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 57 of 76
Mod. 0805 2015-02 Rev.4
Simulations done by Telit show the following:
Inductor
Additional signal loss (dB)
Murata LQG15HS27NJ02
0.65
Quarter wave stub on FR4
0.59
Coilcraft B09TJLC (used in ref. design)
0.37
Table 11-1 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.
11.11.
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 a number of CW jamming signals in each band
(GPS, GLONASS, and BeiDou), 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 its passband.
Because of the nature of the address and data lines, this would be broadband digital noise at a
relatively high level.
Such devices are required to adhere to a regulatory standard for emissions such as FCC Part 15
Subpart J Class B or CISPR 22. However, these regulatory emission levels are far higher than
the GNSS signal.
11.12.
Shielding
Shielding the RF circuitry generally is ineffective because the interference is received by the
GNSS antenna itself, the most sensitive portion of the RF path. The antenna cannot be shielded
because it could not then receive the GNSS signals.
There are two solutions, one is to move the antenna away from the source of interference, and
the other is to shield the digital interference source to prevent it from getting to the antenna.
