User's Manual

GE910 Hardware User Guide

GE910 Hardware User GuideGE910 Hardware User Guide

GE910 Hardware User Guide

1vv0300962 Rev.9-bis3 2013-05-20

Mod. 0805 2011-07 Rev.2

7.4.6. RF Trace Losses

RF Trace losses are difficult to estimate on a PCB without having the appropriate tables or RF

simulation software to estimate what the losses would be. A good rule of thumb would be to

keep the RF traces as short as possible, make sure they are 50 ohms impedance and don’t

contain any sharp bends.

7.4.7. Implications of the Pre-select SAW Filter

The module GE910-GNSS contains a SAW filter used in a pre-select configuration with the

built-in LNA, that is, the RF input of the module ties directly into the SAW filter. Any circuit

connected to the input of the module would see complex impedance presented by the SAW

filter, particularly out of band, rather than the relatively broad and flat return loss presented by

the LNA. Filter devices pass the desired in band signal to the output, resulting in low reflected

energy (good return loss), and reject the out of band signal by reflecting it back to the input,

resulting in high reflected energy (bad return loss).

If an external amplifier is used with the module, the overall design should be checked for RF

stability to prevent the external amplifier from oscillating. Amplifiers that are unconditionally

stable at the output will be fine to use with the module.

7.4.8. External LNA Gain and Noise Figure

The module GE910-GNSS 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. Higher levels can affect the jamming detection

capability of the module. 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 with its internal AGC.

The external LNA, if having no pre-select filter, needs to be able to handle signals other than the

GNSS 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 GNSS

signal itself would start to be attenuated and the performance would suffer.

7.4.9. 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

GE910-GNSS module, but can be easily supplied by the host design.