HE910 Hardware User Guide
1VV0300925 Rev. 32 Page 66 of 111 2019-11-25
7.3.8. External LNA Gain and Noise Figure
The HE910 can be used with an external LNA such as what might be found in an active
antenna. Because of the internal LNA, the overall gain (including signal losses past the
external LNA) should not exceed 14 dB. Levels higher than that can affect the jamming
detection capability of the HE910. If a higher gain LNA is used, either a resistive Pi or T
attenuator can be inserted after the LNA to bring the gain down to 14 dB .
The external LNA should have a noise figure better than 1 dB. This will give an overall
system noise figure of around 2 dB assuming the LNA gain is 14 dB, or if higher the low
gain mode is automatically managed by the HE910 with its internal AGC.
The external LNA, if having no pre-select filter, needs to be able to handle other signals
other than the GPS signal. These signals are typically at much higher levels. The amplifier
needs to stay in the linear region when presented with these other signals. Again, the
system designer needs to determine all of the unintended signals and their possible levels
that can be presented and make sure the external LNA will not be driven into
compression. If this were to happen, the GPS signal itself would start to be attenuated and
the GPS performance would suffer.
7.3.9. Powering the External LNA (active antenna)
The external LNA needs a source of power. Many of the active antennas accept a 3 volt
or 5 volt DC voltage that is impressed upon the RF signal line. This voltage is not supplied
by the HE910, but can be easily supplied by the host design.
7.3.10. External LNA Enable
The HE910 is already provided by an internal LNA. In case the Application needs to
include an additional LNA stage, the module is provided by a digital signal usable to
enable the power supply of the external amplifier. The signal is set to High only when the
GPS receiver is active.
The electrical characteristics of the GPS_LNA_EN signal are:
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