HE910 Hardware User Guide
1VV0300925 Rev. 32
Page
65
of
111
2019-11-25
7.3.5.
GPS Antenna - PCB Line Guidelines
•
Ensure that the antenna line impedance is 50ohm.
•
Keep the antenna line on the PCB as short as possible to reduce the loss.
•
Antenna line must have uniform characteristics, constant cross section, avoid
meanders and abrupt curves.
•
Keep one layer of the PCB used only for the Ground plane, if possible.
•
Surround (on the sides, over and under) the antenna line on PCB with Ground,
avoid having other signal tracks facing directly the antenna line of track.
•
The ground around the antenna line on PCB has to be strictly connected to the
Ground Plane by placing vias once per 2mm at least.
•
Place EM noisy devices as far as possible from HE910 antenna line.
•
Keep the antenna line far away from the HE910 power supply lines.
•
Keep the antenna line far away from the HE910 GSM RF lines.
•
If you have EM noisy devices around the PCB hosting the HE910, such as fast
switching ICs, take care of the shielding of the antenna line by burying it inside the
layers of PCB and surround it with Ground planes, or shield it with a metal frame
cover.
•
If you do not have EM noisy devices around the PCB of HE910, use a strip-line on
the superficial copper layer for the antenna line. The line attenuation will be lower
than a buried one.
7.3.6.
RF Trace Losses
RF Trace losses are difficult to estimate on a PCB without having the appropriate tables or
RF simulation software to estimate what the losses would be. A good rule of thumb would
be to keep the RF traces as short as possible, make sure they are 50 ohms impedance
and don’t contain any sharp bends.
7.3.7.
Implications of the Pre-select SAW Filter
The HE910 module contains a SAW filter used in a pre-select configuration with the built-
in LNA, that is, the RF input of the HE910 ties directly into the SAW filter. Any circuit
connected to the input of the HE910 would see complex impedance presented by the
SAW filter, particularly out of band, rather than the relatively broad and flat return loss
presented by the LNA. Filter devices pass the desired in band signal to the output,
resulting in low reflected energy (good return loss), and reject the out of band signal by
reflecting it back to the input, resulting in high reflected energy (bad return loss).
If an external amplifier is to be used with the HE910, the overall design should be checked
for RF stability to prevent the external amplifier from oscillating. Amplifiers that are
unconditionally stable at the output will be fine to use with the HE910.
If an external filter is to be connected directly to the HE910, care needs to be used in
making sure neither the external filter nor the internal SAW filter performance is
compromised. These components are typically specified to operate into 50 ohms
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.
Summary of Contents for HE910
Page 1: ... 01 2017 Mod 0805 2017 01 Rev 6 HE910 HW User Guide 1VV0300925 Rev 32 2019 11 25 ...
Page 40: ...HE910 Hardware User Guide 1VV0300925 Rev 32 Page 40 of 111 2019 11 25 ...
Page 71: ...HE910 Hardware User Guide 1VV0300925 Rev 32 Page 71 of 111 2019 11 25 ...
Page 111: ... 01 2017 Mod 0805 2017 01 Rev 6 ...
