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 56 of 76
Mod. 0805 2015-02 Rev.4
11.9.
Implications of the Pre-Select SAW Filter
The SE868-V3 modules include a pre-select SAW filter in front of the internal LNA. Thus the
RF input of the module is connected directly to the SAW filter. Any circuit connected to the
RF input pin would see a complex impedance presented by the SAW filter (especially out of
band), rather than the relatively broad and flat return loss presented by an LNA. Filter devices
pass the desired in-band signal, resulting in low reflected energy (good return loss), and reject
the out-of-band signals by reflecting it back to the input, resulting in bad return loss.
If an external amplifier is to be used with the receiver, the overall design should be checked for
RF stability to prevent the external amplifier from oscillating. Amplifiers that are
unconditionally stable at their output will function correctly.
If an external filter is to be connected directly to the module, care needs to be used in making
sure the external filter or the internal SAW filter performance is not compromised. These
components are typically specified to operate into 50
Ω
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.
11.10.
Powering an External LNA (active antenna)
An external LNA requires a source of power. Many active antennas accept a 3 V or
5 V DC voltage that is impressed upon the RF signal line.
Two approaches can be used:
1.
Use an inductor to tie directly to the RF trace. This inductor should be at self-resonant at
L1 (1.57542 GHz) and should have good Q for low loss. The higher the inductor Q, the
lower the loss will be. The side of the inductor connecting to the antenna supply voltage
should be bypassed to ground with a good quality RF capacitor, again with self-resonance
at the L1 frequency.
2.
Use a quarter wave stub in place of the inductor. The length of the stub is designed to be
exactly ¼ wavelength at L1, which has the effect of making an RF short at one end of the
stub to appear as an RF open at the other end. The RF short is created by a high quality RF
capacitor operating at self-resonance.
The choice between the two would be determined by:
•
RF path loss introduced by either the inductor or quarter wave stub.
•
Cost of the inductor.
•
Space availability for the quarter wave stub.
