NEO-M9N - Integration manual
• In-band interference: Although the GNSS band is kept free from intentional RF signal sources
by radio-communications standards, 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 GNSS signal power. Notably, 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 GNSS band.
As an example, GSM uses power levels of up to 2 W (+33 dBm). The absolute maximum power
input at the RF input of the GNSS receiver can be +15 dBm. The GSM specification allows spurious
emissions for GSM transmitters of up to +36 dBm, while the GNSS signal is less than -128
dBm. By simply comparing these numbers it is obvious that interference issues must be seriously
considered in any design of a GNSS receiver. Different design goals may be achieved through
different implementations:
• The primary focus is to prevent damaging the receiver from large input signals. Here the
GNSS performance under interference conditions is not important and suppression of the
signal is permitted. It is sufficient to just observe the maximum RF power ratings of all of the
components in the RF input path.
• GNSS performance must be guaranteed even under interference conditions. In such a case,
not only the maximum power ratings of the components in the receiver RF path must be
observed. Further, non-linear effects like gain compression, NF degradation (desensitization)
and intermodulation must be analyzed.
Pulsed interference with a low-duty cycle such as GSM may be destructive due to the high
peak power levels.
4.7.2 In-band interference mitigation
With in-band interference, the signal frequency is very close to the GNSS frequency. Such
interference signals are typically caused by harmonics from displays, micro-controller operation, bus
systems, etc. Measures against in-band interference include:
• Maintaining a good grounding concept in the design
• Shielding
• Layout optimization
• Low-pass filtering of noise sources, e.g. digital signal lines
• Remote placement of the GNSS antenna, far away from noise sources
• Adding an LTE, CDMA, GSM, WCDMA, BT band-pass filter before antenna
4.7.3 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.
Measures against out-of-band interference include maintaining a good grounding concept in the
design and adding a GNSS band-pass filter into the antenna input line to the receiver.
For GSM applications, such as typical handset design, an isolation of approximately 20 dB can be
reached with careful placement of the antennas. If this is insufficient, an additional SAW filter is
required on the GNSS receiver input to block the remaining GSM transmitter energy.
UBX-19014286 - R07
4 Design
Page 78 of 95
C1-Public
