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For using passive antennas, the pin VANT (DC bias voltage) on the module is
connected to ground, and the antenna is directly connected to the GPS signal input pin
ANT. Sometimes, a passive matching connection is required to match the electrical
circuit to 50 Ohms impedance.
6.2 Active Antennas
For FV-25, the active antenna is integrated with a Low Noise Amplifier (LNA), which
is a built-in component part, in the RF section. Through pin ANT, the module obtains
the incoming signal from the antenna. The power supply for the active antenna is
from pin VANT and, in general, the supply voltage is transmitted by the coaxial RF
cable. The supply voltage in pin VANT is supported by either source. One is from the
external power supply and the other is from the output pin V
RF
(connected with
VANT), which is the power supply from the module for RF section. The voltage
requirements for the antenna and the pins on the module have to be specified.
The use of the active antennas will decrease the “bad” effects, which result from the
cable loss and hardware noises, on the received GPS signals. Therefore, the placement
of the active antenna can be away from the possible noise sources, for example, the
module and peripheral circuits, and the active antenna will have good performance if
it is located far from the noise sources. This will ease the circuit design, and the
received signals is less sensitive to jamming. But the active antenna will increase the
power consumption of the whole system, typically in the range of 5 mA to 20 mA.
It is recommended to use an active antenna if the cable length between module and
antenna exceeds 10 cm. The same advice also goes for users without much experience
on the RF design. For FV-25, the active antenna gain should not exceeds 25 dB
because an saturation (overload) condition might occur for high gain (> 25 dB) cases.
NOTE: It’s better not to disconnect antenna during the operation of the module. The
calculation of the reference floor noise is based on the actual condition after the
power is turned on. Hence, the reacquisition time may be prolonged after
re-connecting the antenna to the module.
NOTE: To verify the reacquisition time, users can use a physical object to block the
antenna from receiving the signal until the module loses the lock of the satellites and
then take the object away from the antenna.