JF2 Hardware User Guide
1vv0300985 Rev.4 2013-04-09
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 28 of 40
Mod. 0805 2011-07 Rev.2
9.10.
RF Interference
RF Interference into the GPS receiver tends to be the biggest problem when determining why
the system performance is not meeting expectations. As mentioned earlier, the GPS signals
are at -130 dBm and lower. If signal higher than this are presented to the receiver it can be
overwhelmed. The JF2 can reject up to 8 CW in-band jamming signals, but would still be
affected by non-CW signals.
The most common source of interference is digital noise. This is 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
GPS signal, and depending upon temperature of the SAW filter, can be within the passband of
the GPS receiver. 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 GPS signal.
9.11.
Shielding
Shielding the RF circuitry generally is ineffective because the interference is getting into the
GPS antenna itself, the most sensitive portion of the RF path. The antenna cannot be shielded
because then it can’t receive the GPS signals.
There are two solutions, one is to move the antenna away from the source of interference or
the second is to shield the digital interference to prevent it from getting to the antenna.