LENA-R8 series - System integration manual
UBX-22015376 - R02
Design-in
Page 63 of 116
C1-Public
•
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
(as for example illustrated in
LENA-R8001M10
31
ANT_GNSS
L1
GND
C1
Figure 41: Simple notch filter for improved in-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-of-band interference
Out-of-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 may compromise the optimal reception of the GLONASS satellites. Again, the effect
can be explained by comparing the LTE frequencies (band 3, 4 and 66 low channel 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-of-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
•
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
for possible suitable examples). It has to be noted that,
LENA-R8001M10 modules already include an internal SAW filter, followed by an additional LNA,
followed by another SAW filter before the u-blox GNSS chipset (as illustrated in
): 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
If all above 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
