SE868Kx-Ax Family Product User Guide
1VV0301201 Rev. 5
Page 58 of 83
2021-07-15
Not Subject to NDA
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 CW
jamming signals in each band (GPS and GLONASS), 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.
Shielding
Shielding the RF circuitry generally is ineffective because the interference is received by
the GNSS antenna itself, which is 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.
