SARA-R5 series - System integration manual
UBX-19041356 - R04
Design-in
Page 70 of 118
C1-Public
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, just after the antenna, at the frequency of the jammer
(for example, as depicted in
, a simple notch filter can be reralized by the series
connection of a discrete capacitor and inductor)
SARA-R51 0M8S
31
ANT_GNSS
L1
GND
C1
Figure 46: Simple notch filter for improved out-of-band jamming immunity against a single jamming frequency
, a simple notch filter can be realized by the series connection of an
inductor and capacitor. Capacitor C1 and inductor L1 values are calculated according to the formula:
𝑓 =
1
2 𝜋 √𝐶 ⋅ 𝐿
For example, a notch filter at ~787 MHz improves the GNSS immunity to LTE band 13 high channel.
Suitable component nominal values are C1 = 3.3 pF and L1 = 12 nH, with tolerance less than or equal to
2 % to ensure adequate notch frequency accuracy.
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 and leakage 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 this requirement cannot be met,
then the antennas should be placed orthogonally to each other and/or on different side of the PCB.
•
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
