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XT65/XT75 Hardware Interface Overview
5.2 GPS Antenna
5.2
GPS Antenna
It is possible to connect active or passive GPS antennas. In either case they must have 50 Ohm impedance. The
application should be designed in a way to achieve a minimum of 6dB decoupling between the GSM/DCS/PCS
antenna path and the GPS antenna path. Please note that the GPS antenna must be isolated for ESD protection
(to withstand a voltage resistance up to 8kV air discharge).
Active versus Passive Antennas
Passive antennas contain only the radiating element, e.g. the ceramic patch or the quadrifilar dipole structure.
Sometimes they also contain a passive matching network to match the electrical connection to 50 Ohms imped-
ance. Note: Passive antenna need not have a DC connection to ground.
Active antennas have an integrated low-noise amplifier (in some cases an additional GPS band pass filter). This
is beneficial in two respects: First, the losses of the cable do no longer influence the overall noise figure of the
GPS receiver system. Secondly, even the receiver noise figure can be higher without sacrificing performance.
Active antennas need a power supply that will contribute to GPS system power consumption, typically in the
region of 5 to 20 mA. The supply voltage is fed to the antenna through the coaxial RF cable.
Inside the antenna, the DC component on the inner conductor will be separated from the RF signal and routed
to the supply pin of the LNA.
The use of an active antenna is always advisable, if the RF-cable length between receiver and antenna exceeds
about 10 cm.
Table 10: GPS antenna: Active versus Passive
Active Antenna
Active antenna connected to the GPS module.
•
A wide range of active patch or quadrifilar
dipole antennas is available in the market.
They differ in size, sensitivity, selectivity and
power consumption
•
Less sensitive to jamming than a passive
antenna, as the placement of the active
antenna is usually some distance away of
other noise or signal radiating devices.
•
Needs more power than a passive antenna
•
Easier and less sensitive to jamming.
•
More freedom to place the antenna
1.
Some cars for instance have a metallic coating on the windshield. GPS reception may not be possible in such a
car. There is usually a small section, typically behind the rear view mirror without the coating for mobile phone
and GPS antennas. The antenna has to be placed with optimal sky visibility. An external antenna (e.g. with a
magnetic base) is easier to use and usually allows a better positioning.
Note: If you are not an expert in RF designs, you should implement an active antenna setup and place the
antenna away from any emitting circuits.
XT65_XT75_HO_v00.130
Confidential / Preliminary
Passive Antenna
Passive patch antennas or quadrifilar dipole antennas con-
nected with a microcoax to the GPS module
•
Passive patch antennas or helical antennas are avail-
able in different form factors and sensitivity
•
Antenna must be connected with a low insertion loss
line to the GPS module to ensure a good GPS sensitiv-
ity.
•
The PCB design with a passive antenna must consider
the sensitivity of the GPS antenna to other radiating cir-
cuits or general signal jamming.
•
Due to the proximity of the GPS antenna to other elec-
tronic circuits, in-band jamming may become a critical
issue.
1
•
Only suitable for RF experts!
•
Needs more experience in RF design.
•
Requires more effort to optimise the circuit design to
minimize jamming into the antenna and the antenna
signal routing.
Page 28 of 67
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2006-10-12
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