SL876Q5-A
Product User Guide
1VV0301333 Rev. 2
Page 51 of 68
2017-06-27
Implications of the Pre-Select SAW Filter
The RF input of the module is connected directly to the SAW filter (after the RF switch).
Therefore, any circuit connected to the RF input pin would see a complex impedance
presented by the SAW filter (especially out of band), rather than the relatively broad and flat
return loss presented by an LNA. Filter devices pass the desired in-band signal, resulting in
low reflected energy (good return loss), and reject the out-of-band signals by reflecting it back
to the input, resulting in bad return loss.
If an external amplifier is to be used with the receiver, the overall design should be checked
for RF stability to prevent the external amplifier from oscillating. Amplifiers that are
unconditionally stable at their output will function correctly.
If an external filter is to be connected directly to the module, care needs to be used in making
sure the external filter or the internal SAW filter performance is not compromised. These
components are typically specified to operate into 50
Ω
impedance, which is generally true in-
band, but would not be true out of band. If there is extra gain associated with the external filter,
then a 6 dB Pi or T resistive attenuator is suggested to improve the impedance match between
the two components.
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,
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.
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.
Summary of Contents for SL876Q5-A
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