User's Manual

HE910 Hardware User Guide

1vv0300925 Rev.28 – 2015-06-24

7.3.7 Implications of the Pre-select SAW Filter

The HE910 module contains a SAW filter used in a pre-select configuration with the built-in LNA, that

is, the RF input of the HE910 ties directly into the SAW filter. Any circuit connected to the input of the

HE910 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 to be used with the HE910, 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 HE910.

If an external filter is to be connected directly to the HE910, care needs to be used in making sure

neither the external filter nor the internal SAW filter performance is compromised. These components

are typically specified to operate into 50 ohms impedance, which is generally true in band, but would

not be true out of band. If there is extra gain associated with the external filter, then a 6 dB Pi or T

resistive attenuator is suggested to improve the impedance match between the two components.

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.