SL876Q5-A
Product User Guide
1VV0301333 Rev. 2
Page 52 of 68
2017-06-27
Powering an External LNA (External Active Antenna)
An external LNA requires a source of power. Many active antennas accept 3 V to 5 V DC 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:
Simulations done by Telit show the following losses:
Table 10-1 Inductor Loss
Since this additional loss occurs after the LNA, it is generally not significant unless the circuit
is being designed to work with both active and passive antennas.
•
RF path loss introduced by either the inductor or quarter wave stub.
•
Cost of the inductor.
•
Space availability for the quarter wave stub.
•
Inductor
Additional signal loss (dB)
Murata LQG15HS27NJ02
0.65
Quarter wave stub on FR4
0.59
Coilcraft B09TJLC (used in ref. design)
0.37
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