EVA-8M and EVA-M8 series - Hardware Integration Manual
UBX-16010593 - R06
Design-in
Page 17 of 47
Early Production Information
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Selectivity and linearity (Robustness against jamming)
•
Robustness against RF power and ESD
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The external LNA (U1) must be placed close to the passive antenna to get best performance.
If Power Save Mode is used and the minimum current consumption has to be achieved, then the
external LNA should also be turned off. 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.
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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
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Applying this sequence results in a permanent change and cannot be reversed. An unstable supply
voltage at the VCC_IO pin while applying this sequence can also damage the receiver.
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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-8M / EVA-M8 series GNSS modules 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-8M / EVA-M8 series GNSS module is exposed to an interference environment, it is
recommended to use additional filtering. Improved interference immunity with good GNSS
performance can be achieved when using a SAW/LNA/SAW configuration between the antenna and
the 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 15.
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 15).
2.6
Safe Boot Mode (SAFEBOOT_N)
If the
SAFEBOOT_N
pin is “low” at start up, the EVA-8M / EVA-M8 series GNSS modules start in Safe
Boot Mode and doesn’t begin GNSS operation. In Safe Boot Mode the module runs from an internal
LC oscillator and starts regardless of any configuration provided by the configuration pins. Thus, it
can be used to recover from situations where the SQI Flash has become corrupted.
Owing to the inaccurate frequency of the internal LC oscillator, the module is unable to communicate
via USB in Safe Boot Mode. For communication by UART in Safe Boot Mode, a training sequence (0x
55 55 at 9600 baud) can be sent by the host to the EVA-8M / EVA-M8 series GNSS modules in order
to enable communication. After sending the training sequence, the host has to wait for at least 2 ms
before sending messages to the receiver. For further information see the
u-blox 8 / u-blox M8 Receiver
Description Including Protocol Specification [3].
