SE873 Family Product User Guide
RF Signals
1VV0301216
Rev.4
Page 45 of 69
2018-08-24
RF Trace Losses
RF Trace losses on a PCB are difficult to estimate without having appropriate tables or RF
simulation software. A good rule of thumb would be to keep the RF traces as short as possible,
make sure they are 50 ohm impedance, and don’t contain any sharp bends.
Figure 9-1 RF Trace Examples
Implications of the Pre-Select SAW Filter
The SE873 modules include a pre-select SAW filter in front of the internal LNA. Thus, the RF
input of the module is connected directly to the SAW filter. 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.
Summary of Contents for SE873 Series
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