SAM-M10Q - Integration manual
If the hardware backup mode is not used, leave the V_BCKP pin open.
3.2 RF interference
The received GNSS signal power at the antenna is very low compared to other wireless
communication signals. The nominal -130 dBm received GNSS signal strength is below the thermal
noise floor, making a GNSS receiver susceptible to interference from nearby RF sources of any kind.
As an example, cellular applications emit signals with power levels of approxi30 dBm, while
the GNSS signal is less than -128 dBm when reaching the antenna. By simply comparing these
numbers it is obvious that interference issues must be seriously considered during the design
phase.
3.2.1 In-band interference
Although the radio communications standards prevent intentional RF signal sources from
interfering the GNSS frequencies, many devices emit RF power into the GNSS band at levels much
higher than the GNSS signal itself.
One reason is that the frequency band above 1 GHz is not well regulated with regards to EMI,
and even if permitted, signal levels are much higher than the GNSS signal power. In particular,
all types of digital equipment, such as PCs, digital cameras, LCD screens, etc. tend to emit a
broad frequency spectrum up to several GHz of frequency. Also wireless transmitters may generate
spurious emissions that fall into the GNSS band.
The section
defines measures against in-band interference during the design phase
of the application.
3.2.2 Out-of-band interference
Out-of-band interference is caused by signal frequencies that are different from the GNSS carrier
frequency. The main sources are wireless communication systems such as LTE, GSM, CDMA,
WCDMA, Wi-Fi, BT, etc. Typically, these systems may emit their specified maximum transmit power
in close proximity to the GNSS receiving antenna, especially if such a system is integrated with
the GNSS receiver. Even at reasonable antenna selectivity, destructive power levels may reach the
RF input of the GNSS receiver. In addition, larger signal interferers may generate intermodulation
products inside the GNSS receiver front-end that fall into the GNSS band and contribute to in-band
interference.
3.2.3 Spectrum analyzer
The UBX-MON-SPAN message can be enabled in u-center to provide a low-resolution spectrum
analyzer sufficient to identify noise or jammers in the reception band. Once enabled, u-center
includes a real-time chart that is updated once per second with the message data. See
for an example.
The shape of the spectrum as well as any RF interference in the form of a spur or intermodulation
product can be easily analyzed with this graphical tool. The vertical axis compares the amplitude
difference in dB for each frequency. A good spectrum shape is characterized by an even noise floor
along with the GNSS band. For example, if any unwanted spur stands out, the vertical axis gives a
rough approximation of the power level in dB compared to the noise floor.
Next to the chart, the center frequency, span, and resolution values set for the spectrum, and the
PGA value are also displayed. The PGA value represents the internal gain set by the receiver, which
depends on the external amplification of the GNSS input signal.
UBX-22020019 - R01
3 Hardware integration
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