SL871 Family Product User Guide
1VV0301170 Rev. 9
Page 60 of 89
2021-07-15
Not Subject to NDA
•
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, also with self-resonance at the L1 frequency.
•
Use a quarter wave stub in place of the inductor. The length of the stub is designed
to be exactly a quarter wavelength at L1 (1.57542 GHz), 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 the good 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.
Simulations done by Telit show the following results:
Inductor
Additional signal loss (dB)
Murata LQG15HS27NJ02 Inductor
0.65
Quarter wave stub on FR4
0.59
Coilcraft B09TJLC Inductor (used in ref. design)
0.37
Table 29: 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 Interference
RF Interference into the GNSS receiver tends to be the biggest problem when
determining why the system performance is not meeting expectations. As mentioned
earlier, the GNSS signals are at -130 dBm and lower. If signals higher than this are
presented to the receiver, the RF front end can be overdriven. The receiver can reject up
to 12 in-band CW jamming signals, but would still be affected by non-CW signals.
The most common source of interference is digital noise, often created by the fast rise
and fall times and high clock speeds of modern digital circuitry. For example, a popular
netbook computer uses an Atom processor clocked at 1.6 GHz. This is only 25 MHz away
from the GNSS signal, and depending upon temperature of the SAW filter, can be within
