JF2 Hardware User Guide
1vv0300985 Rev.4 2013-04-09
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 22 of 40
Mod. 0805 2011-07 Rev.2
9.
RF Front End Design
The JF2 contains an integrated LNA and pre-select SAW filter. This allows the JF2 to work
well with a passive GPS antenna. If the antenna cannot be located near the JF2, then an active
antenna (that is, an antenna with a low noise amplifier built in) can be used. The following
items will be discussed in turn to assist in designing the “RF front end”.
1.
RF signal requirements
2.
GPS antenna polarization
3.
GPS antenna gain
4.
System noise floor
5.
Active versus passive antenna
6.
RF trace losses
7.
Implications of the pre-select SAW filter
8.
External LNA gain and Noise Figure
9.
Powering the external LNA (active antenna)
10.
RF interference
11.
Shielding
9.1.
RF Signal Requirements
The JF2 can achieve Cold Start acquisition with a signal level of -147 dBm at its input. This
means the JF2 can find the necessary satellites, download the necessary ephemeris data and
compute the location within a 5 minute period. In the GPS signal acquisition process,
downloading and decoding the data is the most difficult task, which is why Cold Start
acquisition requires a higher signal level than navigation or tracking signal levels. For the
purposes of this discussion, autonomous operation is assumed, which makes the Cold Start
acquisition level the important design constraint. If assistance data in the form of time or
ephemeris aiding is available, then even lower signal levels can be used to compute a
navigation solution.
The GPS signal is defined by IS-GPS-200E. This document states that the signal level
received by a linearly polarized antenna having 3 dBi gain will be a minimum of -130 dBm
when the antenna is in the worst orientation and the satellite is 5 degrees or more above the
horizon.
In actual practice, the GPS satellites are outputting slightly more power than specified by IS-
GPS-200E, and the signal level typically goes higher as the satellites have higher elevation
angles.
The JF2 will display a reported C/No of 40 dB-Hz for a signal level of -130 dBm into the RF
input.
