ZED-F9P - Integration Manual
UBX-18010802 - R01
4 Electromagnetic
interference on I/O lines
Page 37 of 64
Objective Specification - Confidential
• Out-of-band interference: Typically any kind of wireless communications system (e.g. LTE,
GSM, CDMA, 3G, WLAN, Bluetooth, etc.) may emit its 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. Also, 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.
• 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, like 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 2W (+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 prevention of destruction of the receiver from large input signals. Here
the GNSS performance under interference conditions is not important and suppression of the
GNSS signal is permitted. It is sufficient to just observe the maximum RF power ratings of all
the components in the RF input path.
• GNSS performance must be guaranteed even under interference conditions. In that case,
not only the maximum power ratings of the components in the receive patch 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 like e.g. GSM may be destructive due to the high
peak power levels.
4.2 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 optimisation
• Low-pass filtering of noise sources, e.g. digital signal lines
• Remote placement of the GNSS antenna, far away from noise sources
• Adding a LTE, CDMA, GSM, WCDMA ,BT band-pass filter before antenna
4.3 Out-of-band interference
Out-of-band interference is caused by signal frequencies that are different from the GNSS carrier
The main sources are wireless communication systems such as LTE, GSM, CDMA, WCDMA, WiFi,
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.
