EVA-7M - Hardware Integration Manual
UBX-12003235 - R05
Production Information
Design-in
Page 14 of 42
circuit (see Figure 18) or the 3-pin active antenna supervisor circuit (see Figure 4), has to be added to the active
antenna circuit. These active antenna supervisor circuits also make sure that the active antenna is turned off in
Power Save Mode stages.
2.5.2
Passive Antenna
If a passive antenna is connected to the EVA-7M, it is mandatory to use an additional LNA in front of EVA-7M to
achieve the performance values as written in the
EVA-7M Data Sheet
[1] , see Appendix A. An LNA (U1) alone
would make the EVA-7M more sensitive to out-band jammers, so an additional GNSS SAW filter (F1) has to be
connected between the external LNA (U1) and the EVA-7M RF-input If strong out-band jammers are close to the
GNSS antenna (e.g. a GSM antenna), see section 2.5.3.
The LNA (U1) can be selected to deliver the performance needed by the application in terms of:
•
Noise figure (sensitivity)
•
Selectivity and linearity (Robustness against jamming)
•
Robustness against RF power and ESD
The external LNA (U1) must be placed close to the passive antenna to get best performance.
The
ANT_OFF
pin can be used to turn off an external LNA. The
ANT_OFF
signal must be inverted for common
LNAs which come with an enable pin which has be “low” to turn off.
The the function of the
ANT_OFF
pin can be inverted by sending the following sequence to the receiver:
B5 62 06 41 0C 00 00 00 03 1F 90 47 4F B1 FF FF EA FF 33 98
Note, that applying this sequence results in a permanent change and cannot be reversed.
A pull-down resistor (R7) is required to ensure correct operation of the
ANT_OFF
pin.
ESD discharge into the RF input cannot always be avoided during assembly and / or field use with this approach!
To provide additional robustness an ESD protection diode, as listed in Appendix B.7, can be placed in front of the
LNA to GND.
2.5.3
Improved Jamming Immunity
If strong out-band jammers are close to the GNSS antenna (e.g. a GSM antenna) GNSS performance can be
degraded or the maximum input power of the EVA-7M RF-input can be exceeded. An additional SAW filter (F2)
has to put in front of the external LNA (U1) , see Appendix A. If the external LNA can accept the maximum input
power, the SAW filter between the passive antenna and external LNA (LNA1) might not be necessary. This results
in a better noise figure than an additional SAW filter (F2) in front of the external LNA (U1).
If the EVA-7M is exposed to an interference environment it is recommended to use additional filtering. Improved
interference immunity with good GNSS performance is achieved when using a SAW/LNA/SAW configuration
between the antenna and the EVA-7M RF-input. The single-ended SAW filter (F2) can be placed in front of the
LNA matching network to prevent receiver blocking due to strong interference, see Figure 5.
It should be noted that the insertion loss of SAW filter (F2) directly affects the system noise figure and hence the
system performance. Choice of a component with low insertion loss is mandatory when a passive antenna is
used with this set-up. An example schematic for an improved jamming immunity is shown in Appendix A.3 (see
Figure 14).
2.6
RESET_N
The EVA-7M provides a
RESET_N
pin to reset the system. The
RESET_N
is an input-only with internal pull-up
resistor. It must be at low level for at least 10 ms to make sure
RESET_N
is detected. It is used to reset the
system. Leave
RESET_N
open for normal operation. The
RESET_N
complies with the
VCC_IO
level and can be
actively driven high.
RESET_N
should be only used in critical situations to recover the system. The Real-Time Clock (RTC) will
also be reset and thus immediately afterwards the receiver cannot perform a Hot Start.
