SE868-V3 Product User Guide
1VV0301205 r4 – 2016-01-20
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 54 of 76
Mod. 0805 2015-02 Rev.4
11.5.
External LNA Gain and Noise Figure
The modules can be used with an external LNA (usually built into an active antenna). Because
of the internal LNA, the overall external gain (including signal losses past the external LNA)
should conform to the specification in
Table 4-1 Internal gain value and external gain range
for best performance. Using an external LNA may require setting the internal LNA to low gain
mode, which requires a firmware change. Levels higher than specified may affect the jamming
detection capability and overall performance of the receiver. Contact Telit for details.
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 higher if a low gain mode is
selected within the module. The overall system noise figure can be calculated using the Friis
formula for cascaded noise figure. The simplified formula is:
Equation 11-1 Cascade Noise Figure
Where F is the total system noise figure, F1 is the noise figure of the external LNA, F2 is the
noise figure of the internal LNA, and G1 is the gain of the external LNA. In the GNSS receiver,
digital noise is an additive number and cannot be reduced by reducing the System Noise figure.
Figure 11-1 Cascade Noise Figure – example calculations with external LNA
If the external LNA has no pre-select filter, it must be able to accommodate signals outside the
GNSS passband, which are typically at much higher levels than GNSS. The amplifier must 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 making
sure the external LNA will not be driven into compression. If this were to happen, the GNSS
signal itself would start to be attenuated and the GNSS performance would suffer.
11.6.
System Noise Floor
The receiver will display a reported C/No of 40 dB-Hz for an input signal level of -130 dBm.
The C/No number means the carrier (or signal) is 40 dB greater than the noise floor measured
in a one Hz bandwidth. This is a standard method of measuring GNSS receiver performance.
