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 26 of 40
Mod. 0805 2011-07 Rev.2
9.7.
Implications of the Pre-select SAW Filter
The JF2 module contains a SAW filter used in a pre-select configuration with the built in
LNA, that is the RF input of the JF2 ties directly into the SAW filter. Any circuit connected to
the input of the JF2 would see a 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 JF2, 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 JF2.
If an external filter is to be connected directly to the JF2, care needs to be used in making sure
neither the external filter or 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.
9.8.
External LNA Gain and Noise Figure
The JF2 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 JF2. 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 or the JF2 can be switched into a low gain
mode by issuing an OSP command to do so.
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 selected within the JF2.
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.
