MAX-8 / MAX-M8 - Hardware Integration Manual
UBX-15030059 - R05
Product handling
Page 26 of 31
Production Information
4.4
Applications with cellular modules
GSM terminals transmit power levels up to 2 W (+33 dBm) peak, 3G and LTE up to 250mW
continuous. Consult the data sheet for the absolute maximum power input at the GNSS receiver.
See the GPS Implementation and Aiding Features in u-blox wireless modules [7].
Isolation between GNSS and cellular antenna
In a handheld type design, an isolation of approximately 20 dB can be reached with careful placement
of the antennas. If such isolation cannot be achieved, e.g. in the case of an integrated cellular /GNSS
antenna, an additional input filter is needed on the GNSS side to block the high energy emitted by the
cellular transmitter. Examples of these kinds of filters would be the SAW Filters from Epcos (B9444
or B7839) or Murata.
Increasing interference immunity
Jamming signals come from in-band and out-band frequency sources.
In-band interference
With in-band jamming, the signal frequency is very close to the GNSS constellation frequency used,
e.g. GPS frequency of 1575 MHz (see
Figure 18). Such interference signals are typically caused by
harmonics from displays, micro-controller, bus systems, etc.
1525
1550
1625
GPS input filter
characteristics
1575
1600
0
-110
Jammin
g signal
1525
1550
1625
Frequency [MHz]
Power [dBm]
GPS input filter
characteristics
1575
1600
0
Jamming
signal
GPS
signals
GPS Carrier
1575.4 MHz
Figure 18: In-band interference signals
Figure 19: In-band interference sources
Measures against in-band interference include:
•
Maintaining a good grounding concept in the design
•
Shielding
•
Layout optimization
•
Filtering
•
Placement of the GNSS antenna
•
Adding a CDMA, GSM, WCDMA band pass filter before handset antenna
