low attenuation in
the cellular frequency bands (as for example the Qualcomm B8636, or B8666). It has to be noted
that the addition of an external filter along the cellular RF line has to be carefully evaluated as
further countermeasure only if the conditions above cannot be met in different way, considering
that an external filter may affect the cellular TRP and/or TIS RF performance figures.
As far as Tx power is concerned, SARA-R5 series modules maximum output power during LTE
transmission is 23 dBm. High-power transmission occurs very infrequently: typical output power
values are in the range of -3 to 0 dBm (see Figure 1 in the GSMA official document TS.09 [10]).
Therefore, depending on the application, careful PCB layout and antenna placement should be
sufficient to ensure accurate GNSS reception.