u-blox EVA-8M Series Hardware Integration Manual

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u-blox EVA-8M Series Hardware Integration Manual Manual pdf 17 page image

EVA-8M and EVA-M8 series - Hardware Integration ManualUBX-16010593 - R06 Design-in Page 17 of 47Early Production Information• 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.If Power Save Mode is used and the minimum current consumption has to be achieved, then theexternal 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.☞ The function of the ANT_OFF pin can be inverted by sending the following sequence to thereceiver:B5 62 06 41 0C 00 00 00 03 1F 90 47 4F B1 FF FF EA FF 33 98☞ Applying this sequence results in a permanent change and cannot be reversed. An unstable supplyvoltage at the VCC_IO pin while applying this sequence can also damage the receiver.☞ 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 thisapproach! To provide additional robustness an ESD protection diode, as listed in Appendix B.7, can beplaced in front of the LNA to GND.2.5.3 Improved Jamming ImmunityIf strong out-band jammers are close to the GNSS antenna (e.g. a GSM antenna) GNSS performancecan 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), seeAppendix A. If the external LNA can accept the maximum input power, the SAW filter between thepassive antenna and external LNA (LNA1) might not be necessary. This results in a better noise figurethan 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 isrecommended to use additional filtering. Improved interference immunity with good GNSSperformance can be achieved when using a SAW/LNA/SAW configuration between the antenna andthe RF-input. The single-ended SAW filter (F2) can be placed in front of the LNA matching network toprevent 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 figureand hence the system performance. Choice of a component with low insertion loss is mandatory whena passive antenna is used with this set-up. An example schematic for an improved jamming immunityis 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 SafeBoot Mode and doesn’t begin GNSS operation. In Safe Boot Mode the module runs from an internalLC oscillator and starts regardless of any configuration provided by the configuration pins. Thus, itcan 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 communicatevia USB in Safe Boot Mode. For communication by UART in Safe Boot Mode, a training sequence (0x55 55 at 9600 baud) can be sent by the host to the EVA-8M / EVA-M8 series GNSS modules in orderto enable communication. After sending the training sequence, the host has to wait for at least 2 msbefore sending messages to the receiver. For further information see theu-blox 8 / u-blox M8 ReceiverDescription Including Protocol Specification [3].